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
(BLc) with high yield and high selectivity, whereas cyclic
valero- (VLc) and caprolactone (CLc) 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.