Synthesis, characterisation, and application of novel aluminium initiators for the ring-opening polymerisation of cyclic esters
File(s)
Author(s)
Gesslbauer, Sami
Type
Thesis
Abstract
The salen and salan ligands are ligand families which have been extensively studied in the field
of ring-opening polymerisation (ROP) catalysis. However, little is known about the behaviour
of their ortho-aminophenol analogue in which the nitrogen atoms are directly connected to the
aromatic moieties. This ligand scaffold (herein referred to as catam) has been previously
complexed with early and late transition metals. Interestingly, no aluminium complexes
bearing a catam ligand have been reported and little data exists on the use of catam complexes
in the ROP of cyclic esters. Of those, the monomer lactide (LA) has been garnering
considerable attention as it presents itself as a sustainable alternative to common polymers.
This thesis describes the preparation and characterisation of aluminium complexes bearing
catam ligands and their subsequent application in the ROP of rac-LA and other cyclic esters.
Even though aluminium complexes generally exhibit low activities in the polymerisation of
cyclic esters, many of the described aluminium catam complexes were found to be highly active
at room temperature with stereoselectivities ranging from slightly isoselective to highly
heteroselective, placing them among the most active Al catalysts to date. Chapter 2 focusses
on the catam ligands tethered by either 2,2-dimethyl-1,3-propanediamine or 1,2-ethanediamine
and the synthesis and characterisation of their corresponding Al complexes. The NH moieties
of the ligand were found to form beneficial non-covalent interactions with the substrate, which
is thought to be at the root of the observed activity.
Seeking an improvement in isoselectivity, the synthesis and application of chiral aluminium
complexes bearing a chiral catam ligand is detailed in Chapter 3. Neither the enantiopure nor
the racemic catalysts afforded stereoregular polylactic acid. However, the enantiopure catalysts
were able to polymerise enantiopure LA with unprecedented activities. The lack of selectivity
in the polymerisation of rac-LA was traced back to polymeryl exchange events which lead to
quasi-random insertions of LA. Furthermore, polymerisation of an achiral monomer (ε-
caprolactone) was slower when the racemic catalyst was used which was traced back to
aggregation being favoured when both enantiomers of a catalyst are present.
Chapter 4 describes the synthesis of aluminium complexes bearing a phenylene-tethered catam
ligand. The corresponding Al chloro complexes was found to be redox-active. This feature was
exploited in ROP. The reduced state was able to polymerise rac-LA with good activities whilst
the oxidised state preferentially polymerised cyclohexene oxide. Using an oxidising (e.g.
FcBArF) and reducing agent (e.g. CoCp2), it was possible to readily switch the complex
between both states and selectively polymerise one monomer over the other.
of ring-opening polymerisation (ROP) catalysis. However, little is known about the behaviour
of their ortho-aminophenol analogue in which the nitrogen atoms are directly connected to the
aromatic moieties. This ligand scaffold (herein referred to as catam) has been previously
complexed with early and late transition metals. Interestingly, no aluminium complexes
bearing a catam ligand have been reported and little data exists on the use of catam complexes
in the ROP of cyclic esters. Of those, the monomer lactide (LA) has been garnering
considerable attention as it presents itself as a sustainable alternative to common polymers.
This thesis describes the preparation and characterisation of aluminium complexes bearing
catam ligands and their subsequent application in the ROP of rac-LA and other cyclic esters.
Even though aluminium complexes generally exhibit low activities in the polymerisation of
cyclic esters, many of the described aluminium catam complexes were found to be highly active
at room temperature with stereoselectivities ranging from slightly isoselective to highly
heteroselective, placing them among the most active Al catalysts to date. Chapter 2 focusses
on the catam ligands tethered by either 2,2-dimethyl-1,3-propanediamine or 1,2-ethanediamine
and the synthesis and characterisation of their corresponding Al complexes. The NH moieties
of the ligand were found to form beneficial non-covalent interactions with the substrate, which
is thought to be at the root of the observed activity.
Seeking an improvement in isoselectivity, the synthesis and application of chiral aluminium
complexes bearing a chiral catam ligand is detailed in Chapter 3. Neither the enantiopure nor
the racemic catalysts afforded stereoregular polylactic acid. However, the enantiopure catalysts
were able to polymerise enantiopure LA with unprecedented activities. The lack of selectivity
in the polymerisation of rac-LA was traced back to polymeryl exchange events which lead to
quasi-random insertions of LA. Furthermore, polymerisation of an achiral monomer (ε-
caprolactone) was slower when the racemic catalyst was used which was traced back to
aggregation being favoured when both enantiomers of a catalyst are present.
Chapter 4 describes the synthesis of aluminium complexes bearing a phenylene-tethered catam
ligand. The corresponding Al chloro complexes was found to be redox-active. This feature was
exploited in ROP. The reduced state was able to polymerise rac-LA with good activities whilst
the oxidised state preferentially polymerised cyclohexene oxide. Using an oxidising (e.g.
FcBArF) and reducing agent (e.g. CoCp2), it was possible to readily switch the complex
between both states and selectively polymerise one monomer over the other.
Version
Open Access
Date Issued
2021-04
Date Awarded
2021-08
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Romain, Charles
Britovsek, George
Publisher Department
Department of Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)