The manufacture of plastics is normally based on one of three POLYMERIZATION processes: direct polymerization, polycondensation or polyaddition. Plastics are composed of polymer molecules consisting of chains or networks of chemically linked units or MONOMERS. A reaction which involves the joining together of monomers into a polymer is called a polymerization reaction. If more than one type of monomer is used, the process is called copolymerization and the product is a copolymer.
In direct polymerization reaction, a small number of monomers in the reaction medium are activated by means of heat, pressure or an added initiator (CATALYST). The activated monomers usually have reactive end groups that are either ionic (positively or negatively charged) or have a free electron (an outer electron not involved in a chemical BOND), and they will readily react with the nonactivated monomers.
The result of the reaction between an activated monomer and a nonactivated one is an activated molecule consisting of two monomer units which can then react with another monomer molecule. Chains of monomers build up in this way until they are deactivated, for example, by reaction with a further activated monomer. The resulting polymer molecules may contain many hundreds of monomer units.