Photoinduced copolymerization studies of N-substituted maleimides with various electron donors

Danning Yang

Abstract

Photoinduced copolymerization of N-substituted maleimides and vinyl ethers was investigated systematically. Both photo-differential scanning calorimetry (photo-DSC) and real-time FT infrared spectroscopy (RT-FTIR) were utilized to confirm that copolymerization of the systems proceeds via a free radical mechanism. Transferable hydrogens from the vinyl ethers were found to be critical in obtaining high polymerization rates by examining the polymerizations of various combinations of maleimide/vinyl ether mixtures. Quenching processes of excited triplet maleimide new by different vinyl ethers have been studied using laser flash photolysis. The quenching rate constants were approximately 5 × 10 9 L mol-1 sec-1 , and they are independent of the availability of transferable hydrogens on the vinyl ether. GC-MS studies showed that the reactions of the excited triplet state of the maleimide with vinyl ethers mainly gave the 2 + 2 cycloadduct. Based on results, the initiation mechanism for the copolymerization of the maleimide/vinyl ether system was proposed to be an electron/proton transfer process. In order to have a better understand of photo-initiated copolymerization of multifunctional maleimide and vinyl ether systems, computer modeling was employed to simulate the copolymerization process, especially the sol-to-gel transition. Several kinetic parameters which we difficult or impossible to determine through experimental measurements were simulated as a function of reaction time. A novel system based on a mixture of N-substituted maleimide and vinyl dioxolane monomer was developed. Photo-DSC studies showed that the copolymerization of this system was very efficient compared to a corresponding maleimide/vinyl ether mixture. RT-FTIR data clearly illustrated the occurrence of the ring opening of the vinyl dioxolane monomer during copolymerization. Thus, one can expect less shrinkage in this system compare to direct vinyl addition polymerization. Lower temperatures were found to favor ring opening of the vinyl dioxolane monomer without producing any extractable benzaldehyde molecules. 1 H-NMR spectroscopy was employed to characterize the charge-transfer complex formation between maleimide and various electron donors, and the results showed that the vinyl dioxolane monomer exhibited much greater interaction with the maleimide than the vinyl ether did. (Abstract shortened by UMI.)