Living polymerization cationic

For a long time, cationic polymerization has been considered to be very difficult to control to obtain polymers of narrow molecular-weight distribution. Usually, molecular weights are unpredictable and M /M is far from one. An active species at the propagating polymer end is a carbocation or an onium ion, which reacts with olefin monomers extremely rapidly. The active polymer end is also highly unstable and readily participates in chain-transfer reactions by loss of p-protons leading to uncontrollable molecular-weight distributions. 

The discovery of living cationic polymerization was an epoch-making event in polymer science and technology. There are three key issues in living cationic polymerization  

equilibrium between active species and dormant species  

Initiation is a very important issue in controlling molecular weight and its distribution. To control the degree of polymerization based on the monomer/initiator ratio, the initiation reaction should be very fast and 

The equilibrium between active species and dormant species is also helpful in preventing chain-breaking processes, such as termination and transfer reactions. 

The successful synthesis of AB diblock copolymers by living cationic polymerization is based on the sequential polymerization technique. Vinyl ether derivatives bearing perfluoroalkyl groups are employed as monomers. 1,1,2-Trichlorotrifluoroethane is often effective to avoid precipitates during polymerization and to produce polymers with controlled structures (Choi et al, 1988 Percec and Lee, 1992 Vandooren et al, 1994). Similarly, a block copolymer 

CH20CH2CH2(CF2)mCF3 CH2(0CH2CH2)30CH3 CH20(CH2)2C8Fi7 

On the other hand, the monomer addition order is important and regulated when styrene is used, due to the large reactivity difference between styrene and perfiuoroalkyl methacrylate. Polystyryllithium end capped with 1,1-diphenylethylene can completely initiate the 

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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. 

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

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. 

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]. 

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... 

The metal-centered complexes can also be used as multifunctional initiators. For example, Fe2+(4,4 dichloromethyl-2,2 -bipyridine)3 or the Ru2+ complex have been used as initiators for the living cationic polymerization of 2-ethyl-2-oxazoline [120],... 

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. 

Hoogenboom R, Fijten MWM, Schubert US (2004) The effect of temperature on the living cationic polymerization of 2-phenyl-2-oxazoline explored utilizing an automated synthesizer. Macromol Rapid Commun 25 339-343... 

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