The comparative reactivity of 7-oxa-benzonorbornadiene in metathesis copolymerization reactions using Ru-based initiators has been investigated. It is as reactive as norbornene, about 8 times more reactive than cyclopentene, and at least 19 times more reactive than benzonorbornadiene. The role of the proximate 7-oxygen atom in facilitating the transition state of the [2 + 2] cycloaddition step is discussed. (C) 2003 Elsevier B.V. All rights reserved.