New catalysts bearing chelate ligands for ring opening polymerization studies.

Zhang, Xin

Abstract

Chapter 1: The accumulation of traditional polymer (plastic) pollution has led people to start looking for biodegradable plastics as alternatives. This first chapter provides information on the concept and the development of biodegradable polymers. It also highlights the development of efficient catalysts based on Sn/Al/Ti/Zn metals for producing biodegradable aliphatic polyester and the key ring-opening polymerization mechanisms, including coordination-insertion, cationic, anionic mechanisms. Moreover, this chapter delivers a comprehensive review of metal complexes bearing 2,2’-diphenylglycine or benzilic acid ligands, with particular focus on the binding modes of Ph2C(X)(CO2H) (X = NH2, OH) in these complexes. These complexes’ application as catalysts for the ring-opening polymerisation of cyclic esters is summarised. Lastly, the characterisation methods used in this work are discussed.
Chapter 2: Here, the Schiff-base compounds 2,4-di-tert-butyl-6-(((3,4,5-trimethoxyphenyl)imino)methyl)phenol (L1H), 2,4-di-tert-butyl-6-(((2,4,6-trimethoxyphenyl)imino)methyl)phenol (L2H), 2,4-di-tert-butyl-6-(((2,4-dimethoxyphenyl)imino)methyl)phenol) (L3H) derived from anilines bearing methoxy substituents have been employed in the preparation of alkylaluminium and zinc complexes. Molecular structure determinations reveal mono-chelate aluminium complexes of the type [Al(Ln)(Me)2] (L1, 1; L2, 2; L3, 3), and bis(chelate) complexes for zinc, namely [Zn(Ln)2] (L1, 5; L2, 6; L3, 7). All complexes have significant activity at 50 C and higher activity at 100 C for the ring-opening polymerisation (ROP) of -caprolactone (-CL) with good control over the molar mass distribution (Mw/Mn < 2) and molecular weight. Complex 1 was found to be the most active catalyst, achieving 99% conversion after 18 h at 50 C and giving polycaprolactone with high molecular weight; results are compared against aniline-derived (i.e. non-methoxy containing) complexes (4 and 8). Aluminium or zinc complexes derived from L1 exhibit higher activity as compared with complexes derived from L2 and L3. Complex 1 was also tested as an initiator for the copolymerisation of -CL and glycolide (GL). The CL-GL copolymers have various microstructures depending on the feed ratio. The crosslinker 4,4’-bioxepane-7,7’-dione was used in the polymerisation with -CL using 1, and well-defined cross-linked PCL was afforded high molecular weight.
Chapter 3: This chapter focuses on niobium and tantalum complexes. In particular, reaction of benzilic acid, Ph2C(OH)(CO2H) (L4H2), with equimolar amounts of M(OR)5 (M = Nb, Ta) led, following work-up, to the tetranuclear complexes [Nb4(OEt)8(L4)4(-O)2] (9) or [Ta4(OEt)8(L4)4(-O)2]∙0.5MeCN (10∙0.5MeCN), respectively. Similar use of 2,2’-diphenylglycine (L5H3), Ph2C(NH2)(CO2H) (L5H3), led to the isolation of the dinuclear complexes [Nb2(OEt)4(L5H2)4(-O)]∙2MeCN (11∙2MeCN) or [Ta2(OEt)4(L5H2)4(-O)]∙2.25MeCN (12∙2.25MeCN). The molecular structures of complexes 9-12 are reported. These complexes have been screened for their potential to act as catalysts for the ROP of -CL and rac-lactide (r-LA), with or without benzyl alcohol (BnOH) present. In the case of -CL, complex 9 displayed the best activity with >99 % conversion at 100 C, whilst 10 and 11 were inactive under the same conditions. All complexes show moderate activities towards the ROP of r-LA at 160 C, with 9-11 producing heterotactic enriched PLA while 12 afforded isotactic enriched PLA.
Chapter 4: The reactions of the titanium alkoxides [Ti(OR)4] (R = Me, nPr, iPr, tBu) with the L4H2 or L5H3 have been investigated. Variation of the reaction stoichiometry allows for the isolation of mono-, bi-, tri or tetra-metallic products, the structures of which have been determined by X-ray crystallography. The ability of the resulting complexes to act as catalysts for the ROP of -CL and r-LA has been investigated. In the case of -CL, all catalysts except that derived from [Ti(OnPr)4] and L5H3 i.e. 19, exhibited an induction period of between 60 and 285 min., with 19 exhibiting the best performance (>99% conversion within 6 min.). The PCL products are moderate to high molecular weight polymers. For r-LA, there was no induction period, and systems 13, 15, 16 and 19 afforded conversions of ca. 90% or more, with 16 exhibiting the fastest kinetics. The molecular weights for the PLA are somewhat higher than those of the PCL, with both cyclic and linear PLA products (end groups of OR/OH) identified. Comparative studies versus the [Ti(OR)4] starting materials were conducted and although high conversions were achieved, the control was poor.
Chapter 5: The better performing catalysts from chapters 2-4 are selected and compared with other reported catalysts with similar ligand systems. The evaluation of these catalysts is based on the ROP of -CL. Various factors, including polymerisation rate, molecular weight and distribution of polymer products, catalyst efficiency, and catalyst stability have been considered.

Citation

Zhang, X. New catalysts bearing chelate ligands for ring opening polymerization studies. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4443364

• Thesis Type: Thesis
• Deposit Date: Nov 14, 2023
• Publicly Available Date: Nov 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4443364
• Additional Information: Department of Chemistry; University of Hull
• Award Date: Sep 19, 2023