Silane radical atom abstraction

The use and limitations of Atom Transfer Radical Coupling (ATRC) reactions including polyrecombination reactions for the preparation of telechelic polymers, segmented block copolymers, and polycondensates are presented. Specifically, the preparation of telechelic polymers with hydroxyl, aldehyde, amino and carboxylic functionalities, poly(/i-xylylene) and its block copolymers, and polyesters via ATRC process is described. The method pertains to the generation of biradicals at high concentration from polymers prepared by ATRP or specially designed brfunctional ATRP initiators. The possibility of using silane radical atom abstraction (SRAA) reactions, that can be performed photochemically in the absence of metal catalysts, as an alternative process to ATRC is also discussed. 

In conclusion, it has been demonstrated that ATRC reactions are useful for preparing various macromolecular stmctures such as telechelics and certain polycondensates. The method preserves to the generation of biradicals at high concentration from polymers prepared by ATRP or specially designed bifimctional ATRP initiators. The radical generation process is not limited to the metal catalyzed atom transfer reactions. Silane radical atom abstraction reactions can also be used for the formation of reactive radicals. Aromatic carbonyl assisted photoinduced reactions seemed to a promising alternative route for silane radical generation since it can be performed at room temperature and does not require metal catalysts. 

Silane radical atom transfer (SRAA) was demonstrated as an efficient, metal-free method to generate polystyrene of controllable molecular weight and low polydispersity index values. (TMSlsSi radicals were generated in situ by reaction of (TMSlsSiH with thermally generated f-BuO radicals as depicted in Scheme 14. (TMSlsSi radicals in the presence of polystyrene bromide (PS -Br), effectively abstract the bromine from the chain terminus and generate macroradicals that undergo coupling reactions (Reaction 70). 

The concept of this method is illustrated in Scheme 3.1, where the clock reaction (U R ) is the unimolecular radical rearrangement with a known rate constant ( r)- The rate constant for the H atom abstraction from RsSiH by a primary alkyl radical U can be obtained, provided that conditions are found in which the unrearranged radical U is partitioned between the two reaction channels, i.e., the reaction with RsSiH and the rearrangement to R. At the end of the reaction, the yields of unrearranged (UH) and rearranged (RH) products can be determined by GC or NMR analysis. Under pseudo-first-order conditions of silane concentration, the following relation holds UH/RH = (A H/A r)[R3SiH]. A number of reviews describe the radical clock approach in detail [3,4]. 

High-level computational methods are limited, for obvious reasons, to very simple systems. In the previous section we showed the contribution of the theory for a better imderstanding of the entropic and enthalpic factors that influence the reactions of hydrogen atom with the simplest series of silanes Me4 SiH , where n = 1-3. Calculated energy barriers for the forward and reverse hydrogen atom abstraction reactions of Me, Et, i-Pr and t-Bu radicals with Me4- SiH , where n = 0-3, and (H3Si)3SiH have been obtained at... 

Decomposition of benzoyl peroxide in hexamethyldisilane at 80° C gives, as major products, benzene, benzoic acid, l,2-bis(pentamethyldisilanyl)-ethane and benzylpentamethyldisilane (151). The reaction of hexamethyldisilane in carbon tetrachloride with benzoyl peroxide (at reflux temperature) and with di-tert-butyl peroxide (in a sealed tube at 129° C) gives (chloro-methyl)pentamethyldisilane as the main product arising from the silane (150). In no case are rearrangement products formed. Therefore, in solution at relatively low temperature, the pentamethyldisilanylmethyl radical does not undergo rearrangement as in the thermolysis. The main fate of this free radical is dimerization in the absence of solvent or chlorine atom abstraction when carbon tetrachloride is present. 

Similar results were obtained for the photolysis of poly(n-hexylmethyl-silane) using either methanol or n-propyl alcohol as the trapping reagent (Table II). The additional complexity of the disilane products was anticipated, because silyl radicals can abstract either a hydrogen atom or an alkoxy radical. Although the alkoxy disilanes could be produced by other routes (e.g., alcoholysis of Si-H bonds or addition to an intermediate disilene), these routes were considered unlikely on the basis of appropriate control experiments or the lack of a literature precedent. 

In addition, the silyl radical may abstract an H atom from the surface, generate a surface Si dangling bond, and return into the gas phase as a silane molecule. This abstraction reaction can be represented as... 

A class of HAT reactions for which the additivity postulate appears not to hold are those with strong polar effects. In some HAT reactions, as pointed out by Tedder, ... the rate of atom transfer is very dependent on the degree of polarity in the transition state. For instance, Rong et al. showed that alkyl radicals abstract H faster from thiols than from silanes or stannanes, while the kinetic preference is reversed for perfluoroalkyl radicals. " The more electron rich R" radical preferentially abstracts the electron deficient RS -H while the electron deficient Rp" radical reacts faster with R3Sn -H . Such an inversion of reactivity cannot be accounted for by a cross relation treatment, because from the additivity postulate the reactivity of a reagent is not dependent on its partner. 

The rate of combination of trifluoromethyl radicals to form hexafluoro-ethane has been measured by the flash photolysis of trifluoromethyl iodide coupled with rapid-scan i.r. spectroscopy in the absence of an inert diluent (At, N2, or COt) carbon tetrafluoride and tetrafluoroethylene were also formed, presumably via fluorine atom abstraction from trifluoroiodomethane by hot trifluoromethyl radicals (c/. ref. 27). Photolysis of trifluoroiodomethane has been used in studies on (i) the direction of radical attack on 1,3,3,3-tetrafluoropropene [- CFj CHI CHFCF, (75%) + (CFa)jCH CHFl (25%)] (ii) the rates of hydrogen abstraction from ammonia, ammonia-ethylene oxide, silane, trimethylsilane, tetramethylsilane, and cycloalkanes and (iiQ the competitive addition of the CFj- radical to ethylene and vinylidene fluoride. Radicals formed by photolysis of the fluoroalkyl iodides CFJ, C FJ, n-C,F,I, (CF,)jCFI, (CFaljCHI, (CFalaCDI, (CFa)jCClI, and (CFa)iCPhI (the last was synthesized by treatment of CFs CPh CTj with CsF and iodine in DMF) have been... 

Dissociation of the gases SiH4 and H2 by electron impact will create reactive species (radicals) and/or neutrals (Si2H6 and even higher-order silanes [195-198]). Atomic hydrogen is an important particle because it is formed in nearly all electron impact collisions, and the H-abstraction reaction [199, 200] of (di)silane is an important process, as is seen from sensitivity study. Dissociation of SiHa can create different SiH (with x = 0, 1,2, 3) radicals. Only silylene (SiH2) and... 

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