POLYMERIZATION MECHANISM

    Polymers may be classified broadly as condensation, addition, or ring-opening polymers, depending on the type of polymerization reaction involved in their formation. Condensation polymers are formed from a series of reactions, often of condensation type, in which any two species (monomers, dimers, trimers, etc.) can react at any time leading to a larger molecule. In condensation polymerization, the stepwise reaction occurs between the chemically reactive groups or functional groups on the reacting molecules. In the process, a small molecule, usually water or ammonia, is eliminated. A typical condensation polymerization reaction is the formation of a polyester through the reaction of a glycol and a dicarboxylic acid (Equation 1.1). Examples of condensation polymers include polyamides (e.g., nylon 6,6) polyesters (e.g., poly(ethylene terephthalate); and urea-formaldehyde and phenol–formaldehyde resins.

       (1.1)

Addition polymers are produced by reactions in which monomers are added one after another to a rapidly growing chain. The growing polymer in addition polymerization proceeds via a chain mechanism.

   Like all chain reactions, three fundamental steps are involved: initiation, propagation, and termination. Monomers generally employed in addition polymerization are unsaturated (usually with carbon-carbon double bonds). Examples of addition polymers are polystyrene, polyethylene, polyacrylonitrile, poly(methyl methacrylate), and poly(vinyl chloride).

   As the name suggests, ring-opening polymerization polymers are derived from the cleavage and then polymerization of cyclic compounds. A broad generalization of ring-opening polymerization is shown in Equation 1.2.

      (1.2)

The nature of the cyclic structure is such that in the presence of a catalyst it undergoes equilibrium ring-opening to produce a linear chain of degree of polymerization, n. X is usually a heteroatom such as oxygen or sulfur; it may also be a group such as lactam or lactone. A number of commercially important polymers are obtained via ring-opening polymerization. Thus, trioxane (1) can be polymerized to yield polyoxymethylene (2), the most important member of the family of acetal resins, and caprolactam (3) undergoes ring-opening to yield nylon 6 (4), an important textile fiber used especially for carpets.

  The original classification of polymers as either condensation or addition polymers as proposed by Carothers does not permit a complete differentiation between the two classes or polymers, particularly in view of the new polymerization processes that have been developed in recent years. Consequently, this classification has been replaced by the terms step-reaction (condensation) and chain-reaction (addition) polymerization.  These terms focus more on the manner in which the monomers are linked together during polymerization.