Plastics are polymers. They can be both natural or synthetic. They are usually carbon compounds. They are made by polymerisation of monomers into long chains. There are lots of different sorts of monomers.
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. Most synthetic condensation poymers are alternating copolymers.