29-1 organicpolymerchemistry. 29-2 some definitions polymer:poly+meros polymer: from the greek,...

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OrganicOrganicPolymerPolymerChemistryChemistry

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Some DefinitionsSome Definitions

Polymer:Polymer: From the Greek, polypoly ++ merosmeros, many parts.• Any long-chain molecule synthesized by bonding

together single parts called monomers.

MonomerMonomer: From the Greek, monomono ++ merosmeros, single part.• The simplest nonredundant unit from which a polymer

is synthesized.

Plastic:Plastic: A polymer that can be molded when hot and retains its shape when cooled.

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ContinuedContinued

Thermoplastic:Thermoplastic: A polymer that can be melted and molded into a shape that is retained when it is cooled.

Thermoset plastic:Thermoset plastic: A polymer that can be molded when it is first prepared but, once it is cooled, hardens irreversibly and cannot be remelted.

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Notation & NomenclatureNotation & Nomenclature

Show the structure by placing parens around the repeat unit:• nn = average degree of polymerization.

To name a polymer, prefix polypoly to the name of the monomer from which it is derived.• For more complex monomers or where the name of the

monomer is two words, enclose the name of the monomer in parens, for example poly(vinyl chloride).

n

StyrenePolystyrene

Cln

Poly(vinyl chloride) (PVC)

Cl

Vinyl chloride

synthesized from

synthesized from

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Molecular WeightMolecular Weight

All polymers are mixtures of individual polymer molecules of variable MWs.• number average MW:number average MW: Count the number of chains of a

particular MW (Ni moles), multiply each number by the MW (Mi), sum these values, and divide by the total number of polymer chains.

• weight average MW:weight average MW:

ba ba

Mn =

MiNi

Ni

= (Mass of fraction i)(fraction of all molecules having mass i)

Mw =

MiMiNi

MiNi

= (Mass of fraction i)(f raction of total mass of molecules having mass i)

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ExampleExample

Sample has the unique chainsChainMW, Mi

MolesPresent, Ni

ChainMass, MiNi

120 0.10 12g

140 0.20 28

140 0.10 14

160 0.30 48

160 0.40 64

180 0.20 36

200 0.10 20

Mn = .10(120) + (.20+.10)(140)+ (.30+.40)(160) +.20(180)+.10(200)

(.10+.20+.10+.30+.40+.20+.10)

= 12+28+14+48+64+36+20

1.4=158

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ExampleExample

Now for weight averaged MW

Mw = 120(12) + 140(28+14)+160(48+64)+180(36)+200(20)

(12+28+14+48+64+36+20)

=161

140

ChainMW, Mi

MolesPresent, Ni

ChainMass, MiNi

120 0.10 12g

140 0.20 28

140 0.10 14

160 0.30 48

160 0.40 64

180 0.20 36

200 0.10 20

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Polydispersivity IndexPolydispersivity Index

PDI = Mw/Mn measures the extent of different molecular weights.

PDI greater than or equal to 1.0

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MorphologyMorphology

Polymers tend to crystallize as they precipitate or are cooled from a melt.

Acting to inhibit crystallization are that polymers are large molecules. Complicated and irregular shapes prevent efficient packing into ordered structures.

As a result, polymers in the solid state tend to be composed of • ordered crystalline domainscrystalline domains • disordered amorphous domainsamorphous domains

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Morphology: CrysatlineMorphology: Crysatline

High degrees of crystallinity are found in polymers with • regular, compact structures• strong intermolecular forces, such as hydrogen bonds

and dipolar interactions.

As the degree of crystallinity increases, the polymer becomes more opaque due to scattering of light by the crystalline regions.

Melt transition Melt transition temperature, Ttemperature, Tmm:: The temperature at which crystalline regions melt.• As the degree of crystallinity increases, Tm increases.

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Morphology: AmorphousMorphology: Amorphous

Highly amorphous polymers are sometimes referred to as glassy polymers.• Lacking crystalline domains that scatter light,

amorphous polymers are transparent.• They are weaker polymers, both in terms of their

greater flexibility and smaller mechanical strength.• On heating, amorphous polymers are transformed

from a hard glassy state to a soft, flexible, rubbery state.

GlassGlass transition temperature, T transition temperature, Tgg:: The temperature at which a polymer undergoes a transition from a hard glass to a rubbery solid. Put polystyrene cup in boiling water.

