Electrophilic chain polymerization of 1,3-pentadiene initiated by aluminium triflate were performed in polar and non-polar media. The influence of various parameters such as temperature. initiator and monomer concentrations, were studied. Polymerization was carried out at room temperature (20°) in non-polar media as well in polar media. A mechanism of polymerization is proposed via coinitiation with crystallization water and/or triflic acid coming from triflate synthesis, a transfer reaction to the polymer, an intramolecular cyclization and a crosslinking reaction. It has been shown that the trans-isomer is more reactive than the cis-isomer and gives strictly 1,2 and 1,4 sequences. The cis-isomer gives, besides 1,2 and 1,4 also 3,4 sequences and a higher tendency of cycle formation.