Thermal Degradation of Methacrylic and Acrylic Polymers

The thermal degradation of polymers of acrylic and methacrylic alkyl esters is a process of depo lymerization to monomers at temperatures up to 250C, provided that the alkyl group is small, less than butyl [468]. Poly (t-butyl methacrylate) yields quantitatively isobutene instead. It was shown that thermal depolymerization to monomers is probably common to all poly (methacrylate ester) s. As the size of the alkyl group increases, however, particularly within secondary or tertiary structures, there is increased tendency for the alkyl group to also decompose. This decomposition interferes with the depolymerization process Thermal stability of poly (methyl methacrylate) appears to vary with the molecular weight. For instance, a sample of the polymer of molecular weight of 150,000 when heated in vacuum for 30 min at 318C yields 74.3% volatiles. By comparison, a sample of this polymer of molecular weight of 5,100,000 when heated for 30 min at 319C yielded only 35.2% volatiles [456]. The thermal stability of copolymers of long-chained diol dimethacrylates was investigated [583]. These copolymers included 1,4-butane-, 1,5-pentane-, 1,6-hexane-, 1,8-octane-, 1,10-decane-, or 1,12-dodecanediol dimethacrylates, respectively, as well as 2,2-bis[4-(2-hydroxy-3-methacryloyl oxypropoxy)-phenyl] propane and triethylene glycol dimethacrylate. The polymers were found to be thermally stable up to 250C, as shown by the initial decomposition temperature and their degradation profiles. A quantitative investigation of the thermal degradation of poly (ethyl acrylate), poly (n-propyl acrylate), poly (isopropyl acrylate), poly (n-butyl acrylate), and poly (2-ethylhexyl acrylate) demonstrated that the principle volatile products are carbon dioxide, olefin, and alcohol corresponding to the alkyl group [469, 470]. The following mechanism of degradation was proposed [469, 470]:

More recently, another study was carried out on the thermal decomposition of homopolymers of ethyl methacrylate, n-butyl methacrylate, and 2-hydroxyethyl methacrylate as well as their copolymers [471]. The copolymers of hydroxyethyl methacrylate with ethyl methacrylate and butyl methacrylate were found to degrade by unzipping to yield the monomers similarly to poly(methyl methacrylate). In addition, there is competition between unzipping and cross-linking in binary copolymers of hydroxyethyl methacrylate with ethyl methacrylate and in n-butyl methacrylate. Thermal degradation of nitrile polymers, particularly acrylonitrile, was studied in detail [472–478]. It was shown that there can be two paths of degradation, depending upon the temperature (see also Chap. 8). One reaction takes place at a low temperature, between 100 and 200C, and the other one occurs above 240C. At the low temperature, the polymers develop dark color. There is, however, very little evolution of volatiles. At the high temperature, on the other hand, there are evolutions of volatiles and thermally stable residues develop. The low temperature darkening process of polyacrylonitrile was shown to be intramolecular cyclization and polymerization of the cyanide groups [475, 476]. The overall reaction can be illustrated as follows:

The above shown ladder structure is actually a vary idealized picture. Nevertheless, the formation of fused tetrahydropyridine rings was demonstrated by IR data [475, 476]. At the same time, there are many irregularities in the above shown structure. Also, it was demonstrated that the longer are the isotactic sequences in the polymeric structures, the longer are the sequences of ladder structures that form [479]. High temperature degradation of polyacrylonitrile leads to formation of oligomers. The general form of the oligomerized material can be shown as follows [480]:

where, x = 0–2 and n = 0–5. The above structure occurs as a result of an initiation and termination process of cyclization at frequent intervals along the chain. The growth of the ladder structure, however, terminates due to hydrogen transfer. This process is associated with the atactic sequences in the polymer chain:

Polymethacrylonitirle develops color upon heating as a result of linking up of adjacent carbons and nitrogen atoms in intramolecular cyclization reactions similar to acrylonitrile. This reaction is initiated primarily by impurities that are often present in the polymer both at the end of the chains and at various locations at the backbones. Due to this ring formation, the amount of monomer that can be obtained from the polymer at 200C is approximately 50%. If, however, the polymer is prepared from highly purified monomer, the yield of monomer upon thermal degradation at 300C is 100%. Also, a yellow color does not develop from such a polymer at temperatures of 120–220C [457]. Poly (vinylidine cyanide) has the structure that would suggest formation of a double ladder polymer upon pyrolysis. It was shown, however, in an early study that this does not happen. Instead, the polymer mostly depolymerizes [481, 482].

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مواضيع ذات صلة

Degradation of Polymeric Materials by Ionizing Radiation

Oxidation of Chain-Growth Polymers

Oxidative Degradation of Polymers

Thermal Degradation of Cellulosic Materials

Thermal Degradation of Polyimides, Polyoxidiazoles, and Polyquinoxalines

Thermal Degradation of Aromatic Polysulfones

Thermal Degradation of Epoxy Resins

Thermal Degradation of Polyamides

Thermal Degradation of Polyesters

Thermal Degradation of Poly (Vinyl Acetate)

Thermal Degradation of Chlorocarbon and Fluorocarbon Polymers

Thermal Degradation of Polyolefins and of Polymers from Conjugated Dienes