Correlating electronic properties of a nickel polymerisation catalyst with the branching density of a polyethylene

<p dir="ltr">This work investigates how the electronic properties of a nickel ethylene polymerisation catalyst can be used to exert control on key polymerisation steps namely chain walking and chain propagation and in turn affect branching and polymer molecular weight. Specifically, a series of sterically enhanced 2-(imino)-4-R-pyridine–nickel complexes (R = OMe <b>Ni1</b>, Me <b>Ni2</b>, H <b>Ni3</b>, Br <b>Ni4</b>, CF_{<small>₃</small>}<b>Ni5</b>, NO_{<small>₂</small>}<b>Ni6</b>) that differ only in the electronic properties of the <i>para</i>-R substituent have been prepared and fully characterised. Under activation with EASC (ethyl aluminium sesquichloride), all nickel catalysts exhibited high activities for ethylene polymerisation and produced branched polyethylenes (BPE: branching density range: 61–99/1000 Cs) with a range of molecular weights (4.89–15.4 kg mol^{<small>⁻¹</small>}); the relative order of catalytic activity being: <b>Ni5</b> (CF_{<small>₃</small>}) > <b>Ni4</b> (Br) > <b>Ni1</b> (OMe) > <b>Ni6</b> (NO_{<small>₂</small>}) > <b>Ni3</b> (H) > <b>Ni2</b> (Me). Additionally, cyclovaltametry measurements performed on <b>Ni1</b>–<b>Ni6</b> have been used to measure half-wave potentials (<i>E</i>_{<small>_1/2</small>}) that can serve as a quantitative descriptor of the electronic effect. Moreover, good correlations exist between <i>E</i>_{<small>_1/2</small>}and i) the degree of branching and ii) the polymer molecular weight. Importantly, this study highlights the potential of <i>E</i>_{<small>_1/2</small>}as a predictive tool for rational design of catalysts to make tailored BPE's.</p>