Copolymerization of styrene and p‐divinylbenzene. Initial rates and gel points

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Abstract
Initial rates of polymerization were measured at 70.0 and 89.7°C. for styrene, p‐divinylbenzene, and mixtures of the two monomers initiated with 1% benzoyl peroxide. The rates relative to styrene increased linearly with mole fraction of p‐divinylbenzene: the latter monomer polymerized faster than styrene by a factor of 2.50 at 70.0°C. and 3.54 at 89.7°C. Times to gelation were also measured for those samples containing p‐divinylbenzene, and from these times the corresponding fractional conversion at the gel point was calculated. The conversion at the gel point passed through a minimum with increasing content of p‐divinylbenzene. The gelation behavior is explained by concurrent interchain crosslinking to give a network and intrachain crosslinking to give microgels which accumulate to give macrogel, the latter process being dominant at all but low concentrations of p‐divinylbenzene. The higher the content of crosslinker, the less swollen the microgels, and the greater the conversion required to give gelation by this process. The linear increase in rate with increasing p‐divinylbenzene content was also explained by intrachain crosslinking: growing polymer radicals which undergo this process have a reduced rate of termination. The factor by which the termination rate constant is reduced with increasing divinylbenzene content is the same factor as that by which the conversion required to give gelation by accumulation of microgels is increased.

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