Polymetallic Zr catalysts for the ROP of lactide

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"This is a PhD research project in Chemistry.Single-site, Lewis acidic metal centers are often applied to the ring-opening polymerisation (ROP) of lactide. These are well defined initiators that aim to provide high levels of activity along with control over the polymerisation process. While these initiators show high activity towards the ROP of lactide, some can lack stability outside of the glovebox. Polynuclear or aggregated initiators generally have improved stability but structure-activity relationships are more challenging to establish for these species due to complications associated with defining their nuclearity in solution.In two recent publications1,2 we have shown that polynuclear Ti complexes, bridged by amine bis(phenolate) ligands (Fig 1), are active in the ROP of lactide. The complexes have several advantages over traditional, single site initiators as they are stable to air and moisture, and behave as true catalysts.We now propose to further develop this chemistry by incorporation of Zr into the complex (either exclusively or in tandem with Ti). Zr is well established to be a more active than Ti for the ROP of lactide and exhibits greater stereoselectivity. Zr complexes are generally more susceptible to degrading in the presence of air/moisture so these supramolecular complexes are ideally placed to develop a bench stable but highly active catalyst for the ROP of lactideInitially the student will create several amine bis(phenol) pro-ligands using literature procedures (or minor modifications thereof). We are currently focussing on incorporating amino acids into the ligand framework. These new compounds will provide us with a range of chiral ligands via readily accessible starting materials. Once prepared we will attempt coordination to Zr complexes, once we have a reliable procedure we will attempt to form polymetallic complexes via the formation of oxo bridges (using literature precedent) or carboxylate bridges (from established precedent within the group). The complexes will be characterised by the student using spectroscopic methods (NMR, IR, UV-vis), mass spectrometry, elemental analysis and single crystal x-ray diffraction. Once the complexes have been made we will trial these in the ROP of lactide to ascertain their activity. Polymers will be characterised via NMR and gel permeation chromatography (GPC).