Chain biting

Preparation of graft copolymers by chain biting reactions... 

These reactions are just an example of many other possibilities since a backbone substituent, or the chain backbone directly is attacked, I propose to call these processes chain biting reactions. 

However, metathesis is at least partially intramolecular instead of intermolecular the growing chain bites its own tail ... 

Example 9.4 Show, as above, how short branches arise in polyethylene when a growing chain bites its own back, that is, the radical transfers to an atom a few [5-8] carbon atoms down the chain. 

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. 

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)). 

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. 

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. 

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 ... 

Note that these rates are not true rate constants as these overall rates will contain concentration and pre-equilibria parameters. Nevertheless, longer bridges, and thus wider bite angles lead to a relative increase in the rate of chain transfer. Ester formation for the wide bite angle ligands were assigned to the formation of trans complexes as mentioned above. 

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