Synthesis of Self-Photosensitizing Polyesters Carrying Pendant Norbornadiene (NBD) Moieties and Benzophenone Groups and Their Photochemical Reactions

Abstract

Various epoxy monomers containing benzophenone (BP) groups were synthesized by reactions of potassium or lithium salts of BP derivatives with excess epichlorohydrin or epibromohydrin using tetrabutylammonium bromide (TBAB) as a phase transfer catalyst (PTC). The copolymerization of glycidyl 3-(piperidylcarbonyl)-2,5-norbornadiene-2-carboxylate (GPNC), which was prepared by the reaction of epichlorohydrin with potassium carboxylate of a norbornadiene (NBD) derivative using a PTC, and (glycidyloxy)benzophenone with phthalic anhydride (PAn) proceeded in 93% yield using TBAB as a catalyst in sulfolane at 100 °C for 24 h to give self-photosensitizing polyester P-1 containing a pendant NBD moiety and a pendant 4-oxybenzophenone group. Copolymerizations of various epoxy monomers containing certain NBD moieties and BP groups with PAn produced corresponding polyesters with pendant NBD moieties and BP groups in good yields. The photochemical valence isomerizations of the NBD moieties and the photosensitizing isomerizations of the NBD moieties due to the energy transfer from excited BP groups to the NBD moieties in the polymers were performed in the film state or in dichloromethane solution. From this result, it was found that rates of the photochemical isomerization of the NBD moieties in the polymers were strongly enhanced by the introduction of the pendant BP, N-alkyl-N,N‘,N‘-trimethyl-4,4‘-diaminobenzophenone (TBP), group. Furthermore, the photochemical isomerization of the NBD moieties in the polymers having a pendant TBP group proceeded effectively in a diluted dichloromethane solution compared with the mixtures of NBD polymers and low molecular weight photosensitizers such as 4,4‘-bis(N,N-diethylamino)benzophenone. Glass transition temperatures (Tg's) of the prepared polyesters having NBD moieties and stored thermal energy in the corresponding quadricyclane (QC) residues in the polymers were measured by DSC analysis. Then, it was found that Tg's of NBD polyesters were 30−85 °C, and photoirradiated polyesters having QC residues released their stored thermal energies (about 90 kJ/mol) at temperatures above the Tg's of the corresponding NBD polymers.