New dinitrosylmolybdenum complexes cis-[Mo(NO)(2)Cl-2(MeCN)(2)]Cl and cis-[Mo(NO)(2)Cl-3((PrOH)-Pr-i)](.)3(i)PrOH were synthesized and their catalytic activity was investigated. The cationic complex [Mo(NO)(2)Cl-2(MeCN)(2)](+) was found to catalyze the polymerization of norbornene. However, with EtAlCl2 both the complexes form systems catalytically very active in metathesis (pent-2-ene), polymer degradation by cross-metathesis (polynorbornene and polyphenylacetylene by hex-3-ene), ROMP (norbornene) and polymerization of substituted acetylenes (phenylacetylene, hex-2-yne and tert-butylacetylene). The obtained results indicate that polymerization reaction of tert-butylacetylene is living polymerization, In these catalytic systems, vinyltrimethoxysilane copolymerizes with poly-tert-butylacetylene and parallely induces its partial degradation. The interaction of the precatalysts with EtAlCl2 was also studied. (C) 2002 Elsevier Science B.V. All rights reserved.