P—scission

The proposed mechanism for producing ethanol [64-17-5] from butane involves -scission of a j -butoxy radical (eq. 38). The j -butoxy radicals are derived from j -butylperoxy radicals (reaction 14 (213)) and/or through some sequence involving reaction 33. If 25% of the carbon forms ethanol, over 50% must pass through the j -butoxy radical. Furthermore, the principal fate of j -butoxy radicals must be the P-scission reaction the ethoxy radical, on the other hand, must be converted to ethanol efficiently. 

Diperoxyketals. Some commercially available di(/ f2 -alkylperoxy)ketals and their corresponding 10-h half-life temperatures (deterrnined in dodecane) are hsted in Table 5 (39). Diperoxyketals thermally decompose by cleavage of only one oxygen—oxygen bond initially, usually foUowed by P-scission of the resulting alkoxy radicals (40). For acychc diperoxyketals, P-scission produces an alkyl radical and a peroxyester. 

Because high temperatures are required to decompose diaLkyl peroxides at useful rates, P-scission of the resulting alkoxy radicals is more rapid and more extensive than for most other peroxide types. When methyl radicals are produced from alkoxy radicals, the diaLkyl peroxide precursors are very good initiators for cross-linking, grafting, and degradation reactions. When higher alkyl radicals such as ethyl radicals are produced, the diaLkyl peroxides are useful in vinyl monomer polymerizations. 

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

Many radicals undergo fragmentation or rearrangement in competition with reaction with monomer. For example, f-butoxy radicals undergo p-scission to form methyl radicals and acetone (Scheme 3.6). 

The reactivity of the monomer and the reaction conditions determine the relative importance of P-scission. Fragmentation reactions are generally favored by low monomer concentrations, high temperatures and low pressures. Their significance is greater at high conversion. They may also be influenced by the nature of the reaction medium. 

Only a few diacvl peroxides see widespread use as initiators of polymerization. The reactions of the diaroyl peroxides (36, R=aryl) will be discussed in terms of the chemistry of BPO (Scheme 3.25). The rate of p-scission of thermally generated benzoyloxy radicals is slow relative to cage escape, consequently, both benzoyloxy and phenyl radicals are important as initiating species. In solution, the only significant cage process is reformation of BPO (ca 4% at 80 °C in isooctane) II"l only minute amounts of phenyl benzoate or biphenyl are formed within the cage. Therefore, in the presence of a reactive substrate (e.g. monomer), tire production of radicals can be almost quantitative (see 3.3.2.1.3). 

A slow rate of p-scission also means that the main cage recombination process will be cage return to reform the peroxydicarbonate. Dialkyl peroxides are typically not found amongst the products of peroxydicarbonate decomposition. In these circumstances, cage recombination is unlikely to be a factor in reducing initiator efficiency. 

The low conversion initiator efficiency of di-r-butyl pcroxyoxalatc (0.93-0.97)1-1 is substantially higher than for other peroxyeslers [/-butyl peroxypivalale, 0.63 /-butyl peroxyacetate, 0.53 (60 °C, isopropylbenzene)195]. The dependence of cage recombination on the nature of the reaction medium has been the subject of a number of studies. 12I,1<>0 20CI The yield of DTBP (the main cage product) depends not only on viscosity but also on the precise nature of the solvent. The effect of solvent is to reduce the yield in the order aliphatic>aromatie>protic. It has been proposed199 that this is a consequence of the solvent dependence of p-scission of the f-butoxy radical which increases in the same series (Section 3.4.2.1.1). 

The decomposition of the peroxyketals (53) follows a stepwise, rather than a concerted mechanism. Initial homolysis of one of the 0-0 bonds gives an aikoxy radical and an a-peroxyalkoxy radical (Scheme 3.36).306"08"210 This latter species decomposes by P-scission with loss of either a peroxy radical to form a ketone as byproduct or an alkyl radical to form a peroxyester intermediate. The peroxyester formed may also decompose to radicals under the reaction conditions. Thus, four radicals may be derived from the one initiator molecule. 

Ceric ions react rapidly with 1,2-diols. There is evidence for chelation of cerium and these complexes are likely intermediates in radical generation10 106 The overall chemistry may be understood in terms of an intermediate alkoxy radical which undergoes p-scission to give a carbonyl compound and a hydroxyalkyl radical (Scheme 3.59). However, it is also possible that there is concerted electron transfer and bond-cleavage. There is little direct data on the chemical nature of the radical in termediates. 

Pioneering work by Wallingj94 established that the specificity shown by t-butoxy radical is solvent dependent. Work21 22396 on the reactions of /-butoxy radicals with a series of a-mcthylvinyl monomers has shown that polar and aromatic solvents favor abstraction over addition, and [3-scission over either addition or abstraction. Recently, Weber and Fischer418 and Tsentalovich at a/.410 reported absolute rate constants for [3-scission of r-butoxy radicals in various solvents. These studies indicate that p-scission is strongly solvent dependent while abstraction is relatively insensitive to solvent. 

The rate of fl-scission of benzoyloxy radicals is such that in most polymerizations initiated by these radicals both phenyl and benzoyloxy end groups will be formed (Scheme 3.4). A reliable value for the rate constant for p-xcission would enable the absolute rates of initiation by benzoyloxy radical to be estimated. Various values for the rale constant for p-scission have appeared. Many of the early estimates are low. The activation parameters (in CCI4 solvent) determined by Chateauneuf et a(.m are log]0 A = 12.6 and Ea = -35.97 kJ mol 1 which corresponds to a rate constant of 9xl06 s 1 at 60 °C. 

