Synthesis of polymeric materials derived from abundant natural feedstocks using non-toxic heavy metals

Thesis

The aims of this Thesis were: firstly, to develop new di(amido)siloxane complexes using relatively non-toxic bismuth and scandium metals, and secondly, to examine the activity of the bismuth compounds in ring-opening polymerisation (ROP) and small molecule activation reactions.

This Thesis begins with an introduction to bio-derived polylactide (PLA) produced via the ROP of lactide. An overview of previously reported main group and rare-earth ROP initiators is provided, presenting challenges in the development of systems that combine high activity, selectivity, robustness, and low toxicity. Bismuth compounds are highlighted as they offer favourable characteristics as heavy metal ROP initiators, while demonstrating remarkably low toxicity.

The synthesis and characterisation of a new family of di(amido)siloxane bismuth complexes, (NON^R)BiX, is presented. The ligand scaffold was explored through the incorporation of different substitution patterns; the tunable sterics and electronics of the complexes was found to directly influence their ROP initiating capability.

The ROP of lactide was studied using aryloxide/alkoxide initiators, proceeding with induction periods (< 4 hours) and moderate control (Ɖ_M = 1.33–1.85). A bismuth amide analogue resulted in faster kinetics (four times faster towards L-lactide than its aryloxide counterpart), good polymerisation control (Ɖ_M = 1.36) as well as high heteroselectivity for the ROP of meso-lactide (P_m = 0.86).

The amide initiator was shown to readily insert a single cyclic ester unit into the Bi-N bond. The resulting insertion products were proposed to act as propagating species in ROP reactions, representing rare examples of spectroscopically and/or structurally characterised metal complexes with ligands derived from ring-opened cyclic esters.

The reactivity of the bismuth amide was also explored towards small molecules. Facile addition of C-O, C-S, and C-N bonds across the Bi-N bond was observed for electrophilic, unhindered heteroallenes. Conversely, the insertion reaction proved reversible using a less electrophilic, more sterically bulky ketone, 2-benzoylpyridine.

The ligand scaffold was also used to incorporate scandium, affording the first examples of di(amido)siloxane scandium complexes. Their versatile reactivity was studied in salt-metathesis reactions, yielding both “ate” and neutral compounds. The conformational flexibility of the ligand was discussed in relation with varying steric demands, and the bonding environment was contrasted with related bismuth structures, which contain a +3 metal ion with a considerable size difference (r_Sc3+ = 0.745, r_Bi3+ = 1.03 Å).

Authors: Navickaite, G

• DOI: 10.5287/ora-1reerd7n0
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English