Agostic Bonding and Hydrogen Atom Migrations

In the first step, a cis 1,2-insertion of cyclopentene takes place the intermediate derived from this insertion is sterically too demanding to permit insertion of another monomer molecule and is isomerised before another cyclopentene molecule is inserted. Since only /1-hydrogen atoms are available for the stabilisation of the active species (jS-agostic interaction), /1-hydride transfer, followed by monomer rotation or migration, and then reinsertion occur, leading to 1,3-linked polymer chains. The rotation around the Zr olefin n bond results in the formation of cis-1,3-poly(cyclopentylene) [scheme (3a)] [20] while the n face migration of the olefin across the nodal plane of the double bond results in the formation of trans-1,3-poly(cyclopentylene) [scheme (3b)] [19]. 

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