Intramolecular transfer reaction

Many reactions have been performed in the presence of a solvent. However, the solvent must be chosen carefully to avoid reaction with polymer. For example, the low yield for grafts of acrylonitrile on polyamides in the presence of methanol has been shown to be due to the methanolysis (18,31). Generally speaking, the grafted products are principally obtained however minor amounts and homopolymers can also result. The homopolymerization proceeds by an intramolecular transfer reaction between macroradicals and monomers. The amount of homopolymer depends on the system. Details on systems already investigated will be described in the next section. 

The homopolymerization reaction proceeds through an intramolecular transfer reaction between macroradicals and monomers ... 

The transfer of hydrogen to peroxyl radicals may proceed intra- or inter-molecularly. Intramolecular transfer reaction (isomerization) of peroxyl macroradicals of polypropylene occurs during the oxidation of the polymer in a solution of inactive solvent [75] while the intermolecular transfer is preferred during the oxidation in reactive solvent or in the crystalline state [76]. 

The intramolecular transfer reactions can be more complicated, and an example is the formation of 1-methyl-4-isopropenylcyclohexene (DL-limonene) during the pyrolysis of polyisoprene, which is formed from an intramolecular transfer step as shown below (hydrogen migration is shown by arrows) ... 

In the beginning of this section the AMSA approach will be applied to the description of this model of electrolyte solution. The obtained results will be applied to describe the thermodynamic properties of electrolyte solution and to study the effect of electrolyte solution on intramolecular transfer reactions. Finally, the specific features of the effect of ion association on the properties of electrical double layer will be discussed. 

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. 

The minimum in degradation rate found for subsaturation PVC obtained around 55°C becomes less obvious if the monomer concentration at the reaction site is used as variable instead of the relative monomer pressure, P/PQ. The observed behavior is mainly due to the influence of the polymerization conditions on the formation of thermally labile chlorine, i.e. tertiary chlorine and internal allylic chlorine. Tertiary chlorine is associated with ethyl, butyl and long chain branches. The labile structures are formed after different inter-and intramolecular transfer reactions. Generally, the content increases with decreasing monomer concentration and increasing temperature in accordance with the proposed mechanisms. The content of internal double bonds instead decreases with increasing temperatures. 

While long branches are formed by the normal chain transfer to polymer, as shown above [Eq. (6.124)], reactive radicals like those of polyethylene can also undergo self-branching by a backbiting intramolecular transfer reaction (see Fig. 6.9) in which the chain-end radical abstracts a hydrogen atom from a methylene unit of the same chain resulting in the formation of short branches (as many as 30-50 branches per 1000 carbon atoms in the main chain) that outnumber the long branches by a factor of 20-50. 

Facilitation of intramolecular transfer reactions The proximity of two successive enzymes in the reaction sequence facilitates the direct group transfer. 

The long-chain branching thus produced has been shown by comparison of the radii of gyration with those of poly(methylenes) of the same molar mass, since the latter possess practically no branching. On the other hand, butyl groups are produced by intramolecular transfer reactions ... 

A mechanism involving random homolytic scission of the chain followed by a series of inter and intramolecular transfer reactions has been shown to be applicable to the simple n-alkyl polyacrylates and is suggested to be generally applicable to the lower branched chain esters. 

In common with the polymers of the normal alkyl esters and of the isomeric propyl esters, steric factors reduce the intramolecular transfer reactions and the yields of monomer and low molecular weight products increase as the alkyl chain length is increased. 

Intramolecular transfer reactions are possible with cationic polymerizations. This leads to an isomerization of the monomeric unit. Since the monomeric unit of the resulting polymer often cannot be produced by polymerization of the monomer or unimer of this polymer, reactions of this type are also referred to as phantom or exotic polymerizations. At low temperatures, the isomerization polymerization is preferred to normal propagation, and here two types can occur. An isomerization of bonds occurs in the transannular polymerization of norbornadiene ... 

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

The degradation reactions involved in PS include random scission (which reduces the molecular weight of the polymer), depolymerization (which yields monomer), intramolecular transfer reaction (which produces dimer, trimer, etc.), and intermolecular transfer reaction which reduces the molecular weight of the polymer. The initial degradation products from PVC are Cl radicals and HCl (Owen 1984 Ahmad and Mahmood 1996). The structure and composition of PVC and PS and the interaction of various products formed may give rise to some cross products formed from the radicals or molecules which migrate across these phase boundaries and can play an important role in the degradation of blends. 

Other Anhydrides.—A D-glucosyltransferase from Aspergillus niger acted on maltose to give, among other products, 1,6-anhydro-p-D-glucopyranose (1 %) by an intramolecular transfer reaction. A series of oligomeric ethers (88) was... 

As already discussed (see Section 7.2.4), inter- and intramolecular transfer reactions to polymer are of great importance with regard to the molecular weight distribution (Scheme 7.6). In complete thermodynamic equilibrium, the conversion should be given by the equilibrium monomer concentration and the molecular weight distribution should be the most probable one with D = 2, because of the intermolecular chain transfer, which is responsible for scrambling of the molecular... 

In contrast to the case of ethylenic monomers where transfer results in chain branching, intermolecular transfer reactions in the polymerization of heterocyclics correspond to exchange reactions between chains, with the number of chains remaining constant. Intramolecular transfer reactions—which is generally the case due to a higher probability of collision—yield inactive cycles which can possibly be the monomer (n = 1) and thus correspond to a depolymerization ... 

Only the former of the macroradicals (A) and (B) is believed to lead directly to production of volatile products (Scheme 15). Monomer production involves depropagation of radical (A) by reaction (a) in Scheme 15, whilst backbiting intramolecular transfer (reactions b, c) account for the dimer, etc. Fragments such as the trimer, etc. can be formed in the same way by H abstraction at subsequent tertiary H sites along the chain. 

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