A tight-binding calculation to describe the triblock copolymer xPA(polyacetylene)/nPPP (poly(p-phenylene))/yPA or xPPP/nPA/yPPP is presented. The interfacial coupling between homopolymer segments is attributed to the hopping of r-electrons and the coupling of σ-bonds. The dependence of the band gap of triblock copolymers on the interfacial couplings or on the composite segment lengths is studied. The influence of composite segment lengths on the electron density is also studied. For nPPP/xPA/nPPP structures, the band gap varies with PA segment length over a wide range of 1.32-2.74eV. For nPA/xPPP/nPA structures, the band gap is invariant with PPP segment length. It is found that a spontaneous tunnelling phenomenon could take place in nPA/xPPP/nPA structures. Furthermore, the polaron caused by doping an electron into nPA/xPPP/nPA will tend to be confined in one of the well (PA) parts. This kind of confinement may increase the electron-hole recombination probability.