Kinetic and Mechanistic Studies of Photopolymerizations of Acrylates in the Presence of Iniferters

Abstract

In this particular study, the reaction mechanisms of bulk polymerization of acrylates in the presence of tetraethylthiuram disulfide (TED) are investigated using hexyl acrylate as a model acrylate. TED's presence impacts the termination mechanisms of the acrylate polymerization by two pathways: (1) dissociation of TED to generate dithiocarbamate radicals, which could terminate the acrylic radicals, and (2) chain transfer to TED. The impact of TED concentration as well as the initiation rate on the hexyl acrylate polymerization rate was investigated for polymerizations initiated by both UV and visible light. It was observed that for the polymerizations initiated by UV light, the initial polymerization rate (averaged between 5 and 20% conversion) scaled with the initiation rate by a power of 0.86 ± 0.02 at various concentrations of TED, and a power of 0.45 ± 0.05 for polymerizations in the absence of TED. The initial polymerization rate exhibited a near inverse-linear relationship with the concentration of TED, with the order of dependence ranging from -0.96 ± 0.13 to -1.00± 0.14. Thus, it was found that chain transfer to TED appears to contribute significantly to the termination mechanisms in the polymerization of acrylates. It was observed that TED does not exhibit any absorbance in the visible range (400-500 nm), and hence, the dissociation of TED was eliminated in the polymerizations conducted by visible light. For these polymerizations initiated by visible light, the initial polymerization rate scaled with the initiation rate by a power of 0.84 ± 0.04 to 0.95 ± 0.05 at various TED concentrations ranging from 0.1 to 0.5 wt %. It was also observed that at a light intensity of 25 mW/cm 2 , the log-log plot of the initial polymerization rate as a function of TED concentration exhibited a slope of -1.19 ± 0.03. Thus, the dependence of polymerization rate on the TED concentration was observed to be greater than the expected inverse-linear relationship because of the dependence of the termination kinetic constant on the concentration of TED. Furthermore, it was shown that chain-transfer to TED is the dominant mechanism relative to the dissociation of TED to generate dithiocarbamate radicals, and it appears to contribute significantly to the termination reactions for both polymerizations.