Radical transfer reaction

Atom or radical transfer reactions generally proceed by a SH2 mechanism (substitution, homolytie, bimolecular) that can be depicted as shown in Figure 1.6. This area has been the subject of a number of reviews.1 3 27 97 99 The present discussion is limited, in the main, to hydrogen atom abstraction from aliphatic substrates and the factors which influence rate and specificity of this reaction. 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

Although BDE is by far not the only factor that determines the kinetics of H-transfer reactions, within a given series of simple alkyl radical a correlation seems to hold (Table 6.4). In polymers, where the lifetime of the polymer-bound radicals may be long, radical transfer reactions by intramolecular H-abstrac-tion (primary - secondary —> tertiary) are common (Chap. 9.4). In general, whenever a system starts with a mixed radical system (e.g., in the reaction of OH with 2-PrOH 2-hydroxy-prop-2-yl and 2-hydroxypropyl) a steady-state is approached which is dominated by the lower-energy radical [here 2-hydroxy-prop-2-yl, cf. reaction (21)]. This process is favored by low initiation rates and high substrate concentrations, and these two factors determine whether such an H-transfer manifests itself is also in the final products. 

Based on the IR data of samples before extraction, it is concluded that the allyl groups react rapidly to completion within about 2 minutes, whereas the ester absorption remains constant. More allyl groups per unit of time react than peroxide radical fragments are initiated, it can be concluded that the allyl groups react predominantly via radical addition reactions, probably accompanied by radical transfer reactions. FT-IR analysis after vulcanisate extraction indicates that the co-agent is covalently bound to the elastomer matrix, as shown by the 100% recovery of the ester absorption after 2 minutes of curing. 

Radical transfer reactions involving poly ethers of the poly (ethylene oxide) type are well known (7). Heating polyethylene glycols of various molecular weights at 140 °C. with dicumyl peroxide for 2.5 hours has resulted in a gel fraction explained by transfer at the a-carbon followed by combination of the polymer radicals. Further, poly (ethylene oxide) dissolved in MMA and heated in the presence of benzoyl peroxide results in grafted copolymer. 

RADICAL TRANSFER REACTIONS IN CLASS II AND III RIBONUCLEOTIDE REDUCTASES... 

Effective for fibre type Mainly cellulose, also wool, catalysing their dehydration to char All kinds of fibres, because their flame chemistry is similar (radical transfer reactions)... 

Table 3.4. Radical transfer reactions involving OH radicals and sulfonated substrates demonstrated by endgroup analysis 1401...

The fragment radicals, CHO and C2H5, undergo a number of radical transfer reactions involving O2 and NO as given below ... 

Propagation is the second step in the free radical chain reaction. This step leads to new radicals but also to the formation of small stable molecules. Because the stability of a long chain free radical is usually higher than that of a small free radical, if small radicals are formed in the first step, they usually react with the polymer, forming polymeric chain radicals and small molecules. This type of propagation is known as radical transfer reaction. Using the notation Pn for the polymer and Rn for the polymeric radical, the radical transfer reactions can be indicated as follows ... 

As an example, for poly(methyl methacrylate), the radical transfer reaction may take place as follows ... 

The radical transfer reaction, instead of occurring from a small radical to a large molecule, may occur randomly, and one radical may generate a different radical chain and a new polymeric molecule. This type of chain transfer of an unpaired electron from one molecule to another is known as /nfermo/ecu/ar transfer, which can be written as follows ... 

A radical transfer reaction also occurs in a small proportion, leading to the formation of toluene as shown in the following scheme ... 

Fuchs and Gong reported an acyl radical transfer reaction from aldehydes to acetylenic trifluoromethylsulfones to give acetylenic ketones [43J. In this case, tri-fluoromethyl radical, arising from the a-scission of trifluoromethylsulfonyl radical, abstracts the hydrogen of an aldehyde to form an acyl radical, which then propagates the chain. 

Curran and Schwarz determined the optimal conditions for a similar acyl radical transfer reaction from acyl methyl selenide to methyl acrylate to achieve the... 

Narasaka and Sakurai found that chromium carbene complexes, when exposed to a copper(II) reagent, generate acyl radicals by a one-electron oxidation, and these then undergo addition to electron-deficient alkenes (Scheme 4-27) [50J. The resulting copper(I) species reduces the resulting radical to an anion, and subsequent protonation leads to the addition product. This redox type acyl radical transfer reaction works particularly well for aromatic acyl radical systems, for which decarbonylation is not a problem. Related work has also recently appeared [51]. 

Another aspect49 is the initial presence of persistent species in nonzero concentrations [Y]o, and it will be discussed more closely in section IV. In the absence of any additional initiation, the excess [Y]o at first levels the transient radical concentration to an equilibrium value [R]s = A[I]o/[Y]o. This is smaller than that found without the initial excess and lowers both the initial conversion rate and the initially large PDI. Further, it provides a linear time dependence of ln-([M]o/[M]), which is directly proportional to the equilibrium constant. Later in the reaction course, [Y] may exceed [Y]0 because of the self-termination, then [R] is given by eq 18. If there is additional radical generation, the first stages will eventually be replaced by a second stationary state that was described above. Further effects are expected from a decay or an artificial removal of the persistent species that increases the concentration of the transient radicals and the polymerization rate (see section IV). Radical transfer reactions to polymer, monomer, or initiator have not yet been incorporated in the analytical treatments. 

As AS Ab is not constant in a series of compounds, R—X bond dissociation energy does not linearly depend on ASE° or, a fortiori, on S °(R). On the other hand, the heat of a radical transfer reaction,... 

Radical transfer reactions involve abstraction of an atom or group B by a radical A from a molecule B-C (reaction 6.24). B is nearly always an atom transfer of a group, which would correspond to substitution at a polyvalent atom, though important in nucleophilic and electrophilic reactions, is very uncommon in radical reactions. The atom transferred is almost always a hydrogen or a halogen atom. 

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