Platinum Metals Rev., 1963, 7, (2), 53
The Polymerisation of Acetylenes
A New Catalytic System
It has been known for some years that compounds of metals such as titanium and vanadium play an essential role in the stereospecific polymerisation of α-olefins. Recently it has been recognised that salts of the noble Group VIII metals have related properties; thus for example palladous chloride can trimerise diphenylacetylene to hexaphenylbenzene, and further examples of “co-ordination catalysis” are constantly appearing. A recent paper by L. B. Luttinger and E. C. Colthup of the Central Research Division of the American Cyanamid Company (J. Org. Chem., 1962, 27, 3752–3756) describes a novel method for polymerising mono-substituted acetylenes such as 1-heptyne. The essential ingredients of the catalytic system are (i) a salt of a metal of the platinum group (PdCl2, PdCl2(n -Bu3P)2, RuCl3, PtCl4, OsCl3), (ii) an hydridic reducing agent (NaBH4 or LiAlH4 for preference) and (iii) a suitable solvent to bring these into solution (ethanol, acetonitrile or even in certain cases water). Under favourable conditions the polymerisation proceeds smoothly at room temperature to yield a mixture consisting predominantly of dimers and trimers; total polymer yields range between about 8 and 35 per cent. Palladous chloride di(n -butyl phosphine) gives a much larger yield of polymer than does uncomplexed palladous chloride. Dilute solutions of the metal salts were used, the concentrations ranging between 0.01 and 0.05 M.
A number of other metal salts were investigated, but catalytic activity was found to be confined to the salts of the Group VIII metals. Salts and complexes of nickel and cobalt were particularly active; cobaltous nitrate gave 60 per cent polymer, mainly trimer.
No detailed formulation of the reaction mechanism was presented. The reaction was thought to proceed homogeneously, and to be initiated by reduction of the Group VIII metal ion to a metal hydride. This is consistent with the observation that only hydridic reducing agents were effective. The acetylene is then thought to form a π-complex with the metal hydride, and subsequent steps may resemble those proposed for stereospecific polymerisation. This reaction lengthens the already rapidly growing list of examples of homogeneous co-ordination catalysis.