Cationic Rhodium Complexes with Hemilabile Phosphine Ligands as Polymerization Catalyst for High Molecular Weight Stereoregular Poly(phenylacetylene)

Abstract

A series of cationic complexes [Rh(diene){Ph2P(CH2)nZ}][BF4] (diene = 1,5-cyclooctadiene (cod), tetrafluorobenzobarralene (tfb) or 2,5-norbonadiene (nbd)) containing functionalized phosphine ligands of the type Ph2P(CH2)nZ (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)3NMe2}][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 (Mn) of 970 000 (6) and 1 420 000 (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 (Mw/Mn = 1.20) and incremental monomer addition experiments have demonstrated the quasi-living nature of the polymerization reaction under these conditions. The PPA obtained with these catalytic systems has been characterized by 1H and 13C{1H} NMR spectroscopy and shows a cis−transoidal 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 °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.