The effects of Lewis acid complexation on type I radical photoinitiators and implications for pulsed laser polymerization

Abstract

In the present work, we examine the effects of zinc chloride (ZnCl2) and aluminium chloride (AlCl3) complexation on the photochemistry of two well-known Type I photoinitiators, methyl-4ʹ-(methylthio)-2-morpholinopropiophenone (MMMP) and 2,2-dimethoxy-2-phenylacetophenone (DMPA). High-level ab initio calculations and experimental results demonstrate that Lewis acid complexation has a significant effect on the individual processes that comprise radical photoinitiation. Theoretical calculations predict that ZnCl2 coordinates to MMMP and DMPA to form thermodynamically stable bidentate ketone-amine and ketone-ether chelates, respectively. Meanwhile, the AlCl2 + cation coordinates to MMMP and DMPA to form a tridentate ether-amine-ketone chelate and a bidentate ketone-ether chelate, respectively. We found that addition of ZnCl2 and AlCl3 to solutions containing MMMP significantly increase its molar extinction coefficient (ε) between 350-360 nm. In contrast, the complexation of either ZnCl2 or AlCl3 to DMPA slightly reduces the value of ε in the 350-360 nm range. Time dependent density functional theory (TD-DFT) calculations demonstrate that Lewis acid complexation blue shifts the nπ* excitations of both DMPA and MMMP, while concurrently red shifting the ππ* transitions. Complexation also significantly alters the stability and reactivity of the photoinitiator fragment radicals. Lewis acid complexation localizes and destabilizes acyl radicals, resulting in significantly increased reactivity towards methyl methacrylate (MMA). In contrast, complexation of Lewis acids dramatically reduces the reactivity of the morpholine substituted isopropyl radical and the dimethoxyphenyl radical towards MMA. Alternative complexation at the methyl ester side-chain of MMA has a beneficial effect on the reactivity of all fragments, increasing addition rate coefficients by 2-4 orders of magnitude. We discuss some of the important implications of these findings for pulsed laser polymerization (PLP), and acetophenone photochemistry more generally.

Pulsed laser polymerisation studies of methyl methacrylate in the presence of AlCl3 and ZnCl2 – evidence of propagation catalysis

“…As the photoinitiators 2,2-dimethoxy-2-phenylacetophenone (DMPA) and 2-methyl-4-(methylthio)-2-morpholinopropiophenone (MMMP) interact with AlCl3 and ZnCl2 via complexation at the carbonyl in solution, 20 optimization of the PLP conditions was not straightforward and many of our initial samples failed to meet IUPAC consistency criteria and were not included here It was found that in general the presence of LAs resulted in a "smearing" of the MWD as seen in Figure 1. This was particularly a problem in our PLP experiments at higher pulse rates, which also had a greater tendency to yield molecular weight distributions (MWDs) with no secondary inflection points.…”

“…The reason for these problems is the interaction between the photoinitiator and Lewis acid, which alters the UV-Vis spectrum by relatively stabilizing the π-π* transitions, and destabilizing the nπ* transitions that lead to homolysis. 20 The net result is a decrease in the initiation rate and an increase in the triplet lifetimes, which becomes problematic at low initiator concentrations and high flashing rates.…”

“…Indeed, a recent experimental and theoretical study showed that Lewis acids redshift the UV-absorption profile of acetophenone derivative initiators such as MMMP and DMPA, and alters their energetics and photostabilities. 20 The first unequivocal demonstration of propagation catalysis was made by Clark who carried out quantum chemical calculations to investigate the effect of metal complexation to alkenes more than 25 years ago. 21 The study showed an increase in the rate of radical addition in the presence of a Li + metal cation, and this was attributed to electrostatic interactions.…”

“…In course of this work, we observed that that these stronger Lewis acids have detrimental effects on the PLP experiment by interfering with the photoinitiation process. 20 Thus, as part of the present work, we also identify strategies to overcome these issues.…”

A comparison of methods for theoretical photochemistry: Applications, successes and challenges

“…8 In a similar vein, complexation with Lewis acids has been shown to dramatically alter the photochemical behavior of photoinitiators, blue shifting the nπ* excitations of type I photoinitiators, methyl-4 0 -(methylthio)-2-morpholinopropiophenone (MMMP) and 2,2dimethoxy-2-phenylacetophenone (DMPA), while concurrently red shifting the ππ* transitions. 10 Even when direct conjugation is not possible, the electrostatic effect of charged functional groups can also influence the relative energies of excited states due to their different polarities. Recently Hill and Coote showed used TD-DFT that by including these charged functional groups as nonconjugated substituents, simple pH changes can be used to selectively modify the relative energies (and even ordering) of the nπ* and ππ* states of an acetophenone derivative.…”