[MX2(CO)(3)(DAB)] (M=Mo, W; X=I, Br) (5-8) complexes bearing the 1,4-diazobutadiene (DAB) ligand RN=C(Ph)-C(Ph)=NR [R=(CH2)(3)Si(OEt)(3)] were immobilized in MCM-41 mesoporous silica. The tethering, stepwise procedure started with treatment of the MCM-41 mesoporous material with a toluene solution of the 1,4-diazobutadiene ligand, under reflux. The molybdenum and tungsten organometallic cores were subsequently introduced into the ligand-silicas by pore volume impregnation of a solution of the complexes [MX2(CO)(3)(NCMe)(2)] (M=Mo, W; X=Br, I). The modified materials were extensively characterized by several techniques, such as FTIR, solid-state MAS and CP MAS NMR (C-13, Si-29), powder XRD, and nitrogen adsorption-desorption measurements. These new materials (containing 2.6-2.9 wt.% Mo or 0.4-0.6 wt.% W) catalyze the ring-opening metathesis polymerization (ROMP) of norbornene (NBE) and norbornadiene at 328 K, in contrast with the very low activity exhibited by the precursor complexes and with their behavior at lower temperature. Addition of AlCl3 as a co-catalyst enhanced the catalytic performance of the material MCM-DAB-MoBr2 (12) in the ROMP of NBE. (c) 2006 Elsevier B.V All rights reserved.