The rate constant for p-scisskm is dependent on ring substituents. Rate constants for radicals X-Q.H. CCh are reported to increase in the series where X is / -Fi There is qualitative evidence that the relative rales for p-scission and addition are insensitive to solvent changes. For benzoyloxy radicals, similar relative reactivities are obtained from direct competition experiments10 as from studies on individual monomers when p-scission is used as a clock reaction.399,401... 

The chemistry of atkoxycarbonyloxy radicals in many ways parallels that of the aroyloxy radicals (e.g. benzoyloxy, see 3.4.2.2.1). Products attributable to the reactions of alkoxy radicals generally arc not observed. This indicates that the rate of p-scission is slow relative to the rate of addition to monomers or other... 

Starnes et al.hl have also suggested that the head adduct may undergo p-scission to eliminate a chlorine atom which in turn adds VC to initiate a new polymer chain. Kinetic data suggest that the chlorine atom does not have discrete existence. This addition-elimination process is proposed to he the principal mechanism for transfer to monomer during VC polymerization and it accounts for the reaction being much more important than in other polymerizations. The reaction gives rise to terminal chloroallyl and 1,2-dichlorocthyl groups as shown in Scheme 4.8. 

Stansbury and Bailey. A review by Colombam on addition-fragmentation processes is also relevant. Monomers used in ring-opening are typically vinyl (e.g. vinylcyclopropane - Scheme 4.20 Section 4.4.2.1) or methylene substituted cyclic compounds (e.g. ketene acetals - Section 4.4.2.2) where addition to the double bond is followed by p-scission. 

Mcthylencdioxolanc derivatives also undergo ring-opening. However, the ring-opened radical may undergo a further p-scission (e,g. 50, Scheme 4.32). 

Initiation resulting from insertion of the monomer into the Al—Cl bond is followed by propagation involving insertion between the porphinato-aluminum and the alkoxide group of the growing polymer, coupled with P-scission of the C—O bond of the oxirane monomer (demonstrated by nmr results) it yields a polyether terminated by a CH2C1 end-group. 

Displacement (a-scission) and Free Radical Arbuzov (P-scission) Reactions. 58... 

The p-scission of a phosphoniumyl radical yields a cation and a phosphonyl radical, while its reaction with a nucleophile generates a phosphoranyl radical which can undergo SET reactions and a- or p-fragmentations (Scheme 14). 

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... 

The trapping of alkyl, alkoxyl and alkylthiyl radicals by trivalent phosphorus compounds, followed by either a-scission or p-scission of the ensuing phosphoranyl radical, is a powerful tool for preparation of new trivalent or pen-tavalent phosphorus compounds [59]. However, the products of these reactions strongly depend on the BDE of the bonds, which are either formed or cleaved. For example, the addition of phenyl radicals on a three-coordinate phosphorus molecule occurs irreversibly, while that of dimethylaminyl (Me2N ) or methyl radicals is reversible, the amount of subsequent P-scission (formation of compound C) depending on the nature of Z and R (Scheme 25). For tertiary alkyl radicals and stabilized alkyl radicals no addition is observed (Scheme 25) [63]. 

Scheme 25 Influence of the nature of ligands on the a- and p-scission of phosphoranyl radicals. Reprinted with permission from [63]. Copyright 1997 American Chemical Society...

Scheme 31 Decay of phosphoranyl radicals via a-scission, p-scission and SET. Reprinted with permission from [68]. Copyright 1994 Wiley Interscience...

Roberts et al. [74] took advantage of the rapid and selective p-scissions of phosphoranyl radicals, to develop a radical chain desulfurization affording new substituted a-alkyl acrylates in good to moderate yields (Scheme 37). 

Phosphoranyl radicals were observed by FPR at the end of the sixties [91]. For a long time, phosphoranyl radicals, particularly the alicyclic ones [59], were considered as elusive species. However, recently. Marque et al. [92] observed the first strongly persistent (ti/2=45 min at RT) alicyclic phosphoranyl radicals (Fig. 10) when they irradiated bis(trialkylsilyl)peroxides in the presence of tris(trialkylsilyl)phosphites. The increased lifetime of the ensuing phosphoranyl radicals is a consequence of the presence of four bulky R3SiO groups around the phosphorus. The bulkiness of the substituents hampers the dimerization and the Sh2 reaction of phosphoranyl radicals with the peroxide initiator. Furthermore, the high strength of the P-0 and 0-Si bonds results in slow a- and p-scissions [93]. 

Innumerable reactions occur in acid catalyzed hydrocarbon conversion processes. These reactions can be classified into a limited number of reaction families such as (de)-protonation, alkyl shift, P-scission,... Within such a reaction family, the rate coefficient is assumed to depend on the type, n or m cfr. Eq. (1), of the carbenium ions involved as reactant and/or product, secondary or tertiary. The only other structural feature of the reactive moiety which needs to be accounted for is the symmetry number. The ratio of the symmetry number of the... 

LDPE polymerization reaction consists of various elementary reactions such as initiation, propagation, termination, chain transfer to polymer and monomer, p-scission and so forth [1-3], By using the rate expression of each elementary reaction in our previous work [4], we can construct the equations for the rate of formation of each component. 

---