Polymers (Class 12): Classification, Mechanisms and Key Examples

Four ways of classifying, and why each is asked

The forces classification explains the properties

This is the classification that actually predicts behaviour, so it is worth understanding rather than listing.

• Elastomers have weak forces between coiled chains, so they stretch and recover. Light cross-linking stops the chains sliding permanently past each other.
• Fibres have strong forces, often hydrogen bonding, letting chains pack closely and giving high tensile strength.
• Thermoplastics have intermediate forces and linear chains, so they soften on heating and can be remoulded repeatedly.
• Thermosetting polymers form an extensive cross-linked network on curing. Heating cannot melt them because the network is held by covalent bonds, so they cannot be remoulded.

Addition polymerisation

Unsaturated monomers add to one another with no atoms lost, so the repeating unit has the same formula as the monomer. The usual mechanism is a free-radical chain with the familiar three stages: initiation by a radical from an initiator, propagation as the radical adds across successive double bonds, and termination when two radicals combine.

Ionic mechanisms also exist, and coordination catalysts allow control of chain regularity — which is how high-density linear polymers are made rather than branched ones.

Condensation polymerisation

Bifunctional monomers join with elimination of a small molecule, typically water, at each linkage. Because a molecule is lost, the repeating unit is not the same as the monomers combined — a distinction that is regularly tested.

Polyesters form from a diacid and a diol; polyamides from a diacid and a diamine. The amide linkage in a polyamide allows hydrogen bonding between chains, which is why polyamides make strong fibres while many polyesters are used differently.

Copolymers

A copolymer contains more than one kind of monomer unit. Arrangement matters: random, alternating, block and graft copolymers of the same two monomers have distinctly different properties. Copolymerisation is the standard route to combining the useful properties of two homopolymers.

Biodegradable polymers

Conventional synthetic polymers persist because their backbones resist enzymatic attack. Biodegradable polymers are designed with linkages, usually esters, that organisms can break. They are a favourite question because they connect the chapter to environmental chemistry.

Molecular mass of a polymer

A polymer sample contains chains of many different lengths, so its molecular mass is an average, and two averages are used.

The weight average is always at least as large as the number average, because longer chains contribute more heavily to it. A polydispersity index of exactly one would mean every chain is identical in length, which happens only in certain natural polymers.

Frequently asked questions

Why can thermosetting polymers not be remoulded?

Because their chains are joined by covalent cross-links forming one giant network. Heating supplies enough energy to decompose the material before it can flow.

What makes a polyamide a good fibre?

The amide linkages hydrogen-bond strongly between adjacent chains, allowing close regular packing and giving high tensile strength.

Is the repeating unit always the same as the monomer?

In addition polymerisation, yes. In condensation polymerisation, no — a small molecule is eliminated, so the repeating unit is lighter than the sum of the monomers.

Why is polydispersity index never less than one?

Because the weight average gives greater weight to longer chains, so it cannot be smaller than the number average. A value of one means all chains are identical.

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