Synthesis and polymerisation of a,u-bis(3-pyrrolyl)alkanes Richard C. Foitzik, a Akif Kaynak a and Frederick M. Pfeffer b, * a Deakin University, School of Engineering and Information Technology, Geelong, Victoria 3217, Australia b Deakin University, School of Life and Environmental Science, Geelong, Victoria 3217, Australia Received 29 November 2006; revised 14 February 2007; accepted 1 March 2007 Available online 6 March 2007 Abstract—The synthesis, characterisation and polymerisation studies of a homologous series of a,u-bis(pyrrolyl)alkanes are described. These a,u-bis(pyrrolyl)alkanes were produced using Friedel–Crafts acylation followed by reduction of the carbonyl group using Red- Al Ò . Chemical polymerisation of the resultant dimers using FeCl 3 produced poly(a,u-bis(pyrrolyl)alkane) films, which were characterised by SEM, FTIR and tested for conductivity. Ó 2007 Elsevier Ltd. All rights reserved. 1. Introduction The cross-linking of polymers has proven to increase the strength, brittleness, rigidity or even the elasticity of the re- sultant polymers. For example, one of the first processes to form cross-linked polymers was the addition of sulfur during the vulcanisation of rubber, which produces a harder, more robust form of rubber. 1 The cross-coupling of two pyrrole molecules for the purpose of forming conductive polymers can occur at the nitrogen, C-2 or C-3 positions. Previous work has shown that the intro- duction of an alkyl chain to couple two heterocyclic mole- cules at the C-2 position alters a variety of properties, such as flexibility and elasticity. 1–4 Aromatic spacers have also been used for this purpose and upon polymerisation of such dimers large changes in conductivity and solubility were noted. 5,6 Various pyrrole dimers have previously been syn- thesised by coupling at the C-2 positions, these dimers have been used for multiple applications, the major use being for sensor technologies. 7–11 Generally, polymers that possess a high degree of cross- linking require a monomer that has three or more sites for polymerisation and this is usually achieved by first synthe- sising a dimer. For thiophene and pyrrole this is generally achieved by the fixation of a linker from the C-3 position. Pyrrole dimers can also be produced easily by cross-linking through the nitrogen. 12–22 However, for our future applica- tions of these polymers, the nitrogen position must be unfunctionalised as it is involved in fixation to specific substrates. 23 There has been a large number of publications on cross- linked monomers of thiophene linked through the C-3 position such as a,u-bis(thienyl)alkanes (Fig. 1). These a,u- bis(thienyl)alkane molecules when polymerised are reported to have good conductivity and are highly flexible. 24–32 In contrast, studies on the cross-coupling of pyrrole at the C-3,3 0 -positions are scarce and the published work has been limited to theoretical studies, patent examples or syn- thesis for specific purposes. 33–35 Detailed herein are the synthesis, polymerisation and char- acterisation of a dimer family of a,u-bis(pyrrolyl)alkanes. The ideal structure of a,u-bis(pyrrolyl)ethane (Fig. 2) illus- trates the high degree of cross-linking, the extent of which S (C n H 2n ) S Figure 1. a,u-Bis(thienyl)alkanes. H N N H H N N H H N N H H N N H Figure 2. Ideal polymer structure for poly(1,2-bis(pyrrolyl)ethane). Keywords: Conducting polymers; Cross-linked polymers; Cross-coupled pyrrole; Pyrrole dimers; Pyrrole. * Corresponding author. Tel.: +61 3 52271439; fax: +61 3 52271040; e-mail: thefef@deakin.edu.au 0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2007.03.010 Tetrahedron 63 (2007) 4237–4242