Radical chain propagation

Other experimental data seem to provide support for an implicit penultimate model. Thus, simple (monomeric) model radicals for the propagating radical chain... 

The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H—Si(lll) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si—H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the... 

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

A terminal radical-complex model for copolymerization was formulated by Kamachi. He proposed that a complex is formed between the propagating radical chain and the solvent (which may be the monomer) and that this complexed radical has a different propagation rate constant to the equivalent uncomplexed radical. Under these conditions there are eight different propagation reactions in a binary copolymerization, assuming that the terminal unit is the only unit of the chain affecting the radical reactivity. These are as follows. 

Each of the four propagation reactions has its own rate constant [ky], where subscript i refers to the nature of the propagating radical chain-end and subscript j denotes the nature of the adding monomer. In the mathematical description of copolymer composition as a function of comonomer feed composition, the individual propagation rate constants are not used. Instead, it is common practice to use so-called reactivity ratios. These reactivity ratios are defined as the ratio between the rate constant for homopropagation and that for aosspropagation. The definition of reactivity ratios is mathematically represented as shown in eqn [5] ... 

As two radicals (R - and Rm- in Fig. 1.3.1) collide and radical sites react, the propagating radical chains lose their active sites, and then terminate. This bimolec-ular reaction between radicals occurs either by recombination to form one dead polymer (Rn+m in Fig. 1.3.1) or by disproportionation leading to two dead polymer molecules (Rn and Rm in Fig. 1.3.1) however, the latter is a less prevalent case. 

Reaction with the free radicals formed as a result of bond cleavage by solar radiation produces peroxy radicals. The latter propagate radical chain reactions that multiply the destructive effect of the radiation manifold ... 

This chapter describes the application of electron spin resonance (ESR) spectroscopy and controlled radical polymerization techniques to basic research on the chemistry of radical polymerizations. This combination can provide information on the chain length of propagating radicals, chain-transfer reactions to polymers, and penultimate unit effects in copolymerization, topics that have been difficult or impossible to study by direct detection of radicals. 

Development of controlled radical polymerization techniques has stimulated basic research on radical chemistry in conventional radical polymerizations. Information on the effect of chain lengths on propagating radicals, chain-transfer reactions to polymers, and penultimate unit effects has been obtained from ESR observation of model radicals generated from radical precursors prepared by ATRP. Previously, it has been extremely difficult, even impossible, to obtain such information from ESR spectra during conventional radical polymerizations. The ESR study of radical polymerizations has made remarkable progress as a result of the combination of study of radicals formed as a result of various kinds of controlled radical polymerization techniques. 

As described in Section 4.35.3.2, fep data are derived from PLP-SEC. The experimentally accessible quantities are the time period between two successive laser pulses to and the number of propagation steps between two subsequent laser pulses Li as obtained from SEC. In systems where solubility is no problem, for example, in many bulk polymerizations. Cm is easily identified with overall monomer concentration. However, in solution polymerizations, local monomer concentration in the vicinity of the propagating radical chain end Cm,ioc may differ from the analytic overall monomer concentration CM,a-In particular for systems in which the solvent power of monomer and solvent (e.g., of SCCO2) are markedly different, Cm,ioc and CM.a may not be the same. If the addition of SCCO2 to a polymerizing system leads to a variation in the product fep Cm, which is the quantity directly accessible from PLP-SEC, two limiting situations may occur ... 

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