Poly(2,5-dioctylphenyl-3-thiophene) synthesized by direct arylation polycondensation: End groups, defects and crystallinity

One property central to π-conjugated polymers is the ability of polymer backbones to interact intermolecularly through the direct contact of π-orbitals. Poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) is a remarkable exception as the bulky side chains prohibit main chain π–π interactions. However, due to side-chain crystallization of interdigitated n-octyl side chains, PDOPT is semicrystalline, and thus any deviation from a perfect regioregularity can severely affect crystallization. Here, we synthesize PDOPT via direct arylation polycondensation (DAP) for the first time and analyze the effect of various reaction conditions on molecular weight, end groups, defect structures, crystallinity, and morphology. Despite extensive optimization of PDOPT synthesis via DAP including bulk polymerization, MW is limited to M_n,SEC ∼ 10 kg/mol as a result of dehalogenation of chain ends. Extensive NMR spectroscopy is carried out to analyze defect structures present in PDOPT made by DAP and also made by Kumada catalyst transfer polycondensation (KCTP) for comparison. Because of the complex ¹H NMR spectra arising from the additional phenyl rings, defect structures are identified using well-defined oligomers and ¹³C NMR spectroscopy. For the highest MW DAP samples, we find evidence for internal tail-to-tail defects (TT), while PDOPT made by KCTP appears to carry a TT defect at the chain end. The internal TT defect lowers the thermal transitions and enthalpies and leads to smaller and less defined spherulites in isothermally crystallized thin films. These results suggest that internal TT defects, which do not severely affect structure formation in the well-studied poly(3-hexylthiophene) analog, are much more important to be controlled and eliminated in the case of PDOPT, in which ordering occurs exclusively by side-chain crystallization.