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• Example:Example: poly(ethylene terephthalate), abbreviated PET or PETE, can be made with crystalline domains of 0% to 55%.

OO

OO

nPoly(ethylene terephthalate)

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Completely amorphous PET is formed by quickly cooling the melt.• PET with a low degree of crystallinity is used for

plastic beverage bottles.

More crystallline formed by slow cooling, more molecular diffusion occurs and crystalline domains form as the chains become more ordered.• PET with a high degree of crystallinity can be drawn

into textile fibers and tire cords.

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Step-Growth PolymersStep-Growth Polymers Step-growth polymerization:Step-growth polymerization: A polymerization in which chain growth

occurs in a stepwise manner between difunctional monomers. Many chains initiated at same time. All monomeric -> mostly dimeric -> mostly trimers. Etc.

five types of step-growth polymers:• Polyamides

• Polyesters

• Polycarbonates

• Polyurethanes

• epoxy resins

NH

O

O

O

O O

O

HN

OH

NH

O

O

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Polyamides Nylon 66Polyamides Nylon 66

Nylon 66 (from two six-carbon monomers).

• During fabrication, nylon fibers are cold-drawncold-drawn to about 4 times their original length, which increases alignment, crystallinity, tensile strength, and stiffness.

O

HOOH

OH2N

NH2

Hexanedioic acid(Adipic acid)

1,6-Hexanediamine(Hexamethylenediamine)

+

O HN

NO H

heat

n

Nylon 66

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Nylon 66, source of Hexanedioic AcidNylon 66, source of Hexanedioic Acid

• The raw material base for the production of nylon 66 is benzene, which is derived from cracking and reforming of petroleum.

catalyst

Cyclohexanone

catalyst

Benzene Cyclohexane

Cyclohexanol

+

3H2

HNO3

Hexanedioic acid(Adipic acid)

OH O

COOHCOOH

O2

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Nylon 66, source of the 1,6 hexanediamineNylon 66, source of the 1,6 hexanediamine

• Hexanedioic acid is the starting material for the synthesis of hexamethylenediamine.

O

H2NNH2

O

O

O-NH4+

ONH4

+ -O

4H2 H2NNH2 catalyst

heat

1,6-Hexanediamine(Hexamethylenediamine)Hexanediamide

(Adipamide)

Ammonium hexanedioate(Ammonium adipate)

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Polyamides, Nylon 6Polyamides, Nylon 6

Nylons are a family of polymers, the two most widely used of which are nylon 66 and nylon 6. • Nylon 6 is synthesized from a six-carbon monomer.

• Nylon 6 is fabricated into fibers, brush bristles, high-impact moldings, and tire cords.

Caprolactam

1. partial hydrolysis2. heat n

nNH

O

NOH

Nylon 6

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Polyamides, KevlarPolyamides, Kevlar

Kevlar is a polyaromatic amide (an aramid).

• Cables of Kevlar are as strong as cables of steel, but only about 20% the weight.

• Kevlar fabric is used for bulletproof vests, jackets, and raincoats.

+

1,4-Benzenediamine(p-Phenylenediamine)

1,4-Benzenedicarboxylic acid (Terephthalic acid)

nKevlar

+

O

NH

COHnHOC

O O

nH2N NH2

CNHC

O

2nH2 O

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Polyesters, PETPolyesters, PET

Poly(ethylene terephthalate), abbreviated PET or PETE, is fabricated into Dacron fibers, Mylar films, and plastic beverage containers.

heatHO

O

OH

O

HOOH

O O

OO

n

1,4-Benzenedicarboxylic acid(Terephthalic acid)

+

1,2-Ethanediol(Ethylene glycol)

+2nH2 O

Poly(ethylene terephthalate)(Dacron, Mylar)

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PET, source of glycol and terepthalic acidPET, source of glycol and terepthalic acid

• Ethylene glycol is obtained by air oxidation of ethylene followed by hydrolysis to the glycol.

• Terephthalic acid is obtained by catalyzed air oxidation of petroleum-derived p-xylene.

Terephthalic acidp-XylenecatalystHOC COHCH3

O2H3C

O O

O

CH2=CH2O2

CH2-CH2

H+, H2OHOCH2CH2OH

Oxirane(Ethylene oxide)

1,2-Ethanediol(Ethylene glycol)

Ethylenecatalyst

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Polycarbonates, LexanPolycarbonates, Lexan

• To make Lexan, an aqueous solution of the sodium salt of bisphenol A (BPA) is brought into contact with a solution of phosgene in CH2Cl2.

