Radicals kinetic mechanism, free

For the linear chain reaction case, a free radical kinetic mechanism (e.g., polymeriza-... 

As a means of beginning our discussion, let us choose to illustrate the model with a rather standard free radical kinetic mechanism ... 

Thermal Oxidative Stability. ABS undergoes autoxidation and the kinetic features of the oxygen consumption reaction are consistent with an autocatalytic free-radical chain mechanism. Comparisons of the rate of oxidation of ABS with that of polybutadiene and styrene—acrylonitrile copolymer indicate that the polybutadiene component is significantly more sensitive to oxidation than the thermoplastic component (31—33). Oxidation of polybutadiene under these conditions results in embrittlement of the mbber because of cross-linking such embrittlement of the elastomer in ABS results in the loss of impact resistance. Studies have also indicated that oxidation causes detachment of the grafted styrene—acrylonitrile copolymer from the elastomer which contributes to impact deterioration (34). 

Our acrylic polymerization model was developed to meet the need for solving these problems. Kinetics used are based on fairly well accepted and standard free radical polymerization mechanisms. 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

We continue our study of chemical kinetics with a presentation of reaction mechanisms. As time permits, we complete this section of the course with a presentation of one or more of the topics Lindemann theory, free radical chain mechanism, enzyme kinetics, or surface chemistry. The study of chemical kinetics is unlike both thermodynamics and quantum mechanics in that the overarching goal is not to produce a formal mathematical structure. Instead, techniques are developed to help design, analyze, and interpret experiments and then to connect experimental results to the proposed mechanism. We devote the balance of the semester to a traditional treatment of classical thermodynamics. In Appendix 2 the reader will find a general outline of the course in place of further detailed descriptions. 

Lindemann mechanism, Free radical chain mechanism, Enzyme kinetics, Surface chemistry... 

This process may well be understood in terms of intramolecular rearrangement of the disilane by a free radical chain mechanism, the average kinetic chain length being about 4. 

It is interesting to calculate the approximate impurity levels of the samples for which the kinetic data are presented in Figures 3 and 4. Table III summarizes these approximations, based on Equation 3 and the rate constants given in Reference 36 for the two monomers in question. The comparison has been made on the basis of the slopes of the experimental curves between 104 and 105 rads/hour and the corresponding values of the calculated curves in the same interval. For both monomers, the values of impurity concentrations over which dramatic changes in rate constants given in Ref. 36 for the two monomers in question, ample explanation of the failure of all early studies, especially for styrene, to uncover any mechanism other than the free radical kinetics. 

Chain Mechanisms. The fact that first-order kinetics are observed for a gas-phase reaction does not prove that the unimolecular mechanism described above must be involved. Indeed very many organic decompositions that experimentally are first order have complicated free-radical chain mechanisms. 

Rather than using the halogens themselves, other halo n radical donors are more commonly used in laboratory scale synthesis. One of the simplest of these is CCU, which can chlorinate alkanes by a free radical chain mechanism.The chain lengths are not very long (equations 76-78), because of their slightly endothermic nature and in part because the reaction is also kinetically rather slow. Elevated temperatures are therefore normally required. Nitrosylchloride at 1(X) C has also been used for these reactions. ... 

Several papers have appeared in the literature in recent years showing that certain metal acetylacetonates can function as initiators for the polymerisation of vinyl and diene monomers in bulk and solution (1 - 12). Results for the kinetics of bulk and solution polymerisation are consistent with the view that the reaction occurs by a free-radical mechanism. The usual free-radical kinetics are operative, but an unusual feature is that, in some cases, certain additives such as chlorinated hydrocarbons have an activating effect upon the reaction by inducing more rapid decomposition of the initiator (2,11,12,13). Other additives which have been reported as promotors for the polymerisation include pyridlne(14) and aldehydes and ketones(15). The complexity of the reaction in the presence of such additives is evident from the fact that chloroform has been reported to be an inhibitor for the poly-merlsatlon(3). 

Radiation chemistry, and pulse radiolysis in particular, is now a mature subject that is available as a very valuable and a powerful tool by which fundamental problems in free radical reaction mechanisms can be addressed. This chapter is restricted to studies concerning sulfur-centered radicals and radical-ions performed by radiation chemistry techniques in the first eight years of XXI century (2001-2008). SuMur-centered radicals represent a very interesting class of radicals since they exhibit very interesting redox chemistry, including biological redox processes, and different spectral and kinetic properties as... 

It has been generally assumed that the polymer is formed by a free-radical chain which is initiated by H atoms or alkyl radicals. While this mechanism may be true for the mercury-photosensitized polymerization, recent results on the direct photopolymerization suggest that the two systems may be very similar and that a free-radical chain mechanism is not tenable for the latter case. It is probably most logical to examine this system by first studying the polymer that is formed, and then establishing the kinetics and mechanism of its formation. 

A number of workers (Rice et Rice and Rice Echols and Pease Steacie ° Semenov Benson Purnell and Quinn Sagert and Laidler " ) have proposed free-radical chain mechanisms for the butane pyrolysis, and most of the suggested schemes have many features in common. In the light of present knowledge of the overall kinetics and of the elementary reactions involved there seems little doubt that the following mechanism is close to the truth. 

