A latent and switchable ruthenium indenylidene catalyst bearing an imidazole functionalized Schiff base ligand for emulsion ring opening metathesis polymerization (ROMP) reactions in water is reported. This catalyst shows latent behavior towards olefin metathesis reactions and can be activated by addition of acids. The activity of this catalyst is first tested using ring closing metathesis (RCM) and self-metathesis (SM) and later intensively studied for ring opening metathesis polymerization (ROMP) reactions. By varying the acid/ ruthenium ratio, a high control of ROMP of 5-norbornene-2yl-acetate in both organic and aqueous media was achieved. In emulsion ROMP reactions, the molecular weight of the emulsion polymers can be controlled, while keeping the particle sizes between 11 and 21 nm. In addition to its latent behavior, the catalyst displayed a high affinity to silica gel, which eases the separation of ruthenium residues from the metathesis products. (C) 2014 Elsevier Ltd. All rights reserved.