A binary catalytic system, RuCl2(N-heterocyclic carbene)(p-cymene)/alkyne, was developed for improved molecular weight control in ring-opening metathesis polymerization (ROMP) reactions of norbornene derivatives in organic and aqueous media. Monometallic ruthenium arene compounds were activated using aryl and aliphatic terminal alkynes to form highly active metathesis species. The effects of alkyne structure and concentration on the overall catalytic activity were systematically investigated. The catalytic activity of the metathesis active species can be tuned by varying alkyne substituents. Also, the initiation rate of the ROMP reaction can be tuned by increasing the alkyne-to-Ru ratio. ROMP polymers with a wide range of molecular weights (91-832kDa) were isolated in organic media, whereas polymers with a molecular weight range of 110-280kDa with average particle sizes of 150-250nm were isolated in aqueous media. Copyright (c) 2016 John Wiley & Sons, Ltd.