Titanium-Mediated Living Radical Styrene Polymerizations. VI. Cp2TiCl-Catalyzed Initiation by Epoxide Radical Ring Opening: Effect of the Reducing Agents, Temperature, and Titanium/Epoxide and Titanium/Zinc Ratios

ABSTRACT: The effects of the reducing agent, temperature, and epoxide/Ti and Ti/Zn ratios were investigated for the Cp2TiCl-catalyzed living radical polymerization of styrene initiated by epoxide radical ring opening. No reduction of bis(cyclopenta-dienyl)titanium dichloride occurred with Cu, Devarda’s alloy, Ni, Ce, Cr, Sn, Mo, and ascorbic acid, whereas Al, lithium nitride, Mn, Sm, and Fe led to free-radical or poorly controlled polymerizations. The best results were obtained with Zn alloy, pow-der, or nanoparticles. Nano-Zn provided the lowest polydispersity index values, high-est initiator efficiency (IE), and fastest reaction rate while maintaining a well-defined living polymerization. Progressively lower polydispersity was obtained with an increasing excess of Zn with an optimum at Cp2TiCl/Zn 1/2. This was rationalized through the heterogeneous nature of Zn and its possible involvement in the reversi-ble termination step. The polymerization was insensitive to light or dark conditions, and a linear dependence of Mn on the conversion was observed at all temperatures in the 60–130 8C range with an optimum at 70–90 8C. A stoichiometric 1/2 epoxide/Ti ratio provided low polydispersity (weight-average molecular weight/number-average molecular weight < 1.2) and high IE, whereas increasing the epoxide/Ti ratio to 1/3 maintained a low polydispersity index but decreased the IE. VC 2006 Wiley Periodicals