Alkyl and Metal Halide Compounds

Metal Alkyl and Metal HaBde Compounds.—Initiation reactions involving cationically-polymerizable monomers and metal alkyls or metal halides have always presented investigators with considerable difficulties. The reactive form of the initiator is often obscure and the mechanism of initiation can be influenced strongly by the presence of trace impurities. 

Grattan and Plesch demonstrated that aluminium halides in alkyl halide solvents ionize according to equation (1) and hypothesized that the cation thus 

Weaker Lewis acids derived from AICI3, such as AlEtaCl and AlEts, require the presence of a co-initiator to initiate polymerization. - Similarly, BCI3 is an effective initiator for styrene and isobutylene polymerizations only in the 

The problem of determining the exact nature of the initiating species in cationic polymerizations of alkenes conducted in the presence of metal perchlorates led to the use of a sterically hindered proton scavenger, 4-methyl-2,6-di-t-butyl pyridine. Since initiation occurred in solutions containing this compound it was concluded that protons derived from impurities were not responsible for the reaction and direct initiation by the metal perchlorates was postulated. However, results from experiments with the BClj/HjO system suggest that hindered pyridines act as proton transfer agents. Differences in mechanism between protonic acid — and metal halide - initiated oligomerizations of styrene and DPE - have been defined. 

Neither chromyl chloride nor the mixed initiator diethyl zinc/phosphoryl chloride were found to induce stereoregularity in the polymerization of alkyl vinyl ethers. 

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