Depolymerization degradation

Clarson, S. J. Depolymerization, Degradation and Thermal Properties of Siloxane Polymers. In Siloxane Polymers, Clarson, S. J., Semiyen, J. A., Eds. Prentice Hall, 1993 pp 21 244. 

Nitrocellulose with a decreased molecular weight may be obtained as the result of depolymerization (degradation) of the cellulose before nitration, e.g. by keeping it at a temperature of 150-170°C or by treating it with acids. The resultant hydrocellulose, which usually possesses a lower molecular weight than cellulose, is then subjected to nitration to produce a more soluble substance as compared with the nitration product of a non-depolymerized raw material. 

One area of Robert Simha s activity that is missing here is work on the kinetics and statistics of chemical reactions such as polymerization, copolymerization, depolymerization, degradation, and sequencing of biomacromolecules (e.g., proteins, polynucleotides, DNA). The decision to omit this topic was based, on the one hand, on its chemical character, and on the other, on the vastness of these topics, which would essentially require an additional volume. 

Instantaneous initiation, polymerization and depolymerization, degradative transfer to monomer 133... 

Chemical degradation (141), whether thermally or photo-iaduced, primarily results from depolymerization, oxidations, and hydrolysis. These reactions are especially harmful ia objects made from materials that coataia ceUulose, such as wood, cottoa, and paper. The chemistry of these degradation processes is quite complex, and an important role can be played by the reaction products, such as the acidic oxidation products which can catalyze hydrolysis. 

A second degradation process is oxidation, often photo-induced especially by exposure to light not filtered for uv. The radicals resulting from this reaction promote depolymerization of the cellulose, as well as yellowing and fa ding of paper and media. Aging causes paper to become more crystalline and fragile, and this can be exacerbated particularly if the paper is subjected to poor conditions. 

Deterioration. The causes of degradation phenomena in textiles (155—158, 164) are many and include pollution, bleaches, acids, alkaUes, and, of course, wear. The single most important effect, however, is that of photodegradation. Both ceUulosic and proteinaceous fibers are highly photosensitive. The natural sensitivity of the fibers are enhanced by impurities, remainders of finishing processes, and mordants for dyes. Depolymerization and oxidation lead to decreased fiber strength and to embrittlement. 

The thermal stability of polymers of types (1) and (2) is also dependent on the nature of the substituents on phosphoms. Polymers with methoxy and ethoxy substituents undergo skeletal changes and degradation above about 100°C, but aryloxy and fluoroalkoxy substituents provide higher thermal stability (4). Most of the P—N- and P—O-substituted polymers either depolymerize via ring-chain equilibration or undergo cross-linking reactions at temperatures much above 150—175°C. 

This reaction also plays a role in the degradation of polysulftdes. A back-biting mechanism as shown in equation 6 results in formation of the cycHc disulfide (5). Steam distillation of polysulftdes results in continuous gradual collection of (5). There is an equiUbrium between the linear polysulftde polymer and the cycHc disulfide. Although the linear polymer is favored and only small amounts of the cycHc compound are normally present, conditions such as steam distillation, which remove (5), drive the equiUbrium process toward depolymerization. 

The stmctural architecture of siUcone polymers, such as the number of D, T, and Q sites and the number and type of cross-link sites, can be deterrnined by a degradative analysis technique in which the polymer is allowed to react with a large excess of a capping agent, such as hexamethyidisiloxane, in the presence of a suitable equiUbration catalyst (eq. 38). Triflic acid is often used as a catalyst because it promotes the depolymerization process at ambient temperature (444). A related process employs the KOH- or KOC2H -catalyzed reaction of siUcones with excess Si(OC2H )4 (eq. 39) to produce ethoxylated methylsiUcon species, which are quantitatively deterrnined by gc (445). 

Silicone networks that form the matrix of the adhesives are not susceptible to degrade or to depolymerize when exposed to a wide range of conditions of temperature and relative humidity. Therefore, the cohesive strength will not change, as... 

In transient elongational flow degradation, it was determined in the authors laboratory, by a detailed mass balance, that main chain scission accounted for >95% of the degradation in dilute solution. Any other type of depolymerization, if present, should then be of minor importance. 

