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Title: Computational mechanistic and stereochemical studies of single-site polymerisation catalysts and reactions
Author: Allan, Charlotte Sarah Moreis
ISNI:       0000 0004 2694 0164
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2009
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Computational investigations into four metal-centred polymerisation catalysts are presented. The work investigates how and why the catalysts behave as they do, focusing on specific interactions within the catalyst structure itself and also on the transition states involved in the polymerisation reactions. Density functional theory has been used to examine the effect of the metal, the role of the ligand and the interplay between the two. Each study addresses particular mechanistic and structural questions that have been raised during experimental investigations and that are difficult to answer experimentally. Chapter one provides a general overview of computational techniques used in chemical modelling. The specific methods used in this work are presented as well as a brief review of modern trends. Chapter two investigates an unusual pair of metal-hydrogen interactions in a tin bis(triazenide) complex. We have termed this double M-H interaction “bifurcated” and compared other systems in which this interaction is present (and often unidentified). A variety of computational techniques are used to analyse the nature of the interactions both in qualitative and quantitative terms. The third chapter investigates the mechanism of alkyl transfer in a magnesium bis(imino)pyridine complex. A number of mechanistic pathways are explored to explain the original report of non-electrophilic alkylation at a pyridine nitrogen. We consider in particular how the solvent and the role of other species in the reaction mixture may influence the mechanism. Chapter four describes the inversion of configuration occurring in a pseudo-C3- symmetric zirconium tris(phenolate) complex. Variable temperature NMR spectra and simulations complement DFT calculations to explore the mechanism of inversion. We question the long-held assumption that the inversion process is concerted. In chapter five, the polymerisation of rac-lactide by an aluminium salen-type system is investigated in detail through characterisation of the transition state structures. Specifically, we have aimed to explain the different behaviour of two structurally similar catalytic species’ which produce polymer of different tacticity. Application of a variety of additional computational techniques in a number of these studies supplements the density functional calculations. They provide insight into specific interactions in both starting materials and transition states and detailed information about the reaction mechanisms.
Supervisor: Rzepa, Henry Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral