Polythiophenes random copolymers

Pilston, R.L. 2001. Synthesis and characterization of novel regioregular polythiophenes, random copolymers of thiophene and studies of the GRIM polymerization method for the synthesis of polythiophenes. Carnegie Mellon University. 

This section reviews the synthesis and characterization of random, alternating, block and graft copolymers of thiophenes. A review [2] of processable electronically conducting polymers published in 1991 contains only two references on polythiophene random copolymers and another article [3] describes the synthetic methods used to prepare block and graft copolymers of thiophene. 

Reviewed below are random copolymerizations of thiophene, 3-methylthiophene, 3-butylthiophene, 3-hexylthiophene and 3-octylthiophene. The structure of polythiophene random copolymers is reviewed in Chapter 15 in a more comprehensive way. 

Irreversible undoping that led to the loss of electrical properties in doped poly(alkylthiophene)s on thermal treatment was reported by Inganas and co-workers. The rate of thermal undoping was observed to be faster in the case of longer chains substituted on the polythiophene when compared with nonsubsti-tuted polythiophene [128-131]. Random copolymers of methyl-and octyl-substituted polythiophene [132] and regular copolymers of dimer thiophene-co-octylphenylthiophene [133] demonstrated much better stability. 

The conductive properties of alkylated polythiophenes are known to be unstable, particularly at elevated temperatures. The mechanism of thermal undoping has been associated with thermal mobility. Consequently, various workers have considered synthesis of random copolymers (e.g., thiophene and 3-octylthiophene), with well-distributed octyl side groups leaving space around the main chains to accommodate dopants. 

This chapter is divided into two main parts polythiophene copolymers and polypyrrole copolymers. Each part reviews the random copolymerization of the heterocyclic monomer and important derivatives, principally those with substituents at the 3-position of thiophene and at the 1- and 3-positions of pyrrole. Alternating, block and graft copolymers are covered in both segments. Because applications of conducting polymers are reviewed in Volume 4, only applications unique to specify copolymers are covered here. In addition, a few theoretical studies dealing with conducting copolymers are reviewed. 

Cyclic voltammetry of the copolymer shows three anodic peaks, two matching the oxidation potentials of the parent homopolymers and a third which is intermediate. Authors attributed the data to the formation of blocks of polypyrrole and polythiophene cormected by blocks of random alternating groups of pyrrole and bithiophene. Increasing the amount of bithiophene in the copolymer produced a strong drop in the final conductivity of the materials, from 17 S cm at 1 mol% to I S cm at 14 mol%. 

In Table 9.5, the physical properties of these random, HT poly(3-alkylthiophene) copolymers are given. It can seen that copolymer 17 exhibits a solution Amax of 510 nm, which corresponds to one of the longest conjugation lengths known for a polythiophene in solution. It can also be seen that there is little difference in the absorption maximum between solution and the solid state (25 nm). Apparently 17 has an extended planar conformation even in solution, owing to the lack of steric hindrance caused by alkyl side chains and solution supra-molecular ordering. 

---