Free radical termination

North, A. M., The Influence of Chain Structure on the Free Radical Termination Reaction, Chap. 5 in Reactivity, Mechanism and Structure in Polymer Chemistry, A. D. Jenkins and A. Ledwith, eds., Wiley-Interscience, New York, 1974. 

The termination kinetic constant exhibits a somewhat more complex behavior. From the onset of reaction, termination is diffusion controlled (segmental diffusion controlled). The diffusion of the macroradicals is the controlling step and the primary means of free radical termination. At some later conversion, the termination mechanism changes from segmental to reaction diffusion control. In this region, a plateau in k, occurs. Reaction diffusion is a propagation controlled... 

The polymerization can terminate when the free-radical terminal C of a long chain forms a bond with the... 

It is reasonable to assume that the H-- and -OH-adducts at C(5) of C bind to C(8) of the neighboring G, and the resulting adduct radical is oxidized in subsequent free-radical termination reactions. While the H--adduct product (on the right)... 

In a free-radical chain reaction, initiation steps generally create new free radicals. Propagation steps usually combine a free radical and a reactant to give a product and another free radical.Termination steps generally decrease the number of free radicals. 

The polymerization can terminate when the free-radical terminal C of a long chain forms a bond with the terminal C of another long chain (combination), RC- + CR —- RC—CR. Termination may also occur when the terminal free-radical C s of two long chains disproportionate, in a sort of auto-redox reaction. One C picks off an H from the C of the other chain, to give an alkane at one chain end and an alkene group at the other chain end ... 

Reversible addition-fragmentation chain transfer (RAFT) polymerization using 2,2 -azobisisobutyronitrile and either A, A-dimethyl-5-thiobenzoylthiopropionamide or A-dimethyl-5-thiobenzoylthioacetamide as chain transfer agents has been used to prepare low polydispersity poly(A, A-dimethylacrylamide). The chain transfer agents were unusually effective in suppressing free radical termination reaction, thereby mimicking a living polymerization reaction. 

Radiation Ionization of monomer Difunctional ion radicals and difunctional free radicals Termination of growing chains, of which mechanisms depends on the concentration of impurities... 

Free radicals terminate to form nonradical products by four major mechanisms ... 

Oxidative polymers are formed in autoxidation when the free radicals terminate each other as under autoxidation. When a triacylglycerol molecule breaks down during autoxidation, the partial triacylglycerol molecules are not removed in the deodorization process and can react with each other, forming dimers, trimers, or polymers. 

This reaction is a termination and limits the molecular weight of the final molecule. In solution or melt polymerization the level of free radical concentration must for this reason not be raised over 10 to 10 M to 3deld reasonably high molecular weight (5). In case a ci3 tal surface would contain higher numbers of mobile free radicals, termination would be unavoidable. Also, crystal nucleation by active intermediate oligomers, as is observed in crystallization during step reaction polymerization, would most likely lead to termination. 

Sunlight, especially a small portion of UV light, is the principal instigator of weathering reactions. The immediate consequence of the interaction of wood with light is the generation of free radicals at the exposed surface (7, 19). As these labile free radicals terminate and stabilize, chromophoric and auxochromic groups are formed and discoloration and deterioration occur. 

Hong et al. (1987a) have mathematically analyzed the reaction system involving the generation of radicals on the surface of either Fe203 or Ti02, diffusion of SO5 to the bulk phase, and a free-radical chain reaction according to Eqs. 89-96. An additional free-radical termination step involves the surficial reaction of SO as follows ... 

Radical termination. Free-radical termination reactions are very fast reactions. The combination of reaction speed and the small reactor volume (i.e., the polymer particle) alters the kinetic model in some cases. The simplest model (Smith-Ewart Case 2) is based on the assumption that instant termination occurs when a free radical enters a particle that already contains an active radical. As the particles become larger and/or the radical mobility decreases because of the gel effect, the termination rate becomes slower. 

Phenolic antioxidants function as free radical terminators and in some cases as ntctal chelators. Phenolic acids particularly have been identified as potent antioxidants. Calfeic acid and its esters are good examples of phenolic antioxidants (2),... 

The most distinctive characteristics of NTP as a chemical process will be its capability to induce various chemical reactions at atmospheric pressure and room temperature. These moderate operation conditions enable a rapid start-up of the NTP process by turning on a switch, and vice versa. Since the NTPs use oxygen and water vapor to produce reactive radicals (for example OH, 0( D). 0( P). N, etc.). O3, and excited molecules (Nj ) for the chemical reactions, additional chemicals for oxidants are not necessary. Chemical reactions involving free radicals terminate in short time (usually less than 10 seconds). In most cases the influence of gas residence time on the gas removal efficiency is negligible. This rapid reaction also enables us to make the system compact. One of the important features of NTP is its flexibility in combining NTP with other technology, which is the main topic in this chapter. 

