Catalyst Engineering Empowers the Creation of Biomass-Derived Polyesters and Polycarbonates.

Abstract

The introduction of circular principles in chemical manufacturing will drastically change the way everyday plastics are produced, thereby affecting several aspects of the respective value chains in terms of raw feedstock, recyclability, and cost. The ultimate aim is to ensure a paradigm shift toward plastic-based (consumer) materials that overall can offer a more attractive and sustainable carbon footprint, which is an important requisite from a societal, political, and eventually economical point of view. To realize this important milestone, it is vitally important to control the polymerization processes associated with the creation of novel sustainable materials. In this respect, we realized that expanding the portfolio of biomass-derived monomers may indeed create an impetus for atom circularity; however, the often sterically congested nature of biomass-derived monomers minimizes the ability of previously developed catalysts to activate and transform these precursors. Our motivation was thus spurred by an apparent lack of catalysts suitable for addressing the conversion of such biomonomers, as we realized the potential that new catalytic processes could have to advance and contribute to the development of sustainable materials produced from polycarbonates and polyesters. These two classes of polymers represent crucial ingredients of important and large-scale consumer products and are therefore ideal fits for implementing new catalytic protocols that enable a gradual transition to plastic materials with an improved carbon footprint.When we started our research expedition, the field was dominated by metal catalysts that incorporated preferred, and in some cases even privileged, ligand backbones (such as salens) able to mediate both ring-opening and ring-opening copolymerization manifolds. One major drawback of these aforementioned catalysts is their rather rigid nature, a feature that reduces their ability to act as adaptive systems, especially in cases where bulky monomers are involved. While our initial focus was on the utilization of sustainable metal salen complexes (M = Zn, Fe) for the activation of small cyclic ethers, which are privileged monomers for polyester and polycarbonate production, we were rapidly confronted with severe limitations related to their inability to activate a wider range of complex epoxides and oxetanes, which was imparted by the planar coordination geometry of the salen ligand in most of its applied metal complexes. In our quest to find a catalytically more effective metal complex with the ability to electronically and sterically tune its substrate-binding and substrate-activation potential, we identified aminotriphenolates as structurally versatile, easily accessible, and scalable ligands for various earth-abundant metal cations. Moreover, the ligand backbone allows for switchable coordination environments around the metal centers, thus offering the necessary adaptation in substrate activation events.This Account describes how Al(III)- and Fe(III)-centered aminotriphenolates have conquered a prominent position as catalyst components in the synthesis of new biobased polyester and polycarbonate architectures, thereby changing the landscape of previously difficult to convert biomonomers, and expanding the chemical space of biobased functional polymers. With the ever-increasing influence of legislation and the restrictions placed on the use of fossil-fuel-based feedstock, the polymer industry needs viable alternatives to design materials that are greener, cost-effective, and allow for the exploration and optimization of their recycling and properties.