# Influence of Monomer Sequence on the Cyclization Behavior of Poly(acrylonitrile-_co_-acrylamide)
###### Abstract
In this case, the cyclization reaction in gradient copolymer proceeds in the narrow temperature range with intensive heat release, which are undesirable for production of carbon fibers. Amide group in NIPA unit can probably accelerate the cyclization reaction, as the temperature of cyclization onset was lower than for pure PAN. The semibatch copolymerization leads to the formation of the copolymer with uniform distribution of NIPA along the backbone. The cyclization reaction proceeds in the wide temperature range and with low intensity of heat release, that is preferable for carbon fiber production. The authors relate this effect to the ability of amide moiety to accelerate the cyclization reaction. However, no detailed discussion of this phenomenon was given. The similar approach was implemented in [15; 16] for the copolymers of AN and acrylic acid (AA) produced by RAFT copolymerization. Acrylic acid is able to accelerate the cyclization of nitrile groups similarly to amide-containing AN copolymer [17; 18; 19]. The batch, continuous, and semi-continuous copolymerizations led to the formation of macromolecules with various drift of AA units along the backbone and to the drastic changes in the properties of the copolymers, i.e., their solubility, spinnability, and cyclization behavior [15; 16; 20; 21]. Acrylamide (AAM) is another monomer that can accelerate cyclization reaction. Its ability to accelerate cyclization of nitrile groups was described first in [22] on the example of the copolymers synthesized through copolymerization in water. The copolymers of AN and AAM can be obtained via a radical mechanism only resulting to the formation of atactic copolymer [8]. To eliminate the composition heterogeneity of the copolymers, the synthesis was stopped at low monomer conversions. The nature of impact of AAM on the cyclization was proposed in [23] basing on the study of FTIR spectra of the similar AN and AAM copolymers subjected to thermal treatment under reduced pressure and in air. Cyclization can be observed even at 130 \({}^{\circ}\)C after the induction period of 4 h, while at 160 \({}^{\circ}\)C the induction period is about 1 h. After the induction period is completed, the intensities of the absorption bands corresponding to CN, CO, and NH groups fall drastically, proving participation of AAM in cyclization. The proposed mechanism of cyclization initiated by AAM is rather complicated and involves several steps (Scheme 1) [22]. The influence of the copolymer composition on the cyclization reaction is rather interesting. Below 3 mol. % of AAM in the copolymer, cyclization proceeds slowly at both at 160 and 200 \({}^{\circ}\)C, while at AAM content above 5 mol. % the cyclization rate is higher and it does not depend on the copolymer composition indicating that the fraction of the available nitrile and amide groups that have reacted at the given time seems a constant value [24]. Besides, the extent of the coloration of the copolymer films after the same time of treatment is inversely proportional to the concentration of AAM in the copolymer, suggesting that the extent of conjugation decreases with the rise of AAM content in the copolymer. These observations have been achieved for the copolymers obtained by radical copolymerization in water with monomer reactivity ratios equal to r\({}_{\text{AN}}\) = 1.5 and r\({}_{\text{AAM}}\) = 0.3. In this case, the sequence length of AN units is 40 and 9 and AAM units 1 for the monomer feed with 2.5 and 10 mol. % of AAM. However, the simple calculations of the number of AN units that can participate in cyclization reaction based only on the monomer sequence distribution give overrated results. However, if the stereochemistry (iso-/sindiotactic placements) is taken into account, then the experimental and theoretic values become close.