Novel electrochemically-doped vinyl polymers containing 2,2’:5’,2 ":5 ",2"’-quaterthiophene, 4’,3"’-dioctyl-2,2’:5’,2 ":5 ",2"’:5"’,2""-quinquethiophene, and 4’,3""-dioctyl-2,2’:5’,2 ":5 ",2"’:5"’,2"":5"",2""’-sexithiophene as pendant groups were prepared by anodic polymerizations of the corresponding new vinyl monomers in the presence of tetra-n-butylammonium perchlorate as a supporting electrolyte, and their electrical and electrochromic properties were investigated. The electrochemically-doped polymers, which were obtained as deeply-colored, lustrous films on the surface of the working electrode, were identified as radical-cation salts of pendant oligothiophenes with ClO4- as a dopant, having a partially-cross-linked structure due to the coupling reaction of the radical cation of the pendant oligothiophene moiety. The electrical conductivity of these electrochemically-doped polymers increased with increasing conjugation length of the pendant oligothiophenes; the polymer containing the pendant sexithiophene group exhibited a room-temperature conductivity of ca. 10(-4) S cm(-1) with an activation energy of 0.25 eV. These polymers undergo a reversible clear color change from green to yellow and vice versa on electrochemical dedoping and doping, constituting a novel class of potential electrochromic materials.