Facile synthesis of well-defined block copolymers containing regioregular poly(3-hexyl thiophene) via anionic macroinitiation method and their self-assembly behavior.

The self-assembly of block copolymers into a nanostructure with novel morphology and property has attracted an increasing interest as a new approach for materials science, chemical synthesis, and nanofabrication.1,2 Most theoretical and experimental studies have concentrated on the assembly characteristics of coil-coil block copolymers. Recently, rod-coil block copolymers have received a great deal of attention since they offer an attractive strategy for the organization of many highly functional rod-like polymers such as helical biopolymers and conducting polymers with rigid π-conjugated backbones.1,3-9 In particular, block copolymers containing conducting polymer segments, such as polyfluorene, poly(phenylene vinylene), and polythiophene, are technologically important since these homopolymers may be used in low cost, large area, and flexible electronics.1,10-17 For example, in organic electronic devices such as photovoltaic cells, their efficiency depends critically on charge separation and transport in the nanostructures of the devices. Conducting block copolymers with highly ordered heterogeneous nanostructure may provide an enhanced interfacial area for charge separation as well as an efficient pathway for charge transport.18 In addition, it was shown that integrating a rod-like conducting polymer in copolymer structures with various coil-like polymer blocks displays better mechanical properties which provide the desirable processing property for novel organic optoelectronic device fabrication.16,19 In order to create a highly ordered nanostructure from these structure-directing block copolymers, the synthesis method to precisely control the molecule weight and distribution for each block is greatly needed. Recently, it was shown that coupling of endfunctionalized monodispersed homopolymers may provide a facile synthesis technique for the formation of blocks with novel copolymer nanostructures, while retaining information and control over the length distribution of the individual blocks.15,20-22 However, end-end coupling reactions are often hindered by the chain length effect of end-functionalized homopolymers, leading to a decrease in the coupling efficiency.23 The most widely used technique for the preparation of block copolymers is living anionic polymerization, which often involves sequential addition of vinylic monomers. For block copolymers involving a conducting polymer and a vinylic polymer, a combination of polymerization methods is needed as the conducting polymers are often synthesized using the polycondensation method. Among various conducting polymers, regioregular poly(3-hexyl thiophene)s (P3HT) are widely studied because of their superior optoelectonic property and chemical stability. McCullough et al. and Yokozawa et al. discovered that well-defined end-functionalized P3HTs can be synthesized with low polydispersity via a chain growth mechanism by using a catalyst-transfer polycondensation method mediated by a nickel catalyst (GRIM method).24-26 McCullough et al. further demonstrated that P3HT block copolymers can be synthesized from a linker molecule attached to an end-functionalized P3HT as a macroinitiator via atom transfer radical polymerization (ATRP).13,16 In this communication, we demonstrate that a vinyl endterminated P3HT can be activated at the terminal double bond with sec-butyllithium (s-BuLi) for subsequent anionic polymerization. The resulting poly(3-hexyl thiophyllithium) (3) can then be used as a living anionic macroinitiator for the polymerization of 2-vinyl pyridine (2VP) monomers to form monodispersed P3HT-P2VP diblock copolymers with high yield. This combination synthesis method takes advantage of the fact that both the GRIM method synthesized P3HTs have well-defined chain ends and their activated macroinitiator anions are stabilized by their conjugated segments yet they are strong enough to initiate 2VP for the synthesis of welldefined block copolymers. Different block ratio of P3HT to P2VP can be synthesized by changing the molar ratio of 2VP to P3HT. The chemical structure and the self-assembly behavior of the block copolymers were studied by various techniques such as NMR, GPC, TEM, and SAXS. We showed that these conducting-insulating block copolymers are able to undergo microphase separation and self-assemble into nanostructures of sphere, cylinder, lamellae, and nanofiber structure with increasing rod ratios. Both the synthesis method that employs conducting macroinitiator for anionic polymerization and the block copolymers thus synthesized are unprecedented. We synthesize the diblock copolymer by first synthesizing homopolymer P3HT (1), which is chain-end-functionalized with a vinyl group at the R position (Scheme 1a). The synthesis of 1 from the 2,5-dibromo-3-hexylthiophene monomer and the vinyl end† Institute of Polymer Science and Engineering. ‡ Department of Chemical Engineering. § Department of Materials Science and Engineering. Scheme 1a