Depolymerization reaction

Polymers are very soft materials which are not easily milled under normal conditions. The mastication of natural rubber is the reduction of molecular weight by milling or cutting, the process being invented by Thomas Hancock as early as 1820. Even today this is a major industrial process in the tire and rubber industry. Other polymers are milled under reduced temperature or even in liquid nitrogen to achieve a controlled molecular mass. The degradation of several polymers has been investigated by Dimitrov et al. [104]. 

In the future, it is likely that reactive comminution will be used as a tool for the preparation of novel materials and precursors for catalysts, membranes, high-performance ceramics, hydride storage media, raw material for powder metallurgy, and nanoparticles. The emerging applications in organic synthesis reactions and environmental protection will undoubtedly also lead to broader applications. 

Quicksilver from Cinnabar The First Documented Mechano-chemical Reaction ,/OM, 

Boldyrev, Mechanical Activation and Its Application in Technology, Materials Science Forum Vols., 

Boldyrev, K. Tkacova, Mechanochemistry of Solids Past, Present, and Prospects, Journal 

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Condensation occurs most readily at a pH value equal to the piC of the participating silanol group. This representation becomes less vaUd at pH values above 10, where the rate constant of the depolymerization reaction k 2 ) becomes significant and at very low pH values where acids exert a catalytic influence on polymerization. The piC of monosilicic acid is 9.91 0.04 (51). The piC value of Si—OH decreases to 6.5 in higher order sihcate polymers (52), which is consistent with piC values of 6.8 0.2 reported for the surface silanol groups of sihca gel (53). Thus, the acidity of silanol functionahties increases as the degree of polymerization of the anion increases. However, the exact relationship between the connectivity of the silanol sihcon and SiOH acidity is not known. 

Consider the effects of the N-R group on the structure and reactivity of polysilazanes as susceptible to hydrogen bonding effects. These effects alone should favor ring closure over the respective siloxane analogs. Moreover, for R = H, a new type of depolymerization reaction, analogous to reaction (4), is available as illustrated in reaction (35). Thus, reaction (35) could also contribute to the... 

Although the picture is far from complete, the available evidence suggests that PBPP is rather different from most polyphosphazenes. The polymer may be induced to be an electronic conductor, but perhaps as a result of this tendency to cross-link, the material is more sensitive and difficult to handle than most polyphosphazenes and the thermal depolymerization reaction is inhibited. 

Besides the main depolymerization reactions, side reactions should also be considered in the kinetic description of a PET recycling process. This is emphasized by the results obtained from a PET extrusion model [85] shown in Figures 2.19-2.23. The complete set of reactions summarized below in Table 2.10 have been used, but shear effects have not been taken into account. Chain degradation, accompanied by a significant reduction of intrinsic viscosity, occurs even within residence times of a few minutes. Carboxyl end groups, vinyl end groups and acetaldehyde are formed in amounts depending on residence time, temperature and initial moisture content of the PET flakes. 

Figure 3.5 shows the distribution of cyclics formed typically as a function of time in a PBT depolymerization reaction. It can be seen that initially the levels... 

Co catalysts, metal crystallite size and support effects, 39 242-246 Ru catalysts, metal crystallite size and support effects, 39 237-242 Thiele modulus effect, 39 275 reaction-transport models, 39 222-223 readsorption probability, 39 264-265 secondary chain growth, hydrogenation, and depolymerization reactions, 39 224—225... 

Fig. 5. Kinetics of brain microtubule depolymerization following rapid dilution. (A) Time course of the disassembly reaction with experimental data represented by the data points and the theoretical progress curve indicated by the solid line. (The inset to A shows that the process can be fitted to a simple decaying exponential for part of the depolymerization reaction.) (B) Microtubule length distribution for the sample subjected to rapid dilution in A. (Reproduced from Karr et al. (1980)./. Biol. Chm. 255, 8560-8566.)...

