This invention relates to ruthenium initiators for the ring-opening metathesis polymerization (ROMP) of cyclic olefins that are inactive at room temperature but activatable at elevated temperature (referred to as (“thermally switchable”). In general, these are compounds of ruthenium metal to which are bonded four or five ligands, in which one ligand is a pyridine ring in which the nitrogen is chelated to the ruthenium, or a five or six membered ring incorporating nitrogen and the ruthenium in which the nitrogen is chelated to the ruthenium.