Highly branched polyethylene oligomers via group IV-catalysed polymerization in very nonpolar media

Abstract

Early transition metal catalysts produce high-density and linear low-density polyethylenes with spectacular efficiency. Nevertheless, these catalysts are ineffective in producing low-density polyethylene homopolymers with large –(CH2)xCH3 branch densities (x ≥ 5) or low-molecular-mass (Mn < 1,200 g mol−1) highly branched polyethylenes (HBPEs). The latter are potential alternative synthetic lubricants that have eluded efficient catalytic synthesis. Here we report the synthesis of low-Mn HBPEs with 61–93 branches per 1,000 carbon atoms from abundant ethylene as the primary feedstock using a soluble, highly active ion-paired organozirconium catalyst in a saturated hydrocarbon solvent. The unprecedented activity and branch selectivity reflect previously unrecognized aspects of the cationic catalyst–counteranion pairing in nonpolar media and are characterized spectroscopically and quantum mechanically. The HBPE products are rheologically and tribologically attractive candidates for synthetic lubricants. Low-molecular-weight, highly branched polyethylenes are attractive candidates for synthetic lubricants, but their efficient production is constrained by a lack of effective catalytic methods. While conventional group IV transition metal catalysts produce diverse polyethylenes on a huge scale, they are unable to produce highly branched polyethylenes. Here a hydrocarbon-soluble organozirconium precatalyst and borate cocatalyst produce the desired polyethylenes with excellent activity and branch selectivity.