Design of Green Catalysts for the Conversion of Renewable Resources into Polymers

Ever since the industrial revolution the concentration of carbon dioxide has been increasing monotonically, making it one of the most abundant carbon sources for the future in light of diminishing fossil sources. Due to the chemical inertness of CO2, only ca. 0.5% of its total abundance is currently being used in production of bulk chemicals. In principle, however, CO2 could offer a nontoxic, renewable, and low-cost starting material, to be used, for example, as a building block to produce green polymers in large quantities. A polymer for a polymer? The GreenCAT project aims were to develop green catalysts that convert CO2 and bio-resources into value-added polymers in a sustainable and competitive way, in contrast to the current processes involving the use of hazardous and costly catalysts. The project revolves about an interplay between experiment and theory to design non-toxic and cost-efficient catalysts giving polymers containing up to 30-50 wt% of CO2. As such, the project can contribute to a complementary and alternative CO2 technology that could replace some daily-used polymers based on oil and natural gas. Among all the polymerization methodologies considered involving CO2 so far, the metal-catalyzed copolymerization of epoxides with CO2 appears to be the most energetically efficient and economical approach to achieve the aforementioned aims. However a substantial amount of the metal-based catalysts in use for the copolymerization of epoxides with CO2 are toxic and/or in low abundance in the Earth`s crust, which limits their applications in green and sustainable industrial polymer synthesis. In line with sustainable catalyst development, the project focused on non-toxic (with no known biological role) and highly abundant transition-metals such as titanium. For instance, 6-7% of the worldwide reserves of titanium ore is located in Norway, making this metal among the most attractive transition-metals for the use in sustainable copolymerization catalysis and posing less issues for removing catalyst residues. Recently, we established methods for the preparation of a set of multidentate functionalized N-heterocyclic carbene (NHC) ligands, which were capable of wrapping the titanium atoms and these compounds proved to have great catalytic properties in the copolymerization of CO2 with epoxides. Since then, we published and presented in international conferences comparative studies with a new set of catalysts based on titanium bearing those multidentate functionalized NHC ligands. From these preliminary studies within the project, other NHC complexes of titanium were developed and we demonstrated their ability to achieve efficiently the copolymerization of cyclohexene oxide with CO2 producing selectively a completely alternated poly(cyclohexene-alt-carbonate) under mild conditions (pressure of CO2 <1 bar). The study was supported by a synergistic relationship between experiment and theory, and data from the theoretical studies helped us to rationalize the experiments. In light of these results, new investigations towards other non-toxic metals from group 4 and from late transition metals were explored remaining so far unexplored by other international research groups. Some of the new compounds, particularly the ones from group 4, showed superior catalytic performances by two orders of magnitude while maintaining highly selectivity toward the formation of polycarbonates compared to our previous benchmark titanium catalysts and/or rivaling with the other catalysts based on toxic transition metals. In addition, several key intermediates relevant to polymerization were isolated and characterized with this new series of catalysts from group 4, which benefited to determine some of the most important reaction factors. Notably, new insights based on combined experiments revealed that the catalytic intermediates have high thermal stability, the determinant role of multidentate functionalized NHC ligands on the activity/selectivity, and that at least one of the rate-limiting steps is the insertion of CO2 into metal-alkoxide bonds. Preliminary results on the catalytic performances of our best group 4 systems with other (bio)epoxides were investigated and showed that some of the catalytic systems are highly selective in the coupling of CO2/epoxides with moderate to high activity in either the corresponding cyclic carbonates or polycarbonates depending on the epoxide used. Even though the novel catalysts technology developed in this project is still at its earlier stage of development and can be further improved, the project has opened new avenues for developing a more robust and efficient green catalysts for making CO2-based polymers.

The GreenCAT project has contributed to the field of developing greener catalysts for making selectively polycarbonates. Thus, this novel class of catalysts obtained within the project has opened new avenues for the copolymerization of epoxides with CO2 such as: the use of nontoxic transition metals that are more environmentally benign than the current employed toxic metals, and determined which kind of catalysts pattern is the most suitable for this reaction. These research results attracted several collaborators across Norway, North America and France, and several collaborations have been initiated to extend the potential of this catalyst technology. Even though currently our novel catalyst technology display high performances in term of activity and selectivity toward the formation of polycarbonates in the range of other industrialized catalysts, additional improvements are needed for raising up their industrial viability for manufacturing CO2-based polymers.

The overall aim of GreenCAT project is to provide greener catalysts for the use of CO2 as raw material with (biorenewable) epoxides for manufacturing eco-friendly polycarbonates. Whereas research on copolymerization of CO2 with epoxides is currently focused on the use of expensive, rare and toxic catalysts, our approach will focus on the development of catalysts which are alternatively more favorable ecologically and economically than the current one. We recently found out and applied for the first time a new family of binary-component catalysts supported by an unprecedented combo ligand pattern for the copolymerization with CO2/epoxides with promising activity and selectivity. Based on this discovery, we further want to broaden our research using unexplored analogous chiral combo ligand?s pattern with more attractive metals with the purpose of developing a more economically viable catalysts. In the first phase of GreenCAT project, our main efforts will converge toward the synthesis of active binary-component catalysts supported by the new combo ligand's class for the CO2 and epoxides copolymerization with selected metals, i.e. less-toxic, abundant and inexpensive. The elucidation of the effective mechanism(s) with the binary-component catalysts will be also investigated by a combination of computational work and experimental kinetic studies. In the second phase of the project, we want to build up (assisted by innovative computer methods) novel single-component catalytic systems with the aims to improve robustness and recyclability of the active catalysts, essential criteria a sustainable exploitation. If successful, we plan to widen the scope of reaction to biorenewable epoxides and to study the properties of these new high-tech green copolymers. In addition the GreenCAT project has the positive aspect to offer a complementary and alternative technology for the utilization CO2.