Bimolecular initiating systems

Decompositions of peroxides into initiating radicals are possible through bimolecular reactions involving electron transfer mechanisms. These reactions are often called redox initiations and can be illustrated as follows  

The above can be shown on a decomposition of a persulfate (an inorganic peroxide) by the ferrous ion  

Side reactions are possible in the presence of sufficient quantities of reducing ions  

A redox reaction can also take place between the peroxide and an electron acceptor R-O-O-H + Ce R-CM3 + Ce + H  

Side reactions with an excess of the ceric ion can occur as well  

A redox reaction can also take place between the peroxide and an electron acceptor  

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Thioxanthiones (TX) absorb strongly in the near-ultraviolet region of the spectrum, and their reaction with amines comprises a bimolecular initiator system competitive with photodissociative initiators [138 150. The photochemistry and photophysics of TX in the presence of amines are similar to that observed for other aromatic ketones, and can be summarized by the scheme proposed by Davidson [148] (Scheme 17). 

A bimolecular initiating system, based in 2,2 -azobisisobutyronitrile was reported by Michl and co workers [26]. It consists of weakly solvated lithium in combination with the cyanopropyl radical (from AIBN). The combination can initiate polymerizations of olefins. The reaction was illustrated as follows ... 

The early history of redox initiation has been described by Bacon.23 The subject has also been reviewed by Misra and Bajpai,207 Bamford298 and Sarac.2,0 The mechanism of redox initiation is usually bimolecular and involves a single electron transfer as the essential feature of the mechanism that distinguishes it from other initiation processes. Redox initiation systems are in common use when initiation is required at or below ambient temperature and drey are frequently used for initiation of emulsion polymerization. 

Using the same toluene-benzoyl peroxide system Nakatsuka (105) measured polymerization rate and molecular weight as functions of temperature (40° and 58°) and of the concentration of.three retarders p-nitrophenol, 2,4-dinitrophenol and picric acid. Results were consistent with a kinetic scheme postulating (among other things) bimolecular initiation involving peroxide and monomer and spontaneous unimolecular termination of growing polymer chains. 

Radical-induced decomposition is thermodynamically favorable (Ea = 37.5 kCal), and is also more consistent with the characteristics of bimolecular initiation by hydroperoxides originally proposed by Russell (356), the kinetics measured in lipid oxidation systems, and significant epoxide products reported in many studies. Most importantly, the radical-induced decomposition described in Reaction 63 provides a powerful cascade of reactive radicals to fuel the very rapid increase in oxidation during the bimolecular rate period. 

Alternative bimolecular methods have been reported that involve mixing appropriate ratios of monomer with free-radical initiators (such as benzoyl peroxide) and an excess of the nitroxide stable free-radical moiety. Such bimolecular methods do not afford the same degree of control of molecular weight and polydispersity since the stoichiometry of the mediating system cannot be accurately dehned, which is a crucial factor in these controlled polymerization systems. A wide variety of unimolecular nitroxide based initiator systems have been described in the literature with those based upon the 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) group proving to be the most commonly used. 

Coming back to the a(C-C)/(3(C-H) primary split ratio (Table 3), it would be valuable to compare these values with that obtained either in the thermal pyrolysis of propene or in chemical activated systems. For example, in shock tube experiments (1650-2300 K), the dominant bimolecular initiation reaction leads to the C-C bond rupture, although a possible contribution of the 3(C-H) bond rupture cannot be excluded (50). This is also observed in the decomposition of hot propene formed from ethylcarbene f(E)(C3H ) s 414 kJ/mol] a(C-C)/ P(C-H) = 22 (51). Conversely, hot propene formed by the addition of singlet methylene to ethylene [(EXCsH ) s 464—492 kJ/mol) gives rise to C-H bond... 

This computer program calculates the kinetic parameters of elementary reactions (bimolecular initiations, addition of a free radical to an unsaturated molecule, unimolecular decomposition, cyclization, oxidation of a radical, metathesis and branching) using structure-reactivity correlations. It is part of the EXGAS system. Information cf EXGAS. 

Nevertheless, cobaltous ions form efficient redox initiating systems with peroxydisulfate ions. " Tertiary aromatic amines also participate in bimolecular reaction with organic peroxides. One of the unpaired electrons on the nitrogen atom transfers to the peroxide link, inducing decomposition. No nitrogen, however, is found in the polymer. It is therefore not a true redox type initiation and the amine acts more like a promoter of the decomposition. Two mechanisms were proposed to explain this reaction. The first one was offered by Homer et al... 

Bimolecular Photoinitiator Systems. Bimoiecular photoinitiators are so-called because two molecular species are needed to form the propagating radical a photoinitiator that absorbs the light and a co-initiator that serves as a hydrogen or electron donor. Photoinitiator families include benzophenone derivatives, thioxanthones, camphorquinones, benzyls, and ketocoumarins (5-9) (3). 

