Copolymerization of cyclic esters, epoxides and anhydrides: Evidence of the dual role of the monomers in the reaction mixture

A bimetallic salen aluminum complex (1), where salen is N,N-bis(4,6-di-tert-butyl salicylaldimine)-1,3-propylendiamine, was tested as a catalyst for the ring opening polymerization (ROP) and copolymerization (ROCOP) of cyclic esters, such as l-lactide (L-LA), ϵ-caprolactone (ϵ-CL), and β-butyrolactone (β-BL) with different heterocyclic substrates, such as cyclohexene oxide (CHO) and succinic anhydride (SA). Copolymers ranging from gradient to blocky structures were obtained from the copolymerization of the different couples of lactones. In the copolymerization of CHO with a cyclic ester, block polyether-co-polyesters were prepared by sequential monomer addition while polyesters were obtained, as exclusive products, when both monomers were simultaneously present in the reaction medium. From a mixture of three different monomers: CHO, SA and a cyclic ester, block copolyesters derived from two different catalytic processes, ROCOP of CHO/SA followed by ROP of the lactone, were instead produced. Surprisingly, a perfect selectivity was observed in the CHO/SA copolymerization without any addition of a cocatalyst; this suggested that the lactone comonomer could act as an endogen cocatalyst.