Bonds, random scissioning

Any electromechanical device that utilizes an automated feedback servomotor to regulate the addition of titrant (a standardized solution of acid or base within a syringe) into a reaction vessel or sample to maintain pH. The rate at which the syringe expels its contents allows one to determine the rate of a chemical reaction producing or consuming protons. There are many such enzyme-catalyzed reactions whose kinetics can be examined with a pH Stat. For maximal sensitivity, one must use weakly buffered solutions. In his classical kinetic investigation of DNA bond scission by DNase, Thomas measured the rate of base addition in a pH Stat. The number of bonds cleaved was linear with time, and this was indicative of random scission. 

Tanford presented a cogent kinetic treatment of random scission of a polymer, and the complete analysis is beyond the scope of this Handbook. The basic idea is that a molecule M, can yield a molecule (where x and y indicate the degree of polymerization, and where y > x) in two different ways. The x-mer formation rate from y-mers is twice the rate of bond scission at the concentration of the y-mer thus,... 

At higher temperatures in the absence of air the degradation would be a random scission occurring primarily at the C-0 bond of the PTHF ... 

The General Bond-Weighted Random Scission Model.485... 

THE GENERAL BOND-WEIGHTED RANDOM SCISSION MODEL 18.3.1 Overview... 

Staggs, J. E. J. Discrete bond-weighted random scission of linear polymers. Polymer 2006 47 897. 

The processes discussed above pertain to the main reactions of cotton with alkaline solutions up to temperatures of 120-130°C. Above 170°C, a random scission of glycosidic bonds in accessible regions occurs, leading to the rapid production of shorter chain molecules with new reducing end units. These immediately participate in the peeling and stopping reactions just described. 

Given the extreme simplicity of the chemical structure of a saturated hydrocarbon, thermal activation can only cause the scission ofaC-CorC—H bond. In the former case, the random scission of a C C bond of the carbon chain — the cracking reaction — produces a paraffin and an olefin ... 

Figure 6.25(a) shows a novel process for the continuous production of fuels from waste plastics. The proposed process consists primarily of three reactors. A mixture of waste plastics is fed into a pyrolytic reactor with heat-medium-particles stirred by a helical impeller (Figure 6.25(b)), where melted plastics are hydrothermally decomposed with steam and the random scission of C-C bonds. The produced mixture of a heavy oil containing wax and sublimate material is carried by steam stream to the next reactor, which is filled with an FeOOH catalyst (i.e. a catalytic hydrolysis reactor). The gaseous... 

While condensation polymers such as PET and polyamides can be broken down into their monomer nnits by thermal depolymerization processes, vinyl (addition) polymers snch as polyethylene and polypropylene are very difficnlt to decompose to monomers. This is becanse of random scission of the carbon-carbon bonds of the polymer chains during thermal degradation, which prodnces a broad prodnct range. 

Ladder polymers are double-strand linear polymers. Their permanenee properties are superior even to those of conventional network polymers. The latter are randomly cross-linked, and their molecular weight ean be redueed by random scission events. When a chemical bond is broken in a ladder polymer, however, the second strand maintains the overall integrity of the molecule and the fragments of the broken bond are held in such close proximity that the likelihood of their recombination is enhanced. 

These results provide additional confirmation for the mechanism of pyrolysis of simple polyolefins. The absence of monomer in the volatile products, the maxima in the rate curves, and the sharp decrease in the intrinsic viscosity for linear polymethylene (29) and polypropylene (2, 6, 13, 30) all point to an essentially random scission, due to pronounced intermolecular chain transfer, Equation 2. However, deviations appear when a, the fraction of bonds broken, or, what amounts to the same, the number average DP is examined as a function of time. For small a, the former relation should be one of simple proportionality and hnearity in 1/P. Instead, for both polypropylene (6) and polymethylene [see Figure 5, in (29)] curvature appears, indicating a reduction of the scission rate after an initial period of rapid degradation. For polypropylene this has been interpreted as a breaking of weak and normal bonds. Between 250° and 280° C., one weak link per 2.4 X 10 is found (6). At 295° C., the existence of more than two types of bonds would have to be postulated. 

It should be noted that the weak link mechanism does not oflFer the only possible explanation for departures from random scission in linear polyolefins or from predictions of our kinetic theory, in general. In the case at hand, the effective rate constant for bond breaking is ki -h k R kgR. It follows from Equation 5b that the rate is reduced by a factor (1 — This effect, however, is... 

The presence of double bonds at the end of the polymeric chain and the relatively lower stability of the bond in a p position to the double bond increases the probability of the end scission compared to random scissions in many polymers. 

The reactions taking place during the pyrolysis can be rather complex and may occur with both C-C bond cleavage in the polymer backbone or with C-O bond cleavage at the side chain. Depending on the amount of sample used in the pyrolysis and indirectly on the temperatures to which the polymer was exposed, some variability in the peak intensities is noticed when the pyrolysate from a small sample is compared to that for a larger sample. In various conditions, the polymeric chain can be cleaved by random scission reactions, similar to those for polyethylene or polypropylene, as shown below ... 

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