NOVELTY - A poly(5,5'-bis(thiophen-2-yl)-benzo(2,1-b;3,4-b')dithiophene) comprises a thiophene derivative, and having a number average molecular weight of 5000-200000 g/mol.    USE - A poly(5,5'-bis(thiophen-2-yl)-benzo(2,1-b;3,4-b')dithiophene) for use as semiconductors or charge transport materials, as thin-film transistors, or in semiconductor components for organic LED, for photovoltaic components or in sensors, as an electrode material in batteries, as optical waveguides or for electrophotography applications.    ADVANTAGE - The polymer is easy to synthesize, has high mobilities and good oxidation stability, and can be processed readily.    DETAILED DESCRIPTION - A poly(5,5'-bis(thiophen-2-yl)-benzo(2,1-b;3,4-b')dithiophene) comprises a thiophene derivative of formula (I), and having a number average molecular weight of 5000-200000 g/mol.    R=A, a-X-D, a-X-E, a-X-G or a-X-J;    D=3-10C cycloalkyl (optionally substituted with 1-4R1);    E=6-14C aryl (optionally substituted with 1-4R1);    G=3-12 membered cycloheteroalkyl (optionally substituted with 1-4R1);    J=5-14 membered heteroaryl (optionally substituted with 1-4R1);    A=1-20C alkyl, 2-20C alkenyl, 2-20C alkynyl or 1-20C alkoxy (all optionally substituted with 1-4R1);    R1=S(O)m-1-20C alkyl, S(O)m-O1-20C alkyl, S(O)m-O-6-14C aryl, C(O)-O1-20C alkyl, C(O)-O-6-14C aryl, A, 3-10C cycloalkyl, 6-14C aryl, 3-12 membered cycloheteroalkyl or 5-14 membered heteroaryl;    X=1-6C alkyl or covalent bond;    m=0-2.    INDEPENDENT CLAIMS are included for:    (1) a composition comprising poly(5,5'-bis(thiophen-2-yl)-benzo(2,1-b;3,4-b')dithiophene) dissolved or dispersed in a liquid medium;    (2) a thin film semiconductor comprising poly(5,5'-bis(thiophen-2-yl)-benzo(2,1-b;3,4-b')dithiophene);    (3) a composite comprising a substrate and the thin film semiconductor deposited on the substrate;    (4) a process of preparing a composite comprising dissolving the polymer in a liquid medium to form a solution, depositing the solution on a substrate, and removing the solvent to form a thin film semiconductor on the substrate;    (5) a field effect transistor device comprising the thin film semiconductor or the composite;    (6) an organic LED device comprising the thin film semiconductor or the composite; and    (7) a photovoltaic device comprising the thin film semiconductor or the composite.