Methylaluminoxane (MAO) alone acts as a weak catalyst for the ring-opening metathesis polymerisation of norbornene, thereby providing good evidence that some transient [Al]double bondCdrop metallacarbenes are formed and propagate the reaction. A novel general mechanism of initiation involving generation of metallacyclopentanes, followed by ring contraction to metallacyclobutanes, is proposed. Methylaluminoxane is also a very effective cocatalyst for Mo and W chlorides, and especially for non-ionic carbonyl compounds of these two metals. Remarkably cis/trans highly stereo-blocky polymers are obtained which for the first time show clear resolution (m/r) on some of the C-13 NMR lines. (C) 2003 Elsevier B.V. All rights reserved.