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Organic Organic ChemChemistristry y

William H. Brown &William H. Brown &

Christopher S. FooteChristopher S. Foote

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Organic Organic PolymerPolymer

ChemistryChemistryChapter 24Chapter 24

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Organic Polymer Chem.Organic Polymer Chem. 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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Organic Polymer ChemOrganic Polymer Chem 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

n

StyrenePolystyrene

Cln

Poly(vinyl chloride) (PVC)

Cl

Vinyl chloride

synthesized from

synthesized from

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Notation & NomenclatureNotation & Nomenclature To name a polymer, prefix polypoly to the name of

the monomer from which the it is derived• if the name of the monomer is one word, no parens are

necessary• for more complex monomers or where the name of the

monomer is two words, enclose the name of the monomer is parens, as for example poly(vinyl chloride) or poly(ethylene terephthalate)

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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, multiply each number by the MW, sum these values, and divide by the total number of polymer chains

• weight average MW:weight average MW: record the weight of each chain of a particular length, sum these weights, and divide by the total weight of the sample

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MorphologyMorphology Polymers tend to crystallize as they precipitate or

are cooled from a melt Acting to inhibit crystallization are their very

large molecules, often with complicated and irregular shapes, which prevent efficient packing into ordered structures

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

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MorphologyMorphology High degrees of crystallinity are found in

polymers with regular, compact structures and 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, TMelt transition temperature, Tmm:: the temperature at which crystalline regions melt• as the degree of crystallinity increases, Tm increases

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MorphologyMorphology Highly amorphous polymers are sometimes

referred to as glassy polymers• because they lack crystalline domains that scatter

light, amorphous polymers are transparent• in addition they are weaker polymers, both in terms of

their greater flexibility and smaller mechanical strength

• on heating, amorphous polymers are transformed from a hard glass to a soft, flexible, rubbery state

Glass transition temperature, TGlass transition temperature, Tgg:: the temperature at which a polymer undergoes a transition from a hard glass to a rubbery solid

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MorphologyMorphology Example: poly(ethylene terephthalate),

abbreviated PET or PETE, can be made with % crystalline domains ranging from 0% to 55%

OO

OO

nPoly(ethylene terephthalate)

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MorphologyMorphology Completely amorphous PET is formed by cooling

the melt quickly• PET with a low degree of crystallinity is used for

plastic beverage bottles

By prolonging cooling time, 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

we discuss five types of step-growth polymers• polyamides• polyesters• polycarbonates• polyurethanes• epoxy resins

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PolyamidesPolyamides Nylon 66 (from two six-carbon monomers)

• during fabrication, nylon fibers are cold-drawncold-drawn to about 4 times their original length, which increases 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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PolyamidesPolyamides• 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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PolyamidesPolyamides• adipic acid is in turn the starting material for the

synthesis of hexamethylenediamine

catalyst

heat

4H2

O

H2N NH2

OH2N NH2

1,6-Hexanediamine(Hexamethylenediamine)Hexanediamide

(Adipamide)

OO-NH4

+

O

Ammonium hexanedioate(Ammonium adipate)

NH4+ -O

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PolyamidesPolyamides 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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PolyamidesPolyamides 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

2nH2O

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PolyestersPolyesters 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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PolyestersPolyesters• 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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PolycarbonatesPolycarbonates 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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PolycarbonatesPolycarbonates• to make Lexan, an aqueous solution of the sodium salt

of bisphenol A is brought into contact with a solution of phosgene in CH2Cl2 in the presence of a phase-transfer catalyst

Phosgene

+

Disodium saltof Bisphenol A

+Na-O

CH3

CH3

O-Na+

Lexan(a polycarbonate)

+

Cl Cl

O

nO

CH3

CH3

O

O

2NaCl

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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 (blocks) alternating with rigid urethane units (blocks)• the rigid urethane blocks are derived from a

diisocyanate

+An isocyanate A carbamate

RNHCOR'RN=C=O R'OH

O

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PolyurethanesPolyurethanes• the more flexible blocks are derived from low MW

polyesters or polyethers with -OH groups at the ends of each polymer chain

Low-molecular-weightpolyester or polyether

2,6-Toluenediisocyanate

+

n

A polyurethane

CH3

CNH NHCO-polymer-OCH3

N=C=OO=C=N

O

nHO-polymer-OH

O

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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• within this range, there are a large number of

polymeric materials, and epoxy resins are produced in forms ranging from low-viscosity liquids to high-melting solids

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Epoxy ResinsEpoxy Resins• the most widely used epoxide monomer is the

diepoxide prepared by treating 1 mole of bisphenol A with 2 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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Epoxy ResinsEpoxy Resins• treatment of the diepoxide with a diamine gives the

resin

H2NNH2

A diamine

O

CH3

CH3

O

An epoxy resin

HN

OH OH

NH n

O

CH3

CH3

OOO

A diepoxide

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ThermosetsThermosets Baelekite was one of the first thermosets

