Chain-reaction mechanism dimer

In this article, the features and mechanism of the crystal-to-crystal reactions of 1,3-diene compounds are described on the basis of the molecular packing structure and intermolecular interactions in the crystals for starting materials and products. The dimerization and isomerization of unsaturated compounds as well as addition polymerization via a chain reaction mechanism are ideal sohd-state reactions, because they produce no leaving group during the reac-... 

Termination Ph-CO-OO- + Ph-CO-OO- —> dimeric peroxy compounds Scheme 2.2 Chain Reaction Mechanism for Benzaldhyde Degradation. 

In general, the oxidative pol5mierization of phenols imdergoes a stepwise growth mechanism, although the electrochemical oxidative polymerization has been suggested to be a chain reaction mechanism (34). However, phenolic dimers... 

The mechanism of ion polymerization in formaldehyde crystals proposed by Basilevskii et al. [1982] rests on Semenov s [1960] assumption that solid-phase chain reactions are possible when the arrangement of the reactants in the crystal prepares the configuration of the future chain. The monomer crystals capable of low-temperature polymerization fulfill this condition. In the initial equilibrium state the monomer molecules are located in the lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary dimer cation, the active center shifts to the intersite, and the barrier for the addition of the next link... 

Scheme 22 Mechanism of radical chain reactions of the growth of styrene line along the edge of a dimer (left side) and of the growth of allyl mercaptan line across the dimer rows (right side) of a H-Si(l 0 0)-2 x 1 surface.

Since glutathione is synthesized in cells in relatively huge amounts, it is seldom applied as pharmacological antioxidant. Furthermore, the mechanism of its antioxidant activity is not so simple as that of vitamins E and C. The major reason is that the GS radical formed during scavenging of free radicals by GSH does not disappear by dimerization but participates in the chain reaction, producing superoxide (Reactions (20)-(23)). Furthermore, it has recently been shown that contrary to previous findings the rate constant for the reaction of GSH with superoxide is relatively small (200-1000 lmol-1 s-1) [211,223],... 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

The chemical reaction mechanism of electropolymerization can be described as follows. The first step in course of the oxidative electropolymerization is the formation of cation radicals. The further fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable case the next step is a dimerization reaction, and then stepwise chain growth proceeds via association of radical ions (RR-route) or that of cation radical with a neutral monomer (RS-route). There might even be parallel dimerization reactions leading to different products or to the polymer of a disordered structure. The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction... 

However, this mechanism does not explain the chain reaction. Tabata and coworkers measured the optical spectrum of the dimer cation radical, by pulse radiolysis of benzonitrile solution of the dimer immediately after the pulse. They found only a peak at 770 nm without other peaks, except for a possible small shoulder at 740 nm (which is within the limit of experimental error). Addition of cation scavengers leads to elimination of this spectrum, while oxygen does not remove it, suggesting that the spectrum is due to a cation. This 770-nm peak of the cation of the cyclodimer of VC reminds one of the 770-nm peak found 1.6 jus after the pulse in the case of 1 M VC solution. It should be noticed that while in this second paper the authors also mentioned this shift from 790 nm to 770 nm, the data in their figure show a peak at 790 nm both immediately and 1.6 jus after the pulse. Consequently, Tabata and coworkers suggested that the observed spectrum in pulse radiolysis of aerated solution of VC in benzonitrile is a composite of the spectrum of VC cation together with that of the cation of the cyclodimer of VC. The contribution of each intermediate to the observed spectrum depends on the concentration of VC and how long after the pulse the spectrum was taken. In a dilute solution, the dimer cation will be produced as time proceeds, but it is absent immediately after the pulse. In concentrated solutions, both cations coexist even immediately after a pulse. 

Another example of oligomers formed by condensation of nucleus with side chain is the dimer (90) formed when 2,2,6,6-tetramethyl-4-oxopiperidin-l-oxyl (91) is set aside at room temperature for six months 112 the mechanism for the formation of 90 has been assumed112 to be hydrogen abstraction, followed by the coupling reaction of the A7-oxyl radical with the C-radical derived from 91. 

It seems that formation of dimers and trimers from soybean oil is the slow step of polymerization, an evidence for step polymerization mechanism. Once formed, these dimers and trimers polymerize very quickly to a high molecular weight and exhibits chain reaction kinetics. This phenomenon is caused by the soybean oil molecule itself, a large molecule with low activity, due to mid chain double bond location. However, the formed dimers and trimers have more unsaturated carbon-carbon double bonds per molecule, and they are easily polymerized to high molecular weight polymers. We propose further investigation on the possible combination of two polymerization mechanisms for the polymerization of soybean oil in SCCO2. Kinetic study of soybean oil polymerization currently is carried out in our laboratory. 

Sieger and Calvert originally favoured the latter of these as the predominant primary step, but the more recent study of the photolysis by Dawidowicz and Patrick indicates the former as the predominant step, in view of the identification of biacetyl among the products. Some complications in the mechanism are evident from the presence also of acetone, 2,2-difluoropropane, and a dimer of trifluoroacetone in the reaction products. The possibility of a chain reaction at high temperature has been discussed ... 

Common Reaction Types and Preparative Aspects A. Addition reactions Substitution reactions Dimerization-addition reactions Dimerization-elimination reactions Cleavages Chain reactions Indirect oxidations Voltammetry and Studies of Kinetics and Mechanisms A. Aromatic hydrocarbons Arylalkenes... 

The reactions with formation of polymers also are classified based on another difference in their mechanism. This classification distinguishes step reactions and chain reactions. In step reactions the polymers are built from the monomer by random individual reactions to form dimers, trimers, tetramers, etc., each resulting molecule being able to participate in a subsequent reaction with a monomer or with an oligomer molecule. This type of reaction may start with molecules having two reactive functional groups in one molecule such as an o-aminocarboxylic acid. Another possibility consists of reactions between two different types of bifunctional molecules such as a diamine and a dicarboxylic acid as shown below ... 

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