Hindered radicals

Steric hindrance on the R group affects the rate constants of the formation of these intermediates as well. Thus in the above study the reaction of the R = CH(CH3)C02 with (Cu L2)+ is too slow to be measured, whereas a rate constant of 6.3 x 106 M 1 s-1 was derived for the similar radical R = CH2 CH2CO2 (and even higher rates for less sterically hindered radicals) in the L2 system (92). 

Selectivity may also be achieved by having a very bulky radical that is unable to abstract certain protons. A similar point is observed with the use of sterically hindered bases to remove only primary hydrogens instead of the most acidic ones in the molecule. An example of a sterically hindered radical is that formed from A-chloro-di-/-butyl amine, which abstracts primary hydrogens 1.7 times faster than tertiary ones. 

Inhibition of chain processes is achieved by the addition of free radical scavengers, which react by chain transfer more rapidly than the propagation step. The product of chain transfer is also a free radical, but the key to the transfer agent being a good inhibitor is that it must be a very unreactive radical, e.g., sterically hindered radicals formed from the widely used antioxidants BHT (2,6-di-t-butyl-hydroxy-toluene) and BHA (2,6-di-t-butyl-hydroxy-anisole). 

Termination by disproportionation. In Example 11.3, coupling of two polymer radicals was assumed to be the only termination mechanism, as is indeed essentially true for polymerization of styrene [34], However, various other mechanisms may contribute to termination or even dominate it. The most common of these is disproportionation, mainly observed for tertiary and other sterically hindered radicals [35]. An example is methyl methacrylate [34] (see reaction 11.26 below). In disproportionation, two polymer radicals react with one another, transferring a... 

Exceptional products of dimerization are formed by the sterically hindered radical-anions. For example, protonation of the dimeric dianions of l-Phenyl-2-t-butyl acetylene yielded the following hydrocarbons100)... 

The values of k 2°)/k(l°) and k(3°)//c(r) of Table 4 have been derived mainly from the simplest of alkanes. With highly branched alkanes steric effects become apparent even with the reactive Cl. With sterically hindered radicals such as the amine radical cation derived from N-chloro-2,2,6,6-tetramethylpiperidinium ion, the reactivity of the 2-position of pentane is 5 times that of the more hindered 3-positionIn general, methylene hydrogen atoms near the end of a long alkane chain are more reactive towards radical attack than methylene groups nearer to the center of the chain e.g., see Table 7. 

Most studies on stable nitroxide radical-mediated poljmeiization are made with TEMPO or a substituted TEMPO, such as, 4-methoxy-2,2,6,6-tetramethyl-piperidine-l-oxy (MTEMPO) as the stable counter radical. However, the bond formed between the polymer radical and these nitroxides becomes labile around 120°C. This high temperature favors thermal polymerization as also side reactions, such as transfer or termination by dismutation between the growing chain and the stable radical (Jousset et al., 1997). To minimize these reactions, a more hindered radical, namely, di-ieri-butyl nitroxide (II), produced from the compound (III) can be used. 

The photoinitiated polymerization of a sterically hindered semi-fluorinated monomer, which is characterized by a hindered radical chain propagation site, was followed both in an isotropic and a highly ordered smectic liquid crystalline phase. The polymerization rate is slower in the smectic phase than in the isotropic phase, presumably a result of a decrease in both the propagation and termination rate constants. The decrease in the propagation rate results in a very slow persistent increase in polymer molecular weight after the initiating light source is removed. Polymerization firom the smectic phase of the monomer proceeds in a non-equilibrium matrix. 

We suggest that the explanation for this apparent anomaly is that the transition state for attack by rm-butoxyl radicals is more advanced than that for attack by less sterically hindered radicals and that as a result, the product radical stability plays a more significant role than it usually does in detemiining radical reactivity. A recent paper by the CSIRO group provides some support for this. Thus, while the rerr-butoxyl radical was found to behave as an electrophilic species towards a series of substituted styrenes, p/2m-substituents generally resulted in higher reactivities than those expected on the basis of the results for meta-substituents, suggesting a resonance contribution to the rate of addition. 

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