Kinetics of initiation

The initiation process may involve either a single or a few consecutive elementary reactions. In the latter case, the understanding of this process requires knowledge of afl these steps their chemistry and rate constants. NMR spectroscopy proved to be the experimental method of choice in kinetic and mechanistic studies of initiation. However, UV spectroscopy, polarography, conductivity, and the tracer method were also applied. Because of the complexity of initiation, the kinetic studies were performed only for a limited number of systems. The rate constants of the elementary reactions were only determined for a few simpler systems. 

measuring the concentration of initiator as a function of time, the rate constant of initiation can be determined. In a few systems, when the equilibrium monomer concentration is relatively high (THF, DXL), initiation has been studied under conditions at which propagation is excluded ([M]o lM]e). 

For example, when ethyl triflate (C2HSOSO2CF3) is reacted with THF in CD3NO2 solvent at 25 °C at (IJo = (M)o = 2.0 mole I -, only the formation of the ethyltetrahydrofuranium ion is observed without any subsequent propagation  

Reactions of 1,3-dioxolenium salts with THF and DXL were studied in a similar way. It was found from the NMR spectra that initiation involves attack at C-4 of the salt and opening of the 1,3-dioxolenium ring with the formation of formic acid derivatives (cf. discussion of the ambi-dent reactmty. p. 21)  

The initiation rate constant kj is obtained from the slope of the plot of the left- 

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The kinetics of initiation reactions of alkyllithium compounds often exhibit fractional kinetic order dependence on the total concentration of initiator as shown in Table 2. For example, the kinetics of the initiation reaction of //-butyUithium with styrene monomer in benzene exhibit a first-order dependence on styrene concentration and a one-sixth order dependence on //-butyUithium concentration as shown in equation 13, where is the rate constant for... 

The byproducts of decomposition of certain dialkyldiazcncs can be a concern. Consider the case of AIBN decomposition (Scheme 3.13). The major byproduct is the ketenimine (lO).61 100"102 This compound is itself thermally labile and reverts to cyanoisopropyl radicals at a rate constant similar lo that for AIBN thermolysis.59,60 102 This complicates any analysis of the kinetics of initiation/2,60... 

Thus alkyl radicals do not give unwanted end-group functionality and the kinetics of initiation arc comparatively uncomplicated. However, this situation can be perturbed by substitution at or near the radical center. 

Ideally, the metal complex is a catalyst and, in principle, is only required in very small quantities. However, the kinetics of initiation for the systems described to date dictate that relatively large amounts are used and catalyst initiator ratios are typically in the range 1 1 to 1 10. The most commonly used catalysts are metal... 

Empirical Kinetics of Initiation and Polymerization from Tafel... 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

Compelling evidence suggesting that the breakdown of hydroperoxyl groups is not related to polymer destruction, at least in the initial period of oxidation at temperatures below 400 K, comes from experiments on the initiated oxidation of polymers. It was found that the destruction of polymers develops in parallel with their oxidation from the very onset of the process, but not after a delay related to the accumulation of a sufficient amount of hydroperoxyl groups [129]. These experiments also demonstrated that it is free macroradicals that undergo destruction. Oxidation of polymers gives rise to alkyl, alkoxyl, and peroxyl macroradicals. Which radicals undergo destruction can be decided based on the kinetics of initiated destructive oxidation. 

The ability to conduct radical reactions without the use of tin reagents is important. Allylic triflones have been used to conduct allylation reactions on a range of substrates (39) as a replacement for allyltributylstannane (Scheme 28). The main limitation was that unactivated or trisubstituted triflones failed to undergo reactions. In other nontin radical methods, arenesulfonyl halides have been used as functional initiators in the CuCl/4,4 -dinonyl-2, 2 -bipyridine-catalysed living atom-transfer polymerization of styrenes, methacrylates, and acrylates.The kinetics of initiation and propagation were examined with a range of substituted arylsulfonyl halides with initiator efficiency measured at 100%. 

Data on kinetics of initiation are available only for initiation of BPL polymerization with triphenyl-phosphine and methyl,n-propyl,phenylphosphine [16]. 

