Chapter X Synthetic Polymers.pdf

CHAPTER X

SYNTHETIC POLYMERS

X,l. BRIEF INTRODUCTION TO SUBJECT

Polymerisation involves the chemical combination of a number of

identical or similar molecules to form a complex molecule. The resulting

polymer has a high molecular weight. The term synthetic polymer is usu-

ally employed to denote these compounds of very high molecular weight.

Compounds which polymerise include :

1. Those which contain a reactive multiple bond (C=C, C = C, C=O, etc.).

2. Polyfunctional molecules (glycols, dibasic acids, hydroxy acids,

diamides, etc.).

3. Cyclic compounds capable of undergoing ring opening (alkylene

oxides, lactones, lactams, anhydrides, etc.).

Polymers can be classified as addition polymers and condensation

polymers. Addition polymers are formed by intermolecular reactions of

the monomeric units without the elimination of atoms or groups. An

example is vinyl chloride, which can be made to combine with itself to

yield polyvinyl chloride :

nCH 2 =CHCl —^ — (CH 2 CHCl) n -

The terminal valencies of the chain are saturated by univalent groups

such as hydrogen, halogen, hydroxyl, etc., or an organic fragment derived

from the polymerisation catalyst. Copolymers are obtained by addition

polymerisation of a mixture of two different monomers, for example,

butadiene and styrene. The properties of the product depend upon the

proportions of the two monomers and upon the average molecular weight

of the copolymer. Condensation polymers are produced by reactions

which are attended by the elimination of some simple molecule (e.g.,

water, alcohol or ammonia) between functional groups : such reactions

are esterification, anhydride formation, amide formation, aldol con-

densation, and the like. Only a very limited number of examples can,

of necessity, be given in this volume.

In practice, synthetic polymers are sometimes divided into two classes,

thermo-setting and thermo-plastic. Those polymers which in their original

condition will flow and can be moulded by heat and pressure, but which

in their finished or " cured " state cannot be re-softened or moulded are

known as thermo-setting (examples: phenol formaldehyde or urea formalde-

hyde polymer). Thermoplastic polymers can be resoftened and remoulded

by heat (examples : ethylene polymers and polymers of acrylic esters).

ADDITION POLYMERS

Many of the compounds which undergo addition polymerisation may

be represented by the general formula CH 2 =CR—X, for example,

ethylene (R = H, X = H), isobutylene (R = CH 3 , X = CH 3 ), vinyl

chloride (R = H, X = Cl), vinylidene chloride (R = Cl, X = Cl),

vinyl acetate (R = H, X = OCOCH 3 ), methyl acrylate (R = CH 3 ,

X = COOCH 3 ), and styrene (R = H, X = C 6 H 6 ). At least three types

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[X,l]

SYNTHETIC POLYMERS

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of polymeric structures may result from the polymerisation of unsym-

metrical compounds CH 2 =CRX, depending upon the arrangement of

structural units in the chain :

. . . — CH 2 CR— CH 2 CR—CH 2 CR— CH 2 CR— CH 2 CR— . . .

. .

Type I (X groups in 1 : 3 positions)

. — CRCH 2 — CH 2 CR— CRCH 2 — CH 2 CR— CRCH a —

XX XX

Type II (X groups in 1 : 2 positions)

. — CRCH 2 — CH 2 CR— CH 2 CR— CRCH 2 — CRCH 2 —

X X X X

Type III (X groups in random positions)

The structure of the linear polymer formed under a particular set of

experimental conditions can be formulated in a number of cases after a

detailed examination of its properties.

Examples of addition polymers include :

Ethylene. Under the influence of pressure and a catalyst, ethylene

yields a white, tough but flexible waxy solid, known as Polythene. Poly-

ethylene possesses excellent electrical insulation properties and high

water resistance ; it has a low specific gravity and a low softening point

(about 110°). The chemical inertness of Polythene has found application

in the manufacture of many items of apparatus for the laboratory. It is a

useful lubricant for ground glass connexions, particularly at relatively

high temperatures.

Tetrafluoroethylene. Emulsion polymerisation of tetrafluoroethylene,

catalysed by oxygen, yields polytetrafiuoroethylene (Teflon) as a very

tough horn-like material of high melting point. It possesses excellent

electrical insulation properties and a remarkable inertness towards all

chemical reagents, including aqua regia.

Styrene. Styrene is readily polymerised to a glass-clear resin, poly-

styrene, but the exact nature of the polymer is influenced by the nature of

the catalyst, the temperature, solvent, etc.

Styrene (or vinylbenzene) is prepared technically by the " cracking

dehydrogenation " of ethylbenzene :

Polystyrene niav be represented as :

CH 2 CH a —i—CHCH a ^=|—C - CH 2

| i

C 6 H 5

C 6 H 6

! __\x

Vinyl compounds. Vinyl chloride (prepared from acetylene and

hydrogen chloride) yields polyvinyl chloride (P.F.C.), which is employed

as a rubber substitute and for other purposes. Vinyl acetate (from

j C 6 H 6

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PRACTICAL ORGANIC CHEMISTRY

[X,

acetylene and acetic acid) similarly gives polyvinyl acetate. Vinylidene

chloride CH 2 =CC1 2 affords polyvinylidene chloride.

Acrylic acid derivatives. Acrylic esters polymerise readily under

the influence of oxygen, peroxides, light or heat to give colourless, glass-

like plastics.

