Pure Thermal Initiation

The initiation mechanism for styrene has been established [Graham et al., 1979 Kaufman, 1979 Kothe and Fischer, 2001 Mayo, 1968 Olaj et al., 1976, 1977a,b]. It involves the formation of a Diels-Alder dimer (XII) of styrene followed by transfer of a hydrogen atom from the dimer to a styrene molecule (Eq. 3-63). Whether formation of the 

Diels-Alder dimer or its reaction with styrene is the rate-determining step in initiation is not completely established. The dependence of Rp on [M] is closer to third-order than second-order, indicating that Eq. 3-63b is the slow step. The Diels-Alder dimer has not heen isolated, but ultraviolet spectroscopy of the reaction system is entirely compatible with its presence. There are indications that the photopolymerization of neat styrene proceeds by a similar mechanism. 

The initiation mechanism for methyl methacrylate appears to involve the initial formation of a biradical by reaction of two monomer molecules followed by hydrogen transfer from 

The Diels-Alder dimer has not been isolated but its existence has been confirmed by ultraviolet spectroscopy. 

Problem 6.14 There is evidence [30] that thermal self-initiated polymerization of styrene may be of about five-halves order. Show that this is in agreement with the established initiation mechanism involving a Diels-Alder dimer formation [Eqs. (6.95) and (6.96)]. 

The higher than second-order rate observed for thermal conversion of monomer indicates that Eq. (6.96) is the slow step. Representing the concentration of Diels-Alder dimer (V) by [D] and that of styrene by [M], 

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A series of pulsed electron beam tests were conducted on dextrinated and RD-1333 Pb azide pellets by Avrami et al (Ref 232), From the limited data in Table 14 it can be seen that sample ambient pressure, sample thickness and type of Pb azide are all important factors in the sensitivity of initiation by pulsed electron beam The question arises as to what mechanism can explain the observed pressure, thickness and type of Pb azide dependence. A purely thermal initiation mechanism or a compressive shock initiation resulting from nearly instantaneous energy deposition can account for some of the observations but not all... 

More control over the polymerization process is achieved by anionic rather than purely thermal initiation. Thus, the use of n-BuLi as an initiator in THF solvent at 25 °C gives narrow molecular-weight distributions (pdi <1.2), and the molecular weights can be controlled by the ratio of initiator to monomer because each initiator molecule will... 

The question arises as to what mechanism can explain the observed pressure, thickness, and material dependence. A purely thermal initiation mechanism or a compressive shock initiation (see Chapter 7. Volume 2), resulting from nearly instantaneous energy deposition, can account for some but not all of the observations. 

It is therefore not surprising that the early investigators saw no promise in this mechanism of polymerization of butadiene, isoprene, etc., either by pure thermal initiation or by the use of free radical initiators, such as the peroxides. Instead they turned to sodium polymerization, which, although also rather slow and difficult to reproduce, at least yielded high-molecular-weight rubbery polymers from the dienes. Later, in the 1930s, when emulsion polymerization was introduced, it was found that this system, even though it involves the free... 

This section describes polymerizations of monomer(s) where the initiating radicals are formed from the monomer(s) by a purely thermal reaction (/.e. no other reagents are involved). The adjectives, thermal, self-initialed and spontaneous, are used interchangeably to describe these polymerizations which have been reported for many monomers and monomer combinations. While homopolymerizations of this class typically require above ambient temperatures, copolymerizations involving certain electron-acceptor-electron-donor monomer pairs can occur at or below ambient temperature. 

Aspects of thermal initiation have been reviewed by Moad et al., w Pryor and Laswell, 10 Kurbatov/" and Hall.312 It is often difficult to establish whether initiation is actually a process involving only the monomer. Trace impurities in the monomers or the reaction vessel may prove to be the actual initiators. Purely thermal homopolymerizations to high molecular weight polymers have only been demonstrated unequivocally for S and its derivatives and MMA. For these and other systems, the identity of the initiating radicals and the mechanisms by which they are formed remain subjects of controversy. 

