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For the last two decades, mixtures of D‐ and L‐lactides, the DD and LL cyclic dimers of lactic acid enantiomers, were polymerized under various conditions of temperature, time, and initiators. Consequences of the dimeric structure of lactide molecules on the distribution of chiral units and on the properties of resulting PLAs were discussed occasionally. In the meantime, NMR techniques have grown up and are now powerful tools to investigate configurational structures from stereosensitive resonances. Thus far, correlation between experimental data and theoretical n‐ades were based on Bernoullian‐type enchainments of DD and LL pairs of lactyl units present in PLA stereocopolymer chains obtained by ring‐opening polymerization. Analysis of NMR spectra of racemic PLAs of different origins, including literature data, was reconsidered on the basis of the probabilities to form iso‐ and hetero‐dyads during chain propagation. Consideration of equireactivity and stereodependent reactivity between the DD and LL pairs generated from a racemic mixture of D‐ and L‐lactide diastereoisomers led us to the conclusion that DD/LL and LL/DD heterotactic junctions are preferentially formed when one uses Sn octoate and Zn metal as initiator systems, even though completed macromolecules remain predominantly isotactic because of the isodyads inherently present in lactide diastereoisomers. A simple method is proposed to determine whether homo‐ or cross‐addition of DD and LL pairs is favorized in any poly(racemic lactide) in the absence of transesterification rearrangements. This method is based on the comparison of experimental relative weights of the three distinct groups of resonances due to carbonyl carbon atoms with relative weights calculated according to the Bernoullian statistics. It allows the determination of a coefficient of stereoselectivity Pi from which one can easily evaluate the average length of isotactic blocks Li without any computation approximations. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1651–1658, 1997

Poly(epsilon-caprolactone) (PCL) is known to biodegrade under composting or water sewage plant conditions. However, as compared with poly(alpha-hydroxy acids) derived from lactic and glycolic acids, PCL is much more resistant to chemical hydrolysis and is achiral, a feature that limits very much the possibility of property modulation through the configurational structure of polymer chains. For the sake of enlarging the family of PCL-type polymers, a novel method is proposed which is based on the anionic activation of PCL chain by the removal of a proton from the methylene group in alpha-position of the ester carbonyl present in the main chain, using a nonnucleophilic base such as lithium diisopropyl amide (LDA). This activation leads to a polycarbanion onto which various electrophile groups can be attached. The feasibility of the process was first shown on poly(methyl acrylate), (PMA), whose polyacrylic main chain is resistant to strong bases. The PMA polycarbanion was modified by various electrophiles, namely benzaldehyde, naphthoyl chloride, benzyl chloroformate, and iodomethane. In a second stage, the same reactions were performed successfully on PCL. The degree of substitution depended on the experimental conditions. PCL underwent main chain degradation during the formation of the polycarbanion whereas the reaction with the electrophiles did not cause any further main chain cleavages. The degradation of PCL chains can be limited enough to give access to novel functional PCL polymers.

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Biodegradable copolymers were prepared by ring‐opening polymerization of sequentially added ε‐caprolactone and DL‐lactide in the presence of ethylene glycol or poly(ethylene glycol), using zinc metal as catalyst. Polymerization was performed in bulk and yielded block copolymers with predetermined PEG/PCL/PLA segments. The obtained polymers were characterized by 1H NMR, SEC, IR, DSC, TGA, and X‐ray diffraction. Data showed that the copolymers preserved the excellent thermal behavior inherent to PCL. The crystallinity of PLA‐containing copolymers was reduced with respect to PCL homopolymer. The presence of both hydrophilic PEG and fast degrading PLA blocks should improve the biocompatibility and biodegradability of the materials, which are of interest for applications as substrate in drug delivery or as scaffolding in tissue engineering.

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