ISSN:
0887-624X
Schlagwort(e):
DL-lactide
;
stereocopolymerization
;
poly(lactic acid)
;
configuration
;
stereosequences
;
Chemistry
;
Polymer and Materials Science
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
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
Zusätzliches Material:
5 Ill.
Materialart:
Digitale Medien
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