Living Cyclocopolymerization through Alternating Insertion of Isocyanide and Allene via Controlling the Reactivity of the Propagation Species: Detailed Mechanistic Investigation

Living cyclocopolymerization
through the alternating insertion
of an isocyanide and allene into palladium–carbon bond was
developed based on the controlling the reactivity of the propagation
species using bidentate ligands. We revealed that the rate of the
presented cyclocopolymerization was depended on the ligands of Pd-initiator.
When the palladium–methyl complexes having appropriate cis-chelating
ligand, such as 1,3-bis­(diphenylphosphino)­propane (dppp), were used
as initiator, the cyclocopolymerization of bifunctional aryl isocyanides
(1) that contain both isocyano and allenyl moieties polymerized
to afford poly­(quinolylene-2,3-methylene)­s with controlled molecular
weight and narrow molecular weight distributions. The resulting polymer
was characterized by 1H and 13C NMR analyses,
which clearly showed that the terminal moiety of the polymer formed
well-defined organopalladium complex as the resting state for the
polymerization, which could undergo further polymerization; not only
cyclocopolymerization with 1 but also homopolymerization
of simple aryl isocyanide. In the analysis of the cyclocopolymerization
mechanism, we conclusively demonstrated that the insertion reaction
of isocyanide is the rate-determination step in the cyclocopolymerization,
which proceeds via a five-coordinate intermediate with a geometrical
change. The cis-chelating ligand controls the site interchange reaction,
which dominates the reactivity of propagation species.