These are called linear polymers . Plastics with a linear polymer structure can be melted and resolidified. The other name for this sort of plastic is thermoplastic . Examples of plastics which are thermoplastic are polythene and nylon. Most synthetic condensation poymers are alternating copolymers.
Some monomers join together to form chains with branch links to other chains.
These are called cross-linked polymers . Plastics with this type of structure will not melt but will blister and burn, often giving off toxic fumes. These types of plastics are called thermosetting . Thermosetting plastics include polyurethanes. In fact many plastics fall between these two extremes, the properties depending on the degree of cross linking.