35.2 Predicting the type of polymerisation

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.

A polymer section made by addition retains the carbon backbone from opened C=C bonds and has no alternating small-molecule loss. A condensation section contains ester or amide links formed with elimination of water or HCl.

Work backwards: locate the repeating linkage, restore monomer functional groups, and ask whether a small molecule must have been removed.

A chain with –CH₂–CH₂– repeats comes from addition of ethene; a chain with –CO–NH– links is a condensation polyamide.

Do not classify from the polymer name alone; the repeat structure is the evidence.