Free radical polymerization backbiting reactions

Scheme 4.7 Backbiting reaction or intramolecular transfer to polymer. TABLE 4.5 Simple Kinetic Mechanism for Free Radical Polymerization...

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.

For certain monomers such as vinyl acetate and ethylene, branching is much more significant. The free-radical (high-pressure) polymerization of low-density polyethylene (LDPE) includes a back-biting internal chain-transfer reaction that results in the formation of a short branch. It is this branching that results in an upper limit for the crystallinity of LDPE of about 60%-70% and a melt temperature of 110 C the backbiting reaction preferentially occurs with the formation of an intramolecular six-membered ring that results in preferential formation of a C4 short-chain branch as shown in Scheme 1.41. 

The generally accepted mechanism for the formation of short branching in polyethylene involves a backbiting intramolecular transfer reaction in which a radical at the end of the polymer chain abstracts a hydrogen atom from a methylene unit in the same chain (Fig. 6.9). This is a very important process in the free-radical, high-pressure polymerization of this monomer. Branched polyethylene from this process has lower crystallinity than linear polyethylene produced by a low-pressure process and as a consequence it tends to be less rigid and tougher and form clearer films than the latter. 

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