MECHANISM OF THE STEREOCOMPLEX FORMATION BETWEEN ENANTIOMERIC POLY(LACTIDE)S

Poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) crystallize into a s tereocomplex with a melting point 50 degrees C higher than the crystal s of the enantiomers. The racemic crystal is formed by packing beta-fo rm 3(1)-helices of opposite absolute configuration alternatingly side by side. Single crystals of the stereocomplex exhibit triangular shape . The drastic difference of the powder patterns evidences the differen t packing of the beta-form in the stereocomplex and in crystals of the pure lactides. By force field simulation of the stereocomplex and the PLLA unit cells and of their powder patterns, the reasons for the dif ferent packing could be clarified. Between the beta-helices in the ste reocomplex, van der Waals forces cause a specific energetic interactio n-driven packing and, consequently, higher melting point. Helices of i dentical absolute configuration pack different from pairs of enantiome r beta-helices. Packing favors ct-type helication. A well-defined 10(3 )-helix has not been found. Good agreement with the experimental powde r patterns proves the correctness of the simulations. On the basis of morphology, packing calculations, and atomic force microscopy, we prop ose a model of stereocomplex crystal growth, which explains the triang ular shape of single crystals. Thus, for polymer components beyond cha in folding length, the stereocomplex formation by simultaneous folding of the two types of chains is. plausible. The triangular type of crys tallizing offers favorable position for the polymer loops during the c rystal growth. Our study of the PLA complexation mechanism may offer a chance to predict other polymeric stereocomplexes and their propertie s.