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A full-relaxation optimization of molecule and the popular MM2 force field are em-ployed to obtain the geometry parameters and the conformational energy surface of a mesoor a racemic dyad of poly（methyl acrylate） （PMA） with a specified carbonyl-bond orien-tation in side-groups. It is found that the conformational energy maps calculated hereconsiderably differ from those calculated with the rigid molecular model as reported in theearlier studies. The g

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state cannot be omitted in the obtained contour maps. Two impor-tant conformers tg

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and g

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with energy minima were newly detected for a racemic dyad.The analysis on the conformations with energy minima confirmed that the ester groupsare not always perpendicular to the plane defined by the two adjacent skeletal bonds andmay change their relative orientations to meet the requirement of lower energies duringthe conformational state transition. Instead of the early way of adjusting the interactionenergy parameters to fit the experimental data

we attempt to predict unperturbed chaindimensions via the reliable force field and the configurational statistical mechanics. Theproposed scheme with three rotational states identified from the contour maps allowed usto satisfactorily reproduce the experimental dimensions of random PMA chains.