Lithium anthraquinoids as catalysts in the ROP of lactide and caprolactone into cyclic polymers

Abstract

We describe herein the synthesis and characterization of three novel lithium complexes (2b–d) bearing different hydrazonyl anthraquinonic backbones, which are tested in the ring-opening polymerization (ROP) of lactide and ε-caprolactone towards the synthesis of cyclic polymers. Various analytical methods, including multinuclear (¹H, ¹³C, ¹⁵N, ⁷Li) one- and two-dimensional, homo- and heteronuclear nOe and PGSE diffusion NMR, high-resolution MS spectrometry and FTIR spectroscopy, are used to fully identify the structures of the catalysts. The resulting cyclic polymers were characterized by means of MALDI-TOF, end-group NMR analysis and PGSE diffusion NMR coupled to inverse Laplace algorithms to unravel the weight-average molecular weight and dispersity. In comparison to the previously reported complex (2a), a clear difference in selectivity (linear vs. cyclic) is observed when using ligand backbones with different coordinative abilities and/or steric restrictions.