ELECTRON DIFFRACTION IN POLYMER CRYSTAL STRUCTURE ANALYSIS: SOME RECENT EXAMPLES. Stefano Valdo Meille, Dipartimento di Chimica, Politecnico di Milano, via Mancinelli 7, 20131 Milano, Italy

It is normally assumed that X-ray fiber patterns provide the more readily accessible and reliable diffraction data for polymers. Essential geometric information like the fiber repeat and the probable helical symmetry are immediate results with this technique. It is however inapplicable to low molecular weight or infusible polymers and to phases present in small proportion or else unstable to elongation. Even in favorable cases the determination of lattice constants and of integrated intensities may be problematic because of polymorphism and overlapping of diffraction maxima.

Experience in our laboratory shows that often the key problem in, the structural analysis of polymers is the determination of the unit cell and, the space group, even with relatively simple asymmetric units. Electron diffraction (E.D.) represents the most reliable technique for this purpose and examples involving the ß- and [[gamma]]-polymorphs of isotactic polypropylene and other recent structural investigations will be discussed. Lattice parameters even more so than the space group can hardly be predicted by packing analysis while on the other hand molecular modeling is often adequate to determine molecular conformation also in the crystal.

There is also increasing evidence that with appropriate precautions E.D. intensities are adequate for polymer crystal structure solution and refinement. It appears that the negative impact of dynamic effects and of radiation damage on E.D. data have been overemphasized while background and peak shape problems in fiber diffraction are often underestimated. Combined approaches using parallel structural refinements with E.D. and X-ray powder diffraction data may represent an efficient alternative to traditional refinements based on fiber data.