Deduced initiation kinetics from the complex free radical chain mechanism of decomposition. Small revisions in mechanism of ref. 93 led to a revised. 4-factor. 

When the pyrolytic process does not occur in gas phase, different problems appear. Although equations of the type (6) with k expressed by rel. (5) or (14) can be used in certain cases, these may lead to incorrect results in many cases. Various empirical models were developed for describing the reaction kinetics during the pyrolysis of solid samples. Most of these models attempt to establish equations that will globally describe the kinetics of the process and fit the pyrolysis data. Several models of this type will be described in Section 3.3. A different approach can be chosen, mainly for uniform repetitive polymers. In such cases, a correct equation can be developed for the description of the reaction kinetics. This is based on the study of the steps occurring during pyrolysis involving a free radical chain mechanism. The subject will be discussed in some detail in Section 3.4. 

Thermal decomposition of uniform repetitive polymers was extensively studied in literature [17-19] in relation to the thermal stability of synthetic polymers. A kinetics equation has been developed based on the study of the steps occurring during pyrolysis involving a free radical chain mechanism [17]. For some natural polymers such as rubber, this theory is directly applicable. However, for non-repetitive polymers, or for polymers with more complex decomposition pathways, the theory does not provide appropriate kinetics equations. 

Other reported reactions involving phosphinyl radicals include the reaction of dimethylphosphine and tetramethylbiphosphine with tetrafluoro-ethylene. The gas-phase reaction of dimethylphosphine with the olefin occurs quantitatively, and a free-radical chain mechanism was postulated. Detailed kinetic analysis of the reaction indicated that a rather unusual type of initiation reaction occurs, in which, it is thought, the olefin (26) reacts as a biradical. The... 

Equation (P6.44.23) shows that the distribution function for the degree of polymerization of polymer formed by a free-radical chain mechanism, in which chain transfer is absent, depends only on the kinetic chain length and the ratio of disproportionation to coupling. The plot of vs. x according to Eq. (P6.44.23) gives the... 

Kinetic analysis [1,3-5,7,9], use of inhibitors [1-2,4-5,7,9], CIDNP experiments [16,19] and the stereochemistry [8,12-13,17-18, 30] indicate that the degradation of peracids RCO ,H into alcohols ROH takes place through a free-radical chain mechanism (cf. Schemes 1 and 2). The initiation step (reaction 1) is the thermal homolysis of the weak -O-O- bond of the peracid [32]. The two propagating steps forming the alcohol are ... 

The kinetics of the dehydrochlorination have been studied extensively, but there is no agreement on the mechanism. There is some support for a free-radical chain mechanism, although other investigators favor a unimolecular decomposition mechanism. A radical chain of any considerable kinetic length has been ruled out. 

There are two ways in which stabilizers can function to retard autoxidation and the resultant degradation of polymers. Preventive antioxidants reduce the rate of initiation, e.g., by converting hydroperoxide to nonradical products. Chain-breaking antioxidants terminate the kinetic chain by reacting with the chain-propagating free radicals. Both mechanisms are discussed and illustrated. Current studies on the role of certain organic sulfur compounds as preventive antioxidants are also described. Sulfenic acids, RSOH, from the decomposition of sulfoxides have been reported to exhibit both prooxidant effects and chain-breaking antioxidant activity in addition to their preventive antioxidant activity as peroxide decomposers. 

Scheme 3.11 Basic free-radical copolymerization mechanism, assuming terminal radical kinetics.

The free-radical chain mechanism proposed to interpret the kinetic behavior is depicted by Equations (7) (14). 

Polymers are macromolecules which are composed of smaller molecules linked by covalent bonds. In terms of the reaction kinetics, polymerizations are traditionally classified into several categories stepwise polymerization, free-radical polymerization, ionic polymerization, ring-open polymerization, and coordination polymerization or polyinsertion. Each polymerization method has a combination of requirements for reaction conditions, and they exhibit certain types of product and process features (Caneba, 1992a, 1992b Odian, 1991). Even though in principle, the FRRPP process can be implemented with a wide variety of polymerization mechanisms, its discovery and immediate implementation has occurred in conjunction with free-radical kinetics. 

Fig. 13.1 Free-radictil kinetics mechanism, which includes initiation to generate the primary radical, propagation to form the growing polymer, and termination to form the dead polymer chains...

The values of overall three-halves order and Arrhenius kinetic parameters were interpreted in terms of a free radical chain mechanism, which was subsequently supported by Benson, S.W. and Shaw, R. ( )... 

Kinetic Isotope Effect. The steady-state assumption was applied to the free radical chain mechanism, (a) (h), giving rise to the overall rate equation as... 

The kinetic isotope effect observed on the overall rates of hydrogenolytic demethylation of propylene in the presence of deuterium was successfully interpreted in terms of a free radical chain mechanism. Little differences were inferred to exist between the rates of addition of H or D- to propylene and also between those of unimolecular decomposition of the produced hot n-propyl radicals, and thus the kinetic isotope effect was ascribed mainly to the difference between the steady state concentrations of [H-] in the presence of hydrogen and the concentrations of [D ] + [H ] in the presence of deuterium. In more detail, conversion of rather inactive allyl radical by metathesis with deuterium into an active D is relatively slow. This was concluded to be the main cause of the observed kinetic isotope effect, which agrees well with the calculated... 

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