ch26

77

6 Suspension Polymerization Redox Initiators

Munmaya K. Mishra, Norman G. Gaylord, and Yusuf Yagci

Contents

6.1 Introduction...................................................................................................... 786.2 AcylPeroxide................................................................................................... 79

6.2.1 Fe2+asReductant................................................................................... 796.2.1.1 SuspensionPolymerizationofVinylChloride.......................80

6.2.2 Sn2+asReductant.................................................................................. 816.2.2.1 SuspensionCopolymerizationofAcrylonitrilewith

MethylAcrylateandwithStyrene.........................................846.2.3 Cu2+asReductant:SuspensionPolymerizationofVinylChloride.......846.2.4 TertiaryAmineasReductant................................................................85

6.2.4.1 SuspensionPolymerizationofVinylChloride....................... 896.2.4.2 SuspensionPolymerizationofAcrylonitrile.......................... 896.2.4.3 SuspensionPolymerizationofStyrene................................... 916.2.4.4 SuspensionPolymerizationofMethylMethacrylate.............92

6.2.5 QuaternaryAmmoniumSaltsasReductants........................................926.2.5.1 SuspensionPolymerizationofMethylMethacrylate.............946.2.5.2 SuspensionPolymerizationofStyrene...................................94

6.2.6 NitriteasReductant...............................................................................956.2.6.1 SuspensionPolymerizationofVinylChloride.......................966.2.6.2 SuspensionPolymerizationofVinylPyridine.......................96

6.3 AlkylPeroxide.................................................................................................976.3.1 AlkylBoronasReductant.....................................................................97

6.3.1.1 BulkPolymerizationofMethylMethacrylate...................... 1006.4 Peresters(PeroxyestersofCarbonicAcid).................................................... 101

6.4.1 MercaptansasReductant.................................................................... 1016.4.1.1 SuspensionPolymerizationofVinylChloride..................... 102

6.4.2 SulfideandDithionateasReductant................................................... 1026.4.2.1 SuspensionPolymerizationofVinylChloride..................... 103

6.4.3 AlkylBoraneasReductant................................................................. 1046.4.3.1 BulkPolymerizationofVinylChloride............................... 104

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78 Handbook of Vinyl Polymers: Radical Polymerization and Technology

6.1 IntRoDUCtIon

Theconditionsunderwhichradicalpolymerizationsareperformedarebothofthehomogeneousandheterogeneoustypes.Thisclassificationisusuallybasedonwhethertheinitialreactionmixtureishomogeneousorheterogeneous.Somehomogeneoussystemsmaybecomeheterogeneousaspolymerizationproceedsduetoinsolubilityofthepolymerinthereactionmedia.Heterogeneouspolymerizationisextensivelyusedasameanstocontrolthethermalandviscosityproblems.Threetypesofhet-erogeneouspolymerizationareused:precipitation,suspension,anddispersion.

Thetermsuspensionpolymerization(alsoreferredtoasbeadorpearlpolymer-ization)referstopolymerizationinanaqueoussystemwithamonomerasadispersedphase,resultinginapolymerasadispersedsolidphase.Thesuspensionpolymeriza-tionisperformedbysuspendingthemonomerasdroplets(0.001–1cmindiameter)inwater(continuousphase).Inatypicalsuspensionpolymerization,theinitiatorisdissolvedinthemonomerphase.Suchinitiatorsareoftenreferredtoasoil-solubleinitiators.Eachmonomerdropletinasuspensionisconsideredtobeasmallbulkpolymerizationsystemandthekineticsisthesameasthatofbulkpolymerization.Thesuspensionofamonomerismaintainedbyagitationandtheuseofstabilizers.Thesuspensionpolymerizationmethodisnotusedwithmonomers,whicharehighly

6.5 Peresters(PeroxyestersofCarboxylicAcid)................................................. 1046.5.1 Sn2+SaltsasReductant....................................................................... 104

6.5.1.1 SuspensionPolymerizationofVinylChloride..................... 1076.5.2 MercaptansasReductant.................................................................... 109

6.5.2.1 SuspensionGraftCopolymerizationofStyreneandAcrylonitriletoPolybutadieneLatex................................... 110

6.5.2.2 SuspensionCopolymerizationofAcrylonitrileandStyrene........................................................................... 111

6.5.3 AlkylBoraneasReductant................................................................. 1116.5.3.1 SuspensionPolymerizationofVinylChloride

orItsMixture....................................................................... 1116.5.4 BisulfiteasReductant.......................................................................... 111

6.5.4.1 SuspensionPolymerizationandCopolymerizationofVinylChloride.................................................................. 112

6.5.4.2 SuspensionGraftCopolymerizationofVinylPyridine....... 1126.5.5 MonosaccharideastheReductant....................................................... 113

6.5.5.1 SuspensionPolymerizationofVinylChloride..................... 1136.5.6 MetalMercaptidesasReductant......................................................... 1136.5.7 Ascorbic/IsoascorbicAcidorEstersasReductant............................. 114

6.5.7.1 SuspensionPolymerizationofVinylChloride..................... 1146.6 Hydroperoxides.............................................................................................. 115

6.6.1 SulfurDioxideasReductant............................................................... 1156.6.1.1 BulkPolymerizationofVinylChloride............................... 118

6.6.2 SulfiteasReductant............................................................................. 1216.6.2.1 BulkPolymerizationofAcrylonitrile................................... 121

References.............................................................................................................. 121

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Suspension Polymerization Redox Initiators 79

solubleinwaterorwhereapolymerhastoohighofaglasstransitiontemperature.Themethodisusedcommerciallytopreparevinylpolymerssuchaspolystyrene,poly(methylmethacrylate),poly(vinylchloride),poly(vinylacetate),poly(vinylidenechloride),andpoly(acrylonitrile).Varioustypesofredoxinitiatorareusedtopre-paresuchpolymersbysuspensionpolymerization.Thefollowingexamplesdescribethevarioustypesofinitiatingsystemforsuspensionpolymerization.

Suspensionpolymerizationisessentiallyequivalenttobulkpolymerizationbutisperformedinareactionmediuminwhichthemonomerisinsolubleanddispersedasadiscretephase(e.g.,droplets),withacatalystsystemthatgeneratesorpermitstheentryofradicalspecieswithinthesuspendedmonomerphaseordroplets.Thefollowingreviewpresentsexamplesofinitiatorsforbulkpolymerizationaswellassuspensionpolymerization,asinitiatingsystemssuitableforbulkpolymerizationduetomonomer-solublecatalystsarepotentiallyusefulinsuspensionpolymerization.

6.2 ACylPeRoxIDe

Acylperoxidesmaybedefinedassubstancesofthetype

RCOOCR||||OO

whereRandR′areeitheralkyloraryl.Acylperoxideshavebeenoneofthemostfre-quentlyusedsourcesoffreeradicals,andinterestintheirvariousmodesofdecom-positionhasbeenkeen.Acylperoxides(i.e.,benzoylperoxide[Bz2O2]andlauroylperoxide[LPO])havebeenusedextensivelyastheinitiatorforsuspensionpolymer-izationofstyrene[1–4],vinylchloride[5–7],andvinylacetate[8].

6.2.1 Fe2+asReductant

Kernandotherinvestigators[9,10]foundBz2O2tobeveryeffectiveinbothaqueousandnonaqueousmediawithorwithoutheavymetalsasacomponent,Kern[9]basedhis theory of reaction on Haber’s earlier suggestions and formulated the produc-tionofradicalsasanelectrontransferprocess.HeproposedaHaber–Weisstypeofmechanismfortwo-componentsystems:

Fe RCOO Fe RCOO RCOO22

3+ + -+ → + +( ) . (6.1)

whereRCOO.istheactivespecies.Inthepresenceofathirdcomponent,areducingagent(YH2),thereactioncon-

tinuesasfollows:

Fe YH Fe YH H32

2+ + ++ → + +. (6.2)

Fe YH Fe Y H3 2+ + ++ → + +. (6.3)

RCOO YH RCOOH YH. .+ → +2 (6.4)

The effect of activators like FeSO4 [11, 12] for emulsion polymerization andferricstearate[13]forbulkpolymerizationofvinylmonomersincombinationwith

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acylperoxidehasbeenstudied.TheferrousioncatalyzeddecompositionofBz2O2inethanolhasbeenstudiedinsomedetailbyHasegawaandco-workers[14,15].Thecycle,whichrequiresreducingofFe3_bysolvent-derivedradicals,yieldsasteady-stateconcentrationofFe2_afterafewminutes,shownspectroscopicallytobepro-portionaltotheinitialconcentrationoftheferrousion[14].Thesecond-orderrateconstantforthefollowingreactionwasfoundtobe8.4Lmol_1sec_1at25°C,withanactivationenergyof14.2kcalmol_1:

Bz O Fe BzO BzO Fe2 22 3+ → + ++ - +. (6.5)

BzO EtOH BzOH MeC HOH. .+ → + (6.6)

MeC HOH Fe Fe AcH H. + → + ++ + +3 2 (6.7)

Thesuspensionpolymerizationofvinylchlorideusinglauroylperoxide(LPO)andawater-solubleFe2_salt[16,17]andmonomer-soluble[18–20]Fe2_saltasthereducingagenthasbeenstudied.Inthecaseofamonomer-solublereducingagentlikeferrouscaproate,themechanismofinitiationofthepolymerizationisconsid-eredtobeanone-electrontransferreactioninthemonomerphaseasfollows:

C H COO OOCC H C H COO Fe C H CO11 23 11 23 5 11 2 11 23- + - →( ) OO

C H COO Fe OOCC H

.

( )+ - -11 23 5 11 2

(6.8)

C H COO C H COO Fe C H COO Fe OOC11 23 5 11 2 11 23. ( ) (+ - → - - 55 11 2H ) (6.9)

Das and Krishnan [21] had reported the suspension polymerization of vinylacetateandvinylalcoholusingaredoxpairofBz2O2andferrousoctoate(reducingagent).Ahighdegreeofpolymerizationwasachievedusingthisredox-pairinitiatingsystem.

6.2.1.1 suspensionPolymerizationofVinylChloride

InthesuspensionpolymerizationofvinylchlorideusingLPOandawater-solublereducingagent[16,17],Fe(OH)2(producedbyinsitureactionofaferroussaltandanalkalimetalhydroxide),theconversionwas80%and65%byusingaNamaleate–styrenecopolymerandpoly(vinylalcohol)asthedispersingagent,respectively.ThereactionwasperformedaccordingtotherecipepresentedinTable6.1.

Thesuspensionpolymerizationofvinylchloridewasalsoperformedat–15°Cusingamonomer-solublereducingagentlikeferrouscaproate[18,19].Themolecu-larweightofthepoly(vinylchloride)decreasedastheconcentrationoftheiron(II)systemincreased,becauseofchainterminationreactions.KonishiandNambu[20]also reported low-temperature polymerization of vinyl chloride using the LPO–ferrouscaproateredoxsystem.Thereactionwasstudiedbyvaryingthetemperaturefrom–30°C to+30°Cwithamolar ratioofoxidant to reductantof1:1.Theacti-vationenergyoftheoverallrateofpolymerizationwas6.5kcalmol_1.Theinitialrateincreased,andthedegreeofpolymerizationdecreased,withincreasingratioof

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ferrouscaproatetoLPO.TherelativeefficienciesoftheperoxidewiththereducingagentferrouscaproateweremeasuredandarepresentedinTable6.2.

A moderate rate of polymerization and a maximum yield were obtained byappropriate, continuous charging of the catalyst ingredients instead of the one-timeaddition.Thesyndiotacticitywasincreasedasthepolymerizationtemperaturedecreased.TheinitialratewasincreasedwiththeincreasingrateofferrouscaproatetoLPO,butafterpassingthroughtheratioofunity,themaximumyieldofthepoly-mersuddenlybecamelower.Thiscouldbeattributedtothedecreaseinthenumberof initiating radicalsas shown in reaction (6.9).Theoxidation–reduction reactioninitiatesandthepolymerizationcanproceedreadilyinthemonomerphasebyusingamonomer-solublereducingagent.

6.2.2 sn2+asReductant

Organicperoxidesmaydecomposeinanumberofdifferentwayswhentreatedwithionsofvariableoxidationnumberofdifferentwayswhentreatedwithionsofvari-ableoxidationnumber.Thereactioncanberationalizedonthebasisofthefollowinggeneralreaction:

+ + - - - - +→ + - -+ + -2 22e M R C

O

O O C

O

R M R C

O

On n||

:|| ||

( ) (6.10)

tAble6.2RelativeefficiencyofthePeroxides

Peroxides temperature(°C) RateofPolymerization(%h)

Lauroylperoxide -15 4.5

2,4-Dichlorobenzoylperoxide -15 1.5

Benzoylperoxide -15 1.4

Cumenehydroperoxide -15 0.8

Di-tert-butylhydroperoxide -15 0.7

Source:A.KonishiandK.Nambu,J. Polym. Sci.,54,209(1961).

tAble6.1typicalRecipe:suspensionPolymerizationofVinylChloridea

Ingredients Amount(ppm)

0.03%Aqueousdispersingagent 200

FeSO4 0.15

Vinylchloride 100

Lauroylperoxide 0.2

HCCl–CCl2 40

0.5%AqueousNaOH 1.7

a Polymerizationfor5hat20°C.

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Thereactionofdiacylperoxidewithstannouschlorideinacidsolutioninroomtemperatureorataslightlyelevatedtemperatureisusedinthequantitativeanalysisof theperoxygencompounds [22].The reactionof theperoxygencompoundwithstannouschlorideintheacidmediumisapparentlyrapidandcompleteenoughatroomtemperaturetoserveasaquantitativeassaymethod.However,noinformationisavailableastothenatureofthedecompositionproducts(i.e.,radicalorionic).Intheabsenceofotherevidence,themostreasonablemechanismwouldappeartobeaheterolyticprocessasshowninreaction(6.11):

R C||O

O O C||O

R Sn R C||O

O O||

Sn2 2- - - - - + →+ - - - --→+←

++ +→ - - + - - -

- =

→ - -

R C||O

O R C||O

O Sn

R C O

2 R C||O

3

OO Sn4- ++ (6.11)

Some evidence of the free-radical mechanism of polymerization using a per-oxygen compound and Sn2_ halides exists. The effective polymerization of vinylchlorideinthepresenceoftheperoxyester-SnCl2catalystsystemconfirmsthegen-erationoffreeradicals[23].Thecontrastswiththereportedrapiddecompositionofdiacylperoxidesinsolutionatroomtemperatureinthepresenceofvariousmetalhalides, tononradicalspeciesthroughionicintermediates.Thus,apolarcarbonylinversionmechanismisproposedinthedecompositionofbenzoylperoxideand/orotherdiacylperoxideinthepresenceofaluminumchloride[24–27],antimonypen-tachloride[26–28],andborontrifluoride[25–27].

