Transition metal acetylide catalysts for polymerization of alkynes 3. Polymerization mechanism

A coordination insertion mechanism is proposed for the polymerization of al kynes initiated by transition metal acetylides on the basis of some experim ental results. A transition metal acetylide coordinates with an alkyne resu lting in the formation of a penta-coordinated complex, which is a prelimina ry activation and rate-determining step. The polymeric chain begins by inse rtion of the pi -coordinated monomer molecule into the metal-carbon sigma - bond. For large-bulk monomers such as p-diethynylbenzene and its derivative s, the insertion is almost trans-type to give rich trans polymers. On the o ther hand, for small-bulk monomers such as phenylacetylene, cis insertion e asily occurs to yield cyclotrimers. The propagation of the polymer chain ca n be interrupted when the sigma -bonded monomer transfers its acidic hydrog en to the growing chain. (C) 2001 Elsevier Science B.V. All rights reserved .