The oligomerization mechanisms of several derivatives of pyrrole have been studied by quantum-mechanical methods. Different growth processes are compared from the point of view of thermodynamic stabilities to gain insight into the branching possibilities. For the simple polypyrrole, polymerization may proceed through either an a or a beta carbon with equal probabilities. When an aldehyde group is attached to the nitrogen, beta positions are definitely favoured owing to strong steric hindrance. For 2-pyrrole carboxaldehyde and 3-pyrrole carboxaldehyde, the most stable forms of the bonding are alternating ones that go through alpha-beta pairs.