Flexible Monoarylmethyl\nSubstituents in an Efficient\nSynthesis of Hyperbranched Polyethylene and Polar-Functionalized Polyethylene

Our previous research established the benefits of axially\nflexible\nsubstituents in α-diimine-catalyzed ethylene polymerization,\nnoting their role in enhancing polyethylene branching. In this study,\nwe’ve crafted a set of flexible monoarylmethyl α-diimine\nPd(II) catalysts tailored for the synthesis of hyperbranched and polar-functionalized\npolyethylene. During ethylene polymerization, these catalysts produced\npolyethylenes with high branching degrees (79–95/1000) and\nvariable molecular weights spanning from 7.7 to 144.4 kg/mol. The\npolymerization activities ranged from 1.6 to 15.1 × 10<sup>4</sup> g/(mol Pd·h). When juxtaposed with rigid <i>o</i>-aryl and <i>o</i>-diarylmethyl Pd(II) catalysts, our flexible\ncatalysts notably increased the branching density (94 vs 28–34/1000).\nThese highly branched polyethylenes can effectively modulate the viscosity\nof Polyalphaolefin (PAO) lubricant base stocks. Comparable trends\nemerged in the copolymerization of ethylene with methyl acrylate (MA).\nFurthermore, our flexible Pd(II) catalysts demonstrated superior efficiency\nin facilitating MA insertion during the copolymerization of ethylene\nwith MA, outperforming their rigid analogues. The hyperbranched structure\nof the (co)polymers was verified through <sup>13</sup>C NMR spectroscopy.\nEssentially, flexible substituents can afford the catalytic center\na more adaptable and dynamic environment, thereby favoring branching\nformation and MA insertion.