Light Induces Polymerization of photoinitiator-free Donor/Acceptor Monomer Systems

The photoinduced polymerization of electron donor/electron acceptor monomer systems has been extensively studied in the past few years, in particular the combination of vinyl ethers WE) and Nsubstituted maleimides (Ml) 17 These non-acrylate UV-curable resins have the distinct advantage of undergoing a rapid polymerization when exposed to UV-radiation in the absence of any added photoinitiator, with formation of an alternating copolymer. Some VE/MI based systems proved to be as reactive as typical acrylate-based resins containing a radical-type 8 The reaction kinetics has been thoroughly ated in recent years by means of photocalorimetry 5,6 and real-time
infrared (RTIR) spectroscopy but the actual mechanism of the copolymenzation still remains to be
elucidated. The presence of labile hydrogen atoms on at least one of the monomer molecules was
shown to be essential in order to achieve a rapid polymerization upon UV exposure With respect to the buildup of the alternating copolymer, two mechanisms can be considered the chain reaction can propagate either by a cross-over process where MI° radicals would react with the VE double bonds and VE° radicals with the Ml double bonds, or by homopolymerization of an acceptor-donor complex AD]. Whatever the polymerization mechanism, exactly the same alternating copolymer will be formed. The fact that no ground state AD] complex could be detected by UV spectroscopy in any of the MINE systems studied so far was taken as a strong argument in favor of a copolymerization proceeding by a cross-propagation mechanism.
The main objective of this work was to clarify the mechanism by which the MINE system undergoes
polymerization upon exposure to UV light, in particular regarding the propagation step. In this respect, some UV-curing experiments have been performed in the presence of a radical-type photoinitiator by using filtered UV-radiation, in order to prevent any direct excitation of the maleimide monomer and to get a better control of the initiation rate. The high-speed polymerization of each one of the two monomers was followed quantitatively in real time by means of infrared spectroscopy. By studying the influence of the monomer feed composition on the Ml and VE polymerization rates, we have obtained valuable kinetic data allowing us to elucidate the mechanism of the light-induced copolymerization of the maleimide/vinyl ether system.