Principles of Condensation Polymerization

A condensation polymerization is a form of step-growth polymerization in which monomers and/or oligomers react with each other to form larger structural units while releasing smaller molecules as a byproduct such as water or methanol. A well-known example of a condensation reaction is the esterification of carboxylic acids with alcohols. If both moieties are difunctional, the condensation product is a linear polymer, and if at least one of the moieties is tri- or tetra-functional, the resulting polymer is a crosslinked polymer (i.e. a three-dimensional network). Adding monomers with only one reactive group will terminate a growing chain, and consequently lower the (average) molecular weight. Thus, the average molecular weight and the crosslink density will depend on the functionality of each monomer involved in the condensation polymerization and on its concentration in the mixture.

A classic step-growth condensation is the reaction between a dibasic acid and a glycol, shown below:

HOOC–(CH₂)_n–COOH  +  HO–(CH₂)_m–OH →
HOOC–(CH₂)_n–COO–(CH₂)_m–OH  +  H₂O

The resulting polymer is called a polyester. The water is continuously removed from the reaction system because it is reactive with ester linkages to reverse the reaction.
One of the most common thermoplastic polyesters is poly(ethylene terephthalate), often abbreviated PET or PETE:

Other important condensation reactions are ester interchange¹, etherification, and amidization. For example, linear polyamides can be produced by reacting diamides with dicarboxylic acids:

HOOC–(CH₂)₄–COOH + H₂N–(CH₂)₆–NH₂ → [–HNOC–(CH₂)₄–CONH–(CH₂)₆–]

In this example the product is poly(hexamethylene adipamide), also known as Nylon 6-6, which is one of the most common thermoplastic polyamides. Its chemical structure is shown below

Both Nylon and PETE do not need a catalyst because the acid monomers do catalyze the reaction. However, strong acids such as sulfuric acid further accelerate the reaction. Instead of diacids, diacyl chlorides can also be used. They have the advantage that there is usually no reversal reaction, because the by-product HCl is not reactive with ester linkages.

  

Notes:

1. An ester interchange reaction is a reaction between an ester and another compound such as an alcohol, an acid or another ester (transesterification), to form an ester of different composition.²

2. Transesterification of methyl esters with glycols instead of direct esterification of diacids with dialcohols is sometimes prefered because of the easy purification of diesters by distillation.