2,4,6-Triphenylpyridinium-Substituted Neutral Nickel Catalysts: Ethylene Polymerization, Influence of Activator, Catalyst Decomposition, and End-Group Analysis

Abstract

The reactivity of 2,4,6-triphenylpyridinium-substituted nickel(II)-salicyliminato catalysts in ethylene polymerization has been explored. The known catalyst 6a equipped with a 2,6-diisopropylphenyl group affords polyethylene with M_n = 10–12 kDa at 19 °C, while 8a possessing the bulkier 2,6-diphenylphenyl moiety gives polymer with M_n = 270–310 kDa. The stability of catalyst 6a depends on the borane as activator. If B(C₆F₅)₃ is employed, enhanced stability is observed compared with that using 3H₂O·B(C₆F₅)₃ activator. Catalyst 6a decomposes to form a hydroxyl-bridged dimer 14 which may be reactivated, albeit inefficiently, by an excess borane activator. The polymer formed by 6a contains ca. 3–6% of 2-trifluoromethylphenyl, pentafluorophenyl, and 3,5-bis(trifluoromethyl)phenyl end groups combined. The pentafluorophenyl end group originates from the reaction of 6a with the borane activator and from the reactivation of hydroxyl-bridged dimer 14. 3,5-Bis(trifluoromethyl)phenyl end groups are likely formed in a reaction sequence initiated by the protonation of one aryl group in NaBArF that creates B[C₆H₃(CF₃)₂]₃. The triarylborane can then react with 6a or 14, transferring the 3,5-bis(trifluoromethyl)phenyl group to nickel.