Lactide Polymerisation with Air-Stable and Highly Active Zinc Complexes with Guanidine–Pyridine Hybrid Ligands

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Abstract

New class of air-stable catalysts for lactide polymerisation: Guanidine–pyridine hybrid ligands (picture, left) were used to prepare a series of zinc complexes (e.g., depicted cation [ZnL2(CF3SO3)]+, where L is the quinoline-containing ligand with R1=R2=R3=R4=Me), in which the ligand binds through two different N-donor atoms. The zinc complexes show high activity in ring-opening polymerisation of d,l-lactide (right), giving polylactide with molecular masses up to 176 000 g mol−1 and in high yield.

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The synthesis of zinc complexes of guanidine–pyridine hybrid ligands [Zn(DMEGpy)Cl2] (C1), [Zn(TMGpy)Cl2] (C2), [Zn(DMEGqu)Cl2] (C3), [Zn(TMGqu)Cl2] (C4), [Zn(DMEGpy)(CH3COO)2] (C5), [Zn(TMGpy)(CH3COO)2] (C6), [Zn(DMEGqu)(CH3COO)2] (C7), [Zn(TMGqu)(CH3COO)2] (C8), [Zn(DMEGqu)2(CF3SO3)][CF3SO3] (C9) and [Zn(TMGqu)2(CF3SO3)][CF3SO3] (C10) is reported. These zinc complexes were completely characterised and screened regarding their activity in the ring-opening polymerisation of d,l-lactide. They proved to be active initiators in lactide bulk polymerisation, and polylactides with molecular weights (Mw) up to 176 000 g mol−1 could be obtained. They combine high activity with robustness towards moisture and air. The influence of reaction temperature and of the anionic component of the zinc salt on the activity of the catalyst, as well as the occurrence of undesired side reactions, was investigated. By correlating these findings with the structural study on the zinc complexes we could deduce a structure–reactivity relationship for the zinc catalysts. This study was accompanied by DFT calculations. The bis-chelate triflate complexes C9 and C10, supported by quinoline–guanidine ligands L3 and L4, exhibit by far the highest reactivity. Systematic comparison of these complexes with their mono-chelate counterparts and their bis-guanidine analogues allows the attributes that promote polymerisation by neutral guanidine ligand systems to be elucidated: accessibility to the zinc centre and Lewis acidity.

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