OH

+ CH2OOH

OH

OH

OH

OH

Baekelite

Phenol Formaldehyde

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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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PolyethylenesPolyethylenesCH2=CHCN

CF2 =CF2

CH2=CHC6 H5

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 as shown on mild heating

ΔO

O

O

O

Dibenzoyl peroxide

O

O

2 + 2CO2

A phenyl radical

A benzoyloxy radical

2

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Radical Chain-GrowthRadical Chain-Growth Another common class of initiators are azo

compounds, which also decompose on mild heating or with absorption of UV light

Azoisobutyronitrile (AIBN)

Δ or hνN NN

C

N

C NN CN

+2

Alkyl radicals

•: : ::

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Radical Chain-GrowthRadical Chain-Growth Radical polymerization of a substituted ethylene

• chain initiation

• chain propagation

In-In

In

Δ or hυ 2 In

In

RR+

In

RR

In

RR

etc.

In

R

In

RRn

R

Rn+

+

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Radical Chain-GrowthRadical Chain-Growth• chain termination

InRR

InRR

InR R

InRR

InRR

H

2

+

n n

nn

n

radicalcoupling

dispropor-tionation

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Radical Chain-GrowthRadical 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

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Radical Chain-GrowthRadical Chain-Growth 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

a number of butyl branches on the polymer main chain• these butyl branches are generated by a “back-biting”

chain-transfer reaction in which a 1° radical end group abstracts a hydrogen from the fourth carbon back

• polymerization then continues from the 2° radical

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Radical Chain-GrowthRadical Chain-Growth

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

H H

n

nCH2=CH2

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Radical Chain-GrowthRadical 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

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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

nCH2=CH2

TiCl 4/ Al(CH2CH3)2Cl

MgCl2Ethylene Polyethylene

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Ziegler-Natta PolymersZiegler-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+ +

Mg Cl2 / TiCl4particle

Diethylaluminumchloride

AlCl

Cl

AlCl

Ti Ti+ CH2 = CH2

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Ziegler-Natta PolymersZiegler-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 blow-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 StereochemistryPolymer 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)

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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• the chains of atactic polyethylene, for example, do not

pack well and the polymer is an amorphous glass• isotactic polyethylene, on the other hand, is a

crystalline, fiber-forming polymer with a high melt transition

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Ionic Chain-GrowthIonic Chain-Growth May be either anionic or cationic polymerizations

• cationic polymerizations are most common with monomers with electron-donating groups

• anionic polymerizations: 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 activated alkene

R'

R LiR

R'R'

R

R' R'

etc.

Li ++ +

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Anionic Chain-GrowthAnionic 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 +

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Anionic Chain-GrowthAnionic Chain-Growth To improve the efficiency of anionic

polymerizations, soluble reducing agents such as sodium naphthalide are 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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Anionic Chain-GrowthAnionic Chain-Growth

• 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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Anionic Chain-GrowthAnionic Chain-Growth propagation of the distyryl dianion

Na+

Na+

1. 2n

2. H2O

A distyryl dianion

Polystyrene

nn

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Anionic Chain-GrowthAnionic Chain-Growth 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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Anionic Chain-GrowthAnionic Chain-Growth• termination by carboxylation

:nNa+

CO2H3 O+n

COO- Na +

nCOOH

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Cationic Chain-GrowthCationic Chain-Growth The two most common methods for initiating

cationic polymerization are • reaction of a strong protic acid with the monomer• abstraction of a halide from the organic initiator by a

Lewis acid

Initiation by a protic acid requires a strong acid with a nonnucleophilic anion in order to avoid 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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Cationic Chain-GrowthCationic Chain-Growth• initiation

• propagation

Cl + SnC l4 +

2-Chloro-2-phenylpropane

SnCl5-

n +

+

2-Methylpropene

++

+

n

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Cationic Chain-GrowthCationic Chain-Growth• chain termination

OH+

+SnCl5

-

H2O

H+SnCl5-

n

n

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Prob 24.5Prob 24.5Name each polymer, and draw the structure of the monomer(s) that might be used to make it.

(a) (b)nO

n (c)O

n

O

(d) CFCF2

CF3n

On

(e) (f) OO

O O

n

n(g)

CH2Cl

HN

NH

O

On

(h)

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Prob 24.7Prob 24.7Draw a structural formula for the polymer formed in each reaction.

(a) + HO OH H+OMe

O

MeO

O

(b) + HO OHOH

H+

OMe

O

MeO

O

(c) (d)O CF3SO3H O

KOH

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Prob 24.8Prob 24.8Propose reagents and experimental conditions for the conversion of furan to hexamethylenediamine.

Furfural

oat hulls, corn cobs, sugar cane stalks, etc

Furan Tetrahydrofuran (THF)

(2)(1)

Zn-Cr-Mo catalyst

O CH

O

O

O

H2 SO4

H2 O

1,4-Dichloro- butane

Hexanedinitrile (Adiponitrile)

1,6-Hexanediamine(Hexamethylenediamine)

(3)

(4)C(CH2 )4CN

Cl(CH2)4Cl

H2 N(CH2)6NH2N

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Prob 24.9Prob 24.9Propose reagents for the conversion of 1,3-butadiene to hexamethylenediamine.

