A Polymer is a large molecule built up by the repetition of small, simple chemical units. Repetition can be linear, branched, cross-linked or dendritic. The repeat unit is usually equivalent to the monomer - the starting material from which the polymer is formed. Plastics are one type of polymer These can be broken down into thermoplastics which can repeatedly follow a melt-freeze cycle, and thermosets, which set, or cure once during an irreversible chemical process during the manufacturing phase. Polypropylene, polyethylene and suchlike are thermoplastics, while most elastomers as well as hard plastics like Bakelite are thermosets.