Article pubs.acs.org/Macromolecules
Macrocyclic Trinuclear Nickel Phenoxyimine Catalysts for HighTemperature Polymerization of Ethylene and Isospecific Polymerization of Propylene Zhongtao Chen, Xiangxiang Zhao, Xinyi Gong, Di Xu, and Yuguo Ma* Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Soft Matter Science and Engineering, Key Lab of Polymer Chemistry & Physics of Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China S Supporting Information *
ABSTRACT: A series of macrocyclic multinucleating phenoxyimine ligands and the corresponding neutral binuclear (2Ni2) and trinuclear (3-Ni3) nickel catalysts have been efficiently synthesized. The trinuclear nickel complex 3-Ni3 showed high activity, high thermal stability, and slow chain transfer in ethylene polymerization, thus producing polyethylene with high molecular weight and low branch density. Highly regiospecific and isospecific polymerization of propylene was also achieved with 3-Ni3, generating regioregular and highly isotactic propylene with high Tm and crystallinity. This is the first example of regio- and stereocontrolled propylene polymerization promoted by nickel phenoxyimine catalysts. Statistical analysis suggested selective 1,2insertion and enantiomorphic site control mechanism in the chain propagation step, likely caused by the unique steric effect of macrocyclic ligands and the potential cooperative effect.
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The β-H elimination was proved to be partially suppressed in ethylene polymerization. To further increase the steric effect and slow down the β-H elimination, we designed macrocyclic trinuclear nickel complex as shown in Chart 1. We expect that the nickel coordination planes would be constrained by the macrocyclic ligand to orient in crowded space with short metal−metal distance. Such macrocyclic multinuclear catalysts mimicked the structures of enzyme25,26 and would be more attractive. Rare examples, such as the cagelike tetranuclear palladium catalysts27 and the macrocyclic trinunuclear 2,6bis(imino)pyridyliron catalyst,28 have been reported. They both produced polyethylene with much higher molecular weight in comparison to their mononuclear analogues, which was attributed to steric protection of metal center. In addition, longer lifetime for ethylene polymerization was also observed in the latter system, in accordance with the enhanced thermal stability in those sterically demanding catalysts system.29−33 Consequently, cyclic trinuclear nickel catalyst was expected to show special properties not only in the regio- and stereocontrol of propylene polymerization but also in ethylene polymerization owing to the unique structure. Herein, we report the synthesis of a series of macrocyclic multinucleating ligands and macrocyclic trinuclear nickel complex 3-Ni3. Complex 3-Ni3 showed exceptional thermal stability and activity in ethylene polymerization, yielding
INTRODUCTION While regio- and stereospecific polymerization of propylene have been achieved by early transition metal catalysts, direct copolymerization of propylene with polar monomers to prepare functionalized isotactic polypropylene was difficult as these early transition metal catalysts were easily poisoned by polar monomers.1−3 Late transition-metal olefin polymerization catalysts have attracted tremendous interest due to their low oxophilicity.4−8 However, even polymerization of propylene by late transition-metal catalysts remained challenging due to the propensity to generate amorphous and atactic polymers as a result of poor regioselectivity, poor stereoselectivity, and fast chain-walking process.9−12 Substantial efforts have been devoted to the control of regio- and stereoselectivity in these systems.13−17 Low polymerization temperature (
