Diffusion-Controlled Free Radical Polymerization. Effect on Polymerization Rate and Molecular Properties of Polyvinyl Chloride

At high conversions of monomer (or high polymer concentrations), chain entanglements and a glassy-state transition (significant reduction in free volume) can have a profound effect on the relative rates of free-radical reactions which control the molecular structure of polymer chains. In particular, it will be shown that a diffusion-controlled propagation leads to the production of very low molecular weight chains with one terminal double bond per polymer molecule and to chains with a higher frequency of long-chain branching than might normally be expected at high polymer concentrations. On the basis of these results for diffusion-controlled polymerizations, polymer reactor operating procedures which should give higher productivity and a more thermally stable PVC are suggested. It is also suggested that the short chains and highly branch polymer molecules which are produced under monomer-starved conditions may in large part be due to diffusion-controlled propagation.