Polyolefins are produced today catalytically on a vast scale, and the manufactured polymers find use in everything from artificial limbs and food/medical packaging to automotive and electrical components and lubricants. Although polyolefin monomers are typically cheap (e.g., ethylene, propylene, α-olefins), the resulting polymer properties can be dramatically tuned by the particular polymerization catalyst employed, and reflect a rich interplay of macromolecular chemistry, materials science, and physics. For example, linear low-density polyethylene (LLDPE), produced by copolymerization of ethylene with linear α-olefin comonomers such as 1-butene, 1-hexene, or 1-octene, has small but significant levels of short alkyl branches (C2, C4, C6) along the polyethylene backbone, and is an important technology material due to outstanding rheological and mechanical properties. In 2013, the total world polyolefin production was approximately 211 million metric tons, of which about 11% was LLDPE. Historically, polyolefins were produced using ill-defined but highly active heterogeneous catalysts composed of supported groups 4 or 6 species (usually halides) activated by aluminum alkyls. In 1963, Karl Ziegler and Giulio Natta received the Nobel Prize for these discoveries. Beginning in the late 1980s, a new generation of group 4 molecule-based homogeneous olefin polymerization catalysts emerged from discoveries by Walter Kaminsky, a team led by James Stevens at The Dow Chemical Company, this Laboratory at Northwestern University, and a host of talented groups in Germany, Italy, Japan, the United Kingdom, and the United States. These new "single-site" catalysts and their activating cocatalysts were far better defined and more rationally tunable in terms of structure, mechanism, thermodynamics, and catalyst activity and selectivity than ever before possible. An explosion of research advances led to new catalysts, cocatalysts, deeper mechanistic understanding of both the homogeneous and heterogeneous systems, macromolecules with dramatically altered properties, and large-scale industrial processes. It is noteworthy that many metalloenzymes employ multiple active centers operating in close synergistic proximity to achieve high activity and selectivity. Such enzymes were the inspiration for the research discussed in this Account, focused on the properties of multimetallic olefin polymerization catalysts. Here we discuss how modifications in organic ligand architecture, metal···metal proximity, and cocatalyst can dramatically modify polyolefin molecular weight, branch structure, and selectively for olefinic comonomer enchainment. We first discuss bimetallic catalysts with identical group 4 metal centers and then heterobimetallic systems with either group 4 or groups 4 + 6 catalytic centers. We compare and contrast the polymerization properties of the bimetallic catalysts with their monometallic analogues, highlighting marked cooperative enchainment effects and unusual polymeric products possible via the proximate catalytic centers. Su...

Abstract.We have designed and characterized a new inlet and aerodynamic lens for the Aerodyne aerosol mass spectrometer (AMS) that transmits particles between 80 nm and more than 3 µm in vacuum aerodynamic diameter. The design of the inlet and lens was optimized with computational fluid dynamics (CFD) modeling of particle trajectories. Major changes include a redesigned critical orifice holder and valve assembly, addition of a relaxation chamber behind the critical orifice, and a higher lens operating pressure. The transmission efficiency of the new inlet and lens was characterized experimentally with size-selected particles. Experimental measurements are in good agreement with the calculated transmission efficiency.

The synthesis, structural characterization, and ethylene polymerization properties of two neutrally charged Ni(II) phenoxyiminato catalysts are compared and contrasted. Complex FI-SO2-Ni features a −SO2– group embedded in the ligand skeleton, whereas control FI-CH2-Ni has the −SO2– replaced by a −CH2– functionality. In comparison with FI-CH2-Ni, at 25 °C, FI-SO2-Ni is 18 times more active, produces polyethylene with 3.2 times greater M W and 1.5 times branch content, and is significantly more thermally stable. The FI-SO2-Ni-derived polymer is a hyperbranched polyethylene (148 branches 1000 C–1, M W = 3500g mol–1) versus that from FI-CH2-Ni (98 branches 1000 C–1, M W = 1100g mol–1). DFT calculations argue that the distinctive FI-SO2-Ni catalytic behavior versus that of FI-CH2-Ni is associated with nonnegligible OSO···Ni interactions involving the activated catalyst.

