Chain backbiting

It is proposed that thermal aging of the Cloisite/PDMS elastomers promotes the reformation of the siloxane network into a more thermodynamically stable form though a series of catalytically driven chain backbiting, hydrolysis and recombination reactions. This produces a siloxane network with increased thermal stability which is more intimately associated with the nano-filler. 

T Chain-transfer agent T can represent the monomer (chain transfer to monomer), the same polymer chain (backbiting), a different polymer chain (branching), or any other molecule present in the system with a labile atom (e.g., surfactant, solvent, etc.). Even water is not inert and can actively... 

The anticipated polymer may be predicted with reasonable precision from the monomer structure, as for example in reaction (V). The rational prediction for the structure of polyethylene is clearly (I), and this was the structure first proposed in 1936 from the free-radical reaction ((V) and Figure 1.3). Five years later it was realized that this was not quite correct, and that the polymer in fact contained a few side branches. The simplest explanation of side branching is that every now and then, as the polymer propagates (V), the chain backbites , according to... 

For instance, in polymerization of cyclic acetals the oxo-nium or oxocarbenium cation acting as the active center of propagation can react with the acetal groups located inside the polymer chain (backbiting) or with the end group, the nature of which depends on the used initiator. If... 

Figure 13.2 Intramolecular chain backbiting mechanisms. Upper scheme illustrates the attack of a chain by its own OH-terminated free end to form a cyclic siloxane. Lower scheme illustrates the case where a closed, continuous chain folds back upon itself and rearrangement occurs to form a new Si-O bond and a free cyclic siloxane...

Intramolecular rearrangement of the initially formed radical may occur occasionally (e.g. backbiting - Section 4.4.3) or even be the dominant pathway (e.g. cyelopolymerization - Section 4.4.1, ring-opening polymerization - Section 4.4.2). These pathways can give rise to branches, rings, or internal unsaturation in the polymer chain. 

Copolymerization of macromonomers formed by backbiting and fragmentation is a second mechanism for long chain branch formation during acrylate polymerization (Section 4.4.3.3). The extents of long and short chain branching in acrylate polymers in emulsion polymerization as a function of conditions have been quantified.20 ... 

We can introduce short chain branching into polymers by three methods copolymerization, "backbiting , and chemical modification. The first two occur during polymerization, while the last requires a secondary chemical reaction. Short chain branches have well defined chemical structures, the nature of which we can accurately determine via analytical methods or know, from the structure of the reactants. 

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