Modeling the polymerization kinetics of a dimethacrylate monomer in the after-effect period

摘要

Isothermal differential scanning photocalorimetry was used to monitor the light-induced (366 nm; 1.3 mW/cm~2; 40 deg C, Ar atm.) polymerization of (diethylene glycol) dimethacrylate proceeding in the presence of 0.03 M 2,2-dimethoxy-2-phenylacetophenone and to study variations in the mechanism of termination in the after-effect period after illumination had been discontinued. Illumination was stopped at five moments of the reaction corresponding to different degrees of double bond conversion. Three termination models, mono [I, eq. (1)], bimolecular [II, eq. (2)], and mixed type [III, eq. (3)], were used to evaluate parameters k_t~m and k_p[P~(centre dot]_0; k_t~b[P~(centre dot]_0 and k_p[P~(centre dot]_0; and k_t~m, k_t~p[P~(centre dot]_0 and k_p[P~(centre dot]_0, resp., where k_t~m and k_t~b are the mono and the bimolecular termination rate coefficients, k_p is the propagation rate coefficient and [P~(centre dot]_0 is the macroradical concentration at the start of the dark reaction. The parameters were calculated at various durations of post-polymerization (10-160 s at 10-s increments) starting invariably at the moment of discontinuation of illumination. In this manner, the propagation and termination rate coefficients were averaged over increasingly large dark conversions. Two-stage statistical analysis was used to find the "best" model. Termination was found to follow the mechanism varying with time of dark reaction from the bimolecular to the mixed type, whereby radical trapping is seen to be increasingly important. Model III was the only model to describe correctly the variation of the dark reaction rate coefficients with time, i.e., the increase in k_t~m and decrease in the product k_t~b[P~(centre dot]_0.