SynopsisIt is well known that the reaction rate and molecular weight of vinyl polymers can change markedly during the course of polymerization and that these changes are due to the influence of diffusion on the termination reaction. The chain length dependence of the termination rate constant has been considered in this work and has resulted in a general method of treating the polymerization kinetics and molecular weight distribution. This method is independent of the form of the chain length dependency and is capable of dealing with both disproportionation and recombination modes of termination. A specific model for the termination rate constant with chain length dependence is proposed and is based on free volume theory and entanglement coupling. Master curves for the characteristics of the reaction rate and molecular weight distribution are presented with the application of this model.

SynopsisIt is frequently observed that the gel effect in vinyl polymerizations decreases in intensity at about the 50-70'% conversion level. This is apparent in both rate and molecular weight data. It is postulated that there should be a limitation on the decrease in the termination rate constant to explain this effect. As an extension of the general theory of chain length dependent termination behavior, a general treatment of the gel effect with a limiting value of the termination rate constant is presented. A specific model is proposed for this limited rate constant which is based on the simple consideration that as translational movement of macroradicals becomes increasingly difficult the contributions made by segmental motion derived solely from the propagation reaction will be the prevailing mechanism. The termination reaction changes from chain length dependent to chain length independent during this transition period.

SynopsisBulk polymerization data of methyl methacrylate, ethyl methacrylate, ethyl acrylate, n-propyl acrylate, vinyl acetate, and styrene were compared with the predictions of the theory proposed in the earlier parts of this series (1-111). This theory of polymerization kinetics uses the concepts of free volume and chain entanglements to describe the relationship between chain mobility and chain length dependent termination reactions. Excellent agreement was found between the predictions of the theory and the polymerization rate and molecular weight data of the six polymerization systems studied. Emphasis was placed on the ability to explain the development of higher order molecular weight averages (mw, mz, etc.) because they provide the most crucial tests for such a model. No changes were required in the model as it was applied to the different polymerization systems for a variety of reaction conditions. The theory offers a unified understanding of the diverse polymerization behavior displayed by such systems. METHOD OF ANALYSISThis section deals with the general computational procedures and set of equations used for modeling vinyl polymerizations with the proposed theory. Only an outline is given below because the details of the theory have already been discussed.

SynopsisThe values of the parameters that are necessary to compute the free volume changes accompanying polymerization were determined from viscosity data of concentrated polymer solutions. Relationships applicable above and below the glass transition temperature of the polymer were quantfied.A general treatment of the variation of the propagation rate constant with free volume was postulated, based on the variation of the monomeric diffusivity with free volume. This, in turn, was related to the friction coefficient of a polymer chain segment, the values of which are readily available. A discussion of the reaction behavior in the last stage of polymerization and the occurrence of limiting conversion is also presented.

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