Substituent effect on cationic ring-opening polymerization behavior of seven-membered cyclic carbonates

Abstract

The syntheses and cationic ring-opening polymerizations of the seven-membered cyclic carbonates having phenyl and methyl groups were carried out, and the substituent effect on the polymerization behavior was examined. The cyclic carbonate (2) carrying a phenyl group in position 4 was transformed into the corresponding polycarbonate containing a small amount of a polyether unit by an accompanying elimination of 2-phenyltetrahydrofuran (5) by using TfOMe and TfOEt as initiators. When TfOH and SnCl4 were used as the initiators, the main product was not the polymer but compound 5. It was confirmed that the unit sequence of poly(2) was completely controlled by the stabilization effect of the phenyl group on the propagating cation. Namely, the cationic polymerization of 2 proceeded via the selective cleavage of the bond between the benzyl carbon and the ether oxygen with forming a stable benzyl cation. Using cyclic carbonates carrying a phenyl group in position-5 (3) and a methyl group in position-4 (4) lead to the corresponding polycarbonates in good yields without elimination of tetrahydrofuran derivatives under any conditions. In these cases, the unit sequences could not be controlled. We have demonstrated that the cationic polymerization behavior of seven-membered cyclic carbonates is dramatically affected by the substituents and their positions.