A cobaltocene catalyst effected a cycloaddition copolymerization of 1,11-dodecadiyne (1a) with acetonitrile (2a) in toluene at 150 degrees C to afford a poly(pyridine) with a molecular weight up to 18 000. An excess of 2a to 1a was necessary for the preparation of a soluble poly(pyridine) without diyne trimerization units. The structure of the nitrile influenced remarkably its copolymerizability with 1a : the copolymerizability order was p-tolunitrile > acetonitrile > n-octyl cyanide. The length (m) of a methylene chain tethering two C=C bonds of a diyne HC=CCCH2)(m)C=CH controlled the diyne/nitrile copolymerization : 1,9-decadiyne with m = 6 or 1,7-octadiyne with m = 4 did not copolymerize with 2a effectively to give a diyne trimer or a bicyclic pyridine as a main low-molecular weight product, respectively. The poly(pyridine) formation is the first example of an efficient cycloaddition copolymerization of a terminal diyne.