The iridium derivatives [Ir(diene)(N-N)X] (diene=1,5-cyclooctadiene, 1,5-hexadiene; N-N=1,10-phenantroline, 4,7-dimethyl-1,10-phenantroline, 3,4,7,8-tetramethyl-1,10-phenantroline, 4,7-diphenyl-1,10-phenantroline; X=Cl, Br, I) catalyze the polymerization of pheny- lacetylene. The reaction is highly stereoselective with formation of 100% trans-polyphenylacetylene. The catalytic activity has a pronounced dependence upon the choice of the ligands coordinated to iridium: effect of nature of the diene, of the phenantroline, of the halogen, as well as of the reaction conditions (solvent, temperature, presence of a cocatalyst) have been examined in detail. The evolution of the catalyst precursors after addition of the alkyne has been studied by NMR; the results evidence a different behaviour of cyclooctadiene and hexadiene derivatives, which can be related to the differences observed in the catalytic reactions. (c) 2005 Elsevier B.V. All rights reserved.