Bulk polymerization autocatalytic effect

The polymer concentration at which autocatalytic effects are significant varies from system to system. When a polymer is insoluble in its own monomer, radicals can be occluded in the precipitated polymer-rich phase where they are prevented from terminating by their low mobilities. Bulk polymerization of vinyl chloride provides an example of this. 

There is autoacceleration in the bulk polymerization rate of vinyl chloride. It was suggested by Schindler and Breitenbach that the acceleration is due to trapped radicals present in the precipitated polymer swollen by monomer molecules. This influences the rate of the termination that decreases progressively with the extent of the reaction, while the propagation rate remains constant. The autocatalytic effect in vinyl chloride bulk polymerizations, however, depends on the type of initiator used. Thus, when 2,2 -azobisisobutyronitrile initiates the polymerization, the autocatalytic effect can be observed up to 80% of conversion. Yet, when benzoyl peroxide initiates the reaction it only occurs up to 20-30% of conversion. 

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