The most important types of chain polymerization are radical polymerization, ionic polymerization, and insertion or coordination polymerization. Three steps characterize each method of polymerization. The start reaction or initiation, the growth reaction, and the termination reaction. The first reaction step, i.e. The initiation, is the formation of a reactive radical or ion9. This requires, in general higher activation energy than the subsequent growth reaction in which the monomers add to the growing chain. Consequently the growth rate is very much higher than the rate of initiation. Nomer is activated and opened up so that it can for new bonds and then after the reaction with another monomer it becomes saturated e.g. Ethylene becomes –CH2-CH2– Bi-functional monomers polymerize into linear chain molecules and a thermoplastic material is obtained. Monomer with two functional groups results in cross linked polymers, such as thermosets and elastomers. The termination step can occur either by two growing polymer chains combining with each other in radical polymerization or by addition of a terminating agent or a modifier whose function is to combine with the reactive end of the growing chain and thus saturate its reactivity. Modifying or termination agents can be by products or impurities or specifically added substance that reacts with the reactive end of the growing chains.
