A geometrical and energy analysis on the isolated chain of the isotactic optically active alternating styrene-carbon monoxide copolymer is presented. Relations between the bond and torsion angles of the main chain, which give helical s(2/1) repetition and highly extended chains, have been found. The results of the conformational energy calculations indicate that the best conformation in the crystalline field corresponds to a slight deviation of the torsion angles from 180 degrees, according to the experimental structure, Packing energy calculations for the space group P2(1) and the experimental unit cell with axes a = 5.47 Angstrom, b = 8.367 Angstrom, c = 7.57 Angstrom, and gamma = 110 degrees are presented. The present. analysis indicates that the packing interaction influences the chain conformation. Precisely, the need to optimize intermolecular contacts between the atoms of the phenyl rings of adjacent chains produces a relevant deviation (nearly 30 degrees) of the torsion angle which characterizes the orientation of the lateral phenyl group from the value (approximate to 60 degrees) that keeps the phenyl ring plane bisecting the valence bond at the tertiary carbon. These results are in agreement with those obtained in the refinement of the crystal structure by X-ray diffraction.