Ionic polymerization. VI. The mechanism of molecular termination by aromatic compounds in cationic polymerization of sytrene

Evidence is described concerning the mechanism of molecular termination by aromatic compounds in the cationic polymerization of syrene. Chemical analysis and infrared spectra of the polystyrenes produced by stannic chloride catalysis in the presence of thiophene, anisole, or p ‐chloroanisole indicate that these compounds are incorporated into the polymer in the process of molecular termination, evidently as end groups. It is found that the molecular termination constant of p ‐xylene is independent of the dielectric constant of the medium. Consideration of the relative values of the molecular termination constants of representative aromatic compounds reveals that their order of reactivity corresponds to the order of net electron‐donating character of the substituents attached to the aromatic nucleus. With the aid of the above facts, this leads to the conclusion that these compounds react with the carbonium ion‐anion pairs of the active centers by a mechanism analogous to that of the familiar Frindel Crafts alkylation. Since the three reactive aromatic hydrocarbons studied are found to have no detectable influence on the over‐all rate of polymerization, it is concluded that these compounds are chain transfer agents. p ‐Chloroanisole accelerates the polymerization, while thiophene and, to a small extent, anisole, retard it. The broader applicability of the molecular termination equation to cationic than to free radical polymerization is disscussed, and some suggestions are made concerning the mechanism of spontaneous termination and chain transfer with the monomer in the cationic polymerization of styrene.