Backbiting attack

The resulting polymerization is a copolymerization between the two monomers, not a homopolymerization of MMA. More importantly, this results in a slower reaction (and lower polymer molecular weight) since the carbanion derived from the ketone is not as reactive as the carbanion from MMA. Nucleophilic substitution by intramolecular backbiting attack of a... 

The effect of this backbiting attack of a propagating carbanion is lowering of both polymer molecular weight and polymerization rate. 

Polar monomers, like methyl methacrylate, acrylonitrile, or methyl vinyl ketone, contain substituents that react with nucleophiles. This can lead to terminations and side reactions that compete with both initiation and propagation. An example is a nucleophilic substitution by an intramolecular backbiting attack of a propagating carbanion ... 

Figure 2.2 Mechanism of "backbiting" in formation of short chain branching initiated by attack of radical on a 5 carbon-hydrogen bond. In the reaction above, homolytic bond scission occurs resulting in a free radical on the 5 carbon atom and an n-butyl branch. R is a polymeric alkyl group.

In comparison to the previously discussed monomers, the polymerization of N-substituted aziridines is easier to control since the side reactions by proton transfer are eliminated because of the absence of primary and secondary amines. Nonetheless, termination by nucleophilic attack of the cationic propagating chain end into polymeric tertiary amines results in the formation of unreactive quaternary ammonium groups, that is, termination. As a result, the polymerization of N-substituted aziridines usually stops at limited conversion as was first demonstrated for A-methylaziridine by Jones [129]. Detailed evaluation of the polymerization kinetics as well as the evolution of molar mass during the polymerization revealed that termination mainly occurs via intramolecular backbiting... 

If reaction of backbiting is considered, involving the intrachain acetal group, the macrocyclic oxonium cation is about 50% converted to free macrocycle. This is because for non-strained, sufficiently large cyclic oxonium cations, the reactivities of the two acetal methylene groups at the oxonium oxygen atom - endocyclic and exocyclic - are practically equal, which results in equal probabilities of endo- and exocyclic reactions. The exocyclic attack of monomer leads to free... 

The ROP of six-membered cyclic carbonates with nucleophilic initiators is a chain reaction in which, besides initiation and propagation reactions, transesterification reactions can also take place (Scheme 32).Intramolecular nucleophile (e.g., alkoxide) attacks on carbonyl carbon atom (backbiting) lead to cyclic oligomers, while intermolecular transesterifica-tion leads to a change of the macromolecule length with the consequence that, at equilibrium, the most probable distribution of the molecular weight is obtained (Scheme 32). 

Scheme 1 Major side reactions in the anionic poiymerization of MMA. (a) initiator attack onto the monomer ester group and (b) backbiting intramoiecuiar termination.

The attack of the propagating enolate anion to the antepenultimate ester carbonyl group is the major secondary reaction identified, forming cyclic p-ketoester-terminated polymers. The presence of the backbiting reaction producing cyclic p-ketoester in the case of MMA polymerization has been confirmed using IR spectroscopy as a distinct band at... 

Chain transfer to polymer leads to branched polymer chains and thus greatly affects the physical and mechanical properties of a polymer, such as the ability to crystallise. Transfer to polymer does not necessarily lead to a decrease of molecular mass, but the chain length distribution becomes broader. If we consider transfer from a growing polymer chain to another dead chain, this is intermolecular transfer to polymer. If the radical attacks a proton in the same chain we call this intramolecular transfer to polymer, also called backbiting. 

Figure 13.2 Intramolecular chain backbiting mechanisms. Upper scheme illustrates the attack of a chain by its own OH-terminated free end to form a cyclic siloxane. Lower scheme illustrates the case where a closed, continuous chain folds back upon itself and rearrangement occurs to form a new Si-O bond and a free cyclic siloxane...

Preparation of cyclic oligo(ether sulfone)s by cesium fluoride-catalyzed depolymerisation in aprotic solvents was reported by Colquhoun et al. [36, 37]. The fluoride ion cleaves the C-O bonds at temperatures > 140 °C and the resulting phenoxide ions attack the neighboring ether groups forming cyclics by backbiting . Several cyclic oligoethers up to the pentamer were isolated and characterized by X-ray crystal structure analyses. 

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