Polymerization of Methyl Methacrylate with the Carbon Tetrachloride/Dichlorotris- (triphenylphosphine)ruthenium(II)/Methylaluminum Bis(2,6-di-tert-butylphenoxide) Initiating System: Possibility of Living Radical Polymerization

Introduction. Because of the virtual absence of chain transfer, termination, and other side reactions, living polymerization is among the best methods for the synthesis of polymers with controlled molecular weights and architecture^.^ It is therefore not surprising that, since the discovery of the now classical anionic systems: a number of living polymerizations have been developed by utilizing almost all of the mechanisms of addition and ring-opening polymerization^.^ Perhaps the only remaining frontier in this aspect would be living radical polymerization, for which attempts have in fact been started quite recently in several research groups.6-’l In our view, however, well-defined, highly versatile living radical polymerizations are still beyond our reach, and no general principles have been established to achieve living or controlled radical polymerizations. The primary reasons for the difficulty include the high reactivity or low stability of free radicals and their facile coupling and/or disproportionation reactions. Recently, we have achieved living cationic polymerization of vinyl compounds, where control of the polymerization has long been considered very difficult, due to the instability of the highly reactive carbocationic growing species and their tendency to undergo chaintransfer reactions.12J3 In these living cationic processes, the unstable growing carbocations are considered to be “stabilized” via the reversible formation of the corresponding covalent or dormant species that rapidly exchange with the ionic counterparts.14 For example, the living cationic polymerizations of vinyl ethers with the HCI/ZnClz initiating system involve the intermediate (1) with a carbon-chlorine terminal bond derived from hydrogen chloride (initiator) and the monomer, and the covalent end in 1 is reversibly activated by ZnClz into the carbocationic form (2) that actually propagates with monomer (eq 1). Our recent NMR analysis of