Chain transfer backbiting

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

The occurrence of this chain transfer reaction results in a cis-trans isomerisation of double bonds in the polymer chains however the cis or trans structure of these double bonds has no essential influence on their susceptibility to a backbiting reaction. An important implication of the intermolecular secondary metathesis reaction is, instead, the tendency of the molecular weight distribution in the resulting polymer to attain the equilibrium condition Mw/Mn = 2 [122]. 

Chain transfer may also occur to a carbon atom of the same polymer molecule five, six, or seven positions distant from the original reactive center [45]. This is called backbiting and is regarded as the mechanism of formation of short branches in polyethene polymerization [46,47],... 

Chain transfer to polymer increases with reaction temperatures. The backbiting reaction (6-89) results in the production of polyethylene with more short branches at higher polymerization temperatures. This reaction changes the polymer constitution but not its molecular weight. 

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. 

Metallocene-catalyzed Z-N polymerization is finding increased use on an industrial scale. One application is the production of linear low-density polyethylene (mLLDPE),99 which is a linear polymer with short branches incorporated deliberately at various points along the chain. Short branches are produced by Z-N copolymerization of ethene with 1-butene, 1-pentene, and 1-hexene rather than through radical mechanisms of chain transfer and backbiting. Thus, the process is... 

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. 

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