At higher temperatures (>250°C), endothermic depolymerization reactions dominate and generate volatile anhydrosugars and related monomeric compounds, see pathway... 

As for (i-0-4 ethereal bond cleavage, reaction of the primary cation-radical with solvent water under the same conditions of bio-oxidation was shown to form an arylglycerol and the corresponding phenoxy radical (Kirk et al. 1986, Fabbri et al. 2005) (Scheme 8.22). Since the p-0-4 ethereal bond is the most abundant type of interunit linkage in the lignin polymer, this ethereal bond cleavage represents an important depolymerization reaction. 

Earlier transition metals, as zirconium and hafnium, are still more active in hydrogenolysis, which allows zirconium hydrides to be used in depolymerization reactions (hydrogenolysis of polyethylene and polypropylene) [89], In this case, the zirconium hydride was supported on silica-alumina. Aluminum hydrides close to [(=SiO)3ZrH] sites would increase their electrophilicity and, thus, their catalytic activity. A catalyst prepared in this way was able to convert low-density polyethylene (MW 125000) into saturated oligomers (after 5h) or lower alkanes at 150°C (100% conversion). It was also able to cleave commercial isotactic polypropylene (MW 250000) under hydrogen at about 190 °C (40% of the starting polypropylene was converted into lower alkanes after 15 h of reaction). 

In Figure 1, the pH of the batch diluted solutions Is plotted against the dilution ratio, C/Co, where C Is the molar concentration of Al In the dilute solution and Co Is the molar concentration of Al In Chlorhydrol (6.43 M). The pH of the most concentrated solution was still Increasing after ten days aging. (The higher viscosity of the concentrated solutions may contribute to the slower equillbriatlon). At higher dilution (lower C/Co values), new equilibria can be Introduced due to depolymerization reactions. For the zero day curve, pH continuously Increases with... 

Many, if not most, step polymerizations involve equilibrium reactions, and it becomes important to analyze how the equilibrium affects the extent of conversion and, more importantly, the polymer molecular weight. A polymerization in which the monomer(s) and polymer are in equilibrium is referred to as an equilibrium polymerization or reversible polymerization. A first consideration is whether an equilibrium polymerization will yield high-molecular-weight polymer if carried out in a closed system. By a closed system is meant one where none of the products of the forward reaction are removed. Nothing is done to push or drive the equilibrium point for the reaction system toward the polymer side. Under these conditions the concentrations of products (polymer and usually a small molecule such as water) build up until the rate of the reverse reaction becomes equal to the polymerization rate. The reverse reaction is referred to generally as a depolymerization reaction other terms such as hydrolysis or glycolysis may be used as applicable in specific systems. The polymer molecular weight is determined by the extent to which the forward reaction has proceeded when equilibrium is established. 

Proteins are nature s polyamide condensation polymers. A protein is formed by polymerization of o-artiino acids, with the amino group on the carbon atom next to the carboxylic acid. Biologists call the bond formed a peptide rather than an amide. In the food chain these amino acids are continuously hydrolyzed and polymerized back into polymers, which the host can use in its tissues. These polymerization and depolymerization reactions in biological systems are all controlled by enzyme catalysts that produce extreme selectivity to the desired proteins. 

The number average molecular weight of the PMMA sample was 24,000, and the amount of butyl lsopropenyl ketone unit in this polymer could be calculated to be 0.42mol% of MMA unit if a polymer molecule was assumed to contain one ketone unit. The amount of the ketone formed in the depolymerization reaction was 0.3mol% of MMA formed and corresponded to about 70% of the theoretical value. The depolymerization of the oligomer also gave low-boiling products in 53% yield, which contained about 9% of butyl lsopropenyl ketone. 

On the other hand, if the covalent bonds are almost as stable and as numerous in the gas-phase molecules as in the solid, vaporization takes place readily. Examples are the depolymerization reactions that lake place at a few hundred degrees. For example, red phosphorus sublimes and recondenscs as white phosphorus.25... 

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