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The preparation of polystyrene-b-poly(ethylene oxide)-b-poly(ε-caprolactone) triblock copolymers using three different strategies is described. Experimentally, a method we have called “thermal” is the simplest one of the three employed. However, it presents serious limitations for the preparation of high molecular weight triblock copolymers with low polydispersity. The titration method is a more elegant alternative to prepare the PS-b-PEO-b-PCL triblock copolymers. Nevertheless, the stoichiometric titration of the precursor terminal hydroxyl groups requires much expertise, and it becomes more difficult as the concentration of such terminal groups decreases when the precursor molecular weight increases. Finally, the sequential method may be regarded as the most convenient one. However, the experimental steps involved a solvent exchange during polymerization that must be performed under conditions of strict purity and high vacuum. Several triblock and pentablock copolymers of varying compositions and molecular weights were successfully prepared. The fact that these copolymers contain two crystallizable blocks, i.e., PEO and PCL, makes them very interesting from a structural point of view. The copolymers were found to be phase segregated according to evidence provided by DSC and WAXS. The crystallization and melting behavior of both PEO and PCL blocks is greatly affected by composition. Fractionated crystallization phenomena were detected when either one or both blocks constituted the minor components within the block copolymers.

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Poly(beta-l-aspartate)s are known to take up helical conformations reminiscent of the alpha-helix of polypeptides. The isobutyl, n-butyl, and 2-methoxyethyl esters have been examined by polarized ir spectroscopy in order to discriminate between the left (1L) and right (2R)-handed conformations, which are known to be compatible with the 13/4-helix adopted by these polyamides when crystallized in the hexagonal form. Dichroic ratios obtained from samples stretched in poly(ethylene oxide) together with orientation measurements made by x-ray diffraction were used to estimate the transition moment directions of amide A, I, and II bands with respect to the fiber axis. These were compared to those calculated by modeling simulations to conclude that the right-handed conformation consisting of 14-membered hydrogen-bonded rings is the correct model for the 13/4-helix. These results give definite support to earlier molecular mechanics calculations, which had shown that the 2R model is energetically favored over the 1L by about 2.5 kcal/(mol residue).

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