Homolytic bond scission

Hepuzer et al. [91] have used the photoinduced homolytical bond scission of ACPB to produce styrene-based MAIs. These compounds were in a second thermally induced polymerization transferred into styrene-methacrylate block copolymers. However, as Scheme 24 implies, benzoin radicals are formed upon photolysis. In the subsequent polymerization they will react with monomer yielding nonazofunctionalized polymer. The relatively high amount of homopolymer has to be separated from the block copolymer formed after the second, thermally induced polymerization step. 

The first step in cracking is the thermal decomposition of hydrocarbon molecules to two free radical fragments. This initiation step can occur by a homolytic carbon-carbon bond scission at any position along the hydrocarbon chain. The following represents the initiation reaction ... 

According to the transition state theory, the pre-exponential factor A is related to the frequency at which the reactants arrange into an adequate configuration for reaction to occur. For an homolytic bond scission, A is the vibrational frequency of the reacting bond along the reaction coordinates, which is of the order of 1013 to 1014 s 1. In reaction theory, this frequency is diffusion dependent, and therefore, should be inversely proportional to the medium viscosity. Also, since the applied stress deforms the valence geometry and changes the force constants, it is expected... 

In the absence of solvation mechanisms, the process of homolytic bond scission in organic compounds requires much less energy than heterolytic bond scission... 

Bearing an unpaired electron, the fragments formed from homolytic bond scission are highly reactive and are capable of undergoing any of the chemical reactions normally expected from a macroradical ... 

Homolytic bond cleavage from excited states in irradiated polymers [30] can lead to a pair of free radicals via bond scission, involving main chain or side-chain substituents. 

Table IV compares the X-ray exposure characteristics (at 8.3 X, Al Kai,2 emission line) of the halogenated resists and of PMMA Elvacite 20U1. It can be seen that poly(2-ehloroethyl methacrylates) and poly(2-bromoethyl methacrylates) exhibit a low sensitivity unlike poly(2-fluoroethyl methacrylates) and poly(2-, 2-,2-trifluoroethyl methacrylates) which are more sensitive than PMMA as shown in Figures 2a, 2b, 2c, 2d where the dose-thickness curves of these resists are plotted. The low sensitivity of the PC1EMA and PBrEMA samples may be explained by some competing crosslinking reactions which could occur during exposure as a result of C-Cl and C-Br homolytic bond scissions as noted by Tada...

The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

The reaction temperatures and some of the activation energies cited above seem to be too low to support a radical-chain reaction mechanism. Guryanova found that exchange of radioactive elemental sulfur with the p sulfur atoms of bis-p-tolyl tetrasulfide proceeds at 80-130 °C with an activation energy of only 50 kJ/mol in the case of the corresponding trisulfide the activation energy was determined as 60 kJ/mol. These data sharply contrast with the observation that liquid sulfur has to be heated to more than 170 °C to detect free radicals by electron spin resonance spectroscopy and the activation energy for homolytic SS bond scission has been determined as 150 kJ/mol (see above). 

Chain growth polymerizations (also called addition polymerizations) are characterized by the occurrence of activated species (initiators)/active centers. They add one monomer molecule after the other in a way that at the terminus of each new species formed by a monomer addition step an activated center is created which again is able to add the next monomer molecule. Such species are formed from compounds which create radicals via homolytic bond scission, from metal complexes, or from ionic (or at least highly polarized) molecules in the initiating steps (2.1) and (2.2). From there the chain growth can start as a cascade reaction (propagation 2.3) upon manifold repetition of the monomer addition and reestablishment of the active center at the end of the respective new product ... 

Interestingly, 56 cannot be obtained photochemically from the benzodisilacyclobutene 57 instead silastyrene 64 is formed as an intermediate46. 64 is produced by homolytic bond scission of the Si—Si bond in 57 followed by intramolecular disproportionation of... 

