Step-Growth Polymerization: Overview

Step-growth polymerization is a stepwise condensation of monomers to form polymers, where dimers form first, then trimers, tetramers, oligomers, and eventually long-chain polymers. Unlike chain-growth polymerization, step-growth polymerization requires no initiator and has no termination reactions. However, it requires longer reaction times to achieve high molecular weight polymers.

Step-growth polymerization is called condensation polymerization because each polymerization step involves the loss of small molecules, typically water or HCl. These byproducts are released as monomers bond together, making the reaction a condensation process where atoms combine while releasing small molecular fragments.

Monomers in step-growth polymerization must have two functional groups for bonding. These can be A-A type, B-B type, or A-B type monomers. A-A and B-B type monomers condense to form (A-A-B-B)n polymers, while A-B type monomers undergo self-condensation to form (A-B)n polymers.

A-B type monomers have two different functional groups at each end, allowing them to undergo self-condensation to form (A-B)n polymers. In contrast, A-A and B-B type monomers have identical functional groups at both ends and require pairing with complementary monomers to form (A-A-B-B)n polymers where A groups react exclusively with B groups.

Common step-growth polymers include polyesters, polyethers, urethanes, and polyamides. Poly(ethylene terephthalate) or PET is formed from glycol and terephthalic acid monomers. Nylon 6 is produced from 6-aminohexanoic acid, an AB-type monomer with amine and acid functional groups that undergoes self-condensation.

Step-growth polymerization requires longer reaction times because all monomers are reactive and most are consumed early to form small chains of reactive oligomers. These oligomers then combine in late stages to form long polymer chains. Extended reaction time is necessary to achieve high conversion degrees and high molecular weights.

Step-growth polymerization offers key advantages: no initiator is required to start the reaction, and termination reactions are absent. These features simplify the polymerization process compared to chain-growth methods, which require initiators and involve termination steps, though step-growth does demand longer reaction times for high molecular weight achievement.