Synthesis and characterization of low- and medium-molecular-weight hyperbranched polyethylenes by chain walking ethylene polymerization with Pd–diimine catalysts

Abstract

We report in this article the synthesis and characterization of a range of hyperbranched polyethylenes having various low and medium molecular weights by chain walking ethylene polymerization with Pd–diimine catalysts of reduced ligand steric crowdedness, which are intended for potential applications as novel synthetic base stocks. Four Pd–diimine catalysts featured with different ligand crowdedness, ([(RC6H3NC(R′)–C(R′)NC6H3R)Pd(CH3) (NCMe)]SbF6) (1, R = 2,6-(iPr)2, R′ = CH3; 2, R = CH3, R′ = H; 3, R = 2,6-(iPr)2, R′ = H; 4, R = CH3, R′ = CH3), were employed herein for ethylene polymerizations at different conditions. Generally, reducing ligand steric crowdedness (in the order 1 > 4 > 3 > 2) leads to decreased catalyst activity and dramatically reduced polymer molecular weight. As opposed to high-molecular-weight polymers (weight-average molecular weight (Mw): about 150 kg/mol) obtained with catalyst 1, low-molecular-weight polymers (Mw: below 1.0 kg/mol) were obtained with 2 and 3, and medium-molecular-weight polymers (Mw: about 25 kg/mol) were produced with 4. Despite their various reduced molecular weights, the polymers are all featured with highly branched chain structures with a total branching density of above 100 branches per 1000 carbons. The low- and medium-molecular-weight hyperbranched polymers synthesized with 2–4 display good potential for applications as synthetic base stocks. In comparison with three commercial poly(α-olefin) based synthetic base stocks, they exhibit similar thermal and viscosity properties. Meanwhile, it is also discovered that a subsequent one-pot hydrogenation step can be incorporated in the process after the Pd–diimine catalyzed polymerization step to render nearly fully saturated hyperbranched polymers without the use of additional hydrogenation catalysts.