Multipulse initiation in pulsed laser and quenched instationary polymerization: determination of the propagation and termination rate coefficients for dicyclohexyl itaconate polymerization

The commonly used pulsed laser polymerization technique and the recently introduced method of quenched instationary polymerization were successfully adapted for the measurement of kinetic coefficients in the bulk polymerization of the highly hindered monomer dicyclohexyl itaconate (DCHI) in the temperature range 20-50 °C. The need for very high free radical concentrations in the polymerization system was met via a multipulse laser initiation, in which a burst of laser pulses substitutes for the commonly used single laser pulse. Thus, two new methods are at hand for the investigation of the kinetic parameters. The chain length distributions that are obtained can be easily analyzed, and the data for the propagation rate coefficients, calculated via these two techniques, show excellent agreement. The activation parameters for k_p were calculated as E_a = 22.0 kJ mol^-1 and A = 1.74 × 10⁴ L mol^-1 s^-1 for the first method and as E_a = 22.8 kJ mol^-1 and A = 2.49 × 10⁴ L mol^-1 s^-1 for the second method. The activation energy is comparable with the methacrylate series of monomers. The frequency factor is relatively small and reflects the steric hindrance in the transition state caused by the bulky substitution in the monomer (and/or the radical). The quenched instationary polymerization method was utilized to determine the termination rate coefficient, yielding very low values (200-1000 L mol^-1 s^-1 in the temperature range 20-50 °C). The low k_t may well be related to the monomer viscosity. The temperature dependence of k_t shows the same extent as that of the viscosity, indicating the diffusion control of the termination reaction. Modeling of the chain length distributions obtained by the quenched instationary polymerization experiments indicates about 25% termination via combination.