Unimolecular chain transfer

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... 

To determine the proportions of unimolecular (chain transfer) and bimolecular (coupling) termination, let y fraction of molecules be terminated by the former mechanism. Therefore,... 

Unimolecular Chain Transfer = UMCT) process. The silicon radical thus formed propagates the radical chain via I-abstraction [18],... 

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). ... 

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). 

Assuming that the number average degree of polymerization (DP ) is determined by chain transfer to monomer and assuming unimolecular termination relative to propagation (i.e., chain breaking due to solvent, polymer, impurities are absent), the simple Mayo equation55 ... 

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. 

Spontaneous termination, also referred to as Chain transfer to counterion, is the unimolecular rearrangement of the ion-pair which results in termination of the growing chain and simultaneous regeneration of the catalyst-cocatalyst complex. 

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. 

Multiblock copolymers comprising NIPAAm and N,N-dimethylacrylamide (DMAAm) with variable length of blocks of both components prepared by reversible addition-fragmentation chain transfer polymerization were also investigated and their phase behavior was found to exhibit protein-like behavior exhibiting intramolecular collapse upon heating, forming unimolecular flower-like micelles above the thermal phase transition temperature." ... 

Storey et al. [140] point out that in living polymerizations of monomers like isobutylene that are co-initiated by TiCLt temperatures as low as —80°C, the livingness is limited not by chain transfer to monomer but rather by a unimolecular termination process. Unimolecular terminations often involve p-proton expulsions to produce polymers with terminal unsaturation. They claim, however, that this does not happen here. Rather the normal ferf-chloride chain ends of polyisobutylene formed by this type of polymerization gradually become depleted. They propose, therefore, that an isomerization mechanism takes place instead in the presence of an active Lewis acid, tmder monomer starvation conditions. It can be illustrated as follows [140] ... 

Based on H NMR analysis of low molecular weight polypropylene formed by 40b/MAO at 25 C with a propylene concentration of 0.5 M (in toluene solution), the predominant chain termination pathway appears to be f)-H elimination (i.e., vinylidene end groups are present). The primary kinetic isotope effect observed for 35b/MAO (vide supra) is also indicative of P-H elimination operating as the predominant chain termination pathway. No fl-CH3 elimination was observed for these catalysts specifically, no vinylic end groups are present in the H NMR spectra. For 35a-c and 40c, chain transfer was found to be primarily unimolecular, derived from f)-H elimination. For 40b, a small bimolecular contribution (chain dansfer to monomer) was also suggested, owing to the less than first order dependence of molecular weight on propylene concentration. 

The tendency towards chain transfer to hydrogen seems to be the highest with metallocenes prone to unimolecular p-H elimination. Metallocenes with high selectivity towards bimolecular p-H elimination are less reactive towards hydrogen. 

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