Back-biting mechanism

Acetone is a coproduct of butane LPO. Some of this is produced from isobutane, an impurity present in all commercial butane (by reactions 2, 13, 14, and 16). However, it is likely that much of it is produced through the back-biting mechanisms responsible for methyl ketone formation in the LPO of higher hydrocarbons (216). 

This reaction also plays a role in the degradation of polysulftdes. A back-biting mechanism as shown in equation 6 results in formation of the cycHc disulfide (5). Steam distillation of polysulftdes results in continuous gradual collection of (5). There is an equiUbrium between the linear polysulftde polymer and the cycHc disulfide. Although the linear polymer is favored and only small amounts of the cycHc compound are normally present, conditions such as steam distillation, which remove (5), drive the equiUbrium process toward depolymerization. 

There are still some non-explained observations. For example, syndiotactic PP was reported [45,46] as being more stable than isotactic polymer. At 140°C, the maximum chemiluminescence intensity was achieved after 2,835 min for syndiotactic PP, while isotactic polymer attained the maximum after only 45 min. Atactic PP was reported to be more stable than the isotactic polymer [46]. An explanation has been offered that the structure of isotactic PP is much more favourable for autooxidation, which proceeds easier via a back-biting mechanism where peroxyl radicals abstract adjacent tertiary hydrogens on the same polymer chain. 

The formation of rings by this back-biting mechanism implies that when the reaction mixture is neutralised, there should be a number of linear fragments corresponding to the number of catalyst molecules. Careful examination of the reaction mixtures has shown that if such linear fragments are present, their concentration is very much lower than that of the initiator [13] This, therefore, appears to exclude this and any other kind of ring formation by back-biting this question will be discussed in more detail later. 

Figure 2.9 Back-biting mechanism for the formation of cyclic oligomers...

The polymerization of trioxane in solution has been studied by Okamura (26) and his co-workers and by Kern and Jaacks (56, 58, 63). The initiators were borontrifluo-ride or its complexes and anhydrous perchloric acid. During the polymerization the eight-membered ring, tetroxane, is formed rapidly but this compound takes part in the polyoxymethylene formation. This results in an equilibrium concentration of tetroxane when the rate of formation becomes equal to the rate of consumption (27). Minor amounts of the ten-membered ring, pentoxane, are also formed (28). The authors conclude that tetroxane and pentoxane are formed by a back-biting mechanism. It is assumed that the active species in the reaction is a carbenium ion. Although this ion... 

The important observation was that oligomers with rinpizes 6,12,18, 24... etc. were 10 to 100 times more abundant than the others. This strongly indicates that these oligomers are formed directly from monomer since there are no reasons for supposing that the formation of these rings by a back-biting mechanism from polymer would be especially favored. 

Good evidence exists that cationic polymerization of heterocyclic monomers occurs with cyclic onium salts as the active species. Therefore it is reasonable to believe that also the degradation reactions occur via such ions, so that the most plausible reaction path is the classical back-biting mechanism ... 

Miki, Higashimura, and Okamura (2) reported the formation of tetra-oxane from polyoxymethylene cations by a back-biting mechanism ... 

M of cyclics —1500-1700), but the molecular weight distribution is quite narrow (MJM 1.1-1.25). This is in obvious disagreement with random back-biting mechanism, which should lead to much broader distribution of ring sizes. 

If the reactions in THF and the inclusion of dead material in block copolymers are taken into account, the balance of the evidence at present seems to support the back-biting mechanism. However, the mechanisms are not really mutually exclusive, because they both suggest an alternation between a chlorine-ended chain and a sodium-ended chain. The sodium ended-chain reacts first with the initial dichloride in the rate-determining step, and then the chlorine-ended chain reacts with the sodium surface. The difference lies in the mode of formation of the cyclic materials and the importance of any condensation reactions. Both mechanisms might be operative, although one may dominate, depending on the conditions or substituents. 

Thus, manganese-ion catalysis would be expected to suppress the rate of formation of ketones and secondary alcohols (which are ketone precursors) by eq. (6) and the production of methyl ketones by the back-biting mechanism (eq. (16)). Moreover, it increases the rate of attack on all ketones, formed by whatever mechanism. These proposals are consistent with the preferred use of manganese-ion catalysts for the production of synthetic fatty acids [10]. Carbonyl impurities are especially critical for this product. 

Formation of butyl branches takes place by a back-biting mechanism via a six-membered ring, while transfer to polymer from macroradicals is a reasonable source to LCB with tertiary chlorine (7 9). We have suggested an alternative mechanism which also explains the formation of internal double bonds and LCB with tertiary hydrogen (7, 8). This mechanism is based on transfer to polymer from chlorine atoms produced in the mechanism for transfer to monomer ... 

Formation of EO/THF macrocyclics probably occurs via tail-biting and back-biting mechanisms as outlined in Figure 5. The back-biting reaction route appears most likely for this system. Since no THF homocyclic oligomers have been found in this copolymerization and are extremely limited in THF... 

The ACE mechanism, developed in the absence of hydroxyl groups, leads to the formation of cyclic oligomers, at a high yield, by the back biting mechanism (reaction 7.20). 

Among these, the latter 1,5-transfer is reasonably accepted as the main path to form the trimer by a back-biting mechanism through a six-membered ring intermediate. However, the former 1,3-transfer has not been clearly accepted as the main path of the dimer formahon by a reasonable mechanism and/or intermediates. 

Scheme 1.6 Expected formation mechanism of lactide (back-biting mechanism).

Scheme 6.20.2 Back-biting mechanism and vinylidene formation in the free-radical ethene polymerization process leading to the characteristic structural features of LDPE.

Scheme XXIL A proposed back-biting mechanism for the metathesis polymerization of alkynes. Substituents have been omitted for clarity.

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