Crosslinking in isoprene polymerization

Abstract

AbstractA study has been carried out on the emulsion polymerization of isoprene in an attempt to evaluate the relative rate of the crosslinking reaction and its variation with temperature. From the relation between primary chain length and gel formation during polymerization, the following values were obtained for the relative crosslinking rate constant, i.e., ratio of the velocity constants of the crosslinking and propagation reactions. Temperature, °C. K = kx/kp 60 3.4 × 10−5 70 4.3 × 10−5 80 5.2 × 10−5 The above data lead to a calculated value for the difference in the activation energy between the crosslinking and the propagation reaction (Ex − Ep) of 5.0 kcal. From these data and the known value of the propagation rate constant, it is possible to calculate the following absolute rate expression for the crosslinking reaction itself: It is noteworthy that the relative crosslinking rate in isoprene polymerization is only about one‐sixth that found for butadiene (K = 2 × 10−4 at 60°C.) apparently owing largely to a higher steric factor. Hence longer chains are possible for polyisoprene before network formation leads to gelation. In the Mutual Recipe, the rate per particle for isoprene polymerization is about three times as fast as for butadiene, although it is still short of the rate found in the ideal low temperature systems.