Initiation in Anionic Chain-Growth Polymerization

Anionic initiations can take different paths. That depends upon the initiator, the monomer, and the solvent [143]. As a rough generalization, however, it is possible to separate the initiation reactions into two types (1) those that take place through an addition of a negative ion to an olefin, and (2) those that result from an electron transfer. 

The polymerization reactions can, furthermore, occur in two ways. One way is in a homogeneous environment, with both monomer and initiator soluble in the solvent. The other way is in a heterogeneous one, where only the monomer is soluble. Organolithium compounds are examples of soluble initiators, while metal dispersions are examples of insoluble ones. 

The homogeneous polymerizations can be separated further into those carried out in non-polar solvents and those in polar ones. In non-polar solvents they are confined mostly to organolithium initiators. Other organometallic compounds require polar solvents for solubility. 

The breakdown of the aggregates was shown to dramatically enhance the reactivity of the organometaUic compounds [150, 151], For instance, polymerizations of styrene in benzene with butyUithium are slow reactions. When, however, these polymerizations are carried out in tetrahydro-furan they are extremely rapid. Tetrahydrofuran is, of course, a Lewis base. Nevertheless, the breakdown of the aggregates even in such Lewis bases as tetrahydrofuran or diethyl ether are not complete, though the clusters are smaUer and more solvated. Differences in reactivity, however, can be observed even in different nmi-polar solvents [152]. 

The reaction rates depend to a great extent on the nature of the organometaUic compounds, such as polarity of the bonds and the degree of solvation. In polar solvents, where free solvated ion-pairs predominate, the mechanism of initiation may simply consist of a direct addition of the anion to the monomer. If the solvents are nmi-polar, on the other hand, the initiation is more complex. In these solvents, the metal cation coordinates with the monomer first. This is foUowed by a rearrangement [153]  

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