## 4 Conclusions  
Various amounts of VDPE monomer were incorporated with styrene and DVB into the copolymer structure using the developed copolymerization strategy. Moreover, the content of DVB was changed giving different cross-linking degrees of the produced copolymer materials. Finally, they were converted into a catalytically active form by hydrolysis of diisopropyl ester functionalities and ring closure towards the formation of NHPI groups. A wide series of the synthesized materials allowed the analysis of the influence of the copolymer composition on its catalytic activity in the aerobic PMT oxidation. Obviously, the observed PMT conversions were correlated with the NHPI species concentration. It was however found that too high NHPI and DVB contents resulted in a clear decrease in the catalytic activity due to limited availability of N-OH functionalities embedded inside the copolymer structure. Furthermore, at the high NHPI loadings the formation of noticeable number of inactive (C=O)-N=O functionalities was confirmed by FT-IR and XPS. The presence of Co(II) cations in the reaction mixture was necessary for the functioning of NHPI centers anchored in the copolymer structure. At a properly selected Co(II)/NHPI molar ratio, the optimal concentration of PINO radicals was maintained, which activated C-H bonds in PMT through hydrogen atom transfer. The formed CH\({}_{2}\)\({}^{\bullet}\) radicals reacted with molecular O\({}_{2}\) giving corresponding hydroperoxide compound. The Co(II)/Co(III) redox system facilitated the formation of p-anisyl alcohol, p-anisaldehyde, and p-anisic acid, being the main products of the studied reaction. It was observed that along with the reaction time, the copolymer catalysts were deactivated gradually due to decomposition of NHPI groups. The FT-IR results revealed that the main cause of this negative effect was the opening of amide ring leading to the formation of carboxyl groups and hydroxylamine. The latter compound remained in the catalyst structure, especially with high cross-linking degree, additionally complex Co(II) ions, which was confirmed by XPS and UV-Vis-DR techniques. The spent copolymer catalyst could, however, be relatively easily regenerated by repeating the last two steps of NHPI group formation. Taking into account high catalytic activity of the developed copolymer catalysts compared to other systems described in the literature (Table 3), as well as the efficient regeneration procedure, they can be candidates for commercial use. A very significant advantage of their application is the lack of necessity to use a solvent, which reduces the costs of separation of the reaction mixture. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Catalyst** & \begin{tabular}{c} **Reaction** \\ **Conditions** \\ \end{tabular} & \
\begin{tabular}{c} **Substrate to** \\ **Oxidation** \\ \end{tabular} &
\begin{tabular}{c} **Conversion** \\ **[\%]** \\ \end{tabular} & **Ref.** \\ \hline NHPI\({}_{30}\)-co-S\({}_{63}\)-co-DVB\({}_{7}\) & 80 \({}^{\circ}\)C, 1 atm, 6 h, no solvent & p-methoxytoluene (15.8 mmol) & 23.6 & This work \\ \hline NHPI immobilized onto polystyrene resins by amide bond (100 mg) & 80 \({}^{\circ}\)C, 1 atm, 6 h, AcOH (10 mL) & p-methoxytoluene (5 mmol) & 8.3 & [19] \\ \hline NHPI immobilized onto polystyrene resins by ester bond (100 mg) & 80 \({}^{\circ}\)C, 1 atm, 6 h, AcOH (10 mL) & p-methoxytoluene (5 mmol) & 6.7 & [19] \\ \hline NHPI immobilized onto polyacrylamide methacrylate (GMA) and methyl methacrylate (MMA) copolymers (1 g) & 100 \({}^{\circ}\)C, 1 atm, 25 h, AcOH (30 mL) & ethylbenzene (8 mmol) & 20.1 & [65] \\ \hline _N,N_-dihydroxypropylmellitimide (NDHPI) immobilized on SBA-15 (2 mol\%) & 90 \({}^{\circ}\)C, 1.6 MPa, 7 h, MeCN (5 mL) & toluene (55 mmol) & 14.8 & [38] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of activity in the aerobic oxidation of aromatic compounds for selected catalysts described in the literature and the NHPI-containing copolymer studied in this work.