Introduction

Abstract

Polymeric materials consist of long chain molecules formed by the chemical combination of small molecules called monomers. The reaction leading to the formation of polymer molecules is called polymerization. These have been broadly classified^1,2 as chain growth and step growth polymerizations. In chain growth polymerization, there are growth centers in the reaction mass, to which monomer molecules add on successively. Usually, a high molecular weight product is produced right from the very beginning of the reaction with the quantity of unreacted monomer in the reaction mass decreasing slowly with time. Depending upon the nature of the growth centers, chain growth polymerizations can be further classified into radical, cationic, anionic, and stereoregular polymerizations.^1–8 In step growth polymerization, on the other hand, the growth of molecules usually occurs through the reaction of functional groups, e.g., -COOH, -NH₂, -OH, etc., located on the molecules. Polymer formation can occur through this mechanism only when the monomer has at least two functional groups. When there are more than two functional groups in the monomer molecules, the resultant polymer is either branched or cross-linked in structure, while for bifunctional monomers, linear polymer molecules are formed. In contrast to chain growth polymerization, the growth of the polymer molecules in this case occurs by the reaction between any two molecules (whether polymeric or monomeric) in the reaction mass, and there is a slow increase in the average molecular weight of the polymer.