Homo- to Co-Polyesters Conversion in the Absence of Reagents Facilitated by Hydrogen Transfer Catalysis

A ruthenium-catalyzed hydrogen transfer ester metathesis (HTEM) is reported that allows for the transfer of a linear polyester such as polycaprolactone (PCL) without the need for any stoichiometric reagent into a novel type of co-polyester, consisting of an additional hexylene adipate repeating unit. For PCL and polyhexylene adipate (PHA) as starting materials, the reaction yields polyesters of almost identical PHA/PCL ratios after elongated reaction times, which is indicative for a thermodynamic minimum ratio between the two polyesters. Mechanistic investigations show that the formation of the chemically modified polyester relies on two catalytic reactions, a hydrogen transfer ester metathesis and a transesterification catalyzed by the base co-catalyst. Evidence is provided that the hydrogen transfer ester metathesis proceeds via a reversible aldehyde formation.