Efficient Modulation of Polyethylene Microstructure by Proper Activation of (α-Diimine)Ni(II) Catalysts: Synthesis of Well-Performing Polyethylene Elastomers

The activation of a prototypical nickel(II) Brookhart catalyst by either methylalumoxane (MAO) or diethylaluminum chloride (AlEt2Cl) under a variety of conditions showed that a proper choice of the mode of activation is a powerful tool to modulate the polymer microstructure. In particular, use of AlEt2Cl instead of MAO resulted in the production of more branched polyethylenes with a higher content of long chain branches and even some "branches on branches". Characterization of these materials by NMR, thermal, X-ray diffraction, and mechanical analyses provided insight into the relationships between the microstructure and the crystallization behavior and the elasticity of the polymers. For these branched polyethylenes, a transition from plastomeric toward elastomeric behavior occurs for branch concentrations much lower than for ethylene-propylene copolymers and like those observed for ethylene copolymers with bulkier comonomers. For elastomeric materials, reduction of branch concentration implies two relevant advantages: (i) reduction of glass transition temperature becoming closer to that of polyethylene; (ii) more efficient radical cross-linking with reduction of degradation reactions. An additional advantage is, of course, a polymer production process involving only ethylene.