Article pubs.acs.org/Macromolecules
Poly(3-(2,5-dioctylphenyl)thiophene) Synthesized by Direct Arylation Polycondensation: End Groups, Defects, and Crystallinity Daniel Schiefer,† Hartmut Komber,‡ Fanuel Mugwanga Keheze,§ Susanna Kunz,† Ralf Hanselmann,† Günter Reiter,§,∥,⊥ and Michael Sommer*,†,∥,⊥ †
Institut für Makromolekulare Chemie, Universität Freiburg, Stefan-Meier-Straße 31, 79104 Freiburg, Germany Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany § Institut für Physik, Universität Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany ∥ Freiburger Materialforschungszentrum FMF, Stefan-Meier-Straße 21, 79104 Freiburg, Germany ⊥ Freiburger Zentrum für interaktive Werkstoffe und bioinspirierte Technologien FIT, Georges-Köhler-Allee 105, 79110 Freiburg, Germany ‡
S Supporting Information *
ABSTRACT: 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-(2,5dioctylphenyl)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 noctyl 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 Mn,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.
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of interest for light-emitting applications.6,7 Moreover, its onedimensional semiconducting character makes PDOPT an ideal model to elucidate effects of dimensionality on optical and electronic properties and finally to allow contributions from inter- and intrachain processes to be disentangled. PDOPT has first been synthesized using oxidative coupling with ferric chloride leading to limited degrees of regioregularity (RR).4 Recently, we reported the successful synthesis of PDOPT via Kumada catalyst transfer polycondensation (KCTP)8 using uncommon Ni(II) catalysts with pyridinebased hybrid ligands. Using KCTP, control over molecular weight (MW) with number-average molecular weights Mn up to 40 kg/mol and high degrees of RR approaching unity were
INTRODUCTION Conjugated polymer backbones generally interact with each other on the basis of π−π stacking, resulting in 2-dimensional charge transport properties.1,2 Bulky substitution of conjugated backbones can prevent π−π stacking, enabling for instance high quantum yields and, more generally, the investigation of properties of isolated conjugated polymer backbones in the solid state.3 Poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) represents a rare exception within the class of conjugated polymers.4 In PDOPT, the bulky 2,5-dioctylphenyl side chain attached to every thiophene unit twists out of the backbone plane and thus acts as a spacer between two chains stacked on top of one another. As a result, the nominal π−π distance increases from 0.34 nm in poly(3-hexylthiophene) (P3HT) to 1.48 nm in PDOPT.5 Suppressing π−π interactions has been reported to render PDOPT highly photoluminescent, which is © XXXX American Chemical Society
Received: August 18, 2016
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DOI: 10.1021/acs.macromol.6b01795 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 1. (a) General procedure for PDOPT synthesis via DAP in solution, (b) end groups of PDOPT made in different solvents at 0.05 M, and (c) MALDI-ToF mass spectra (tetramer region) as a function of the solvent used.
possible.9 However, one drawback of the KCTP method was a significant degree of chain termination, leading to bromideterminated chains and significantly larger dispersities Đ for conversions >50%. While for conversions
