NOVELTY - Polymer (A) comprising one or more benzo(1,2-b;4,5-b')dithiophene units (I), is new.    USE - (A) is useful: as charge transport, semiconducting, electrically conducting, photoconducting or light emitting material in an optical, electro optical, electronic, electroluminescent or photoluminescent device; or in a component of such a device, and in an assembly comprising such a device or component, where the device is organic field effect transistors, thin film transistors, organic LED's, organic light emitting transistors, organic photovoltaic devices, organic solar cells, laser diodes, Schottky diodes, photoconductors and photodetectors, bulk heterojunction organic photovoltaic devices or inverted bulk heterojunction organic photovoltaic devices, the component is charge injection layers, charge transport layers, interlayers, planarizing layers, antistatic films, polymer electrolyte membranes, conducting substrates or conducting patterns, and the assembly is integrated circuits, radio frequency identification tags or security markings or security devices, flat panel displays or backlights, electrophotographic devices, electrophotographic recording devices, organic memory devices, sensor devices, biosensors and biochips (all claimed).    ADVANTAGE - (A) is easy to synthesize, suitable for mass production, shows good structural organization and film-forming properties, exhibits good electronic properties (i.e. high charge carrier mobility), good processability (i.e. high solubility in organic solvents), improved bandgap, and high stability in air.    DETAILED DESCRIPTION - Polymer (A) comprising one or more benzo(1,2-b;4,5-b')dithiophene units of formula (I), is new.    X1, X2 = C(R1R2), Si(R1R2), C=C(R1R2) or C=O;    A1, A2 = mono- or bicyclic aromatic or heteroaromatic having 5-12 ring atoms and optionally substituted;    R1, R2 = H, 1-30C-cyclic alkyl, where one or more non-adjacent CH2 are optionally replaced by -O-, -S-, -C(O)-, -C(S)-, -C(O)-O-, -O-C(O)-, -NR1a-, -SiR1aR2a-, -CF2-, -CHR1a=CR2a-, -CY1=CY2- or -C=C-, O and/or S atoms are not linked directly to one another, and one or more H atoms are optionally replaced by F, Cl, Br, I or CN, or aryl, heteroaryl, aryloxy or heteroaryloxy with 4-20 ring atoms, which is optionally substituted, preferably by halo or by one or more of the aforementioned alkyl or cyclic alkyl;    Y1, Y2 = H, F, Cl or CN; and    R1a, R2a = H or 1-40C-carbyl (optionally substituted) or hydrocarbyl, preferably H or 1-12C-alkyl.    INDEPENDENT CLAIMS are included for:    (1) a mixture or polymer blend comprising (A) and one or more compounds having semiconducting, charge transport, hole/electron transport, hole/electron blocking, electrically conducting, photoconducting or light emitting properties;    (2) a formulation comprising (A), mixtures or polymer blends, and one or more solvents, preferably organic solvents;    (3) aryl or heteroaryl monomer of formula (R7-(Ar1)a-U1-(Ar2)c-R8) (VI); and    (4) the preparation of (A).    U1 = unit;    Ar1-Ar3 = aryl or heteroaryl that is different from U1, preferably the aryl or heteroaryl has 5-30 ring atoms and is optionally substituted with one or more groups Rs1;    Rs1 = F, Br, Cl, -CN, -NC, -NCO, -NCS, -OCN, -SCN, -C(O)NR1aR2a, -C(O)X1a, -C(O)R1a, -NH2, -NR1aR2a, -SH, -SR1a, -SO3H, -SO2R1a, -OH, -NO2, -CF3, -SF5, optionally substituted silyl, carbyl or 1-40C-hydrocarbyl that is optionally substituted with one or more hetero atoms;    X1a = halo, preferably F, Cl or Br;    R7, R8 = Cl, Br, I, O-tosylate, O-triflate, O-mesylate, O-nonaflate, -Si(CH3)2F, -Si(CH3)F2, -O-SO2Z1, -B(OZ2)2, -CZ3=C(Z3)2, -C-CH, -C-CSi(Z1)3, -ZnX1a or -Sn(Z4)3; either    Z1-Z4 = alkyl or aryl (both optionally substituted); or    Z2Z2 = a cyclic group; and    a, c = 0-2.