Diffusion and gelation in polyadditions. II. Theory of the viscosity‐conversion curve

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Abstract
The curve governing the viscosity as a function of conversion in the system methyl methacrylate (95% moles)/ethylene dimethacrylate (5% moles) up to the gel point is predicted theoretically. The classical theory of gelation enters critically into the theory. The excellent experimental verification of the curve thus confirms the applicability of the classical theory and the absence of interference by diffusion control. As the gel point is reached at just over 1% conversion, these experiments require further refinements in the viscodilatometric technique used. The reproducibility during the induction period preceding polymerization of the constant initial volume is attested by a standard deviation of 0.5% calculated on the subsequent shrinkage up to the gel point. The standard deviation of the viscosity during the induction period was 1%. The theory of the viscosity/conversion curve makes a correction for nonideal solution behavior, but this is small here. No allowance is required for chain transfer effects or ring formation within a primary polyaddition chain, which are serious obstacles when at tempting to predict the critical conversion absolutely from the classical theory. Formation of rings involving several primary polyaddition chains is neglected, and this seems justified by the agreement between theory and experiment, and by the statistical theory to be given in the fourth paper of this series. The initial intrinsic viscosity of the system is found to be substantially lower than for pure methyl methacrylate polymerization. This reduction can be formally evaluated in terms of a chain transfer constant to ethylene dimethacrylate monomer. However, the value obtained is too high in comparison with suitable model compounds for this monomer, including ethylene diacetate and difumarate. A large part of the reduction in viscosity is, therefore, attributed to the compacting of the primary chains through formation of internal rings of substantial size.