Polymers, living cationic

Chain transfer is far more important than in the anionic case, so we do not encounter living polymers in cationic systems. 

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. 

Interest in anionic polymerizations arises in part from the reactivity of the living carbanionic sites4 7) Access can be provided to polymers with a functional chain end. Such species are difficult to obtain by other methods. Polycondensations yield ro-functional polymers but they provide neither accurate molecular weight control nor low polydispersity. Recently Kennedy51) developed the inifer technique which is based upon selective transfer to fit vinylic polymers obtained cationically with functions at chain end. Also some cationic ring-opening polymerizations52) without spontaneous termination can yield re-functional polymers upon induced deactivation. Anionic polymerization remains however the most versatile and widely used method to synthesize tailor made re-functional macromolecules. 

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. 

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. 

Kinetics of Living Cationic Polymerisations, P.H. Plesch, Polymer Bulletin, 1988, 19,145-148. 

Much research has already been devoted in the past couple of years to (i) the immobilization of ATRP active metal catalysts on various supports to allow for catalyst separation and reycycling and (ii) ATRP experiments in pure water as the solvent of choice [62]. A strategy to combine these two demands with an amphiphilic block polymer has recently been presented. Two types of polymeric macroligands where the ligand was covalently linked to the amphiphilic poly(2-oxazo-line)s were prepared. In the case of ruthenium, the triphenylphosphine-functiona-lized poly(2-oxazoline)s described in section 6.2.3.2 were used, whereas in the case of copper as metal, 2,2 -bipyridine functionalized block copolymers were prepared via living cationic polymerization [63] of 2-methyl-2-oxazoline and a bipyridine-functionalized monomer as shown in Scheme 6.8. 

Aoshima, S. Yoshida, T. Kanazawa, A. Kanaoka, S. New stage in living cationic polymerization. J. Polym. Sci. Part A Polym. Chem. 2007, 45, 1801-1813. 

Star and miktostar polymers have been synthesized by living cationic polymerization using dienes and trienes as coupling agents and/or multifunctional initiators [Faust and Shaffer, 1997 Hadjichristidis, et al., 1999 Hadjikyriacou and Faust, 2000 Kennedy and Jacob, 1998 Puskas et al., 2001 Sawamoto, 1991]. Multifunctional halides such as hexaiodo-methylmelamines have also been used to obtain star and comb polymers [Ryu and Hirao, 2001 Zhang and Goethals, 2001], Hyperbranched and dendritic polymers have also been studied. 

Another approach to block copolymer is the mutual termination of a living cationic polymer with a living anionic polymer, but the efficiency of coupling is generally low because of proton transfer from the carbocation to the carbanion, resulting in two homopolymers instead of the block copolymer [Richards et al., 1978]. 

Surface-initiated living cationic polymerization of 2-oxazolines on planar gold substrates has been reported by Jordan et al (Fig. 9). SAMs of initiators on a planar gold substrate have been used to initiate the living cationic ringopening polymerization of 2-ethyl-2-oxazoline. The polymer chain end was functionalized with an alkyl moiety by means of a termination reaction in order to form an amphiphilic brush-type layer. The resulting layers (thickness... 

Another way of synthesizing block copolyers it to have two polymers which possess mutually reacting chain ends. A picturesque example is the mutual deactivation of living" cationic polytetrahydrofuran and of "living" anionic polystyrene.12... 

A recent contribution reported by Aoshima and Tsukuda showed the aerobic oxidation of alcohols such as benzyl alcohol catalyzed by gold nanoclusters. These stable and durable clusters of less than 4 nm were prepared using thermosensitive vinyl ether star polymers previously obtained by living cationic polymerization. 

The synthesis of A2B miktoarm star polymers has been discussed and exemplified using PIB as a component. The synthesis involves a quasi living cationic polymerization of isobutylene from a monofunctional cationic initiator. This initiator also contains a blocked hydroxyl group. Eventually, the blocked hydroxyl group of the initiator is deblocked, and functionalized with a branching agent. This activated reagent is then used for an atom transfer radical polymerization process of /erf-butyl acrylate (18). 

R.F. Storey, D.W. Baugh, and K.R. Choate, Poly(styrene-b-isobutyl-ene-b-styrene) block copolymers produced by living cationic polymer-... 

M. Sawamoto, T. Enoki, and T. Higashimura, End-functionalized polymers by living cationic polymerization. 1. Mono- and bifunctional poly(vinyl ethers) with terminal malonate or carboxyl groups, Macromolecules, 20(l) l-6, January 1987. 

---