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.

Two bis(β-ketoamino)copper [ArNC(CH₃)CHC(CH₃)O]₂Cu (1, Ar = 2,6-dimethylphenyl; 2, Ar = 2,6-diisopropylphenyl) complexes were synthesized and characterized. Homo- and copolymerizations of methyl acrylate (MA) and 1-hexene with bis(β-ketoamino)copper(II) complexes activated with methylaluminoxane (MAO) were investigated in detail. MA was polymerized in high conversion (>72%) to produce the syndio-rich atactic poly(methyl acrylate), but 1-hexene was not polymerized with copper complexes/MAO. Copolymerizations of MA and 1-hexene with 1,2/MAO produced acrylate-enriched copolymers (MA > 80%) with isolated hexenes in the backbone. The calculation of reactivity ratios showed that r(MA) is 8.47 and r(hexene) is near to 0 determined by a Fineman-Ross method. The polymerization mechanism was discussed, and an insertion-triggered radical mechanism was also proposed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1113–1121, 2010

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)

Homo- and copolymerization of ethylene and norbornene were investigated with bis(β-diketiminato) titanium complexes [ArNC(CR₃)CHC(CR₃)NAr]₂TiCl₂ (R = F, Ar = 2,6-diisopropylphenyl 2a; R = F, Ar = 2,6-dimethylphenyl 2b; R = H, Ar = 2,6-diisopropylphenyl 2c; R = H, Ar = 2,6-dimethylphenyl 2d) in the presence of methylaluminoxane (MAO). The influence of steric and electric effects of complexes on catalytic activity was evaluated. With MAO as cocatalyst, complexes 2a–d are moderately active catalysts for ethylene polymerization producing high-molecular weight polyethylenes bearing linear structures, but low active catalysts for norbornene polymerization. Moreover, 2a–d are also active ethylene–norbornene (E–N) copolymerization catalysts. The incorporation of norbornene in the E–N copolymer could be controlled by varying the charged norbornene. ¹³C NMR analyses showed the microstructures of the E–N copolymers were predominantly alternated and isolated norbornene units in copolymer, dyad, and triad sequences of norbornene were detected in the E–N copolymers with high incorporated content of norbornene. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 93–101, 2008

A series of 2-aminopyridine Ni(II) complexes bearing different substituent groups {(2-PyCH₂NAr)NiBr, Ar = 2,4,6-trimethylphenyl (3a), 2,6-dichlorophenyl (3b), 2,6-dimethylphenyl (3c), 2,6-diisopropylphenyl (3d), 2,6-difluorophenyl (3e); (2-PyCH₂NHAr)₂NiBr₂, Ar = 2,6-diisopropylphenyl (4a)} have been synthesized and investigated as precatalysts for ethylene polymerization in the presence of methylaluminoxane (MAO). High molecular weight branched polymers as well as short-chain oligomers were simultaneously produced with these complexes. Enhancing the steric bulk of the ortho-aryl-substituents of the catalyst resulted in higher ratio of solid polymer to oligomer and higher molecular weight of the polymer. With ortho-haloid-substitution, the catalysts afforded a product with low polymer/oligomer ratio (3b) and even only oligomers (3e) in which C₁₄H₂₈ had the maximum content. Compared with complex 3d containing ionic ligand, complex 4a containing neutral ligand exhibited obviously low catalytic activity for ethylene polymerization. The molecular weight, molecular weight distribution, and microstructure of the resulted polymer were characterized by gel permeation chromatography and ¹³C NMR spectrogram. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1618–1628, 2008