Polymer molecules can be connected in different architectures even when their repeat-unit chemistry is similar.

A linear polymer has long chains without permanent covalent links between most chains. A branched polymer contains side chains attached to a main backbone. A crosslinked polymer contains covalent bridges joining chains into a network.

Crosslinks strongly restrict large-scale chain motion. As crosslink density increases, a polymer generally becomes less able to flow or dissolve and more able to recover elastically from deformation.

This structural distinction underlies three broad classes:

• thermoplastics consist mainly of separate chains and can soften enough to be reshaped when heated;
• thermosets form densely crosslinked networks during curing and do not melt into a processable liquid on reheating;
• elastomers have flexible chains with a relatively sparse network of crosslinks, allowing large reversible extensions while preventing irreversible chain flow.

Branching also changes how efficiently chains pack. For example, strong branching can reduce crystallinity and density compared with a more linear version of the same polymer chemistry.

Polymer behavior therefore depends on connectivity at a scale larger than the individual covalent bond: whether chains are independent, branched, entangled or permanently joined into a network.