Abstract
The physical properties of poly(lactic acid) (PLA) are influenced by its stereoregularity and stereosequence distribution, and its polymer stereochemistry can be effectively studied by NMR spectroscopy. In previously published NMR studies of PLA tacticity, the NMR data were fitted to pair-addition Bernoullian models. In this work, we prepared several PLA samples with a tin catalyst at different L,L-lactide and D,D-lactide ratios. Upon analysis of the tetrad intensities with the pair-addition Bernoullian model, we found substantial deviations between observed and calculated intensities due to the presence of transesterification and racemization during the polymerization processes. We formulated a two-state (pair-addition Bernoullian and single-addition Bernoullian) model, and it gave a better fit to the observed data. The use of the two-state model provides a quantitative measure of the extent of transesterification and racemization, and potentially yields useful information on the polymerization mechanism.
Highlights
Poly(lactic acid) (PLA) is a biodegradable and thermoplastic aliphatic polyester derived from renewable resources, such as corn, cassava, and sugarcane
We introduced a two-state model, consisting of both the pair-addition Bernoullian and single-addition Bernoullian models
In the NMR analysis of polymers, statistical models are useful in providing a theoretical framework for analysis, checking the consistency of results, and deriving information on polymerization mechanisms
Summary
Poly(lactic acid) (PLA) is a biodegradable and thermoplastic aliphatic polyester derived from renewable resources, such as corn, cassava, and sugarcane. A better understanding of PLA stereochemistry is useful both in fundamental studies of structure/property relationships and in commercial product development. NMR is known to be one of the best techniques for the analysis of polymer stereochemistry and tacticity [9,10]. There have been a large number of NMR tacticity studies of PLA (mostly in CDCl3 ) at the tetrad level for CH protons and carbon, and partially at the hexad level for carbonyl carbon [11,12,13,14,15,16,17,18,19,20,21,22,23,24]. Pyridine-d5 has been found to have a notable solvent effect on the NMR spectra
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