However,radicalgenerationhasbeenconfirmedinthepolymerizationofvari-ousmonomersinthepresenceofacatalystsystemconsistingofanaluminumalkylandeitheradiacylperoxideoraperoxyester(i.e.,peroxygencompoundscontain-ingcarbonylgroups)[29–31].TheproposedmechanismofdecompositioninvolvescomplexationoftheAIR3withthecarbonylgroupoftheperoxideaswellaswiththemonomer,resultinginanelectronshift,whichweakenstheperoxylinkage:

AlR3

O M C–O–l–O–

(6.12)

Althoughthismechanismmaybeoperativetosomeextent,aredoxmechanismanalogoustothatnormallyinvokedinredoxcatalystsystemscontainingaperoxy-gencompoundfortheinitiationofpolymerization,consideredtobeatwo-electrontransfer,probablyplaysamajorrole:

R C

O

OO C

O

R e R C

O

O R C

O

O- - - - + → - - + - --|| || || || .2 (6.13)

Sn Sn e2 4 2+ +→ + (6.14)

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2 R C||O

OO C||O

R Sn 2 R C||O

O 2 R C||O

2- - - - + → - - + -+ - -- +• +O Sn4

(6.15)

Another — a one-electron, transfer mechanism — may be suggested for theformationoffreeradicalsasfollows:

R C||O

OO C||O

R Sn R C||O

O R C||O

O2- - - - + → - - + - -+ - .++ +Sn3 (6.16)

AsSn3_isveryunstableafterformation,itmayundergoreactionintwowaysinwhichitmayagainbereducedtoSn2_oroxidizedtoaSn4_state.Thereactionsareasfollows:

R C

O

OO C

O

R Sn R C

O

O R C

O

O- - - - + → - - + - -+ +|| || || ||

3 ++ +Sn2

(6.17)

The R C

O

- - -⋅||

radical,reaction(6.16),and R C

O

OO- - ⋅||

radical,reaction(6.17),mayreactwithacylperoxideasfollows:

. || || ||(

||)O C

O

R R C

O

OO C

O

R R C

O

- - + - - - - - - - - - - - -→ - 22O R C

O

OO+ - -|| . (6.18)

O O C

O

R R C

O

OO C

O

R R C- - - + - - - - - - - - - - - -→ -|| || ||

(||OO

O R C

O

O O- + - - +)|| .

2 2

(6.19)

or,intheotherstep,Sn3_producedinreaction(6.16)maybeoxidizedtotheSn4+stateasfollows:

2 2 23R C

O

OO C

O

Sn R C

O

O R- - - + - - - - - - - -→ - - + -+ -|| || ||

CC

O

O Sn|| .- + +4 (6.20)

Themechanismofpolymerizationmayberepresentedasfollows:

Initiation

M R Mki

+ →. . (6.21)

Propagation

M M Mkp. .+ → (6.22)

M M Mn

k

n

p. .- + →1 (6.22a)

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Termination

M M Deadx y. .+ → polymer (mutual) (6.23)

M R Deadx. .+ → polymer (linear) (6.23a)

where M is the monomer, R. is the initiating radical, and ki and kp are the rateconstants.

6.2.2.1 suspensionCopolymerizationofAcrylonitrilewithMethylAcrylateandwithstyrene

Kidoetal.reportedthesuspensioncopolymerizationofacrylonitrile–methylacrylate[32]andacrylonitrile–styrene[33]usingdilauroylperoxideandtheSnCl2redoxsystem.

Inthecaseofsuspensioncopolymerizationofacrylonitrileandmethylacrylate,mixtureof40–85%acrylonitrileand15–60%methylacrylatewerepolymerizedinanH2Osuspensionusinginorganicdispersantsaccordingtothetypicalrecipepre-sentedinTable6.3toproduce100-µsphericalcopolymerbeads.

In the case of suspension copolymerization of acrylonitrile–styrene mixturesof10–40wt%acrylonitrileand40–90wt%styrenearepolymerizedinH2OinthepresenceofinorganicdispersingagentsaccordingtothetypicalreciperesentedinTable6.4 to produce transparent copolymer beads containing >90% 100–400-µmeshparticles.

6.2.3 cu2+asReductant:suspensionpolymeRizationoFVinylchloRide

Recently,Cozens[34]hasreportedthesuspensionpolymerizationofvinylchlorideusingaLPO–Cu2+metalchelateredoxpairsystem.Thesuspensionpolymerizationofvinylchloride[35]wasalsostudiedusingadiacylperoxidesuchasBz2O2–Cu2_as the redox initiator. The microsuspension polymerization of vinyl chloride wasperformedat40–60°C.Theconversionof85%wasobtainedafter10hofpolymer-izationaccordingtothetypicalrecipepresentedinTable6.5.

tAble6.3typicalRecipe:suspensionCopolymerizationofAcrylonitrileandMethylAcrylatea


Water 200

SnCl2⋅2H2O 0.02

75:25Acrylonitrile-methylacrylate 250

Dilauroylperoxide 0.5

HCC–CCl2 40

0.5%AqueousNaOH 1.7

a Polymerizationfor1hat250°Cfollowedby15hat60°Catstirringrateof1000rpm.

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6.2.4 teRtiaRyamineasReductant

Theuseoftertiaryaminesascocatalystswithmetalionsinaqueouspolymerizationhasbeenthesubjectofstudyofvariousworkers[36].Nonucleophilicdisplacementinperoxidicoxygenhasreceivedmoreattentionthanthatbyamines[37].Extensivestudieswithacylperoxidewereperformedbyseveralworkers[38–51].

Theamine–peroxidecombinationasan initiator forvinylpolymerizationhasbeeninvestigatedextensivelybyvariousworkers.Solutionpolymerizationofvinylchloride[52]andstyreneandmethylmethacrylate[53],bulkpolymerizationofsty-rene[54],anddead-endpolymerizationofstyreneandmethylmethacrylate[55]wereperformedusing thebenzoylperoxide–dimethylaniline initiating system.Lal andGreen[56]havereportedtheeffectofvariousamineacceleratorsonthebulkpolym-erizationofmethylmethacrylatewithbenzoylperoxide.At about the same time,ImotoandTakemoto[57]hadreportedthesolutionpolymerizationofacrylonitrileinthepresenceofasubstitutedbenzoylperoxide–dimethylanilineredoxsystem.Inanotherarticle,Takemotoetal. [58]have reported the solutionpolymerizationof


Ingredients Amount(g)

Water 700

Vinylchloride 675

Benzoylperoxide 0.675

CuSO4⋅5H2O 45mg


tAble6.4typicalRecipe:suspensionCopolymerizationofAcrylonitrileandstyrenea


Water 150

Hydroxylapatite 2

Polyethyleneglycolalkylaryletherphosphate 0.01

SnCl2×2H2O 0.02

Acrylonitrile 25

Styrene 75

tert-Dodecylmercaptan 0.5

Dilauroylperoxide 0.71

HCC–CCl2 240

a Polymerizationfor1hat25°Cfollowedby15hat60°Catstirringrateof400rpm.

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styreneusingbenzoyl peroxide andvariousdi-n-alkylaniline redox systems. In aseriesofarticles,O’DriscollandMcArdlereportedonthebulkpolymerizationofstyreneat0°C[59]andhighertemperatures[60]usingbenzoylperoxide–dimethyl-aniline,andthebulkpolymerizationofstyrene[61]usingsubstituteddiethylanilineandbenzoylperoxide.Theefficienciesoffree-radicalproductionbyvarioussubsti-tutedbenzoylperoxidesand substituteddi-n-alkylanilineshavealsobeen studied[59–65].Recently,thefeasibilityofthetriethylamine–benzoylperoxide[55]redoxsystemtoinducephotopolymerizationinsolutionhasbeenreported.

Thepresenceoffreeradicalsinthereactionoftertiaryaminesandbenzoylperoxidehasbeenobservedbyelectronspinresonance(ESR)spectroscopy[67–69].Thereac-tionofamineswithacylperoxideismuchmorerapidthanthethermaldecompositionoftheperoxidealone[70].Forexample,benzoylperoxide[53]withdimethylanilineat0°Cinstyreneorchloroformexhibitsanapparentsecond-orderrateconstantof2.3×10_4sec_1.However,acetyl[41]andlauroylperoxide[71,72]reactsomewhatslower.

Recently,Morsietal.[73]havestudiedtherateofchargetransferinteractionsinthedecompositionoforganicperoxides.O’DriscollandRichezza[74]havealsoreportedtheultravioletabsorbancestudyofthecomplexformationbetweenbenzoylperoxideanddimethylaniline.AccordingtoHornerandSchwenk[45],themechanismforthepolym-erizationofvinylmonomersbybenzoylperoxideanddimethylanilineisasfollows:

Ø–C–O–O–C–Ø Ø–N CH3

CH3 O OØCOO + ØCOO Ø–N

CH3

CH3

+ .+ – . (6.24)

Ø–NCH3

CH3

Ø–NCH3

CH3

.+ ØCOO – .+ ØCOOH (6.25)

Ø–N–MØ–NCH3

CH3

CH3CH3

.+ + M + .

(6.26)

Ø–N–M

CH3

CH3

+ nM + .

Ø–N–Mn+1

CH3

CH3

+ . (6.27)

2Ø–N–Mn+1

CH3

CH3

+ . Disprop. or 1 to 2 PolymersCombination (6.28)

wheresteps (6.24)and (6.25) represent the formationof free radicals, step (6.26)theinitiationofthemonomer,step(6.27)thechainpropagation,andstep(6.28)theterminationbycombinationordisproportionation.

They suggested that the dimethylaniline radical is the initiator. However, themechanismwaslaterquestionedbyImotoetal.[52].Theysuggestedthattheactive

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radical(benzoateradical)producedbytheinteractionbetweenbenzoylperoxideanddimethylanilineinitiatesthevinylchloridepolymerization.

Inalaterstudy,Horner[38]postulatedthedetailedreactionmechanismofter-tiaryaminewithbenzoylperoxideandpicturedtheinitiationofpolymerizationbybenzoate radical.Mechanisticallyspeaking, thefirst stageof theamine–peroxidereactionis,unquestionably,nucleophilicattackontheO–Obond.ImotoandChoe[75] have studied the detailed aspects of the mechanism of the reaction betweensubstitutedbenzoylperoxideinthepresenceofdimethylaniline(DMA).Themecha-nismofthereactionofBz2O2withsubstituteddimethylanilinewasstudiedbyHorneretal.[44,45].Theyhaveindicatedthatthehighertheelectrondensityofthelonepaironthenitrogenatomofsubstituteddimethylaniline,thestrongerthepromotingeffectoftheamineonthedecompositionrateofBz2O2.ItwasshownthatthemoreabundantthequantityofDMA,thefasterthedecompositionvelocityofBz2O2.

Intheirstudy,ImotoandChoe[75]suggestedthereversibleformationofacom-plexintermediateIII,whichsubsequentlydecomposesintofreeradicalsasfollows:

CH3 O–CO

OO–CCH3

N

CH3

CH3

NO–CO

OC

O

DMA (II)Bz2O2 (III)

+

(6.29)

CH3

CH3

NO–CO

OC

O(III)

_

_

_

_

(6.30)

CH3

CH3

N

O

–C

(IV) (V)

__

+ O –COO+

Ѳ

(6.31)

CH3

CH3

NO

–C

(I, DMA)

Ionic Decomp.

(VI)

(IV)

_ _ O +

.

(6.32)

CH3 –N–CH3

O–CO

–H+

Rearrangement

–

(IV)

(VII)

(6.33)

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Decomposition Radical

(IV)

(VIII)

CH3

CH3

N. .

O –C

(IX)

O + +

(6.34)

AlthoughitappearsclearthatBz2O2andDMAundergoabimolecularreactionthatgivesrisetofreeradicals,theexactnatureoftheprocessiscontroversial.Thus,Horner[38]hasproposedtheformationofa“complex”inreaction(6.35)astherate-determiningstep,whichsubsequentlygivesrisetotheobservedproducts.

NCH3

CH3

.+C6H5C

C6H5CO

O

O

–

(6.35)

Imoto and Choe [75] have suggested the reversible formation of a complex,whichsubsequentlydecomposesintofreeradicals.However,WallingandIndictor[53] have suggested a new approach toward the free-radical mechanism betweenbenzoylperoxideanddimethylamine.Theysuggestedthattherate-controllingstepisanucleophilicdisplacementontheperoxidebyDMAtoyieldaquaternaryhydrox-ylaminederivative.Thereactionisasfollows:

NOCOC6H5 C6H5COO–

CH3

CH3

Bz2O2 + DMA

+

(6.36)

Sucha formulationparallels thatproposed for thebimolecular reaction [76]ofperoxidesandphenols,and,asitleadstoanionicproduct,shouldhaveaconsiderablenegativeentropyofactivation.Ashasbeenpointedoutpreviously[77],italsoaccountsfortheacceleratingeffectsofelectron-supplyinggroupsontheamineandelectron-withdrawinggroupson theperoxide, andparallels aplausible formulationof threeother reactions: the reactionofperoxideswith secondary amines, the formationofamineoxidesinthepresenceofhydrogenperoxides,andtheinitiationofpolymeriza-tionbyamineoxidesinthepresenceofacylatingagents[78].Theproductofreaction(36)hasonlyatransientexistenceanddecomposesbyatleasttwopossiblepaths:

NOCOC6H5

CH3

CH3

C6H5N + C6H5COO.CH3

CH3

+

.+ (6.37)

NOCOC6H5

CH3

CH3+ C6H5COOHC6H5N

CH3

CH2

+ +

(6.38)

DK3074_C006.indd 88 12/18/07 9:34:22 PM


Reaction(6.37),whichgivesHorner’s[38]intermediate,representsafree-radicalpathandwouldaccountfortheinitiationofpolymerization.Asnosignificantamountofnitrogenisfoundintheresultingpolymers,theaminefragmentmaywelldisap-pearbyreactingwithperoxide.Reaction(6.38)representsanonradicalbreakdownandwouldaccountforthelowefficiencyofthesystemasapolymerizationinitiator.Admittedly,thesameproductscouldarisefromaradicaldisproportionationclosertothatsuggestedbyHorner,butinthelattercase,thereactionwouldhavetooccurinthesamesolvent“cage”asreaction(6.37),becauseotherwise,reaction(6.39)wouldcompetewiththeinitiationofpolymerizationandtheefficiencyofthelatterwouldnotshowtheindependenceofBz2O2andDMAconcentrationactuallyobserved.

