NOVELTY - A conjugated polymer having cyclic repeating unit (I) is new.    USE - New conjugated polymer is used for polymer blend and formulation used as charge transport, semiconducting, electrically conducting, photoconducting or light emitting materials in optical, electrooptical, electronic, electroluminescent or photoluminescent devices e.g. organic field effect transistor, thin film transistor, integrated circuit, logic circuit, capacitor, radio frequency identification tag, organic light emitting diode, organic light emitting transistor, flat panel display, backlight of display, organic photovoltaic device, solar cell, laser diode, photoconductor, photodetector, electrophotographic device, electrophotographic recording device, organic memory device, sensor device, light emitting diode, Schottky diode, planarizing layer, antistatic film, polymer electrolyte membrane, conducting substrate, conducting pattern, electrode material in batteries, alignment layer, biosensor, biochip, security marking, security device and devices for detecting and discriminating DNA sequences (all claimed).    ADVANTAGE - The conjugated polymer has excellent photoconducting and semiconducting properties.    DETAILED DESCRIPTION - A conjugated polymer having cyclic repeating unit of formula (I) is new.    one of X1,X2 and one of X3,X4=sulfur, selenium or oxygen, and the other is CH, CR' or N;    R=linear, branched or cyclic 1-35C alkyl, in which non-adjacent carbon atoms are optionally replaced by -0-, -S-, -CO-, -CO-O-, -O-CO-, -O-CO-O-, -CR"=CR"'- or -CC- and in which H atoms are optionally replaced by fluoro, chloro, bromo, iodo or cyano, or represents aryl, heteroaryl, aryloxy, heteroaryloxy, arylcarbonyl, heteroarylcarbonyl, arylcarbonyloxy, heteroarylcarbonyloxy, aryloxycarbonyl or 2-40C heteroaryloxycarbonyl optionally substituted by non-aromatic groups (R1);    R'=linear, branched or cyclic 1-15C alkyl, in which non-adjacent carbon atoms are optionally replaced by -0-, -S-, -CO-, -CO-O-, -O-CO-O-, -CR"=CR"'- or -CC- and in which H atoms are optionally replaced by fluoro, chloro, bromo, iodo or cyano;    R",R"'=H, optionally substituted carbyl or optionally substituted hydrocarbyl optionally having hetero atoms;    R1=H, halo, -CN, -NC, -NCO, -NCS, -OCN, -SCN, - C(=O)NR"R"', -C(=O)X', -C(=O)R', -NH2, -NR'R"', -SH, -SR', -SO3H, -SO2R', -OH, -NO2, -CF3, -SF5, optionally substituted silyl, carbyl or optionally substituted 1-40C hydrocarbyl optionally having hetero atoms, or P'-Sp-;    P'=polymerizable group;    Sp=spacer group or single bond;and    X'=halo.    INDEPENDENT CLAIMS are included for the following: (i) monomer (M) of formula: R2-A-R3; (ii) polymer blend containing the conjugated polymer and one or more additional polymer having semiconducting, charge transport, hole/electron transport, hole/electron blocking, electrically conducting, photoconducting or light emitting properties; (iii) formulation comprising the conjugated polymer or polymer blend, and one or more organic solvents; and (iv) preparation of conjugated polymer.    A=repeating unit (I).