Depolymerization mechanism

Depolymerization mechanisms, 456 Depolymerized PET products, applications of, 531-532 Deuterium NMR, 328 Dexon, 28... 

Preston JF, Rice JD, Ingram LO, Keen NT (1992) Differential depolymerization mechanism of pectate lyase secreted by Erwinia chrysanthemi EC 16. J Bacteriol 174 2039-2042... 

Figure 10. A proposed depolymerization mechanism of lignin by laccase. (1) Side chain cleavage (2) fi-O-4 ether cleavage.

Erwinia chrysanthemi, interest in the maceration process has led to the cloning and sequencing of four separate genes coding for different secreted pectate lyases (18,19), The production of these enzymes, either by the parent organism, or as recombinant enzymes in E. coli, has now led to their detailed characterization with respect to depolymerization mechanism. 

As mentioned previously, when polystyrene is subjected to the temperatures of a flame it pyrolyzes by a depolymerization mechanism to give monomer and oligomers [14]. The combustion of these volatile products in the vapor phase above the sample supplies heat back to the solid sample (Figure 29.3). If the energy supplied by combustion is sufficient to maintain the pyrolysis process, the flame is self-sustaining even after the test flame has been removed. In order to make polystyrene more flame retardant, the cycle of pyrolysis and combustion must be broken. Flame retardants may act in either the vapor or solid (condensed) phase. 

Figure 1. Idealized plots relating molecular weight ratio to depolymerization mechanisms. Key A, end-group depolymerizationfollowed by complete unzipping B, random scission followed by complete unzipping C, end-group depolymerization followed by incomplete unzipping and D, random depolymerization followed by incomplete...

The author thanks Martel Zeldin for his help in preparing the section on differentiation of depolymerization mechanisms. 

This latter depolymerization mechanism which takes place at the chain ends was proposed by Verkhotin158 and Aleksandrova154 and their respective coworkers. The most complete study is that of Grassie and MacFarlane151 who took into account the numerous data accumulated over the years, the importance of residual catalysts and the need of analysis by several techniques (TG, TV A, IR, GLC, GC-MS, NMR and Osmometry). They showed the necessity of a precise and reproducible method of polymerization and of precise control of the conditions under which depolymerization must be carried out in order to clearly establish mechanisms of reactions. 

The depolymerization mechanism from the polymer end has been recently revisited in the design of positive electron beam resists. 2-Phenylallyl-termi-nated poly(a-methylstyrene) was prepared by living anionic polymerization, which exhibited a significantly lower depolymerization temperature on TGA than the H-terminated counterpart [340]. The 2-phenylallyl-terminated polymer depolymerized completely when treated with n-BuLi in THF at room temperature. A single-component resist (without PAG) formulated with the 2-phenylallyl-terminated poly(a-methylstyrene) demonstrated a higher e-beam sensitivity (500 pC/cm2 at 20 keV) than the one based on the H-terminated polymer when developed with methanol/methyl isobutyl ketone (2/3 vol/vol) [340]. However, the sensitivity of the non-catalyzed single-component system... 

At 200°C the initial volatilization rate does decrease after 50 hr, but not to the extent observed at lower temperatures. The total weight loss after 764 hr is 40%. This weight loss is attributed to the formation of cyclic oligomers produced by depolymerization of PFAP(II). After 50 hr the chain scission degradation process has lowered the molecular weight to ca. 450 repeat units. Thereafter, the depolymerization mechanism is viewed as an unzipping reaction initiated at the ends of the low molecular weight chains. 

Kalnin sh, K.K. Polymerization/Depolymerization mechanism for "living" sodium poly-a-Methylstyryl and sodium polystyryl. Russian J. Appl. Chem. 2001, 74,1913. 

The pyrolysis of polar macromolecules tends to be more complex from a mechanistic standpoint than that of nonpolar materials, since many of the products formed go through a cyclic intermediate. However, compounds such as polymethacrylates generally decompose via a depolymerization mechanism that produces mostly monomers. Analytical pyrolysis can be used to extract a great deal of information concerning nonvolatile samples, for instance, sample identification through fingerprinting, quantitative measurement of copolymers, and structural information related to branching and stereochemistry. 

A single and relatively sharp peak in the TIC curve is usually an indication of a single-step thermal degradation mechanism. One possibility is depolymerization mechanism yielding mainly the monomer. In this case, the pyrolysis mass spectra are usually simple and resemble that of the monomer. Thus, comparison of the spectrum with the mass spectrum of the monomer is always necessary. 

The rate of monomer production by pyrolysis of some polymers (e.g., PMMA, poly-a-methylstyrene, FIFE) is explained by a chain reaction mechanism. The monomer is evolved at the end of the chain reaction by rupture of C-C bonds according to an unzipping reaction mechanism without transfer all along the chain [6]. The depolymerization mechanism is as follows ... 

According to Her [1], the dissolution of amorphous silica above pH 2 is catalyzed by OH ions that are able to increase the coordination of silicon above four weakening the surrounding siloxane bonds to the network. This general nucleophilic mechanism could presumably occur via Sjv2-Si, S/ 2 Si, or S/ 2 -Si transition states or intermediates and could equally well explain alkoxide ion- or fluorine ion-catalyzed depolymerization mechanisms. 

We can give here an example of topolytic activity of ceUulases (which means the capability of the enzyme to run reactions on the surface of insoluble substrate without deep destmction of cellulose structure), particularly of endo-P-l,4-glucanases (EGs). These enzymes have hydrolytic activities toward polyglucans containing p-l,4-glycosidic bonds, which include cotton and wood cellulose, different soluble cellulose derivatives, p-glucans of oat and barley, and other polysaccharides. Hydrolysis of cellulose by EGs occurs by endo-depolymerization mechanism. 

---