Neutral nickel (II) catalysts are sufficiently tolerant of water (and other protic compounds) that polymerization can be carried out in the presence of water. Single component neutral nickel (II) catalysts based on salicylaldimine ligands are capable of producing high molecular weight polymers, and have been used to produce high molecular weight polyethylene in mini-emulsion and micro-emulsion media. By using a displaceable hydrophilic ligand (e.g. TPPTS) catalysts can be made water soluble, negating the need for any organic phase. These water soluble catalysts become hydrophobic under polymerization conditions upon initiation of chain growth. In the absence of organic media, crystallization of the formed polyethylene is faster than the growth of the polymer chain. The ordered deposition of the chain leads to the production of the polyethylene in the form of highly ordered nanocrystals, free from entanglements and possessing extremely high crystallinity. Careful catalyst design is needed to produce suitably high molecular weight polymers. Of particular importance is the suppression of β-hydrogen elimination, a common chain transfer pathway with late transition metal catalysts, which can be controlled by the choice of remote substituents. Through careful catalyst design and control of reactor conditions, ‘ideal polyethylene nanocrystals’ with the melting behavior of linear ultra-high molecular weight polyethylene (UHMWPE) can be synthesized. The creation of disentangled UHMWPE in aqueous media is unique to these catalysts and can in principle be used in blending applications with other aqueous polymer dispersions. The potential for creating composites with these nanocrystals was previously demonstrated with post-polymerization functionalization with graphene. Despite the potential presented by these systems, little is understood about the catalytic species in water. Beyond further suppression of β-hydrogen elimination and chain transfer, of particular interest are the activation and deactivation processes, the nature of the hydrophobic species and how interactions of the active species with water effect the polymerization.

Aqueous polyethylene nanocrystal dispersions from catalytic polymerization

253rd National Meeting (2017)

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