The polymerization of 1,3-pentadiene initiated by AlCl3 was investigated in a nonpolar medium at room temperature. It was shown that, besides propagation, the system undergoes various side reactions such as cross-linking, cyclization, double bond isomerization,.... An examination of the various possible structures of the polymer showed that the comparison of the H-1 and C-13 quantitative NMR spectroscopies allows the determination of the relative importance of cyclization and double bond isomerization. It is shown that some of these side reactions can be kept under control : for instance crosslinking could be avoided at low monomer concentration, while cyclization could be strongly decreased at high monomer concentration and by catalyst sublimation. Double bond isomerization was eliminated in the presence of a transfer agent such as methyl-2-butene-2.