TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - The molar ratio of thiol-containing compound and bromine-containing compound is 1:1-1.5:1. The solvent is chosen from methylbenzene, tetrahydrofuran, benzene, N,N-dimethylformamide. The catalyst is organic palladium catalyst or a mixture of organic palladium catalyst and organic phosphine ligand. The content of catalyst is 0.01-5 mol% with respect to bromine-containing compound. The mole ratio of organic palladium catalyst and organic phosphine ligand is 1:2-1:20. The thiol-containing compound (II) is obtained by dissolving thiol-containing compound of formula (IV) into predetermined amount of dichloromethane, obtaining a mixture, dripping obtained mixture with an injector into anhydrous methylene chloride containing 1,3-dicyclohexylcarbodiimide and 4-dimethylamino pyridine, reacting for 8-24 hours, obtaining thiol-containing compound of formula (V), adding thiol-containing compound (V) and benzothiophene-containing compound of formula (VI) in a mole ratio of 1:1 into a solvent, heating the mixture at 78-100 degrees C, refluxing, adding reducing agent, reacting for 10 minutes, obtaining thiol-containing compound of formula (VII), dissolving obtained thiol-containing compound (VII) into anhydrous tetrahydrofuran under protection of nitrogen, cooling the solution to -78 degrees C, slowly adding hexane solution containing n-butyl lithium into the solution, stirring the solution at -78 degrees C for 2 hours, adding trimethyl tin-chloride reagent into the solution, reacting for 30 minutes, restoring to room temperature, and reacting at room temperature. The mole ratio of n-butyl lithium and thiol-containing compound (VII) is 1:2.5. The mole ratio of trimethyl tin-chloride and thiol-containing compound (VII) is 2.5:1. The solvent is ethanol or propanol. The reducing agent is potassium hydroxide or sodium hydroxide. The mole ratio of the reducing agent and thiol-containing compound (IV) is 5:1. The mole ratio of thiol-containing compound (III) with respect to 1,3-dicyclohexylcarbodiimide and dimethylamino pyridine is 3:3:1.    TECHNOLOGY FOCUS - POLYMERS - Preparation (claimed): A thiol-containing compound of formula (II) and bromine-containing compound of formula (III) are added into solvent in the mol ratio of 1:1-1.5:1, and refluxed at 60-120 degrees C for 12-72 hours in presence of catalyst under aerobic environment, to obtain benzo dithiophene copolymer (I). The solvent is methylbenzene, tetrahydrofuran, benzene and/or N,N-dimethylformamide. The catalyst is organic palladium catalyst or a mixture of organic palladium catalyst and organic phosphine ligands. The dosage of the catalyst is 0.01-5 mol% of the raw material (III). The organic palladium catalyst is tris(dibenzylideneacetone)dipalladium(0), tetrakis(triphenylphosphine)palladium, or bis(triphenylphosphine)palladium(II) dichloride. The mol ratio of the mixture of organic palladium catalyst and organic phosphine ligand is 1:2-1:20. The organic phosphine ligand contains tris(o-tolyl)phosphine or tricyclohexyl phosphine. Preferred Definitions:    n=8-60;    R1=H, ethyl, propyl, butyl, hexyl, hexadecyl, or methyl;    R2=H, amyl, propyl, dodecyl, or methyl;    R3=H, methyl, 3-methyl thienyl, dodecyl, 14-alkoxy, octyloxy, 2-methyl thienyl, methoxy, 16-alkoxy, or hexyl;    R4=methyl, H, 3-methyl thienyl, ethoxy, or octyl;and    R5=n-butyl, methyl, 2-methyl butyl, 2,4-dimethyl-3-ethyl heptyl, 2,2,4-trimethyl amyl, hexadecyl, acetyl, methyl, or butyl.