C H N

CH

CH

C H COO C H N

CH

CH6 5

3

3

6 4 6 5

3

2

|

|

. |⋅+ + → =

+

+

C H COOH6 5 (6.39)


No induction period existed in the solution polymerization of vinyl chloride [52]initiatedbythebenzoylperoxide–dimethylanilinesysteminvarioussolventssuchastetrahydrofuran, ethylenedichloride,dioxane, cyclohexanone,methylethylketone,and so forth. The initial rate of polymerization and the conversion was directlyand inversely proportional to the temperature, respectively. The polymerizationwasrestrictedtoonly20%conversion,probablyduetothecompleteconsumptionof benzoyl peroxide. Without the monomer, the extent of decomposition on ben-zoylperoxide reachesaconstantvalue regardlessof the temperatureandamountofdimethylaniline.Itwasseenthatthegreatertheamountofdimethylaniline,thefastertheinitialrateofpolymerizationandthelowertheconversion.Thedegreeofpolymerizationofvinylchlorideobtainedby theredoxsystembenzoylperoxide–dimethylanilinewasgenerallylowerthanthepolymerobtainedbythebenzoylper-oxidesystemalone.Theactivationenergyofthepolymerizationbytheredoxsystemwaslowerthanthatofthebenzoylperoxidealoneinitiatedpolymerizationandfoundtobe12.5kcalmol_1.Theinitialrateofpolymerizationcouldbeexpressedas

d PVCdt

k Bz O DMAt

( )( ) ( )/ /

=→0

2 21 2 1 2 (6.40)

TheresultsinthesolutionpolymerizationofvinylchloridearesummarizedinTable6.6.

6.2.4.2 suspensionPolymerizationofAcrylonitrile

Thesolutionpolymerizationofacrylonitrile[57]hasbeenstudiedinbenzeneat40°Cbyadilatometerusingdimethylanilineandvarioussubstitutedbenzoylperoxide.ItwasfoundthattheinitialrateofpolymerizationincreasedwithincreasingthemolarratioofDMA/Bz2O2from0–5bykeepingtheBz2O2concentrationat5.57×10–5molL_1.Ontheotherhand,afteraconsiderablepolymerizationtimehaselapsed,

DK3074_C006.indd 89 12/18/07 9:34:24 PM


thepolymeryieldsinthepresenceofalargequantityofDMAfrequentlybecamesmaller than theyieldobtained in thepresenceofsmallerquantityofDMA.Therelationbetweentheinitialratesofpolymerization Rp

0 , andconcentrationofBz2O2andDMAmaybeexpressedas

R k Bz O DMAP0

2 21 2 1 2= ( ) ( )/ / (6.41)

Theinitialratewasalsofoundtobedirectlyproportionaltothemonomerconcen-tration.Onthebasisofthekineticdata,arateequationmaybederivedasfollows:

Bz O DMA BzOk2 2

1+ → . (6.42)

BzO M P initiationk. . ( )+ →1 1 (6.43)

P M P propagationn

k

np. . ( )+ → +1 (6.44)

P P P P (or P termination)n n

k

n n n n

t. . ) (+ → + ′ + ′ (6.45)

Introductionofthesteadystateleadsto

R k P M kk M

kBzO Mp n p

i

t

= =

( . ) ( )

( )( .) ( )

/

/

1 2

1 2 (6.46)

Again,assumingthesteadystatefor ( .),BzO thefollowingequationwillbedrawn:

d BzO

dtk Bz O DMA k BzO Mi

( .)( )( ) ( .)( )= - =1 2 2 0 (6.47)

tAble6.6solutionPolymerizationofVinylChlorideintetrahydrofuran;bz2o2=0.52mol(%)

DMAbz2o2

temperature(°C)

InitialRate(%min)

MaximumConversion(%) DP

0.16 50 0.430 11.5 75

1.00 50 0.509 9.5 110

1.20 50 0.500 8.6 70

1.61 50 0.590 7.8 —

0.80 20 0.037 >26 85

0.80 30 0.120 <25 —

0.80 40 0.200 20.5 60

0.80 50 0.400 17.5 75

0.80 60 0.600 15.5 —

Source:M.Imoto,T.Otsu,andK.Kimura,J. Polym. Sci.,15,475(1955).

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From the previously mentioned equation, the following expression is readilyobtained:

Rk k

kM Bz O DMAp

t

= 11 2

1 2 2 21 2 1 2

/

// /( )( ) ( ) (6.48)

6.2.4.3 suspensionPolymerizationofstyrene

Thepolymerizationofstyreneinsolution[53,58]andbulk[54,55,59–61]bytheredoxsystembenzoylperoxide–di-n-alkylanilinehasbeenstudiedconsiderablybymanyresearchers.Differentdialkylanilines(DAAs)suchasdimethylaniline(DMA),diethylaniline (DEA), di-n-butylanilines, di-n-octylaniline, and di-n-decylanilinecombinedwithbenzoylperoxidehavebeenstudiedforthesolutionpolymerizationofstyrene[58]inbenzeneat30°C.ItwasfoundthattheinitialrateofpolymerizationincreasedwithadecreaseofthemolarratioofBz2O2.ThedegreeofpolymerizationdecreasedwiththedecreaseofmolarratioofBz2O2/DAA.TheinitiatorefficiencyseemedtoincreasegraduallywiththenumberofcarbonsofthealkylgroupsintheDAAs,withtheexceptionofdi-n-octylaniline.

Thekineticsofthebulkpolymerizationofstyrenehasbeenstudiedindetailat30°Cand60°Cbyadilatometer[60]usingthebenzoylperoxide–dimethylanilineredoxsystem.Also,theinitiatingefficiencyofthering-substituteddiethylanilines–benzoylperoxidesystem[61]at30°Cforstyrenepolymerizationahsbeenreported.Amathematicaltreatmentforthefree-radicalproductionbyBz2O2–DMAhasbeenderivedforthestyrenepolymerization[55,59]at0°C.Theinitialratesofpolymer-ization Rp

0 for30°Cand60°Careasfollows:At30°C,

R Bz O DMAp0 3

2 20 4971 67 10= × -. ([ ][ ]) . (6.49)

At60°C,

R Bz O DMAp0 3

2 20 4185 25 10= × -. ([ ][ ]) . (6.50)

The results of O’Driscoll and Schmidt [60] were different from those ofMeltzer and Tobolsky [54] for Rp

0 (initial rate of polymerization) as a functionof the catalyst concentration. In the latter work, it was shown that the rate lawR K Bz O DMAp

a02 2= ([ ][ ]) held over a wide range of catalyst concentrations and

temperatures.Thevalueofexponentawas0.5atlowtemperatures,asexpectedforabimolecularreactionbetweenamineandperoxide.At30°C,45°C,and60°C,thevaluesfoundbyMelzerandTobolsky[54]were0.39,0.38,and0.33,respectively.However,accordingtoO’DriscollandSchmidt[60],thesevalueswere0.5and0.42at30°Cand60°C,respectively.Thelowervalueat60°Cmaybeattributedtotheexistenceofaninductionperiodatthelowercatalystconcentration.

Inconclusion, itwasdemonstrated that thekineticsofpolymerizationare thesameathigherand lower temperature.Theefficiencyof the reaction in initiatingpolymerizationappearstofallslightlywithincreasingtemperature.

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6.2.4.4 suspensionPolymerizationofMethylMethacrylate

LalandGreen[56]havestudiedextensivelythebulkpolymerizationofmethylmeth-acrylateat25°Cusingvariousamines,mostly tertiaryamines.The totalyieldofpolymerdependsontheheatdevelopedduringpolymerizationaswellasthepro-duction of free radicals. The rate of polymerization increased or decreased withthesubstitutionattheparapositionofdimethvlanilinebyelectron-donatinggroupsor electron-withdrawing groups, respectively. Aliphatic tertiary amines are muchlessreactivethanaromatictertiaryaminesforacceleratingpolymerization,whereasprimaryamines,aliphaticaswellasaromatic,actasinhibitors.Substitutionofthemethylgroupsindimethylanilinebyethylgroupsdoesnotchangethereactivityofthe amine for accelerating the polymerization; however, when propyl groups aresubstitutedformethylgroups,thereactivityissomewhatreduced.Replacementofmethylgroupsindimethylanilinebydroxyethylgroupsdoesnotmateriallyaffectthereactivityoftheamineforacceleratingpolymerization.Tribenzylaminedecom-posesbenzoylperoxideveryrapidly(lessthan5min),butnopolymerisobtainedinthebulkpolymerizationofmethylmethacrylate.Theaminemayfunctionasitsowninhibitor.Themolecularweightsofthepolymersobtainedareintheneighborhoodof120,000±10,000inthecaseoftrialkylamines.

Veryrecently,thefeasibilityofaliphatictertiaryaminelikethetriethylamine–benzoylperoxide redox-initiating system inphotopolymerizationofmethylmeth-acrylate [66]hasbeen reported. In thedilatometric studyofmethylmethacrylatepolymerizationat35°Cwithvarioussolvents,theinitiatorexponentwas0.34.Themonomerexponentdependsonthesolventsused.Inacetonitrile,pyridine,andbro-mobenzene,themonomerexponentwas0.5,0.67,and1.1,respectively,withintheconcentrationrangestudied.Benzeneandchloroformgivefirst-orderdependenceofrateon[monomer]andbehaveasnormal(inert)diluents.Theactivationenergy3.2kcalmol_1.

6.2.5 QuateRnaRyammoniumsaltsasReductants

Quaternarysaltsincombinationwithbenzoylperoxideareknowntoinducevinylpolymerizationinemulsionsystems[79,80].Quaternarysaltsarealsopotentialpho-toinitiatorsforvinylpolymerization[81].Theuseofquaternarysaltsincombinationwithperoxidesasredoxinitiatorsforsuspensionpolymerizationofstyrene[82]andpolymerization of methyl methacrylate [83, 84] in bulk or in solution have beenexplored.

The polymerization of MMA with the cetyltrimethyl ammonium bromide(CTAB)–Bz2O2 redox system [84] and the cetylpyridinium bromide (CPB)–benzoylperoxideredoxsystem[83]wasstronglyinhibitedbyhydroquinone,whereastheinhibitoryeffectofairoroxygenwasmarginal.Aradicalmechanismisthusindi-cated.End-groupanalysisforaminoendgroupsbythedyetechnique[85]clearlyindicatedtheincorporationofbasic(amino)endgroups.WhenadilutesolutionofBz2O2wasmixedwithanequalvolumeofadilutesolutionofquaternarysalt,forexample,CPB, theUVabsorption spectrumof themixturewasnot theaverageof thespectraof thetwosolutions.Theabsorbancedifferencemaybeattributedtotherapidequilibriumbetweentheformationofacomplexandthecomponents.

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Thus,thespecieseffectiveforinitiatingpolymerizationappearstobethecomplexof the peroxide and CPB, which subsequently decomposes by a radical mecha-nism.Theconcentrationoftheinitiatingspecies[I]inthepolymerizationmaybeexpressedas

[I]=Kc[CPB][Bz2O2] (6.51)

whereKcistheequilibriumconstantforcomplexation:

CPB + Bz2O2

Kc [CPB . . . Bz2O2]Initiating Complex (I)

(6.52)

The mechanism is similar to that of the cetyltrimethyl ammonium bromide–benzoylperoxideredoxsystem.Theradicalgenerationprocessmaybeconsideredtoincludethefollowingsteps:

1. Complexation

+ C6H5—C—O—O—C—C6H5

C16H33(CH3)3N+ Br–

CTAB

Kc [CTAB . . . Bz2O2]Initiating Complex (I)

O O

Complexation

Bz2O2

(6.53)

2. Radicalgeneration

[C16H33(CH3)3N+]C6H5–CO

O–

Radical GenerationRoute1

(I) + Br + C6H5–CO

O. (6.54)

Route 2(I) C6H5–C

O

O–

N—CH3 CH3

CH3

N–CH3

C6H5–C O

O–

C6H5–C O

O. C6H5–C O

O–

CH3

CH3

CH3–N CH3

CH2

+ C16H33Br

+

+

+

+ +

N—CH3 CH3

CH3

.

.

.

(6.55)

(6.56)

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Theradicalgenerationstepisapparentlyinfluencedbymonomer(M)andsol-ventmolecules(S),whichpossiblycompeteinreactionwiththeinitiatingcomplex(I).Theradicalgenerationreactionsinfluencedbymonomerandsolventmaythenbeexpressedas

( )I M radicalsk+ →1 (6.57)

( )I S radicalsk+ →2 (6.58)

TherateofinitiationRimaythenbewrittenas

Ri=Kc[CATB][Bz2O2](k1[M]+k2[S]) (6.59)

6.2.5.1 suspensionPolymerizationofMethylMethacrylate

The polymerization of methyl methacrylate was studied dilatometrically at 40°Cunderbulkandhigh-dilutionconditionsusingCPB–Bz2O2[83]andCTAB–Bz2O2[84]redoxsysteminpolarsolventssuchasalcohol,acetone,ordimethylformamide.Theeffectofseveralsolvents/additivesonthepolymerizationrevealedthatdimethylformamide(DMF),acetonitrile,andpyridineactedasrate-enhancingsolvents;ben-zene, methanol, chloroform, and acetone acted as inert diluents; formamide andacetamidecausepronounced retardation. In thecaseof theCTAB–Bz2O2 systemunderbulkcondition(usingDMF10%ofthetotal)),theratewaspracticallyinde-pendentof[Bz2O2]upto0.025M,whereasthekineticorderwithrespecttoCTABwasabout0.16 foraconcentrationup to0.001M.AthighdilutionDMF50%ofthetotal),therateofpolymerizationwasproportionalto[Bz2O2]0.5and[CTAB]0.5.Atthehigh-dilutionconditioninDMF(50%v/v),Rpincreasedwith[Bz2O2]upto0.025 M and remained constant with a further increase in [Bz2O2]. Rp increasedwithincreasing[CTAB]upto0.001Mandthendecreasedwithafurtherincreasein[CTAB].ItwasfoundthatRpincreasedwithincreasingDMFcontentuptoabout30%.ThisacceleratingeffectofDMFwasnotapparentwithfurtherdilutionandtheusualeffectofmonomerconcentrationwasfound,theorderwithrespecttothemonomerbeingunity.Theoverallactivationenergywas11.2kcalmol_1.

