Mechanism-Inspired Design of Bifunctional Catalysts for the Alternating Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides

Abstract

Advances in catalysis have enabled the ringopening copolymerization of epoxides and cyclic anhydrides to afford structurally and functionally diverse polyesters with controlled molecular weights and dispersities. However, the most common systems employ binary catalyst/cocatalyst pairs which suffer from slow polymerization rates at low loadings. Inspired by new mechanistic insight into the function of binary metal salen/nucleophilic cocatalyst systems at low concentrations, we report a bifunctional complex in which the salen catalyst and an aminocyclopropenium cocatalyst are covalently tethered. A modular ligand design circumvents the extended linear syntheses typical of bifunctional catalysts, enabling systematic variation to understand and enhance catalytic activity. The optimized bifunctional aluminum salen catalyst maintains excellent activity for the ring-opening copolymerization of epoxides and cyclic anhydrides at low concentrations (≥0.025 mol %), and the aminocyclopropenium cocatalyst suppresses undesirable transesterification and epimerization side reactions, preserving the integrity of the polymer backbone.

Heterocycle/Heteroallene Ring‐Opening Copolymerization: Selective Catalysis Delivering Alternating Copolymers

“…Coates and co-workers apply catalyst design to minimize transesterification side-reactions. [78][79][80] Tolman, Coates and co-workers reported an excellent ROCOP mechanistic investigations using Al(III) salen/PPNCl bicomponent catalysts. [78] A bis(carboxylate) aluminate resting state was proposed in the initial stages of the catalysis, even when a large excess of epoxide was present.…”

“…The rate determining step was proposed as epoxide insertion into the aluminium-carboxylate intermediate producing a for a bicomponent catalyst). [79] These properly designed tethered catalysts showed much less transesterification, epimerisation and chain-end coupling reactions than bicomponent analogues. The performances were rationalised by control over metallate equilibria avoiding formation of free alkoxide chains.…”

Triblock polyester thermoplastic elastomers with semi-aromatic polymer end blocks by ring-opening copolymerization

“…This catalyst performance is at the upper end in ROCOP, particularly given that most catalysts operate effectively only when exposed to <200 equivalents of anhydride. 10,41,42,[75][76][77] To assess the polymerization kinetics, a reaction was conducted with regular removal of aliquots which were analyzed by 1 H NMR spectroscopy to determine conversion with time.…”

Heterodinuclear catalysts Zn(ii)/M and Mg(ii)/M, where M = Na(i), Ca(ii) or Cd(ii), for phthalic anhydride/cyclohexene oxide ring opening copolymerisation

“…24 Several adaptations have been reported to improve activity, notably by the groups of Lee and Coates, in which the nucleophilic co-catalyst is appended to the phenol or diamine backbone moieties, respectively ( . 33,34 Nonetheless such tethered co-catalyst systems require lengthy and sometimes difficult syntheses.…”