More than half of the 170 million metric tons of polymers produced each year are polyolefins. Current technology uses highly active cationic catalysts, which suffer from an inability to tolerate heteroatoms such as oxygen, nitrogen, and sulfur. These systems require scrupulously clean starting materials and activating cocatalysts. A family of catalysts has been developed whose members are tolerant of both heteroatoms and less pure starting materials. These heteroatom-tolerant neutral late transition metal complexes are in fact highly active systems that produce high-molecular-weight polyethylene, polymerize functionalized olefins, and require no cocatalyst.
Synthesis of 1d,e; To a solution of 2-(9-phenanthrenyl)phenol or 2-(9anthracenyl)phenol (25 mmol) and 2,6-lutidine (4.6 g, 43 mmol) in toluene (50 mL) under an atmosphere of Ar was slowly added SnCl 4 (0.75 mL, 6.4 mmol). The solution was stirred at rt for 20 min. Paraformaldehyde was added (4.3 g, 140 mmol) and the reaction was stirred at 1 10C for 12 h. After cooling to rt, the reaction mixture was poured into water (300 mL), acidified to pH 1 with concentrated HCI, extracted with diethyl ether (500 mL), separated, the combined organic layers washed twice with sat. brine, and dried over Na 2 SO 4. The solvent was removed by rotary evaporation to yield a yellow oil. The crude product was purified by flash chromatography on silica gel eluting with hexanes:ethyl acetate (9:1) to yield a white solid.
Neutral Ni(II) salicylaldimine catalysts (pendant ligand ϭ NCMe or PPh 3 ) were used to copolymerize ethylene with monomers containing esters, alcohols, anhydrides, and amides and yielded linear functionalized polyethylene in a single step. ␣-Olefins and polycyclic olefin comonomers carrying functionality were directly incorporated into the polyethylene backbone by the catalysts without any cocatalyst, catalyst initiator, or other disturber compounds. The degree of comonomer incorporation was related to the monomer structure: tricyclononenes Ͼ norbornenes Ͼ ␣-olefins. A wide range of comonomer incorporation, up to 30 mol %, was achieved while a linear polyethylene structure was maintained under mild conditions (40°C, 100 psi ethylene). Results from the characterization of the copolymers by solution and solid-state NMR techniques, thermal analysis, and molecular weight demonstrated that the materials contained a relatively pure microstructure for a functionalized polyethylene that was prepared in one step with no catalyst additive.
A facile synthesis of nickel salicylaldimine complexes with labile dissociating ligands is described. In addition to producing highly active ethylene polymerization catalysts, important insights into the effect of ligand size on catalyst stability and information on the mechanism of polymerization are provided.
Probing chemical reaction kinetics in the near-solid state (small molecules and polymers) is extremely challenging because of the restricted mobility of reactant species, the absence of suitable analytical probes, and most critically the limited temperature stability of the materials. By limiting temperature exposure to extremely short time frames (sub-millisecond), temperatures in excess of 800 °C can be accessed extending kinetic rate measurements many orders of magnitude. Here we demonstrate measurements on a model system, exploiting the advantages of thin-films, laser heating, and chemically amplified resists as an exquisite probe of chemical kinetic rates. Chemical reaction and acid diffusion rates were measured over 10 orders of magnitude, exposing unexpected and large changes in dynamics linked to critical mechanism shifts across temperature regimes. This new approach to the study of kinetics in near-solid state materials promises to substantially improve our understanding of processes active in a broad range of temperature-sensitive, low-mobility materials.
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