Polymers
Polymers tests classification, addition and condensation polymerisation, monomer identification, rubber, molecular mass and biodegradable polymers.
Core rules
- Addition polymers form without loss of a small molecule (polyethylene, PVC, polystyrene).
- Condensation polymers lose water or another small molecule (nylon-6,6, terylene, bakelite).
- Homopolymer: one monomer; copolymer: two or more monomers.
- Natural rubber is cis-polyisoprene; vulcanisation with sulphur creates cross-links.
- Number-average and weight-average molecular mass differ; polydispersity is their ratio.
- Biodegradable polymers include PHBV and Nylon-2-nylon-6.
Worked example
Identify the monomers of nylon-6,6 and the type of polymerisation. Nylon-6,6 is made from adipic acid (hexanedioic acid) and hexamethylenediamine. The amine and carboxylic acid groups condense with loss of water, so it is a condensation polymer. Reasoning: the amide linkages in the chain are formed by elimination of water between the two bifunctional monomers.
Traps
Calling PVC a condensation polymer, forgetting the small molecule lost in condensation, and confusing the monomer of natural rubber (isoprene) with that of neoprene (chloroprene). Do not assume a higher molecular mass always means a higher melting point; branching and intermolecular forces matter.