1,6-Hexanediamine(Hexamethylenediamine)

Butadiene 1,4-Dichloro-2-butene

3-Hexenedinitrile

(1) (2)

(3)

CH2=CHCH=CH2 ClCH2CH=CHCH2 Cl

H2 N(CH2)6NH2N CCH2CH=CHCH2C N

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Prob 24.10Prob 24.10Propose reagents and experimental conditions for the conversion of butadiene to adipic acid.

HOOCCOOH

1,3-Butadiene Hexanedioic acid(Adipic acid)

?

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Prob 24.12Prob 24.12Propose a mechanism for the step-growth reaction in this polymerization.

Ethylene glycol

Poly(ethylene terephthalate)

Dimethyl terephthalate

+n

+n 275°C

COCH2CH2OC 2nCH3OH

nHOCH2CH2OH

Methanol

O O

COCH3

OCH3OC

O

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Prob 24.13Prob 24.13Identify the monomer(s) required for the synthesis of each step-growth polymer.

C C-O-CH2 CH2 OO O

nKodel

(a polyester)

(a)

C(CH2)6CO

NHO

CH2 NHn

(b)

Quiana(a polyamide)

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Prob 24.14Prob 24.14Draw a structural formula for the repeating unit of Nomex.

+

1,3-Benzenediamine 1,3-Benzene-dicarbonyl chloride

NH2H2N ClCl

O O

polymerizationNomex

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Prob 24.15Prob 24.15Propose a mechanism for this Beckmann rearrangement which converts cyclohexanone oxime to caprolactam, the monomer from which nylon 6 is synthesized.

NOH

O

NH2OH H2SO4 NH

O

Cyclohexanone Cyclohexanone oxime

Caprolactam

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Prob 24.17Prob 24.17Propose a mechanism for the formation of this polycarbonate.

F F

O

O

O

+

n+

An aromaticdifluoride

O

Sodium carbonate

O

A polycarbonate

Na2CO3

2NaF

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Prob 24.18Prob 24.18Propose a mechanism for the formation of this polyurea. To simplify, consider the reaction of one -NCO group with one -NH2 group.

NCOOCN H2NNH2

NH

NH

NH

HN

OO

n

+

1,2-Ethanediamine

Poly(ethylene phenylurea)

1,4-Benzene-diisocyanate

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Prob 24.19Prob 24.19When equal molar amounts of these two monomers are heated, they form an amorphous polyester. Under these conditions, polymerization is regioselective for the 1°-OH groups. Draw a structural formula for the repeat unit of this polyester.

O

O

O

HO OHOH

+heat a polyester

Phthalicanhydride

1,2,3-Propanetriol (Glycerol)

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Prob 24.21Prob 24.21Propose a mechanism for formation of this polymer.

H2N

O

NH2

O O O

N Nn

base+

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Prob 24.22Prob 24.22Draw a structural formula for the polymer resulting from base-catalyzed polymerization of each monomer. Will the polymer by optically active?

O

O

O

O

(a) O

(R)-Propylene oxide

(b)

(S)-(+)-Lactide

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Prob 24.23Prob 24.23The polymer on the left is an insoluble, opaque material that is difficult to process into shapes. The polymer on the right is an transparent material that is soluble in a number of organic solvents. Explain the difference in physical properties between the two in terms of their structural formulas.

O O O

O O O

n n m

Poly(3-hydroxybutanoic acid) Poly(3-hydroxybutanoic acid -3-hydroxyoctanoic acid) copolymer

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Prob 24.25Prob 24.25Draw a structural formula for the repeat unit of the polymer formed in each reaction.

(a)O

OAIBN

70°C(b)

CN

Li

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Prob 24.26Prob 24.26Select the member of each pair that is more reactive toward cationic polymerization.

(a)OCH3

or (b)OCH3 OCCH3

or

(d)

(c) or

orCH3O

O

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Prob 24.33Prob 24.33Natural rubber is the all-cis polymer of isoprene. Draw a structural formula for the repeat unit of natural rubber.

Poly(2-methyl-1,3-butadiene) (Polyisoprene)

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Prob 24.34Prob 24.34Radical polymerization of styrene gives a linear polymer. Show by drawing structural formulas how incorporation of a few percent 1,4-divinylbenzene in the polymerization mixture gives a cross-linked polymer.

+

Styrene 1,4-Divinylbenzene

a copolymer of styreneand divinylbenzene

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Prob 24.37Prob 24.37From what two monomer units is this polymer made?

C C NN

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Prob 24.38Prob 24.38Draw a structural formula for the repeat unit in the polymer formed by ring-opening metathesis polymerization of each monomer.

(a) (b) (c) (d)O

O

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OrganicOrganicPolymerPolymer

ChemistryChemistryEnd Chapter 24End Chapter 24