Photopolymerization of acrylic systems using sensitization of substituted maleimides
The use of N-substituted maleimides as photoinitiators for free-radical polymerization has gained recent attention as an alternative to conventional (cleavage and hydrogen-abstraction type) photoinitiators. Based on reports of triplet sensitizers to increase the yields of 2+2 cylcoadducts of substituted maleimides, sensitization of susbtituited maliemides (N- and 2,3- positions) to initiate the polymerization of acrylic systems, as well as the elucidation of the initiation mechanism was chosen as the focus of this dissertation research. A series of photopolymerizations was performed using various sensitizers and substituted maleimides in acrylic systems. These polymerizations were compared for efficiency to an efficient cleavage photoinitiator, 2,2-dimethoxy-2-phenylacetophenone (DMPA) at equal absorbance. It was found that under certain conditions, sensitized maleimide systems were comparable to DMPA. Laser flash photolysis was used to characterize the kinetic rate constants for quenching of the excited state triplets of sensitizers by susbstituted maleimides and N-methyl-N,N-diethanolamine (MDEA). Rate constants for quenching of excited state triplets of N- and 2,3-disubstituted maleimides by MDEA were obtained. Further, the triplet energy of N-methylmaleimde (MMI) was estimated by quenching a series of sensitizers. Fluorescence spectroscopy of purified MMI showed no fluorescence at room temperature. Purified MMI did not show any phosphorescence emission at liquid nitrogen temperatures in a glass.