Carbocation-oxonium ion equilibria are obvious complicating factors in studies of the kinetics of initiation of polymerisation and useful thermodynamic data for such equilibria involving Ph3C+ and a variety of linear and cyclic ethers have been reported by Slomkowski and Penczek (132). A dramatic increase in rates of initiation of polymerisation of THF induced by Ph3C+ salts is observed on addition of small amounts of epoxides such as propylene oxide (113a,b), which compete favourably with THF in the primary carbocation-oxonium ion equilibria and simplify the initiation reaction ... 

The kinetics of initiation reactions of alkyllithium compounds often exhibit fractional kinetic order dependence on the total concentration of initiator, consistent with initiation by the unassociaied form or the alkyllithium. 

Trityl salts have often been used to study the kinetics of initiation by following the disappearance of the salt and the monomer by UV spectroscopy. For example, trityl salts absorb strongly (e = 40,000 M l-cm-1) at 410-440 nm, in contrast to either their adducts or triphenylmethane. 

Kinetics of initiated radical polymerization of styrene is well studied at the initial stages. At the high-conversion of polymerization the growth of viscosity of media- influences the mechanism and kinetics of process greatly (12),In our research we have attempted to obtain tne empirical dependence of some rate constants of initiated polymerization of styrene on conversion with the aim of using them at mathematical simulation of the process. 

If the dielectric constant of the medium is taken as a measure of its ability to solvate and thus stabilize ionic or polarised species, one can broadly classify the solvents relevant to this context into three classes non-polar solvents (e = 2—6), solvents of modest polarity (e = 7-15) and polar solvents (e > 15). Table 5 gives examples for each class with the value of the dielectric constant at room temperature. The polarity of the medium can drastically influence both the kinetics of initiation and the position of the equilibria among the various species formed therefrom. One often encounters limiting situations vdiete a given monomer-catalyst system will yield no polymerisation in a... 

Lorimer and Pepper have recently reported a study of the polymerisation of styrene by perchloric acid in methylene chloride at low temperature. Althou the stop-flow apparatus they used was not very fast, some rough ideas on the kinetics of initiation could be obtained. Assuming that the initial increase in absorption at 340 nm was due to the protonation of styrene, unhampered by side reactions (esterification), the approximate rate law observed at —80 °C,... 

The study of the polymerisation of cyclopentadiene by stable carbenium salts has been the exclusive domain of S walt and his group. In 1967, they reported the first observations on a system involving trityl hexachloroantimonate in methylene chloride between — 70 and 20°C Because of the interesting features of this polymerisation, the work was pursued by kinetic and mechanistic investigations. In I969 it was shown that initiation took place by direct addition and that transfer and termination were imimpor-tant particularly in the first stipes of the process. The kinetics of initiation were followed by visible spectroscopy and provided furthw evidence for a one-to-one reaction. Thus,... 

The recent explosion of interest in cationic research does not, however, stem from further refinements in the clean handling of sensitive materials. Wistfully, in this respect, the authors record a steady decline in standards. As a result, spectacular improvements due to modern spectroscopic stopped-flow methods, and to instrumental advances in unravelling chemical mechanisms generally, have perhaps been slower than elsewiiere to emerge in studies of cationic polymerisation mechanism. Only in 1976 do we find — in the work of Kunitake and Takarabe — the first thorough examination of the kinetics of initiation of a dimerisation ( ) based on spectroscopic stopped-flow measurements. 

In vinyl polymerization, the kinetics of initiation was studied by Sigwalt and Vairon for cyclopentadiene 4) and p-methoxystyrene using the spectrophoto-metric method. 

With a decreasing content of monomer in the polymer phase, k2 proved to decrease while F grew. This agrees with the known data on the influence of the viscosity of the medium on the kinetics of initiator decay. 

Bryers, J.D., and Characklis, W.G., 1981, Kinetics of initial biofilm formation within a turbulent flow stream, in Somerscales, E.F.C. and Knudsen, J.G. eds. Fouling of Heat Transfer Equipment. Hemisphere Pub. Corp. 313 -333. 

The kinetics of melt mechanical degradation have been discussed along with the kinetics of initiation under reactive processing conditions [Goldberg and Zaikov, 1987]. The maximum initiation rate is heating-rate dependent and for optimum results this must accord with the material residence time. [Turcsanyi, 1993]. A theoretical model on the effects of thermal processing of blends was proposed by Loev et al. [1989]. 

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