Well-controlled polymerization of phenylacetylenes with organorhodium(I) complexes: Mechanism and structure of the polyenes

A tetracoordinate rhodium complex, Rh(C=CC6H5)(nbd)[P(C6H5)(3)] (nbd = 2,5-norbornadiene), combined with 4-(dimethylamino)pyridine (DMAP) is an excellent initiator for the stereospecific living polymerization of phenylacetylene and its m- and p-substituted derivatives. The rhodium initiator can be generated efficiently by dissociation of triphenylphosphine from isolable Rh(C=CC6H5)(nbd)[P(C6H5)(3)](2) or by reacting Rh(CH3)(nbd)[P(C6H5)(3)](2) or [Rh(OCH3)(nbd)](2)/P(C6H5)(3) With one equivalent of phenylacetylene. The use of a phenylethynyl group, triphenylphosphine, and NBD ligand attached to the rhodium center is crucial for the well-controlled polymerization of phenylacetylenes. An additive, DMAP, is necessary to attain low polydispersities of the polymer products. An active rhodium(I) complex bearing a growing polymer chain, NBD, and P(C6H5)(3) was isolated from a reaction mixture and was characterized by NMR, GC-MS, XPS, and elemental analyses. The isolated active polymer initiates the further polymerization of the same monomer or substituted ones with an almost 100% initiation efficiency to give higher molecular weight homopolymers or block copolymers, respectively. Detailed NMR structural analysis of the products indicated that the polymerization with the rhodium(I) complexes proceeds via a 2,1-insertion mechanism to provide stereoregular poly(phenylacetylene)s with cis-transoidal backbone structure.