Chemistry Reference
In-Depth Information
It will be clear from these examples that this classification is now somewhat
arbitrary. Many authors still refer to addition and condensation polymers in their
current, conventional sense, however, and it
is therefore necessary for
the
informed reader to understand this usage
7.2
Step-Growth and Chain-Growth Polymerizations
A more useful distinction is based on polymerization mechanisms rather than
polymer structures
[2]
. Most polymerizations can be classified as step-growth or
chain-growth processes. A few reactions possess some characteristics of both
mechanisms but the great majority of polymer syntheses can be characterized
conveniently into one class or the other. We will see later that the nature of the
polymerization is very different in the two cases, and the general operations
required to produce high-molecular-weight polymers of good quality and in good
yield are quite different for step-growth and chain-growth syntheses.
The distinguishing features of step- and chain-growth mechanisms are listed
below. A subsequent example is provided to illustrate the differences, which may not
be entirely clear from this bald summary of the characteristics of each reaction type.
The following features characterize step-growth polymerizations:
1.
The growth of polymer molecules proceeds by a stepwise intermolecular
reaction. Only one reaction type is involved in the polymerization.
2.
Monomer units can react with each other or with polymers of any size.
Polymer molecules grow over the course of the whole reaction, and such
growth is in a series of fits and starts as the reactive end of a monomer or
polymer encounters other species with which it can form a link.
3.
The functional group on the end of a monomer is usually assumed to have the
same reactivity as that on a polymer of any size.
4.
A high conversion of functional groups is required in order to produce high-
molecular-weight products. Average polymer molecular weight rises steadily
during the course of the polymerization.
5.
Many step-growth polymerizations involve an equilibrium between reactants on
the one hand and macromolecular products and eliminated small molecules on
the other [cf. reactions (1-6) and (1-7)]. In such cases, all molecular species are
present in a calculable distribution, and the course of the polymerization is
statistically controlled. High polymer cannot coexist with much monomer in
equilibrium systems. Such step-growth polymerizations are evidently reversible
and also involve interchange reactions (Section 7.4.1) in which terminal
functional groups react with linking units in other molecules to produce changes
in molecular weight distributions.
6.
Condensation polymers by the above definition are usually produced by step-
growth polymerizations but not all step-growth syntheses are condensation
reactions. Thus, there is no elimination product in polyurethane synthesis from
a diol and a diisocyanate [cf. reaction (1-12)]:
