ABSTRACT
In the preceding chapters we have examined the two main classes of polymerization, namely step-
growth and chain-growth polymerizations, with the latter exemplified by the free-radical mech-
anism. These are the workhorses of the polymer industry, permitting rapid and facile production of
large quantities of useful materials. One common feature that emerged from the discussion of these
mechanisms is the statistical nature of the polymerization process, which led directly to rather broad
distributions of molecular weight. In particular, even in the simplest case (assuming the principle of
equal reactivity, no transfer steps or side reactions, etc.), the product polymers of either a poly-
condensation or of a free-radical polymerization with termination by disproportionation would
follow the most probable distribution, which has a polydispersity index (Mw/Mn) approaching 2. In commercial practice, the inevitable violation of most of the simplifying assumptions leads to even
broader distributions, with polydispersity indices often falling between 2 and 10. In many cases the
polymers have further degrees of heterogeneity, such as distributions of composition (e.g., copoly-
mers), branching, tacticity, or microstructure (e.g., cis 1,4-, trans 1,4-, and 1,2-configurations in polybutadiene).
