A Highly Efficient Catalyst for the Synthesis of Alternating Copolymers with Thieno[3,4-c]pyrrole-4,6-dione Units via Direct Arylation Polymerization

π-Conjugated polymers with a donor–acceptor (DA) combination of repeating units possess a narrow HOMO–LUMO gap, thus resulting in a high device performance in solar cells. This paper reports an improved catalytic system for the synthesis of DA polymers containing 5-(2-hexyldecyl)-5 H -thieno[3,4- c ]pyrrole-4,6-dione-1,3-diyl (TPD) group as the acceptor unit via palladium-catalyzed direct arylation polymerization. Although a related study has been reported ( Angew. Chem. Int. Ed. 2012, 51, 2068), we attempted to reduce the catalyst loading because the palladium residue in π-conjugated polymers has been known to produce a detrimental effect on device performance. As a result, the amount of palladium could be reduced to 1/8 by using PdCl 2 (MeCN) 2 and P(C 6 H 4 - o -OMe) 3 ( L1 ) as catalyst precursors. The polymerization smoothly proceeds at 100 °C in THF in the presence of pivalic acid and Cs 2 CO 3 to afford TPD-based DA polymers 3a – 3d containing the following donor units in almost quantitative yields: 4,4′-dioctyl-2,2′-bithiophene-5,5′-diyl ( 3a, M n = 36800, M w / M n = 2.20), 4,8-bis(2-ethylhexyloxy)benzo[1,2- b:4,5- b ′]dithiophene-2,6-diyl ( 3b, M n = 31100, M w / M n = 2.44), 3,4-(2,2′-dioctylpropylenedioxy)thiophene-2,5-diyl ( 3c, M n = 68200, M w / M n = 3.04), and 2,5-bis(2-ethylhexyloxy)-1,4-phenylene ( 3d, M n = 65500, M w / M n = 2.21). A detailed analysis of the structure of 3a is reported.