The synthesis of branched polyethylenes by ethylene polymerization with new tandem catalyst systems consisting of methylaluminoxane-preactivated linked cyclopentadienyl-amido titanium catalysts [Ti(eta(5):eta(1)-C5Me4SiMe2NR)Cl-2 (R = Me or Bu-t)] supported on pyridylethylsilane-modified silica (PySTiNMe and (PySTiNBu)-Bu-t) and homogeneous dibromo nickel catalyst having a pyridyl-2,6-diisopropylphenylimine ligand (PyminNiBr(2)) in the presence of modified methylaluminoxane was investigated. Ethylene polymerization with only PyminNiBr2 yielded a mixture of 1- and 2-olefin oligomers with methyl branches [weight-average molecular weight (M-w) similar to 460)] with a ratio of about 1:7. By the combination of this nickel catalyst with PySTiNtBu, polyethylenes with long-chain branches (Mw = 15,000-50,000) were produced. No incorporation of 2-olefin oligomers was observed in the C-13 NMR spectra. Unexpectedly, the combination of the nickel catalyst with PySTiNMe produced lower molecular weight polyethylenes with only methyl branches. The molecular weight distributions of branched polyethylenes obtained with both PySTiNMe and PySTiNtBu combined with the nickel catalyst were broad (weight-average molecular weight/number-average molecular weight < 9). Bimodal gel permeation chromatography (GPC) curves were clearly observed in the PySTiNMe system, whereas GPC curves with small shoulders in low molecular weight areas were observed for PySTiNtBu. The synthesis of branched polyethylenes with tandem catalyst systems of corresponding homogeneous titanium catalysts and the nickel catalyst was also investigated for comparison. (C) 2003 Wiley Periodicals, Inc.