Structural defects in polythiophenes: Monte Carlo simulations with quantummechanical growth probabilities

Thermodynamical stability of all possible isomers of small oligomers of thi ophene including branched forms is studied by ab initio calculations. The r elative energetics of isomers is fitted to an additive potential function. The probability of various growth mechanisms for polythiophene can be under stood in terms of Boltzmann factors derived from this potential function. A Monte Carlo type simulation is employed for growing in a canonical ensembl e of polythiophenes. The conformational properties of such grown polymers a re analyzed in terms of temperature and the length of the chain. Specifical ly, the extent of mislinkages such as alpha-beta bondings and the branching ratio is discussed. The results are compared to those obtained from polypy rrole for ab initio and density functional theory generated potential funct ions. In polythiophene, 10% branching is predicted at room temperatures whe reas similar figures for polypyrrole is about 20%.