Variable-temperature FTIR measurements were performed on amorphous films of isotactic and syndiotactic PMMA, followed by a detailed analysis of the temperature dependence of integrated band intensities for C-O stretching modes in the region 1050-1300 cm(-1). The use of computerized FTIR instrumentation and modern methods of quantitative band-fitting analysis in combination with the recent IR and ab initio results for simple esters allowed us to achieve, for the first time, the unambiguous conformational assignment for the ester bands of PMMA. This, in turn, enabled reliable IR spectroscopic determination of conformational energies for the PMMA, backbone and the ester side group. The backbone conformational characteristics for i-PMMA and s-PMMA are found to be closely similar (in the staggered bonds approximation); the trans-trans state is strongly preferred over the trans-gauche state in both polymers, and the preference is only slightly higher for s-PMMA. At the same time, the two stereoregular forms appear markedly different in the conformational characteristics of ester side group. The minimum energy conformation of the ester moiety is trans in i-PMMA but cis in s-PMMA. Moreover, the absolute value of the conformational energy difference between the cis and trans states is 1.84 kcal/mol for i-PMMA, compared with only 0.46 kcal/mol in s-PMMA. Besides, the FTIR data shows that the ester groups of i-PMMA do not contribute to the glass transition, whereas those of s-PMMA form intermolecular associates (presumably, by carbonyl-carbonyl dipole interactions) and, in this way, elevate the glass transition temperature relative to that expected from the backbone conformational energy alone. This finding discards the thesis of O'Reilly and Mosher (Macromolecules 1981, 14, 602) that the Gibbs-DiMarzio theory fails to explain the difference in T-g between i-PMMA and s-PMMA.