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Title: Novel phosphasalen ligands for the ring-opening polymerisation of lactones
Author: Coleman, Charlotte
ISNI:       0000 0004 6501 0461
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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Bio-derived polymers offer a sustainable alternative to petroleum-derived polymers. One such polymer, polylactide (PLA) is synthesised from the ring-opening polymerisation (ROP) of rac-lactide (rac-LA). The monomer contains two stereocentres which gives a range of tacticities. The isotactic ROP of rac-LA yields the most desirable polymer properties (physical and mechanical properties) and is quantified using a Pi (probability of isotactic enchainment) value. This thesis investigates tri- and tetradentate ligands coordinated to zinc(II), aluminium(III) and indium(III) to understand the ligand effects on both rate and isoselectivity. The first results chapter, Chapter 3, outlines the synthesis of a tetradentate salen ligand, its analogous phosphasalen ligand, where the imine bonds are replaced with iminophosphorane bonds and a hybrid ligand containing a mixture of both imine and iminophosphorane bonds. The salen, phosphasalen and hybrid ligands are coordinated to both aluminium and indium and used as initiators for the ROP. The indium hybrid initiator yields the highest isoselectivity (Pi = 0.71) which is significantly higher than its salen and phosphasalen analogues (Pi = 0.51 - 0.52) Chapter 4 investigates structure-activity relationships of the indium hybrid initiator, by modification of the ligand scaffold. Bulkier ortho-phenolate substituents do not affect rate or isoselectivity. Electron-withdrawing chloride substituents increase rate but decrease isoselectivity (Pi = 0.61). Modification of the phosphorus substituents yields the highest isoselectivity value (Pi = 0.75). It may be hypothesised that isoselectivity depends upon steric hindrance at both ortho-substituents and at phosphorus. Furthermore, the synthesis of a ferrocene containing indium hybrid complex is developed for future use in redox-switch catalysis. Chapter 5 describes the synthesis of two tridentate half-phosphasalen ligands and their coordination chemistry with zinc(II) and indium(III). Zinc initiators are active but display poor polymerisation control and low isoselectivity values (Pi = 0.67). Zinc complexes, with a less sterically encumbered ligand, preferentially form a homoleptic complex, which cannot be used in ROP. Indium initiators are slower than analogous half Schiff base initiators reported in the literature but show an improved isoselectivity value (Pi = 0.72).
Supervisor: Williams, Charlotte Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Inorganic Chemistry ; Catalysis