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

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(cyclopentadienyl)titanium dichloride occurred with Cu, Devarda's alloy, Ni, Ce, Cr, Sri, 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, powder, or nanoparticles. Nano-Zn provided the lowest polydispersity index values, highest 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 reversible termination step. The polymerization was insensitive to light or dark conditions, and a linear dependence of Mu on the conversion was observed at all temperatures in the 60-130 degrees C range with an optimum at 70-90 degrees C. 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. (c) 2006 Wiley Periodicals, Inc.