Dictionary of Chemistry [6th Ed.]

430

p

POLYMERS

Polymers are substances that have *macromolecules composed of many repeating

units (known as ‘mers’). A large number of naturally occurring substances are

polymers including *rubber and many substances based on glucose, such as the

polysaccharides *cellulose and *starch (in plants) and *glycogen (in animals).

*Proteins, nucleic acids, and inorganic macromolecular substances, such as

*silicates, are other examples.

Synthetic polymers

One of the unique features of the chemistry of carbon is its ability to form long

chains of atoms. This property is the basis of an important area of industrial

chemistry concerned with the manufacture of polymeric materials with a variety

of properties (seeplastics). The molecules in these materials are essentially long

chains of atoms of various lengths. In some polymers, cross-linkage occurs

between the chains. Synthetic polymers are formed by chemical reactions in

which individual molecules (monomers) join together to form larger units (see

polymerization). Two types of polymer, homopolymersand heteropolymers, can

be distinguished

Homopolymers

These are polymers formed from a single monomer. An example is *polyethene

(polyethylene), which is made by polymerization of ethene (CH 2 :CH 2 ). Typically

such polymers are formed by *addition reactions involving unsaturated

molecules. Other similar examples are *polypropene (polypropylene),

polystyrene, and *polytetrafluoroethene (PTFE). Homopolymers may also be made

by *condensation reactions (as in the case of *polyurethane).

HH

CC

HH

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

C

H

H

Addition polymerization of ethene to form polyethene: a homopolymer

H 2 N (CH 2 ) 6 NH 2

O

C

HO

(CH 2 ) 4

O

C

OH

+

1,6-diaminohexane hexanedioic acid

H

N(CH 2 ) 6 NC(CH 2 ) 4 CN(CH 2 ) 6 N nH 2 O+

HO OH H

Condensation polymerization to form nylon: a heteropolymer

Formation of polymers