Abstract
Four dimeric lanthanide alkoxide complexes bearing ONNO bipyrrolidine salan ligands (LMeH2/LtBuH2) have been prepared with Nd, Sm and Yb. Depending on the metal and substituents, these complexes adopt varying coordination geometries. While investigating the hydrolytic degradation of these complexes, three dimeric mixed alkoxide/hydroxide and bis-hydroxide products were also prepared, isolated and characterised. Despite paramagnetism, 1H NMR and diffusion ordered spectroscopy (DOSY) allowed additional characterisation alongside elemental and single-crystal X-ray diffraction analyses. These systems were very active for the controlled ring-opening polymerisation (ROP) of rac-lactide (LA), under industrially relevant melt conditions and in solution, yielding complete conversion within 5 minutes at [Ln] : [LA] ratios of up to 3000 : 1 in toluene, and at 80 °C, whilst retaining low dispersities (Đ = 1.1). 1H DOSY NMR spectroscopy was employed to monitor polymer growth from the metal centres in situ, and revealed a dinuclear catalytically active species.
Highlights
With the dependence on fossil fuel resources and the environmental persistence of most commodity polymers, our societal reliance on plastics is unanimously viewed as unsustainable.[1]
1H NMR and diffusion ordered spectroscopy (DOSY) allowed additional characterisation alongside elemental and single-crystal X-ray diffraction analyses. These systems were very active for the controlled ring-opening polymerisation (ROP) of rac-lactide (LA), under industrially relevant melt conditions and in solution, yielding complete conversion within 5 minutes at [Ln] : [LA] ratios of up to 3000 : 1 in toluene, and at 80 °C, whilst retaining low dispersities (Đ = 1.1). 1H DOSY NMR spectroscopy was employed to monitor polymer growth from the metal centres in situ, and revealed a dinuclear catalytically active species
While all possible ligand/metal combinations were attempted, only [{LMeYb (OiPr)}2], [{LMeSm(OiPr)}2], [{LtBuSm(OiPr)}2], and [{LtBuNd (OiPr)}2] dialkoxide species could be isolated as crystals, with poor to very good yields (47, 51, 7 and 86% yield, respectively). These complexes were all characterised by elemental analysis and single crystal X-ray diffraction which are in agreement with the expected products (Scheme 2)
Summary
With the dependence on fossil fuel resources and the environmental persistence of most commodity polymers, our societal reliance on plastics is unanimously viewed as unsustainable.[1] Supported by public opinion, this has triggered a concerted research effort between academia and industry towards the development of sustainable alternatives from renewable resources.[2,3,4,5] Polylactide (PLA), a thermoplastic aliphatic polyester derived from lactic acid, is arguably one of the most widely studied degradable and renewable polymers. It is noteworthy that while subtle changes in ligands have been extensively shown to induce a change in stereoselectivity,[16,17,18,19,20] systematic studies across a range of metals ( period or series) are less common. Several examples have shown that the metal can significantly influence
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