The alkenyl substituted phenoxy-imine complexes [2-C3H5-6-(2, 3, 5, 6-C6F4H-NCH)C6H3O](2)TiCl2 (C3H5=CH2 CHCH2 or CHCHCH3) are synthesized and characterized by H-1 NMR, C-13 NMR, and elemental analysis. When activated by MAO, they show high activity for the polymerization of ethylene to UHMWPE under different conditions (temperatures and polymerization time). Most of the resulting polymers have high molecular weights (>1.0 x 10(6) gmol(-1)) and high melting points as well as crystallinity. To clarify the effect of the alkenyl group on the catalytic performance and the resultant polymer microstructure, the corresponding saturated complexes of type [2-C3H7-6-(2, 3, 5, 6-C6F4H-NCH)C6H3O](2)TiCl2 where C3H7=-CH2CH2CH3 or CH(CH3)(2) were synthesized and tested as catalysts in ethylene polymerization under the same reaction conditions. The microstructure and morphologies of these two species of PE samples were fully compared by the analysis of C-13 NMR, GPC, DSC, and SEM. As a result, the allyl substituted complex show the highest activity to prepare the highest molecular weight polyethylene of all the catalysts. An interesting feature of the UHMWPE produced by these four catalysts is that they contain only a few short-chain branches (mainly methyl, isobutyl and 2-methylhexyl branches) in a low amount (<2.7 branches/1000 C). (c) 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3808-3818