Phosgene

+

Disodium salt of Bisphenol A

+Na-O

CH3

CH3

O-Na+

Lexan (a polycarbonate)

+

Cl Cl

O

nO

CH3

CH3

O

O

2NaCl

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Phase transfer catalysis.Phase transfer catalysis.

The sodium salt of bisphenol A is water soluble while the phosgene is not. Immiscible. No reaction.

Solution: Phase transfer catalysis. NBu4+ and a

negative ion can go back and forth between the two phases.

NBu4+ brings bisphenolate ion into organic phase

Reaction occurs with phosgene producing Cl-

NBu4+ brings chloride ion into water phase.

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Polycarbonates, LexanPolycarbonates, Lexan

Lexan is a tough transparent polymer with high impact and tensile strengths and retains its shape over a wide temperature range.• It is used in sporting equipment, such as bicycle,

football, and snowmobile helmets as well as hockey and baseball catcher’s masks.

• It is also used in the manufacture of safety and unbreakable windows.

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PolyurethanesPolyurethanes

A urethaneurethane, or carbamate, is an ester of carbamic acid, H2NCH2COOH.• They are most commonly prepared by treatment of an

isocyanate with an alcohol.

Polyurethanes consist of flexible polyester or polyether units alternating with rigid urethane units.• The rigid urethane units are derived from a

diisocyanate.

+An isocyanate A carbamate

RNHCOR'RN=C=O R'OH

O Addition to the N=C bond

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PolyurethanesPolyurethanes

• The more flexible units are derived from low MW polyesters or polyethers with -OH groups at the ends of each polymer chain.

CH3N=C=OO=C=N nHO-polymer-OH

CNH NHCO-polymer-OCH3 OO

Low-molecular-weightpolyester or polyether

2,6-Toluenediisocyanate

+

n

A polyurethane

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Epoxy ResinsEpoxy Resins

Epoxy resins are materials prepared by a polymerization in which one monomer contains at least two epoxy groups.• Epoxy resins are produced in forms ranging from low-

viscosity liquids to high-melting solids.

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• The most widely used epoxide monomer is the diepoxide prepared by treating one mole of bisphenol A with two moles of epichlorohydrin.

O

CH3

CH3

OOO

A diepoxide

OCl Na+-O

CH3

CH3

O-Na+

the disodium salt of bisphenol A

Epichlorohydrin

+

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• Treatment of the diepoxide with a diamine gives the resin.

H2NNH2

A diamine

O

CH3

CH3

O

An epoxy resin

H N

OH OH

N H n

O

CH3

CH3

OOO

A diepoxide

Note the regioselectivity

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ThermosetsThermosets

Bakelite was one of the first thermosets.

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Chain-Growth PolymersChain-Growth Polymers

Chain-growth polymerization:Chain-growth polymerization: A polymerization that involves sequential addition reactions, either to unsaturated monomers or to monomers possessing other reactive functional groups.

Reactive intermediates in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes.

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Chain-Growth PolymersChain-Growth Polymers

We concentrate on chain-growth polymerizations of ethylene and substituted ethylenes.

• On the following two screens are several important polymers derived from ethylene and substituted ethylenes, along with their most important uses.

R

An alkene

R

n

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PolyethylenesPolyethylenes

CH2=CH2

CH2=CHCH3

CH2=CHCl

CH2=CCl2

MonomerFormula

Common Name

Polymer Name(s) andCommon Uses

Ethylene

Propylene

Vinyl chloride

1,1-Dichloro-ethylene

Polyethylene, Polythene;break-resistant containersand packaging materials

Polypropylene, Herculon;textile and carpet fibers

Poly(vinyl chloride), PVC;construction tubing

Poly(1,1-dichloroethylene), Saran; food packaging

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PolyethylenesPolyethylenes

CH2=CHCN

CF2=CF2

CH2=CHC6H5

CH2=CHCOOEt

CH3

CH2=CCOOCH3

Acrylonitrile

Tetrafluoro-ethylene

Styrene

Ethyl acrylate

Methylmethacrylate

Polyacrylonitrile, Orlon;acrylics and acrylates

Poly(tetrafluoroethylene), PTFE; nonstick coatings

Polystyrene, Styrofoam;insulating materials

Poly(ethyl acrylate); latex paintsPoly(methyl methacrylate), Plexiglas; glass substitutes

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Radical Chain-GrowthRadical Chain-Growth

Among the initiators used for radical chain-growth polymerization are diacyl peroxides, which decompose on mild heating.

