Termination mechanism during the photo-induced radical cross-linking polymerization in the presence and absence of oxygen

The objective of this study was to characterize changes in the termination mechanism and in the rate coefficients during a light-induced polymerization occurring under continuous illumination and during the after-effect (after the irradiation has been discontinued) in the presence or in the absence of oxygen. The illumination was stopped at various double bond conversions. The polymerization of a model monomer-(diethylene glycol) dimethacrylate was analyzed using three termination models: monomolecular (I), bimolecular (II) and mixed (III) and the following parameters were estimated: k(t)(m) and k(p)[P(.)](0) (Model I), k(t)(b) [P(.)](0) and k(p)[P(.)](0) (Model II) and k(t)(m), k(t)(b) [P.](0) and k(p)[P(.)](0) (Model III), where k(t)(m) is the monomolecular termination rate coefficient, k(t)(b) is the bimolecular termination rate coefficient, kp is propagation rate coefficient and [P(.)](0) denotes macroradical concentration at the beginning of the dark reaction. The parameters of the models were averaged over increasingly large dark conversion. Multistage statistical analysis of the results showed that over the conversion range studied, the bimolecular reaction was the main way of termination during continuous illumination, but during the after-effect termination mechanism changed with time of dark reaction from the bimolecular to the mixed one indicating an increasing importance of radical trapping. The obtained results suggest that oxygen accelerates the bimolecular termination, but pseudo first-order reaction with oxygen also contributes to the overall termination process. Moreover, the postpolymerization is more sensitive to negative influence of oxygen than the polymerization under continuous irradiation.