polymers
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29-1
OrganicPolymerChemistry
29-2
Some Definitions
Polymer: From the Greek, poly + meros, many parts.• Any long-chain molecule synthesized by bonding
together single parts called monomers. Monomer: From the Greek, mono + meros, single
part.• The simplest nonredundant unit from which a polymer
is synthesized. Plastic: A polymer that can be molded when hot
and retains its shape when cooled.
29-3
Continued
Thermoplastic: A polymer that can be melted and molded into a shape that is retained when it is cooled.
Thermoset plastic: A polymer that can be molded when it is first prepared but, once it is cooled, hardens irreversibly and cannot be remelted.
29-4
Notation & Nomenclature
Show the structure by placing parens around the repeat unit:• n = average degree of polymerization.
To name a polymer, prefix poly 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 Weight
All polymers are mixtures of individual polymer molecules of variable MWs.• 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:
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)
29-6
Example
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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Example
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 Index
PDI = Mw/Mn measures the extent of different molecular weights.
PDI greater than or equal to 1.0
29-9
Morphology
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 domains • disordered amorphous domains
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Morphology: 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 temperature, Tm: The temperature at which crystalline regions melt.• As the degree of crystallinity increases, Tm increases.
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Morphology: 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.
Glass transition temperature, Tg: 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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Morphology
• Example: poly(ethylene terephthalate), abbreviated PET or PETE, can be made with crystalline domains of 0% to 55%.
OO
OO
nPoly(ethylene terephthalate)
29-13
Morphology
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.
29-14
Step-Growth Polymers 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
29-15
Polyamides Nylon 66
Nylon 66 (from two six-carbon monomers).
• During fabrication, nylon fibers are cold-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 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
29-17
Nylon 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)
29-18
Polyamides, 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
29-19
Polyamides, 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
29-20
Polyesters, 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)
29-21
PET, 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
29-22
Polycarbonates, 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
29-23
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.
29-24
Polycarbonates, 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.
29-25
Polyurethanes
A urethane, 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
29-26
Polyurethanes
• 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
29-27
Epoxy 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.
29-28
Epoxy Resins
• 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
+
29-29
Epoxy Resins
• 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
29-30
Thermosets
Bakelite was one of the first thermosets.
29-31
Chain-Growth Polymers
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.
29-32
Chain-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
29-33
Polyethylenes
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
29-34
Polyethylenes
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
29-35
Radical 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
29-36
Radical Chain-Growth
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
: :
29-37
Radical Chain-Growth
Radical polymerization of a substituted ethylene.• chain initiation
• chain propagation
In-In
In
or h2In
In
RR+
In
RR
In
RR
etc.
In
R
In
RRn
R
Rn+
+
29-38
Radical Chain-Growth
• chain termination.
InRR
InRR
InR R
InRR
InRR
H
2
+
n n
nn
n
radical coupling
dispropor- tionation
29-39
Radical Chain-Growth
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
29-40
Radical Chain-Growth
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
29-41
Radical Chain-Growth
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.
29-42
Ziegler-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
29-43
Ziegler-Natta Polymers
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
29-44
Ziegler-Natta Polymers
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!
29-45
Polymer Stereochemistry
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)
29-46
Polymer 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.
29-47
Ionic 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
29-48
Anionic 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 ++ +
29-49
Anionic Chain Growth
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 +
29-50
Anionic Chain Growth
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
:
29-51
Anionic Chain Growth
• The styryl dianion then propagates polymerization at both ends simultaneously.
A styrylradical anion A distyryl dianion
Styrene
Na+
Na+Na+
Na+
29-52
Anionic Chain Growth
Propagation of the distyryl dianion.
Na+
Na+
1. 2n
2. H2O
A distyryl dianion
Polystyrene
nn
29-53
Anionic Chain Growth
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.
29-54
Anionic Chain Growth
• Termination by carboxylation.
:nNa+
CO2 H3O+nCOO- Na+
nCOOH
29-55
Anionic Chain Growth
• Termination by ethylene oxide.
Na+
O
CH2CH2O-Na+
H2OCH2CH2OH
n n
n
29-56
Cationic 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.
29-57
Cationic 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
+
29-58
Cationic Chain Growth
• initiation
• propagation
Cl + SnCl4 +
2-Chloro-2-phenylpropane
SnCl5-
n +
+
2-Methylpropene
++
+
n
29-59
Cationic Chain Growth
• chain termination
OH+
+SnCl5
-
H2O
H+SnCl5-
n
n
29-60
Ring-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 metathesis polymerization (ROMP).
LiAl(C7H15)4
TiCl4,
Norbornene
n
n
ROMP polymer
1,2-Addition polymer
29-61
Ring-Opening Metathesis Polymerization
• 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
29-62
Ring-Opening Metathesis Polymerization
• 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)
> >>
29-63
Ring-Opening Metathesis Polymerization
• 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
29-64
Ring-Opening Metathesis Polymerization
• poly(phenylene vinylene) is prepared as follows.
OAc
OAcROMP
AcO OAc
HH
-2nCH3COOH
n
heat
nPoly(phenylene vinylene)
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