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The quality and performance of polymers are critically linked to the cure characteristics of the polymerization system. Improvements in monomer curing efficiency would allow optimum polymer properties to be achieved with minimized irradiation times and intensities, which has significant industrial relevance. The development of monomers that exhibit high reactivities and superior mechanical properties and aging characteristics is the ultimate goal. In part, this entails
the use of monomers that react rapidly to very high conversions and result in a mechanically and chemically stable three-dimensional network in the process. This development and eventual use requires, above all, an intimate understanding of the polymerization kinetics and the underlying mechanisms that define those kinetics. Additionally, it is essential to understand how the kinetics and corresponding mechanisms are affected by changes in monomer structure and functionality.
The effect of monomer functionality on the rate of polymerization has been studied 1-3]. As the
functionality is increased, the rate of polymerization also increases, at the cost of a greater degree of residual unsaturation and the formation of a hard but typically brittle polymer. Functionality, in these works, is defined as the number of vinyl containing reactive substituents present on the monomeric starting material. Recent studies have been aimed at the development of monoacrylates that exhibit rates of cure equivalent to that of multiacrylate monomers but with higher extents of cure and equivalent if not superior mechanical properties. It has been shown that by including certain chemical groups such as carbonates 4-12J, carbamates 2,9,10, and
oxazolidones 9,10,12] into the structural unit of a monovinyl acrylate, the reactivity of the resulting monomer is well in excess of that of other commonly used monovinyl systems. In fact, the reactivity of these monomers in air, as opposed to an inert atmosphere, rivals that of commonly used multivinyl 9,10,13J. Additionally, gel fractions have been measured for the polymer product in several of these systems. Interestingly, a number of these monomers are, for the most part, insoluble in organic solvents, which is indicative of the formation of a crosslinked network. This result is unexpected for the polymerization of monoacrylate monomers and indicates that secondary reactions resulting in crosslink formation are occuring. The reason for this enhanced reactivity and gelation behavior is not fully 14]. The hypothesis of Decker et a!. is that the reactivity is a result of an efficient chain transfer reaction that involves labile hydrogens from the newly introduced functional 7,12]. This hypOthesis is supported by polymerization results involving an initiator dependent on the hydrogen donor characteristics of the system,benzophenone. Benzophenone was very successful in rapidly polymerizing the new monomer systems, but no polymer was formed when polymerizations of HDDA and TMPTA were 1 2. Additionally, the high rates have been' attributed to the presence of an inefficient termination process in these new systems. This hypothesis is supported by kinetic data that show a decrease in the termination kinetic constant during the polymerization, while the propagation kinetics closely follow that of standard a1cr6ylic] 1 3,15]. More recent work hypothesizes that many of these results are explained by hydrogen. A more detailed kinetic analysis of such monovinyl systems is imperative to the comprehension of the underlying kinetics responsible for . the desirable qualities of these polymerization systems. Its value will be even more pronounced from the perspective of learning how to design materials rationally. The aim of this research is to examine the mechanisms hypothesized as contributors to the exceptional kinetics and material properties of these monovinyl systems in a controlled and systematic manner. The contributions of such effects will be evaluated using small changes in monomer structure and functionality as well as via the introduction of controlled amounts of crosslinking and chain transfer into the polymerizing system.
