Highly Electrophilic Organometallics for Carbocationic Polymerizations: From Anion Engineering to New Polymer Materials

Abstract

Ion-ion interactions are a crucial but often overlooked aspect of many polymerization reactions. The precise nature of cation-anion binding is as yet poorly understood, and little is known of the extent of ionic interactions in the typically nonaqueous, low-polarity reaction media of most polymerizations. Nevertheless, adequate control of cation-anion interactions can greatly enhance the productivity and efficiency of chemical processes and can provide low-energy alternatives to current methods. This is illustrated here with the carbocationic polymerization of isoalkenes. Carbocationic polymerizations involve, as the name implies, carbocations as propagating species. Of the various types of substrates that can be polymerized cationically, the copolymerization of isobutene to isobutene-isoprene rubber stands out as the only large-scale, industrially important implementation of this reaction type. The products, elastomers with controlled degrees of unsaturation for subsequent cross-linking, have excellent gas barrier and mechanical dampening properties that make them indispensable components in polymer composites. For such applications, the polymer molecular weight has to be high, ∼5 × 10(5) g/mol, with 1-2 mol % isoprene. Cationic polymerizations are however notoriously difficult to control. As a means of suppressing chain transfer, the process is carried out at temperatures as low as -100 °C, with aluminum chloride initiators in chloromethane. Current industrial production of isobutene-isoprene butyl rubber is thus highly energy intensive and produces aluminum and chloride effluent. This Account summarizes how highly electrophilic organometallics coupled with new types of very weakly coordinating counteranions can provide the basis for a more environmentally friendly, lower energy alternative. Because any copolymerization of two monomers, here primarily isobutene and isoprene, leads to two different propagating species, each of which is characterized by different chain growth and chain termination kinetics, variation of the associated counteranions can give rather unexpected results. With judicious choice of the initiator and the counteranion, new chemistry can be injected into such processes and can open avenues to new families of polymer materials. Mechanistic investigations of the initiation process with zirconocene hydrides illustrate the complexity of this first step. Replacing aluminum with zinc initiators not only provides a nontoxic alternative but also generates a system in which the polymer molecular weight is much less affected by temperature and comonomer concentration, which can lead to a range of products, from oligomeric lubricant precursors to C═C-rich high-molecular-weight elastomers. The key in all these cases is the construction of either preformed or in situ-generated complex anions that are resistant to electrophilic or redox degradation and are capable of stabilizing tightly associated carbocations. Such initiator systems allow much more benign operating temperatures, reduce the need for chlorocarbon solvents, and can operate at concentrations as low as 5 × 10(-5) M. Along the way are provided the first examples of structurally characterized sec-alkyl carbocations and carbocation salts of organometallic zincates.