Chain-terminating reactions

New radicals are introduced by thermolysis of the hydroperoxide by chain-branching decomposition (eq. 4). Radicals are removed from the system by chain-termination reaction(s) (eq. 5). Under steady-state conditions, the production of new radicals is in balance with the rate of radical removal by termination reactions and equation 8 appHes for the scheme of equations 1—5 where r. = rate of new radical introduction (eq. 4). 

This proposal, however, has been criticized on the basis of transition state theory (74). Hydroperoxy radicals produced in reaction 23 or 24 readily participate in chain-terminating reactions (eq. 17) and are only weak hydrogen abstractors. When they succeed in abstracting hydrogen, they generate hydrogen peroxide ... 

Kadical-addition reactions to unsaturated molecules-. Chain-termination reactions-. 

The pinacol formation reaction follows a radical mechanism. Benzopinacol, benzophenone and the mixed pinacol are formed jointly with many radical species [72, 74]. In the course of the reaction, first a high-energy excited state is generated with the aid of photons. Thereafter, this excited-state species reacts with a solvent molecule 2-propanol to give two respective radicals. The 2-propanol radical reacts with one molecule of benzophenone (in the ground state, without photon aid) to lengthen the radical chain. By combination of radicals, adducts are formed, including the desired product benzopinacol. Chain termination reactions quench the radicals by other paths. 

The rates of each of these steps must exceed competing chain termination reactions in order for good yields to be obtained. The most important competitions are between (a) the addition step k1 and reaction of the intermediate R- with Bu3SnH, and (b) between the H abstraction step k2 and addition to another molecule of the alkene. If... 

More recently, a number of different copolymer structures have been prepared from butadiene and styrene, using modified organolithiums as polymerization initiators ( 4). Organolithium initiated polymerizations have gained prominence because stereo-control is combined with excellent polymerization rates, and the absence of a chain termination reaction facilitates control of molecular weights and molecular weight distributions ( 5). 

The propagation of a a-th type block of a macroradical may be interrupted either because of the addition of monomer or owing to the loss of an active center caused by the chain termination reaction. The probabilities of these events within the interval dxa = dl/0a are equal to Vap(l)dxa and Tadra = ktaR adra, respectively. Hereafter, kla is the constant of the chain termination reaction while R a stands for the concentration of a-th type active centers in the surface layer of the a-th phase. Function wa(r ), having the sense of the probability for a a-th type terminal block of a macroradical to attain length rj, reads as... 

Interestingly enough, quantity Ha (Eq. 84) has a rather transparent probabilistic meaning. In fact, the growth of the terminal a-th type block of a macroradical may be over either by the transition of an active center into another phase, or by its vanishing due to the chain termination reaction. The probabilities of these events, coinciding with the probabilities that a block chosen at random will be either internal or external, are equal to Ha and 1 -Ha, respectively. 

In contrast to the failure to metathesize terminal olefins, internal olefins such as cis-2-pentene can be metathesized to the extent of 50 turnovers. The chain terminating reaction in this case is rearrangement of intermediate ethylidene and propylidene complexes (equation 4). Both rearrangement of intermediate trisub-... 

Typically, a chain reaction involves a number of steps which, depending on their role in the overall chain process, are classified as chain initiation, chain propagation, and chain termination reactions. 

Another important characteristic of inhibitors is the time of their inhibition action. If an inhibitor is consumed only in chain termination reactions, this time is determined by the initial concentration [InH]0, stoichiometric coefficient of inhibition / and Vj. In this case, the rate of inhibitor consumption is vInH = v //. Side reactions of InH with dioxygen and hydroperoxide shorten the inhibitory period and increase the rate of inhibitor consumption. Therefore, an inhibitor is efficient when it provides a minimal chain length v and its own loss in side reactions w is low. Assuming that an efficient inhibitor has w < 0.25, we get the inequality 4k 2[InH][02] < v which can be transformed, by substituting the correlation equation from Table 14.7, into the following equation... 

This second molecule might be a monomer, polymer, or solvent. Because of chain transfer the end of one polymer chain might be a hydrogen atom, and the beginning of the next the radical formed by removing the hydrogen atom from the solvent molecule. In the same paper, he proposed the two most probable chain termination reactions, mutual combination and disproportionation. 

The mechanistic issues to be discussed are the initiation modes of the reaction, the propagation mechanism, the perfect alternation of the polymerisation reaction, chain termination reactions, and the combined result of initiation and termination as a process of chain transfer. Where appropriate, the regio- and stereoselectivity should be discussed as well. A complete mechanistic picture cannot be given without a detailed study of the kinetics. The material published so far on the kinetics comprises only work carried out at temperatures of -82 to 25 °C, which is well below the temperature of the catalytic process. 

The common chain termination reactions comprise (3-hydrogen elimination... 

They conclude that, at the low-temperature end of the effective temperature window, the NO reduction effectiveness is limited principally by the rates of the chain-termination reactions that compete with the preceding branching sequence. In addition, below about 1100K, hydrogen abstraction by OH is so... 

The measured G(x) value of representative epoxy polymers is approximately 10, but this value depends strongly on the structure of the polymer, its glass transition temperature and other characteristics. Since the crosslinking reaction that characterizes the COP resist functionality is a chain reaction, in theory, a single, electron-initiated event could result in the insolublization of an entire film of the resist material. Fortunately, because of the existence of chain terminating reactions, this does not occur and high resolution imaging of the resist material can be accomplished. 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

H +C>2 - OH+ O, so called because the disappearance of one chain carrier leads to the appearance of two. If chain carriers are produced at a rate faster than they are removed (by chain-breaking or chain-terminating reactions), a branching-chain explosion can occur without any preliminary temperature rise at all (hence "isothermal )... 

It should be recognized from the results that cationic polymerizations are usually initiated at low temperatures in order to suppress chain-terminating reactions and also to keep the reaction from becoming explosive in nature. These low temperatures thus favor high molecular weight polymer formation. 

A reaction of chain transfer to alcohol or other proton-donor compounds is the most common process among the chain termination reactions in the anionic polymerization of the epoxy compounds in proton media, including the action of TA ... 

---