## 2 Results and Discussion
### Stability of Copolymer Catalysts during Aerobic Oxidation of PMT  
Stability of copolymer catalysts in the aerobic PMT oxidation was analyzed using 30% NHPI_5% DVB material. The catalytic test was interrupted after 1, 2, or 6 h, respectively, and then the catalyst was separated from the reaction mixture, washed with a solvent and dried. During the studied period, the PMT conversion increased from 6.1% (after 1 h) by 13.2% (2 h) finally to 18.5% (6 h). With the increase in the PMT conversion, a decrease in the N content in the catalyst was observed. The fresh 30%NHPI_5%DVB catalyst contained 4.0 wt% of nitrogen corresponding to 2.84 mmol of N-OH groups in 1 g of the catalyst. After 1 h of the reaction the number of N-OH groups dropped to 2.29 mmol/g (3.2 wt% of N). The prolonged reaction time resulted in a further decrease in the N-OH amount to 1.79 mmol/g (2.5 wt% of N) and 0.93 mmol/g (1.3 wt% of N), after 2 and 6 h of the catalytic test, respectively. The changes in the catalyst structure were studied by FT-IR (Figure 7). After the catalytic reaction, a new intense IR band appears at 1675 cm\({}^{-1}\) corresponding to the vibrations  
Figure 6: Influence of Co(OAc)\({}_{2}\)/NHPI molar ratio on conversion of PMT achieved in the presence of 30%NHPI_5%DVB after 6h at 80 \({}^{\circ}\)C (0.1 g copolymer catalyst, 5.0 mg AIBN, 2.0–6.5 mg Co(OAc)\({}_{2}\)-4H\({}_{2}\)O, 2.0 mL PMT, 1 atm O\({}_{2}\)). of carboxylic groups that may be derived from p-anisic acid (one of the PMT oxidation products) deposited on the catalyst surface. On the other hand, NHPI decomposition may occur during the reaction according to the mechanism proposed by [PERSON] et al. [56], who described the opening of imide ring leading to the formation of carboxyl groups and free hydroxylamine. For a proper diagnosis of this effect, the catalyst after 6 h of the reaction was rinsed copiously with MeCN, which effectively removed p-anisic acid. The FT-IR spectrum of 30%NHPI_5%DVB_6h_MeCN reveals an IR band at 1686 cm\({}^{-1}\) confirming the presence of carboxyl groups formed by the NHPI decomposition. Furthermore, the FT-IR spectra of the 30%NHPI_5%DVB catalyst after 6 h of the catalytic test show bands at 3444 cm\({}^{-1}\) and 3393 cm\({}^{-1}\) attributed to asymmetric and symmetrical NH stretching vibrations, as well as at 1633 cm\({}^{-1}\) assigned to the deformation vibrations of primary NH\({}_{2}\) groups present in released NH\({}_{2}\)OH molecules deposited inside the copolymer structure. Additionally, UV-VIS measurements were performed for the fresh and used 30%NHPI_5%DVB copolymer material (Figure 8). Figure 8: UV-Vis spectra collected for the 30%NHPI_5%DVB copolymer—fresh and after various times (1, 2, and 6 h) of the aerobic PMT oxidation at 80 \({}^{\circ}\)C (0.1 g copolymer catalyst, 5.0 mg AIBN, 5.0 mg Co(OAc)\({}_{2}\)-4H\({}_{2}\)O, 2.0 mL PMT, 1 atm O\({}_{2}\)). Figure 7: FT–IR spectra collected for the 30%NHPI_5%DVB copolymer—fresh and after various times (1, 2, and 6 h) of the aerobic PMT oxidation at 80 \({}^{\circ}\)C (0.1 g copolymer catalyst, 5.0 mg AIBN, 5.0 mg Co(OAc)\({}_{2}\)-4H\({}_{2}\)O, 2.0 mL PMT, 1 atm O\({}_{2}\)), as well as 30%NHPI_5%DVB after 6 h of the catalytic reaction, separated, and washed with MeCN. In the shaded areas, the new bands are indicated. The UV-Vis spectra of the spent catalysts exhibit an increase in intensity of absorption bands in the wavelength range of 400 to 600 nm compared to the fresh sample. The observed effect can be attributed to the electronic transitions characteristic of Co(II) and Co(III) ion complexes with carboxyl and amine ligands, such as Co(II) with COO\({}^{-}\) which presents an absorption band at 415 nm [57]. The previously published papers reported absorption bands at 520 nm for Co(II) complexes with NH\({}_{2}\) amine ligands [58], at 550 nm for Co(III) with ethylenediamine ligands [59] and at 470 nm for Co(III) in Co(NH\({}_{3}\))\({}_{6}\)\({}^{3+}\) complex [60].