Dimethacrylate monomers are commonly used as the organic phase of dental restorative materials but many questions remain about the underlying kinetics and network formation in these highly crosslinked photopolymer systems. Several novel experimental and modeling techniques that have been developed for other multifunctional (meth)acrylates were utilized to gain further insight into these resin systems. Specifically, this work investigates the copolymerization behavior of bis-GMA (2,2-bis[p-(2-hydroxy-3-methacryloxyprop-1oxy)-phenyl]propane) and TEGDMA (triethylene glycol dimethacrylate), two monomers typically used for dental resin formulations. Near-infrared spectroscopy, electron paramagnetic resonance spectroscopy, as well as dynamic mechanical and dielectric analysis were used to characterize the kinetics, radical populations, and structural properties of this copolymer system. In addition, a kinetic model is described that provides valuable information about the network evolution during the formation of this crosslinked polymer. The results of these numerous studies illustrate that all of the aforementioned techniques can be readily applied to dental resin systems and consequently can be used to obtain a wealth of information about these systems. The application of these techniques provides insight into the complex polymerization kinetics and corresponding network formation, and as a result, a more complete understanding of the anomolous behaviors exhibited by these systems, such as diffusion controlled kinetics and conversion dependent network formation, is attained.

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.

The importance of kinetic chain length on termination kinetics has been investigated in cross-linking free radical polymerizations both experimentally and using a kinetic model. The model differs from previous multivinyl polymerization models via incorporation of chain length dependent termination (CLDT) kinetics. Experimentally, kinetic chain length distributions were manipulated, and the impact these changes have on the relationship between polymerization rate, R p, and initiation rate, Ri, was examined over the entire conversion range for both dimethacrylate and diacrylate polymerizations. While the acrylate polymerization exhibits little change, the methacrylate polymerization exhibits significant deviation from the classical dependence of R p on Ri; this dependence continuously changes throughout polymerization. Once the polymerization becomes reaction diffusion controlled, changes in Ri no longer effect the kinetics. The transition from chain length dependent to independent kinetics is predicted when CLDT is incorporated, thus predicting polymerization kinetics more accurately over a range of polymerization conditions.