Development of thermally stable nickel-based catalysts is highly desirable for industrial gas-phase olefin polymerizations. On the basis of the strategy of promoting the thermostability of nickel catalyst by the ligand backbone, we herein reported novel dibenzobarrelene-derived α-diimine nickel precatalysts for ethylene polymerization. Increasing the steric bulk on the ligand backbone was expected to inhibit the N-aryl rotation of the α-diimine ligands by the repulsive interactions, thus enhancing thermal stability (100 °C) and living fashion a temperatures up to 80 °C. Bulk ligand backbone also improved tolerance of nickel catalyst toward polar groups, and the α-diimine nickel catalyst containing a 2,6-t Bu-dibenzobarrelene backbone catalyzed living copolymerization of ethylene and methyl 10-undecenoate.

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1-Hexene polymerizations were carried out with amine−imine nickel complexes [(ArNC(R 1 )−(R 2 R 3 )CNHAr)-NiBr 2 , 1a, R 1 = R 2 = R 3 = Me, Ar = 2,6-(iPr) 2 C 6 H 3 ; 1b, R 1 = R 2 = R 3 = Me, Ar = 2,6-(Me) 2 C 6 H 3 ; 2a, R 1 = Me, R 2 = R 3 = H, Ar = 2,6-(iPr) 2 C 6 H 3 ; 3a, R 1 = Me, R 2 = tBu, R 3 = H, Ar = 2,6-(iPr) 2 C 6 H 3 ] in the presence of MMAO or Et 2 AlCl. The ligand-directed regioselectivity involving insertion fashion and chain walking in amine−imine nickel-catalyzed 1-hexene polymerization is clearly observed. Catalyst 1a with two methyl substituents on the bridging carbon can polymerize 1-hexene to afford semicrystalline "polyethylene" with long methylene sequence (−(CH 2 ) n −, n = 40−74) via a combination of 90% selectivity of 2,1-insertion fashion and precise chain walking, whereas catalyst 3a with a tert-butyl on the bridging carbon can polymerize 1-hexene in 80% selectivity of 1,2-insertion to produce amorphous polyolefin with predominant methyl branches through 2,6-enchainment.Precise control of polyolefin microstructure by olefin coordination polymerization, including molecular weight and polydispersity, stereochemistry, and branch structure, is challenging and increasingly attractive because polyolefin architecture is closely related to mechanical and rheological properties. 1 Early transition metal catalysts have been developed that give precise control over polymer stereochemistry. 2 On the contrary, late transition metal catalysts such as α-diimine nickel and palladium show a distinguishing chainwalking mechanistic feature involving interaction between the metal center and β-H of the growing polymer chain. 3 In terms of α-olefins polymerization with α-diimine nickel and palladium catalysts, polymer branch structure including branching distribution and density is closely relative to the regioselectivity involving insertion fashion of α-olefin and chain-walking process. 4 In the case of abstraction of insertion from secondary carbon, 1,2-insertion of α-olefin and subsequent β-hydride elimination followed by metal migration up to the primary carbon atom can lead to 2,ω-enchainment to give methyl branch in the polymer chain, while 2,1-insertion of α-olefins can result in 1,ω-enchainment to give a linear polymer chain without branches. Amorphous poly(α-olefin)s are generally produced by late transition metal catalysts because of poor regioselectivity involving low selectivity of insertion fashion and uncontrolled chain walking. 5 Currently, high selectivity of 1,2-insertion fashion of α-olefin have been achieved using symmetric α-diimine nickel and α-keto-β-diimine nickel catalysts at low temperature of −78°C . 4e,6 Isotactic polypropylenes with mirror regioerrors were obtained because of restriction of chain walking at low temperature. One high regioselectivity example involving 1,2-insertion and chain walking is the aminobis(imino)-phosphorane nickel system, which can produce polyolefins with methyl branches at well-defined intervals through 2, ω-enchainment. 4a In comparison with t...

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