Page 243 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

752 Polymers, Synthesis

SCHEME 1 Synthesis of polymers.

I. REVIEW OF FUNDAMENTAL merisms. The nature of the chain isomerism plays a major
DEFINITIONS role in defining the chain symmetry and hence the physi-
cal behavior of macromolecules, even when the structures
The major routes to polymerization are summarized in are nominally very similar.
Scheme 1 (see below). Step-growth polymerization is il-
lustrated by the reaction of ethylene glycol and tereph- B. Composition
thalic acid to produce poly(ethylene terephthalate) (PET)
The chemical composition defines whether a material will
plus water as a by-product. This reaction is also referred
have, for example, weak intermolecular forces, as illus-
to as a condensation polymerization. The pioneers in the
trated by a hydrocarbon elastomer (e.g., polybutadiene),
field called these types of reaction condensation pro-
or be strongly hydrogen bonded, as in the case of a fiber-
cesses since a small molecule (i.e., water) was elimi- forming polyamide. The term is also related to the ques-
nated in the process. The alternative approach to pro- tion of whether one is dealing with a single repeating unit,
ducing high-molecular weight linear chains is illustrated which is called a homopolymer, or more than one chem-
by the transformation of acrylonitrile to polyacryloni- ical repeating unit in the chain structure. The presence of
trile. Chain-growth polymerizations are also referred to
two dissimilar units defines a copolymer, three a terpoly-
as addition or vinyl polymerizations. The terms conden-
mer, and so on. The arrangement of the monomer unites
sation and addition thus have their basis in composition
may be statistical, alternating, or blocklike. Sequential ar-
and structure, whereas the more modern terms step- and
rangements that are particularly long are referred to as
chain-growth polymerization are related to the kinetics
segmented or blocklike structures. Fundamentally, this is
and the mechanisms of the processes. In fact, step-growth
related to the sequence distribution of the two species in
polymerization terminology is considered more univer-
a copolymer chain. If long, chemically distinct sequences
sal and includes polymerization reactions between func-
are pendant to the main chain, they are referred to as graft
tional groups in the absence of condensate formation, e.g.,
copolymers. These features are illustrated in Table I.
polyurethane formation from diisocyanates and diols.
C. Degree of Polymerization (Molecular Weight)
A. Chain Isomerism
The number of repeating units in the macromolecule or
This refers to the possibility of geometric (cis, trans, polymeric chain structure is referred to as the degree
etc.),stereochemical(isotactic,syndiotactic,heterotactic), of polymerization. The molecular weight of the chain is
or regio (head-to-tail, head-to-chain configurations) iso- then a simple product of the degree of polymerization
TABLE I Chemical Compositions of Polymers
Polymer type Composition

Homopolymer AAAAAAAAAAAAAAAAAAAAAAAAAAAA
Statistical or random copolymer AABABBBABBABABBBABABAABBBBB
Block or segmented copolymer AAAAAAAA AAAAABBBBBB BBBBBBBBB
Graft copolymer AAAAAAAAAAAAAAAAAAAAAAAAA
B B
B B
B B
B B