Radical Polymerization Initiated by Bergman Cyclization

The diradical generated by the Bergman cyclization of 3,4-benzocyclodec-3-ene-1,5-diyne is shown to initiate the radical polymerization of several monomers. Methacrylates are polymerized to high molecular weight by the diradical initiator much more efficiently than other monomers. The relation between the rate of polymerization and the degree of polymerization indicates that the polymer primarily propagates as a monoradical. This monoradical growth is in agreement with established theory predicting that diradical initiators can produce high polymer only through chain transfer followed by monoradical growth due to the rapid intramolecular termination of short diradical chains. In agreement with this mechanism, the polymerization rate of acrylonitrile initiated by the diradical is shown to increase by more than 20-fold upon addition of a chain transfer agent. Small molecule products consistent with intramolecular termination of diradical oligomers were isolated, and the structures of these molecules suggest how the diradical self-terminates in the absence of chain transfer.