However, in thecaseofCPB–Bz2O2 initiatingsystem,Rpwasproportional to([CPB][Bz2O2])0.18both innear-bulkandhigh-dilutionconditions.The [CPB]wasbetween0.1×10_3and8×10_3Mand[Bz2O2]withbetween3×10_3and100×10_3M.Theactivationenergyforpolymerizationwas13.6kcalmol_1.DMF,acetonitrile,andpyridineactedasrate-enhancingsolventsintheredoxpolymerization,whereasformamideandacetamidebehavedasretardingadditives.

6.2.5.2 suspensionPolymerizationofstyrene

In the case of suspension polymerization of styrene [82] using the Bz2O2–laurylpuridinium chloride redox system, about 100% conversion with 1-mm-diameterpolystyrenebeadswereobtainedusingthetypicalrecipepresentedinTable6.7.

DK3074_C006.indd 94 12/18/07 9:34:41 PM


6.2.6 nitRiteasReductant

Thereactionbetweenhydrogenperoxideandsodiumnitritewasstudied indetailbyHalfpennyandRobinson[86]in1952.Thecharacteristicsofthereaction,par-ticularlyinthepresenceofabromideion,andtheevolutionofoxygenwithcertainconcentrationofperoxidessuggestedthepossibleformationoffreeradicals.Theydemonstratedtheiroccurrencebyobservingthepolymerizationofmethylmethac-rylate.Intheearly1950s,Schulzetal.[87]hadreportedtheacroleinpolymerizationbyH2O2–NaNO2asaredoxinitiator.Thereportsoftheuseofnitritesasareducingagentinpolymerizationareveryfew.Patentliteraturereportsthesuspensionpolym-erizationofvinylpyridine[88]andvinylchloride[89]usingNaNO2asthereducingagentincombinationwithacylperoxideslikeBz2O2orlauroylperoxide.Themecha-nismmaybewritteninonestepasfollows:

R C

O

OO C

O

R NO R C

O

O NO R C- - - - + → - - + + --|| || || . . |

2 2

||O

O- - (6.60)

AccordingtoHalfpennyandRobinson[86]inthelightofthemechanismfortheH2O2–nitriteredoxsystem,thevariousstepsofthereactionsystemmaybewrittenasfollows:

NO H HNO2 2- ++ → (6.61)

HNO R C

O

OO C

O

R R C

O

O NO R C

O

2 2+ - - - - → - - + - -|| || || ||

OOH (6.62)

R C

O

O NO R C

O

O NO- - → - - +|| || . .

2 2 (6.63)

R C

O

O NO R C

O

NO- - + → -|| . . ||

2 3

(6.64)

tAble6.7typicalRecipe:suspensionPolymerizationofstyrenea


Water 1000

Styrene 900

3(Mgsilicate 0.5

Benzoylperoxide 2.7

Laurylpyridiniumchloride 0.03

tert-Butylperbenzoate 3.6

a Polymerizationfor10hat80–120°C.

DK3074_C006.indd 95 12/18/07 9:34:44 PM


Whenperoxideisabundant,theacylradicalprovidesameansofoxygenlibera-tionaccordingtothefollowingreactions:

R C

O

O R C

O

OO C

O

R R C

O

O C

O

- - + - - - - → - - - -|| . || || || ||

RR R C

O

O O+ - - -|| . (6.65)

R C

O

OO C

O

R R C

O

O O R C

O

O R- - - - + - - - → - - + -|| || || . || . CC

O

O C

O

O|| ||- - + 2 (6.66)

The preceding free radicals take part in the initiation and the terminationprocessesinpolymerization.


Vinylchloride[89]withorwithoutacomonomerhasbeensuspensionpolymerizedusingamixed-catalystsystem(i.e.,lauroylperoxideand2-ethylhexylperoxydicar-bonate)withareducingagentNaNO2intworeactorsmaintainedatdifferenttemper-atures.ThepolymerizationwasperformedaccordingtothetypicalrecipepresentedinTable6.8.After50h,theconversionvaluesinthefirstandsecondreactorswere15%and90%,respectively,andnoside-walldepositionwasnoted.Thepolymerhadaweight-averagedegreeofpolymerization(DP)of1020,aplasticizerabsorbabilityof29.2%,athermalstabilityof75min,andgeltimeof2.5min.Theproductpre-paredbypolymerizationat58°Cinbothreactorshadaweight-averageDPof1010,aplasticizerabsorptionof24.8%,athermalstabilityof65min,andageltimeof4.0min.

6.2.6.2 suspensionPolymerizationofVinylPyridine

Vinylpyridinewithorwithoutcomonomerswaspolymerizedbysuspensionpolym-erization[88]inH2Ointhepresenceoffattyacidestersorphthalatestogivepoly-mersinsphericalpowderform.Thepolymerizationwasperformedaccordingtothe



Water 140

Vinylchloride 100

2-Ethylhexylperoxydicarbonate 0.04

Lauroylperoxide 0.01

80%Saponifiedpoly(vinylacetate) 0.07

NaNO2 0.002

a Polymerizationat61°Cinthefirstreactorandat57°Cinthesecondreactorfor50h.

DK3074_C006.indd 96 12/18/07 9:34:46 PM


recipeprovidedinTable6.9toyieldafinalproductof850mlofyellowtransparentsphericalcopolymerbeads.Whendioctylphthalatewasomitted,asimilarcomposi-tionyieldedlargelumps.

6.3 AlKylPeRoxIDe

Alkylperoxidesareextensivelyusedforthesuspensionpolymerizationofstyrenicmonomers[90–93]andvinylchloride[94,95].

6.3.1 alkylBoRonasReductant

Alkylboroncompoundscaninitiatethepolymerizationofvinylmonomersinthepresenceofasuitablecocatalyst.Acommonfeatureofthecocatalyst(i.e.,peroxides[96–98],hydroperoxides[99],amines[100],andorganichalides[101])isthatitcanbeconsideredan“electron-donating”compound.Inmostofthesystemsinvestigated,ithasbeenestablishedthatthereactionisafree-radicalpolymerization[101–104],buttherateequationisnotsimple,suggestingacomplexmechanisminwhichcoor-dinationoftheorganometalliccompoundisarate-determiningstep[100,101,105].Furthermore, the reaction order changes when the organometallic compound tococatalystratiochangesforperoxides[97],oxygen[102–104,106,107],hydroperox-ides[99],hydrogenperoxide[108],andorganichalides[101].Thischangesoforder,whichmaybeaconsequenceofcomplexformationisattributedtovariouscauses[101,106,108],butinmostcases,nosatisfactoryexplanationisgiven.

Theoxidationoftrialkylboronsbymolecularoxygengenerallyproducesalkoxyboroncompoundsviaintermediateperoxides[109,110],althoughMirviss[111]hasreportedhydrocarbonsamongtheproducts.Inaddition,vinylmonomerspolymerizeatroomtemperatureinthepresenceoftrialkylboronsandair[112].Freeradicalsareevidentlyproducedatsomestageinthereaction.Freeradicalshavebeenassumedtoarisefromthehemolyticdecompositionofperoxidicintermediates[111,113]eventhoughtheseperoxidesareverystableatroomtemperature[110].Othershavesug-gested that thefreeradicalsareproduced ina reactionbetween theperoxideandunoxidizedtrialkylboron[98,114].

tAble6.9typicalRecipe:suspensionPolymerizationofVinylPyridinea


Water 800

4-Vinylpyridine 247

Styrene 62

Benzoylperoxide 4

Dioctylphthalates 50

NaCl 234

NaNO2 2.9

Hydroxyethylcellulose 4.5

a Polymerizationfor9hat80°Catastirringrateof250rpm.

DK3074_C006.indd 97 12/18/07 9:34:47 PM


Thehighrateofperoxideformationintheoxidationoftriethylboronseemstoruleoutthepossibilityofalong-livedoxygen–triethylboroncomplexthatrearrangestotheperoxide.Thishasbeenstatedbyvariousauthors[109–111].OnlyZuttyandWelch [109]haveprovidedexperimentalevidence in thecaseof tri-n-butylboron.Atransientintermediatecannotbeexcluded.Noindicationexiststhatfreeradicalsariseduring theoxidationof triethylboron.Both triethylboronandperoxidewererequiredtoinitiatevinylpolymerizationofmethylmethacrylateinagreementwithBawnandco-workers[98].Theresults[104]indicatethattheethylradicalwaspro-ducedinareactionbetweentriethylboronandtheperoxide,whereintheperoxidewasreduced.Noevidenceexistedforthepresenceoftheethoxyradicalarisingfromhemolyticdecompositionoftheperoxide.Iodidewasanefficienttrapfortheethylradical. From the work of Hansen and Hamman [104], it is indicated with someuncertaintythatthereductionisa1:1reaction.Itisunlikelythattheethylradicalwastheonlyoneproduced,butthestructureofacompanionradicalcouldnotbeascertained.Theamountof iodineconsumed indicates that the radicalswerenotproducedineachreactiveact.Apossibleexplanation[115]isthatthereductionisa“cagereaction”:

C2H5O OB

B–C2H5 B–OC2H5 + C2H5. + BO.

(6.67)

The formationof a cage shouldbeespecially favoredbycoordinationofoneof the oxygen atoms to boron. Radical recombination would lead to the alkoxycompoundscommonlyobserved,whereasdiffusionfromthecagecouldleadtotheproductsderivedfromfreeradicals.

Recently,Abuinetal. [96] investigated thekinetic featuresofbulkpolymer-ization of methyl methacrylate using triethyl boron (TEB)–di-tert-butylperoxidemixture as the radical initiator. From their data, it can be seen that, working ata constant di-tert-butylperoxide concentration, the reaction rate increases as theTEB concentration increases, reaching a maximum and then decreasing withfurtherTEBaddition.IftheTEBonlymodifiestheinitiationrate,asimplefree-radicalmechanismwouldpredictthatatagiventemperature,thefollowingequationshouldholdtrue:

Rλ=constant (6.68)

whereRisthemeasuredpolymerizationrateandλisthemeanchainlength.However,theirdatashowthat,atahighTEBconcentration, theproduct(Rλ)

decreaseswhenTEBincreases.Thiseffectcanberelatedtotheoccurrenceofchaintransfertotheorganometalliccompound.Thedatacanbetreatedaccordingtothefollowingreactionscheme:

TEB X R+ = 2 . (6.69)

R M M. .+ = (6.70)

M M M. .+ = (6.71)

DK3074_C006.indd 98 12/18/07 9:34:50 PM


M M polymer. .+ = (6.72)

M M Q polymer. .+ = + (6.73)

M TEB P polymer. .+ = + (6.74)

P M M. .+ = (6.75)

Q M M. .+ = (6.76)

P M polymer. .+ = (6.77)whereM,X,and R. representmonomer,peroxide,andradical,respectively.

Thechaintransferreactionsareconsideredtobe

M TEB MH P. .+ = + (6.78)

M TEB C H MB C H. . ( )+ = +2 5 2 5 2 (6.79)

Areactionsimilartoreaction(6.79)hasbeenreportedasbeingextremelyfastforseveralradicalsconjugatedtoacarbonylgroup[116].

Initiation by TEB–di-tert-butyl peroxide (DTP) shows [97] the followingmaincharacteristics:

1. Atlow(TEB/DTP),theinitiationstepfollowsaratelawrepresentedby

Ri=k80(TEB)(DTP) (6.80)

2. When theperoxideconcentration iskeptconstant, the rateof initiationincreases,reachingamaximumandthendecreasingwhentheTEBcon-centrationincreases.

Thefollowingmechanismisconsistentwiththesefindings:

TEB+MMA=C1 (6.81)

2TEB+DTP=C2 (6.82)

C1+DTP=radicals (6.83)

whereC1andC2representcomplexedformsoftheTEB.Theproposedmechanismgivesthefollowingexpressionfortherateofinitiation:

Rk K MMA DTP TEB

K TEBi = +83 81 0

8221

( )( ) ( )

( ) (6.84)

where(DTP)0isthetotalperoxideconcentration,(TEB)istheconcentrationofTEBuncomplexed,andK81andK82aretheequilibriumconstantsofreactions(6.81)and(6.82),respectively.TheconcentrationofuncomplexedTEBcanbeobtainedfrom

DK3074_C006.indd 99 12/18/07 9:34:56 PM


thetotalconcentration(TEB)0bysolving

(TEB)(TEB)[1 K (MMA)] 2K (DTP) (TEB)

1081 82 0

2

=+ +

+KK (TEB)822 (6.85)

Similarly,forarateatlow(TEB)0,Eq.(6.84)reducesto

Rk K MMA DTP TEB

K MMAi = +83 81 0 0

811

( )( ) ( )

( ) (6.86)

Similarly,themaximumrateforagiven(TEB)0couldbederivedtobe

( )( )( )

max /Rk K MMA DTP

Ki = 83 81 0

821 22

(6.87)

6.3.1.1 bulkPolymerizationofMethylMethacrylate

Methyl methacrylate [97] was bulk polymerized at 50°C using t-butyl peroxide–triethylboron(TEB)astheinitiator.Therateofinitiationbythemixtureoftrieth-ylboronandt-butylperoxidewasfirstorderwithrespecttoperoxide.Theorderintriethylboronchangesfrom1atalowtriethylboron/peroxideratiotonearly0atahightriethylboron/peroxideratio.Theresults[97]aregiveninTable6.10.

Abuinetal.[96]alsoreportedthebulkpolymerizationofmethylmethacrylateat20°Cusingtriethylboron–di-t-butylperoxideatvarioustriethyl-boronconcentration.Theamountofpolymerproducedwasproportionaltothereactiontime.TheresultsarepresentedinTable6.11.

