Back-bite 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). 

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. 

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]. 

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]. 

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. 

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 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 ... 

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

So far, not many systems are known to meet the conditions of the growth mechanism with activated monomer. Polymerizations of oxirane, chlo-romethyloxirane [145, 146] and of lactams [147] belong to this class. The existence of an electroneutral propagating macromolecule and of an activated monomer results in reduced probability of back-biting reactions and in easier preparation of macromers [145] by means of exchange reactions. 

It is known that in a solid state polymerization occurs along the certain axis of the monomer crystal and extended chain crystals are formed [146]. In the polymerization from liquid phase, on the other hand, lamellar crystals are formed. This indicates that back-biting reaction proceeds not at random but in a specific manner, forced by the nature of crystals. The model of growth of lamellar crystals of polyoxymethylene is known [147]. According to this model, the subsequent layers are formed on the surface of the crystal by growth of folded chain as shown schematically below ... 

From the point of view of the mobility of active sites and the reactivity of acetal oxygen, the probability of back-biting reaction is the highest at the surface of the crystal, i.e., at the chain folds. 

Back-biting reaction occurring during cationic polymerization of lactams is detrimental to preparation of block copolymers of two different lactams by sequential polymerization. Block copolymers can be obtained only in those systems in which the rate of polymerization of the second monomer is much higher than the rate of chain transfer to polymer resulting in transamidation [219]. 

Y. Ishihara, H. Nanbu, K. Saido, T. Ikemura, and T. Takesue, Back biting reactions during the catalytic decomposition of polyethylene, Bull. Chem. Soc. Jpn., 64, 3585-92 (1991). 

This is a phenomenon opposite to that described in the previous section. A linear polymer is rapidly formed and then the equilibrium concentration of macrocydes is slowly reached by back-biting reactions according to the scheme... 

In the polymerization of dialkyldichlorosilanes with sodium in refluxing toluene, propagation and a concurrent back-biting reaction to cyclic material could give the range of products found if the products are kinetically, instead of thermodynamically, determined (12). No evidence for depolymerization has been found for the reaction in toluene solution. 

Equation (16) can lead to the formation of difunctional acids, lactones, methyl ketones with carbon chains shorter than the starting hydrocarbon, and other compounds [44, 45]. Back-biting reactions have the unique characteristic that it is not possible to dilute the site of a back-biting attack with respect to the attacking radical. In the normal chain propagation (eq. (2)), such dilution with respect to the attacking radical can be achieved by, for example, holding the conversion at a lower level. 

In addition to short branches from back-biting reactions, long branches are seen to be formed both in LDPE and in PVC from intmmolecular chain transfer to polymer. These are much less frequent ( 1 per 2000 repeat units) but have a great effect on the viscosity of the melt and thus the processing. This is discussed in more detail in later chapters. 

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