Zwitterionic Ring-Opening Polymerization of N-Substituted Eight-Membered Cyclic Carbonates to Generate Cyclic Poly(carbonate)s.

The zwitterionic ring-opening polymerization of N-functionalized eight-membered cyclic carbonates with N-heterocyclic carbenes (NHC) in the absence of alcohol initiators generates cyclic polycarbonates of Mn ∼ 30-100 kDa. The polymerization behavior of these eight-membered cyclic azacarbonates depends sensitively on the nature of the nitrogen substituent. The N-benzyl-substituted eight-membered cyclic carbonate (8CCBn) polymerizes readily with 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene to generate cyclic polycarbonates with molecular weights of Mn = 14 000 to 96 000 Da. In contrast, the N-phenyl-substituted cyclic carbonate (8CCPh) catalytically dimerizes in the presence of the NHC to afford the crystalline cyclic dimer. The zwitterionic ring-opening copolymerization of δ-valerolactone (VL) and the cyclic carbonates afford gradient cyclic copolymers. The cyclic topology of both the homopolymers and copolymers was supported by MALDI-TOF MS and intrinsic viscosity measurements. 13C NMR and differential scanning calorimetry of the cyclic copolymers are indicative of a gradient sequence distribution as a consequence of the more rapid enchainment of the cyclic carbonates relative to valerolactone.