Polymerization living ionic method

Living ionic methods, however, have limitations as to the types of monomers that can be polymerized resulting from the incompatibility between the reactive centers and monomers. Radical polymerizations, on the other hand, do not really suffer from these drawbacks because a free radical is less discriminating re-... 

Methods of obtaining block copolymers by radical processes have been developed rather lately about other processes, and especially ionic methods. This may be due to the nature of the radical, which is an intermediate with a very short lifetime, and a very high non-selective reactivity. These characteristics do not favor a well-controlled architecture as in the case of living carbanions appearing in anionic polymerization. However, the recent development of new... 

The methods for graft polymerization of cellulose can be generally classified into three major groups such as (i) free-radical polymerization, (ii) ionic and ring opening polymerization, and (iii) living radical polymerization. 

Living ionic polymerization methods provide unique control of average... 

Living ionic polymerization is frequently used to synthesize amphiphilic block copolymers in the absence of irreversible chain transfer and chain termination, where aU polymer chains are instantaneously initiated and grow simultaneously [30]. The presence of counterions in the reaction medium associated witli the propagating chains preserves the electroneutrality of the entire system. However, these living polymerization methods are affected significantly by the nature of the solvent and the presence of water, carbon dioxide, and/or impurities. Moreover, they have limited apphcations for the synthesis of block copolymers with functional groups as side chains. 

In order to reduce the contribution of termination processes (Scheme 2.19), controlled/living radical methods establish a fast equilibrium between dormant and active chain ends. Essentially, this equilibrium decreases the relative proportion of propagating radicals and allows them to reversibly deactivate rather than permanently terminate. The radical chain end concentration for CRPs (10 -10" M) is also much less than for ionic polymerizations (10 -10 M). 

Many of the aforementioned characteristics of CRP are in common with those of previously developed (pseudo)living ionic polymerization techniques. However, it is the relative ease with which CRP can be employed to achieve these characteristics that has led to the continued success and growth of the field. Indeed, because most of the methods described in the chapters that follow can be conducted under relatively non-stringent conditions in a variety of solvents and at a wide range of temperatures, the barrier to entry to the field is minimal. Precision polymer synthesis is now possible in laboratories without access to expensive experimental equipment or extensive expertise. 

A major difference between radical polymerization and the various ionic methods is that, in the latter, the incoming monomer must fit between the growing chain end and an associated ion or complex. The growing radical chain, however, has no snch impediment at the growing end although the precise nature of the growing chain end in radical and living radical polymerization is not completely understood. 

A number of techniques for the preparation of block copolymers have been developed. Living polymerization is an elegant method for the controlled synthesis of block copolymers. However, this technique requires extraordinarily high purity and is limited to ionically polymerizable monomers. The synthesis of block copolymers by a radical reaction is less sensitive toward impurities present in the reaction mixture and is applicable to a great number of monomers. 

---