Flexible Monoarylmethyl Substituents in an Efficient Synthesis of Hyperbranched Polyethylene and Polar-Functionalized Polyethylene

Our previous research established the benefits of axially flexible substituents in α-diimine-catalyzed ethylene polymerization, noting their role in enhancing polyethylene branching. In this study, we’ve crafted a set of flexible monoarylmethyl α-diimine Pd(II) catalysts tailored for the synthesis of hyperbranched and polar-functionalized polyethylene. During ethylene polymerization, these catalysts produced polyethylenes with high branching degrees (79–95/1000) and variable molecular weights spanning from 7.7 to 144.4 kg/mol. The polymerization activities ranged from 1.6 to 15.1 × 104 g/(mol Pd·h). When juxtaposed with rigid o-aryl and o-diarylmethyl Pd(II) catalysts, our flexible catalysts notably increased the branching density (94 vs 28–34/1000). These highly branched polyethylenes can effectively modulate the viscosity of Polyalphaolefin (PAO) lubricant base stocks. Comparable trends emerged in the copolymerization of ethylene with methyl acrylate (MA). Furthermore, our flexible Pd(II) catalysts demonstrated superior efficiency in facilitating MA insertion during the copolymerization of ethylene with MA, outperforming their rigid analogues. The hyperbranched structure of the (co)polymers was verified through 13C NMR spectroscopy. Essentially, flexible substituents can afford the catalytic center a more adaptable and dynamic environment, thereby favoring branching formation and MA insertion.