Carbonium tetrachloride

It was known that polystyrene and poly- >-methoxystyrene initiated by tin tetrachloride have a branched structure, due to aromatic substitution in the course of the polymerization (186). Haas, Kamath and Schuler (93, 124) studied the ionic chain transfer reaction between a polystyrene carbonium chain and poly-/>-methoxystyrene. They were able to separate the homopolymers from the graft copolymers by extraction with methylcyclohexane. 

Plesh (188) treated polyvinyl chloride and vinyl chloride-vinylidene chloride copolymers with aluminium chloride or titanium tetrachloride for initiating the carbonium polymerization of styrene. As expected (218)... 

This situation is somewhat reminiscent to that encountered in enzyme chemistry where the active biocatalyst is a combination of an apo-enzyme and a coenzyme, the components alone being complete inactive. Substrate specificity, which is so characteristic for enzymatic processes is also high in carbonium ion chemistry. For example styrene is polymerized by titanium tetrachloride—water, but not by titanium tetrachloride— alkyl chlorides 37) however, with stannic chloride catalyst alkyl chlorides are effective cocatalysts 88). In the same vein Plesch (93) showed that water is a better cocatalyst than acetic or chloroacetic acid in conjunction with titanium tetrachloride in isobutene polymerization, but Russel (94) found just the opposite with stannic chloride. 

If a carbonium ion were involved, we should expect the halide to be derived from the more stable, and therefore the more highly branched, alkyl group. It is also pertinent that anisole is not cleaved by dry hydrogen chloride or hydrogen bromide in carbon tetrachloride solution unless a small amount of pyridine, dimethylaniline, or aniline is added.18 Furthermore, the presence of these bases increases the rate of cleavage of ethers in acetic acid solution. All these experiments imply that more than two molecules participate in the formation of the transition complex leading to etherification or cleavage (see p. 83). 

The carbonium ion and the perchlorate ion remain closely associated as an ion pair. The most important types of initiators for cationic polymerization are Lewis acids or Friedel-Crafts catalysts such as BF3, AICI3 or SnCU. For Friedel-Crafts catalysts, proton transfer cannot take place without a source of ions. It is found BF3 will catalyse the polymerization of styrene in carbon tetrachloride only when traces of water are present. 

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