In this letter, we report a simple approach to fabricating scalable flexible tactile sensors using a nanofiber assembly of regioregular poly(3-hexylthiophene) (P3HT). Uniform P3HT nanofibers are obtained through a continuous electrospinning process using a homogeneous solution of high-molecular-weight P3HT. The P3HT nanofibers are oriented by collecting them on a rotating drum collector. Small physical inputs into the self-standing P3HT nanofiber assemblies give rise to additional contact among neighboring nanofibers, which results in decreased contact resistance in directions orthogonal to the nanofiber orientation. The P3HT nanofiber assemblies could detect pressure changes and bending angles by monitoring the resistance changes, and the sensor responses were repeatable.

Poly(3-hexylthiophene) was synthesized by the polycondensation of 2-bromo-3-hexylthiophene with a nickel catalyst and (2,2,6,6-tetramethylpiperidin-1-yl)magnesium. The polymerization proceeded at room temperature in a highly regioregular manner.Oligothiophenes and polythiophenes attract considerable attention in materials science. Regioregular polythiophene, which involves head-to-tail (HT) repeating unit such as poly(3-hexylthiophene) (HT-P3HT) (1) (Chart 1) is of particular interest since the compound shows remarkable physical properties materials such as organic TFTs, thin-film organic solar cells, and conductive polymers.1 Debrominative polycondensation of 2,5-dibromo-3-hexylthiophene (2a) with a Grignard reagent in the presence of a transition-metal catalyst is a method of choice for the preparation of 1.1d,2 A regiochemical error in the halogenmetal exchange causes incomplete conversion to the polymer 1. Although use of the 5-iodinated derivative 2b is a solution to such problem, 3 preparation of 2b requires multistep procedures as well as inferior atom efficiency. On the other hand, the dehydrobrominative method to afford 1 with 2-bromo-3-hexylthiophene (3) is a potentially atom-economic pathway. Deprotonation of 3 with lithium amide and following metal exchange with zinc 1d or magnesium 2a2f,3 also gives 1 although extremely low temperature is necessary. A palladium-catalyzed direct polycondensation in the presence of an alkali metal carbonate has recently been shown to lead to polymer, 4 however, high temperature is necessary to afford the polythiophene in a reasonable yield with high degree of polymerization. It is thus highly intriguing to develop milder dehydrobrominative polymerization (around room temperature), which would be a goal for the efficient preparation of regioregular polythiophenes. We envisaged that our recent efforts on the development of transition-metal-catalyzed CH functionalization of heteroaromatic compounds 5,6 can be applied to the synthesis of HT-P3HT (1) and herein disclose that the employment of Knochel-Hauser base 7 and a nickel-catalyst is a practical dehydrobrominative polycondensation method toward highly regioregular poly(3-substituted thiophene)s at room temperature with high atom efficiency.We first examined palladium-catalyzed polymerizaton of 3a in the presence of lithium t-butoxide. 8 The polymerization occurred at 50°C to afford the corresponding polymer 1 with M n of 7600. In contrast to the above undesired results, it was found that the use of (2,2,6,6-tetramethylpiperidin-1-yl)magnesium chloride¢LiCl (4), which was developed by Knochel for the proton abstraction of various sp 2 CH bonds, 7 induced polymerization highly efficiently. When the reaction was carried out with 0.5 mol % [NiCl 2 (dppe)] as catalyst, the corresponding HT-P3HT (1) was obtained in quantitative yield in contrast to debrominative polymerization with a Grignard reagent which does not reach complete conversion due to regiochemical error in the halogenmetal exchange. The obtained polymer 1 exh...

High-molecular-weight head-to-tail-type regioregular polythiophenes are synthesized by [CpNiCl(SIPr)]-catalyzed dehydrobrominative polycondensation. Polycondensation of 2-bromo-3-hexylthiophene at the CH bond proceeds with an equimolar amount of TMPMgCl¢LiCl and a catalytic amount of [CpNiCl(SIPr)] to afford the regioregular poly(3-hexylthiophene) (HT-P3HT) with up to M w = 815000. A self-standing HT-P3HT film is obtained with the regioregular HT-P3HT of M w = 414000, while the attempted formation of the corresponding film with lower-molecular-weight HT-P3HT (M w = 38000) is unsuccessful.Polythiophenes have attracted considerable attention recently because of their remarkable physical properties as electronic materials.1 The formation of head-to-tail (HT)-type polythiophenes such as poly(3-hexylthiophene) (P3HT) (1) has been particularly important, since superior material performances have been generally found in HT-P3HT materials because of their highly planer and ³-conjugation-extended characteristics. 2In contrast with the extensive spectroscopic and electronic characterizations of HT-type polythiophenes, the mechanical performance of HT-polythiophene as a self-standing film has rarely been studied owing to the preparative difficulties involved in the synthesis of high-molecular-weight HT-P3HT.3 Although regioregular HT-P3HT was shown to be prepared by nickelcatalyzed polycondensation of the corresponding dibromothiophene with a Grignard reagent (the GRIM method), 4 it had been difficult to obtain HT-P3HT with a molecular weight as high as 100000.5 Very recently, Seferos and co-workers revealed that the synthesis of higher-molecular-weight P3HT could be achieved with [(o-tolyl)NiCl(dppp)] as an initiator, affording polythiophene with a molecular weight up to ca. 200000. 6 This prompted us to report our separate findings on the development of a new class of nickel catalysts bearing a cyclopentadienyl (Cp) ligand for the polymerization of thiophene derivatives to yield highmolecular-weight HT-P3HT. We have reported recently that nickel-catalyzed dehydrobrominative or dehydrochlorinative polycondensation is also a powerful tool (in addition to the GRIM method) for the synthesis of HT-type polythiophenes using the KnochelHauser base, chloromagnesium 2,2,6,6-tetramethylpiperidide lithium chloride salt (TMPMgCl¢LiCl), 7 in the presence of a nickel(II) catalyst. 8 The method allows polymerization at a lower reaction temperature within a shorter polymerization period with high atom efficiency. In our studies on the dehydrobrominative synthesis as well as the aforementioned GRIM method, 9,10 it is remarkable that the average molecular weight of HT-P3HT is controllable by the ratio of monomer feed and catalyst loading, because of the characteristics of living polymerization. In the course of our studies on the synthesis of HT-P3HT, we found unexpectedly that P3HT of an extremely high molecular weight (>100000) was obtained irrespective of the catalyst loading when a nickel catalyst bearing a cyclopentadienyl (Cp) a...

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