Chain initiation purely thermal

Gray fit Yang (Ref 1), a mathematical model was proposed to unity the chain and thermal mechanisms of explosion. It was shown that the trajectories in the phase plane of the coupled energy and radical concentration equations of an explosive system will oive the time-dependent behavior of the system when the initial temperature and radical concentration are given. In the 2nd paper of the same investigators (Ref 2), a general equation for explosion limits (P—T relation) is derived from a unified thermal and chain theory and from chis equation, the criteria of explosion limits for either the pure chain or pure thermal theory can be deduced. For detailed discussion see Refs... 

Unlike ionic polymerizations, radical chain polymerizations have so far been found to occur only with unsaturated compounds. In some cases they can be induced purely thermally, or by means of light or high-energy radiation generally, however, radical initiators such as peroxo compounds, azo compounds, and redox systems are used. 

Tetralin hydroperoxide (1,2,3,4-tetrahydro-l-naphthyl hydroperoxide) and 9,10-dihydroanthracyl-9-hydroperoxide were prepared by oxidizing the two hydrocarbons and purified by recrystallization. Commercial cumene hydroperoxide was purified by successive conversions to its sodium salt until it no longer increased the rate of oxidation of cumene at 56°C. All three hydroperoxides were 100% pure by iodometric titration. They all initiated oxidations both thermally (possibly by the bi-molecular reaction, R OOH + RH — R O + H20 + R (33)) and photochemically. The experimental conditions were chosen so that the rate of the thermally initiated reaction was less than 10% of the rate of the photoreaction. The rates of chain initiation were measured with the inhibitors 2,6-di-ter -butyl-4-methylphenol and 2,6-di-fer -butyl-4-meth-oxyphenol. None of the hydroperoxides introduced any kinetically first-order chain termination process into the over-all reaction. 

The dissociation process is described by a free radical chain mechanism. The thermo-oxidative dissociation is initiated by the oxidation of the aliphatic moieties by a subsequent cleavage of the hydroperoxides formed. With increasing time of oxidation the temperature of the onset of degradation is lower as compared with that for a purely thermal degradation. 

The second step (eq. 4.5) is a thermal oxidative process. This initiates the reaction of ZDDP with oxygen, and enhances the decomposition. Since oxygen and/or hydroperoxide is present in the oil, decomposition is not a pure thermal degradation. The main products on the surface are zinc polyphosphates with minor amounts of zinc sulfides. As the rubbing continues, the polyphosphate layer comes into closer contact with water in oil and is hydrolyzed to give short-chain polyphosphates (eq. 4.6). 

The DR and AC intermediates of the photopolymerization reaction are stable only at low temperatures. At temperatures above about 100 K they react to form long macromolecules by subsequent addition of monomer molecules. The 10 K optical absorption spectra of Fig. 17 show the result of the thermal reaction as a function of the time at 100 K The initial spectrum showing only the dimer A absorption has been prepared at 10 K by only one UV-excimer laser pulse at 308 nm. Only pure thermal addition polymerization reactions are observed within the DR-series A, B, C,. .. No chain termination reactions are detectable in the optical spectra. The final product P is situated in the vicinity of the final polymer absorption. 

Chain reactions, essentially polymerizations, can be achieved with medium doses, as a result of the chemical amplification by purely thermal processes of radiation-induced initiation (Scheme 2). Processes involving single steps or short kinetic chain length reactions require much higher doses.This is generally the case for the radiation cross-linking of rubbers and thermoplastics. 

Depolymerization is the reverse reaction to polymerization it consists of unzipping the monomeric units after initiation of the reaction either at random or at the chain ends. The initiation can be the consequence of a photochemical reaction but depolymerization itself is a purely thermal reaction which is discussed in Chapter 1 of this book. It is usually negligible at room temperature. Photochemically initiated depolymerization has been studied in the case of polymethylmethacrylate [9], poly-a-methylstyrene [10] and polymethylisopropenylketone [11]. 

Since pure PP does not Incorporate chromophorlc groups, it is clear that photolnltlatlon of radical degradation processes must Involve chromophorlc impurities. There has been a great deal of discussion of this in the past and hydroperoxides or carbonyl structures formed by oxidation of the parent polymer and transition metal residues from the polymerization catalyst seem to be the most likely candidates. It is not appropriate to discuss this aspect in the present paper, suffice it to say that the association of methane with photolnltlatlon, but not thermal Initiation, suggests that photolnltlatlon Involves C-CH3 bond scission to form chain side radicals in contrast to thermal Initiation which involves scission of the C-C bond In the main chains. 

The pure PFAP (II) exhibits a rapid decrease in viscosity from 2.35-0.38 dl/g over a span of 32 hr at 177°C. In contrast, the PFAP (II) samples containing 1, 2, and 3 wt % of stabilizer exhibit viscosity decreases to 1.20, 1.88, and 1.93, respectively, under the same conditions. These plots indicate that the zinc complex reacts with the weak links (III and IV) to deactivate them and thus prevent chain scission. The addition of the stabilizer in excess of 2 wt % doesn t appear to improve the thermal stability. Evidently the zinc complex has deactivated all of the weak links that are capable of reaction under these conditions. The fact that the PFAP (II) containing 2 and 3 wt % stabilizer still exhibits a small initial decrease in viscosity to 1.93 indicates that a small amount of weak links have not been deactivated. 

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