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

Inter- and intramolecular ester exchange reactions in the ring-opening polymerization of (D,L)-lactide using lanthanide alkoxide initiators

“…[23] Isoselectivity corresponding to a first-order Markov propagation was observed in our group. [36] With other metallic initiators usually a syndiotactic tendency was reported as described by Kricheldorf et al, [37] Coudane et al [38] and Thakur et al [39] Coudane et al [38] used a method proposed by Kasperczyk [40] for the calculation of a coefficient of probability P i reflecting the probability of isotactic (0.5 a P i a 1.0) or syndiotactic (0 a P i a 0.5) enchainments. For a random type, a Bernoullian enchainment, P i is equal 0.5.…”

Polymeric nanomedicines based on poly(lactide) and poly(lactide-co-glycolide)

“…Such stereo-regular concerns affect the mechanical, thermal, and biological properties of PLA. [25] PLLA is considered more biocompatible since the naturally occurring lactic acid is in the L form. From a physical level of understanding, PLA homopolymers degrade more slowly than PGA homopolymers on the basis of crystallinity as well as steric inhibition by the PLA methyl group to hydrolytic attack.…”

Structurally diverse dysprosium and yttrium complexes containing an amine-bis(benzotriazole phenolate) ligand: Synthesis, characterization and catalysis for lactide polymerization

“…To test the ability of molecular weight control by 6 in combination with the extra alcohol, 9-anthracenemethanol (9-AnOH) as the chain transfer agent was added to bulk polymerization mixture. i P r is the probability of racemic linkages between monomer units and is determined from the methine region of the homonuclear decoupled 1 H NMR spectrum [37,38]. j Because of no epmerization during ROP of L-LA, the isotactic poly(lactide) was produced.…”