Nickel complexes bearing amine–imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et₂AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine–imine nickel precursor with a tert-butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition-metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di- and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature.

A novel polymerization methodology for efficient synthesis of hyperbranched polyethylene amphiphiles by chain walking polymerization (CWP) followed by RAFT polymerization has been developed. Hyperbranched polyethylene with hydroxyl ends (HBPE-OHs) is first synthesized via chain walking copolymerization of ethylene with 2-hydroxyethyl acrylate with Pd-α-diimine catalyst. The hydroxyl groups of hyperbranched polyethylene are then converted into thiocarbonyl thio moieties by an esterification reaction with trithiocarbonate 3-benzylsulfanylthiocarbonyl sulfanylpropionic acid (BSPA). The hyperbranched polyethylene with thiocarbonyl thio moiety ends (HBPE-BSPAs) is used as a macro-RAFT agent for the synthesis of hyperbranched polyethylene amphiphiles, HBPE-PDMAEMAs, by RAFT polymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA). The resultant HBPE-PDMAEMAs can self-assemble to form supramolecular polymer vesicles in aqueous solution. A preliminary investigation on thermo- and pH-responsive behaviors of the polymer is also reported.

1-Hexene polymerizations catalyzed by α-diimine nickel complexes after activation with modified methylaluminoxane were performed at various reaction temperatures. Effects of catalyst structure and polymerization temperature on activity and polymer microstructure were evaluated in detail. Bulky catalyst 1b with camphyl backbone exhibited good control ability and greatly enhanced thermal stability to be capable of polymerizing 1-hexene at 80°C. The poly(1-hexene)s with long methylene sequences and dominate branches (methyl and butyl) were synthesized using catalyst 1b. Differential scanning calorimetry analysis further confirmed that long polymethylene block ((CH₂)_n, n > 20) was formed in the poly(1-hexene)s with melting point of 64°C obtained by catalyst 1b on the basis of initial branched model polyethylene. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Several N-functionalized bis(phosphanyl)amine ligands respectively containing benzyl, furfuryl, thiophene-2-methyl, thiophene-2-ethyl, and 2-picolyl groups (1a–e) were synthesized and characterized. The ligands reacted with (DME)NiBr₂ in CH₂Cl₂ to give their corresponding nickel complexes [Ph₂PN(R)PPh₂NiBr₂] [R = CH₂C₆H₅ (2a), CH₂C₄H₃O (2b), CH₂C₄H₃S (2c), CH₂C₅H₄N (2d), and CH₂CH₂C₄H₃S (2e)]. The structures of these complexes were established by single-crystal X-ray crystallography. All these nickel complexes were highly active towards ethylene oligomerization in the presence of methylaluminoxane or Et₂AlCl, producing a high content of butene (C4). Especially for 2e, which contains a thiophene-2-ethyl pendant group, the oligomerization products obtained at –40 °C contained 95.9 mol-% C4 fraction with 100 mol-% 1-butene. Over 50 °C, however, these nickel complexes underwent Friedel–Crafts alkylation of toluene with ethylene and the olefin oligomers.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Ethylene was selectively oligomerized by nickel complexes such as (PPh₃)₂NiBr₂ and (PPh₃)₂NiCl₂ immobilized in chloroaluminate ionic liquid in biphasic catalytic reactions. The influence of reaction parameters such as reaction media, reaction temperature and Et₂AlCl:Ni molar ratio was also evaluated. Turnover frequency up to 24000 mol C₂H₄/(mol Ni h) was achieved under mild reaction conditions (0.5 atm and 40 °C). GC-MS analyses showed that the obtained oligomers completely consist of C4 and C6. The olefinic products can be easily separated from the catalytic ionic liquid phase by simple decantation, and the nickel catalyst can be reused without a significant decrease in turnover frequency and change of the distribution of the olefinic products. Copyright © 2009 John Wiley & Sons, Ltd.

A series of new N,N,S tridentate nickel(II) complexes [(ArN = CHC₆H₄NPh-2-SPh)NiBr] (Ar = 2,6-diisopropylphenyl, 1; Ar = 2,6-dimethylphenyl, 2; Ar = Ph, 3) and palladium(II) complexes [(ArN = CHC₆H₄NPh-2-SPh)PdCH₃] (Ar = 2,6-diisopropylphenyl, 4; Ar = 2,6-dimethylphenyl, 5; Ar = Ph, 6) were synthesized and characterized. X-ray diffraction analyses of the single-crystal structures revealed that tridentate complexes 1, 2, 4, 5, and 6 featured a distorted square-planar coordination of the central metal. Compared to the bidentate metal complexes bearing anilido–imine ligands, the N,N,S tridentate metal complexes showed better stability. In the presence of methylaluminoxane (MAO), nickel complexes 1–3 showed moderate activity toward ethylene oligomerization at atmosphere pressure, but the palladium complexes were inactive. However, the nickel complexes exhibited high activity up to 8.20 × 10⁶ g/(mol of Ni) h and palladium complexes showed very high activity up to 2.68 × 10⁸ g/(mol of Pd) h toward norbornene polymerization with MAO as cocatalyst. The obtained polynorbornenes are vinylic addition polymers with high molecular weights. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)