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

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Abstract

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)-5H-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 PdCl2(MeCN)2 and P(C6H4-o-OMe)3 (L1) as catalyst precursors. The polymerization smoothly proceeds at 100 °C in THF in the presence of pivalic acid and Cs2CO3 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, Mn = 36800, Mw/Mn = 2.20), 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl (3b, Mn = 31100, Mw/Mn = 2.44), 3,4-(2,2′-dioctylpropylenedioxy)thiophene-2,5-diyl (3c, Mn = 68200, Mw/Mn = 3.04), and 2,5-bis(2-ethylhexyloxy)-1,4-phenylene (3d, Mn = 65500, Mw/Mn = 2.21). A detailed analysis of the structure of 3a is reported.

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