PHOTOINDUCED FREE RADICAL POLYMERIZATION USING SELF-INITIATING MONOMERS

摘要

Historically, a number of self photo-initiating carbon-carbon double bond (C=C) containing monomers have examined during a fairly long period of time with respect to their relative efficiencies in initiating free radical polymerization and these findings are well documented in the literature. Traditional monomer systems involve maleimides and α,β-substituted maleimides and maleates/fumarates mostly in vinyl ether combinations. In this paper, a comprehensive study has been undertaken in investigating the initiating efficiencies of α,β-substituted alkenyl acrylates (X_1-C=C(Y,)COO(X_2-C=C-Y_2)). In order to evaluate the photochemistry involved in the excitation process, focus has been emphasized on vinyl acrylate (VA) as a model compound. VA exhibits fast rates of polymerization and high degrees of conversion are obtained. Real Time Infra Red analysis (RTIR) was used throughout the investigation in recording the kinetics of the homo and co-polymerization of VA and in combinations with various co-monomers in the presence and absence of Phi. In the homopolymerization of VA, the acrylate C=C was found to polymerize much faster than the vinyl ester C=C. The enhanced reactivity of the acrylate C=C in VA is mainly due to the very close proximity of the two C=C bonds exhibiting large differences in electron densities over the two C=C bonds within the same molecule. Modeling studies are compared to real time recorded kinetics, showing good correlation. Furthermore, a mechanistic investigation of the nature of the excited state conformation in VA also indicates fundamental differences in the "efficiencies of initiation", which can be caused by a biradical versus a conventional "monoradical" pathway of initiation seen by the use of commercially available PHI's. Finally, useful future implications in the use of VA and derivatives thereof as initiating monomers in practical acrylate based formulations will be discussed in the paper.