Scission process

There are, indeed, many biological implications that have been triggered by the advent of fullerenes. They range from potential inhibition of HIV-1 protease, synthesis of dmgs for photodynamic therapy and free radical scavenging (antioxidants), to participation in photo-induced DNA scission processes [156, 157, 158, 159, 160, 161, 162 and 163]. These examples unequivocally demonstrate the particular importance of water-soluble fullerenes and are summarized in a few excellent reviews [141, 1751. 

This result should be vahd for sufficiently high density 0 where correlations, brought about by the mutual avoidance of the chains, are negligible. Due to the recombination-scission process a polydisperse solution of living polymers should absorb or release energy as the temperature is varied. This is reflected by the specific heat Cy, which can be readily obtained from Eq. (9) as a derivative of the internal energy U... 

One of the peculiar features of sulfonyl radicals is the a-scission process, namely... 

Example 4. Depolymerization under Pressure.62 PET resin was depolymerized at pressures which varied from 101 to 620 kPa and temperatures of 190—240° C in a stirred laboratory reactor having a bomb cylinder of2000 mL (Parr Instrument) for reaction times of 0.5, 1, 2, and 3 h and at various ratios of EG to PET. The rate of depolymerization was found to be directly proportional to the pressure, temperature, and EG—PET ratio. The depolymerization rate was proportional to the square of the EG concentration at constant temperature, which indicates that EG acts as both a catalyst and reactant in the chain scission process. 

If the intersystem crossing process is efficient at this excitation, then the Norrish type II rearrangement must occur from the triplet state. This is further substantiated by a reduction in loss of tenacity with increasing concentration of triplet state quencher. The reduction in loss of tenacity may be equated with interruptions of the chain scission process(es). 

We also note two reports on the degradation of stacked films of oriented PET. In the first [36], photodegradation is said to be two-step in nature with weak links cleaving first followed by a much slower cleaving of the strong links . The chain scission process was tracked versus exposure time by measurement of solution viscosity [37], In the second paper [38], the surface nature of the degradation was reported and the rationale elucidated for why surface degradation causes overall mechanical failure limited by that layer, much as we have discussed in the impact properties of the PECT (see above). 

The close correspondence between the fragments molecular weights, 500-750 observed (873.2 calculated) and 2000-2500 observed (1987.6 calculated), leads to the conclusion that linear chain fragments and X-units are apparently both formed after the scission process begins. 

Note 2 Some main-chain scissions are classified according to the mechanism of the scission process hydrolytic, mechanochemical, thermal, photochemical, or oxidative scission. Others are classified according to their location in the backbone relative to a specific structural feature, for example, a-scission (a scission of the C-C bond alpha to the carbon atom of a photo-excited carbonyl group) and P-scission (a scission of the C-C bond beta to the carbon atom bearing a radical), etc. 

The kinetics of the decomposition of PPC has been estimated from several studies. An analysis from TGA shows that the activation energy for end-capped PPC at temperatures over approximately 250°C is in the range of 130 kJ/mol, a relatively low value (for a chain scission process) [19]. The same analysis for uncapped PPC is complicated by non-linear behavior. Results consistently indicate that, at lower temperatures, a different decomposition reaction takes place than at higher temperatures. 

Op the foregoing basis, the fundamental difference with respect to the energy transfer and bond scission processes between the paraffins on the one hand, and substrates such as alcohols, ethers, amines, olefins, etc., on the other, would appear to be that in the paraffins the excitation... 

Thermal analysis (DSC, DMTA, DETA, etc.) gives valuable information on eventual changes of glass transition temperature, which can be interpreted in terms of crosslinking/chain-scission processes. 

In general, we have outlined how the conversion of isobutane on sohd acid catalysts takes place according to well-established carbenium ion transition state chemistry. The difficulty with using isobutane conversion as a probe of catalyst performance is that many combinations of oligomcrization// -scission processes with isomerization steps are possible, resulting in a wide variety of adsorbed species and observable reaction products. For example, the following products are observed from isobutane conversion in the presence of ultrastable Y zeolite at temperatures near 520 K (where the reaction is initiated by the addition of isobutylene to the feed) ... 

The arguments outlined previously lead to 88 steps for oligomerization and /3-scission processes involving 74 adsorbed hydrocarbon species. Although the choice of these steps is arbitrary, we later show that relatively few of them are kinetically significant. 

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