Polyacrylonitrile degradation reaction

FIGURE 12.15 Differential scanning calorimetric scans of polyacrylonitrile homopol5mer in nitrogen atmosphere. Exothermic peak arises from the cyclic degradation reaction. 

If a blend of poly(methyl methacrylate) and polyacrylonitrile is thermally degraded, the behaviour of the poly (methyl methacrylate) is profoundly affected, but that of the polyacrylonitrile is unaltered. The methacrylate units react with ammonia arising from the polyacrylonitrile, and the amide-ester copolymer so formed undergoes complex degradation reactions, rather than the depolymerization to monomer which takes place with unchanged poly(methyl methacrylate). 

There is much evidence that weak links are present in the chains of most polymer species. These weak points may be at a terminal position and arise from the specific mechanism of chain termination or may be non-terminal and arise from a momentary aberration in the modus operandi of the polymerisation reaction. Because of these weak points it is found that polyethylene, polytetrafluoroethylene and poly(vinyl chloride), to take just three well-known examples, have a much lower resistance to thermal degradation than low molecular weight analogues. For similar reasons polyacrylonitrile and natural rubber may degrade whilst being dissolved in suitable solvents. 

Structural changes in the polymer, which will accompany the formation of small molecule products from the polymer, or may be produced by other reactions, can cause significant changes to the material properties. Development of colour, e.g. in polyacrylonitrile by ladder formation, and in poly(vinyl chloride) through conjugated unsaturation, is a common form of degradation. 

Poly(methyl methacrylate) depolymerizes at elevated temperature under the influence of ultraviolet light of 259.7 nm [549]. Irradiation of polyacrylonitrile, however, leads to chain scission at the acrylonitrile units. The difference between thermal and ultraviolet light degradation of polyacrylonitrile is principally in the different sites of initiation and the fact that the reaction occurs at 160°C in the presence of light and at 280°C in the darkness [550]. 

Although elimination reactions are clearly degradation, leading to color and char formation, they can have advantages. Conjugated sequences and ladder structures confer thermal stability on the polymers, forming stable chars. This is the basis of the preparation of carbon fibers by pyrolysis of polyacrylonitrile. It is also one reason for the use of plasticized PVC in electrical insulation in a... 

The degradation of polyacrylonitrile deuteriated in the alpha position has been followed by Fourier transform i.r. spectroscopy. The results are consistent with imine-enamine tautomerism followed by oxidation to give pyridone structures. A variety of thermo-analytical techniques has been employed to study thermal and thermo-oxidative reactions and it is concluded from these that a carboxylate moiety is responsible for initiating cyclization in an oxidizing atmosphere. The role of oxygen is said to be three-fold, creation of the carboxylate initiator sites, dehydrogenation, and crosslinking. 

Two investigations have been made on polyacrylonitrile in fibre form. The first reports on rates of formation of degradation products and the second on exothermic reactions and discolouration. The latter paper also reports the effect of copolymerization of acrylonitrile with 2-vinylpyridine upon these reactions. 

Ouyang, Q. Cheng, L. Wang, El. Li, K. Mechanism and kinetics of the stabilization reactions of itaconic acid-modified polyacrylonitrile. Polymer Degradation and Stability 2m, 93, 1415-1421. 

For copolymers of butadiene and acrylonitrile, cyclisation will be more facile due to the pendant nitrile group as in the case with polyacrylonitrile polymers [65,55]. This mechanism of decomposition suggests that the greater the alternating nature of the polymers, the greater will be the extent of cyclisation reactions. The cyclised polymers would undergo further thermal degradation to produce various products. 

Random chain scission results from homolytic scission of weak points of the chain. Consequently, this type of degradation produces substances of different composition and molecular weights corresponding to infra- and intermolecular radical transfer reactions. Degradation of polyolefins and of polyacrylonitrile (PAN) are typical examples of random chain scissions. 

---