1-Hexene polymerizations catalyzed by a-diimine nickel complexes after activation with modified methylaluminoxane were performed at various reaction temperatures. Effects of catalyst structure and polymerization temperature on activity and polymer microstructure were evaluated in detail. Bulky catalyst 1b with camphyl backbone exhibited good control ability and greatly enhanced thermal stability to be capable of polymerizing 1-hexene at 80 degrees C. The poly(1-hexene)s with long methylene sequences and dominate branches (methyl and butyl) were synthesized using catalyst 1b. Differential scanning calorimetry analysis further confirmed that long polymethylene block (?(CH2)n?, n > 20) was formed in the poly(1-hexene)s with melting point of 64 degrees C obtained by catalyst 1b on the basis of initial branched model polyethylene. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012