Stereoregular polymerization of phenylacetylene using alkynyl and methyl rhodium(I) complexes with functionalized phosphine ligands: linear vs. branched poly(phenylacetylene)s

The alkynyl [Rh(CC-Ph)(nbd){Ph₂P(CH₂)₃Z}₂] and [Rh(CC-Ph)(cod){Ph₂P(CH₂)₃Z}] (cod = 1,5-cyclooctadiene, nbd = 2,5-norbornadiene) and methyl complexes [Rh(CH₃)(cod){Ph₂P(CH₂)₃Z}] featuring functionalized phosphine ligands (Z = NMe₂, OEt) have been prepared. These complexes efficiently catalyze the polymerization of phenylacetylene in the absence of a base to afford stereoregular poly(phenylacetylene)s with high molar masses. Polymer characterization by SEC-MALS and A4F-MALS revealed a bimodal molar mass distribution due to the presence of a high molar mass polymer fraction. The diene ligand in the alkynyl complexes influences the morphology of the polymers. The poly(phenylacetylene)s prepared with the square-planar alkynyl complexes [Rh(CC-Ph)(cod){Ph₂P(CH₂)₃Z}] having cod as the diene are linear, whereas the catalysts [Rh(CC-Ph)(nbd){Ph₂P(CH₂)₃Z}₂] with nbd as the diene afford PPAs with a fraction of high molar mass branched polymer. However, neither the catalytic performance nor the PPA morphology is affected by the donor function of the functionalized phosphine ligand (–NMe₂ or –OEt). The alkynyl complexes having nbd as the diene ligand, [Rh(CC-Ph)(nbd){Ph₂P(CH₂)₃Z}₂], are much more active than the square-planar alkynyl and methyl complexes having cod as the diene. These catalysts polymerize phenylacetylene at a faster rate than the catalyst [Rh(CC-Ph)(nbd)(PPh₃)₂] bearing a non-functionalized phosphine ligand and afford polymers with a much higher molecular weight.