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.