The temperature induced conformational transition of stereoregular PMMA in diluted or semi-diluted solution was studied by viscometry, NMR and FT-IR, The conformational energies of polymer chains were calculated from infrared spectral subtraction by following the method described by O'Reilly and Mosher [O'Reilly JM, Mosher RA, Macromolecules, 14 (1981) 602; O'Reilly JM, Teegarden DM, Mosher RA. Macromolecules, 14 (1981) 1693]. The fraction of sequences containing gauche conformation that can be deduced shows a large increase during the conformational transition. This increase in the probability of gauche conformations accounts for an increase in the chain flexibility deduced from the decrease of the calculated characteristic ratio. The increment in the hydrodynamic volume of the chain in chloroform that occurs at the transition temperature can be correlated with an increase in the density of the polymer-solvent specific interactions. The range of temperatures of the conformational transition, namely, 20-40 degrees C, was found to be independent of the quality of the solvent (theta or athermic solvent). However, the cooperativity of the conformational changes sigma is shown to depend on the solvent for the i-PMMA, Further, the aggregating character of the solvent (o-dichlorobenzene) shifts the observed transition for the i-PMMA solution towards higher temperatures which is ascribed to the disassociation of the aggregated structures.