Post-polymerization modification of unsaturated polyesters by Michael addition of N-heterocyclic carbenes

Abstract

The post-polymerization modification of unsaturated polyesters, three polymaleates and a polyfumarate, was performed by the Michael addition of N-heterocyclic carbenes (NHCs) to produce thermally stable unsaturated polyesters. The model reactions of NHCs, 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT), and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp), with dibutyl fumarate and dibutyl maleate selectively proceeded to give the corresponding Michael adducts, whereas 1,3-di-tert-butylimidazol-2-ylidene (ItBu) exclusively catalyzed the isomerization of dibutyl maleate. Based on these results, reactions of the unsaturated polyesters with TPT and IDipp were investigated. The polyesters bearing NHCs as side chains were obtained in good-to-high yields. The number of NHC-incorporated units can be controlled by the amount of added NHCs, and the obtained polyesters showed better solubility in common organic solvents. It is noteworthy that the glass transition temperatures (Tgs) were widely tunable and increased linearly with increasing NHC content because of the stiff and bulky structure. In addition, reactions of the unsaturated polyesters with TPT in the presence of water allowed Michael addition and transfer hydrogenation to occur simultaneously. Radical cross-linking of the NHC-incorporated unsaturated polyesters with styrene produced thermally stable and fully miscible polyester resins. The Michael addition of N-heterocyclic carbenes (NHCs) to the unsaturated polyesters, polymaleates and a polyfumarate, produced the polymers bearing the NHCs as side chains in good-to-high yields. The glass transition temperatures of the obtained polymers were widely tunable and increased linearly with the increasing NHC content. The stiff and bulky structures of the NHCs can reduce the mobility of the polymer main chain. The simultaneous Michael addition and transfer hydrogenation of the unsaturated polyesters have also been performed by the reaction with an NHC in the presence of water.