  We study the energetics of acetylene (${\rm C_2H_2}$) molecules inside a
carbon nanotube (CNT) using the {\it ab initio} pseudopotential method. The
encapsulation energy of a single ${\rm C_2H_2}$ molecule into the nanotube and
the formation energy of ${\rm (C_2H_2)}_n$@CNT are calculated. We investigate
whether a polyacetylene chain can be produced by fusion of the close-packed
acetylene molecules inside the CNT and find that there is practically no
activation barrier to polymerization. We propose to employ this method to
obtain straight, perfectly isolated, and single-stranded polyacetylene chains
encapsulated inside CNTs, which may be used for molecular electronic devices.
