No abstract

A new ligand–metal combination (see picture, Ni green, Br brown, N blue, O red, C gray) was designed to be cationic and to benefit from removal of electron density by the action of a Lewis acid on the ligand framework. In the presence of various activators, the resulting catalytic site is highly active for ethylene polymerization and capable of polymerizing α‐olefins to high molecular weights.

We report the design, synthesis, and physical/ mechanical properties of graft copolymers containing semicrystalline polypropylene side chains and amorphous ethylene/α-olefin copolymer backbones. These materials, a new class of semicrystalline, polyolefin-based thermoplastic elastomers, are made in two steps. First, allyl-terminated syndiotactic or isotactic polypropylene macromonomers are synthesized with controlled microstructure and molecular weight using bis(phenoxyimine)-titanium or chiral ansa-zirconocene catalysts, respectively. Second, a pyridyl-amido hafnium catalyst is used to copolymerize the macromonomer, ethylene, and an α-olefin with precise control of composition and side chain incorporation. With highly crystalline polypropylene side chains and amorphous backbones of low glass-transition temperatures (<−55°C), the samples have strain-to-break values up to 1400% and elastic recovery above 85% at maximum strains up to 1000%. The synthetic method described herein does not require the use of a living polymerization catalyst; in addition, the mechanical properties of these graft copolymers exceed those of the best linear block polyolefins.T hermoplastic elastomers (TPEs) are an important class of industrial materials that are an attractive alternative to vulcanized rubber. 1 TPEs can be melt-processed, do not require vulcanization, and, unlike thermoset rubbers, can be recycled. TPEs are blocky polymers that derive their elastomeric properties from a combination of "hard" segments with high melting points (T m ) or high glass-transition temperatures (T g ) and "soft" segments with low T g . Elastomeric properties can be obtained in structures having at least two hard sequences separated by a soft sequence and are typically linear triblock or multiblock copolymers. In the solid state, the hard segments are dispersed throughout the amorphous matrix and form physical cross-links that produce recoverable elasticity after straininduced deformation. 1 A well-known example of a commercial TPE is a triblock polystyrene-block-poly(ethylene-co-butene)-block-polystyrene copolymer (SEBS). Although many commercial TPEs are prepared via anionic polymerization and contain polystyrene, there has been longstanding interest in the synthesis and properties of TPEs based on other monomers. Bates and co-workers prepared model polyolefin TPEs by anionic polymerization followed by hydrogenation. 2 More recently, bio-derived TPEs have been reported. 3Given the low cost of ethylene, propylene, and α-olefins, considerable academic and industrial efforts have focused on the development of catalysts and reaction conditions to prepare blocky polyolefins. 4 Several approaches are shown in Scheme 1. Early efforts focused on elastomeric polypropylene (Scheme 1A) prepared with nonliving catalysts. Natta reported the first elastomeric polypropylene in 1959 using TiCl 3 /AlR 3 catalyst mixtures, 5 and more recently similar materials were obtained using zirconium and titanium alkyl complexes supported on alumina. 6 In the 1990s, the synth...

The reaction mechanism of the Ru(H)2(CO)(PPh3)3-catalyzed addition of C–H bonds in aromatic esters to olefins was studied by means of deuterium-labeling experiments and measurement of the 13C kinetic isotope effect. The deuterium-labeling experiment revealed a rapid equilibrium among the intermediates prior to the reductive elimination, and the 13C NMR kinetic isotope effect indicated that C–C bond formation is the rate-determining step in this reaction.