Insertion polymerization of electron-deficient vinyl monomers

• Applicant: Professor Dr. Stefan Mecking
• Subject Area: Preparatory and Physical Chemistry of Polymers
• Term: from 2010 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 164875938

Catalytic polymerization of ethylene and propylene is one of the most well-studied chemical reactions. By stark contrast, an insertion polymerization of electron deficient polar-substituted vinyl monomers is a long-standing challenge. Only recently, linear copolymerizations of ethylene with a scope of monomers including even acrylic acid have been realized. In the first funding period a comprehensive picture was gained of factors governing insertion chain growth and catalyst deactivation. Catalyst design was advanced and novel polymeric materials evolved. We now focus on overcoming the identified intrinsic limitations of insertion (co)polymerization of polar vinyl monomers.Coordination of the functional group of the free monomer, and in particular of the repeat units gener¬ated by insertion, blocks coordination sites reversibly and hinders catalysis. To overcome this problem, a selective binding of the functional groups to appropriate sites in the catalyst is pursued. An interaction of functional groups of the monomer with carefully placed complementary hydrogen-bonding motifs in the bidentate ligand of neutral Pd(II) catalysts is sought to favor olefin coordination over binding of the incoming monomers functional groups. In extension of this concept also to the polar groups of already incorporated monomer, a diversion of their binding away from chelating coordination blocking the active sites is aimed for. An alternative approach to resolve this problem - potentially also providing new copolymer microstructures - is a combination of ethylene insertion chain growth with free-radical growth of polar monomer segments. Monteil (Macromolecules 2011, 44, 3293) suggested that neutral Ni(II) salicylaldiminato complexes polymerize ethylene and methyl methacrylate (MMA) to (multi)block copolymers. Our preliminary studies confirm that an insertion chain growth of ethylene and radical polymerization of MMA occur simultaneously, and both reactions are interdependent. To unravel the mechanisms operative here and devise generic concepts to insertion/radical 'shuttling' polymerizations, polymerizations with (13C labeled) Ni-aryl and Ni-methyl precursors will be pursued, combined with polymer endgroup analysis. Further, the role of the metal in generating and scavenging radicals will be illuminated by studies of (salicylaldiminato)Ni(I) species prepared for this purpose. This comprises their reactivity under polymerization conditions as well as stoichiometric reactions with the active species of insertion chain growth.Stereocontrol by space-filling repulsive interactions, established for propylene, is not applicable here. We have devised an asymmetric substitution of moderate bulk at a P-donor adjacent to the bound olefin as a concept to control acrylate insertion. This serves as a starting point to develop stereoregular chain growth via remote rigid linkers. In further course, the aforementioned concepts of attractive interactions can come into play.

• DFG Programme: Research Grants