O

O

O

O

O

O

2CO2

Dibenzoyl peroxide

2 +

A phenyl radical

A benzoyloxy radical

2

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Another common class of initiators are azo compounds, which also decompose on mild heating or with absorption of UV light.

Azoisobutyronitrile (AIBN)

or hN NN

CN

C NN CN+2

Alkyl radicals

: :

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Radical polymerization of a substituted ethylene.• chain initiation

• chain propagation

In-In

In

or h 2In

In

RR+

In

RR

In

RR

etc.

In

R

In

RRn

R

Rn+

+

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• chain termination.

InRR

InRR

InR R

InRR

InRR

H

2

+

n n

nn

n

radical coupling

dispropor- tionation

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Radical reactions with double bonds almost always gives the more stable (the more substituted) radical.• Because additions are biased in this fashion,

polymerizations of vinyl monomers tend to yield polymers with head-to-tail linkages.

head-to-tail linkages

R R R R R R R

R R R

head-to-tail linkages head-to-head linkage

R R R R R R R

R R R

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Chain-transfer reaction:Chain-transfer reaction: The reactivity of an end group is transferred from one chain to another, or from one position on a chain to another position on the same chain.• Polyethylene formed by radical polymerization exhibits

butyl branches on the polymer main chain.

A six-membered transition state leading to 1,5-hydrogen abstraction

H H

n

nCH2=CH2

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The first commercial polyethylenes produced by radical polymerization were soft, tough polymers known as low-density polyethylene (LDPE).• LDPE chains are highly branched due to chain-transfer

reactions.• Because this branching prevents polyethylene chains

from packing efficiently, LDPE is largely amorphous and transparent.

• Approx. 65% is fabricated into films for consumer items such as baked goods, vegetables and other produce, and trash bags.

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Ziegler-Natta PolymersZiegler-Natta Polymers

Ziegler-Natta chain-growth polymerization is an alternative method that does not involve radicals.• Ziegler-Natta catalysts are heterogeneous materials

composed of a MgCl2 support, a Group 4B transition metal halide such as TiCl4, and an alkylaluminum compound.

CH2=CH2

TiCl4/ Al(CH2CH3)2ClMgCl2 n

Ethylene Polyethylene

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Mechanism of Ziegler-Natta polymerization.Step 1: Formation of a titanium-ethyl bond

Step 2: Insertion of ethylene into the Ti-C bond.

Ti Cl Ti+ +

MgCl2/ TiCl4particle

Diethylaluminumchloride

AlCl

Cl

AlCl

Ti Ti+ CH2=CH2

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Polyethylene from Ziegler-Natta systems is termed high-density polyethylene (HDPE).• It has a considerably lower degree of chain branching

than LDPE and a result has a higher degree of crystallinity, a higher density, a higher melting point, and is several times stronger than LDPE.

• Appox. 45% of all HDPE is molded into containers.• With special fabrication techniques, HDPE chains can

be made to adopt an extended zig-zag conformation. HDPE processed in this manner is stiffer than steel and has 4x the tensile strength!

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Polymer Polymer StereochemistryStereochemistry

There are three alternatives for the relative configurations of stereocenters along the chain of a substituted ethylene polymer.

HR RH HR RH HR

Syndiotactic polymer(alternating configurations)

HR HR HR HR HR

Isotactic polymer (identical configurations)

HR HR HR HR RH

Atactic polymer(random configurations)

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Polymer StereochemistryPolymer Stereochemistry

In general, the more stereoregular the stereocenters are (the more highly isotactic or syndiotactic the polymer is), the more crystalline it is.• Atactic polypropylene, for example, do not pack well

and the polymer is an amorphous glass.• Isotactic polypropylene is a crystalline, fiber-forming

polymer with a high melt transition.

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IonicIonic Chain Growth Chain Growth

Either anionic or cationic polymerizations• Cationic polymerizations are most common with

monomers with electron-donating groups.

• Anionic polymerizations are most common with monomers with electron-withdrawing groups.

