Iron and titanium amino-phenolate complexes in controlled alkene polymerisation
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Date
28/11/2019Author
Coward, Daniel Luke
Metadata
Abstract
Transition metals have long been used as both catalysts and mediators in the polymerisation
of many monomers. This thesis explores the use of iron and titanium amino-phenolate
complexes as mediators in the polymerisation of alkene monomers.
Initial work focused on understanding the fundamental mechanisms behind the
organometallic-mediated radical polymerisation (OMRP) of styrene, methyl methacrylate
(MMA) and vinyl acetate using a novel and fully characterised tert-butyl substituted aminebis(phenolate) iron(II) complex. A range of temperatures and conditions were explored to
elucidate the equilibrium between propagation and termination reactions in the
polymerisation. It was found that in the polymerisation of MMA, propagation was favoured
at low conversion with good control and reasonable dispersities. Mechanistic studies suggest
that propagation proceeds through a reversible-termination OMRP mechanism. At higher
conversions, irreversible termination reactions become dominant. The polymerisation
temperature significantly affects the nature of termination, dictating whether termination is
either bimolecular or via catalytic chain transfer (CCT). The polymerisation of styrene shows
well-controlled behaviour with dispersities as low as 1.27, which is the first time this has been
achieved for iron-mediated OMRP. The use of alternative initiation methods, such as
macroinitiators and photoinitiators, is also discussed.
A family of titanium(III) amino-phenolate complexes were used as mediators in the
polymerisation of methacrylates. Well-controlled polymerisations were achieved, with linear
first-order kinetics and dispersities as low as 1.09. The nature of the substituents on the
ligand greatly affects the tacticity of the resultant polymer, with large bulky groups having a
more significant effect in promoting isotactic polymer. Detailed experimental and
computational studies suggest that the polymerisation mechanism is not radical or ionic, but
instead proceeds through a coordinating-type mechanism. This mechanism is suggested to
be bimetallic, involving a titanium(IV)-enolate complex and an MMA-coordinated
titanium(III) species, with polymerisation propagating via a group transfer mechanism, which
is rarely exhibited in transition metals. This also likely represents the first example of the
initiation of a coordination polymerisation with a conventional azo initiator, without the need
for pyrophoric or expensive activators.