Titanium/methylaluminoxane-based catalyst systems polymerising norbornene by vinyl addition were modified to initiate ring-opening metathesis polymerisation by interrupting the course of polymerisation with phenylacetylene. The products showed nearly 30% ring-opened structures according to their H-1 NMR spectra. The change in mechanism of polymerisation after the addition of the reactivity transfer reagent is seen from the kinetic data.