A new siloxy-substituted alpha-diimine compound and its corresponding Ni(II) complex, {bis[N,N'-(4-tert-butyl-diphenylsiloxy-2,6-dimethylphenyl)imino]acenapht hene} dibromonickel (6), were successfully synthesized and the molecular structure of 6 characterized by Xray crystallography. The precatalyst 6 activated by methylaluminoxane (MAO) or diethylaluminum chloride (DEAC) was tested in the polymerization of ethylene, showed to be highly active (e.g. 2.2 X 10(7) and 1.8 X 10(7) g polymer (mol Ni.h.bar)(-1), respectively) and led to a branched polyethylene (ca. 35-55 branches/1000 C). The catalyst system 6/methylaluminoxane (MAO) catalyzes, at -11 degrees C, living polymerization of propylene, to a polypropylene showing a syndiotactic-rich microstructure (P-r = 0.74). 1-Hexene was also successfully polymerized via a living process, both at -11 and + 16 degrees C. The C-13 NMR spectra of the poly(1-hexene)s obtained at room temperature show a microstructure almost exclusively composed by n-butyl and methyl branches, the latter being present in a much higher number. Diblock polypropylene-block-poly(1-hexene) and triblock poly(1-hexene)-block-poly(propylene-ran-1-hexene)-block-poly(1-hexene) copolymers have also been synthesized and characterized by GPC/SEC, DSC and NMR. (c) 2005 Elsevier Ltd. All rights reserved.