

FOLLOWUS
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China
lxh83@suda.edu.cn (X.H.L.)
tuyingfeng@suda.edu.cn (Y.F.T.)
Received:31 March 2021,
Revised:2021-5-6,
Accepted:11 May 2021,
Online First:23 July 2021,
Published:18 October 2021
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Tu, Y. Y.; Wan, X. T.; Huan, J.; Zhu, X.; Li, X. H.; Tu, Y. F. The effect of trifluoroacetic acid on molecular weight determination of polyesters: an in situ NMR investigation. Chinese J. Polym. Sci. 2021, 39, 1590–1596
Yan-Yan Tu, Xue-Ting Wan, Jie Huan, et al. The Effect of Trifluoroacetic Acid on Molecular Weight Determination of Polyesters: An
Tu, Y. Y.; Wan, X. T.; Huan, J.; Zhu, X.; Li, X. H.; Tu, Y. F. The effect of trifluoroacetic acid on molecular weight determination of polyesters: an in situ NMR investigation. Chinese J. Polym. Sci. 2021, 39, 1590–1596 DOI: 10.1007/s10118-021-2605-0.
Yan-Yan Tu, Xue-Ting Wan, Jie Huan, et al. The Effect of Trifluoroacetic Acid on Molecular Weight Determination of Polyesters: An
Due to the poor solubility of aromatic polyesters in common organic solvents
trifluoroacetic acid is usually used as a co-solvent to increase their solubility for characterizations. However
only few studies have reported the side reactions induced by it. We present here the application of
in situ
1
H-NMR techniques to explore its effect on the hydroxyl end-groups
which are usually used for the molecular weight determination of polyesters by end-group estimation method. Using bis(2-hydroxyethyl) terephthalate (BHET) as model compound
1
H quantitative NMR results show the peak integration of hydroxyethyl end-groups decreased with time
via
a pseudo-first-order kinetics in d-trifluoroacetic acid/d-chloroform mixture solvent (1:10
V
:
V
). This is due to the esterification of hydroxyethyl groups with trifluoroacetic acid
revealed by the
1
H-
13
C gradient-enhanced heteronuclear multiple bond correlation (gHMBC) spectrum. The mixtures of dimethyl terephthalate and BHET with different molar ratios were used to represent poly(ethylene terephthalate) (PET) with different degrees of polymerization
and the effect of trifluoroacetic acid on the estimation of hydroxyethyl groups and subsequent molecular weight determination of polyesters was studied. Our results show that if a relative error of 5% is allowed
the NMR measurements must be finished within 1.3 h of solution preparation at 25 °C in the mixture solvent. The results were confirmed in PET sample
while in poly(ethylene adipate)
the obtained esterifaction constant is faster that those in aromatic system. The results can be applied to other polymer systems with alcohol functionalized groups
and used as a guideline for the characterization of polyesters and polyethers by end-group estimation method.
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