Intramolecular backbiting

Short branching has also been verified in poly(vinyl acetate), poly(vinyl chloride), and various polyacrylates. Branching in poly(vinyl acetate) and polyacrylates involve the intramolecular backbiting mechanism as in polyethylene [Adelman and Ferguson, 1975 Heatley... 

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... 

Figure 25.12 Intramolecular backbiting process for the formation of cyclic oligomers from PET pyrolysis. (Reprinted from Polymer, Vol. 43, B. J. Holland etai, 2002, pp. 1835-1847, with permission from Elsevier)...

The a and j3 methylene CHj groups (endo- and exocyclic ones) change their diem-ical environment. In the new environnffint their chemical shifts in the H- and C-NMR spectra differ from the previous ones. The corre ronding ectra cannot, however, discriminate between the intermolecular and intramolecular (backbiting) chain transfer, because in the non-strained rit formed by back-biting. 

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... 

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 ... 

Polycondensation chemistry results in the formation of both cyclic and linear oligomers [19]. In the case of ADMET polymerization, cyclic polymers or oligomers are formed through an intramolecular backbiting reaction between... 

Free radical intramolecular backbiting creates a butyl branch on polymer backbone. (Note ethylene insertion continues on the newly formed free radical five carbons from the chain end). 

Transfer to monomer and intramolecular (backbiting) reactions are considered to be responsible for the formation of the short side chains observed in PVC. Note that transfer reactions do not change the total number of live polymer chains and, therefore, the rate of polymerization and the number-average molecular weight will not be affected. 

Transfer may be intramolecular ( backbiting ) giving small fragments such as dimer, trimer, etc, or intermolecular, producing a macroradical in which the radical centre is not at the chain end this undergoes scission, so that intermolecular transfer leads to a rapid drop in the molecular weight (MW) of the polymer. 

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