A composition curable by a metathesis reaction upon mixing its components and comprising an olefin-containing substrate, a metathesis catalyst, and a reaction control agent for slowing the progress of the metathesis reaction. The metathesis catalyst is a ruthenium or osmium curbene complex catalyst, such as one containing an alkylidene ligand with basicity higher than that of tricyclohexylphosphine (PCy.sub.3) and a neutral electron donor ligand with a basicity lower than that of PCy.sub.3, whereby the catalyst initiates the metathesis reaction of the composition upon mixing with the substrate. The reaction control agent is an organic compound that contains carbon-carbon double and/or triple bonds and one or more Group 14 atoms and is present in an amount effective to slow the progress of the metathesis reaction. The olefin containing substrate may be, for example, one or a combination of a di-, tri- or quadri-functional polydimethylsiloxane end-capped with norbornenylethyl groups or a polydimethylsiloxane tethered and end-capped with norbornenylethyl groups.