The polymerisation of cyclic and strained monomers such as bicyclo[2.2.1)hept-2-ene (norbornene, NBE) and bicyclo[2.2.1]hepta-2,5-diene (norbornadiene, NBDE) with the catalytic systems Mo(CO)(5)PPh(3)/alkylaluminums was studied. From the polymers produced we demonstrate the existence of an antagonism between the ROMP (Ring-Opening-Metathesis-Polymerisation) mecanism and the olefin addition polymerisation mecanism. In addition ROMP mecanism is affected by the solvent. These catalysts also promotes the isomerisation of double bonds, as indicated by the ESR signal observed for the conjugated crosslinked polyNBDE produced by ROMP.