Back-bite

The theoretical explanation of the butane reaction mechanism is as fully developed as is that of acetaldehyde oxidation (51). The theory of the naphtha oxidation reaction is more troublesome, however, and less well understood. This is largely because of a back-biting reaction which leads to cycHc products (52). 

Butane. The VPO of butane (148—152) is, in most respects, quite similar to the VPO of propane. However, at this carbon chain length an important reaction known as back-biting first becomes significant. There is evidence that a P-dicarbonyl intermediate is generated, probably by intramolecular hydrogen abstraction (eq. 32). A postulated subsequent difunctional peroxide may very well be the precursor of the acetone formed. 

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. 

Polymerisation could proceed from the radical in the normal way or alternatively chain transfer may occur by a second back-biting stage either to the butyl group (Figure 10.2(a)) or to the main chain (Figure 10.2(h)). 

Back-biting reactions, 63 Back-end cure, 235 BAK, 28 Bakehte resins, 1 Barrier properties, 26 Batch polymerization, autoclave cycle for, 167... 

Heterochain polymers produced by ring-opening polymerization contain the hetero-atoms in the main chain as well as in the monomer and the polymer chain competes with the monomer for the reaction with the propagating species. This competition leads to polymer transfer and back-biting reactions during the polymerization. Heterochain polymers are also susceptible to depolymerization by the ionic active species which are easily formed during processing. 

Alcohol functions have also been introduced via hydrosilylation reactions, for example, the reaction of T8[OSiMe2H]8 with allyl alcohol and allyloxy ethanol (Table 19). In the first case, it has been postulated that the compound T8[OSiMe2 (CH2)30H]8 is not very stable due to back-biting of the -OH groups on the silicon corners (Figure 31). Nevertheless, it reacts with polymers such as polyvinyl pyrrolidone to give polymer hybrids (Table 19, entries 4 and 5). 

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 radical at the end of the growing polymer chain can also abstract a hydrogen atom from itself by what is called back biting. This leads to chain branching. 

PE can be produced using Ziegler-Natta catalysts (organometallic complexes of transition metals) in which no radicals are produced, no back biting occurs, and, consequently, there is no chain branching. 

Figure 1.25 Minimum-energy diastereoisomeric monomer free intermediates for butadiene polymerization catalyzed by titanium complexes presenting Cp group as ancillary ligand. Chiralities of coordination of allyl groups (assumed to be si) and back-biting double bonds (si or re) are indicated, in order to easily visualize possible stereoregularity (iso or syndio) of model chains. In fact, like and unlike chiralities would possibly lead to isotactic and syndiotactic enchainments, respectively.

The stereoselectivity mechanisms for polymerizations of dienes present several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of allylic type rather than of o type, as for the monoalkene polymerizations. There is experimental evidence, also supported by molecular modeling studies, that a relevant role for chemoselectivity and stereoselectivity is also played by the chirality of the back-biting coordination to the metal of the double bond of the polydienyl chain closest to the coordinated allyl group. 

The production of these branches is ascribed to intramolecular transfer, i.e., back-bite. CH2 —CH, CH. — CH,... 

On the basis of this argument it is probable that the tris-p-halogen substituted trityl salts will be useful initiators for the polymerisation of isobutylene since their ionisation potentials probably exceed 690 kj mol1 [38], and the p-substitution would inhibit the back-biting reactions which make both trityl and dityl salts unstable [39]. 

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. 

When one considers how cyclic polymers could be formed from propagating species (VII) one is faced with exactly the same problem as with the carboxonium ion theory either there must be a 100% efficient end-to-end ring-closure (which in any case would be confined to polymers with an initial hydroxyl group) or there must be back-biting and the formation of linear fragments, as illustrated by Jaacks [17a]. 

According to this view the macrocyclic molecules found in the reaction product are formed by a back-biting reaction (3) in which the formal CH2 group adjacent to the tert.-oxonium ion reacts with an oxygen atom along the chain, generating a macrocyclic tert.-oxonium ion ... 

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

During polymer chain growth, a back-biting process can lead to cyclic carbonate formation. In general, this process is more facile for aliphatic epoxides than for alicyclic epoxides and when the growing polymer chain dissociates from the metal center (Scheme 3). 

Scheme 3 Two modes of back-biting for cyclic carbonate formation...

---