CO2-Based Polycarbonates through Ring-Opening Polymerization of Cyclic Carbonates Promoted by a NHC-Based Zinc Complex.

A backbone-substituted N-heterocyclic carbene (NHC) zinc complex, in combination with alcohol initiators, has been shown to be an effective catalyst for the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) to poly(trimethylene carbonate) (PTMC) devoid of oxetane linkages. The ROP of TMC proceeded in solution to give PTMC, possessing controlled molecular mass (2500 < Mn < 10000) and low dispersity (D ∼ 1.2). Changing the alcohol initiators, PTMCs with different end-groups were obtained, included a telechelic polymer. The results of MALDI-ToF and NMR analysis confirmed the controlled/living nature of the present ROP catalytic system, where side reactions, such as inter- and intramolecular transesterifications, were minimized during the polymerization. Solution studies in different solvents demonstrated the polymerization reaction to proceed via a mechanism first order in monomer and in catalyst. The zinc complex was also able to convert substituted cyclic carbonates, which were purposely synthesized from renewable feedstocks such as CO2 and 1,3-diols. For the asymmetric 2-Me TMC monomer, good regioselectivity was observed (Xreg up to 0.92). The excellent control of the polymerization process was finally brought to light through the preparation of polycarbonate/polyether triblock copolymers by using polyethylene glycol (PEG) as a macroinitiator and of well-defined di- and triblock polycarbonate/polylactide copolymers by sequential ROP of TMC and L-LA.