Synergic Heterodinuclear Catalysts for the Ring-Opening Copolymerization (ROCOP) of Epoxides, Carbon Dioxide, and Anhydrides

Abstract

Conspectus The development of sustainable plastic materials is an essential target of chemistry in the 21st century. Key objectives toward this goal include utilizing sustainable monomers and the development of polymers that can be chemically recycled/degraded. Polycarbonates synthesized from the ring-opening copolymerization (ROCOP) of epoxides and CO 2 , and polyesters synthesized from the ROCOP of epoxides and anhydrides, meet these criteria. Despite this, designing efficient catalysts for these processes remains challenging. Typical issues include the requirement for high catalyst loading; low catalytic activities in comparison with other commercialized polymerizations; and the requirement of costly, toxic cocatalysts. The development of efficient catalysts for both types of ROCOP is highly desirable. This Account details our work on the development of catalysts for these two related polymerizations and, in particular, focuses on dinuclear complexes, which are typically applied without any cocatalyst. We have developed mechanistic hypotheses in tandem with our catalysts, and throughout the Account, we describe the kinetic, computational, and structure–activity studies that underpin the performance of these catalysts. Our initial research on homodinuclear M(II)M(II) complexes for cyclohexene oxide (CHO)/CO 2 ROCOP provided data to support a chain shuttling catalytic mechanism, which implied different roles for the two metals in the catalysis. This mechanistic hypothesis inspired the development of mixed-metal, heterodinuclear catalysts. The first of this class of catalysts was a heterodinuclear Zn(II)Mg(II) complex, which showed higher rates than either of the homodinuclear [Zn(II)Zn(II) and Mg(II)Mg(II)] analogues for CHO/CO 2 ROCOP. Expanding on this finding, we subsequently developed a Co(II)Mg(II) complex that showed field leading rates for CHO/CO 2 ROCOP and allowed for unique insight into the role of the two metals in this complex, where it was established that the Mg(II) center reduced transition state entropy and the Co(II) center reduced transition state enthalpy. Following these discoveries, we subsequently developed a range of heterodinuclear M(III)M(I) catalysts that were capable of catalyzing a broad range of copolymerizations, including the ring-opening copolymerization of CHO/CO 2 , propylene oxide (PO)/CO 2 , and CHO/phthalic anhydride (PA). Catalysts featuring Co(III)K(I) and Al(III)K(I) were found to be exceptionally effective for PO/CO 2 and CHO/PA ROCOP, respectively. Such M(III)M(I) complexes operate through a dinuclear metalate mechanism, where the M(III) binds and activates monomers while the M(I) species binds the polymer change in close proximity to allow for insertion into the activated monomer. Our research illustrates how careful catalyst design can yield highly efficient systems and how the development o...

Rational Optimization of Bifunctional Organoboron Catalysts for Versatile Polyethers via Ring-Opening Polymerization of Epoxides

“…Inspired by the state-of-the-art bimetallic systems based on Al, Cr, and Co for the ROP of epoxides, where the distance between the two metal centers is pivotal to the catalytic reactivity, − we then explored the influence of the distance between B centers and N + on ROP of PO. Considering the commercial availability of 9-BBN, we focused on BBN structure-based organoboron catalysts, though the Bora-containing catalyst 4 shows much higher reactivity than the BBN-containing counterpart.…”

“…Though the dinuclear organoboron compounds displayed high efficiency for polyether production, systematical investigations of the relationship between catalyst structure and catalytic performance are needed, since understanding at the molecular level of the enchainment of ether would be beneficial for designing advanced catalytic systems. In addition, the construction of di- and multinuclear catalytic systems has become one of the most prevailing strategies in the design of new catalysts in epoxide-involved ROP and ROCOP reactions. − Therefore, understanding how the polymerization proceeds in the presence of di- and multinuclear organoboron catalysts would inspire chemists to explore superior catalysts.…”

5-Methylene-1,3-dioxane-2-one: A First-Choice Comonomer for Trimethylene Carbonate

“…APCs are typically prepared according to two different polymerization routes: (i) copolymerization of epoxides and CO 2 , and (ii) ring-opening polymerization (ROP) of cyclic carbonates (CC), mainly six-membered-ring cyclic carbonates, 6CCs. Although having the advantage of using CO 2 as feedstock, the former strategy is a synthetic challenge as the used catalytic systems may suffer from low functional compatibility, and in many cases formation of the corresponding five-membered-ring cyclic carbonate (5CC) is in competition with polymerization.…”

“…Working at high CO 2 pressure is often required for selective polymerization to take place. Nevertheless, important progresses have been achieved over the last decade in this regard, as well as in the utilization of bio-resourced epoxides. , The alternative strategy, ROP of 6CCs, has also received great attention not only with trimethylene carbonate (TMC) but also with a variety of substituted/functionalized 6CCs. − Controlled ROP of these 6CCs under mild conditions has strongly benefited from the major progress done in organocatalytic ROP. A variety of organocatalysts are now available to promote the controlled polymerization of lactones and cyclic carbonates.…”

Mechanistic Basis for the High Enantioselectivity and Activity in the Multichiral Bimetallic Complex-Mediated Enantioselective Copolymerization of meso-Epoxides

“…More specifically, this takes place by the nucleophilic attack of the growing carboxylate species at one metal center toward the activated epoxide at the other (Scheme ). A similar mechanism with sequential coordination of the growing polymer chain at bimetallic centers of dinuclear zinc catalysts for CHO/CO 2 copolymerization was proposed by Williams and co-workers. − Moreover, this chain shuttling mechanism was not only applied to dinuclear zinc catalysts but also suitable for other metal centers, such as Co and Mg, especially heterobimetallic catalysts, showing more active than monometallic dinuclear complexes. − …”