Abuinetal.[96]havealsocomparedtherateofpolymerizationinitiatedbythemixturecontainingdifferentperoxidesand it is foundthat theratewithdimethylperoxide is nearly32 times faster thanwithdi-tert-butylperoxide as a cocatalyst.Thisdifferencecanbeattributed to the sterichindrance introducedby thebulkytert-butyl groups. Similarly, it is interesting to note the difference between TEBand triethyl aluminum (TEA) as the cocatalyst with peroxides. With TEB, alkylandacylicperoxidesshowsimilarcocatalyticactivities[97].Ontheotherhand,ithasbeenreportedthatTEAisonlyactivewhenacylperoxidesareemployed[117].

tAble6.10bulkPolymerizationofMethylMethacrylateat50°C

[t-butylPeroxide](moll-1)

[teb](moll-1)

time(min)

Conversion%(%l-1)

e(kcalmol-1)

0.062 0.034 60 3.2

6 4.1

135 9.7 10.0

0.062 0.063 60 6.0

60 5.5

100 10.3

120 10.9

DK3074_C006.indd 100 12/18/07 9:34:59 PM


ThisdifferencecanberelatedtothemonomericstateofTEB;TEAismainlypresentasadimmer[117].

6.4 PeResteRs(PeRoxyesteRsoFCARbonICACID)

Althoughperoxydicarbonatesareusefullow-temperatureinitiatorsforvinylpolym-erization[118,119],littlehasbeenpublishedaboutthecharacteristicsoftheirther-maldecomposition.Therateofdecompositionwasdeterminedforseveralofthesecompounds([ROC(O)O]2,R=Et,i-Pr,PhCH2,NO2CMe2CH2[120];R=i-Pr[104])intheearly1950s.Razuvaevetal.havesinceaddedotherstothelist(R=Me,Bu,i-Bu,t-Bu,amyl,cyclohexyl[122],andPh[123,124]).Abeliefexiststhatperoxydicar-bonatesareparticularlysensitivetoradical-induceddecomposition[119–121]).Thisisundoubtedlytrueforthepuresubstances[119,120].Theadditionof1%ofiodinetopurediisopropylperoxydicarbonatereduces[120] therateofdecompositionbyafactorof60.Amongall thepercarbonates,phenylperoxydicarbonate[123,124]mayprovetoactdifferentlybecausedecarboxylationyieldstheresonance-stabilizedphenoxyradical.Thisperoxideissaidtobemorelabilethanotherperoxydicarbon-ates and inhibits [123] instead of initiates polymerization [124]. Peroxydicarbon-atesareveryefficientinitiatorsforthesuspensionpolymerizationofvinylchloride[125–139],vinylidienefluoride[16],andstyrene[17],andthecopolymerizationofvinylacetate[18,19]withothermonomers.Peroxydicarbonates[140–143]arealsoprovedtobeefficient-radicalinitiatorsinconjunctionwithvariousreducingagentsforvinylpolymerization.

6.4.1 meRcaptansasReductant

MercaptanshavebeenprovedtobeanefficientreducingagentwithH2O2[144,145]toinitiatevinylpolymerization.IthasalsobeenusedwithBz2O2[146]foremulsionpolymerizationofvinylmonomers.Theactivationofpersulfatebyreducingagentsuchasthiols[147–155]hasbeenextensivelystudiedandthecombinationhasbeenusedforvinylpolymerization.Starkweatheretal.[156]andKolthoffetal.[157,158]

tAble6.11bulkPolymerizationofMethylMethacrylateat20°C

[teb](10-2M) [DtP]a(M)

RateofPolymerization)b(×10-6msec-1)

MeanChainlength

1.18 0.062 3.65 5000

6.80 0.062 4.50 4000

15.40 0.062 4.14 3800

24.50 0.062 2.74 5500

38.80 0.062 2.50 5000

a DTP=di-t-butylperoxide.b Averagedovera185-minreactiontime.

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have demonstrated the catalytic effect of thiols in persulfate-initiated emulsionpolymerizationofstyrenewithorwithoutbutadiene.

Theuseof2-mercaptoethanolasreducingagentinconjunctionwithperoxydicar-bonate for the suspension polymerization of vinyl chloride [159, 160] has beenreported in thepatent literature.During theredoxreaction,hydrogen isextractedfromthiolbythehomolysisofthe–S–Hgrouptogiveasulfurradical.Themecha-nismmaybeproposedasfollows:

R O C

O

O O C

O

O R R S H RS R O C

O

- - - - - - - + - - → + - - -|| || . ||

OOH R O C

O

O+ - - -|| .

(6.88)


Suspensionpolymerizationofvinylchloridehasbeenreportedusing2-mercapto-ethanolasareductantwithbis(2-ethylhexyl)peroxycarbonate[160]anddiisopropylperoxydicarbonate[159].Thus,inthecaseof(2-ethylhexyl)peroxycarbonate[160],mixturesofvinylchloridewithorwithoutcomonomers100,C2–6compounds,having–SHand–OHgroups0.001–0.1ppm,C4–18alkylvinylether0.01–1.0ppm,andbenzylalcoholwithorwithoutC1–4alkylsubstituents0.01–1.0ppmarestirredtogivePVCorcopolymershavingequallygoodporosity,heatstability,andprocessability.

Inthecaseofdiisopropylperoxydicarbonate[159],an80–20:20–80mixtureofpartiallysaponifiedpoly(vinylacetate)andacelluloseetherwasusedasthedispers-ingagent.Thesuspensionpolymerizationorcopolymerizationofvinylchloridewasperformedinthepresenceofacompoundhavinga–SH,–OH,or–CO2Hgroupstoreducetheamountofchaintransferagentrequired.

6.4.2 sulFideanddithionateasReductant

Theoxyacidsofsulfursuchassulfite,bisulfite,bisulfate,thiosulfate,metabisulfite,anddithionateprovedtobeefficientreducingagentsintheredox-initiatedpolym-erizationofvinylmonomers.Numerousarticlesintheseareashavebeenreportedin the literature. Palit et al. [161–164] and Roskin et al. [165–167] have reportedthepolymerizationofvinylmonomersusingthepersulfate–dithionateredoxsystem.Chaddha et al. [168] also reported the persulfate–sulfide redox system to initiatepolymerization.Theuseofsulfide[169,170]anddithionate[171]asreducingagentsinconjunctionwithorganichydroperoxide,likecumenehydroperoxideandironsaltinemulsionpolymerization,hasbeendescribed.TadsaandKakitanihavereportedthe suspension polymerization of vinyl chloride by percarbonate–sodium sulfide[172]andpercarbonate–sodiumdithionate[173]systems.

Thegeneralinitiationreactioninthesesystemscanbeschematicallyrepresentedas

R O C

O

O O C

O

O R A R O C

O

O R O- - - - - - - + → - - - + -- -|| || ||

2 -- - + -C

O

O A|| . .

(6.89)

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whereAissulfideordithionate;

R O C

O

O O C

O

O R S S O R O C

O

- - - - - - - + + → - -- -|| || ||

23 6

2 -- + - - -

+ +

-

- -

O R O C

O

O

S S O

|| .

. .3 6 (6.90)

Theseindicatedradicalsinitiatepolymerization.


Thepresenceofsulfideordithionatealsopreventsscaleformationduringpolymer-ization.Thus,inthesystemdioctylperoxydicarbonate–sodiumsulfide[172],vinylchloride with or without vinyl comonomers was polymerized in the presence of0.1–1000ppm(basedonmonomers)inorganicsulfidesaccordingtotherecipepre-sentedinTable6.12togivePVCwithgoodheatstability,withnoscaleformation,comparedwith450gm_2forasimilarrunwithoutNa2Sx.

Similarly,inthecaseofdioctylperoxydicarbonate–Na2S3O6[173],thepolymer-izationwasperformedaccordingtotherecipeinTable6.13togivePVCwithgoodheatstability.Scaleformationintheprecedingpolymerizationwas5gm_2,com-paredwith550gm_2forasimilarrunwithoutNa2S3O6.

tAble6.12typicalRecipe:suspensionPolymerizationofVinylChloridebyDioctylPeroxydicarbonate–sodiumsulfidesystema


Water 150

Vinylchloride 100

Partiallysaponifiedpoly(vinylacetate) 0.1

Dioctylperoxydicarbonate 0.05

Na2Sx 0.01

a Polymerizationunderstirringfor5.8hat58°C.

tAble6.13typicalRecipe:suspensionPolymerizationofVinylChloridebyDioctylPeroxydicarbonate-na2s3o6systema


Water 150

Vinylchloride 100

Partiallysaponifiedpoly(vinylacetate) 0.1

Dioctylperoxydicarbonate 0.04

Na2S3O6 0.001

a Polymerizationunderstirringfor6.4hat58°C.

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6.4.3 alkylBoRaneasReductant

Despitethegreatnumberofinvestigations[106,112,114,174–177]inwhichalkylboroncompoundswereusedasinitiatorsofvinylpolymerization,mostofthemainfeatures of the mechanism involved for the initiating system, such as alkyl boroncompoundsintheabsenceofair[106,113,114,175,178–181],peroxidesorhydroper-oxidesinconjunctionwithtrialkylboron(A3B)compounds[98,99,182]havenotyetbeendemonstrated.Thereportsonapercarbonate–alkylboronredoxsystemforvinylpolymerizationareveryfew.Thebulkpolymerizationofvinylchloridebytheredoxsystemconsistingofdiisopropylperoxydicarbonate–triethylboronhasbeenreportedby Ryuichi and Isao [183]. Ryabov et al. [184] also reported the low-temperaturepolymerizationofvinylchloridebythedicyclohexylperoxydicarbonate–tri-n-butyl-boronredoxsystem.InlightofthemechanismdescribedbyContrerasetal.[97],thefollowingmechanismmaybesuggestedforthepercarbonate–alkylboronsystem:

A B R O C

O

O O C

O

O R A BO C O R A R3 2+ - - - - - - - → - - - + +|| || . -- - -O C

O

O|| .

(6.91)

R O C

O

O R O CO- - - → - +|| . .

2 (6.92)

6.4.3.1 bulkPolymerizationofVinylChloride

Inthecaseofthediisopropylperoxydicarbonate–triethylboronredoxsystem[183],26.4%di-butylphthalatesolutioncontaining0.01624gofdiisopropylperoxydicar-bonatewaschilledto–78°Cinapressurevessel,15gofvinylchloridewasaddedfollowedby6.928×10_5molEt3Binhexaneundernitrogen,andthemasswaskeptat–20°Cfor7htogive4.4%polymerization.Similarpolymerizationat10°Cwith0.02706gofEt3Bproduced16.64%polymer,butnopolymerwasobtainedwiththeuseofdiisopropylperoxydicarbonatealone.

6.5 PeResteRs(PeRoxyesteRsoFCARboxylICACID)

Peroxyesters of carboxylic acids have been extensively used for the suspensionpolymerizationofvinylmonomers(Table6.14).Severalpatentshaveappearedonthesuspensionpolymerizationofvinylchloride[185–188],styrenicmonomers[189,190],andmethylmethacrylate[191].

6.5.1 sn2+saltsasReductant

A decomposition of a peroxyester by a stannous salt involves 2 mol of peresterbecausetheoxidationofstannousiontostannicisatwo-electrontransfer;thatis,

Sn R C

O

O O R Sn R C

O

O O R2 42 2 2+ + -′ - - - - → ′ - - + -|| || . (6.93)

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Thestoichiometryshownindicatesthata2:1perester/Sn2_moleratioshouldresultincompleteperesterdecomposition.However,thisisnotinaccordwiththeexperi-mentalobservationinthereaction[192,193]betweent-butylperoctoate(TBPO)andstannousoctoate(SnOct);thatis,thedecompositionoccursrapidlytotheextentofapproximately40%andthentheTBPOconcentrationremainsunchanged.Thismaybeattributedtotherequirementfortheavailabilityofstannousionandthefailureofstannousoctoatetoundergoionicdissociation;thatis,stannousoctoatemaypossesssomecovalentcharacter.

Itisnoteworthythattheanalyticalprocedureforthequantitativedeterminationof TBPO involves reaction with excess stannous chloride in an aqueous medium,followedbybacktitrationofexcessstannousions.Theaqueousmediumresultsinthehydrolysisofstannouschloride toproduceasolutionofstannoushydroxide inaqueoushydrochloricacid.Stannousoctoatemaynothydrolyzeinaneutralaqueousmedium.Thus,theabsenceofcompletedissociationand/orhydrolysispreventsthestoichiometricinteractionofstannousoctoateandTBPO.ThepossiblepresenceofaTBPO–stannousoctoatecomplexmayalsoplayaroleinthefailuretocompletethereaction,assuggestedbytheobservedpresenceofresidualperoxideandresidualstan-nousionsafterthedecompositionofTBPOhasproceededtothemaximumextent.

Thepresenceofavinylchloridemonomer(VCM)hasbeenshowntoreduceeventhelimitedextentofBPOdecompositionbystannousoctoate.ThismaybeattributedtoaVCM–stannousoctoatecomplex,thepresenceofwhichhasbeenexperimentallyconfirmed[192].Apparently,thestannousoctoateinthiscomplex,whichcontainsVCMandstannousoctoateina1:2molarratio,dissociatesorhydrolyzesorinteractsinsomeothermannerwithTBPO(e.g.,bycomplexationwiththecarbonylgroup)toanevenlesserextentthanstannousoctoateintheabsenceofVCM.

