Styrene overall mechanism

Based on the above reactions, an overall mechanism for the hydrogenation of NBR catalyzed by Wilkinson s catalyst was proposed (see Scheme 19.4), which is also applicable to the kinetic performance of the homogeneous hydrogenation of PB [88] and styrene-butadiene copolymers [89], where K2 and Ks vanish. 

The conversion of ethylbenzene to styrene is a highly. endothermic reaction which obeys the following overall mechanism ... 

NMP was first viable for styrene and its substituted derivatives [211], but was extended to acrylates, acrylamides, and some other vinyl monomers [212], Hawker et al. [7] reviewed the overall mechanism of NMP as well as several nitroxides and their reactivity. 

For the styrene (Sty)-vinyltriethoxysilane (VTES)-MA system, it was pointed out that didonor-acceptor combinations may polymerize via both a CTC-donor and/or CTC-CTC mechanism, depending on the initial monomer feeds.Studying the Sty-VTES-MA reaction, from both perspectives, it was concluded that the experimental data explained satisfactorily some of the assumptions in copolymerizations involving charge-transfer complexes. However, the data did not provide convincing proof that CTC formation preceded polymerization and whether all of the monomer molecules add to the growing chain in the form of a CTC. Also, the data did not provide the possibility of judging the effect of CTC formation on the overall mechanism of free-radical copolymerization. 

Equation (1.20) is frequently used to correlate data from complex reactions. Complex reactions can give rise to rate expressions that have the form of Equation (1.20), but with fractional or even negative exponents. Complex reactions with observed orders of 1/2 or 3/2 can be explained theoretically based on mechanisms discussed in Chapter 2. Negative orders arise when a compound retards a reaction—say, by competing for active sites in a heterogeneously catalyzed reaction—or when the reaction is reversible. Observed reaction orders above 3 are occasionally reported. An example is the reaction of styrene with nitric acid, where an overall order of 4 has been observed. The likely explanation is that the acid serves both as a catalyst and as a reactant. The reaction is far from elementary. 

Figure 26 Overall anionic chain polymerisation mechanism of styrene initiated by n-butyllithium.

Both Odian and Silverman models satisfactorily explain most of the observed results in all solvents (Tables I-III, VI) used in the present study, however there are some exceptions especially when solvents other than the alcohols are used (Table VI). Thus the Odian mechanism is not consistent with the DMF data nor can the Silverman model account for the acetone results. In addition, in further preliminary studies with grafting of styrene to polyethylene (10) in solvents other than those reported here both Odian and Silverman mechanisms are deficient. The problem is that possible contributions from the radiation chemistry of the components in the grafting reaction need to be considered in formulating a complete mechanism for the overall process. 

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

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. 

In the presence of sonication there is an overall increase in conversion with increase potential, whilst the opposite is true for copolymerisation performed in silent conditions. The lower degree of conversion at the lower potential is not without precedent, since simple mechanical stirring retards styrene polymerisation [82]. 

The above process, Eq. (25), is in conflict with the currently available theoretical results (Table 5) regarding the dissociation enthalpies of aggregated organolithiums. A similar conclusion was reached by Brown in 1966 70>. This assessment is fortified by the fact that the measured 941 energy of activation for the reaction of styrene with n-butyllithium, 18 kcal/mole, is a value far lower than that required if the calculated dissociation enthalpy of the n-butyllithium aggregates is included in the overall energetics of the initiation event. Thus, it would seem that any mechanism which involves only unassociated organolithiums as reactive entities is invalid. 

Intramolecular protonation on the more hindered face of a steroid from a neighbouring hydroxyl group best explains a reversal of diastereoselectivity in the Birch reduction of styrene double bonds.266 The kinetics and product distribution of lithium metal reduction of benzaldehyde to benzyl alcohol in THF have been studied electron transfer from Li to PhCHO occurs in a slow step, but absorption of the PhCHO onto the metal surface is also crucial in determining the overall kinetics. The proposed mechanism successfully accounts for the formation of minor products, benzoin and... 

Overall, the transformations are equivalent to carbene additions to the styrenes. However, a carbene mechanism can be ruled out since the only alkenes which are successful are those carrying anion-stabilising groups. 

To begin, let s consider the anionic polymerization of styrene. For an initiator, we will choose an organometallic compound an organic compound bonded to a metal atom) such as butyllithium, C4H9 Li+. Although the details differ, you should recognize the overall similarity of the mechanism for this anionic polymerization to that for the free radical polymerization of ethylene, above (initiation, propagation, and termination). 

While ionomers of many types have been made and characterized [1,2,3], there is little work on the overall relaxation mechanisms. For polymers with low ionic concentrations, there is general agreement on the fundamental relaxation step. The stress relaxes by detachment of an ion pair from one cluster and reattachment to another. For the styrene/methacrylic acid Na salt (ST/-MAA-Na) system, there is a secondary plateau in the relaxation modulus which depends on the ionic content and can be described as a rubbery modulus [4], While a rubbery modulus with stress relaxation due to ionic interchange has been invoked earlier, it does not adequately describe the relaxation curves. A different approach is taken here. 

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