Molecular sulfur consists of stable eight-membered rings at low temperatures, but it is known to undergo homolytic cleavage in the melt with generation of free-radical terminated sulfur chains (I). In their reactivity toward tetrafluoroethylene, the radicals present in sulfur at 150°C or higher resemble thiyl radicals such as CHaS and CFaS (2), so that thermally initiated addition of sulfur to tetrafluoroethylene is accomplished easily. Under static conditions the reaction is carried out best... 

The polymerization rate is directly proportional to the monomer concentration for ideal free radical polymerization kinetics. Deviations from this first-order kinetics can be caused by a whole series of effects which must be checked by separate kinetic experiments. These effects include cage effects during initiator free radical formation, solvation of or complex formation by the initiator free radicals, termination of the kinetic chain by primary free radicals, diffusion controlled termination reactions, and transfer reactions with reduction in the degree of polymerization. Deviations from the square root dependence on initiator concentration are to be primarily expected for termination by primary free radicals and for transfer reactions with reduction in the degree of polymerization. 

A list of common types of termination reactions is given at the bottom of Fig. 3.25. Any time two radicals meet, they may combine and terminate the reaction. A very low free radical concentration is thus important for producing molecules of high molar mass. The free radical terminations include recombination, disproportionation, transfer, and the reverse of initiation. The ionic chain reactions differ from the free radical reactions by needing a counterion to neutralize the charge on the active chain end. Obviously, the actual reaction configuration is complicated by the presence of the counterion. The main difference between the two ionic types of chain reactions... 

For the two different monomers, there ate four propagation reactions in Fig. 3.45. The chance that -Mj, the free radical terminating a chain ending in Mj, reacts with the same monomer Mj is Pn. The chance to continue with monomer M2, is P12 which equals (1 - P,). The analogous probabilities for - Mj are P22 and Pjj (= 1 - P22)- The probabilities are equal to the rates of the appropriate reaction divided by the sum of both possible rates, as indicated in Fig. 3.45 for P,. Three more defiititions are given in Fig. 3.45, two for the reactivity ratios, rj and, r2, and the third, for the monomer ratio X, also called the feed ratio (x = f/fj). The basic copolymer equation is shown in the box at the bottom, where dlMJ/dlMj] = F,/F2 is the composition ratio. 

It can how see from Fig. l(a, b, c), additives MP into oxidation of toluene in presenee Ni(acac)2 result in increase in initial rates of aceumula-tion of BH, BZA and BAL. This allows to assume inerease in the activity of Ni(acac)2 upon its complexation with in chain initiation (1.1), BH decomposition (1.3-1.3 ), and free radical termination (1.5.) (Scheme 1). 

Light stabilizers are used to protect plastics, particularly polyolefins, from discoloration, embrittlement, and eventual degradation by UV light. The three major classes of light stabilizers are UV absorbers, excited state quenchers, and free-radical terminators. Each class is named for the mechanism by which it prevents degradation. The major types included in each light stabilizer class may be categorized by their chemistries, as shown in Table 4.17. 

Hindered amine light stabilizers (HALS). Hindered amine light stabilizers are the newest type of UV light stabilizer. They were introduced in 1975 by Ciba and Sankyo. HALS do not screen ultraviolet light, but stabilize the resin via free-radical termination. HALS are used at lower levels than benzophenones and benzotriazoles, and are widely used in polyolefins for their cost-effectiveness and performance. The successful growth of HALS has been directly related to their substitution for benzophenones and benzotriazoles in many applications as well as their blending with benzophenones. 

Calculations have shown that in the conversion range of a-methylacrolein of up to 35% (for t < 10" s) (see also Figure 6.4) the contributions of steps (1) and (8), relevant to free radicals generation are negative, while contributions of steps (15) and (16) on free radicals termination are positive. From this follows the conclusion about the possibility of controlling (in the considered case raising) the selectivity of chain reaction by the stimulation of steps (15, 16) that lead to the termination of free radicals. Calculations carried out also confirm that the introduction of inhibitors leads to the increase in reaction selectivity (Table 6.4). 

The mechanism is not fully understood. Hydroperoxide decomposition and free radical scavenging certainly play a part, as also does the regeneration of HALSs, where UV absorbers are frequently consumed as a result of their operation. There are several theories for how this works - possibly by energy transfer, free radical termination, or peroxide decomposition. Significant stabilization is achieved at relatively low concentrations and it appears that the HALS is actually regenerated by the stabilization process, rather than consumed by it. Theory suggests that the hindered amine oxidizes to form amine-ether, which is a non-radical species. 

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