# Kinetics of PTSA-Catalysed Polycondensation of Citric Acid with 1,3-Propanediol
## 3 Results and Discussion  
### Spectral Analysis  
#### 3.1.1 FTIR  
As a result of the syntheses that were performed, products in the form of a colourless or slightly yellowish resin with a high viscosity that increased with the reaction time were obtained. The formation of polyester was confirmed based on the FTIR spectra of the selected samples (Figure 2). The spectra of the substrates included:  
1. For citric acid: * around 3500 cm\({}^{-1}\) (A), a narrow band of stretching vibrations of O-H groups that did not participate in hydrogen bonding; * a broad band near 3250 cm\({}^{-1}\) (B) that extended down to about 2600 cm\({}^{-1}\) (C) and originated from O-H groups bound by hydrogen bonding; in this area, bands originating from the free-standing hydroxyl group and the O-H bond in the carboxyl group, as well as a stretching vibration band of aliphatic C-H bonds, most likely overlapped; * a characteristic narrow band of high intensity around 1715 cm\({}^{-1}\) (D) of the stretching vibrations of the C=O bond in a carboxylic group; a double band indicates non-equivalence of carboxylic groups (\(\alpha\)-C(O)O-H and \(\beta\)-C(O)O-H), which could also have resulted from the presence of hydrogen bonds; then, the band of the carbonyl group of the associated molecules appeared at higher frequencies than with non-associated ones; * bands in the range of 1430-1100 cm\({}^{-1}\) (E,F) corresponding to deformation vibrations of the C-O-H group and stretching vibrations of C-O bonds. 2. For 1,3-propanediol: * a characteristic broad band of high intensity in the range of 3600-3000 cm\({}^{-1}\) (G) originating from O-H bond stretching vibrations; the presence of hydrogen bonds significantly influenced the shape of the band;  
Figure 1: Comparison of the \({}^{13}\)C-NOE NMR spectra in the range of signals from carbonyl carbon atoms—from the top: citric acid (orange line), triethyl citrate (green line), and poly(1,3-propanediol citrate) (dark blue line). band of about 2900 cm\({}^{-1}\) (H) of stretching vibrations of aliphatic C-H bonds; deformation vibration band of an O-H bond near 1430 cm\({}^{-1}\) (I); a strong C-O stretching vibration band around 1050 cm\({}^{-1}\) (J); a broad band corresponding to the deformation vibrations of the hydrogen-bonded O-H group in the range of 550-750 cm\({}^{-1}\) (K). Figure 2: FTIR spectra of citric acid, 1,3-propanediol, and selected products of their polycondensation. The significant bands are marked with letters (A–R) and described in the text below. The FTIR spectra of the substrates were compared with those of the products. For this purpose, two samples (from the initial and final phases) from each measurement series were selected. The most important bands were distinguished as follows:  
* a broad band in the range of 3700-3100 cm\({}^{-1}\) (L) that formed as a result of the overlapping of bands corresponding to stretching vibrations of O-H bonds in molecules of the unreacted substrates, i.e., A, B, and G;
* a band of approximately 2900 cm\({}^{-1}\) (M) of stretching vibrations of C-H aliphatic bonds in both a polyester chain and the molecules of substrates;
* a band at about 2600 cm\({}^{-1}\) (N) from O-H groups bonded by hydrogen bonding, indicating the presence of unreacted acid (the corresponding band is marked with C);
* a strong narrow band around 1730 cm\({}^{-1}\) (O) corresponding to the stretching vibrations of the C=O bond of the ester; the visible broadening of the band on the right-hand side is due to the incompletely reacted acid, where the C=O bond is part of the carboxyl group. In addition to the noticeable shift in the frequency of the stretching band of the carbonyl group, another confirmation of the polyester being obtained is the bands coming from the two groupings containing the C-O bond:  
* at about 1050 cm\({}^{-1}\) (R), the C-O bond stretching vibration band in the O-C-C grouping (Figure 3a);
* at about 1240 cm\({}^{-1}\) (P), the C-O bond stretching vibration band in the ester grouping C-C(O)-O (Figure 3b).