Condensation (Step-reaction) Polymerization

A large number of important and useful polymeric materials are not formed by chain-growth processes involving reactive species such as radicals, but instead by condensation polymerization reactions. These polymerizations often occur with loss of a small-molecule byproduct, such as water or methanol. In a condensation polymerization, the primary requisite is that the monomer, or pair of monomers, must possess two or more condensable functional groups. For example, a monomer such as formaldehyde can be regarded as bifunctional since it is capable of forming two bonds with other units. The products of these reactions are termed polyesters, polyamides, polyethers, etc., depending upon the inter-unit linkage, or upon the reaction involved: esterification, amidation, etc. Cutting across these various chemical types, two classes of condensation polymers are clearly distinguishable: linear polymers formed from reactants which are exclusively bifunctional, and non-linear polymers formed from reactants which are trifunctional or higher. Generally speaking, if both monomers are bifunctional, the condensation product is a linear polymer, and if at least one of the monomers is tri- or tetra-functional, the resulting polymer is a crosslinked polymer.

Condensation polymerization offers the possibility of constructing polymer molecules of accurately known structure. Furthermore, the synthesis follows “Carothers equation” DP = 1/(1-p), which describes the degree of polymerization (DP) as a function of conversion (p, the fraction of monomer transformed to polymer). Thus the average molecular weight of condensation polymers can be controlled, to a large extent, by tuning the degree of condensation. This is the crucial difference between step-growth and chain-growth polymerizations. In chain growth reactions, as conversion (p) increases, additional polymer chains are formed; all growing chains, on average, have the same life time, and therefore the average molecular weight does not vary much with conversion significantly. The Carothers equation is simple but imposes the strict conditions that required by step reactions for polymer synthesis, e.g., fast reaction, high purity monomer and large conversions. In fact the majority of commercial polymers are prepared by chain-growth polymerizations, only a few are made by step-growth reactions.*

Some commercial polymers synthesized by step-growth polymerization. Reprinted from Ref 1.*

Reference:
Ramakrishnan, S. “Condensation polymerization.”Resonance 2017.

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