The ring-opening polymerization of norbornene was investigated in refluxing benzene solution with [MI2(CO)(3)(L')] (M = Mo, W; L = L' = NCCH3, PPh3, AsPh3; L = NCMe, L' = PPh3, AsPh3) as catalysts. Turnover numbers (TNs) after 20 min ranged from 40 to 380 with the highest value recorded for [WI2(CO)(3)(NCMe)(2)]. Under similar conditions Grubbs' catalyst, [RuCl2(PCy3)(2)(CHPh)], was at least 1000-fold more active. A first order relationship was obtained between the initial polymerization rate and [WI2(CO)(3)(NCMe)(2)] concentration. The catalytic activity of [WI2(CO)(3)(NCMe)(2)] showed exponential decay with t(1/2) = 600 s. (C) 2002 Elsevier Science B.V. All rights reserved.