Some monomers join together to form long unbranched chains. 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.
Some monomers join together to form chains with branch links to other chains.
These are called cross-linked polymers and 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.