Backbiting reaction

ROMP now represents a well-understood technique and titanium-, molybdenum-, and ruthenium-based systems permit living polymerisations. Living in this context means that a controlled initiation takes place and that chain transfer as well as chain terminating reactions, due to possible side reactions (backbiting and secondary isomerisation) of the active metal carbene species with the inner double bonds of the polymer chains, are absent during propagation or more realistically are very limited. 

Other reactions Backbiting reaction Propagation toward formation of a short branch... 

RAFT end groups are known to be unstable at very high temperatures (>200 °C). Thermal elimination has been used as a means of trithiocarbonate end group removal. For ps430,4W direct elimination is observed (Scheme 9.54). For poly(butyl acrylate)464 the major product suggests a hoinolysis/backbiting/ i-scission reaction is involved (Scheme 9.55). 

In many polymerizations cyclic material is produced by a concurrent backbiting reaction as linear polymer is formed. For example dioxan Is formed In the cationic polymerization of ethylene oxide to polyoxyethylene, and polyoxyethylene can be degraded to... 

Much is left not undestood in this reaction. It would appear that more than one route leads to the higher MW material, most likely two to account for the appearance of the two higher MW peaks in the GPC. The low molecular weight cyclics are perhaps the products of a simultaneous growth and backbiting mechanism In one of these routes. One of the mechanisms shows some evidence of ionic or alkalimetalsilyl intermediates, and this maybe also the route to the cyclics because this is promoted in THF solution. This reaction would then form most of the products and be the major factor deciding the kinetics of the reaction. 

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. 

Figure 5.11 Example of backbiting reaction to form a butyl branch during the high pressure polymerization of polyethylene...

An example of a backbiting reaction that creates the short chain branches is shown in Fig. 18.5. In this example the growing end of a polyethylene chain turns back on itself and abstracts a hydrogen atom from the carbon atom located four bonds away from the chain end, as shown in Fig. 18.5 a). Chain growth proceeds from the newly formed unpaired electron, leaving a pendant butyl group, as shown in Fig. 18.5 b). There are many variants of backbiting, which create a variety of short chain branches. 

A complicating feature of PBT chemistry is the degradation of hydroxybutyl end groups via a reaction which results in the formation of THF and carboxylic acid end groups. This reversion is often called a backbiting reaction ... 

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