Living cationic polymerization of p-methylstyrene by hydrogen iodide/zinc halide initiating systems

Abstract Living cationic polymerization of p ‐methylstyrene has been investigated with use of hydrogen iodide/zinc halide initiating systems (HI/ZnX 2 ; X = Cl, I) in toluene and methylene chloride solvents. The best results were obtained with HI/ZnCl 2 below 0°C where the zinc salt was employed in excess over hydrogen iodide (HI/ZnCl 2 = 1/5 molar ratio). Under these conditions, the number‐average molecular weights \documentclass{article}\pagestyle{empty}\begin{document}$ \left( {\overline {M_n } } \right)$\end{document} of the produced polymers increased proportionally to monomer conversion, further increased upon addition of fresh feeds of p ‐methylstyrene into completely polymerized reaction mixtures, and were in good agreement with the calculated values assuming that one living polymer chain forms per molecule of hydrogen iodide; the molecular weight distributions (MWDs) of the polymers were fairly narrow \documentclass{article}\pagestyle{empty}\begin{document}$ \left( {{{\overline {M_w } } \mathord{\left/ {\vphantom {{\overline {M_w } } {\overline {M_n } }}} \right. \kern-\nulldelimiterspace} {\overline {M_n } }} = 1.1 - 1.3} \right) $\end{document} throughout these processes. In contrast, when equimolar mixtures of hydrogen iodide and zinc chloride or iodide were employed, the polymerizations were rather slow even in methylene chloride at +25°C, and the product polymers exhibited bimodal MWDs, the lower‐molecular weight fraction of which was mediated by long‐lived growing species. Addition of tetra‐ n ‐butylammonium iodide as a common ion salt ( n Bu 4 NI/HI = 1/200 molar ratio) led to unimodal MWDs consisting of the long‐lived lower polymer fraction alone.