Synthesis of cyclic polyesters: effects of alkoxy side chains in salicylaldiminato tin(ii) complexes

Abstract

A new class of salicylaldiminato tin(II) catalysts having different alkoxy side chains has been developed. The ligands were modified to have different lengths and flexibilities such as –(CH₂)₂– (2a), –(CH₂)₃– (2b), –(ortho-C₆H₄)CH₂– (2c) and –(CH₂)₂–O–(CH₂)₂– (2d). Complexes 2a, b were characterized crystallographically revealing a more constrained environment around the metal in complex 2a. These catalysts are active for the solvent-free polymerization of L-lactide and ε-caprolactone. Complex 2a having a shorter side chain was shown to better promote intramolecular transesterification affording cyclic polylactides and cyclic poly(ε-caprolactone). Complexes 2b and 2d having longer side chains produced cyclic poly(ε-caprolactone) as a major product but failed to give cyclic polylactides.