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

Abstract

As an alternative to asymmetric iniferters like XDT (p-xylylene bis(N,N-diethyldithiocarbamate), the use of DMPA (2,2-dimethoxy-2-phenylacetophenone) in 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 carbon radicals while the TED would photocleave and terminate via dithiocarbamyl (DTC) radicals. In this contribution, we report that for the case of TED and DMPA photoinitiated systems, the nature of the living polymerization and 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 365nm were performed using differential scanning calorimetry. TED was added in incremental amounts to determine its effect on the polymerization as a function of wavelength and concentration. The results suggest that termination via DTC radicals 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 TED are all referred to as chain breaking reactions. The model fit the data well after the maximum rate of polymerization. Furthermore, the model predicts that termination involving TED and its derivatives dominates over carbon radical–carbon radical termination when the ratio of TED:DMPA is 1:2 or higher 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 traditional termination. Infrared analysis of several samples proved that TED:DMPA combinations greater than 1:2 can prevent trapped carbon radicals from persisting in polymer networks after cure.