Hydrogen Borrowing Catalysis for the Modification, Depolymerization, and Synthesis of Polyesters

A ruthenium-catalyzed hydrogen transfer ester metathesis (HTEM) is reported that allows for the isomerization of different types of linear polyesters such as polylactones without the need for any stoichiometric reagent, forming novel types of copolyesters containing additional alkylene dicarboxylate (AD) repeating units. Depending on the thermodynamic boundary conditions given by the polyester, the cocatalyst, and the potential resting state, the reported HTEM reaction equilibrates toward a thermodynamic minimum, which is otherwise not accessible by a regular transesterification or hydrolysis. It is for example demonstrated that poly(butylene succinate) (PBS) is depolymerized to cyclic butyrolactone (BL_c) with high yield and high selectivity, whereas cyclic valero- (VL_c) and caprolactone (CL_c) can be polymerized by HTEM to poly(lactone-alkylene dicarboxylate) copolyesters. Mechanistic investigations show that the formation of the chemically modified polyester relies on a two-fold catalytic reaction: a HTEM via a hydrogen borrowing process and a concomitant transesterification catalyzed by the base cocatalyst. Evidence is provided that hydrogen transfer ester metathesis proceeds via a reversible aldehyde formation. The described HTEM represents an unprecedented, catalyzed hydrogen borrowing process within polymers and bears significant importance regarding a dynamic postsynthetic modification of polyesters.