Living cationic

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

Since the discovery of living cationic systems, cationic polymerization has progressed to a new stage where the synthesis of designed materials is now possible. The rapid advances in this field will lead to useful new polymeric materials and processes that will greatiy increase the economic impact of cationic initiation. 

Cationic Polymerization. For decades cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition (see Elastomers, synthetic-BUTYLRUBBEr). More recently, development has led to the point where living cationic chains can be made, with many of the advantages described above for anionic polymerization (27,28). 

Considerable advances have taken place in the 1990s with regard to cationic polymerisation of styrene. Its uses to make block copolymers and even living cationic polymerisation have been reported (171). 

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

A living cationic polymeriza tion of isobutylene and copolymeriza tion of isobutylene and isoprene has been demonstrated (22,23). Living copolymerizations, which proceed in the absence of chain transfer and termination reactions, yield the random copolymer with narrow mol wt distribution and well-defined stmcture, and possibly at a higher polymerization temperature than the current commercial process. The isobutylene—isoprene copolymers are prepared by using cumyl acetate BCl complex in CH Cl or CH2CI2 at —30 C. The copolymer contains 1 8 mol % trans 1,4-isoprene... 

Besides being used as initiators and monomers, azo compounds may also be used for terminating a cationic polymerization. Thus, the living cationic polymerization... 

Of recent interest is the living cation of tetrahydrofuran. The oxonium ion with suitable counter ions is stable and exists in equilibrium with the monomer47. Bifunctional initiators were found to produce the polytetrahydrofuran dication. 

It is to be noted that N-vinylcarbazole (NVC) undergoes also living cationic polymerization with hydrogen iodide at —40 °C in toluene or at —78 °C in methylene chloride and that in this case no assistance of iodine as an activator is necessary 10d). NVC forms a more stable carbocation than vinyl ethers, and the living propagation proceeds by insertion between the strongly interacting NVC-cation and the nucleophilic iodide anion. 

Polystyrene-polytetrahydrofuran block copolymers121122 are an interesting case of coupling between functional polymers The mutual deactivation of living anionic polystyrene and living cationic polyoxolane occurs quantitatively to yield polystyrene-polyoxolane block copolymers. Since either of the initial polymer species can be mono- or difunctional, diblock, triblock or multiblock copolymers can be obtained. 

Yijin X. and Caiyaun P., Block and star-hlock copolymers by mechanism transformation. 3. S-(PTHF-PSt)4 and S-(PTHF-PSt-PMMA)4 from living CROP to ATRP, Macromolecules, 33, 4750, 2000. Feldthusen J., Ivan B., and Mueller A.H.E., Synthesis of linear and star-shaped block copolymers of isobutylene and methacrylates hy combination of living cationic and anionic polymerizations. Macromolecules, 31, 578, 1998. 

Allcock HR, Crane CA, Morrissey CT, Nelson JM, Reeves SD, Honeyman CH, and Manners I. Living cationic polymerization of phosphoranimines as an ambient temperature route to polyphosphazenes with controlled molecular weights. Macromolecules, 1996, 29, 7740-7747. 

The possibility, through living cationic polymerization processes, to produce linear chain phosphazene copolymers [486]... 

Based on the synthesis of polyphosphazenes and of diblock copolyphosp-hazenes by the living cationic polymerization of phosphoranimines [237,241], the triblock poly(phosphazene-ethylene oxide) copolymer XVIII was synthesized by Allcock [223]. 

Because very rapid depolymerization occurred at higher temperatures, it was necessary to control the temperature within the narrow range of 50 10°C. Even so, the of the polymer was no greater than 15,000 because of rapid degradation by the living cationic end group. 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophihc stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophihc anions, such as ammonium and phosphonium derivatives. 

Kwon, Y. and Faust, R. Synthesis of Polyisobutylene-Based Block Copolymers with Precisely Controlled Architecture by Living Cationic Polymerization. Vol. 167, pp. 107-135. 

Living cationic polymerization, 14 271-272 Living free-radical polymerization (LFRP), 23 388-389... 

Cationic complexes are key intermediates in a great variety of organic transformations such as isomerizations, rearrangements, addition reactions, aromatic substitutions, polymerization and others. Long-lived cationic complexes are important structural models for these intermediates. Studies of such complexes by modem physical methods provide valuable insight regarding their structure and reactivity. 

There are numerous publications on the chemistry of long-lived cationic complexes see, for example (7-70). Most of our recent results in this area have been published in Russian and are probably not easily accessible to most foreign researchers. It was for this reason that we enthusiastically accepted the invitation made by Prof. Ken Laali to contribute a brief review summarizing our recent studies on the structure and reactivity of long-lived cationic organic complexes. We limit this account to the data obtained after 2000 but include some earlier unpublished results that were not included in our earlier reviews (5, 77). 

Cyclobutenyl carbocations have been under study in our laboratory for some years (11). In (34), unusually strong temperature dependence of l3C chemical shifts (up to 0.077 ppm/K) was observed for long-lived cations 17a-c. The authors suggested that this resulted from a very low inversion barrier for the four-membered ring (Scheme 13). 

---