8334 | New J. Chem., 2018, 42, 8334--8337 This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 Cite this: New J. Chem., 2018, 42, 8334 Synthesis of low to high molecular weight poly(1-hexene); rigid/flexible structures in a di- and mononuclear Ni-based catalyst series† M. Khoshsefat, ab S. Ahmadjo, * a S. M. M. Mortazavi, a G. H. Zohuri c and J. B. P. Soares b 1-Hexene was polymerized with mono- (MC n , n = 1–3) and dinuclear (BC n , n = 1–7) a-diimine Ni-based catalysts bearing different backbones, ortho-substituents and spacers between the active centers. Significantly, catalyst BC 2 through an optimum bulkiness in its structure and electronic and steric effects, had the highest activity among all the dinuclear and mononuclear analogues, and made poly(1-hexene) with high M w (1.7 Â 10 6 g mol À1 ) and a narrow MWD (2.2). Moreover, a lower level of branching density was observed for the samples obtained by catalysts BC 1 and BC 2 (83–85/1000C) in comparison to MC 1–3 (107–120/1000C) and BC 3–7 (131–150/1000C). The properties of polyolefins in particular can be dramatically tuned by the type of catalyst used in the polymerization. 1,2 Among these polyolefins, poly(1-hexene) or poly(1-octene) have several applications as lubricants, adhesives, linings, elastomers, and drag-reducing agents. 3 The microstructures of these polymers can be controlled by changing the catalyst structure and polymerization conditions. The ability of late transition metal catalysts to yield branched polyolefins without using comonomers, be activated with different cocatalysts, and synthesize functional copolymers has been considered their main advantages. 4 Moreover, the backbone structure, spacer nature, and substituent positions of these catalysts determine their behaviour and the properties of the produced polymers. Besides, cooperative effects in multinuclear catalysts can affect the polymer architecture. 1 Cooperative effects are expressed when secondary interactions take place between weakly basic monomer substituents (such as C–H or –Ph struc- tures) and a second metal center. The steric and electronic effects of the bulky groups surrounding the metal center, along with the nuclearity effect in terms of synergistic reactivity patterns involving two or more metal centers, affect propagation, chain transfer, and chain walking rates, and consequently the molecular weight and branching frequency of the produced polymers. These phenomena depend strongly on the catalyst architecture, and in dinuclear complexes on interactions between the active centers. Although the background of this work is consistent with our and other previous reports on different types of substituents, bridges and backbones in late transition metal catalysts based on Ni which were used in the (co)polymerization of ethylene and a broad distribution of products is reported, the effect of length, nature and groups on the bridge structure is still an ambiguous point regarding the previous results. 5–12,14 In addi- tion, there should be an optimum electronic environment for active sites to show efficient synergistic and cooperative effects. It could be concluded that the structural features such as the absence or non-effective protection of axial sites can make the catalyst an oligomer producer or a producer of mixtures of oligomers, waxes and low to high molecular weight polymers. Moreover, the length and nature (rigid/flexible) of the bridge in regard to cooperative effects are very crucial, where a long distance between the active sites can cause an independent behavior of each center. This behavior can also be observed if bulky substituents are replaced on the ortho position of the aromatic rings in the bridge structure. In contrast, the perfor- mance of the metal centers in close proximity is different. As the nature and length of the bridge are important, the presence of substituents also has a high impact on the behavior of the catalysts. Besides, the effect of monomer length could be remarkable as well as an agostic interaction between the atom of the pendant groups and the second metal center. Herein, we investigated how different backbones, substituents, and linkage structures affect the behaviour of a-diimine Ni-based catalysts at various [Al]/[Ni] molar ratios for the production of poly(1-hexene). Organic ligands and complex structures bearing acenaphthene and methyl groups on the backbone, methyl and isopropyl groups on the aryl rings, and rigid/flexible bridges with different distances between the metal centers were studied. a Department of Catalyst, Iran Polymer and Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran, Iran. E-mail: S.ahmadjo@ippi.ac.ir b Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada c Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, P.O. Box: 91775, Mashhad, Iran † Electronic supplementary information (ESI) available: Experimental details including the synthesis route and 1 H NMR, FT-IR and mass spectra and elemental analysis of the ligands and complexes. See DOI: 10.1039/c8nj01678j Received 7th April 2018, Accepted 19th April 2018 DOI: 10.1039/c8nj01678j rsc.li/njc NJC LETTER Published on 24 April 2018. Downloaded by University of Alberta on 7/24/2018 5:32:00 AM. 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