OR SR

Styrene IsobutyleneVinyl ethersVinyl thioethers

StyreneCOOR COOR CN COOR

CN

Alkyl methacrylates

Alkyl acrylates

Acrylonitrile Alkylcyanoacrylates

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Anionic Chain GrowthAnionic Chain Growth

Anionic polymerization can be initiated by addition of a nucleophile, such as methyl lithium, to an alkene.

R'

R LiR

R'R'

R

R' R'

etc.

Li ++ +

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An alternative method for initiation involves a one-electron reduction of the monomer by Li or Na to form a radical anion which is either reduced or dimerized to a dianion.

+

A radical anion

A dianion

Butadiene

Li +

Li +

Li +

Li

Li

A dimer dianion

radical couplingto form a dimer

Li +Li +

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sodium naphthalide may be used.

The naphthalide radical anion is a powerful reducing agent and, for example, reduces styrene to a radical anion which couples to give a dianion.

THF

Sodium naphthalide(a radical anion)

Na+Na+

Naphthalene

:

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• The styryl dianion then propagates polymerization at both ends simultaneously.

A styrylradical anion A distyryl dianion

Styrene

Na+

Na+Na+

Na+

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Propagation of the distyryl dianion.

Na+

Na+

1. 2n

2. H2O

A distyryl dianion

Polystyrene

nn

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Living polymer:Living polymer: A polymer chain that continues to grow without chain-termination steps until either all of the monomer is consumed or some external agent is added to terminate the chains.• after consumption of the monomer under living

anionic conditions, electrophilic agents such as CO2 or ethylene oxide are added to functionalize the chain ends.

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• Termination by carboxylation.

:nNa+

CO2 H3O+nCOO- Na+

nCOOH

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• Termination by ethylene oxide.

Na+

O

CH2CH2O-Na+

H2OCH2CH2OH

n n

n

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CationicCationic Chain Growth Chain Growth

The two most common methods for initiating cationic polymerization are:• Addition of H+. Reaction of a strong proton acid with

the monomer.

• Ionization, as in SN1. Abstraction of a halide from the organic initiator by a Lewis acid.

Initiation by a proton acid requires a strong acid with a nonnucleophilic anion in order to avoid completion of the addition to the double bond• Suitable acids include HF/AsF5 and HF/BF3.

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Cationic Chain GrowthCationic Chain Growth

• Initiation by a protic acid.

• Lewis acids used for initiation include BF3, SnCl4, AlCl3, Al(CH3) 2Cl, and ZnCl2.

H3CR

R

H+BF4-

+ BF4-R

R

R

R

n BF4-

H3C R

R R R R R

+

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• initiation

• propagation

Cl + SnCl4 +

2-Chloro-2-phenylpropane

SnCl5-

n +

+

2-Methylpropene

++

+

n

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• chain termination

OH+

+SnCl5

-

H2O

H+SnCl5-

n

n

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Ring-Opening Metathesis PolymerizationRing-Opening Metathesis Polymerization

• During early investigations into the polymerization of cycloalkenes by transition metal catalysts, polymers that contained the same number of double bonds as the monomers used to make them were formed.

• for example:

• this type of polymerization is known as ring-opening ring-opening metathesis polymerization (ROMP)metathesis polymerization (ROMP).

LiAl(C7H15)4

TiCl4,

Norbornene

n

n

ROMP polymer

1,2-Addition polymer

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• ROMP polymerizations involve the same metallocyclobutene species as in ring-closing alkene metathesis reactions.

M CH2

A metalcarbene

M CH2CH2M

CH2M

CH2M

continuedpolymerization

n

redrawn

ROMP polymer from cyclopentene

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• all steps in ROMP are reversible, and the reaction is driven in the forward direction by the release of ring strain that accompanies the opening of the ring.

Ring strain[kJ (kcal)/mol]

125 (29.8) 113 (27) 24.7 (5.9) 5.9 (1.4)

> >>

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• ROMP is unique is that all unsaturation present in the monomer is conserved in the polymer.

• polyacetylene is prepared by the ROMP technique.

Cyclooctatetraene

metal-nucleophiliccarbene catalyst

nPolyacetylene

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• poly(phenylene vinylene) is prepared as follows.

OAc

OAcROMP

AcO OAc

HH

-2nCH3COOH

n

heat

nPoly(phenylene vinylene)

29-1 organicpolymerchemistry. 29-2 some definitions polymer:poly+meros polymer: from the greek,...

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