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

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 epsilon-caprolactone towards the synthesis of cyclic polymers. Various analytical methods, including multinuclear (H-1, C-13, N-15, Li-7) 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.