Chain-transfer reactions unimolecular

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si-H moiety has been reported as a key step in several unimolecular chain transfer reactions.59,60 In particular, the 1,5-hydrogen transfer of radicals 14-17 [Eq. (5)], generated from the corresponding iodides, was studied in... 

Chain transfer reactions are bimolecular or unimolecular (spontaneous). Typical bimolecular chain transfer reactions are transfer to monomer, initiator, and external chain transfer agents (especially impurities), and intermolecular chain transfer to polymer typical unimolecular chain transfer reactions are transfer to counterion in ionic polymerizations, intramolecular chain transfer to polymer, and transfer to solvent (pseudo xmimolecular). 

Allyltin compounds behave as excellent unimolecular chain transfer (UMCT) reagents [49] which serve as radical acceptors and sources of tin mediators [50]. Since acyl radicals are nucleophilic radicals, the addition reaction to allyltin, which is regarded as an electron rich alkene, is not a rapid process. Ryu, Sonoda, and coworkers found that unsaturated ketones can be synthesized by a three-component coupling reaction, comprised of alkyl halides, CO, and allyltin reagents [51]. Because of the slow direct addition of alkyl radicals to allyltin compounds [50b], radical car-bonylations with allyltin can be conducted at relatively low CO pressures, and high substrate concentrations (0.1-0.05 M) were used to ensure the chain length. 

Treatment of a y-iodoallylic alcohol with NaH and -Bu2SiHCl afforded the corresponding di-f-butyloxysilanes which were exposed to UV irradiation in the presence of 10% hexabutylditin in the so-caUed unimolecular chain transfer (UMCT) reaction of silicon hydrides to afford the reduced alkene (eq 1). ... 

Process (i) is a unimolecular process, while (ii) is a bimolecular process and the rate depends on monomer concentration. Frequent chain-transfer reactions bring about low molecular weight polyolefins. If chain transfer is negligible or very slow, the polymerization can be living , as observed in group 5 metallocene-diene complexes [30, 31]. j3-Methyl elimination is also reported in bis(pentamethylcyclopentadienyl)metallocene catalysts [32,33]. 

The DPs obtained in cationic polymerizations are affected not only by the direct effect of the polarity of the solvent on the rate constants, but also by its effect on the degree of dissociation of the ion-pairs and, hence, on the relative abundance of free ions and ion-pairs, and thus the relative importance of unimolecular and bimolecular chain-breaking reactions between ions of opposite charge (see Section 6). Furthermore, in addition to polarity effects the chain-transfer activity of alkyl halide and aromatic solvents has a quite distinct effect on the DP. The smaller the propagation rate constant, the more important will these effects be. 

Products are olefins and the corresponding acids. These reactions are among the most widely studied and best understood of all gas phase unimolecular reactions. With few exceptions they are experimentally and kinetically well behaved cleanly first-order, no surface sensitivity, and no free radical chain complications. Reactions involve 1,5-hydrogen transfer from the f -carbon to the carbonyl oxygen, migration of the carbonyl Jt-bond, rupture of the ester (C-O) bond, and formation of a (Cg-Cf) 7t-bond. All present evidence favors a mechanism in which the above occur in a concerted manner. However, a two-step consecutive mechanism (see later) cannot be entirely ruled out at this time. 

The most ubiquitous chain-release reactions occurring in ct-olefin polymerizations are the unimolecular and bimolecular /3-hydride transfers after primary insertion. These are shown in Scheme 11. 

Ionic-polymerization Kinetics. The kinetics of ionic polymerization share some common principles with that of the free-radical reaction. Both are based on the basic steps of initiation, propagation, termination, and chain transfer, and in both the ultimate average molecular weight depends on the ratio of the reaction rates of propagation and termination. There are, however, important differences. In ionic polymerization the termination step appears to be unimolecular, while it is bimolecular in free-radical type polymerization. The dependence of the kinetic scheme of the reaction on the various parameters is therefore different in the two reactions. Likewise, the fact that a cocatalyst has to be brought into the ionic reaction scheme has to be taken into account. 

It includes a unimolecular initiation reaction (ui) leading, by transfer reactions, which have not been written, to the free radical chain carriers 6 or p. The chain is propagated by the reactions (bs) and (me) and gives rise to products m and 6H. The free radical p decomposes by a monomolecular reaction, whereas the free radical 6 reacts in a bimolecular reaction the initial letters of these two adjectives have given their Greek symbols to the two types of radical. The free radicals 6 and 11 are called chain carriers. The termination reactions include the two by two recombination or... 

The intramolecular process is relatively easy to study quantitatively. This is because the products of the chain transfer (by unimolecular transesterification) are cyclic compounds and their concentration can be measured, for example, by standard chromatographic methods. Thus, propagation and formation of cyclic oligomers are competitive reactions taking place simultaneously. 

As stated in section I, the termination mode of the particular monomer determines the number of functionalities per macromolecular chain. Most monomers undergo both unimolecular and bimolecular termination reactions. It is often observed that both respective monofunctional and bifunctional polymers are formed and well-defined functional polymers cannot be prepared. The use of allylmalonic acid diethylester in free-radical polymerization has been proposed to overcome the problems associated with the aforementioned functionality. In the presence of the allyl compound, the free-radical polymerization of monomers, regardless of their termination mode, proceeds entirely with the unimolecular termination mechanism, as shown in Scheme 9. Because allyl compounds lead to degradative chain transfer, the resulting allyl radical is quite stable due to the allyl resonance. Monofunctional polystyrene, polyvinylacetate, and poly(t-butyl methacrylate) were prepared by using this approach [33]. Subsequently, various macromonomers were derived from these polymers. 

The termination and the transfer reactions occur quite normally in cationic polymerization. The termination reaction is unimolecular—unlike in radical polymerization, where it is bimolecular. It occurs by the abstraction of a proton from the carbonium ion end of the growing polymer chain by the gegen ion, which always stays in its vicinity. How readily this occurs once again depends on the stability of the carbonium ion end of the growing polymer chain, the nature of the gegen ion, and the dielectric constant of the medium. The information on k, is quite scanty. 

Experiments in which radical scavengers are added indicate that a chain reaction is involved, because the reaction is greatly retarded in the presence of the scavengers. The mechanism shown below indicates that one of the steps in the chain process is an electron transfer and that none of the steps involves atom abstraction. The elimination of nitrite occurs as a unimolecular decomposition of the radical anion intermediate, and the SrnI mechanistic designation would apply. 

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