A high purity approach to poly(3-hexylthiophene) diblock copolymers

Abstract

The combination of superior optoelectronic properties and enhanced solubility for regioregular poly(3-hexylthiophene) (rr-P3HT) has enabled materials based on P3HT to be widely used for organic field-effect transistors, chemical sensors, and photovoltaic solar cells. Significantly, the optoelectronic performance of rr-P3HT thin film has been shown to depend on molecular weight, processing conditions, and end group functionalities which places significant demands on developing synthetic approaches that do not lead to defects in the P3HT chain. This is especially true for the preparation of P3HT block copolymers where control of the polymeric chain ends is essential and the successful incorporation of nonconductive vinyl blocks, such as polystyrene, leads to modification of the mechanical and electronic properties. Starting with the pioneering work of McCullough and coworkers, a variety of groups have prepared rr-P3HT-based block copolymers using a combination of Grignard metathesis method (GRIM) and controlled radical polymerization (i.e., atom transfer radical polymerization, ATRP) or anionic polymerization. More recently, P3HTblock-poly(lactic acid) copolymers have also been synthesized from telechelic P3HT derivatives by ring-opening polymerization. For vinyl-based systems, controlled radical polymerization (CRP) such as ATRP has a number of advantages such as wide monomer selection, tolerance for various functionalities, and simplified synthetic procedures. As demonstrated by several research groups, P3HT-based block copolymers have been successfully synthesized by CRP and in these cases, a monofunctional P3HT macroinitiator is initially prepared containing an initiating fragment at one chain end, for example a 2-bromopropionyl bromide unit for ATRP procedures. In many examples, the second P3HT chain end arises from the 2,5-dibromo-3-hexyl thiophene monomer and the resulting x-bromo chain end is undesirable for a variety of reasons. For example, the thiophene-bromine bond can be active in a variety of side reactions which may lead to the introduction of defects, such as coupling between P3HT chains, either during synthesis of the macroinitiator or during the subsequent ATRP reaction. The presence of small amounts of coupled P3HT having a higher molecular weight and two active initiating chain ends would lead to an ABA block structure that can greatly affect the phase behavior of P3HT block copolymers. Previously, McCullough and coworkers have elegantly Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 46, 8200–8205 (2008)