LIVING CATIONIC POLYMERIZATION OF STYRENE IN THE PRESENCE OF TETRABUTYLAMMONIUM SALTS

The effect of tetrabutylammonium chloride on the kinetics and the molecular weight distribution in the cationic polymerization of styrene initiated by I-phenylethyl chloride/SnCl4 was investigated. This ''living'' system resembles classic cationic systems but proceeds with a lower instantaneous concentration of active carbocations. The carbenium ions form covalent species reversibly. The fraction of unpaired carbocations can be suppressed by addition of salts with common anions. When molecular weights are low enough, the transfer reactions cannot be easily detected and polymers with low polydispersities are formed. For this system, the upper limit of the molecular weight control is in the range of M(n) almost-equal-to 5000 at 20-degrees-C. Polymerization rates decrease approximately 5 times in the presence of a small amount of salt ([salt]/[SnCl4] < 0.1), remain fairly constant until [salt]/[SnCl4] almost-equal-to 1, and then rapidly decrease at ratios [salt]/[SnCl4] > 1. The changes in the structure of Lewis acid and anions were monitored by Sn-119 NMR. One average signal was observed at low salt concentrations due to a rapid exchange between SnCl4 and SnCl5-. At ratios [salt]/[SnCl4] > 1 very broad signals were found which were converted to a sharp signal of the SnCl62- anion at ratios [salt]/[SnCl4] greater-than-or-equal-to 2. The unusual kinetics and Sn-119-NMR data were explained by nearly equal ionization ability of SnCl4 and the SnCl5- anion and by the formation of inactive oligomeric anions (Cl-[SnCl5-]n) at ratios [salt]/[SnCl4] > 1.