Combinations of the ansa-chloroneodymocene precursor [(Cp-CMe2-Flu)Nd(mu-Cl)](2) (2) and a dialkylmagnesium (1-100equiv. versus Nd) have been explored for styrene oligomerization and compared to the single-component polymerization catalyst [Cp-CMe2-Flu]Nd(allyl)(THF) (1). Binary systems derived from 2 and Mg(n-Bu)(2) or Mg(allyl)(2) are moderately active at 60 degrees C (1-8 kg PS mol Nd-1 h(-1)), yielding soluble oligostyrenes (M-n = 1600-6500 g mol(-1), M-w/M-n = 1.3-2.5), which have an unprecedented high degree of syndiotacticity (P-r = 94%, determined by C-13 NMR spectroscopy) and are selectively end-capped by butyl or allyl groups (identified by H-1 NMR and/or MALDI-TOF-MS). The formation of the oligostyrenes is proposed to arise exclusively from a coordination/insertion mechanism, involving in situ generated Nd-alkyl species that are active for syndiotactic styrene polymerization and undergo chain transfer via transmetallation to excess dialkylmagnesium reagent. (C) 2007 Elsevier B.V. All rights reserved.