Polymerisation is the chemical process by which small molecules called monomers combine to form high molecular mass substances known as polymers. Two major mechanisms of polymerisation are chain-growth polymerisation and step-growth polymerisation. The distinction between these mechanisms was clarified through the classical concepts developed by Carothers and later extended and mathematically described by Flory. Although both processes produce polymers, they differ significantly in the nature of the monomers, the sequence of reactions, molecular-weight development, stoichiometric requirements, and reaction kinetics. Carothers originally classified polymer-forming reactions largely on the basis of the functionality of the reacting molecules and the manner in which the polymer chains were constructed. His treatment was particularly important for step-growth polymerisation. According to Carothers, molecules containing two or more functional groups can react with one another to form progressively larger molecules. For example, a diacid can react with a diol to form a polyester. The reaction initially produces a dimer, which can react with another monomer or oligomer, and the process continues through successive reactions between functional groups. In step-growth polymerisation, any two molecular species having appropriate reactive functional groups may react with each other. Thus, monomer–monomer, monomer–oligomer, oligomer–oligomer, or oligomer–polymer reactions can occur at any stage. There is no special distinction between the beginning and the end of chain formation. The polymer molecular weight therefore increases gradually throughout the reaction. At low conversion, the reaction mixture contains mainly monomers and small oligomers. As conversion increases, larger oligomers are produced, and very high molecular weights are obtained only at very high conversion. A typical example is the formation of nylon-6,6 from hexamethylenediamine and adipic acid. Both reactants contain two functional groups, and amine and carboxylic acid groups react to form amide linkages. Similarly, polyethylene terephthalate is produced by the reaction of