Simple yttrium salts as highly active and controlled catalysts for the atom-efficient synthesis of high molecular weight polyesters

Controlled Catalysis Delivering High Molecular Weight Polyesters as Recyclable Alternatives to Polystyrenes

An organometallic Al(III)K(I) catalyst shows exceptional control in the epoxide/anhydride ring opening copolymerization (ROCOP), producing high molecular weight polyesters (Mn ∼ 100 kg·mol-1). The catalysis is highly effective using cyclohexene oxide, cyclopentane oxide, substituted cyclohexene oxide, and butylene oxide, each combined with phthalic anhydride. The polyesters show entanglement molecular weights, determined by oscillatory shear rheology, from 13 to 50 kg·mol-1 with cyclopentene and substituted cyclohexene moieties being particularly effective (highly entangled). The lead polyesters show high glass transition temperatures (94 °C < Tg < 137 °C), high tensile strengths (40 MPa < σ < 47 MPa) and tensile modulii (0.6 GPa < Ey < 0.9 GPa); their properties are similar to polystyrene. The polyesters are all recyclable by repeated cycles of compression molding, and show equivalently high thermal-mechanical performances even over repeated recycles.

From Polymers to Rings and Back Again: Chemical Recycling of Polyesters to Macrolactones

Cyclic anhydride and epoxide ring-opening copolymerization is a versatile and controlled route to make polyesters, gaining attention in different application sectors. But so far, the chemical recycling of these polyesters to cyclic monomers is under-explored. Here, the catalytic chemical recycling of aliphatic polyesters to selectively form 16- and 18-membered lactones is presented. The recycling reactions are catalyzed using commercial tin(II) octoate and conducted in the polymer melt (230 °C) resulting in high conversions to the macrolactones (>90%). The recycled macrolactones undergo catalyzed ring-opening polymerizations to produce polyesters with equivalent properties to the virgin materials.

Record-High-Molecular-Weight Polyesters from Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides Catalyzed by Hydrogen-Bond-Functionalized Imidazoles

Polyesters, with potential for degradability and sustainability, are some of the most versatile polymer materials. However, the limitation of molecular weight (MW) presents a barrier to their applications. The synthesis of polyesters with high MW by the ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is promising but rare and challenging. Herein, we report a series of air-stable, hydrogen-bond-functionalized imidazole catalysts for the copolymerization. These catalysts can produce polyesters (4 examples) using cyclohexane oxide (CHO), propylene oxide (PO), phenyl glycidyl ether (PGE), 4-vinyl-1-cyclohexene 1,2-epoxide (VCHO), and phthalic anhydride (PA) with record-high MW: Mn = 171.2 kDa for poly(CHO-alt-PA), Mn = 518.5 kDa for poly(PO-alt-PA), Mn = 100.5 kDa for poly(PGE-alt-PA), and Mn = 236.4 kDa for poly(VCHO-alt-PA). Furthermore, it can achieve an unprecedented efficiency of 15.6 kg of polyester/g of catalyst at a molar ratio of catalyst/PA/PO = 1:40000:60000. The record-high MW achieved can be attributed to the unique anionic-cationic coexisting ROCOP mechanism, which can reduce transesterification, chain transfer, and annulation side reactions. All high Mn polyesters showed excellent thermal stability, high tensile strength, and a Young's modulus comparable to some commodity thermoplastics like polystyrene and polylactic acid.

Effects of Lewis acidity and size of lanthanide salts for ring-opening copolymerization

Lanthanide chloride salts, with a cocatalyst, are used as a near "one-size-fits-all" catalyst system to perform the ring-opening copolymerization of diverse epoxides and cyclic anhydrides. Variation of the metal in the lanthanide series leads to subtle changes in selectivity for random, gradient, and block copolymers when monomer mixtures are used.

Simple bifunctional salts for synthesising block copolymers from anhydrides/epoxides and vinyl acetate

Herein, we report the first synthesis of poly(ester-block-vinyl acetate) via epoxide/anhydride ring-opening copolymerisation and reversible addition-fragmentation chain transfer polymerisation. This was achieved using simple, robust and bifunctional alkali metal carboxylates featuring a xanthate unit.

Choline Halide-Based Deep Eutectic Solvents as Biocompatible Catalysts for the Alternating Copolymerization of Epoxides and Cyclic Anhydrides

Aliphatic polyesters have received considerable attention in recent years due to their biodegradability and biocompatible, mechanical, and thermal properties that can make them a suitable alternative to today's commercialized polymers. The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is a route to synthesize a diverse array of polyesters that could be useful in many applications. However, the catalysts used rarely consider biocompatible catalysts in the case that any are left in the polymer. To the best of our knowledge, we report the first example of using deep eutectic solvents (DESs) as biocompatible catalysts for this target ROCOP with polymerization activity for at least six diverse monomer pairs. Choline halide salts are active for this polymerization, with dried salts showing polymerization slower than that of those conducted in air. Hydrogen bonding with water is hypothesized to enhance the rate-determining step of epoxide ring opening. While the presence of water improves the rate of polymerization, it also acts as a chain transfer agent, leading to smaller molar mass polymers than intended. Combining the choline halide salts with urea or ethylene glycol hydrogen bond donors in air led to DES catalysts that reacted similarly to the salts exposed to air. However, when generating these DESs in air-free conditions, they showed similar rates of polymerization without a drop in polymer molar mass. The hydrogen bonding provided by urea and ethylene glycol seems to promote the rate increase without serving as a chain transfer agent. Results reported herein display the promising potential of biocompatible catalyst systems for this ROCOP process as well as introducing the use of hydrogen bonding to enhance polymerization rates.

Al(III)/K(I) Heterodinuclear Polymerization Catalysts Showing Fast Rates and High Selectivity for Polyester Polyols

Low molar mass, hydroxyl end-capped polymers, often termed "polyols," are widely used to make polyurethanes, resins, and coatings and as surfactants in liquid formulations. Epoxide/anhydride ring-opening copolymerization (ROCOP) is a controlled polymerization route to make them, and its viability depends upon catalyst selection. In the catalysis, the polyester polyol molar masses and end-groups are controlled by adding specific but excess quantities of diols (vs catalyst), known as the chain transfer agent (CTA), to the polymerizations, but many of the best current catalysts are inhibited or even deactivated by alcohols. Herein, a series of air-stable Al(III)/K(I) heterodinuclear polymerization catalysts show rates and selectivity at the upper end of the field. They also show remarkable increases in activity, with good selectivity and control, as quantities of diol are increased from 10-400 equiv. The reactions are accelerated by alcohols, and simultaneously, their use allows for the production of hydroxy telechelic poly/oligoesters (400 < Mn (g mol-1) < 20,400, Đ < 1.19). For example, cyclohexene oxide (CHO)/phthalic anhydride (PA) ROCOP, using the best Al(III)/K(I) catalyst with 200 equiv of diol, shows a turnover frequency (TOF) of 1890 h-1, which is 4.4× higher than equivalent reactions without any diol (Catalyst/Diol/PA/CHO = 1:10-400:400:2000, 100 °C). In all cases, the catalysis is well controlled and highly ester linkage selective (ester linkages >99%) and operates effectively using bicyclic and/or biobased anhydrides with bicyclic or flexible alkylene epoxides. These catalysts are recommended for future production and application development using polyester polyols.

Understanding differences in rate versus product determining steps to enhance sequence control in epoxide/cyclic anhydride copolymers

One-pot synthesis of random, gradient, and block polyesters via the ring opening copolymerization of epoxides and cyclic anhydrides is investigated using simple yttrium salt catalysts. Impact of rate versus product determining steps is discussed.