Alkyls in Initiations of Cationic Polymerizations

Initiations of polymerizations of vinyl and other monomers by metal alkyls generally take place by anionic mechanisms. This is discussed further in this chapter. There are, however, reports in the literature of cationic polymerizations that are initiated by some metal alkyls. These are polymerizations of monomers like vinyl ethers, o- and p-methoxystyrene, and isobutylene that are initiated by compounds like dialkyl aluminum chloride. 

One explanation is as follows. These metals are strong electron acceptors. Their valence shells and their unfilled orbitals can accommodate electrons from donor molecules. As a result, the organometallic compounds behave like Lewis acids. This was observed in a polymerization of isobutyl vinyl ether. While triethylaluminum is not by itself an initiator for the polymerizations, the reactions will take place in the presence of typical electron acceptors  

The catalyst components must be combined in the presence of the monomer. This is due to high instability of the carbon cation complexes [A1(C2H5)3C1] CH30CH2] . Also, polymerizations by 

C2H5AICI2-H2O show rate decreases as the reaction progresses. When, however, additional water is added, rapid polymerizations start again.  

Substances that generate cations can vary widely. They can be molecules that dissociate into ions or react with other compounds, like solvent or monomer, to form cations. Iodine is an example of such a substance. In a system of n-butyl vinyl ether-iodine-diethyl ether, the iodine apparently forms an inactive 7r-complex with the solvent first. It subsequently dissociates and rearranges into 

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