A new in-situ generated catalytic system for the ROMP of cycloalkenes is prepared by combination of Ru(methallyl)2{Ph2P(CH2)nPPh2} (n = 1, 2, 4, 6) and a bulky 1,3-bis(2,4,6-trimethylphenyl)imidazolinium salt. Cyclooctene is polymerised at 80 °C for 20 min in 75–88% yield or at room temperature, after the activation of this catalytic system by preliminary heating at 80 °C in chlorobenzene.
