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Solution-grown, chain-folded lamellar crystals of poly--propiolactone (PPL) have been crystallized isothermally from cyclohexanone at 55°C. The lathlike crystals have been studied by transmission electron microscopy (imaging and diffraction), atomic force microscopy, and X-ray diffraction. A new crystal structure, the γ-structure, has been discovered. Using the experimental diffraction data and computerized modeling the structure has been refined, including the detailed geometry of the adjacent re-entry folds. The chain is in an all-trans or 1-fold helix conformation and the structure consists of a two-chain, C-faced, orthorhombic unit cell with the following parameters: a ) 0.700 ( 0.002 nm, b ) 0.490 ( 0.002 nm, and c (chain axis) ) 0.493 ( 0.002 nm. The setting angles, with respect to the a axis, are ( 51.5°for the corner and center chains, respectively. The lamellae are 5 nm in thickness and the chains run orthogonal to the lamellar surface, although there is evidence for slight c-axis shearing of the chain-folded sheets. The general fold direction is along the a axis (long axis of the crystal) and the chain folds successively in the (110) and (11 h0) planes. Comparisons are made with previous structures for PPL fibers and related chain-folded polymers.

Stereoblock poly(lactic acid) consisting of D- and L-lactate stereosequences can be successfully synthesized by solid-state polycondensation of a 1:1 mixture of poly(L-lactic acid) and poly(D-lactic acid). In the first step, melt-polycondensation of L- and D-lactic acids is conducted to synthesize poly(L-lactic acid) and poly(D-lactic acid) with a medium-molecular-weight, respectively. In the next step, these poly(L-lactic acid) and poly(D-lactic acid) are melt-blended in 1:1 ratio to allow formation of their stereocomplex. In the last step, this melt-blend is subjected to solid-state polycondensation at temperature where the dehydrative condensation is allowed to promote chain extension in the amorphous phase with the stereocomplex crystals preserved. Finally, stereoblock poly(lactic acid) having high-molecular-weight is obtained. The stereoblock poly(lactic acid) synthesized by this way shows a higher melting temperature in consequence of the controlled block lengths and the resulting higher-molecular-weight. The product characterization as well as the optimization of the polymerization conditions is described. Changes in M(w) of stereoblock poly(lactic acid) (sb-PLA) as a function of the reaction time.

Cover: The cover depicts the novel synthetic scheme for the stereoblock poly(lactic acid) (sb-PLA) by solid-state polycondensation of a stereocomplexed mixture of poly(L-lactic acid) (PLLA), and poly(D-lactic acid) (PDLA), having medium molecular weight. First, meltpolycondensation of L-and D-lactic acids is conducted to obtain the PLLA and PDLA, respectively. In the second step, these polymers are melt-blended to form the stereocomplex. Then, the solid-state polycondensation of the melt-blend affords the sb-PLA with high molecular weight and high melting temperature without formation of single polymer crystals of PLLA or PDLA.Further details can be found in the Full Paper by K. Fukushima, Y. Furuhashi, K. Sogo, S. Miura, and Y. Kimura* on page 21.

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