In this study, a practical and inexpensive procedure for the modification of the Grubbs first generation catalyst is reported. A highly controllable metathesis catalyst system was obtained by in situ modification of the Grubbs catalyst with tridentate Schiff base (O-N-O) ligands. The latent catalyst was activated by the addition of HCl, which allowed control of its initiation rate by varying the HCl/Ru ratio. Due to the superior control over the catalyst initiation step, improved molecular weight control in the ring-opening metathesis polymerization (ROMP) of cyclooctene (COE) was achieved using these complexes. ROMP polymers obtained by these catalyst systems have relatively narrow molecular weight distributions, and the molecular weight of the polymers can be controlled on a wide scale. In addition, the reversible inhibition and activation of the catalysts were studied the in ring-closing metathesis reaction (RCM) of diethyldiallylmalonate. RCM of diethyldiallylmalonate can be halted by the introduction of tridentate Schiff base ligands to the reaction medium. The reaction can be switched on again by the introduction of trace amounts of HCL. (C) 2013 Elsevier B.V. All rights reserved.