The reaction of alpha,alpha＇-dibromometaxylene (DBMX) with sodium cyclopentadienide (CpNa) gives several oligomers, which were characterized by Size Exclusion Chromatography (SEC) and Liquid Secondary Ion Mass Spectroscopy (LSIMS). The obtained oligomers came from two competitive mechanisms after an anionic substitution of the DBMX by CpNa. The first was based on the proton transfer between the monosubstituted DBMX and CpNa giving a transcient monobrominate carbanion which is the propagating species of a polyanionic substitution. The second one is a classical Diels-Alder reaction between two oligomers containing substituted cyclopentadienes (CPDs) and also between an oligomer and the CPD formed in situ by hydrogen transfer. (C) 2000 Elsevier Science Ltd. All rights reserved.