Bulk polymerization of isoprene: Kinetic constants from the dead‐end theory

Abstract

The dead‐end radical polymerization theory of Tobolsky was applied to the bulk polymerization of isoprene. 2‐Azobisisobutyronitrile and benzoyl peroxide were used as initiators in the temperature range 60–90°C. Isoprene shows a dead‐end conversion under all the conditions we used. No Tromsdorff effect was observed in this system. The rate of decomposition of each initiator (k_d)at different temperatures was obtained solely from the conversion‐time curve without further assumptions. The k_d values thus obtained agree well with published data based on other methods. Knowing k_d it was possible to compute (k_p/k)f^1/2, where k_p is the specific rate of propagation, k_t the specific rate of termination, and f the efficiency of the initiator. This allowed comparison of the efficiency of benzoyl peroxide with 2‐azobisisobutyronitrile as free‐radical initiators in the bulk polymerization of isoprene. k_p/k values for the azo‐initiated polymerization were also obtained and were found to be much lower for isoprene in comparison to reported values for styrene or methyl methacrylate. The high values of k_p/k found for isoprene explain the low conversions obtained when isoprene is polymerized in bulk. From reported values of k_p it was possible to calculate k_t. This is the first time where the specific rate of termination of isoprene is reported. The rate constant k_t was found to be 1.34 × 10⁸ liters/mole‐sec. and is independent of temperature in the range 60–90°C. High k_t values account for the low molecular weights obtained during the bulk polymerization of isoprene. The theory was also used to predict the course of isoprene polymerization as a function of time using previously determined values of (k_p/k)f^1/2 and k_d for the initiator used. The entire course of the conversion curves are well reproduced. The molecular weight distribution is also predicted as a function of conversion.