Chain depolymerization

Paraformaldehyde [30525-89-4] is a mixture of polyoxymethylene glycols, H0(CH20) H, with n from 8 to as much as 100. It is commercially available as a powder (95%) and as flake (91%). The remainder is a mixture of water and methanol. Paraformaldehyde is an unstable polymer that easily regenerates formaldehyde in solution. Under alkaline conditions, the chains depolymerize from the ends, whereas in acid solution the chains are randomly cleaved (17). Paraformaldehyde is often used when the presence of a large amount of water should be avoided as in the preparation of alkylated amino resins for coatings. Formaldehyde may also exist in the form of the cycHc trimer trioxane [110-88-3]. This is a fairly stable compound that does not easily release formaldehyde, hence it is not used as a source of formaldehyde for making amino resins. 

Rhombic sulfur is a brittle, crystalline solid at room temperature. Heating to 113 °C causes it to melt to a reddish-yellow liquid of relatively low viscosity. Above approximately 160 °C, the viscosity increases dramatically because of the free-radical polymerization of the cyclic molecules into long, linear chains.6,8 14 30 47-51 At this point, a degree of polymerization of approximately 105 is obtained. If the temperature is increased to above approximately 175 °C, depolymerization occurs, as evidenced by a decreasing viscosity. A similar type of depolymerization occurs with the polysiloxanes discussed in Chapter 4. In thermodynamic terms, the negative -TAS term overcomes the positive AH term for chain depolymerization. (The temperature at which the two terms are just equal to one another is called the ceiling temperature for the polymerization.)... 

Thermogravimetric analysis is another way to determine whether PEtG is well end-capped or not. Indeed, a PEtG which is not end-capped or not properly end-capped will not be stable even at 50°C since chain depolymerization could occur, while end-capped PEtG is stable up to 200°C. 

It seems that the method of preparation of a polymer has significant influence on its mode of degradation. Thus, a sample of poly(methyl methacrylate), prepared by a free radical reaction, suffers rapid depolymerization at about 275 "C and a second mode of initiation of chain depolymerization between 350 and 400 °C, while the sample, prepared by anionic polymerization, undergoes depolymerization as a whole above 350 °C. Ab-... 

In degree of polymerization (DP) studies of borax treatments and ammonium dihydrogen orthophosphate (53), cellulose treated with the acid charred and depolymerized very rapidly. Its DP value decreased from 1110 to 650 after only 2 min of heating at 150 °G. Gellulose treated with borax showed a DP reduction from 1300 to 700 after 1 h of heat treatment at 150 °G. Both these compounds catalyzed the suppression of levoglucosan formation but they had different effects on the chain depolymerization reaction (53). 

Chains depolymerize up to the end and the instantaneous degree of polymerization is equal to the initial degree of polymerization x0. Also it can be shown [2] that... 

Early work on polyethylene and polypropylene has been reviewed by Madorsky[38] and Winslow and Hawkins [39]. Initiation by random scission or at weak links has been proposed. Since little monomer is evolved, chain depolymerization seems to be of minimal importance. The low molecular weight products formed are the result of inter- or intramolecular free radical transfer. 

In this polymer, every alternate carbon of the chain is quaternary and no reactive hydrogen atoms are present. Formation of radicals by scission of the main chain results in monomer formation by a chain depolymerization [51, 52, 54, 55], but the polymer is not quantitatively converted into monomer as happens with polymethylmethacrylate. A range of products from C4 to C2 0 is also evolved [52, 53]. 

The degradation behaviour of polymethylmethacrylate is easily characterized by thermal volatilization analysis [87] (Fig. 29). Monomer is obtained in very high yield in all cases. A polymer sample prepared by a free radical reaction undergoes a rapid depolymerization at about 275°C as indicated by the first peak. The second peak, situated between 350 and 400°C, corresponds to a second mode of initiation of chain depolymerization. For samples prepared by anionic polymerization, the first peak is not observed. Depolymerization of the whole sample occurs above 350°C. 

The thermal degradation of polymethylmethacrylate was investigated many years ago by isothermal methods [88, 89]. The two mechanisms of chain depolymerization were already identified at that time and analysed. At temperatures below 270°C, the reaction is initiated at the double bonds situated at chain ends and formed by radical disproportionation... 

Although monomeric alcohols can be acetylated with acetic acid using a mineral acid catalyst, acetic anhydride is needed to speed the acetylation of cellulose to reduce a competing depolymerization reaction. There will be some chain depolymerization, but it can be controlled so that the end products will have adequate average chain length. 

The actual degradation mechanism is more complex than implied by this simple reaction. If substituents reactions occur, they generally ensue at temperatures (J < 150 °C) below that of degradation reactions in which the backbone bonds are broken. Consequently, the reactivity of the substituents relative to that of the polymer backbone will largely dictate whether a particular polymer undergoes thermal degradation by substituent reactions or by reactions involving the backbone (e.g., chain depolymerization and random scission) [1-11]. 

PTFE is a highly crystalline polymer that is devoid of crosslinks and branching. PTFE undergoes nearly 100% conversion to monomer at elevated temperatures. Thermal degradation by chain depolymerization at the chain ends probably starts at low temperatures (250-350 °C), while random-scission cleavage likely becomes more pronounced at higher temperatures. Although PTFE is the most stable of... 

Polymethyl methacrylate particles of colloidal dimensions are rather unstable under an electron beam in high vacuum, presumably because of free radical chain depolymerization. Shadowing is thus required. However they even appear to degrade under shadowing by metals such as Pt-Pd alloy, probably as a result of the heat released upon condensation of the metal vapor. Thus electron micrographs of PMMA particles less than 0.30 ym diameter have not been published. This is partly due to background noise due to support films or metal shadowing of various kinds which otherwise would be useful for small PMMA particles. 

The monomer formed during UV irradiation is mainly due to chain depolymerization after photolytic scission of the main chain ... 

Random scission can be thought of as the converse of stepwise polymerization. Chain rupture occurs at random points along the chain leaving fragments which are large compared to the monomer units. Chain depolymerization can be considered the reverse of chain polymerization and involves successive release of monomer units from a chain end in a depropagation reaction. 

Chain reaction (depropagation, unzipping, unzippering, chain depolymerization). Stepwise reaction. 

In the first treatment of a chain depolymerization (28,29), a set of differential equations was derived to describe a general-chain mechanism. This treatment is based on initiation, depropagation, transfer, and termination reactions. In a similar treatment (30), a different method was used for solving the equations. Both methods apply the stationary-state hypothesis to radical... 

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