Since the previous progress report was submitted (Sept. 06), we have focused mainly on the dissemination of work highlighted in that report in the primary literature:

Specifically, a communication describing the reactions of both hindered and unhindered carbocations, partnered with weakly coordinating anions (WCA), with sterically hindered pyridines has been accepted for publication in Macromolecules and is in press at the current time. This work is important in that it demonstrates that carbocations partnered with WCA are not inert towards reaction with sterically hindered pyridines (as has been maintained by some workers in this field for many years), though these reactions are much slower than propagation.  In this same paper we also provide an example of a sufficiently hindered carbocation that is inert towards direct reaction with a sterically hindered pyridine.

The implications of this work as far as living polymerization are as follows. Unlike living polymerization using conventional Lewis acids and alkyl halide initiators, the chelating diborane initiators do not suffer facile hydrolysis and if they do, the hydrolysis by-products are insufficiently acidic to react irreversibly with proton traps. Thus, the latter additives are free to degrade propagating chain ends, if present in sufficiently high amounts. Even without this complication, the carbocations partnered with WCA that are formed in situ from alkyl halides are generally formed irreversibly from these initiators and so there is no dormant alkyl halide in equilibrium with propagating carbocation as required for a quasi-living polymerization.

A full paper has been submitted for publication (Organometallics) detailing the use of various alkyl halide or pseudohalide initiators in isobutene polymerization, along with detailed spectroscopic and structural studies of the ion-pairs formed. This work reveals that the sterically hindered, bridged counter-anions formed from cumyl ethers or azide can act as weak bases towards propagating chain-ends rather than nucleophiles (as required for a living polymerization). In the presence of proton traps, the reversible deprotonation that occurs involving the counter-anion, can be rendered irreversible, by subsequent proton transfer to the hindered pyridine if present in sufficient amounts

Finally, a manuscript is in preparation which studies the mechanism for chain transfer in isobutene polymerizations involving both protic initiation or initiation involving cumyl chloride. In the former case, adventitious water is shown to be a potent chain transfer agent when present at ppm levels in molecular form, while indirect evidence suggests that a classical process referred to as chain termination to counter-anion, may actually be a chain transfer process if  reaction of the resulting Brønsted-Lewis acid pair with monomer is competitive with any proton scavengers which may be present.

Unfortunately, we were not able to develop any living initiator systems during the grant period. On the other hand our work draws attention to the fact, that despite having developed quasi-living conditions for isobutene polymerization, much of the mechanistic understanding of that process is still fragmentary and in some cases even erroneous. The well-defined initiators we have developed promise to place cationic polymerization on a much better mechanistic footing than has been possible in the past.