Thermal degradation reactions

The use of aliphatic monomers for hyperbranched polyesters has been debated because aliphatic monomers are said to be prone to thermal degradation reactions such as decarboxylation, cyclization, or dehydration [77]. The only commercial hyperbranched polymer is a hydroxy-functional aliphatic polyester, Boltorn, available from Perstorp AB, Sweden. 

The exothermic heat of reaction for PVC is relatively high, and thus so is the ceiling temperature. PVC resins, however, will dehydrohalogenate at temperatures considerably lower than the ceiling temperature, forming FICl gas and charred material. In this case, thermal degradation reactions occur at temperatures less than the ceiling temperature. 

The Ln(tritox)3 system underlies a thermal degradation reaction (Sect. 6.3). Attempted sublimation yielded Ln(ditox)3, the dimeric composition of which... 

Octacarbonyldicobalt decomposes readily at about 50°-60°C to give dodecacarbonyltetracobalt, and this thermal degradation reaction provides a convenient synthesis of Co4(CO)i2 on a small scale (103). Larger quantities of Co4(CO)i2 are better prepared by the reduction with hydrogen of an appropriate C0X2 compound (e.g., X = 2-ethylhexanoate or acetylacetonate) in the presence of Co2(CO)b [Eq. (10)] (104, 105). 

The flux due to surface thermal degradation reaction is given by... 

To obtain a more clearly defined picture of these structural features and to establish the relationship between the chemical structure of asphaltene and its reactivity under a variety of conditions, the potential of chemical and thermal degradation reactions as diagnostic tools has been studied. The specific subject of this investigation was the high molecular weight, sulfur rich asphaltene from the Athabasca bitumen. 

Tarl reacts with H O, CO2, O2, Hi and suffers thermal degradation reactions too, such as cracking. A secondary and more-refractory-to-be destroyed tar, which will be called tar2, is produced together with Hi, CO, CH. jStochioraetry of these reactions is not well established yet,... 

In addition to the depolymerization reaction discussed earlier, other reactions may be favoured. These are elimination from a side chain and cyclization. For example, propylene is eliminated from the side chain of poly (isopropyl acrylate) as shown in Scheme 1.51(a), leaving poly(acrylic acid). This occurs in all polymers having ester side groups with P-hydrogens available to form a six-membered transition state, as shown. Thus both acrylate and methacrylate polymers will undergo this reaction and, since depolymerization is the dominant thermal-degradation reaction in methacrylates, elimination of alkenes is more important in the poly(acrylates). 

The lactone monomer can be recovered from the thermal degradation reaction if removed quickly (6). As other workers have shown (7), cyclic oligomers are also evolved, but we have found that these, in turn, are decomposed into carbon dioxide and isobutene (via the lactone) when carboxylate ions are present. When metal salts are used as initiators, no termination reactions are possible and, consequently, the polymers have an extremely poor thermal stability. 

The duration of the polymerization depends on the catalyst concentration, reaction temperature, size of the batch of polymer being produced, and the amount of surface area generated in the polymerization autoclave. In comiitercial opetation, it is desirable not to have too long a polymerization qycle for economic reasons. In a cycle that is too long, the competing and irreversible thermal degradation reaction will have sufficient time to lower the viscotity and to cause discoloration. 

There are basically three types of thermal degradation reactions for vinyl polymers [36,37] (1) nonchain scission (2) random chain scission and (2) depropagation. In practice, mechanisms 2 and 3 blend into one another, with many polymers showing evidence of both processes. 

The aqueous aroma composition (solid, liquid, paste) includes components/supports able to form by freezing an amorphous matrix or a gel to immobilize aroma compounds. It will be transformed into an aroma powder, without modification of aroma, for easy storage. After freezing of the aroma composition, there is drying by sublimation of ice, usually under vacuum. The driving force for sublimation is the difference in pressure between the water vapor pressure at the ice interface and the partial pressure of water vapor in the chamber. Because of the low processing temperatures, thermal degradations reactions are excluded. 

In DPMS, polymers are introduced into the ion source via the direct insertion probe (Figure 5.1a) and the temperature is increased gradually rmtil thermal degradation reactions occur. The volatile products formed are ionized (both positive and negative ions are produced) and extracted from the ion source (Figure 5.1b) almost as soon as they are formed, by applying a positive or negative potential to the repeller. 

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