A seenes of cationic complexes [Rh(diene){Ph2P(CH2)(n)Z}][BF4] (diene = 1,5-cyclooctadiene (cod), tetrafluorobenzobarralene (tfb) or 2,5-norbonadiene (nbd) containing functionalized phosphine ligands of the type Ph-2(CH2)(n)Z(n = 2, or 3; Z = OMe, NMe2, SMe) have been prepared and characterized These complexes have shown a great catalytic activity for phenylacetylene (PA) polymerization. Catalyst screening and optimization have determined the superior performance of complexes containing a P, N-functionalyzed phosphine ligand, [Rh(diene){Ph2P(CH2)(3)NMe2}][BF4] (diene = cod, 5; tfb, 6, nbd, 7), and tetrahydrofuran as solvent The influence of the diene ligand and the effect of temperature, PA to rhodium molar ratio, addition of water or a cocatalyst, DMAP(4-(dimethylamino)pyridine), have been studied Diene ligands strongly influence the catalytic activity and complexes 6 and 7 are far more active than 5 Both complexes gave poly(phenylacetylene) (PPA) with very high number-average molecular weights (M-n) of 970 000 (6) and 1420 000 (7) The addition of DMAP resulted in a dramatic drop in the PPA molecular weight, 106 000 (6) and 233 000 (7). The PPA obtained with the system 6/DMAP showed a narrow molecular weight distribution (M-w/M-n = 120) and incremental monomer addition experiments have demonstrated the quasi-living nature of the polymerization by H-1 and C-13{H-1} NMR spectroscopy and shows a CIS-transidal configuration with a high level of steroregularity (CIS content superior to 99%) TGA, DSC, and IR analysis have revealed a thermal CIS <-> trans isomerization process at 150 degrees C The mechanism of PA polymerization has been investigated by spectroscopic means, under stoichiometric and catalytic conditions, and shows an active role of the hemilabile phosphine ligand both in the initiation and, probably, in the termination steps through proton transfer processes involving the hemilabile fragment of the ligand