# 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 copolymer films were studied by DSC in the dynamic conditions under argon atmosphere. Figure 6a presents DSC thermograms of the copolymers at the heating rate of 10 K\(\cdot\)min\({}^{-1}\); the DSC curve of PAN is given for a comparison. Introduction of AAM affects the mechanism and the rate of the cyclization reaction. For random copolymers, at AAM content in the copolymer below \(\sim\)9 mol. % both radical and ionic mechanisms of cyclization are observed (curves 2 and 3). The low-temperature peak on the DSC thermogram is responsible for ionic mechanism of cyclization, while high-temperature peak is responsible for radical mechanism of cyclization. The ionic mechanism becomes predominant with further increase in the AAM molar part in the copolymer (curve 4). The increase in the AAM content in the copolymer leads to the rise of the heat flow corresponding to the ionic cyclization and to suppress the heat flow corresponding to the radical mechanism (Table 4). The chain unit distribution also affects the ratio of the radical and ionic mechanisms (curves 5-7). For block-gradient copolymer (curve 7), the central block of gradient copolymer is attached to the terminal blocks of PAN, resulting in the separate manifest of ionic and radical mechanisms. In gradient copolymers (samples 5 and 6), the AAM content increases from the center of the macromolecule to its tails. In this case, the radical cyclization is suppressed practically. The DSC data are summarized in Table 4.  
Figure 5: Schematic presentation of the random (**a**), gradient (**b**), and block-gradient (**c**) copolymers of AN and AAM obtained in the presence of BTC. rate (20 K\(\cdot\)min\({}^{-1}\)), which render it impossible to determine the activation energy of the radical cyclization and allow the estimation of ionic cyclization only. The corresponding thermograms for other copolymers are presented in ESI (Figure S4). To calculate the activation energy, the dependence of \(\ln(\phi/\mathrm{T_{p}}^{2})\) on \(\mathrm{T_{p}}^{-1}\) was plotted (Figure 6c). According to Equation (1), the activation energy of ionic cyclization was determined from the slope of the linear dependence. As is seen from Table 4, the activation energy of radical cyclization (determined for pure PAN) is about 20 kJ\(\cdot\)mol\({}^{-1}\) higher than that of ionic cyclization initiated by the amide group. The calculated value of ionic cyclization (89 \(\pm\) 3 kJ\(\cdot\)mol\({}^{-1}\)) is close to what was determined before for AN-AAM--alkyl acrylate systems [33]. In contrast to previous reports [48], the activation energy of ionic cyclization is not influenced by AAM content in the copolymer; also, it is not influenced by the monomer sequence distribution. It may be supposed that for the investigated systems the development of the conjugation starts after achieving a certain number of the cycles formed through the reaction of amide and nitrile groups (Scheme 1). The analysis of the chemical changes that occur during cyclization reaction may lead to a deeper understanding of the formation of the ladder structure in AN and AAM copolymers depending on their microstructure. Figure 7 shows the FTIR spectra of random (a), gradient (b), and block-gradient (c) copolymers registered after isothermal heating of the samples at 200 \({}^{\circ}\)C at various times. Figure 7: FTIR spectra of AN and AAM copolymers subjected to thermal treatment in Ar atmosphere at 200 \({}^{\circ}\)C: (**a**) random copolymer 2 (\(\mathrm{F_{AAM}}\) = 7.8 mol. %); (**b**) gradient copolymer 6 (\(\mathrm{F_{AAM}}\) = 9.2 mol. %); and (**c**) block-gradient copolymer 7 (\(\mathrm{F_{AAM}}\) = 6.0 mol. %). Time of thermal treatment is indicated in the figures. In the spectra of non-treated copolymers (curves 1), the characteristic absorbance bands of AN and AAM units are observed.