DIFFUSION CONTROLLED VINYL POLYMERIZATION

It is well known that the polymerization rate and the molecular weight distribution of vinyl polymers can change markedly during the course of polymerization and that these changes are due to the influence of self-diffusion upon the termination reaction. This phenomenon is commonly referred to as the gel effect and in order to explain the polymerization behavior after the onset of the gel effect, the chain length dependence of the termination reaction should be considered. A new method of handling polymerization kinetics with the chain length dependence termination reaction is proposed, which is largely independent of the form of the chain length dependency and is capable of dealing with both disproportionation and recombination modes of termination with chain transfer reaction to monomer. The vinyl polymerization kinetics is modelled for each of the four distinct phases which show different polymerization kinetics-physical property interactions. During Phase I, no interaction is significant and the polymerization kinetics conforms to the conventional kinetic and the molecular weight distribution to the Schulz-Flory most probable distribution. During the Phase II, the termination reaction is controlled by the translational diffusion of the macroradicals. The polymerization kinetics begin to deviate from the conventional kinetics and the termination reaction rate constant shows chain length dependence and conversion dependence. The chain length dependence is modelled with the chain entanglement concept and the conversion dependence with the free volume theory. During the Phase III, the gel effect disappears due to the change of the controlling mechanism of termination from translation diffusion to the excess chain mobility of the chain ends coupled with the propagation reaction. The resulting termination rate constant lacks chain length dependency and is named as the residual termination. During the Phase IV, the propagation reaction and other elementary reactions become diffusion controlled, further slowing down the polymerization rate. A method of estimating the diffusion controlled propagation reaction is proposed. These models, with the aid of general method of polymerization kinetics, were integrated to simulate the vinyl polymerization systems over the whole range of conversion. Methyl methacrylate, ethyl acrylate, n-propyl acrylate, vinyl acetate, ethyl methacrylate, and styrene polymerization data are analyzed with the integrated model which has only one adjustable parameter and excellent agreements are observed