Thermodynamical stability of all possible isomers of small oligomers of thiophene including branched forms is studied by ab initio calculations. The relative energetics of isomers is fitted to an additive potential function. The probability of various growth mechanisms for polythiophene can be understood in terms of Boltzmann factors derived from this potential function. A Monte Carlo type simulation is employed for growing in a canonical ensemble of polythiophenes. The conformational properties of such grown polymers are analyzed in terms of temperature and the length of the chain. Specifically, the extent of mislinkages such as alpha-beta bondings and the branching ratio is discussed. The results are compared to those obtained from polypyrrole for ab initio and density functional theory generated potential functions. In polythiophene, 10% branching is predicted at room temperatures whereas similar figures for polypyrrole is about 20%.