BACKGROUND OF THE DISCLOSURE

Aliphatic polyesters are receiving increased attention as sustainable alternatives to petroleum-based plastics due to their potentially renewable monomers, ease of recycling, and biodegradability. Yet many industrial methods of polyester production employ energy-intensive polycondensation reactions that produce small molecule byproducts, creating a pressing need for more sustainable synthetic routes. The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides provides a lowertemperature and atom-economical chain-growth approach to aliphatic polyester synthesis, producing materials with controlled molecular weights and low dispersities. Additionally, this strategy enables a wide range of monomer combinations, permitting the synthesis of polyesters with readily tunable renewable content and thermomechanical properties. Current catalyst/cocatalyst systems for ROCOP (FIG. 1) typically exhibit moderate to poor activities at low catalyst loadings and often fail to prevent deleterious side reactions, such as epoxide homopolymerization, transesterification, or epimerization.

In 1985, propylene oxide (PO) and phthalic anhydride (PA) were polymerized using an aluminum porphyrin complex in conjunction with a tetraalkylammonium salt. Subsequent efforts have resulted in diverse metal and organocatalysts, the most successful of which are based on the salenMX framework (salen=N,N′-bis(salicylidene)ethylenediamine). In these systems, the Lewis acid/nucleophilic cocatalyst pair initiates polymerization, activates epoxide towards ring-opening, and modulates the reactivity of the propagating chainends. Earlier mechanistic studies demonstrated that a mixed alkoxide/carboxylate intermediate preferentially ring-opens cyclic anhydride to generate a (bis)carboxylate resting state. From this species, epoxide binding at the Lewis acid is fast relative to rate-limiting epoxide ring-opening by a cocatalyst-associated carboxylate (FIG. 2). Therefore, dilution of the catalyst/cocatalyst pair at low loadings is anticipated to inhibit nucleophilic attack by the cocatalyst-associated propagating chain at the metal-bound epoxide. Nevertheless, performing polymerizations at decreased catalyst concentrations is highly desirable to reduce cost, minimize catalyst residue, and access high molecular weight materials.

To subvert the effects of dilution on binary catalyst activity in epoxide/CO2copolymerization, others have developed highly active cobalt salen catalysts in which the Lewis acid and nucleophilic cocatalyst are covalently tethered. Extending two of these bifunctional systems to terpolymerizations with cyclic anhydrides afforded block and gradient poly(carbonateco-ester) copolymers with attenuated activity at increasing ratios of anhydride:CO2. Others have used a cobalt salen complex bearing four quaternary ammonium substituents in the absence of CO2. While they initially obtain turnover frequencies (TOFs) as high as 1600 h−1at 80° C. ([Co]0:[PA]0:[PO]0=1:6400:85000), the catalytic activity decreases with prolonged reaction times. This single report suggests the potential efficacy of the bifunctional strategy for the ROCOP of epoxides and cyclic anhydrides at low catalyst loadings. However, it is unclear from these experiments whether the high polymerization rates were due to the covalent anchor, the catalyst:cocatalyst stoichiometry, or the inherent activity of the cobalt salen unit.

SUMMARY OF THE DISCLOSURE

The present disclosure provides catalysts and/or co-catalysts for making polymers. The present disclosure also provides methods of making the catalysts and methods of using the catalysts.

In an aspect, the present disclosure provides catalysts for making copolymers (e.g., polyesters or polycarbonates).

A catalyst of the present disclosure comprises a metal (e.g., metal ion, such as, for example, Al, Co, Cr, Fe, Zn, Mn, Ti, Ni, Ga, Sm, Y, V, and the like) salen complex group (e.g., an aluminum salen complex), a bridging group (e.g., a backbone, such as, for example, a tetherable backbone), and one or more co-catalyst groups (e.g., a substituted or unsubstituted cyclopropenium group), where the metal salen complex group is attached (e.g., covalently bonded) to the bridging group and the bridging group is attached (e.g., covalently bonded) to the co-catalyst group.

In an aspect, the present disclosure provides methods of making catalysts.

A method may comprise contacting a bridging group precursor (e.g., a backbone group, such as, for example, a tetherable backbone group) with one or more (e.g., 1 or 2) substituted or unsubstituted salicylaldehydes that may be the same or different, such that a first reaction product is formed; contacting the first reaction product with an alkyl halide-functionalized co-catalyst that may have one or more substituents (e.g., an alkyl halide-functionalized cyclopropenium or an alkyl halide-functionalized cyclopropenium having one or more substituents) such that a second reaction product is formed; contacting the second reaction product with a Lewis acid such that the catalyst is formed; and optionally, isolating the catalyst.

In an aspect, the present disclosure provides methods of using catalysts of the present disclosure to produce (e.g., synthesize) polymers (e.g., polyesters and polycarbonates).

Methods of making a polyester may comprise polymerizing an epoxide and a cyclic anhydride in the presence of a catalyst of the present disclosure, a catalyst of the present disclosure and a cyclopropenium co-catalyst, or a catalyst (e.g., a metal salen catalyst, a porphyrin, a trialkyl borane, and the like) and a cyclopropenium co-catalyst.

