Kinetic and mechanistic studies of iniferter photopolymerizations

An investigation of the rate behavior of bulk photopolymerizations of methacrylates initiated by p-xylylenebis(N,N-diethyl dithiocarbamate) (XDT) is presented. Kinetic studies of photopolymerizations of monomethacrylates and dimethacrylates were performed by differential scanning calorimetry. A comparison of the polymerization rate behavior between the iniferter polymerizations and the conventional radical polymerizations is presented to propose a mechanism for the kinetics of the iniferter polymerization. The iniferter polymerization mechanism involves two termination pathways: (1) carbon-carbon radical combination and (2) carbon-dithiocarbamyl (DTC) radical termination. At lower conversions, when the viscosity of the polymerizing system is lower and thus diffusional resistance to termination is low, path 1 is the significant termination mechanism. As the viscosity increases with further conversion, path 2 begins to dominate termination. Therefore, the characteristic autoacceleration effect observed in polymerizations of the mono- and multifunctional methacrylates studied with regular initiators is dramatically reduced or absent in some cases. Additional evidence for preferential cross-termination (i.e., carbon-DTC radical termination) in the presence of an excess of DTC radicals is presented by examining polymerization initiated by a combination of XDT and tetraethylthiuram disulfide (TED). Further, the effects of varying concentration of XDT, intensity of initiating ultraviolet light, and reaction temperature on the rate behavior were examined. The reaction behavior observed in these studies is well explained by the proposed mechanism.