Zn(II) chlorido complexes with novel aliphatic, chiral bisguanidine ligands as catalysts in the ring-opening polymerisation of rac-lactide using FT-IR spectroscopy in bulk

Eight new zinc complexes of bisguanidine ligands have been structurally characterised and tested for the polymerisation of lactide. Initially this necessitated the preparation of the new six bisguanidine ligands (TMG2thf, DMEG2thf, trans-TMG2(1,2)ch, trans-DMEG2(1,2)ch, R,R-TMG2(1,2)ch, R,R-DMEG2(1,2)ch). With these ligands in-hand zinc chlorido complexes could be obtained, which were characterized by X-ray crystallography and NMR spectroscopy. Furthermore, two new zinc chlorido complexes are reported, based on previous bisguanidine ligands ((1,3)TMG2ch, (1,3)DMEG2ch). All complexes show a distorted tetrahedral coordination geometry. These eight complexes are utilised as catalysts in melt polymerization of unsublimed, technical rac-lactide at 150°C. The most promising complexes were tested with different purities of lactide (technical and recrystallized) and with or without the addition of a co-initiator. The conversion was determined via 1H-NMR or FT-IR spectroscopy. Kinetic measurements show first-order behaviour with respect to lactide. The end group of the polymer was determined by MALDI-ToF measurements. Moreover, the impact of chiral complexes was examined with regard to the tacticity of the polymer. Complexes containing a thf-backbone show the highest polymerisation activity with recrystallized rac-lactide and benzyl alcohol as co-initiator.