Acetonitrile ligated molybdenum (III) complexes of the structure [MoCl(NCCH3)(5)](2+) bearing different weakly coordinating anions [B(C6F5)(4)](-) (WCA a), [B{C6H3(m-CF3)(2)}(4)](-) (WCA b) and [(C6F5)(3)B-C3H3N2-B(C6F5)(3)](-) (WCA c) were applied as homogeneous catalysts of the polymerization of isobutylene. High monomer conversions were obtained in short reaction times (<30 min). The molecular weight of the resulting polyisobutylene is nearly independent of parameters such as temperature, solvent, monomer concentration, but is strongly influenced by the type of WCA and by chain transfer reactions which were observed in these systems. Highly reactive low molecular weight polyisobutylenes (M-n < 2000 g/mol) were obtained with a high content of exo double bond end groups as shown by H-1 NMR analysis. Furthermore, experiments were performed to reduce the isomerization of these exo end groups into other internal double bonds by varying the polymerization parameters. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3775-3786, 2010