Polimeros

Anal. Chem. 1997, 69, 4303-4309

Articles

A Quantitative Method for Determination of Lactide Composition in Poly(lactide) Using 1H NMR
Khalid A. M. Thakur,* Robert T. Kean, and Eric S. Hall

Central Research, Cargill Incorporated, P.O. Box 5699, Minneapolis, Minnesota 55440
Matthew A. Doscotch and Eric J. Munson*

Department of Chemistry, University of Minnesota, 207 Pleasant StreetSE, Minneapolis, Minnesota 55455

A method has been developed to quantitatively determine the composition of D-lactide and meso-lactide stereoisomer impurities in poly(lactide) containing predominantly L-lactide. In this method, the stereosequence information obtained from a few well-resolved resonances in the 1H NMR spectrum representing RR and R stereogenic defects is used. The D-lactide andmeso-lactide as minor components lead to RR and R stereogenic defects, respectively, which influence the isotactic chain length distribution and hence affect the polymer properties. Analytical equations relating the stereosequence probability to the lactide feed composition are not available due the complicated kinetics involved for the melt polymerization; viz. the preference for syndiotacticlactide addition decreases with reducing residual lactide concentration in the batch process. Hence, empirical correlations were determined by least-squares fit to the predictions for the specific stereosequence probabilities provided by Monte Carlo calculations of a number of lactide stereocopolymerizations. The Monte Carlo calculations simulate the kinetics observed for melt polymerization at 180 °Ccatalyzed by Sn(II) bis(2-ethylhexanoate) (Sn(II) octoate) in a 1:10 000 catalyst/lactide ratio.
Poly(lactide) is a well-known bioresorbable polymer which has been explored for use in several biomaterials applications and drug delivery systems, including in vivo degradable/resorbable medical implants and sutures.1-4 Due to its biodegradability and ecosystemfriendly properties,2,5 poly(lactide)(PLA, Chart 1) can also be an ideal replacement for nondegradable polymers in numerous applications such as yard waste bags, food containers, agricultural mulch films, etc. Recent technological developments have made
(1) Vert, M.; Schwarch, G.; Coudane, J. J. Macromol. Sci. Pure Appl. Chem. 1995, A32, 787-796. (2) Miyoshi, R.; Hashimoto, N.; Koyanagi, K.; Sumihiro, Y.; Sakai, T. Int. Polym. Process.1996, 11, 320-328. (3) Mainilvarlet, P.; Rahm, R.; Gogolewski, S. Biomaterials 1997, 18, 257266. (4) Hoogsteen, W.; Postema, A. R.; Pennings, A. J.; ten Brinke, G. Macromolecules 1990, 23, 634-642. (5) Chang, Y.-N.; Mueller, R. E.; Iannotti, E. L. Plant Growth Regul. 1996, 19, 223-232. S0003-2700(97)00792-0 CCC: $14.00 © 1997 American Chemical Society

Chart 1

PLA products economicallycompetitive with petroleum-derived plastics.6,7 High molecular weight PLA is prepared by ring-opening polymerization8-11 of lactic acid cyclic dimers (lactides) which exist as RR, SS, or RS stereoisomers. The RR configuration of the cyclic dimer is referred to as D-lactide while the SS configuration is referred to as L-lactide. An equimolar ratio of (RR)- and (SS)lactide is referred to as racemic orD,L-lactide, and the (RS)-lactide is referred to as meso-lactide. A number of physical properties of PLA are linked to its stereosequence distribution.7,12-14 For example, pure isotactic poly(L-lactide) (PLLA) crystallizes at a faster rate and to a larger extent than when L-lactide is polymerized with small amounts of either D-lactide or meso-lactide.7,13 Hence the isotactic S length distribution,which is determined primarily by the amount of L-lactide in PLA, may be linked to its crystallization properties.13,15,16 In the polymer, to a first approximation, the lactide stereoisomer composition may be used to represent the stereosequence distribution.
(6) Gruber, P. R.; Hall, E. S.; Kolstad, J. J.; Iwen, M. L.; Benson, R. D.; Borchardt, R. L. U.S. Patent 5,142,023, 1992. (7) Kolstad, J....