However, Pacansky and his coworkers77 studied the degradation of poly(2-methyl-l-pentene sulfone) by electron beams and from infrared studies of the products suggest another mechanism. They claim that S02 was exclusively produced at low doses with no concomitant formation of the olefin. The residual polymer was considered to be essentially pure poly(2-methyl-l-pentene) and this polyolefin underwent depolymerization after further irradiation. However, the high yield of S02 requires the assumption of a chain reaction and it is difficult to think of a chain reaction which will form S02 and no olefin. 

Plutonium(IV) polymer is a product of Pu(IV) hydrolysis and is formed in aqueous solutions at low acid concentrations. Depolymerization generally is accomplished by acid reaction to form ionic Pu(IV), but acid degradation of polymer is strongly dependent on the age of the polymer and the conditions under which the polymer was formed (12). Photoenhancement of Pu(IV) depolymerization was first observed with a freshly prepared polymer material in 0.5 HClOh, Fig. 3 (3 ). Depolymerization proceeded in dark conditions until after 140 h, 18% of the polymer remained. Four rather mild 1-h illuminations of identical samples at 5, 25, 52, and 76 h enhanced the depolymerization rates so that only 1% polymer remained after the fourth light exposure (Fig. 3). 

The effects of four 1 h, low intensity, UV light exposures to enhance the degradation of freshly prepared Pu(IV) polymer. The darkened line represents depolymerization under dark conditions. The solution contained 0.0093 M total Pu and 0.47 11 HClOi at 22°C (3). 

The effects of UV irradiation to enhance the degradation of aged Pu(IV) polymer in HClOi. Depolymerization under dark conditions for each experiment is shown by a data point directly above the last light sample point (4). 

Diol-functionalized telechelic polymers have been desired for the synthesis of polyurethanes however, utilizing alcohol-functionalized a-olefins degrades both 14 and 23. Consequently, in order for alcohols to be useful in metathesis depolymerization, the functionality must be protected and the oxygen atom must not be /3 to the olefin or only cyclic species will be formed. Protection is accomplished using a/-butyldimcthylsiloxy group, and once protected, successful depolymerization to telechelics occurs readily. 

CL must be very carefully purified to exclude small concentrations of (1) ferric ions which would catalyze die thermal oxidative degradation of polycaprolactam and (2) aldehydes and ketones which would markedly increase oxidizability of CL. The impurities in CL may retard die rate of CL polymerization as well as having a harmful effect on die properties of die polymer fiber. In die vacuum depolymerization of nylon-6, a catalyst must be used because in die absence of a catalyst by-products such as cyclic olefins and nitrides may form, which affects the quality of die CL obtained.1... 

Acids such as sulfuric or nitric acids or bases such as sodium hydroxide may catalyze the hydrolysis of PET. It has been demonstrated that the rate of alkaline PET hydrolysis increases in the presence of quaternary ammonium compounds.26 27 Niu et al.26 reported an increase in the rate of alkaline PET degradation in the presence of dodecylbenzyldimethylammonium chloride at 80°C. Polk et al.27 reported increases in the rate of sodium hydroxide depolymerization of PET in the presence of trioctylmethylammonium chloride, trioctyl-methylammonium bromide, and hexadecyltrimethylammonium bromide at 80° C. 

The mass spectrum of polymeric sulfur S, prepared from either liquid sulfur or by extraction of commercial flowers of sulfur , has been measured and interpreted in terms of Ss, Sy, and Ss molecules leaving the polymer on heating and depolymerization [203]. This result is in agreement with depolymerization studies in solution which also show Ss and Sy as the major thermal degradation products [174]. 

Polymer Degradation and Stability 75,No.l,2002,p.l85-91 STUDY ON METHANOLYTIC DEPOLYMERIZATION OF PET WITH SUPERCRITICAL METHANOL FOR CHEMICAL RECYCLING... 

Because very rapid depolymerization occurred at higher temperatures, it was necessary to control the temperature within the narrow range of 50 10°C. Even so, the of the polymer was no greater than 15,000 because of rapid degradation by the living cationic end group. 

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