The effect of wavelength on the polymerization of multi(meth)acrylates with disulfide/benzilketal combinations

As an alternative to asymmetric iniferters like XDT (p-xylylene bis(N,N-die thyldithiocarbamate), the use of DMPA (2,2-dimethoxy-2phenylacetophenone) i n combination with TED (tetraethylthiuram disulfide) has been proposed as a means of studying living radical polymerizations and creating crosslinked polymers without trapped radicals. This dual initiator system ideally could imitate XDT. The DMPA would photocleave and initiate the reaction via carb on radicals while the TED would photocleave and terminate via dithiocarbamy l (DTC) radicals. In this contribution, we report that for the case of TED and DMPA photoinitiated systems, the nature of the living polymerization an d the polymerization kinetics are dependent on the wavelength of light used . Kinetic studies of TEGDMA (triethylene glycol dimethacrylate) photocured with monochromatic light at wavelengths 290, 310, 324, and 365 nm were perf ormed using differential scanning calorimetry. TED was added in incremental amounts to determine its effect on the polymerization as a function of wav elength and concentration. The results suggest that termination via DTC rad icals becomes more prominent as the irradiating wavelength is increased. A model was developed that incorporates three modes of chain breaking, where carbon-carbon termination, carbon-DTC termination, and chain transfer to TE D are all referred to as chain breaking reactions. The model fit the data w ell after the maximum rate of polymerization. Furthermore, the model predic ts that termination involving TED and its derivatives dominates over carbon radical-carbon radical termination when the ratio of TED:DMPA is 1:2 or hi gher in TED. Therefore, TED:DMPA combinations can be used as an alternative to conventional iniferters as long as a sufficient amount of TED has been added so that carbon-sulfur chain breaking reactions dominate over traditio nal termination. Infrared analysis of several samples proved that TED:DMPA combinations greater than 1:2 can prevent trapped carbon radicals from pers isting in polymer networks after cure. (C) 2000 Elsevier Science Ltd. All r ights reserved.