Zwitterionic Nickel(II) Catalysts for CO-Ethylene Alternating Copolymerization.

A new zwitterionic nickel(II) catalyst that comprises a partially fluorinated tetrakis(aryl)borate center in a bidentate phosphine ligand and a cationic Ni center has been developed and studied for CO-ethylene copolymerization in the context of comparison with a previously reported zwitterionic catalyst that carries a nonfluorinated borate anion. The crystal structures of several zwitterionic and related nickel compounds are characterized. Partial fluorination of the tetrakis(aryl)borate only brings a modest increase in productivity (2700 vs 1600 g of polyketone per gram of Ni, g (g of Ni)-1). Like the nonfluorinated catalyst, the fluorinated zwitterionic catalyst is extremely active at the beginning of the polymerization but deactivates rapidly. Deactivation of the two catalysts apparently follows different mechanisms. Stoichometric decomposition studies show that the partially fluorinated tetrakis(aryl)borate in the Ni compounds is stable under acidic conditions either directly introduced by addition of an acid or created by a CO atmosphere. In contrast, the nonfluorinated tetrakis(aryl)borate is readily decomposed by an acid or under acidic conditions created by CO. For the new catalyst system with the partially fluorinated tetrakis(aryl)borate anion, the deactivation likely involves initially redox processes and eventually ligand redistribution around Ni, as inferred from the stoichiometric decomposition studies. It turns out that such a process allows the deactivated catalyst to be reactivated by H2. When the polymerization is carried out in the presence of H2, the productivity of the new zwitterionic catalyst can reach 6400 g (g of Ni)-1. The zwitterionic catalyst with the nonfluorinated tetrakis(aryl)borate anion cannot be reactivated by H2. A cationic analogue of the zwitterionic catalysts is also studied for comparison. Its productivity for CO-ethylene copolymerization (230 g (g of Ni)-1) is about 1 order of magnitude lower than that of the zwitterionic catalysts, demonstrating the critical role of the zwitterionic character in attaining the aforementioned high productivity. At the productivity level of the zwitterionic catalysts, which to our knowledge is among the highest observed for Ni catalysts, an unacceptable amount of residual Ni(II) species is left in the product, causing the alternating CO-ethylene copolymer to begin to decompose near its melting temperature and hence making melt processing difficult. [ABSTRACT FROM AUTHOR]