The origin of living polymerization with an o-fluorinated catalyst: NMR spectroscopic characterization of chain-carrying species

摘要

To characterize the origin of living polymerization with nonmetallocene titanium-based catalysts containing o-fluoroaryl substituents, ethene polymerization by an o-fluorinated bis(enolatoimine) titanium catalyst and its nonfluorinated counterpart has been studied by multinuclear NMR spectroscopy by using methylaluminoxane (MAO) or AlMe _3/CPh _3B(C _6F _5) _4 as activators. Formation of ion pairs of the type [TiL _2Me][MeMAO] and [TiL _2Me][B(C _6F _5) _4] has been observed for both catalysts. These ion pairs react with ethene to afford the chain-propagating species [TiL _2P][MeMAO] and [TiL _2P][B(C _6F _5) _4], respectively (P=growing polymeryl chain). For the o-F-substituted catalyst species of the second type, NMR spectroscopy provides evidence that the o-F substituents interact with the metal center. This interaction is proposed to keep the polymerization catalysis living by suppressing chain transfer to AlMe _3 and β-hydrogen transfer processes. Never-ending story: Multinuclear NMR spectroscopic characterization of the chain-propagating species in the o-F-substituted bis(enolatoimine) titanium catalyst system for living ethene polymerization and its non-fluorinated (non-living) counterpart is reported. For the living polymerization catalyst, weak coordination of aryl o-F substituents to Ti prevents the metal center from coming into contact with the β-CH _2 group of the growing polymer chain and thus hampers the chain termination.