Norrish type II photoinitiators are bimolecular initiators. Generally an aromatic ketone is used in combination with a tertiary amine. Both aliphatic and aromatic tertiary amines can be used. A well-known example of such an initiating system is benzophenone with dimethylaminoethanol. 

As for other Type II initiating systems, quenching by the monomer has to be taken into account, provided monomers with low triplet energies are used. Thus, the bimolecular rate constants of the reaction of various thioxanthones with styrene are between 3 X 10 and 6 X 10 L mol sec . For acrylonitrile, for example, values in the range between 4 X 10 and 4 X 10 L mol" sec are found, indicating very little quenching [64]. 

Emulsion polymerization has proved more difficult. N " Many of the issues discussed under NMP (Section 9.3.6.6) also apply to ATRP in emulsion. The system is made more complex by both activation and deactivation steps being bimolecular. There is both an activator (Mtn) and a deactivator (ML 1) that may partition into the aqueous phase, although the deactivator is generally more water-soluble than the activator because of its higher oxidation state. Like NMP, successful emulsion ATRP requires conditions where there is no discrete monomer droplet phase and a mechanism to remove excess deactivator built up in the particle phase as a consequence of the persistent radical effect.210 214 Reverse ATRP (Section 9.4,1,2) with water soluble dialky 1 diazcncs is the preferred initiation method/87,28 ... 

The isomerization of A to B yielded kinetic data that conformed to a first-order rate law. but the apparent first-order rate constant depended on the initial concentration of A. The authors propose competing unimolecular and bimolecular processes, and they show that the system reduces to a first-order expression when the equilibrium constant K is unity that is,... 

In thermal polymerization where the rate of initiation may also vary with composition, an abnormal cross initiation rate may introduce a further contribution to nonadditive behavior. The only system investigated quantitatively is styrene-methyl methacrylate, rates of thermal copolymerization of which were measured by Walling. The rate ratios appearing in Eq. (26) are known for this system from studies on the individual monomers, from copolymer composition studies, and from the copolymerization rate at fixed initiation rate. Hence a single measurement of the thermal copolymerization rate yields a value for Ri. Knowing hm and ki22 from the thermal initiation rates for either monomer alone (Chap. IV), the bimolecular cross initiation rate constant kii2 may be calculated. At 60°C it was found to be 2.8 times that... 

Co(NH3)5OH] + + + Brit may be regarded as bimolecular and irreversible. Determine the ratio of the reaction rate in a system initially containing 10 moles/m3 [Co(NH3)5Br](N03)2, 100 moles/m3 NH4OH,... 

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

It can be observed that the initial rate of polymerization decreases and the autoacceleration peak is suppressed as the TED concentration is increased. The TED molecules generate dithiocarbamyl (DTC) radicals upon initiation. As a result, termination may occur by carbon-carbon combination which leads to a dead polymer and by carbon-DTC radical reaction which produces a reinitiatable ( living ) polymer. The cross-termination of carbon-DTC radicals occurs early in the reaction (with the carbon-carbon radical termination), and this feature is observed by the suppression of the initial rate of polymerization. As the conversion increases, the viscosity of the system poses mass transfer limitations to the bimolecular termination of carbon radicals. As has been observed in Figure 3, this effect results in a decrease in the ktCC. However, as the DTC radicals are small and mobile, the crosstermination does not become diffusion limited, i.e., the kinetic constant for termination of carbon-DTC radicals, ktCS, does not decrease. Therefore, the crosstermination becomes the dominant reaction pathway. This leads to a suppression of the autoacceleration peak as the carbon-DTC radical termination limits the carbon radical concentration to a low value, thus limiting the rate of polymerization. This observation is in accordance with results of previous studies (10) with XDT and TED, where it was found that when there was an excess of DTC radicals, the carbon radical concentration was lower and the cross-termination reaction was the dominant termination pathway. 

Photosensitization of diaryliodonium salts by anthracene occurs by a photoredox reaction in which an electron is transferred from an excited singlet or triplet state of the anthracene to the diaryliodonium initiator.13"15,17 The lifetimes of the anthracene singlet and triplet states are on the order of nanoseconds and microseconds respectively, and the bimolecular electron transfer reactions between the anthracene and the initiator are limited by the rate of diffusion of reactants, which in turn depends upon the system viscosity. In this contribution, we have studied the effects of viscosity on the rate of the photosensitization reaction of diaryliodonium salts by anthracene. Using steady-state fluorescence spectroscopy, we have characterized the photosensitization rate in propanol/glycerol solutions of varying viscosities. The results were analyzed using numerical solutions of the photophysical kinetic equations in conjunction with the mathematical relationships provided by the Smoluchowski16 theory for the rate constants of the diffusion-controlled bimolecular reactions. 

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