CRYSTAL-STRUCTURE OF STEREOCOMPLEX OF POLY(L-LACTIDE) AND POLY(D-LACTIDE)

An equimolar mixture of poly(L-lactide) and poly(D-lactide) was crystallized into a stereocomplex whose crystal system is triclinic (P1) with cell dimensions: a = 0.196 nm, b = 0.916 nm, c (chain axis) = 0.870 nm, alpha = 109.2-degrees, beta = 109.2-degrees, and gamma = 109.8-degrees. In the unit cell, a poly(L-lactide) segment and a poly (D-lactide) segment are contained as a pair and packed laterally in parallel fashion. The L- and D-poly(lactides) in the complex take a 3(1) helical conformation, which is a little extended from a 10(3) helix in the homopolymer crystal with the alpha-form. Homopolymers are also able to take the 3(1) helical conformation and form the beta-form crystal. The 3(1) helix in the homopolymer crystal is less stable than the 10(3) one, and hence the beta-form is easily transformed to the alpha-form by annealing.