Dictionary of Chemistry [6th Ed.]

431

p

Heteropolymers

These are also known as copolymers. They are made from two (or more)

different monomers, which usually undergo a condensation reaction with the

elimination of a simple molecule, such as water. A typical example is the

condensation of 1,6-diaminohexane (hexamethylenediamine) with hexanedioic

acid (adipic acid) to form nylon 6,6. The reaction occurs between the amine

groups on the diaminohexane and the carboxyl groups on the hexanedioic

acid, with elimination of water molecules (see diagram opposite).

The properties of a polymeric plastic can most easily be modified if it is a

copolymer of two or more different monomers. A well-known example is ABS

(acrylonitrile–butadiene–styrene) copolymer, commonly used for the body

shells of computers and other electronic apparatus. Its properties can be

preselected by varying the proportions of the component monomers.

Stereospecific polymers

In both normal polyethene and nylon the polymer molecules take the form of

long chains of various lengths with no regular arrangement of the subunits.

Such polymers are said to be atactic. If the constituent subunits repeat along

the chain in a regular way, a stereospecific polymer may result. The polymer

may be isotactic, with a particular group always along the same side of the

main chain, or syndiotactic, with the group alternating from side to side of the

chain. Stereospecific polymerization can be performed by use of certain

catalytic agents (seeziegler process).

alternating A B A B A B A B

random A A B A B B B A

block A A B B B B A A

graft A A A A A A A A A

BB B

BB B

Types of copolymer depending on the arrangement of the monomers A and B

isotactic syndiotactic

HHH

HHH

HHH

XXX

HHH

HHH

H

H

H

X

X

X

X

H

Types of stereospecific polymer

Structure of polymers