Late Metal Sandwich Catalysts for Olefin Polymerization

ConspectusPolyolefins are by far the most ubiquitous industrially produced polymers and are primarily produced by early transition metal catalysts. These catalysts are not functional group tolerant, and copolymerization of ethylene and polar vinyl monomers is quite challenging. Furthermore, early metal catalysts convert ethylene to linear polyethylene, and introduction of branches requires addition of comonomers. In this Account, we describe our efforts in designing and implementing new Pd(II) and Ni(II) olefin polymerization catalysts based on mechanistic understanding of the chain growth process. The original hindered nickel- and palladium-aryl-substituted diimine complexes were discovered in 1995. The key to the success of these now "classic" systems in generating high polymers rather than dimers or oligomers was realizing that incorporation of ortho-disubstituted aryl groups partially blocks the axial sites of the metal and thus retards the rate of chain transfer relative to propagation. Two key features of these late metal catalysts distinguish them from early metal complexes. First, they tolerate certain functional groups, which allows copolymerization of olefins with polar comonomers. Second, they can form a branched polymer from ethylene without the need to add α-olefin comonomers. Importantly, for nickel catalysts, branching levels can be modulated by changing reaction conditions, such as temperature and monomer pressure.Based on molecular modeling, we speculated that 8-(arylnaphthyl) substitution in α-diimine catalysts should result in sandwich-type structures and thus exhibit much more efficient blocking of the axial sites relative to the classical ortho-disubstituted aryl diimines. This analysis proved to be quite fruitful. In this Account we describe the synthesis of palladium and nickel sandwich catalysts, mechanistic investigations of their catalytic behavior, and their use in building new polymer structures. The enhanced axial shielding by the two capping aryl groups in these catalysts results in exceptionally slow rates of chain transfer and, consequently, formation of extremely high molecular weight polymers with very narrow molecular weight distributions, features characteristic of living polymerizations. This behavior, coupled with the ability (particularly for nickel) to control polymer branching densities and thus mechanical properties through pressure and temperature variations permits generation of ultrahigh molecular weight polyethylenes (M_n's over 10⁷ Da) with branches ranging from 9 to 100 per 1000 carbons and T_m values from 17 to 132 °C. Furthermore, the living nature of the polymerization and the variation of branching with pressure has permitted the synthesis of diblock and multiblock polymers with narrow dispersities and complete control of molecular weights as well as specification of hard and soft segment lengths. Such structures are receiving extensive attention as polyolefin compatibilizers.