Synthesis and structure analysis of polyesteramides modified with bio-based diaminopentane hexanedioic salt

2.2.1 Synthesis of polyesteramides series 1

BHET was first developed by the esterification of PTA and EG in the presence of Sb₂O₃ catalyst. Later, polyesteramides was produced by polymerization of BHET and DHS as follows:

The complete process flow chart of synthesis is given in Figure 1. Certain weight proportions of PTA and EG were reacted in the presence of Sb₂O₃ catalyst at 230~240°C for 2.5~3.0 h for esterification. BHET was obtained when the volume of water generated in the esterification condenser was over 90% of the theoretically calculated volume of water. Nitrogen was used as a protector during the whole esterification progress. After esterification, when the reaction temperature was below 150°C, DHS in a certain molar ratio (Table 1) was added into the reaction chamber. In order to prevent rapid oxidation of DHS at high temperature, a significant amount of nitrogen was added to the reactor and the inside gas was replaced with nitrogen twice as soon as reactants feeding was finished. Then the reaction mixture was stirred for 20 min at constant pressure to ensure a homogenous mixture. Temperature and pressure were maintained in the range of 265~275°C and 50 Pa, respectively for 2.5~3 h. When the reaction was over, a small amount of nitrogen was added into the reactor to maintain the micro-positive pressure to discharge the product. Then the polyesteramides series 1 were obtained by grain-sized dicing and drying.

Figure 1:

Flowchart of preparation of polyesteramides series 1.

2.2.2 Synthesis of polyesteramides series 2

Polyesteramides series 2 were produced by polymerization of DHS and low molecular weight LPET which was generated by the pre-polycondensation of BHET. BHET was obtained as a result of esterification between PTA and EG and the chemical equations are shown as follows.

Figure 2 shows the synthesis process of polyesteramides series 2. The process of esterification was the same as that of polyesteramides series 1. When the reaction was over, the temperature of the reactor was regulated between 270 and 280°C and meanwhile the vacuum pump with vacuum degree under 50 Pa was turned on. Half an hour later, low molecular weight LPET was obtained. DHS in a certain molar ration (Table 1) was added when the temperature was below 150°C followed by nitrogen replacement of internal gas twice for preventing quick oxidation of DHS. The reaction mixture was constantly stirred for 20 min at constant pressure and the temperature of the reactor was controlled in the range of 265~275°C for 3 h under 50 Pa vacuum pressure. After the reaction was over, a small amount of nitrogen was added into reactor to attain micro-positive pressure to discharge the product and the polyesteramides series 2 were obtained by grain-sized dicing and drying.

Figure 2:

Flowchart of preparation of polyesteramides series 2.

4.2.1 ¹H-NMR analysis

¹H-NMR spectra of polyesteramides are shown in Figure 3. It is apparent from Figure 3 that δ=7.92 ppm ~8.23 ppm at position 1 is the proton peak (5) of hydrogen in the benzene ring of PTA, δ=4.67~4.93 ppm at potion 2 is the proton peak of aliphatic hydrogen of EG, δ=4.0~4.5 ppm is the peak of hydroxyl group and methylene of DEG, which was the by-product of the reaction (6). During the synthetic process, EG was hydrolyzed to produce DEG as follows.

Figure 3:

¹H-NMR spectra. (A) Polyesteramides series 1; (B) Polyesteramides series 2.

δ=3.72 ppm in position a is the proton peak of -CH₂- in pentanediamine, δ=1.64 ppm in position c is the proton peak of -CH₂- in pentanediamine, δ=2.57 ppm and δ=2.81 ppm in position d′ and d, respectively, are the proton peaks of -CH₂- in adipic acid, δ=1.79 ppm in position b is the proton peak of -CH₂- of pentanediamine, and δ=1.89 ppm in position e is the proton peak of -CH₂- of adipic acid. It is evident, from the ¹H-NMR spectra that the expected chemical structure was formed.

Polyesteramides composition can be quantitatively calculated, as the area of proton peak in NMR spectra is proportional to the number of H. The practical ratio of bio-based diaminopentane hexanedioic salt to PTA was determined by the peak area ratio of a to 1 in Figure 3. It is obvious from Table 3 that with increasing DHS quantity, the response rate slightly decreases from 90% to 83%. The response rates of polyesteramides in series 1 and 2 have no obvious difference.

4.2.2 Analysis of HMBC spectra

2D-NMR unfolds the resonance signals on the same frequency axis which is overlapped in the 1D-NMR significantly, improving the resolution of the spectra and providing much information about the dynamics and structure that the 1D test fails to provide (7), (8), (9), (10). Herein, one axis of the 2D-NMR spectra is the ¹³C chemical shift and another one is the ¹H chemical shift. Their cross peak reflects the coupling relationship between ¹³C and ¹H, which were separated by 2~4 chemical bonds, and is denoted by ⁿJ_CH. The ¹H-NMR analysis in 4.2.1 only explained that there were polyester and polyamide units in the synthesized products, while the further study here on the products by means of HMBC analysis will certify relevance of polyester and polyamide.

There are four relevant reactions among DHS, BHET and LPET. It can be observed from Figure 4 (the chemical shifts of carbon and hydrogen in the designated positions have been marked), relevance 1 is the structural formula of the product generated by PTA and diaminopentane; relevance 2 is the structural formula of the product generated by EG and adipic acid; relevance 3 is the structural formula of the product generated by diaminopentane and adipic acid; and relevance 4 is the structural formula of the product generated by PTA and EG.

Figure 4:

Relevance of carbon and hydrogen in polyesteramides. (A) Relevance 1; (B) relevance 2; (C) relevance 3; (D) relevance 4.

The 2D-NMR spectra of sample 1 of polyesteramides in series 1 and sample 4 of polyesteramides in series 2 are elaborated in Figure 5. Based on the triple-bond long-range coupling relationship denoted by ³J_C1H2 between H of δ=3.72 ppm in ¹H-NMR and C of δ=168.2 ppm in ¹³C-NMR, it can be determined that the chain segment of PTA is directly linked to that of diaminopentane, providing evidence of the reaction between them. The triple-bond long-range coupling relationship denoted by ³J_C3H4 between H of δ=4.79 ppm in ¹H-NMR and C of δ=177.6 ppm in ¹³C-NMR describes that chain segment of adipic acid is directly linked to that of EG. The triple-bond long-range coupling relationship denoted by ³J_C5H6 between H of δ=3.72 ppm in ¹H-NMR and C of δ=177.6 ppm in ¹³C-NMR defines that the chain segment of PA56 is among the reaction products. The triple-bond long-range coupling relationship denoted by ³J_C7H8 between H of δ=4.67 ppm in ¹H-NMR and C of δ=168.2 ppm in ¹³C-NMR, confirms that the chain segment of PET is also among the reaction products. Accordingly, it is certified that the above-mentioned four reactions succeed and that the bio-based DHS participated in the polymerization.

Figure 5:

HMBC spectra of sample 1 and 4. (A) Sample 1; (B) Sample 4.