Initiation depolymerization

For random initial depolymerization followed by complete unzipping, Z can be related to DP through a parameter, B, that is related to the polydispersity of the polymer sample. 

The thermal degradation of irradiated isotactic PMMA has also provided information on the structure of the end groups formed at the site of main-chain scission [408]. The weight loss of non-irradiated PMMA at 250°C has been shown by Grassie and Melville [409] to be mainly due to depolymerization initiated at the carbon—carbon double bonds situated at the chain ends. A small proportion of randomly initiated depolymerization also occurs at this temperature. In agreement with this mechanism, the rate of volatilization has been found to be much higher for atactic than for isotactic PMMA, the latter having no double bonds at the chain ends. If 4.3 scissions per chain are produced by 7-irradiation in the isotactic sample, the rate of monomer evolution is identical to that of the initial unirradiated isotactic sample. This proves that chain ends of the type... 

Depolymerization is the reverse reaction to polymerization it consists of unzipping the monomeric units after initiation of the reaction either at random or at the chain ends. The initiation can be the consequence of a photochemical reaction but depolymerization itself is a purely thermal reaction which is discussed in Chapter 1 of this book. It is usually negligible at room temperature. Photochemically initiated depolymerization has been studied in the case of polymethylmethacrylate [9], poly-a-methylstyrene [10] and polymethylisopropenylketone [11]. 

The seaweed polyglycosiduronic acid alginic acid, whichj is composed of D-mannuronic and L-guluronic acid residues, when treated with a partially purified preparation of a cell-free extract of an adapted pseudomonad, is initially depolymerized to a series of oligosaccharides each of which contains a 4-deoxy-L-er2/terminal position, and then to the h-erythro isomer of the ulosonic acid (107), which is the product formed from both of the constituent residues. 

Brown The determinations of size of the cell-wall capillaries and the cellulases of fungi indicate that the enzymes that have been isolated to date are so large that they probably can penetrate only a few cell-wall capillaries in wood and cotton. This conclusion is supported by Cowling s DP data for the action of the white-rot fungus, Polyporus versicolor, on wood (Figure 13). But it is also contradicted by the same type of data for the effect of both the brown-rot fungus, Poria monticola, on wood (Figure 13) and of Myrothecium verrucaria on mercerized cotton as shown by Selby (60). Thus we believe that the catalysts responsible for the initial depolymerization of cellulose in wood and cotton by these two... 

Activation of iodosylbenzene in the presence of KBr has been explained by the initial depolymerization of (PhIO) with the formation of a highly reactive intermediate PhI(Br)0 K+, which reacts with alcohols to yield the corresponding carbonyl compound with regeneration of KBr [11]. 

The main difference between homo- and copolymers is the fact that copolymers exhibit higher thermal-oxidative resistance. Figure 5.196. Thermal-oxidative degradation in polyoxymethylene leads to the formation of unstable end groups that initiate depolymerization under the formation of formaldehyde. In homopolymers, this results in total decomposition. In copolymers, depolymerization proceeds only to the next comonomer unit (mostly polyethylene, comonomer content approx. 0.5 to 5 wt.%). That widens the processing window for copolymers and reduces the risk of mold fouling [771]. 

Activation energies for depolymerization processes show considerable variation. Purity, method of preparation, and molecular weight critically affect the mode and ease of initiating depolymerization and, to some extent, the depolymerization mechanism. In many cases only an average activation energy has meaning valid over a limited temperature range (38). 

Random macromolecular fragmentation to form defective groups which subsequently initiates depolymerization 351... 

That implies that aU the PS— > bonds are broken in the low temperature step, confirming that they are less stable than the C—C bonds in the polystyrene chain. Therefore, the decomposition mechanism can be rationalized by taking into account the well-known thermal decomposition of PS through a radical initiated depolymerization (Scheme 5.21). 

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