If the initial reactions of coal are purely thermal, one might expect that the H-donor level will be of minor importance if times are kept short. In fact, all coals contain a certain portion of material that is extractable by pyridine. On heating coals to liquefaction temperatures, some additional material also becomes soluble in even non-donor solvents. Thus, there is a portion of all coals which can be solubilized with little dependence on the nature of the solvent. 

If the reacting system is initiated under conditions similar to point 4, pure thermal explosions develop and these explosions have thermal induction or ignition times associated with them. As will be discussed in subsequent paragraphs, thermal explosion (ignition) is possible even at low temperatures, both under the nonadiabatic conditions utilized in obtaining hydrocarbon-air explosion limits and under adiabatic conditions. 

Although the initiation step under purely thermally induced conditions such as those imposed by shocks has not been formulated, it is expected to be a reaction that produces O atoms. The high-temperature mechanism would then be reactions (8.105), and (8.124)-(8.126), with termination by the elimination of the O atoms. 

Miyata and Nakashio [77] studied the effect of frequency and intensity on the thermally initiated (AIBN) bulk polymerisation of styrene and found that whilst the mechanism of polymerisation was not affected by the presence of ultrasound, the overall rate constant, k, decreased linearly with increase in the intensity whilst the average R.M.M. increased slightly. The decrease in the overall value of k they interpreted as being caused by either an increase in the termination reaction, specifically the termination rate constant, k, or a decrease in the initiator efficiency. The increase in kj(= kj /ri is the more reasonable in that ultrasound is known to reduce the viscosity of polymer solutions. This reduction in viscosity and consequent increase in Iq could account for our observed reductions [78] in initial rate of polymerisation of N-vinyl-pyrrolidone in water. However this explanation does not account for the large rate increase observed for the pure monomer system. 

For a purely photochemical polymerization, the initiation step is temperature-independent (Ed = 0) since the energy for initiator decomposition is supplied by light quanta. The overall activation for photochemical polymerization is then only about 20 kJ mol-1. This low value of Er indicates the Rp for photochemical polymerizations will be relatively insensitive to temperature compared to other polymerizations. The effect of temperature on photochemical polymerizations is complicated, however, since most photochemical initiators can also decompose thermally. At higher temperatures the initiators may undergo appreciable thermal decomposition in addition to the photochemical decomposition. In such cases, one must take into account both the thermal and photochemical initiations. The initiation and overall activation energies for a purely thermal self-initiated polymerization are approximately the same as for initiation by the thermal decomposition of an initiator. For the thermal, self-initiated polymerization of styrene the activation energy for initiation is 121 kJ mol-1 and Er is 86 kJ mol-1 [Barr et al., 1978 Hui and Hamielec, 1972]. However, purely thermal polymerizations proceed at very slow rates because of the low probability of the initiation process due to the very low values f 1 (l4 IO6) of the frequency factor. 

Ej has a value of about —60 kJ mol-1 for thermal initiator decomposition, and Xn decreases rapidly with increasing temperature. Ej is about the same for a purely thermal, self-initiated polymerization (Fig. 3-16). For a pure photochemical polymerization Ej is positive by approximately 20 kJ mol-1, since Ed is zero and X increases moderately with temperature. For a redox polymerization, Ej is close to zero, since Ed is 40-60 kJ mol-1, and there is almost no effect of temperature on polymer molecular weight. For all other cases, Xn decreases with temperature. 

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. 

Diazo compounds generally do not undergo [3 + 2] cycloaddition with unactivated nitriles under purely thermal, noncatalyzed conditions. The formation of 4-R-5-trimethylsilyl-l//-l,2,3-triazoles from the reaction of diazo(trimethylsilyl)-methyl lithium and a broad range of nitriles [RCN R = alkyl, aryl, SEt, OPh, PO(OEt)2] appears to be an exception, but this reaction most likely occurs in a stepwise manner with initial nucleophilic attack at the nitrile (275). 

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. 

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... 

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). 

Pure thermal oxidation predominates at very low dose rates (typically lower than about 10 Gy.s ) and is characterized by a large TOL, typically 6 mm. At intermediate dose rates, no one initiation process can be neglected relative to the other one. The model covers all of these domains. 

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