Self-initiated photopolymerization of hyperbranched acrylates

Self-initiated photopolymerization of hyperbranched acrylates (HBAs) has been investigated. The photopolymerization progress was monitored by differential photo-calorimetry (DPC). It was shown that HBAs were able to self-initiate photopolymerization rapidly under the irradiation of medium-pressure mercury lamp. This behavior was explained by the longer wavelength absorption (∼286 nm) of HBAs, which results from the overlapping of π-conjugated orbitals of acrylate groups. Autoacceleration effect was observed. The autocatalytic model was used to analyze the photopolymerization kinetics and found to well match the experimental data. It was found that acceleration stage ended at low conversion (αRm < 0.11), and the final conversion αf was lower than 40% in most cases. This was attributed to the poorer ability of HBAs to generate primary radicals and thus the insufficiency of free radicals in polymerization. Factors in favor of generating primary radicals extended the autoacceleration stage to higher conversion and gave higher final conversion. The values of activation energy were calculated to be in the range of 5.7-7.5 kJ mol−1, analogous to those for the free radical photopolymerization of acrylate monomers in the presence of photoinitiator.