ADDITION POLYMERIZATION

Addition polymerization is employed primarily with substituted or unsubstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds.

In addition polymerization, a chain grows simply by adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I^Z) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows by attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer:

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, “Free Radical Polymerizations”). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent:

Propagation then occurs by successive monomer molecules additions to the intermediates. Three addition modes are possible: (a) Head to tail; (b) Head to head, and (c) tail to tail. The head-to-tail addition mode produces the most stable intermediate. For example, styrene polymerization mainly produces the head-totail intermediate:

Head-to-head or tail-to-tail modes of addition are less likely because the intermediates are generally unstable:

Chain growth continues until the propagating polymer chain terminates.