Ethylene polymerization backbiting

Figure 2.2 Backbiting mechanism in free radical ethylene polymerization 27...

Branching can occur in the presence of chain transfer to polymer reactions. For example, in ethylene polymerization, the radical centers backbite their own chains and form short-chain branches. It is these short-chain branches that limit chain crystallization and result in LDPE. Because of no control over backbiting sites, the branch lengths are ill defined and vary a lot from site to site and from chain to chain. 

In many polymerizations cyclic material is produced by a concurrent backbiting reaction as linear polymer is formed. For example dioxan Is formed In the cationic polymerization of ethylene oxide to polyoxyethylene, and polyoxyethylene can be degraded to... 

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 formation of a short branch in polyvinyl acetate, as in the case of high-pressure polymerization of ethylene, by the backbiting intramolecular radical transfer is also operative [61] ... 

Similar phenomena, although less effective, take place in the polymerization of all monomers leading to macromolecules with heteroatoms in the chains. A related phenomenon is chain transfer to the same macromolecule, called backbiting and bearing a resemblance to the intramolecular transfer in, for example, polymerization of ethylene. However, in the latter, branching results from backbiting and in ROP/ROMP formation of cydic molecules is the dominating process. 

Backbiting is an intramolecular chain transfer reaction. If transfer reactions occur between different chains, long-chain branched polymers are formed. Well-known examples include ethylene and vinyl acetate. Vinyl acetate polymerization could lead to gel formation under certain conditions. It should be pointed out that chain transfer to polymer reaction alone generates only T-type branch structures that do not result in gel formation. Theoretically, some mechanism such as radical termination by combination that brings two chains together to form H-type branch structures is an essential condition for gelation. 

The commercial production of LDPE is by free-radical polymerization. Supercritical bulk ethylene is fed into a tubular reactor operated at steady state. The polymers experience both short-and long-chain branching. The short-chain branches are a consequence of backbiting and the long-chain branches are a consequence of chain transfer to polymers. The low density of the product is a consequence of short-chain branching. 

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