Because most heteroatom-carbon single bonds are less stable than carbon-carbon bonds, traceless linkers can be synthesized on the basis of nearly all heteroatoms. The enthalpies of C-X bonds are, however, only relevant for homolytic bond scission. Many linkers are cleaved heterolytically, and kinetic stability toward hetero-lytic bond cleavage is decisive in these linkers. 

The rate-limiting nucleophilic addition leading to homodimers formally includes the reversible homolysis of the nucleophilic adduct (110) (e.g., Scheme 11). Indeed the successful isolation of the labile intermediates 5c and 5d from both cyclic cations 3 and 4 indicates that the nucleophilic adduct (feLmo) is the critical intermediate leading to the homodimers 6 [compare Eq. (51) with Eq. (49)]. Bond homolysis (111) as the mechanism for the decomposition of the nucleophilic adduct is experimentally supported by kinetics, ESR studies, as well as spin and chemical trapping. Furthermore, the low sensitivity of the first-order rate constant kH (Fig. 7) on solvent polarity and the marked effect of solvent viscosity are both earmarks of the homolytic bond scission (107,108). 

These reactions are reversible and proceed at moderate temperatures (0-120°C) when compounds with cumulated S-S bonds (polysulfanes, elemental sulfur) are considered. The activation energy and therefore the rate of reaction at a certain temperature very much depend on the particular compound. Interconversion reactions are promoted by UV radiation as well as by cationic, anionic, and nucleophilic catalysts. The reaction mechanisms will, of course, be different in these various cases. The various reaction types possible under noncatalyzed conditions and with exclusion of light have been critically reviewed by Steudel in 1982. " Originally it had been thought that homolytic bond scission is the first and rate-determining step in all cases. Dissociation energies... 

A homolytic cyclopropane C-C bond rupture that would furnish in turn a 1,3-diradical is also conceivable. However, it is always difficult to establish whether a purely diradical or ionic mechanism is in operation. Between these two extremes there exists a graded continuum of polarized diradicals of which the zwitterion represents the end of the spectrum. In addition, the continuous development of radical character during the formation of the transition state of a homolytic bond scission, called the continuous diradical, has been postulated to explain the behavior of some reactions. Alternatively, the contribution of a truly concerted transformation cannot be overlooked. ... 

The results clearly indicate that quenching of benzophenone triplets in polar solvents is a bimolecular process. This means that the ions exist mostly as separated ions and that the electron transfer process occurs mainly as an inter-ion-pair reaction [66, 67]. Analysis of the products after irradiation of A-(p-benzoylbenzyl)-A,A,N-tri-n-butylammonium triphenyl-n-butylborate (BTAB, Figure 14) indicates formation of p,p -(benzoyl)dibenzyl, p-pentylbenzophenone and tributylamine. This indicates that electron transfer from the borate anion to the acceptor excited state leads to homolytic C-N bond scission and formation of the tertiary amine. 

Figure 2.2 Mechanism of "backbiting" in formation of short chain branching initiated by attack of radical on a 5 carbon-hydrogen bond. In the reaction above, homolytic bond scission occurs resulting in a free radical on the 5 carbon atom and an n-butyl branch. R is a polymeric alkyl group.

Possible causes of radical formation in the polymer chain are residues of transition metals and peroxides used to catalyze polymerization and thermo-oxidative processes. Whatever causes homolytic bond scission to produce free radicals (eq 8.1 and 8.2), oxygen reacts with the resultant radical to generate hydroperoxide moieties. Key reactions involved in the initiation and propagation of radicals are illustrated in eq 8.1-8.4, where is a polymeric fragment ... 

Excepting di-t-butyl ether (see below), the most important primary process is the homolytic C-0 bond scission (reactions 27 and 28). 

W. A. Lee, T. C. Bruice, Homolytic and heterolytic oxygen-oxygen bond scissions accompanying oxygen transfer to iron(III) porphyrins by percarboxylic adds and hydroperoxides. A mechanistic criterion for peroxidase and cytochrome P-450, ]. Am. Chem. Soc. 107 (1985) 513. 

---