Polymerizations of 1,3-dienes using in situ generated catalyst [(2-methallyl)Ni][B(Ar-F)(4)], 6, (Ar-F = 3,5-bis(trifluoromethyl)phenyl) as well as [(2-methallyl)Ni(mes)][B(ArF)4], 14, (mes = mesitylene) are reported. Highly sensitive complex 6 polymerizes butadiene (BD) at -30 degrees C to yield polybutadiene with a M-n of ca. 10 K and 94% cis-1,4-enchainment while less reactive isoprene (IP) was polymerized at 23 degrees C to yield polyisoprene with M-n ca. 7 K. Complex 6 was also shown to polymerize a functionalized diene, 2,3-bis(4-trifluoroethoxy-4-oxobuty1)-1,3-BD, to polymer with M-n = 113 K. The stable and readily isolated arene complex 14 initiates BD and IP polymerizations at somewhat higher temperatures relative to 6 and delivers polymers with higher molecular weights. Complex [(allyl)Ni(mes)][B(Ar-F)(4)], 13, catalyzes polymerization of styrene to yield polystyrene with high conversion, M-n's = ca. 6 K and MWD = 2. The pi-benzyl complex [(eta(3)-1-methylbenzyl)Ni(mes)] [B(Ar-F)(4)], 19, was detected as an intermediate following chain transfer by in situ NMR studies. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1901-1912, 2010