Backbiting transfer

It has been known for some time that intramolecular atom transfer, or backbiting, complicates polymerizations of E (Scheme 4.40 - Section 4.4.3.1), VAc and VC (see 4.4.3.2). Recent work has shown that backbiting is also prevalent in polymerization of acrylate esters (Section 4.4.3.3) and probably occurs to some extent during polymerizations of most monosubstituted... 

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... 

A realistic RI5 model is used to estimate the relative probabilities of the formation of various types of short branches in ethylene-vinyl acetate copolymers that are rich in ethylene. Butyl is predicted to be the most common short branch in all of the copolymers examined, although it Is less common in the copolymers than in low-density PE. The major factor responsible for the suppression of the Rq4 backbiting intrachain radical transfer is the increased preference for trans states at the main chain bonds flanking the attachment site for an Isolated acetoxy side chain. 

For acrylate polymerization by GTP the cyclohexanone that results from backbiting is an a-ketoester with an active hydrogen that reacts with the methoxide thus thwarting the transfer process (Scheme 15). 

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]. 

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