Short-chain branching intramolecular

The kinetic scheme of this process was studied in greater detail by German researchers102 the formation of long-chain branches (chain transfer on the polymer) and short-chain branches (intramolecular chain transfer) was taken into account. 

It has been shown that intramolecular chain transfer to polymer occurs during the polymerisation of vinyl acetate, lea ding to short-chain branching (81,235—238). The number of short-chain branches has been estimated by nmr to be in the range of 0.12—1.7 mol % (81). The number of short-chain branches increases significantly at low monomer concentration. 

The predominating type of nonlinearity in polyethylene appears to consist of short chain branches three or four chain atoms in length formed by intramolecular chain transfer as follows ... 

These steps are illustrated in Fig. 2.3 and were described in Section 2.3. When chain transfer involves intermolecular or intramolecular reactions, long and short chain branches are created. 

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. 

In addition, short-chain branches of C5 and C7 are seen at levels of 1 per 1000 carbon atoms, compared with a maximum of 15 n-butyl branches. This also arises from a backbiting reaction of the propagating radical and the resultant intramolecular chain transfer, and the relative amounts of the branches of various lengths may vary depending on the conditions of synthesis. 

Chain transfer to polymer also is possible and results in the formation of branched polymer molecules. This can occur intramolecularly, in which case it is known as back-biting and mainly produces short-chain branches, as for example in the polymerization of ethylene ... 

Short-chain branching, presumably caused by intramolecular chain transfer between a terminal free radical and a hydrogen atom further back in the chain. For example, one reaction in polyethylene is postulated as follows (Billmeyer, 1962, pp. 364-375) ... 

A transfer to polymer produces a free radical at the point of transfer and more monomer can add on at this point. This causes branching in the polymer. If the transfer reaction occurs intramolecularly as in poly(ethylene), then, short chain branching is the result ... 

A transfer to polymer produces a radical site on the polymer, and this site can add on further monomer, thereby producing branched polymer. If transfer to polymer occurs intramolecularly as in poly(ethylene) (see Section 25.2.1), then short-chain branching will result ... 

The newly produced free radicals start the polymerization of ethylene and thus produce long-chain branching. Short-chain branching occurs by intramolecular transfer reactions ... 

Rigo et aL [45] proposed a mechanism of branching involving chain transfer. Long-chain branches may be the result of transfer involving two polymer chains short-chain branches were attributed to an intramolecular chain-transfer mechanism. Presumably chain transfer involving monomeric vinyl chloride may also lead to branching. 

Scheme 4.12. Formation of a mid-chain radical by intramolecular chain transfer to polymer. Monomer addition to the new radical structure creates a short-chain branch in the polymer.

Kinetic treatment of these more complex mechanisms is often difficult. Equations (32)-(34) are a network of reactions developed to treat intramolecular transfer, short-chain branch formation, and jS-scission for butyl acrylate polymerization [47]. 

Intramolecular chain transfer leads to short-chain branching with mainly butyl groups at the branches ... 

Intramolecular CT (backbiting), inteimolecular transfer to polymer, and p-scission largely affect polymer stmcture. Backbiting occurs by 1,5-H shift and results in the formation of short-chain branches, mostly n-butyl side groups. The backbiting reaction has been thoroughly studied by Goto et al. and rate coefficients are available as a ftinaion of pressure and temperature. 

Intramolecular reaction or backbiting, which gives rise to short chain branches (length 5 carbons). 

Roedel (74) proposed an intramolecular chain-transfer or backbiting reaction, such as illustrated in step 1 of Fig. 13, to account for the short-chain branches found in polyethylene. Subsequent infrared (75, 76), pyrol57sis (77), and irradiation (75, 78, 79) studies have shown that these ort branches consist entirdy (within limits of detection) of 2- and 4-carbon units. Inasmuch as Roedel originally predicted that the branches should be mostly 4 and 5 carbons in length, and inasmuch as the absence of methyl, propyl, and amyl branches indicates great specifidty in branch formation, the radical transfer mechanism for short-chain branching has been questioned by Wickham (80) and van dec Molen (43). 

In practice, the hnear polymer we might expect for alkenes is not the major product of the free radical process. (Cationic polymerization is generally used to prepare linear addition polymers of alkenes.) The product chains have many alkyl branches, which most often are the four-carbon-atom butyl groups produced by short chain branching. These products are the result of intramolecular hydrogen abstraction by way of a six-membered transition state that generates a secondary radical from a primary radical. 

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