A bimolecular termination

Where B = pyridine, piperidine or 1-methylimidazole, in methylene chloride solution, but under normal conditions rapid irreversible autoxidation takes place 232) leading to the formation of the well characterised 247, 248) fi-oxo product, (TPP)Fe(IlI)—0—Fe(III) (TPP) and since the rate of oxidation decreases 249, 250) with increasing excess of axial base, B, it follows 232, 251) that a five co-ordinate species, Fe(II) (Base)TPP, is probably involved as an intermediate which can then undergo a bimolecular termination reaction with Fe(II) (Base)02TPP, followed by autoxidation. Firstly 251),... 

It is not possible to give here a detailed account of, nor to take issue with, every aspect of the interpretation which the authors give to their results. Their main conclusion is that the inverse correlation between DP and conductivity proves that the principal chain breaking reaction must be a bimolecular termination between free cations at the growing end of the chain and free anions in the solution. However, the arguments which lead to... 

Coombes and Katchalski [29] have considered a slightly more complex version of this mechanism in which a second propagation coefficient operates above a critical degree of polymerization. Katchalski et al. [30] calculated the molecular weight distribution obtained in a system following scheme (12) but also including a bimolecular termination step. Various authors have analysed more complex systems in which the initiator is a polymeric species. Thus Gold [31] has shown that initiation by a poly a-amino acid with a Poisson distribution leads to a polymeric product with an over-all Poisson distribution, and Katchalski et al. [32] demonstrated that in multichain polymers synthesized from polyfunctional initiators Poisson distributions also arise. 

Disulfonyl halides such as MI-22 to MI-26 are effective bifunctional initiators for various monomers including methacrylates, acrylates, and styrenes, because the sulfonyl halide part, as pointed out for their monofunctional versions, can induce fast initiation without a bimolecular termination reaction between the sulfonyl radicals.240-343... 

HBrOj is an intermediate in the first pathway.) Self-acceleration is limited by a bimolecular termination... 

To identify the mechanism(s) responsible for loss of surface-tethered radicals in SI-PMP, several irreversible termination reactions that can result into the permanent loss of surface-tethered radicals, including (a) bimolecular termination, (b) chain transfer to monomer, (c) chain transfer to dithiocarbamyl radical, (d) chain transfer to an adjacent polymer chain, and (e) chain transfer to solvent, were considered by Rahane et al. in their development of a kinetic model to describe SI-IMP [81]. The decrease in the concentration of surface-tethered radicals by chain transfer to monomer and by bimolecular termination reactions is captured by Equation 12.2 ... 

Comparison of the PMMA layer thicknesses measured using variable angle eUipsomehy as a function of expx>sure time with model predictions (thin lines) for (a) bimolecular termination and (b) chain transfer to monomer. Irradiation intensity is 5 mW/cm and methyl methacrylate concentrations in toluene are ( ) 1.1 ( ) 2.34, and ( ) 4.68 M. (Reprinted with permission from Rahane S. B., et at. Macromolecules, 38,8202-8210,2005, copyright (2005) American Chemical Society.)... 

Solution copolymerization of 1,3-dioxane monomers 27A--27D with MA were carried out over the temperature range of 50 90°C with different AIBN and comonomer concentrations/ " The reactivity of the monomers with MA decreased in the order 27A > 27B > 27C > 27D. For all four monomers, the copolymerization rates were maximum when equivalent amounts of monomer were present and first order with respect to the initial monomer concentrations. It was also observed that the rate of polymerization (Rp) showed a square-root dependence on the initiator concentration, suggesting the presence of a bimolecular termination step. From these facts, a simplified rate expression was derived ... 

Kinetic results were consistent with a bimolecular termination reaction whereas reaction products and mechanisms were something of a mystery. At that time it was known that the termination rate constant for autoxidation of cumene ( ) is about three orders of magnitude smaller than the termination rate constant for autoxidation of tetralin (7.). It was, however, generally accepted that the tennination rate constants for tertiary ( ) and secondary (9 ) alkylperoxy radicals are insensitive to the structure of the hydrocarbon residue in the radical. 

In previous calculations, in orda to show what kind of relationships are needed to fulfill the major condition of controlled process (instantaneous initiation) it has been assumed that the concentration of active centas was constant. However, the growing centas are maaoradicals, and, along with the process of monoma addition, termination takes place in a bimolecular termination reaction of macroradicals (fe,). The rate constant of termination, fe for the polystyryl radical is 10 mol" 1 s". ... 

A kinetic analysis of the two modes of termination is quite straightforward, since each mode of termination involves a bimolecular reaction between two radicals. Accordingly, we write the following ... 

The polymerizations (a) and (b) owe their success to what has become known as the persistent radical effect."1 Simply stated when a transient radical and a persistent radical are simultaneously generated, the cross reaction between the transient and persistent radicals will be favored over self-reaction of the transient radical. Self-reaction of the transient radicals leads to a build up in the concentration of the persistent species w hich favors cross termination with the persistent radical over homotermination. The hoinolermination reaction is thus self-suppressing. The effect can be generalized to a persistent species effect to embrace ATRP and other mechanisms mentioned in Sections 9.3 and 9.4. Many aspects of the kinetics of the processes discussed under (a) and (b) are similar,21 the difference being that (b) involves a bimolecular activation process. 

As the polymerization reaction proceeds, scosity of the system increases, retarding the translational and/ or segmental diffusion of propagating polymer radicals. Bimolecular termination reactions subsequently become diffusion controlled. A reduction in termination results in an increase in free radical population, thus providing more sites for monomer incorporation. The gel effect is assumed not to affect the propagation rate constant since a macroradical can continue to react with the smaller, more mobile monomer molecule. Thus, an increase in the overall rate of polymerization and average degree of polymerization results. 

It should be clear from Section IV. B that a major difficulty involved in preparing monomeric iron-dioxygen adducts is the prevention of bimolecular termination reactions, leading via autoxidation to the formation of a ju-oxo dimer, thus... 

We saw previously that a major factor in inhibiting the bimolecular termination reaction was the presence of sufficiently bulky ligands so that a monomeric dioxygen adduct could be isolated 135). A number of synthetic metal porphyrins 239) have been prepared recently which satisfy the above requirement, and bind molecular oxygen we shall now proceed to discuss these. 

A structural feature of the T7 DNA which is important in DNA replication is that there is a direct terminal repeat of 160 base pairs at the ends of the molecule. In order to replicate DNA near the 5 terminus, RNA primer molecules have to be removed before replication is complete. There is thus an unreplicated portion of the T7 DNA at the 5 terminus of each strand. The opposite single 3 strands on two separate DNA molecules, being complementary, can pair with these 5 strands, forming a DNA molecule twice as long as the original T7 DNA. The unreplicated portions of this end-to-end bimolecular structure are then completed through the action of... 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

If we use initiators R-R which have very high reactivities for the chain transfer reaction to the initiator and/or primary radical termination, i.e., ordinary bimolecular termination is neglected, it is expected that a polymer will be obtained with two initiator fragments at the chain ends (Eq. 7) ... 

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

These are two possibilities for the polymerization of MA deviated from the ideal living radical polymerization (i) the chain end of poly(MA) formed primary radical termination with a DC radical does not dissociate or dissociates at an unfavorable position like 41 (ii) bimolecular termination leading to the deactivation of the iniferter sites occurs preferentially to the primary radical termination with the DC radical which reproduces the iniferter site. 

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