# Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-_co_-acrylamide)
## 3 Results and Discussion
### Thermal Treatment of the Copolymers in the Inert Atmosphere: Cyclization Reaction
The following bands are observed: stretching vibrations of CN groups at 2244 cm\({}^{-1}\), stretching vibrations of CH groups at 2990-2920 cm\({}^{-1}\), bending vibrations of CH\({}_{2}\) groups are at 1453 and 1360 cm\({}^{-1}\), mixed vibrations at 1257 and 1073 cm\({}^{-1}\), stretching vibrations of C=O at 1685 and 1613 cm\({}^{-1}\), and N-H stretching vibrations at 3462 and 3357 cm\({}^{-1}\)[23]. Upon heating at 200 \({}^{\circ}\)C, the chemical structure of the copolymers changes and the new bands appear in the spectra and/or the intensity of the existing bands changes (curves 2-6). The reduction of nitrile stretching absorbance at 2244 cm\({}^{-1}\) and carbonyl stretching absorbance at 1685 cm\({}^{-1}\) is observed as compared with intensity of vibrations of CH and CH\({}_{2}\) groups (2990-2920, 1453 cm\({}^{-1}\)). The similar trend is observed for N-H stretching vibrations at 3462 and 3357 cm\({}^{-1}\). A new low intense band appears at 1745 cm\({}^{-1}\) and its intensity decreases upon further heating. The bands at 3190, 1581, 1540, 1370, 1247, 1220, 1150, 795, 740, and 660 cm\({}^{-1}\) appear and their intensity increases with the rise of duration of heating. These bands, except bands at 1745 and 1220 cm\({}^{-1}\), belong to a pyridine-type structure formed in pure PAN. The absence of the absorbance at 1700 cm\({}^{-1}\) indicates the existence of amide-iminol tautomerism. It is important to note that the band at 2244 cm\({}^{-1}\) resembles its shape and no traces of the band 2200 cm\({}^{-1}\) are observed; the latter corresponds to the nitrile group conjugated to the olefinic group. The absence of the bands at 810 and 960 cm\({}^{-1}\) referred to CH bending vibrations in olefins along with the absence of the band at 2200 cm\({}^{-1}\) confirms that no dehydration takes place. Similar trends are observed for other studied copolymers and for all the samples upon the heating at 250 \({}^{\circ}\)C (Figures S5 and S6, ESI). All these changes are typical for cyclization reaction of AN and AAM copolymers [22; 23]. Thus, the copolymer composition and copolymer microstructure have no impact on the chemical changes of the copolymers during cyclization. However, the rate of formation of ladder structure depends on the factors mentioned above (Figure 8). The addition of AAM in the PAN structure results in noticeable acceleration of cyclization reaction. The conversion of nitrile groups increases with the rise of content of AAM in the copolymer (Figure 8). However, if we compare copolymers of various microstructures, we can see that conversion of nitrile groups decreases in the following order of copolymers: random \(>\) gradient \(>\) block-gradient. Another way to the quantitative analysis of the kinetics of the cyclization reaction is the analysis of the dependence of the stabilization  
Figure 8: Dependence of the proportion of the unreacted nitrile groups \(\phi_{\rm CN}\) on the time of heat treatment at 200 \({}^{\circ}\)C for: (**a**) PAN, random copolymer 2 (F\({}_{\rm AAM}\) = 7.8 mol. %) (2), gradient copolymer 6 (F\({}_{\rm AAM}\) = 9.2 mol. %) (6), and block-gradient copolymer 7 (F\({}_{\rm AAM}\) = 6.0 mol. %) (7); (**b**) random copolymer 1 (F\({}_{\rm AAM}\) = 3.4 mol. %) (1), random copolymer 2 (F\({}_{\rm AAM}\) = 8.8 mol. %) (2), and gradient copolymer 4 (F\({}_{\rm AAM}\) = 10.2 mol. %) (4). index \(\mathrm{E_{s}}\) on the time of heat treatment, which gives information about development of the ladder structure. Figure 9 presents corresponding dependences for copolymers obtained at 200 (a) and 250 \({}^{\circ}\)C (b). The higher the \(\mathrm{E_{s}}\) value, the more developed the ladder structure. At 200 \({}^{\circ}\)C both the copolymer composition and microstructure affect the \(\mathrm{E_{s}}\) (Figure 9a). The dependences have S-shape. The rise of AAM content in the copolymer leads to the growth of \(\mathrm{E_{s}}\) value. A similar trend is observed: the \(\mathrm{E_{s}}\) value decreases from block-gradient to gradient to random copolymer.