Computationally Guided Design of Catalysts for Fluoroolefin Polymerization
Final Report Abstract
The main scope of this collaborative DFG/SNF funded research project was the development of transition metal catalysts for the coordination polymerization of fluoroolefins. Polymers of these monomers e.g. polyvinylfluoride or ‐tetrafluoroethene (Teflon) are chemically highly resistant. Currently, their industrial production relies on radical polymerization steps leading to cross‐linked and/or highly branched polymers. Since coordination polymerization is anticipated to provide access to novel polymer structures with unprecedented properties, the development of this type of catalysts is highly desirable. This is best exemplified by comparison with the discovery of Ziegler‐Natta catalysts, which allowed to access new linear polyolefins leading to an extreme boost of the polymer chemistry field. The fluoroolefin monomers impose serious problems to classical group IV transition metal Ziegler‐Natta catalysts however, since the formation of highly stable group IV metal fluorine bonds through C‐F bond cleavage leads to rapid and irreversible catalyst deactivation. The project consisted of both i) a theoretical and ii) an experimental part: The experiments were mostly carried out at the University of Delaware, USA, under the guidance of Klaus Theopold, who is an renowned expert for chromium and its related polymerization chemistry. The theoretical work was carried out at the University Hamburg. The search started at neutral diimido chromium(VI) dialkyl catalysts, for which Siemeling et al. had previously reported the polymerization of polar monomers, e.g. acrylonitrile. The major goal was the optimization of the system for fluoroolefin polymerization, which was attempted by an automated throughput quantum‐chemical search. Key to success is to find a catalyst system which has a high rate constant for chain propagation kprop and a small one for chain termination (kterm), i.e. kprop >> kterm. The corresponding elementary steps were identified and revealed that chain propagation proceeds by 1,2‐ rather than 2,1‐insertion of the olefin. The activation barriers for the chain propagation and termination steps were found to be in the same range for a small model system. As expected the latter proceeds by ß‐F‐elimination and generates an energetically favorable chromium fluorido olefin complex. With the aid of CCSD(T) and demanding CASPT2 calculations the appropriate DFT functional was chosen for the simulation of the in vitro process. In the course of these investigations two important bond (angles) parameters of the catalyst were identified, which have a strong influence on the barriers for chain termination and propagation. Based on two‐dimensional scanning of both angles (α and ß) for both processes, optimal parameters for α and ß were derived with low and high activation barriers for the 1,2‐olefin insertion and ß‐fluorine elimination steps, respectively. With this knowledge an automated search for an optimized catalyst was initiated. The latter consisted of an automatic assembly of the catalyst from a ligand library, successive pre‐optimizations of the geometries at the MM3 (forced field) and PM6 (semi‐empirical) levels and full optimizations of the ground and transition states with DFT methods. From this quantum‐chemical high‐throughput search, three potential candidates with optimized chelating diimido ligands were eventually identified in silico. Due to time constraints of the project their “in vitro genesis” could not be witnessed so far. We are confident however that these complexes are synthetically accessible and have high potential for the polymerization of fluoroolefins. We will therefore eagerly continue our experimental investigations. In addition to this first approach, we developed the genetic algorithm based program system “ChemScreen‐GA” for the quantum‐chemical optimization of catalytic properties. Since our first test for a cationic chromium ethylene polymerization catalyst was highly successful, we are confident to tackle more challenging process in the future, e.g. the production of methanol by CO hydrogenation or the partial oxidation of methane. This code will be made available under GNU public license.
Publications
- 2009, 238. ACS Meeting in Washington. “Multidimensional reaction barrier surfaces: An approach to computational catalyst design”
Jan‐Philipp Werner, Peter Burger, Klaus Theopold, John Young
