Phenoxy imine NOON complexes of heavy alkaline earth ions for the ring-opening polymerisation of cyclic esters

Abstract

The polymerisation of lactide monomers by heavier alkaline earth complexes supported by a phenoxy-imine NOON ligand, ^DippLM(thf)_x (^DippL = [2,7-(CNDipp)-1,8-O-C₁₀H₄]²⁻ where Dipp = 2,6-ⁱPr-C₆H₃; M = Ca (1), Sr (2) and Ba (3)) is described. Complexes 1–3 all proved active initiators for the ROP of LA converting over 500 eq. at ambient temperatures. The dependency on monomer concentration was found to vary depending on the alkaline-earth metal used; calcium-based initiator 1 exhibited first-order dependency with respect to monomer in all cases, while there was a second order dependancy for the heavier strontium analogues. Addition of benzyl alcohol as a co-initiator resulted in an ∼8-fold and ∼6-fold increase in the observed rate for 1 and 2 respectively. In these systems, presence of the expected –OCH₂Ph/–OH end groups, formed via the activated monomer pathway, was confirmed by ¹H and ¹³C{¹H} NMR spectra of the isolated polymers. Further investigations with complex 2 suggested that without addition of co-initiator, the ROP could proceed via an anionic mechanism in which the initiating step would be monomer deprotonation. In all cases, poor polymerisation control was observed. GPC analysis of the polymers indicated the presence of different initiating species, which may be due to the complex speciation of complexes 1–3 or the formation of a mixture of linear and cyclic PLA.