ItwouldappearthatthefailuretoachievethetheoreticalcompletedecompositionofTBPOata2:1TBPO/Sn2_ratioisduetotheunavailabilityofSn2_intheconcen-trationnecessarytoachievetheindicatedstoichiometry.Thus,theTBPO–stannousoctoatecomplexandtheVCM–stannousoctoatecomplexreducetheavailabilityofstannousoctoateforTBPOdecomposition.Further,ifthedecompositionofTBPOrequiresthepresenceofastannousion,theincompletehydrolysisordissociationofstannousoctoateperseorcompletedwithTBPOand/orVCM,underthedecomposi-tioncondition,reducestheavailabilityofastannousion.

tAble6.14PeroxyestersforVinylMonomersPolymerization

Initiators Monomers

tert-Buperoxyneodecanoate Vinylchloride

2,4,4-Trimethylpentylperoxyphenoxyacetate Vinylchloride

Diphenylperoxyoxalate Vinylchloride

Di-tert-Budiperoxyazelate Styrene

Di-tert-Buperoxyhexahydroterephthalate a-Methylstyrene/styrene/acrylonitrile

tert-Bu2-ethylhexaneperoxoate Styrene

tert-Buperoxy-2-ethylhexanoate Methylmethacrylate

DK3074_C006.indd 105 12/18/07 9:35:06 PM


It is obvious that a route to effective, stoichiometric decompositionofTBPOin thepresenceof a stannous salt requires completedissociationorhydrolysisofthelatterthroughachangeinreactioncondition(e.g.,anacidicpH)ortheuseofamorerapidlyhydrolyzedstannoussalt.Itshouldbenotedthatstannouschloride,whichgeneratesanacidicmediumonhydrolysis,quantitativelydecomposesTBPO.Further,stannouslauroate,whichcontainsthelauroatemoiety, inthepresenceofemulsifiers such as sodium lauryl sulfate or dodecylbenzene sulfonate results inamore rapidpolymerization rateandahigherconversionofVCMthanstannousoctoate,indicativeofagreateravailabilityoftheeffectivereductant(i.e.,astannousion).Thestannouslauratemaybesolubilizedintheaqueousphaseandtheresultantmicroenvironmentpromoteshydrolysisand/ordissociation,incontrasttothesitua-tionwithwater-insolublestannousoctoateandstearate.

Becauseitisnecessarytoincreasetheavailabilityofthestannoussaltforhydro-lysisand/ordissociation,itisdesirabletoutilizeanadditivewhichcompeteswithTBPOandVCMincomplexformationwithstannoussalts.Inthisconnection,ithasbeennotedthatthedecompositionofTBPOinthepresenceofstannousoctoatepro-ceedstoagreaterextentwhenanestersuchanethylacetateispresent.Further,thesuspensionsystemcontainsesterssuchassorbateestersandanetherealcompound(i.e.,methylcellulose)andpromotesamorecompletedecompositionofTBPOandpolymerizationofVCM.Bothestergroupsandetherealoxygenhavetheabilitytocomplexwithmetalcompounds,includingstannicandstannousderivatives,andcanthereforeeffectivelycompetewithTBPOand/orVCMincomplexformation.

AnimportantpointtobeconsideredisthenecessitytogenerateradicalsfromTBPOonacontinuousbasistoachieveeffectivepolymerization.Ifthehydrolysisand/or dissociationof a stannous salt occurs rapidly andpromotes rapidTBPOdecomposition, the resultant radicalsmaybegenerated too rapidly for effectiveinitiation of VCM polymerization to high conversions to high-molecular-weightPVC.Thus,itisnecessarytopromotetheformationorreleaseofreactivereduc-tantatthedesiredratethroughoutthepolymerizationperiod.ItisalsonecessarytoprovideforcompletedecompositionofTBPOtoyieldPVCthatdoesnotcontainresidualTBPO.

The mechanism for the polymerization of VCM in the presence of TBPO–stannousoctoatemaybedescribedasfollows.Thet-butoxyradicaladdstoVCMandinitiatespolymerization,inlieuofhydrogenabstraction.Thereactionbetweenthesubstrateradical(i.e.,aVCMradicalorthepropagatingchainradical)andthestan-nicionresultsintheterminationoftheradicalchainreactionandregenerationofthestannousion.Thus,thelatterisavailablefordecompositionofBPOtogenerateaddi-tionalchain-initiatingt-butoxyradicals.However,theterminationofchaingrowthresultsinaninefficientconsumptionofradicals:

2 2 22 4R C

O

O O tBu Sn R C

O

O Sn tBu- - - - + → - - + ++ - +|| ||

OO. (6.94)

tBuO CH C

Cl

H tBuO CH C

Cl

H.

| |

.+ = → -2 2 22 (6.95)

DK3074_C006.indd 106 12/18/07 9:35:08 PM


tBuO CH C

Cl

H nCH CH

Cl

tBuO CH CH

Cl

- + = → -2 2 2 2|

.

|[

|]]

|

.n CH C

Cl

H2 - (6.96)

R–C–O– –O–C–R CH2CH–O–C–R + Sn2+ 2

CH2CH. Cl

O O

.CHCH2 Cl

Sn4+

(6.97)

CH2–CH.Cl

CH2CH2 Cl

CH=CH Cl

.CH2– CH Cl

+ + (6.98)


Gaylordetal.havereportedthesuspensionpolymerizationofvinylchlorideusingtheredoxsystem,suchast-butylperoxyoctoate–SnCl2[23,194]andt-butylperoxyoctoate–stannouscarboxylate[192,193].ThepolymerizationofVCMinthepresenceoftheredoxsystemhasseveralunusualcharacteristicsthatcanbeexplainedonthebasisofthepreviousdescription.

1. The redoxpolymerizationofVCMrequires considerablyhigherTBPOconcentrationthantheconventionalthermalpolymerization.Thedecom-position of TBPO at 50°C requires the presence of stannous octoate(SnOct) in some specific and reactive form, presumably stannous ions.Inviewoftheunavailabilityofstannousoctoateinthisform,duetoitscomplexation with TBPO and VCM as well as its failure to hydrolyzeand/ordissociate,theconcentrationofactivereductantismuchlowerthantheamountofstannousoctoatecharged.BecausetheTBPO/SnOctratioismaintainedconstant,it isnecessarytoincreasetheamountofTBPOsothatasufficientamountofactivereductantisavailable.Itisalsopos-siblethatthestannicspeciesgeneratedbytheBPOoxidationofstannousoctoate participate in the termination of propagating chains or interactwithradicalsgeneratedfromTBPO.Itiswellknownthatmetalsinthehighervalancestate(e.g.,ferricandstanniccompounds)reactwithfreeradicalsand,asaresultofelectrontransfer,convertthelattertocationicspeciesthatcannotaddmonomerssuchasVCM.Itis,therefore,neces-sary to increase theTBPOconcentration toprovide additional radicalsand the propagating chains therefrom, to compensate for those lost byelectrontransfer.ThecoordinationorcomplexationofstannousoctoatewiththechlorineatomsappendedtothePVCchainsmayalsoreducetheconcentrationof stannousoctoateavailable forTBPOreduction, as thePVCparticlesareinsolubleinVCMandthereforeremovetheappendedstannousoctoatefromtheactivelocusofpolymerization.

2. Theredoxpolymerizationgenerallydoesnotgotocompletionexceptafterextremelylongreactiontimesandeventhen,thereactionmixturecontainsa large amount of undecomposed TBPO. Based on the decomposition

DK3074_C006.indd 107 12/18/07 9:35:11 PM


studies,italsocontainsresidualstannousspecies.Thepresenceofunde-composedTBPOandstannousspeciesatthelevelingofforterminationofVCMpolymerizationisconsistentwiththepresenceofTBPO–SnOctcomplexand/ortheunavailabilityofreactivestannousspeciestocompletethe stoichiometric decomposition. Unhydrolyzed or PVC-bonded stan-nousoctoatemaybeinthesystem,butnotcapableofreducingTBPO.

3. Althoughdecompositionstudiesshow that thedecompositionofTBPOinthepresenceofstannousoctoateproceedsrapidlytoabout40%duringthefirst2handthenremainsessentiallyunchangedoverthenext20h,thepolymerizationcontinuesformorethan10h.Theindicatedrapiddecom-positionofTBPOinthepresenceofstannousoctoatedoesnotoccurwhenVCMispresent.Infact,thedecompositionrateisgreatlyreduced.ThisisactuallydesirablebecausetherapiddecompositiongeneratesradicalsatafasterratethanVCMcanaddtoit.Further,thepresenceofthesuspendingagentsresults in interferencewith theVCM–stannousoctoatecomplex,possiblybyformingasuspendingagent–stannousoctoatecomplexwhichslowlymakesactive stannous reductantavailableand thereforeextendsthetimeforradicalgeneration,analogoustothebehaviorofperestersinthermaldecomposition.

ThesuspensionpolymerizationofVCM[23,194]inabottlewasperformedwiththesuspensionrecipegiveninTable6.15.TheattemptedpolymerizationofVCMinthepresenceof0.055ml(0.23mmol)TBPO(0.5%byweightofVCM),intheabsenceofstannouschloridedehydrate,failedtoyieldanypolymerafter20hat50°C.ThisisconsistentwiththeTBPOhalf-lifeof133hat50°C(10hhalf-life74°C).Thesus-pensionpolymerizationofVCMinthepresenceof0.5wt%peroxyoctoate(POT),stannouschloridedehydrate0.052g(TBPO/SnCl2molarratio=1)andglacialaceticacid2mlresultedina5%yieldofpolymerafter13hat50°C.ThelowyieldofPVCindicatedthattheTBPO–SnCl2interactioneitheryieldedpredominantlynonradicalproductsorproceededsorapidlyintheVCMdropletastoprecludeeffectivepolym-erization.WhenthesuspensionpolymerizationofVCMinthepresenceof0.5wt%TBPOandVCM-insolublestannouschloridedehydrate(POT/SnCl2molarratio=1)wasconductedintheabsenceofaceticacid,theyieldofPVCwas82%after13h

tAble6.15typicalRecipesuspensionPolymerizationofVinylChloridea

Ingredients Amount(ml)

Water 21

Polyoxyethylenesorbitanmonostearate(1%aqueoussolution) 1

Sorbitanmonostearate(1%aqueoussolution) 2

MethocelA15(15cpsviscositygrademethylcellulose)(1%aqueoussolution) 2

Vinylchloride 10g

TBPO 0.055


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at50°C.Thismaybeattributedtotheinteractionofthemonomer-insolubleSnCl2orthehydratedionsthereofwiththeTBPOintheVCMatthewater–monomerdropletinterfacetogenerateradicalsataslowusefulrate.

In the caseofperester–stannous carboxylate redox system [192,193]withorwithoutacomplexingagent forvinylchloridepolymerization, thepolymerizationrecipewasthesameasdescribedearlierfor theSnCl2asreductant.Ineachcase,10gofVCMwastakenintheexperimentforpolymerizationat50°C.Theconver-sionwas increased in thepresenceofcomplexingagents.Someof theresultsarepresentedinTable6.16.

6.5.2 meRcaptansasReductant

Theuseofmercaptansasthereducingagentintheemulsionoraqueouspolymer-izationofvinylpolymerization isnotnew. Its efficiencyas the reducingagent in

tAble6.16VinylChloridePolymerizationat50°CwithPerester–stannousCarboxylateRedoxsystemwithorwithoutComplexingAgent

stannousCarboxylate(mmol) tbPoa(mmol)

ComplexingAgentbtype

(mmol) time(h) Conversion%

Octoate0.23 0.46 — 9 60

Octoate0.23 0.46 — 11 80

Octoate0.23 0.46 — 20 92

—— 0.46 — 20 0

Octoate0.115 0.23 — 12 45

Stearate0.115 0.23 — 18 30

Laurate0.23 0.46 — 2 10

Laurate0.23 0.46 — 6 40

Laurate0.23 0.46 — 8 65

Laurate0.23 0.46 — 10 90

Laurate0.23 0.46 — 15 96

Laurate0.115 0.23 — 9 45

Laurate0.115 0.23 — 16 51

Laurate0.115 0.23 DOP(0.23) 9 68

Laurate0.115 0.23 DOP(0.23) 16 73

Laurate0.046 0.092 — 7 15

Laurate0.046 0.092 DOP(0.092) 7 23

Laurate0.115 0.23 — 9 60c

Laurate0.115 0.23 DOP(0.23) 9 83c

Laurate0.115 0.23 DOA(0.23) 9 65

Laurate0.115 0.23 2-EHA(0.23) 9 64

Laurate0.115 0.23 TEP(0.23) 9 65

a TBPO=t-butylperoxyoctoate.b DOP=dioctylphthalate;EHA=ethylhexanoicacid;TEP=triethylphosphate.c Polymerizationat55°C.

DK3074_C006.indd 109 12/18/07 9:35:13 PM


conjunctionwithoxidantslikeH2O2[144,145],Bz2O2[146],andK2S2O8[147–158]forvinylpolymerizationhasbeenreported.Thefeasibilityofmercaptans[195–197]asthereductantwithperoxyestersofcarboxylicacidhasbeendescribedforsuspen-sioncopolymerizationorgraftcopolymerization.Thegeneralmechanismmaybewrittenas

R C

O

O O R R S H RS R C

O

OH R O- - - - ′ + - - → + - - + -|| || . (6.99)

Theprecedingradicalstakepartintheinitiationofpolymerization.

6.5.2.1 suspensionGraftCopolymerizationofstyreneandAcrylonitriletoPolybutadienelatex

In the suspension graft copolymerization [195], ABS polymers having enhancedphysicalpropertieslikehighimpactstrengthandlowpolybutadienecontentarepre-pared by graft copolymerization of styrene and acrylonitrile onto a polybutadinepolymer latex (particle size 1000–3000 Å, gel content 20–85%, and swell index≈18–150)inanaqueousmediuminthepresenceofasuspendingagentandacata-lyst.Thesuspensiongraftcopolymerizationwasperformedaccordingtothetypi-calrecipepresentedinTable6.17toyieldABSresinbeads.Thedriedbeadswereblendedwithantioxidantandextrudedtoformpellets,whichweremoldedtoformasamplehavingIzodimpactstrength>13ft-lb/in.,tensilestressatyield4500,elasticmodulus2.25×105,andshear–Izodratio<4.2.

tAble6.17typicalRecipe:suspensionGraftPolymerizationofstyreneandAcrylonitriletoPoly(butadiene)latexa


PartAStyrene 521

Acrylonitrile 211

Antioxidant(asa10%solutioninacrylonitrile) 3.5

tert-Butylperoxypivalate 1.8

tert-Butylperoctoate 0.54

tert-Dodecylmercaptan 2.4

PartbWater 3050

Pliolite 2104

Latex 281

2.5%Solutionofpoly(vinylalcohol) 700

0.5%Solutionofpoly(ethyleneoxide) 100

a PartAwasaddedtoPartBandthemixturewasmaintainedat68°Cfor4handat100°Cfor1h.

DK3074_C006.indd 110 12/18/07 9:35:14 PM


6.5.2.2 suspensionCopolymerizationofAcrylonitrileandstyrene

Theacrylonitrile–styrenecopolymer[196]waspreparedbysuspensionpolymerizationinthepresenceof0.005–0.05%(basedonmonomers)tert-Bu,3,5,5-trimethylperhex-anoate,andtert-butylperacetateat110–140°CaccordingtoatypicalrecipepresentedinTable6.18togiveacopolymer(unreactedmonomer0.1%)withloweryellowneonandhazethanacontrol(unreactedmonomer)preparedwithouttert-Buperacetate.

