Hydrogen-bonding and resonance stabilisation effects in cationic bis(iminium) phenoxide diacids

Journal article

Plastics have become essential to 21st century human life; it is therefore unrealistic to reduce the usage of such single-life polymers enough to offset their hazardous environmental impacts. This has prompted research into the development of greener, non fossil-fuel based alternatives. Polylactide (PLA) is one such material; it is a biorenewable, biocompatible, aliphatic polyester that is most efficiently produced via the ring-opening polymerisation (ROP) of lactide (LA). The use of alkaline-earth metals, particularly the heavier congeners, in catalysts for this process has been understudied even though they offer advantages like high activity, low toxicity, low cost and lack of colour. The aims of this Thesis were therefore: (i) to prepare new alkaline-earth-based bimetallic, bis(phenoxy-imine) systems, (ii) to test their performance as initiators in LA ROP and (iii) to evaluate the benefits of bimetallic complexes/cooperativity for this type of polymerisation. The Thesis begins by offering a brief introduction to lactide as a biorenewable monomer in ring-opening polymerisation. The general ROP mechanisms are presented alongside a discussion of polymer characterisation techniques. An overview of previously reported catalysts for the ring-opening polymerisation of lactide is also described. A series of alkaline-earth complexes based on the “NOON” motif (H2DippL; 2,7-(HC=NDipp)-1,8-OH-C10H4) were synthesised and fully characterised. These compounds were applied towards the ROP of L-, D-, rac- and meso-LA; a variety of polymerisation conditions (temperature, monomer stereochemistry and initiator concentration) were studied to determine the kinetics, mechanism and overall rate equation of the polymerisation. It was found that all of the prepared initiators were active however, they either suffered from low activity (kobs = 0.0046–0.091 h–1 (Mg)) or poor polymerisation control (1.23 Related alkaline-earth “NON” (HR,DippL; 1-OH-2,6-(HC=NDipp)-4-R-C6H2 where R = H, Me, tBu) systems were then prepared and fully characterised in attempts to improve polymerisation control and activity. The homo vs. heteroleptic nature of these complexes was found to differ between metals as a result of varying steric shielding and bonding lability. In most cases, these monometallic species showed improved polymerisation activity (kp = 966 vs. 3.19 M–1 h–1 (Mg)) and control over their “NOON” counterparts. A variety of techniques were used to characterise the resultant polymers in order to deduce tacticity (Pr = 0.48-0.67), molecular weights and chain end groups. The “NON” framework was then explored, via several synthetic routes, for the formation of cationic, bimetallic alkaline-earth complexes. The generation of protonated proligands which could then be utilised for the insertion of multiple metals (Zn/Ca) by protonolysis was found to be a promising protocol

Authors: Jones, RL; Morris, LJ; Collins Rice, CG; Turner, ZR; O'Hare, D

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Royal Society of Chemistry
• Journal: Physical Chemistry Chemical Physics
• Volume: 25
• Issue: 22
• Pages: 15463-15468
• Publication date: 2023-06-07
• DOI: 10.1039/d3cp01881d
• EISSN: 1463-9084
• ISSN: 1463-9076
• Language: English
• Keywords: Organic chemistry; Acetonitrile; Hydrogen bond; Cationic polymerization; Molecule; Protonation; Catalysis; Medicinal chemistry; Stereochemistry; Ion; Tautomer; Resonance (particle physics); Iminium; Crystallography; Chemistry; Polymer chemistry; Nuclear magnetic resonance spectroscopy