Abstract
Cobalt porphyrins mediated vinyl acetate radical polymerization has been performed in different solvent environments and was found that a high monomer conversion (69%) with linear increased molecular weight versus conversion, narrow molecular weight distribution (Mw/Mn = 1.12 ~ 1.34), and little molecular weight deviation could be approached in bulk condition. However, different levels of molecular weight deviation occurred when polymerization was performed in solution, indicating that the control of vinyl acetate radical polymerization was affected by chain transfer to solvent. Besides, non-coordinating solvents caused a difference in polymerization rate mainly due to the different propagation rate constant (kp), but the coordinating solvents such as THF blocked the reaction site of cobalt porphyrins and interfered the whole polymerization behavior. The effect of radical concentration and the ratio of [VAc]0 / [CoII]0 have been also discussed. The higher radical concentration shortened the induction period, increased the polymerization rate and caused a slight deviation of molecular weight. But the control efficiency of cobalt porphyrins to vinyl acetate polymerization was not significantly affected by the ratio of [VAc]0 / [CoII]0. The coordinating effect of pyridine was demonstrated by the shift of equilibrium from dormant organo-cobalt(III) species to active cobalt(II) species. The equilibrium constant (Keq) was changed from < 10-12 to 5.2 × 10-8 when 25 equivalent of pyridine was added into the polymerization so that the major control process was switched from degenerate transfer to reversible termination. Polymerization of methyl acrylate mediated by cobalt porphyrins has been conducted in different solvents. The polymerization behaviors were similar in non-coordinating solvents. However, the induction period was shortened with coordinating solvents such as acetonitrile, anisole and THF which blocked the reaction site of cobalt porphyrins. When THF was used as an additive, the induction period was slightly decreased, but it showed no effect to the polymerization rate and the controlled efficiency.