The complex (μ-Me 2 C-3,3′){(η 5 -cyclopentadienyl)[1-Me 2 Si-( t BuN)](TiMe 2 )} 2 (3) was prepared as a new binuclear catalyst motif for homogeneous olefin polymerization. Complex 3 exists as rac-3 and meso-3 diastereomers, which can be separated and characterized by solution NMR spectroscopy and single-crystal X-ray diffraction. While meso-3 has high thermal stability, rac-3 undergoes thermolysis in solution to quantitatively form the dimeric methylidene complex (μ-Me 2 C-3,3′){(η 5 -cyclopentadienyl)[1-Me 2 Si( t BuN)][(μ-CH 2 )Ti]} 2 (rac-4). Activation of rac-3 and meso-3 with 1 equiv of Ph(5; rac-5 and meso-5, respectively). Interestingly, meso-5 is stable in the presence of an additional 1 equiv of Ph] 2 (rac-6) as indicated by multinuclear NMR spectroscopy and DFT computation. meso-3 reacts with 2 equiv of B(C 6 F 5 ) 3 to yield meso-[(μ-CMe 2 -3,3′){(η 5 -cyclopentadienyl)[1-Me 2 Si( t BuN)]} 2 (μ-CH 2 )(μ-CH 3 )Ti 2 ] + MeB(C 6 F 5 ) 3 − (meso-7) containing the same meso-5 cation but with a MeB(C 6 F 5 ) 3 − counteranion. These findings, along with catalytic results, indicate that rac-3 and meso-3 remain structurally intact during polymerization, consistent with the observed diastereoselectivity effects. Under identical ethylene/1octene copolymerization conditions, only activated bimetallic rac-3 produces appreciable polymer, with meso-3 exhibiting low activity, but both yield polymer with a branch density >2× that of the monometallic control [(3-t Bu-C 5 H 3 )SiMe 2 N t Bu]TiMe 2 (Ti 1 ). In ethylene/styrene copolymerizations, rac-3 produces polymers with 3.1× higher M n and 2.1× greater styrene incorporation versus Ti 1 , while meso-3 catalyzes only ethylene-free styrene homopolymerization. In 1-octene homopolymerizations, meso-3 + B(C 6 F 5 ) 3 (i.e., meso-7) produces highly isotactic poly-1-octene (mmmm 91.7%), while rac-3 + Ph 3 C + B(C 6 F 5 ) 4 − (i.e., rac-5), rac-3 + B(C 6 F 5 ) 3 (i.e., rac-7), and meso-3 + Ph 3 C + B(C 6 F 5 ) 4 − (i.e., meso-5) produce only atactic poly-1-octene. These bimetallic polymerization catalysts exhibit distinctive cooperative effects influencing product M n , tacticity, and comonomer selection, demonstrating that binuclear catalyst stereochemical factors are significant.

The synthesis and characterization of the neutrally charged electron-deficient nickel(II) phenoxyiminato catalyst {2-(hydroxydiphenylmethyl)-4-tert-butyl-[6-(2,6-diisopropylphenyl)]salicylaldiminato}-methyl(trimethylphosphine)nickel(II) (1b) with an intramolecular hydrogen bond directed toward the active catalytic site is reported. At room temperature, catalyst 1b exhibits 2.5Â greater ethylene polymerization activity and 2Â greater polyethylene product branching than an analogous catalyst without the hydrogen bond (2b). Furthermore, catalyst 1b produces substantially greater polyethylene yields in the presence of polar additives such as ethyl ether, acetone, and water than does 2b under identical conditions. This enhanced polymerization activity in the presence of polar additives suggests that the hydrogen bonding proximate to the metal center significantly modifies the relative rates of competing enchainment and chain transfer processes.