Simulated dilatometry techniques have been applied to compute the glass transition temperatures, T(g)s, of the two poly(methyl methacrylate) (PMMA) chain tacticities, and poly(methyl acrylate) (PMA). Since the difference in T(g)s between the two configurations was accurately simulated, further analysis could be carried out. This article more particularly deals with energetic and structural analysis of the difference. Thus this analysis showed that the non-bond energy and the bending angle energy associated with the intradiad backbone angle, principally contribute to the energetic difference between the two PMMA configurations. Following the free volume theory, these two energetic variations allow an increase in T-g in comparison to PMA, and an enlargement of the difference in the T(g)s between the two PMMA configurations. Actually, these two energetic contributions stem from the substitution of the hydrogen atom attached to the chiral carbon atom in the PMA repeat unit by a methyl group. The same behavior is encountered with the two poly(ethyl methacrylate) (PEMA) chain tacticitics.