Unimolecular chain

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... 

A similar phenomenon was postulated by Thomas and Pellon (10) to account for data obtained in the kinetics of acrylonitrile polymerization. They felt that it was possible to obtain unimolecular chain termination by a process of burial. This was conceived as a mechanism by which the growing chain became shielded from further growth by coiling or by embedding itself in the solid phase. At room temperatures we feel the... 

Unimolecular chain reactions such as (13) have the form... 

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. 

Metallocene d° complexes of the early metals, which are very important catalysts for olefin polymerization, are not known to form H2 complexes, but a Tire metallocene H2 complex is stable calculationally from Car-Parrinello ab initio molecular dynamics.154 This methodology155 can reveal novel reaction pathways that traditional static computations may overlook. An allyl H2complex computed to be stable in the gas phase at 300 is located as a secondary minimum in -elimination, the most common assumed unimolecular chain termination process in Ziegler-Natta ethylene polymerization. Static calculations show that the H2... 

The detrimental influence of decreasing propene concentration" - and of increasing amoimts of CFl2-CI2 solvent on syndiospecificity is in line with unimolecular chain back-skip (also called site epimer-... 

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). ... 

Subsequently, another MA uptakes these chains from the solvent phase and thus becomes larger. The rate of the chain uptake process is evidently proportional to the critical micellar concentration (cmc) or the concentration of unimolecular chains that are solubilized under a given set of solvation conditions. If a solvent is sufficiently poor for the insoluble block(s), the cmc can be very low. Thus, the chain-exchange kinetics can be slow for block copolymers. Fusion or fission of micelles would not occur to a significant extent for MAs whose cores are protected from contacting one another by thick repulsive coronas. 

Figure 2.2 shows a hypothetical first-order plot for an ideal living polymerization (Theoretical), for a polymerization in which unimolecular chain termination is operable (Termination), and for two polymerizations that are free of termination but characterized by different degrees of slow initiation. For all four plots, f] = 0.01 mol L . ... 

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). 

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