6.5.3 alkylBoRaneasReductant

Alkylboraneasthereductantinredoxpolymerizationiswellknown.Ithasbeenusedpreviouslyinconjunctionwithalkylperoxideandperoxyesterofcarbonicacid.The mechanism of alkyl borane–peroxyester of carboxylic acid is similar to thatpreviouslydescribed.Suspensionpolymerizationorcopolymerizationofvinylchlo-rideby theredoxsystemsuchasmonotertiarybutylpermaleate–Et3BorBu3Boriso-Bu3Bhasbeenreportedinpatentliterature[198].

6.5.3.1 suspensionPolymerizationofVinylChlorideorItsMixture

VinylchloridebyitselformixedwithC2H4,propylene,orisobutyleneispolymer-ized[198]inthepresenceofEt3B,Bu3Boriso-Bu3B,andmono-tert-Bupermaleateat-30°Cto+80°C.Thus,500gofvinylchloridewaspolymerizedat20°Cfor10hwith1200cm3ofdouble-distilledH2O,and1.5gofH2O-soluble suspension sta-bilizerwasintroduced.Bu3B(3.14g)wasintroducedwiththeexclusionofoxygenand4.0gofmono-tert-Bupermaleatewasadded;75.6%polymerconversionwasachieved.Muchloweryieldsofpolymerwereobtainedwhenaperacetateorperhen-zoatewasusedinsteadofthepermaleate.

6.5.4 BisulFiteasReductant

Bisulfiteisoneoftheoldestreducingagentsusedinpolymerization.Vinylpolym-erizationsusingbisulfiteasthereductantinconjunctionwithK2S2O8astheoxidant

tAble6.18typicalRecipe:suspensionCopolymerizationofstyreneandAcrylonitrilea


Water 25,000

Ca3(PO4)2 150

Styrene 11,000

Actylonitrile 6000

tert-Butyl3,5,5-trimethylperhexanoate 25

tert-Butylperacetate 15

tert-C12H25SH 50

Styrene 1600a Polymerizationfor5hat100°Cand2hat125°C.

DK3074_C006.indd 111 12/18/07 9:35:14 PM


havebeenreported[155,199–215]asearlyas1946.TheuseofbisulfitewithH2O2

[199,216,217]aswellaswithorganicperoxidelikeBz2O2[218]toinitiatepolym-erization is also well known. The persulfate-bisulfite system has been used forthepolymerizationof acrylonitrile [204,219–222],methyl acrylate [223], styrene[211], chlorotrifluoroethylene [201], andso forth.Thebisulfite–persulfatecombi-nation alongwithFe2_ has been alsoused to polymerize acrylonitrile [207, 224].Bisulfiteisalsousedwithotheroxidantslikeperoxydicarbonate[225],Cr2O3[226],oxygen [227], and KBrO3 [227] for redox initiation. These initiating systems areonlyrestrictedtoemulsionoraqueouspolymerization.Veryfewreportshavebeenpublishedaboutsuspensionpolymerizationofvinylchloride[228]andsuspensiongraftpolymerizationofvinylpyridine topolyolefins [229]usingaperesterof thecarboxylicacid-NaHSO3redoxsystem.

Inthelightofthemechanismofpersulfate–bisulfiteredoxinitiation,themecha-nismfortheperester–bisulfiteredoxsystemmaybesuggestedasfollows:

R C

O

O O R HSO HSO R C

O

O R O- - - - ′ + → - - + ′-|| . || .

3 3

(6.100)

Theprecedingradicalstakepartintheinitiationprocess.

6.5.4.1 suspensionPolymerizationandCopolymerizationofVinylChloride

Vinylchloride[228]waspolymerizedandcopolymerizedinsuspensionatlowtem-peratureinthepresenceofaperoxide,areducingagent,andacopperaccelerator.Thus, the vinyl chloride-2-ethylhexyl acrylate copolymer was prepared in 100%yieldbyusingtherecipegiveninTable6.19.

6.5.4.2 suspensionGraftCopolymerizationofVinylPyridine

Vinylpyridine-graftedpolyolefins[229]havingimproveddyeabilitywerepreparedwith>0.02wt%basedonthemonomerofaperestercatalystand>0.1wt%based

tAble6.19typicalRecipe:suspensionPolymerizationofVinylChloride-2-ethylhexylAcrylateCopolymera


Water 70

Vinylchloride 28.5

2-Ethylhexylacrylate 1.5

FluoronicF-68 0.3

Lupersol-11(t-butylperoxypivalate) 0.06

NaHSO3 0.5

CuCl2(2H2O(0.00039%withrespecttothemonomer)

a Polymerizationat61°F.

Au: Missing data here?Au: Missing data here?

DK3074_C006.indd 112 12/18/07 9:35:16 PM


on themonomerofa reducingagentpromoter selected from lower-valent saltsofmultivalentmetals,hydrosulfite,oralkalimetalformaldehydesulfoxylate.Thus,thepolypropylene–styrene–vinylpyridine-graftcopolymerpreparedin thepresenceof1wt%sodiumhydrosulfiteand0.5wt%tert-butyl2-ethylperhexanoateat90°Cwasmelt-spunintofiberswhichweredyedtoalight-fastwash-resistantdeepredshadewithCapracylRedG.

6.5.5 monosacchaRideastheReductant


AprocessforthebulkorsuspensionpolymerizationofvinylchlorideinthepresenceofaredoxcatalystsystemconsistingofaperoxyesterandamonosaccharideorcarboxylicacidestersofmonosaccharidewasdescribedbyGaylord[230].Themonosaccharideswhich were used as reductants include pentoses and hexoses wherein the carbonylgroup is either an aldehyde or ketone; that is, polyhydroxy aldehydes commonlyreferredtoasaldosesandpolyhydroxyketonescommonlyreferredtoasketoses.

Representativemonosaccharidesorreducingsugars includearabinose,xylose,lyxose,ribose,glucose,mannose,allose,galactose,tallose,altrose,idose,fructose,andsorbose.Thepreferredconcentrationofperoxyesterisgenerallybetween0.5%and 1% by weight of the vinyl chloride. The peroxyester/reductant mole ratio isgenerally1/0.1–1.Thepreferredtemperatureforthesuspensionpolymerizationwasinthe20–60°Crangeandtheweightratioofmonomerandwaterwasabout2:1.

Although the peroxyester–monosaccharide or peroxyester–monosaccharide–carboxylicacidestercatalystsystemisusefulinthebulkandsuspensionpolymer-izationofvinylchloride,theredoxsystemmayalsobeusedinthecopolymerizationofvinylchloridewithvinylidenechloride,vinylacetate,andothermonomerswhichundergocopolymerizationwithvinylchloride.

6.5.6 metalmeRcaptidesasReductant

Gaylordetal.[231]describedthebulkorsuspensionpolymerizationofethylenicallyunsaturatedmonomers,particularlyvinylchloride,usingacatalystsystemconsist-ingofamonomer-solubleperoxyesterordiacylperoxideandareducingagentwhichisastannousorantimony(III)mercaptide.

Theperoxygencompound/reductantmoleratiowasabout1:0.1–1.Theconcen-trationofperoxyesterwasabout0.05–1%byweightofthevinylhalidemonomer.Theconcentrationofbothperoxygencompoundandreductantmaybereducedbytheadditionofcomplexingagentsthatcontainsuitablefunctionalgroups.Alterna-tively, theadditionofcomplexingagents increasestherateofpolymerizationatagivenconcentrationofperoxygencompoundandreductant.

Therateofdecompositionofaperoxygencompoundsuchast-butylperoxyoc-toateinthepresenceofastannousorantimony(III)mercaptideisdecreasedinthepresenceofvinylchloride,presumablyduetotheformationofacomplexbetweenthereductantandthemonomer.However,whenacomplexingagentcontainingcar-bonylfunctionality(e.g.,aketone, lactone,carboxylicacid,orcarboxylicester) ispresent,thecomplexformationisdecreasedandtherateandextentofdecomposition

Au: Initial caps correct?Au: Initial caps correct?

DK3074_C006.indd 113 12/18/07 9:35:16 PM


of theperoxygencompoundincreases,even in thepresenceof themonomer.Theincreasedrateandextentofdecompositionofaperoxyesterordiacylperoxide inthepresenceofthecomplexingagentisaccompaniedbyanincreaseintherateandextentofpolymerizationofvinylchloride.

Thecomplexingagentswhichmaybeusedintheprocessofthepresentinven-tion are organo-soluble and contain carbonyl groups or phosphorus–oxygen link-ages.Thus,ketones,carboxylicacidsandesters,andphosphateestersareeffectivecomplexing agents. The lattermay be saturated or unsaturated, cyclic or acyclic,branchedorlinear,substitutedorunsubstituted.

6.5.7 ascoRBic/isoascoRBicacidoResteRsasReductant

Ascorbicacidhasbeenusedextensivelyasasolereducingagentorincombinationwithcupric, ferrous,or ferricsalts for thepolymerizationofvinylchloride in thepresenceofwater-solublecatalysts includinghydrogenperoxide[232–235],potas-siumpersulfate[236],cumenehydroperoxide[237],acetylcyclohexanesulfonylper-oxide [238], and a mixture of hydrogen peroxide and acetyl cyclohexanesulfonylperoxide[239].

Ascorbicacidhasalsobeenusedasacomplexingagentinthepolymerizationofvinylchloride[240]inthepresenceofadiacylperoxideandvariouswater-solublemetal salts. Similarly, 6-O-polmitoyl-l-ascorbic acidhas been used as a reducingagentinthepolymerizationofvinylchlorideinthepresenceofhydrogenperoxide[241]andmethylethylketoneperoxide[242].


Gaylord[243]hasdescribedthebulkorsuspensionpolymerizationofvinylchlorideusingacatalystsystemconsistingofamonomer-solubleperoxyesterordiactylper-oxideasoxidantanda6-O-alkanoyl-l-ascorbicacidasareducingagent.

Bulk or suspension polymerization may be performed at temperatures in the20–60°Crange.Gaylord[244]hasalsodescribedtheuseofisoascorbicacidasthereducingagentincombinationwithaperoxygencompoundasthecatalystsystemforthesuspensionpolymerizationofvinylchloride.

Acomparisonof theresultsobtainedwithascorbicacidandisoascorbicacid,in the suspension polymerization of vinyl chloride at 50°C, in the presence oft-butylperoxyoctoate(t-BPOT)ataperoxyester/reductantmoleratioof2:1isgiveninTable6.20.Theuseofisomeric6-O-alkanoy-d-ascorbicacidhasbeenfoundto

tAble6.20PolymerizationofVinylChlorideat50°C

Reductant t-bPot(wt%) time(h) Conversion%

Ascorbicacid 0.3 8.5 40.5

0.1 8.5 20.0

Isoascorbicacid 0.1 8.5 70.5

0.1 16.0 70.5

0.05 16.0 40.0

DK3074_C006.indd 114 12/18/07 9:35:17 PM


result inasignificantlyhigherrateofpolymerization,permittingtheuseof lowerconcentrationsofperoxyestertoachievefasterreaction.

6.6 HyDRoPeRoxIDes

Generally,hydroperoxidesarederivativesofhydrogenperoxide,withonehydrogenreplacedbyanorganicradical:

H O OH R O O Hhydrogen hydroperoxide- - - - -

peroxide

Hydroperoxidechemistryhaditsheydayinthedecade1950–1960,followingthefirmestablishmentof thesecompoundsas reactive intermediates in theautoxida-tionofolefins.Afterward,manyreportsregardingvinylpolymerizationinvolvinghydroperoxidealoneorcoupledwithasuitablereducingagenthaveappearedintheliterature.

6.6.1 sulFuRdioxideasReductant

ManyreportshavebeenpublishedontheuseofSO2asthereductanttoinitiatethepolymerization.Forexample,Polishworkers[245]studiedtheemulsionpolymeriza-tionofthestyrene-SO2systemusingcumeneandpinenehydroperoxide.GomesandLourdes [246] investigated the liquid SO2–cumene hydroperoxide system. Ghoshetal.usedtheSO2incombinationwithhetero-cycliccompoundsbypyridine,tet-rahydrofuran,andN-N′-dimethylformamideforphotopolymerization[247–249]aswellasaqueouspolymerization[250].Mazzolinietal.[251–254]reportedtheorganichydroperoxide–SO2redoxpairandanucleophilicagenttopolymerizevinylchlorideinbulkatsubzerotemperatures.PatronandMoretti[255]havealsoreportedonthebulkpolymerizationofvinylchlorideusingthesametypeofsystemat20°C.

ThedecompositionoforganichydroperoxidesbytheactionofSO2dependsonthe reaction medium; for example, cumyl hydroperoxide (CHP) is quantitativelydecomposedintophenolandacetoneifthereactionisperformedinananhydrousweaklynucleophilicornon-nucleophilicmedium(e.g.,CCl4,CH3CN,CH3CH2Cl,CH CHCl).2 - Thistypeofdecomposition,whichproceedsthroughanionicmecha-nismwithoutformationofradicals,canalsobeobtained[256,257]withperchloricacid,ferricchlorideinbenzene,andsulfuricacid.Itis,therefore,inferredthatSO2behavesasastrongacid toward thedecompositionofCHPinanhydrous,weaklynucleophilicornon-nucleophilicsolvents.

Foraredoxreactiontotakeplace,accordingtotheLewistheoryofacidsandbases, it is necessary that the reductant (SO2) acts as a base toward the oxidant(hydroperoxide), to allow the transfer of electrons from the former to the latter[258].Theconditionisfulfilledbytheadditiontothereactionmediumofastronglynucleophilic agent N_ (e.g., OH_) to transform the SO2 into the conjugate baseNSO2

- . When water is added to the system hydroperoxide–SO2 and the concen-trationoftheformerincreases,theabsorbanceat272mµ,characteristicofphenol,diminishes,whereasanewabsorbancemaximum,rangingbetween237and255mµ,emergesduetoamixtureof1-methylstyrene(3%),acetophenone(60%),andcumylalcohol(37%).Whenwaterisaddedtothesystemhydroperoxide–SO2inanorganic

DK3074_C006.indd 115 12/18/07 9:35:20 PM


medium,asituationanalogoustotheemulsionpolymerizationbyhydroperoxideandSO2isinduced[259].ThisdemonstratesthepossibilityofswitchingthemechanismofreactionbetweenhydroperoxideandSO2inanessentiallyorganicmediumfromanionicmechanismtoaradicalone,thusofferingawayfortheinitiationofvinylpolymerizationatlowtemperature.

