APPLICATION OF THE VIRTUAL BOND METHOD TO THE CRYSTALLINE CONFORMATIONS OF ISOTACTIC POLYSTYRENE AND POLY(METHYL METHACRYLATE)

The application of the virtual bond method of analysis to the crystalline conformations of vinyl polymers is discussed. The method can be used if the screw symmetry and the pitch of the helix are known from diffraction data and involves the rotation of the repeat unit about its end-to-end vector or the virtual bond. It enables the determination of the favorable conformations of the helix, by allowing the variation of the skeletal bond angle at the methylene carbon atom. The crystalline conformations oi isotactic polystyrene with 12 repeat units per turn and 3 repeat units per turn are discussed. The analysis is restricted to showing that the 12-unit helix is stereochemically possible. Rigorous comparison with the X-ray data is not attempted due to the possible presence of head-to-head sequences in the polymer and its influence on the crystalline domains. The calculations on the five-unit helix of isotactic PMMA show that it occurs close to an unfavorable minimum on the (øi, øi+1)surface. The calculated energy of the ten-unit helix is lower than that of the five-unit helix of PMMA. The side group rotations play an important role in determining the relative positions of these helical conformations on the overall (øi, i+1) domain. © 1979, American Chemical Society. All rights reserved.