Characterization of poly(phenylacetylene) (PPA) samples produced using Rh(I) complexes featuring hemi-labile phosphine ligands by size exclusion chromatography, multi-angle light scattering, (SEC-MALS), or asymmetric field flow fractionation (A4F)-MALS has revealed that some of these PPA samples contain a mixture of linear and branched polymer. The occurrence and extent of branching is dependent on both catalyst structure and polymerization conditions. The levels of branching are consistent with either terminal branching through copolymerization of macro-monomer or chain transfer to polymer, where the branched species are less reactive towards further polymerization than the linear chains. The MM dependence of B, the number of branches per molecule, or λ, the number of branches per repeat unit, suggests that the latter explanation may be correct but further work is needed.

Dinuclear phosphine-amido, [Rh 2 (diene){μ-NH(CH 2 ) 3 PPh 2 } 2 ], and cationic phosphine-amino complexes, [Rh(diene){Ph 2 P(CH 2 ) 3 NHR}] + (diene = cod, nbd, tfb) and [Rh{Ph 2 P(CH 2 ) 3 NHR} 2 ] + , have been prepared from the corresponding amino-functionalized phosphines Ph 2 P-(CH 2 ) 3 NHR (R = H, Me) and suitable rhodium(I) precursors. The dinuclear [Rh 2 (diene){μ-NH(CH 2 ) 3 PPh 2 } 2 ] complexes bearing π-acceptors diene ligands such as nbd or tfb exhibit a remarkable catalytic activity in phenylacetylene (PA) polymerization affording stereoregular polyphenylacetylenes with, unlike the cod precursor, unimodal molar mass distributions of very high molecular weights, M w up to ≈ 1.2 × 10 6 , and moderate polydispersity indexes. These complexes are more active than the mononuclear phosphino-anilido [Rh(diene){Ph 2 P(C 6 H 4 )NMe}] complexes, which are in turn more active than the cationic complexes [Rh(diene){Ph 2 P(CH 2 ) 3 NHMe}] + , [Rh(nbd){Ph 2 P(CH 2 ) 3 NH 2 }] + , and [Rh(nbd){Ph 2 P(C 6 H 4 )NHMe}] + bearing the same diene ligand. In contrast, complexes [Rh{Ph 2 P(CH 2 ) 3 NHR} 2 ] + (R = H, Me) without a diene ligand have been found to be inactive in PA polymerization. The excellent catalytic performance of [Rh 2 (diene){μ-NH(CH 2 ) 3 PPh 2 } 2 ] (diene = nbd, tfb) complexes is a consequence of the mode of activation of PA that likely results in the formation of unsaturated alkynyl species [Rh(diene)(CC-Ph)L] (L = PA, THF), which may be competent for PA polymerization.

The mono- and dinuclear rhodium­(I) complexes featuring 2-(diphenylphosphino)­pyridine ligands, [Rh­(cod)­(Ph2PPy)]+ and [Rh­(nbd)­(μ-Ph2PPy)]2 2+ (cod = 1,5-cyclooctadiene, nbd = 2,5-norbornadiene), have been prepared in order to be evaluated as phenylacetylene (PA) polymerization catalysts. In contrast with compound [Rh­(nbd)­{Ph2P­(CH2)2Py}]+, featuring a 2-(2-(diphenylphosphino)­ethyl)­pyridine ligand, that showed a moderate catalytic activity, both [Rh­(diene)­(Ph2PPy)]n n+ (n = 1, cod; n = 2, nbd) complexes showed no catalytic activity due to the formation of unusual dinuclear species [Rh2(diene)2(μ-Ph2PPy)­(μ-CC-R)]+, supported by a Ph2PPy bridging ligand and an alkynyl ligand coordinated in a μ-η1:η2 fashion, which are inactive in PA polymerization. However, compounds [Rh­(diene)­(Ph2PPy)]n n+ efficiently polymerize PA in the presence of a cocatalyst as iPrNH2 affording highly stereoregular poly­(phenylacetylene) (PPA) of M w = 3.42 × 105 (cod) and 2.02 × 105 (nbd) with polydispersities of 1.39 and initiation efficiencies of 4–7%. NMR studies on the polymerization reaction have allowed identification of the alkynyl species [Rh­(CCPh)­(cod)­(Ph2PPy)] as the likely initiating species involved in the generation of the rhodium-vinyl species responsible for the propagation step. The iPrNH2 cocatalyst is possibly involved in the efficient proton transfer from the coordinated PA to iPrNH2 that allows for a significant concentration of the key initiating species [Rh­(CCPh)­(cod)­(Ph2PPy)]. The distinct behavior of compounds [Rh­(diene)­(Ph2PPy)]n n+ as PA polymerization catalysts is a consequence of the binucleating ability of the Ph2PPy ligand in combination with the low basicity of the pyridine fragment which allows for the stabilization of the inactive alkynyl-bridge dinuclear species.