Nanocrystal Formation in Aqueous Insertion Polymerization

Abstract

High molecular weight linear polyethylene, in the form of 7 nm thin single-lamella nanocrystals with a diameter of up to 200 nm, is generated via aqueous catalytic polymerization using the water-soluble catalyst precursor [κ2-N,O-{2,6-(3′,5′-(F3C)2C6H3)2C6H3-N═C(H)-(3,5-I2-2-O-C6H2)}NiCH3{P(3-C6H4SO3Na)3}] (1-TPPTS). Studies under true pressure reactor conditions using a 13C-labeled catalyst precursor 1-13C-TPPTS show that in the first 30 s, 21–28% of the catalyst precursor initiates polymer chains. Catalyst deactivation occurs preferentially by hydrolysis, resulting in the formation of fully saturated polyethylene chains. The catalyst lifetime can be extended for more than 10 h upon addition of weakly coordinating compounds like N,N-dimethylformamide. Alternatively, at strongly basic conditions (pH 12.5) nanoparticle dispersions consisting of polyethylene with molecular weight of up to Mn = 7 × 105 g mol–1 and Mw/Mn = 1.2 are accessible after 60 min polymerization time. “Living”-type polymerization also can be performed by substituting water with deuterium oxide as the reaction medium. Here, catalyst lifetimes of more than 24 h are observed, and dispersions with more than 20 wt % nanocrystals are accessible at turnover frequencies (TOF) of ∼4000 ethylene (catalyst precursor)−1 h–1. A pressurizable sampling chamber was used to draw samples at high pressure and enabled the in-depth analysis and monitoring of the polymerization reaction and the particle formation in a time range of 30 s–24 h. It was shown that under most reaction conditions studied each nickel center produces one single-chain polyethylene particle, and the diameter of the resulting nanoplatelet is determined by the molecular weight of the polyethylene chain.