The importance of incorporating chain length dependent termination (CLDT) behavior into
the interpretation of the kinetics of cross-linked systems has been examined. Kinetic chain length
distributions were varied in a variety of di(meth)acrylate photopolymerizations via the manipulation of
the initiation rate and the chain transfer rate. Shifting the kinetic chain lengths toward shorter chains
has little visible effect on multiacrylate systems. In contrast, similar changes in the corresponding
multimethacrylate polymerizations changed the kinetics significantly. Shorter kinetic chains led to the
delayed onset of reaction−diffusion-controlled termination behavior, as well as an increase in the ratio
of k
t/k
p[M] at all conversions prior to the onset of reaction−diffusion control. Additionally, the magnitude
of the kinetic constant ratio in the reaction−diffusion-controlled regime was affected by the kinetics at
low conversion in the polymerization of a rubbery system, PEG(600)DMA. This behavior was independent
of the method used to alter the kinetic chain length distribution and thus implies that CLDT may
potentially impact the network formation in polymerizations occurring above the T
g of the system. These
results illustrate that, although counterintuitive, CLDT is an important factor in cross-linking free radical
polymerizations.
Experimental investigations were made into the effects of monomer structure and functionality on free-radical polymerization kinetics. A more comprehensive understanding of how structural characteristics, monomer traits, and polymerization conditions influence the polymerization mechanisms and network evolution was desired. Variations in the nature of the monomers' secondary functionality and the terminal substitution were the primary variables examined. The three factors hypothesized as important to the advantageous polymerization characteristics observed are hydrogen bonding, hydrogen abstraction, and the electronic characteristics of the monomer. The experimental evaluations presented clearly demonstrate that each of these mechanisms contributes to the reactivity of these monomers and the networks that they form. The combination of these factors leads to crosslinked network formation and enhanced polymerization kinetics, i.e., monovinyl monomers with reactivities that rival those of commonly used divinyl monomers.
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