To incorporate an acceptor type polythiophene segment onto a supramolecular block copolymer for potential light harvesting applications, effective synthetic routes for the end-functionalized and acceptor-substituted polythiophenes are critical. The Ullmann coupling reaction can be utilized to obtain electron-deficient polythiophenes and to attach terminal thiophene units that carry functional groups. In this article, the reactions involving a 2,5-dibromothiophene monomer containing an electron-withdrawing fluorinated ester and 5-bromo-2-thiophenecarboxaldehyde (the endcapper) were studied in detail. It was found that the Ullmann coupling reaction of the dibromide is very fast (completed in a few minutes) and the terminal bromine group does not survive long under the reaction condition. These findings lead to the development of an effective procedure for aldehyde end-capping of electron-deficient polythiophenes. Polymers with molecular weights around 4000 Da are routinely obtained. (c) 2006 Wiley Periodicals, Inc.