More about the stereodependence ofDD andLL pair linkages during the ring-opening polymerization of racemic lactide

Abstract

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

Microstructure of Poly(lactide). Phase-Sensitive HETCOR Spectra of Poly(meso-lactide), Poly(rac-lactide), and Atactic Poly(lactide)

“…Again on the basis of the approximate integrations of the two lines, it was concluded that four of the five tetrads of poly( meso -lactide) were spectroscopically degenerate; all collapsed into the higher field resonance, so that the lower field line represented only the sis tetrad. A similar conclusion was inferred for the two lines observed in the 13 C{ 1 H} spectrum of poly( rac -lactide) 9a. An illustration of these assignments is shown in Figure .…”

“…In this case, if the triad/pentad model is correct, one would expect the pentads sisi and isis to be statistically favored over iisi and isii in poly( rac -lactide) and an overall statistical relationship where ii : iisi : sisi : isis : isii = X :Y:Z:Z:Y . This roughly fits the spectral data in ref b. Similar arguments have been proposed in support of tetrad assignments.…”

“…These spectra qualitatively resemble the spectra reported by Kricheldorf and Boettcher 35 for poly( rac -lactide) in which transesterification reactions are presumed to have randomized the stereochemical microstructure. It is important to note that neither of these spectra are consistent with a random, i.e., Bernoullian, distribution of tetrad stereosequences as assigned by Kricheldorf 9a 14

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Melt polycondensation of L‐lactic acid to poly(L‐lactic acid) with Sn(II) catalysts combined with various metal alkoxides

“…The carbonyl signal (a) shows only one peak due to the single isotactic L ‐sequence of the lactate units, while the carbonyl signal (b) shows several signals at the foot (169–169.5 ppm) because of the presence of a small amount of racemic ( r ) and meso ( m ) sequences of lactate units. The signal assignment shown in the figure was made according to the literature 15–18. These data support the view that that SnCl 2 ·2H 2 O–Ti(O i Pr) 4 induced racemization of lactyl units during polycondensation.…”

Ring‐opening polymerizations of L‐Lactide catalyzed by zinc caprylate: Syntheses of cyclic and linear poly(L‐lactide)s

“…15 That study included ZnO and Zn stearate which is comparable with ZnCap 2 with regard to its basicity. It was found that the stearate did not cause racemization at 120 C and slight racemization at 150 C. However, the catalyst concentration was much higher corresponding to a LA/Cat ratio of 100/1 and the reaction time was 48 h. In later experiments conducted with a LA/Cat ratio of 1000/1 no racemization was found at 150 C even after 96 h. With a LA/Cat ratio of 4000/1 racemization did not occur up to 192 h. In the present work high resolution 13 C NMR spectra of four PLAs (No. 2A, B and 4A, B, Table 3) were recorded with a 600 MHz FT-NMR spectrometer and more than 10,000 scans, so that a signal-to-noise ratio > 150/1 was achieved.…”