Secondary propagating radicals

Metalloporphyrins behave differently, and the mechanism is dependent upon the nature of the metal.301 In addition to the formation of M—C adducts, there are other unspecified reactions with metalloporphyrins which have not been characterized spectroscopically. It is presumed that Scheme 5 may be included in the processes that affect CCT in the case of vinylic monomers that yield a secondary propagating radical. 

The influence of the catalyst system was also investigated by comparing CnCl- and CuBr-based systems at different ratios of [Cn(I)]/[Cn(II)]. The resnlts are summarized in Table 2. In all cases CuBr provides a narrower molecnlar weight distribution of the resulting polymer than CnCl for the ATRP of NlPAAm in water, independent of the ratio [Cu(I)]/[Cn(II)] nsed. This effect is generally observed for acrylate polymerization. When CnCl is nsed the low rate of activation of the dormant species combined with the high reactivity of the secondary propagating radical lead to a lower control as compared to CuBr. ... 

Two secondary propagating reactions often accompany the initial peroxide decomposition radical-induced decompositions and -scission reactions. Both reactions affect the reactivity and efficiency of the initiation process. Peroxydicarbonates and hydroperoxides are particularly susceptible to radical-induced decompositions. In radical-induced decomposition, a radical in the system reacts with undecomposed peroxide, eg ... 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

The reaction of radicals with nitroxides is reversible. 09 This means that the highest temperature that the technique can reasonably be employed at is ca 80 °C for tertiary propagating species and ca 120 °C for secondary propagating species.22 These maximum temperatures are only guidelines. The stability of alkoxyamines is also dependent on solvent (polar solvents favor decomposition) and the structure of the trapped species. This chemistry has led to certain alkoxyamines being useful as initiators of living polymerization (Section 9.3.6). At elevated temperatures nitroxides are observed to add to monomer albeit slowly. 3IS 5" 523... 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

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... 

Two secondary propagating reactions often accompany the initial peroxide decomposition radical-induced decompositions and /3-scission reactions. Both reactions affect the reactivity and efficiency of the initiation process. 

The second route of getting free radical center near is mediated by low-molecular free radicals or compounds which are either present in the polymer or are gradually formed there by fragmentation reactions. While tertiary peroxy radicals propagate only by abstraction of hydrogen atom from surrounding C —H bonds, secondary peroxy radicals may easily cleave to hydroxy radicals and ketones as follows ... 

SCHEME 14.1 The general stmcture of an acrylic polymer and the estahhshed photodegradation mechanism via Norrish I a-cleavage of the carhonyl side chain, leading to main-chain polymeric radical a and oxo-acyl radical b. The secondary P-scission rearrangement reaction leading to the propagating radical c is also shown. 

Peroxyl Radicals Secondary peroxyl radicals, as are found in most lipid acyl chains, recombine rapidly (2k = 10 -10 M s ) (192, 362) to form a variety of products, including alcohols and ketones (Reaction 67) (361, 362, 366), ketones and alkanes (Reaction 68) (60, 292), or acyl peroxides and peroxyl radicals (Reaction 69) (264, 367, 369). The alcohols thus produced are indistinguishable from H abstraction products of an original LO, but the ketones and dialkyl peroxides are unique to recombination reactions. As any R3OO and RO released from Reaction 68 or Reaction 69a react further, peroxyl radical recombinations also have the potential for propagating lipid oxidation (Section 3.1.4). 

In heterolytic bond cleavage, a bond breaks such that both electrons in the bond stay with one of the atoms in homolytic bond cleavage, a bond breaks such that each of the atoms retains one of the bonding electrons. An alkyl peroxide is a radical initiator because it creates radicals. Radical addition reactions are chain reactions with initiation, propagation, and termination steps. Radicals are stabilized by electron-donating alkyl groups. Thus, a tertiary alkyl radical is more stable than a secondary alkyl radical, which is more stable than a primary alkyl radical. A peroxide reverses the order of addition of H and Br because it causes Br, instead of H, to be the electrophile. The peroxide effect is observed only for the addition of HBr. 

As we saw in the case of the reaction of methane with a mixture of Br2 and Cl2, the only kinetically viable first propagation steps in this case are reactions of Cl atoms with propane. Reactions of Br atoms are far too slow to compete. It is this first step that determines the ratio of primary to secondary alkyl radicals that form. Therefore, the selectivity observed is that of Cl atoms. The two radicals both proceed to react rapidly with either molecular halogen, Cl2 or Br2. Because the ratio of radicals present was determined in the prior step, the ratios of chloropropane isomers and of bromopropane isomers obtained are essentially the same, and reflect the selectivity of chlorination. 

A secondary alkyl radical is more stable than a primary radical and is formed faster. Bromine adds to C-1 of 1-butene faster than it adds to C-2. Once the bromine atom has added to the double bond, the regioselectivity of addition is set. The alkyl radical then abstracts a hydrogen atom from hydrogen bromide to give the alkyl bromide product as shown in step 4 of Mechanism 6.8. Steps 3 and 4 propagate the chain, making 1-bromobutane the major product. 

Percent product distribution propanal 50.6 1.2, 2-hydroxy-butanal 7.0 0.3, l-hydroxy-butan-2-one 24.2 0.7, 1,2 dihydroxybutane 18.2 0.7. Computer assisted analysis of the product distribution showed that addition of the OH radical occurs to 26 % at the inner and to 74% at the outer position of the double bond. These reactions produced the corresponding primary and secondary hydroxy-alkylperoxy radicals. The branching ratio for the radical propagating channel of the self-reaction of the secondary peroxy radicals was determined to be issa/ iss = 0.75 0.02 28 % of the hydroxy-alkoxyl radical thus formed reacted with oxygen to produce hydroxyketone. If it is assumed that the rate coefficient for the reaction of the hydroxy-alkoxyl radical with oxygen is 8 x 10 cm molecule s the rate coefficient for the decomposition of this radical to produce propanal is 1 x 10 s V... 

Propagating radicals which undergo depolymerization exclusively are relatively stable. For example, the radically initiated polymerizations methyl methacrylate and a-methyl styrene are both reversible (see Figure 1.22). This stems largely from the fact that the active radical resides on a tertiary carbon atom which is more stable than the corresponding secondary radicals involved in the polymerization of methacrylate and styrene, the reversal of propagation being accompanied by many side reactions in each of these cases. 

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