A series of mononuclear (M 1 and M 2 ) and dinuclear (C 1 -C 6 ) Ni α-diimine catalysts activated by modified methylaluminoxane were used in polymerization of ethylene. Catalyst C 2 bearing the optimum bulkiness showed the highest activity (1.6 × 10 6 g PE (mol Ni) −1 h −1 ) and the lowest short-chain branching (32.5/1000 C) in comparison to the dinuclear and mononuclear analogues.Although the mononuclear catalysts M 1 and M 2 polymerized ethylene to a branched amorphous polymer, the dinuclear catalysts led to different branched semicrystalline polyethylenes. Homogeneity and heterogeneity in the microstructure of the polyethylene samples was observed. Different trends for each catalyst were assigned to syn and anti stereoisomers. In addition, thermal behavior of the samples in the successive self-nucleation and annealing technique exhibited different orders and intensities from methylene sequences and lamellae thickness in respect of each stereoisomer behavior. Higher selectivity of hexyl branches obtained by catalyst C 2 showed a cooperative effect between the centers. The results also revealed that for catalysts C 5 and C 6 , selectivity of methyl branches led to very high endotherms and crystalline sequences with melting temperatures higher than that of 100% crystalline polyethylene indicating ethylene/propylene copolymer analogues. For catalysts C 3 and C 4 , more vinyl end groups were a result of the long distance between the Ni centers. Kinetic profiles of polymerization along with a computational study of the precatalysts and catalysts demonstrated that there is a direct relation between rate constant, energy interval of catalyst and precatalyst, and interaction energy of Et···methyl cationic active center (Et···MCC or π-Comp.). Based on this, narrow energy interval (activation energy) of precatalyst and catalyst leads to fast and higher activation rate (catalyst M 2 ), and strong interaction of ethylene and catalyst leads to high monomer uptake and productivity (catalyst C 2 ). Moreover, theoretical parameters including electron affinity, Mulliken charge on Ni, chemical potential and hardness, and global electrophilicity showed optimum values for C 2 .

Polymerization of 1-hexene was carried out using a mononuclear (MN) catalyst and two binuclear (BN 1 and BN 2 ) α-diimine Ni-based catalysts synthesized under controlled conditions. Ethylaluminium sesquichloride (EASC) was used as an efficient activator under various polymerization conditions. The highly active BN 2 catalyst (2372 g poly(1-hexene) (PH) mmol −1 cat) in comparison to BN 1 (920 g PH mmol −1 cat) and the MN catalyst (819 g PH mmol −1 cat) resulted in the highest viscosity-average molecular weight (M v ) of polymer. Moreover, the molecular weight distribution (MWD) of PH obtained using BN 2 /EASC was slightly broader than those obtained using BN 1 and MN (2.46 for BN 2 versus 2.30 and 1.96 for BN 1 and MN, respectively). These results, along with the highest extent of chain walking for BN 2 , were attributed to steric, nuclearity and electronic effects of the catalyst structures which could control the catalyst behaviour. Differential scanning calorimetry showed that the glass transition temperatures of polymers were in the range − 58 to −81°C, and broad melting peaks below and above 0°C were also observed.In addition, longer α-olefins (1-octene and 1-decene) were polymerized and characterized, for which higher yield, conversion and molecular weight were observed with a narrower MWD. The polymerization parameters such as polymerization time and polymerization temperature showed a significant influence on the productivity of the catalysts and M v of samples.

No abstract

Chain walking mechanism with regard to dinuclear Ni-based catalyst stereoisomers in the polymerization of α-olefins: the effects of bridge, backbone and ortho-substituents.

Mononuclear Fe and Ni based catalysts (M 1 and M 2) in the form of single and dual catalytic systems were employed in the presence and absence of diethyl zinc (DEZ) for polymerization of ethylene. In addition, corresponding homo-(M 3 and M 4) and heterodinuclear catalysts (M 7) along with the mononuclear analogues (M 5 and M 6) were used to explore the effect of adjacency of second metal center on the chain transfer efficiency. DEZ had stronger influence on the behavior of mono-and dinuclear Fe-based structures and corresponding thermal and microstructural properties of the PE samples than Ni complexes. A mechanism was proposed for M 5 as the vinylterminated polymer chains increased in the presence of DEZ. More interestingly, M 7 not only showed a cooperative effect for production of a random copolymer containing short and long chain branches but also at low and high concentration of DEZ, a blocky copolymer was obtained through CCTP and CSP. These results were confirmed by 13 C NMR, DMTA, SSA and CEF.