Polymerization, anionic controlled method

Perhaps the most controlled method for the direct introduction of functionality is by living anionic polymerization. Such polymerization proceeds with an absence of chain transfer and termination, leading to polymers with predictable MW s, narrow MWD s, defined architectures and a high degree of functional purity. It is not uncommon to prepare linear or star polymers witfi MWD s <1.1 and functional purity > 95%. ... 

One of the limitations of anionic polymerization with respect to preparation of block copolymers is the rather limited range of monomers that can be polymerized anionically to form polymers with well-defined stmctures. One solution to this problem is to utilize anionic polymerization to form a well-defined polymer that is functionalized with an end group that can be used to initiate polymerization via another polymerization method, for example, controlled free-radical polymerization. One such functional group is the aminoxy group which can be used to initiate nitroxide-mediated radical polymerization (NMP). °° PSLi has been reacted with 4-methoxy-2,2,6,6-tetramethylpiperidin-1-oxyl (MTEMPO), a stable nitroxide free radical, in THF at -78 °C as shown in eqn [30]. The mechanism of this functionalization was presumed to occur... 

As discussed so far in this section, the helical polymethacrylates are synthesized mainly using anionic polymerization techniques. However, more versatile, inexpensive, and experimentally simple free radical polymerization has been proved to be an alternative, effective way to prepare helical polymethacrylates from some monomers. Although the stereochemical control of radical polymerization is generally more difficult compared with that in other types of polymerization, an efficient method would make it possible to synthesize helical, optically active polymers having functional side chains by direct radical polymerization without using protective groups. 

After all this discussion about radical polymerization and new methods to develop processes to obtain better control of the polymerization, the question remains Why Why should one use these novel methods to polymerize vinyl monomers The answer that first comes to mind is supplementation of anionic and cationic polymerization as the primary means of obtaining well-defined (co)polymers, in these cases by radical polymerization processes which are more tolerant of impurities, functional groups and are applicable to a wider range of monomers. This increased level of control over radical polymerization will allow industry to tailor a material to the requirements of a specific application using the most robust polymerization process available, ensuring the polymers have the optimal balance of physical and chemical properties for a given application. 

The main techniques for synthesis of block copolymers in research labs around the world are presently anionic polymerization and controlled radical polymerization methods. The older technique of anionic polymerization is still used widely in the industrial manufacture of block copolymers. Cationic polymerization may be used to polymerize monomers that eannot be polymerized anioni-cally, although it is used for only a limited range of monomers. A summary of block copolymer synthesis techniques has been provided by Hillmyer [5]. 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

J. Boylan and G. Banker. Molecular-scale drug entrapment as a precise method of controlled drug release II Entrapment of anionic drugs by polymeric gelation. J. Pharm. Sci. 62 1177-1184, 1973. 

The controlled radical polymerization techniques opened up a new era in polymer synthesis, and further growth and developments are certain. However, the control of the molecular characteristics and the variety of macro-molecular architectures reported by these methods cannot be compared with those obtained by other living polymerization techniques such as anionic polymerization. 

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