Alkoxy-Substituted Titanium(IV) Chlorides as Lewis Acid Activators for Living Cationic Polymerization of Isobutyl Vinyl Ether: Control of Lewis Acidity in the Design of Initiating Systems

Abstract

Titanium(IV)-based new initiating systems for living cationic polymerization of isobutyl vinyl ether (IBVE) have been developed by using a series of isopropoxytitanium chlorides [TiCl 4-n (OiPr) n ; n = 0-4] in conjunction with the IBVE-HCl adduct in CH 2 Cl 2 at -15 °C. The 1 H and 13 C NMR analysis of the titanium compounds showed that their Lewis acidity decreased with the increasing number (n) of isopropoxyl groups and affected the polymerizations accordingly. For example, TiCl 4 and TiCl 3 (OiPr), the strongest Lewis acids among the five, induced very rapid polymerizations to give polymers with broad molecular weight distributions (MWDs) (M w /M n ∼1.6). In contrast, the reaction rate with TiCl 2 (OiPr) 2 , a weaker Lewis acid, was moderate. The MWDs of the polymers were narrow throughout the reaction (M w /M n < 1.2), and the number average molecular weights increased in direct proportion to monomer conversion. The polymerization with TiCl(OiPr) 3 was slower, and the MWDs became slightly broader. No polymer was obtained with a much weaker Lewis acid, Ti(OiPr) 4 . Another modified titanium chloride, TiCl 2 (OPh) 2 , also led to a controlled polymerization of IBVE which is similar to the one with TiCl 2 (OiPr) 2 . These results indicate that living cationic polymerizations of IBVE can be achieved by controlling the Lewis acidity of the titanium(IV) activators with electron-donating groups.