Graft/block copolymers of polyacetylene and polyisoprene or polystyrene have been prepared by intercepting a growing polyacetylene chain with a polymer-bound ketone or epoxide, thus forming a new carbon-carbon bond. The catalyst employed is a modification of the Shirakawa catalyst for acetylene polymerization. The molecular weight of the polyacetylene block was varied by changing the ratio of monomer to graft sites in the polymerization, thus generating a series of samples of increasing polyacetylene block lengths. These samples exhibit visible absorption spectra ranging from that characteristic of low molecular weight trans-polyenes to that of conventional trans-polyacetylene. Graft copolymers in which the polyacetylene block length is relatively long were shown in solution to form aggregates which appear to be amorphous. When longer block lengths were prepared, the polyacetylene blocks formed crystalline domains with a characteristic trans-polyacetylene structure. The graft copolymers contain unusually low spin densities (<1/300000 polyacetylene C atoms). Addition of bromine to graft copolymer solutions initially results in the formation of a charge-transfer complex and subsequently bromination. The formation of these complexes could be partially reversed (compensated) by the addition of ammonia.