AbstractAn anionic to metathesis transformation reaction has been devised to study the mechanism of initiation of the metathesis ring‐opening polymerisation of cyclic alkenes and, by inference, the metathesis of alkenes also. The products of the reaction have been analysed by g.p.c. using a dual detector system to establish that block copolymers are formed when polystyryl lithium and tungsten hexachloride are used as initiator for the polymerisation of cyclopentene. A mechanism is proposed for the incorporation of polystyrene segments during the initiation step involving the formation of a bimetallic bridged complex that rearranges to form the active centre. There is also evidence that polystyrene segments are introduced during the termination reaction of the kinetic chai