Methyl acrylate is usually prepared from ethylene chlorohydrin thus :

NaCN

CH.OH,

* CH 2 OHCH 2 CN + NaCl

H,S0 4

Methyl methacrylate is obtained commercially from acetone cyanohydrin :

HCN

•> CH 2 =CHCOOCH 3

(CH 3 ) 2 CO

CH.OH

* (CH 3 ) 2 CCN — --*. CH 2 =CCOOCH 3 + (NH 4 ) 2 S0 4

| H.SO, |

OH CH 3

The action of sulphuric acid alone upon acetone cyanohydrin affords

a-methylacrylic acid. The methyl methacrylate polymers are the

nearest approach to an organic glass so far developed, and are marketed

as Perspex (sheet or rod) or Diakon (powder) in Great Britain and as

Plexiglass and Lucite in the U.S.A. They are readily depolymerised to

the monomers upon distillation. The constitution of methyl methacry-

late polymer has been given as :

H—

CH 3 CH 3

-CH a —C—CH 2 —C

I I

COOCH 3 COOCH 3

—CH=C

COOCH,

COPOLYMERS

Emulsion polymerisation of a mixture of butadiene and styrene gives

a synthetic rubber (Buna S ; GBS rubber), which is used either alone or

blended with natural rubber for automobile tyres and a variety of other

articles.

A mixed polymer of butadiene and acrylonitrile (Perbunan, Hycar,

Chemigum) may be vulcanised like rubber and possesses good resistance

to oils and solvents in general.

Copolymerisation of vinyl acetate and vinyl chloride yields resins of

desirable properties : they are strong and adhesive, thermoplastic, and

are suitable for the manufacture of synthetic fibre ( Vinyon).

Vinylidene chloride and vinyl chloride lead to the copolymer known as

Saran. Other commercial copolymers are produced from vinyl chloride

and acrylonitrile (Dynel), and from maleic anhydride and styrene.

CONDENSATION POLYMERS

These may be produced from a great variety of poly-functional com-

pounds : to obtain satisfactory products, the reactants must be pure.

A few examples follow.

Phenol - aldehyde polymers. L. Baekland (1909) first demon-

strated the possibilities of the reaction between phenol and formaldehyde

from the commercial view point. Condensation in the presence of either

CH 2 OHCH 2 C1

SYNTHETIC POLYMERS

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alkaline or acid catalysts gives valuable polymers, covered by the general

name Bakelite. Baekland recognised three distinct stages in the reaction

of phenol and formaldehyde in alkaline solution. The initial product,

Bakelite A, is a liquid or semi-solid, and is converted by continued heating

into an intermediate, Bakelite B, a relatively insoluble, fusible solid :

when the latter is subjected to heat and pressure, it is converted into

Bakelite C> an insoluble and infusible plastic. A possible formula is :

CH 2 —

The condensation proceeds somewhat differently with an acid catalyst;

the product is termed Novolak.

If formaldehyde is replaced by furfural, the furfural - phenol polymer

(U.S.A. Durite) results. The above polymers are largely used for mould-

ing purposes.

Urea - formaldehyde polymers. Formalin and urea (usually in

the molecular proportions of 3 : 2) condense in the presence of ammonia,

pyridine or hexamine to give urea - formaldehyde polymers, known

commercially as Beetle or Plaskon, and are widely used as moulding

powders. It is believed that the intermediate products in the condensa-

tion are methylol-urea and dimethylol-urea :

NH 2

\ OH " \ \

NHCH 2 OH

ECHO,

+ CO

NH 2 NH 2 NHCH 2 OH

Urea Methylol-urea Dimethylol-urea

Polymerisation may occur as a result of dehydration of these compounds

to methylene and dimethylene urea or more probably by a stepwise loss

of water between the molecules of methylol and dimethylol-urea.

NH 2

N=CH 2

\ \

N=CH 2 N=CH 2

Methylene urea Dimethylene urea

Melamine - formaldehyde polymers. Melamine (2:4: 6-triamino-

1:3: 5-triazine), obtained by heating dicyandiamide under pressure,

condenses with formalin to give melamine - formaldehyde polymers

(Beetle - Melamine), which have similar uses, but better stability to heat

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and light, to urea - formaldehyde plastics. The polymerisation probably

proceeds through the intermediate hexarnethylol-melamine :

NH 2

3 C=NH

H 0 N— C^ /C— NH 2

NHCN

Dicyandiamide

Melamine

N(CH 2 OH) 2

HCHO N N

— " I II

(HOCH 2 ) 2 N—C\ T/ C— N(CH 2 OH) 2

Hexamethylol-melamine

Polyesters from polybasic acids and polyhydric alcohols. Alkyd

resins. The condensation of polyhydric alcohols and polybasic acids or

anhydrides leads to polyesters known as alkyd resins. The most common

member of the group is a glycerol - phthalic acid polymer, and this has

led to the term glyptal resins being frequently applied to the whole group.

By controlling the relative amounts of, for example, glycerol and

phthalic anhydride and the experimental conditions of the reaction,

various polymers of different properties are obtained. Under mild

conditions (ca. 150°) only the primary alcohol groups are esterified and

the secondary alcohol group remains free. The structural unit of the

resulting linear polymer is :

—CO—C 6 H 4 —CO—0—CH 2 —CHOH—CH 2 —OOC—C 6 H 4 —CO—

OCOCH 2 —CH—CH 2 OOC—

These are comparatively soft materials and they are soluble in a number

of organic solvents. Under more drastic conditions (200-220°) and with

a larger proportion of phthalic anhydride, the secondary alcohol groups

are esterified and the simple chains become cross-linked ; three dimen-

sional molecules of much higher molecular weight are formed :

/\—COO—

\)— COOCH 2 —CH—CH 2 00(

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