35.2.1—The type of polymerisation reaction

An alkene monomer with a C=C bond normally undergoes addition polymerisation: the π bond opens and no small molecule is lost. Monomers with two functional groups undergo condensation polymerisation and release a small molecule.

Inspect the starting structures before looking at the product. C=C suggests poly(alkene); –OH/–COOH/–NH₂ pairs suggest polyester or polyamide.

Ethene → poly(ethene) by addition; hexane-1,6-diamine + hexanedioic acid → polyamide by condensation.

A polymer being “formed from two monomers” does not automatically make it condensation; the deciding evidence is the new link and small-molecule loss.