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| - Conformational and Packing Modeling of Optically Active Polyesters. 1.Planar Zigzag Structure of Isotactic Polylactone
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| - The optically active and crystalline poly(α-methyl-α-ethyl-β-propiolactone) (PMEPL) showsconformational changes and polymorphic behavior depending on the preparation method of the sample.The melt-crystallized isotactic PMEPL exhibits a monoclinic lattice and an extended planar zigzagconformation. Using computational modeling, we have solved and refined its crystal structure. Boththe two-step and single-step procedures were used to evaluate packing energies. The results indicatethat two chains with the same chirality and opposite directions pass through a monoclinic unit cell havinglattice dimensions a = 9.10, b = 7.44, c(fiber period) = 4.84 Å and β = 83.1°. The space group is P21 withunique axis b. The final crystal structure was confirmed by comparison with electron diffraction dataand X-ray powder diffraction spectra. Both procedures result in crystal structures in good agreementwith the experiments. The final discrepancy R factors are 0.16 for the flexible model and 0.20 for therigid model when compared to X-ray data and 0.20 for the flexible model and 0.23 for the rigid modelwhen compared to electron diffraction data.
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