Determination of the absolute rate constant of propagation for ion pairs in the cationic polymerization of p-methylstyrene

The cationic polymerization of p-methylstyrene (p-MeSt) was studied in dichloromethane, chloroform, and methylcyclohexane/methyl chloride 60/40 (v/v) at different temperatures with various initiators in conjunction with different Lewis acids, TiCl4, BCl3, SnBr4, and SnCl4. Well-controlled living cationic polymerization was obtained in dichloromethane, in conjunction with SnCl4 as Lewis acid, and the living nature of the polymerization was verified by linear first-order ln([M](0)/[M]) vs time and linear number-average molecular weight (M-n) vs conversion plots in the temperature range of -15 to -70 degrees C. The number-average molecular weight of the polymers increased in direct proportion to monomer conversion up to M-n approximate to 90 000 and agreed reasonably well with the calculated molecular weight, assuming that one polymer chain forms per molecule of initiator. The kinetics of p-MeSt polymerization suggests that the polymerization is first order with respect to SnCl4 concentration. Employing the model compound 1-chloro-1-(4-methylphenyl)ethane in conjunction with SnCl4 in CH2Cl2, UV-vis spectroscopy was used to follow the capping reaction with 2-phenylfuran to determine the equilibrium constant of ionization (K-i) at -30 degrees C. From the K-i value and the apparent rate constant of propagation (k(app)) the absolute rate constant of propagation for ion pairs, k(p)(+/-) = 6.8 X 10(8) L mol(-1) s(-1) at -30 degrees C, was calculated as a lower limit. The absolute rate constant of propagation for ion pairs, k(p)(+/-), was also determined at different temperatures from competition experiments, where polymerizations were carried out in the presence of 2-phenylfuran as capping agent. Gel permeation chromatography and NMR spectroscopy suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting conversion and limiting number-average degrees of polymerization k(p)(+/-) = 1 x 10(9) L mol(-1) s(-1) was calculated using the known rate constant of capping. The k(p)(+/-) value remained unaffected in the temperature range of -15 to -70 degrees C, indicating that propagation does not have an enthalpic barrier.