A systematic series of well-defined aromatic polyesters and a poly-para-phenylene with aliphatic side chains were investigated. Films were mechanically characterized by DMTA. The chain modulus was estimated using an extrapolation method based on the aggregate model. The extrapolated values are comparable with theoretically calculated moduli, but display a fairly large uncertainty. Comparison of the theoretical values with the experimentally attained moduli showed an increasing discrepancy for higher molecular rigidity. The theoretical modulus increases with increasing molecular rigidity, while the experimental moduli (at low temperature and corrected for the cross-sectional area of the side chains) do not exhibit a specific dependence on the molecular rigidity, and range between 140 and 180 GPa. This important effect was ascribed to increasing processing difficulties, which is shown by the maximum draw ratio obtained.