METATHESIS POLYMERIZATION - STEPWISE OR CHAIN GROWTH REACTION

The metathesis polymerization of cycloolefins may follow a stepwise or a chain growth mechanism depending on the type of double bond involved (strained or unstrained) and the catalyst (high or low activity). Strained cycloolefins/catalysts of low activity/catalysts with low transfer ability, in the kinetically controlled regime of the polymerization, yield high molecular weight polymers following a chain growth mechanism; in the ideal case, the molecular weight distribution of the polymers is identical with a Poisson distribution. Under suitable conditions, equilibrium may be attained during the course of the reaction, which yields cyclic oligomers corresponding to a ring-chain equilibrium and a most probable distribution of the polymers. On the other hand, with cycloolefins of low ring strain/highly active catalysts, the reaction yields cyclic oligomers in the initial steps of the polymerization, the dimers and trimers being particularly preferred and produced in greater amounts than those corresponding to their equilibrium concentration. In the further course of the reaction they are consumed in a step-wise manner, to form high molecular weight material exhibiting a most probable molecular weight distribution and to result in a homologous series of cyclic oligomers corresponding to a ring-chain equilibrium.