AccordingtoMazzolinietal.[251],thekineticexpressionsforthecontinuousbulkpolymerizationofvinylchloridebythehydroperoxide–SO2nucleophilicagentmaybeasfollows:

Productionofradicals:

ROOH CH OSO CH OH R SO+ → + +- -3 2 3 3

. . (6.101)

and d R dt K ROOH CH SOd( .) / ( )( ),= -2 3 2whereKd is thevelocity constant for the

radicalproduction.Initiationofpolymerization:

R M M. .+ → (6.102)

whereMisvinylchloridemonomerand

d M

dtK R Ma

( .)( .)( )= (6.103)

whereKaisthevelocityconstantformonomeradditiontoprimaryradicals.Propagation:

M M M. .+ → (6.104)

and

d M

dtK M Mp

( )( .)( )= (6.105)

whereKpisthevelocityconstantforthepropagationreaction.Termination:

M M P. .+ → (6.106)

d M

dtK Mr

( .)( .)= 2 2 (6.107)

whereKristhevelocityconstantforthecombinationreaction.

Understationaryconditions(input=_output+_reactionamount),thebalanceforthecatalystwillbe

F0(C)0=F(C)+Kd(C)(S)V (6.108)

DK3074_C006.indd 116 12/18/07 9:35:25 PM


where F0=feedrateofallliquidstreamstoreactor,volumeperunittime F = output rate of the liquid fraction at overflow from reactor, volume

perunittime (C)0=hydroperoxideconcentrationinliquidfeed (C)=hydroperoxideconcentrationinreactor(orinreactoroverflow) (S)=concentrationofcompound CH OSO3 2

- inreactor(orinreactoroverflow) V=volumeoccupiedbyliquidphaseinreactor

Atsufficientdwelltimeand ( )/( )ROSO CHP2- molarratios,F(C)isnegligible

ifcomparedwithF0(C)0andKd(C)(S)V.Asthecatalystdecompositionapproachescompletion,anexpressionfor(C)canthusbeassumed:

( )( )

( )C

F C

K S Vd

= 0 0 (6.109)

Thebalanceforthemonomeris

F M F M K M M Vp0 0( ) ( ) ( .)( )= + (6.110)

(M)0(monomerconcentrationinfeed)and(M)(monomerconcentrationinliquidphaseofoverflow)beingequalforabulkpolymerization,themonomerconversioncanbeexpressedas

cF F

F=

-0

0

(6.111)

orfromthepreviousequation c K M V Fp= ( .) / .0

Thebalancefor ( .)M is

2 2 2fK C S V F M K M Vd r( )( ) ( .) ( .)= + (6.112)

wherefistheefficiencyofinitiatingradicals(i.e.,thefractionofradicalstakingpartinpolymerchaininitiation).

Theterm2fKd(C)(S)Vcanbeassumedtobeequalto2f(C)0F0. 2 2K M Vr (.) is

equal to twice the number of macromolecules formed per unit time. Both canbe experimentally estimated.F M( .) appears to be negligible if compared with2fKd(C)(S)Vand 2 2K M Vr (

.) . Thentheprecedingequationbecomes

2Kr(M.)2=2fKd(C)(S) (6.113)

orsubstitutingthevalueof(C),

22

2

0 0

K MV

fF Cr (.)

( )= (6.114)

DK3074_C006.indd 117 12/18/07 9:35:32 PM


Then,theconversioncanbeexpressedas

CK

Kf C V Fp

r

=

-1 2

1 201 2 1 2

01 2

// / / /( ) (6.115)

forconversion,notexceedingabout20%,Vcanbeassumedas

V=V0(1-c) (6.116)

thusV0/F0beingtheconventionaldwelltime,O,afinalequationmaybewrittenas

cc

K

Kf c Qp

r( )( )/ /

/ / /

1 1 2 1 21 2

01 2 1 2

-=

(6.117)

Inotherwords,theconversionisproportionaltothesquarerootofthehydroper-oxideconcentrationandthedwelltime.

ThefollowingmechanismcanbeproposedfortheradicaldecompositionofthehydroperoxidebySO2andnucleophilicagent:

SO CH O CH OSO2 3 3 2+ →- - (6.118)

ROOH CH OSO ROOSO CH OH RO+ → + →

+

- -3 2 2 2

.

-- +SO CH OH.3 3

(6.119)

or

Ph–C–O–OH CH3OHO–CH3CH3

CH3–+

Ph–C–OO–SCH3 O

CH3

S+

OO(–) (–)

–

+ +O

Ph–C–O. + .SO–3

CH3

CH3

(6.120)

Theoxycumylradicalmayfurtherdecomposeinto1-methylstyrene,acetophe-none,andcumylalcohol,ortheradicalitself,itsfragments ( . , .),CO OH3 orradicalsderivedfromchaintransferreactionsmayinitiatepolymerization.The -SO.

3 radicaliseasilyidentifiedasanendgroupinthepolymerchain.Therate-determiningstepforthewholecatalytikreactionappearstobetheformationofthecomplex(I),asindicatedbythefactthatanasymptoticlimitforthepolymerizationrateisreachedonlywhenthe ( . ) /( )CH OSO CHP3 2 ratioisinconsiderableexcessoverthestoichio-metricratioof1.

6.6.1.1 bulkPolymerizationofVinylChloride

During the bulk polymerization of vinyl chloride [251], when cumyl or tert-BuhydroperoxidesandSO2areusedwithethers,ketones,oralcohols,sulfonegroups

DK3074_C006.indd 118 12/18/07 9:35:37 PM


areincorporatedinthepolymerchainbecauseofcopolymerizationofSO2.Whenthe hydroperoxides and SO2 are used with MeO or EtO (from Na or Mg alkox-ides),SO2copolymerizationiscompletelysuppressed,providedthe MeO SO- / 2 orEtO SO- / 2 ratio isat least1:1.When the feed rateofhydroperoxide isconstant,themaximummonomerconversion incontinuousbulkpolymerization is reachedwhentheSO2/hydroperoxideratiois≥1.5:1.Thepercentageconversionforthevari-ousnucleophilicagentsusedarepresentedinTable6.21.

Thehydroperoxide-SO2systemreactedinthevinylchloridemonomerat-30°C.Withoutanynucleophilicagent, thereactionproceedsviatheusualionicpathandno polymerization is detected. With the addition of alcohols, ketones, and ethers,the redox reaction is promoted and substantial quantities of polymer are formed.Whenweaknucleophilicagents, likeethersandketones,areused,polymerizationyields are low.High conversionswereobtainedwith alcohols.Thebest yieldwasobtainedbytheadditionof5%methanolonthemonomerweight.Thepolymeriza-tionrate,atconstantCHPandSO2concentrations,approachesthemaximumwhenthe(CH3O−)/(SO2)ratioisatleast1,employingeithersodiumormagnesiummethox-ideata(CH3O−)/(SO2)ratioof1.TheSO2iscompletelytransformedintothesaltofmethylsulfurousacid.Thesystematicpolymerizationstudywasperformedusinga(CH3O_)/SO2ratioof1:1toassurecompleteneutralizationofSO2andavoiditscopo-lymerization.Thesyndiotacticityindexwas2.1–2.2forpolymerspreparedat-30°C,and2.4–2.5forpolymerspreparedat-50°C.TheglasstransitiontemperatureTgwas100°Cforpolymersobtainedat–30°C,and104°Cforpolymersobtainedat-50°C.

Thepreviouslydescribedcatalytic systemwas also effective [251]withothervinylmonomersoverwidetemperatureranges.TheresultsaregiveninTable6.22.

IntwootherpatentsreportedbyMazzolinietal.[252,254]forbulkpolymerizationofvinylchloride,theyusedthesametypeofcatalyticsystemasdiscussedpreviously.

tAble6.21InfluenceofnucleophilicAgentsonbulkPolymerizationofVinylChloridea

nucleophilicAgents %(oMW) Conversion%

None 0.00 0.00

Acetophenone 0.60 1.90

Cyclehexanone 0.49 5.10

Acetone 0.29 1.20

Methylethylether 0.36 3.70

Ethylether 0.52 3.20

Methanol 0.16 6.80

Methanol 5.00 10.50

Butanol 0.36 6.00

Dimethylamine 0.22 0.40

Dimethylformamide 0.36 0.51

a Temperature=-30°C;CHP=0.15%(OMW);SO2=1.6%(OMW);nucleophilicagentasspecified;additiontimeofcatalystcomponentsintomonomer=1h.Totalreactiontime=2h.(OMW=onmono-merweight.)

DK3074_C006.indd 119 12/18/07 9:35:39 PM


Thus, vinyl chloride was polymerized at-30°C in the presence of a mixture ofcumenehydroperoxideortert-Buhydroperoxide,amethanolicsolutionofSO2,andamethanolicsolutionofNaOMe,NaOEt,orKOMe.

InanotherGermanpatent,Mazzolinietal.[253]reportedthelow-temperaturebulkpolymerizationofvinylchlorideinthepresenceofacatalystsystemconsist-ingof anorganichydroperoxide,SO2, andat leastonealkalimetal alcoholate ata [ROX]–[SO2]/[R′OOH]mole ratioof0–0.5 and0.005–1%mercaptocompoundwhichgaveadegreeofconversion>18%andapolymerwithoutstandingphysicalandchemicalproperties.ThetypicalrecipeforthepolymerizationispresentedinTable6.23.

PatronandMoretti[255]alsoreportedthebulkpolymerizationofvinylchlorideat>0°Cinthepresenceofacatalystsystemconsistingofanorganichydroperoxide,SO2,andanalcoholormetalalcoholate.A25%conversionwasobtainedat25°C.ThePVCrecoveredhadanintrinsicviscosityof1.3andbulkdensityof0.41gcm–3.

tAble6.22PolymerizationofVinylMonomersbytheCHP/so2/Mg(oCH3)2Catalyticsystema

Monomer temperature(C) Conversion%

Vinylacetate -30 22.0

-60 6.0

Vinylformate -30 19.0

Acrylonitrile -30 23.0

Styrene 50 15.5

Acrylamide(30%inmethanol) -30 21.0

2-Hydroxyethylacrylate 20 56.5

-30 27.5

tert-Butylaminoethylmethacrylate

20 46.5

a CHP= 0.25% (OMW); SO2= 0.2% (OMW); Mg(OCH3)= 0.14% (on moles). Catalyst additiontime=5h.Totalreactiontime=5h.(OMW=onmonomerweight).

tAble6.23typicalRecipe:bulkPolymerizationofVinylChloridea

Ingredients Amount(ghr-1)

Liquidvinylchloride(at(30°C) 200,000

Cumenehydroperoxide 240

SO2 150

MeONa 136

Mercaptoethanol 60

a 36.2kgh−1(22.5%conversion);PVChasintrinsicviscosity1.38dlg−1.

DK3074_C006.indd 120 12/18/07 9:35:40 PM


They [260] also reported the bulk polymerization of vinyl chloride by taking amixturecontainingliquidvinylchlorideat-30°Candacatalystcompositioncon-tainingcumenehydroperoxide,SO2,Namethylate,and2-mercaptoethanolthatwascontinuouslyfedtoareactor.Themolarweightconcentrationratioofthecatalystcompositionwas(NaOME)(SO2)/cumenehydroperoxide=0.1.ThepolymerizationyieldedPVCwithanintrinsicviscosityof1.3dlg–1.

6.6.2 sulFiteasReductant

Theoxyacids of sulfur such as sulfite [155, 199, 220, 253–265] forman efficientredoxsysteminconjunctionwithpersulfatestoinitiatevinylpolymerization.Sully[266]examinedthe Cu SO2

32+ -- systeminair.The CIO SO3 3

2- --systemhasbeenusedinthepolymerizationofacrylonitrile[267,268]andacryl-

amide [268, 269]. The KBrO3–Na2SO3–H2SO4 system is also an effective redoxinitiator[270],givingrise topolymerscontainingstrongacidendgroups.All thepreceding initiating systemshavebeen employed in aqueousor emulsionpolym-erization.Reportsof theuseof sulfiteas the reductantwithorganicperoxidesorhydroperoxidesareveryfew.Melacinietal.[271]havereportedthebulkpolymer-ization of acrylonitrile by redox system such as cumene hydroperoxide–dimethylsulfite.Themechanismofinitiationmaybedescribedas

ROOH SO RO OH SO+ → + +- - -32

3. .

(6.121)

Theseradicalstakepartintheinitiationstep.t-Butylhydroperoxide(tert-BHP)formsfreeradicalswithSOCI2inthepresenceofmethanolwhichinitiatesthepolym-erizationofvinylchloridesuccessfully [272]. Itwasproposed thatasafirst step,SOCI2reactswithmethanoltoyieldmethylchlorosulfitewithwhichtert-BHPreactstoformmethyltert-butylperoxysulfite,whichdecomposestogivefreeradicals.

6.6.2.1 bulkPolymerizationofAcrylonitrile

Acrylonitrile[271]polymerswerepreparedinbulkinhighyieldsundercontrolledconditionsatroomtemperatureto60°Cin30–90minusingradicalcatalystswithdecomposition rate constants >1 hr-1. Thus, 1600 g of acrylonitrile containing300ppmofwaterwaskept at 50°C, and3.2gof cumenehydroperoxide, 23.2gofdimethylsulfite,and18.1gofmagnesiummethylatein150cm3ofMeOHwereadded.Theconversionachievedin15minrepresentedafinalconversionof77%inacontinuouspolymerizationsystem.

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Au: I inserted “JP” in all refs. with “Jpn. Kokai Tokkyo Koho” – edits OK?

Au: I inserted “JP” in all refs. with “Jpn. Kokai Tokkyo Koho” – edits OK?

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Au: Please spell out journal title in Ref. 39, 50, and 51.

Au: Please spell out journal title in Ref. 39, 50, and 51.

Au: Please spell out journal titles in Refs. 39-46.

Au: Please spell out journal titles in Refs. 39-46.

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Au: Edit OK?Au: Edit OK?

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