Depolymerization, free radical

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... 

A probable pathway for the degradation of hemicelluloses via free-radical intermediates has been proposed by Fengel and Wegener (1989) and is shown in Figure 5.1. Hemicellulose polymers are depolymerized to form oligosaccharides and monosaccharides, which are dehydrated to form furfural (pentoses) and hydroxymethyl furfural (hexoses). 

S " ") and depolymerization (S " " - Sg + Sy -t- Sg 4-. ..). This free radical mechanism has often been discussed on the basis of the observed first order rate equations found for the formation of it-sulfur and the decomposition of organic polysulfides as well as on the basis of the experimentally determined apparent activation energies (50-150 kJ/mol ). However, this type of mechanism seems rather unlikely at least at moderate temperatures sinc (a) no free radicals have been detected in liquid sulfur below 170 (b) only the... 

There is evidence that both ionic and free radical species are involved in the degradation and depolymerization of poly (olefin sulfone) s by high energy radiation (70). Thus, the yields of olefins from poly (1-butene sulfone) at 30 °C (the sample was heated to 70 °C during removal of the gaseous products) are shown in Table II. The butene is not solely 1-butene, but comprises significant proportions of all three isomers, 1-butene, 2-butene and isobutene. 

The formation of the coal extract was interpreted as a free-radical chain reaction leading to the depolymerization of coal and aromatization of some of the hydroaromatic structures. It was suggested that phenanthrene possibly plays the role of a chain carrier in this process. 

Ulanski P, Merenyi G, Lind J, Wagner R, von Sonntag C (1999) The reaction of methyl radicals with hydrogen peroxide. J Chem Soc Perkin Trans 2 673-676 Ulanski P, Bothe E, Hildenbrand K, von Sonntag C (2000) Free-radical-induced chain breakage and depolymerization of polyfmethacrylic acid). Equilibrium polymerization in aqueous solution at room temperature. Chem Eur J 6 3922-3934... 

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.)... 

The HA polymer can be degraded nonenzymatically by a free radical mechanism,226 particularly in the presence of reducing agents such as thiols, ascorbic acid, ferrous, or cuprous ions. This mechanism of depolymerization requires the participation of molecular oxygen. The use... 

This equation permits the calculation of equilibrium constants for polymerization-depolymerization from copolymer composition data extrapolated to zero Mi feed. The agreement between equilibrium constants calculated in this manner from free radical copolymerizations and those obtained from anionic homopolymerizations is shown in Table II, and again emphasizes the thermodynamic character of this work. 

Especially important are the radical-redox reactions by which transition metals promote free radical generation by catalyzing the decomposition of the peroxides formed during bleaching. This faster free radical generation results in higher oxidation rates and a more extensive depolymerization of the cellulose. 

End-chain scission the polymer is broken up from the end groups successively yielding the corresponding monomers. When this polymer degrades by depolymerization, the molecules undergo scission to produce unsaturated small molecules (monomers) and another terminal free radicals. (Polymethylmethacrylate, polytetrafluorethylene, polymethacrylonitrile, polyethylstyrene, polystyrene, polyisobutene)... 

The observed pyrolysis product distribution of PP is developed by a free radical mechanism [28, 29] which begins with the homolytic breakage of the polymer chain drawn in Scheme 12.3 where X represent a methyl group in this case. Primary macroradical 1 is formed only in the initial step, thus its decomposition plays a minor role in PP. Secondary macroradicals may decompose to propene by depolymerization reproducing 2, or... 

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