Conformational energy calculations have been carried out for monomeric and trimeric oligomers of PMMA and for four-bond segments (embracing two repeat units) embedded in stereoregular PMMA chains, including both isotactic (meso) and syndiotactic (racemic) stereoisomeric forms. All incident interactions were taken into account. In each conformational domain the energy was minimized with respect to all bond angles and torsion angles including the torsional rotation χ of the ester group about the bond joining it to the chain backbone. Although in most of the conformations the plane of the ester group tends to occur approximately perpendicular to the plane defined by the adjoining skeletal bonds, substantial departures occur when one of these bonds is in a gauche g state. In consequence of this departure from χ = 0 or π, the energy of the g conformations is not excessive, as was originally concluded. The intradiad bond angle τ’ = 124 ± 1° in all conformations after energy minimization. The interdiad bond angle τ = 106° when both adjoining skeletal bonds are trans t, τ ≈ 111° when one bond is t and the other g or g, and τ ≈ 116° when both are g or g. The spatial configurations found to be of lowest energy for stereoregular chains are in excellent agreement with crystallographic studies on i-PMMA and with results of wide-angle X-ray scattering of s-PMMA. The backbone torsional angles for the various energy minima can be represented approximately by six discrete states that form the basis for a rotational isomeric state treatment. Conformations near trans are preferred; the preference is pronounced for s-PMMA. Characteristic ratios and their temperature coefficients calculated according to the six-state scheme are in satisfactory agreement with experimental results. Parameters used in these calculations follow directly from the conformational energy calculations; adjustments are not required. © 1986, American Chemical Society. All rights reserved.