Back biting reactions

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. 

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). 

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. 

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 ... 

It might be expected that short-chain branches would be formed in the polymerization of vinyl acetate by a cyclic mechanism like that proposed by Roedel (6) for ethylene polymerization, and this possibility has been mentioned by several authors. There appears to be no evidence that such short-chain branches occur if they are absent this is perhaps attributable to steric repulsion between the substituents that might prevent the formation of the cyclic transition state required for the back-biting reaction. 

Polymerisations of 8,9, and 10 were rapid but stopped at limited conversions owing to a termination process involving an intramolecular back-biting reaction, the tertiary amino groups on the polymer backbone being more... 

These structures might of course be regarded as the first propagating species of the polymerisation of 23 initiated by methyl iodide. The ring closure of the open chain form is a process similar to a back-biting reaction, and as such is likely to be thermodynamically favoured by the presence of a ring substituent at the point of closure (157,158). Thus the polymerisation of 23 with I" counterion must involve a similar equilibrium in propagation and any data for rate constants will therefore be of a composite nature. 

Scheme 7. Back-biting reactions during the polymerization of s-caprolactone...

The A-B di-block copolymer of -CL and oxepan-2,7-dione has been synthesized using aluminum isopropoxide as initiator [114] (Scheme 16). In order to prepare the ABA tri-block copolymer, a difunctional initiator [Et2AlO(CH2)4OAlEt2] was used to polymerize B followed by the addition of monomer A. However, the rate of polymerization was lower than in the Al(0 Pr)3-initiated system. Increasing the temperature to 70 °C increased the rate but a broadening of MWD was observed due to intramolecular back-biting reactions and intermolecular transesterification reactions. The addition of 1 equiv. of pyridine with respect to Al increased the polymerization rate and reduced the MWD from 1.95 to 1.25 [95]. 

It is well known from the ROP of lactones and lactides that the catalyst or initiator causes transesterification reactions at elevated temperatures [39], or at long reaction times (Scheme 5) [40]. Intermolecular transesterification reactions modify the sequences of copolylactones and prevent the formation of block copolymers. Intramolecular transesterification reactions, i.e., back-biting, cause... 

Recently, tin(II) butoxide was used in the polymerization of l-LA [88]. The initiation is fast and quantitative and no transesterification or back-biting reactions are observed. The reaction proceeds with acyl-oxygen bond scission with retention of the configuration, and can be used both in bulk and solution (THF, 20-80 C) polymerization. It is possible to control the molecular weight in the range of 103 to 106 with a MWD of 1.15-1.85. The polymerization is very fast, kp=5 x 10 1 mol-1 L s-1, with only the rare earth alkoxides being faster. 

Other reactions may be taken into consideration, with an effect on polymer structure, namely the formation of short- and long-chain branches. A complete list of reactions in S-PVC polymerization may be found in Kiparissides et al. [5]. On the above basis kinetic equations may be written. To keep it simple the chain transfer, back-biting and inhibition reactions are disregarded, while termination is considered to occur only by disproportionation. The elementary reaction rates for initiator decomposition and free radicals generation are as follows ... 

Scheme 20 Proposed mechanism for back-biting reactions occurring in polycyclotrimer-ization of aliphatic diynes...

A unique odd-even effect of the monomers in the diyne polycyclotrimer-ization was observed. In the aliphatic diynes with an odd number of methylene spacers, their triple bonds locate in the same side, which facilitates the back-biting reaction (Scheme 21). In contrast, in the diynes with an even number of methylene units, their triple bonds locate in the opposite sides these unfavorable positions frustrate the back-biting reaction. Consequently, hb-P32(4) possessed triple bond residues in its final structure, whereas its... 

Scheme 21 Odd-even effect in the back-biting reaction...

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 occurrence of a back-biting reaction is further indicated by the observation that in copolymerizations of trioxane or tetroxane with vinyl monomers various 1,3-dioxane derivatives are formed (29,56). With styrene for example, the derivative was 4-phenyl-l,3-dioxane, the formation of which was explained as follows ... 

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