Monomer-induced switching of stereoselectivity and limitation of chain growth in the polymerization of amine-containing: Para -substituted phenylacetylenes by [Rh(norbornadiene)Cl]₂

A series of para-substituted phenylacetylenes bearing various amine-containing pendant groups 1-4 can serve as both a monomer and cocatalyst in the polymerization catalyzed by [Rh(norbornadiene)Cl]₂ alone in CH₂Cl₂, affording polyphenylacetylenes (PPAs) with different stereoselectivities and molecular weights. The polymerization of monomer 4 having a long and flexible pendant group produces high molecular weight PPAs with high cis-transoid configurations, similar to those obtained from the coordination-insertion polymerization of these monomers 1-4 by using the [Rh(norbornadiene)Cl]₂/cocatalyst systems. However, the stereospecific transformation from the cis-transoid to the trans-cisoid configuration and the growth limitation of the polymer chain are observed in the polymerization of monomers 1-3 containing short and rigid pendant groups. The resulting PPAs have trans-cisoid selectivities of up to 66% and low molecular weights. A metathesis mechanism is suggested, in which the steric repulsion between the pendant group of the monomer and the propagation chain originates from the successive 1,2-insertion of monomers 1-3 to the Rh-carbene active species gives a rational explanation for the tendency of stereospecific transformation and the growth limitation of the polymer chain.