Abstract5‐Phenyl‐2‐pentene (5Ph2P) was found to undergo monomer‐isomerization polymerization with TiCl3–R3Al (R = C2H5ori‐C4H9, Al/Ti>2) catalysts to give a polymer consisting of exclusively 5‐phenyl‐1‐pentene (5Ph1P) unit. The geometric and positional isomerizations of 5Ph2P to its terminal and other internal isomers were observed to occur during polymerization. The catalyst activity of alkylaluminum examined to TiCl3was in the following order: (C2H5)3Al>(i‐C4H9)3Al>(C2H5)2AlCl. The rate of monomer‐isomerization polymerization of 5Ph2P with TiCl3–(C2H5)3Al catalyst was influenced by both the Al/Ti molar ratio and the addition of nickel acetylacetonate [Ni(acac)2], and the maximum rate was observed at Al/Ti = 2.0 and Ni/T