Carbocationic living cationic polymerization

Transfer reactions should be absent in living polymerizations but are usually significant in classic carbocationic polymerizations. However, they appear to be suppressed in new living cationic polymerizations. This... 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

One of the most important breakthroughs in cationic polymerization is the discovery of living cationic polymerization. The inherent and serious drawback of cationic vinyl polymerization is instability of the carbocationic intermediates, which causes the chain transfer leading to the formation of polymers of broad molecular weight distribution. Higashimura, Sawamoto, and coworkers proposed and verified experimentally that living cationic polymerization can be attained by stabilizing the carbocationic intermediate by nucleophilic interaction with a suitably nucleophilic counter anion or an externally added Lewis base (B) (Scheme 3) [96-98]. 

In both methods the positive charge of the carbocationic center is reduced and thereby the acidity of the p-proton is reduced to suppress the chain transfer. As a result, good molecular weight control and molecular weight distribution control are attained. On the basis of the principles, a number of initiating systems have been developed for living cationic polymerization [99]. 

One of the major drawbacks of the controlled/living cationic polymerization based on stabilization of the carbocationic intermediates is slow propagation. Because the concentration of the active propagating species is very low because of the equdibrium between active species and dormant species, overall polymerization reactions are much slower than those without the equilibrium. Another important drawback of the controlled/living polymerization is the use of additives such as Lewis bases. Such additives remain in polymer products and are generally rather difhcult to remove from the polymer products. 

Controlled/Living Cationic Polymerization Without Stabilization of Carbocationic Intermediates Using Microflow Systems... 

Cationic polymerization without stabilization of a carbocationic intermediate can be carried out in a microflow system. Good molecular weight control and molecular weight distribution control are attained by virtue of characteristic features of microflow systems (microflow-systempolymerization technology, MCPT). Conventional controlled/living cationic polymerization based on cation stabilization can be also carried out in a microflow system. 

Except for some heterocycles (2), living cationic polymerization has been considered almost impossible, particularly for vinyl monomers, which generate unstable carbocationic intermediates. Despite this pessimistic view, we have recently found that living cationic vinyl polymerization is indeed possible by stabilizing the growing carbocations with nucleophilic counteranions or with externally added bases (3). 

The living cationic polymerizations discussed above are invariably based on the nucleophilic iodide counteranion (activation of the carbon-iodine terminal bond Eq. 3). It is expected, however, that similar living processes are equally possible with other counteranions that can exert, as the iodide anion does, a suitably strong nucleophilic interaction with the growing carbocation. We have in fact found the phosphate anions to meet this requirement (10). Similarly to hydrogen iodide, monoacidic phosphate esters [H0P(0)R 2 R alkyl, alkoxyl, etc.] like diphenyl phosphate ( ) form a stable adduct 5) with a vinyl ether (Eq. 5). Zinc chloride or iodide then activates the phosphate bond in 5 by increasing its polarization (as in 6), and living cationic polymerization proceeds via an intermediate (7) where the carbocationic site is stabilized by a phosphate anion coupled with the zinc halide activator. 

Matyjaszewski, K. and M. Sawamoto, Controlled/Living Carbocationic Polymerization, Chap. 4 in Cationic Polymerizations Mechanisms, Synthesis, and Applications, K. Matyjaszerski, ed., Marcel Dekker, New York, 1996. 

Mitsuo Sawamoto, born in Kyoto, Japan (1951), received his B.S. (1974), M.S. (1976), and Ph.D. (1979) degrees in polymer chemistry from Kyoto University under the direction of Toshinobu Higashimura. After postdoctoral research with Joseph P. Kennedy at the Institute of Polymer Science, The University of Akron, Akron, OH (1980-81), he joined the faculty of the Department of Polymer Chemistry, Kyoto University, in 1981 as a research instructor. He was promoted to Lecturer (1991), to Associate Professor (1993), and to Professor (1994), his current position, of the same department. Sawamoto also serves as one of the three Editors of the Journal of Polymer Science, Part A Polymer Chemistry (1995-present) and as an Editorial Advisory Board member of Macromolecular Chemistry and Physics, the Journal of Macromolecular Science, Chemistry, and e-Polymers, and is the recipient of the 1991 Award of the Society of Polymer Science, Japan, the 1998 Divisional Award of the Chemical Society of Japan, the 2001 Aggarval Lectureship in Polymer Science, Cornell University, and the 2001 Arthur K. Doolittle Award of the ACS PMSE Division. With more than 250 articles and reviews, his research interest covers living radical and cationic polymerizations, precision polymer synthesis, and the chemistry of radical and carbocationic reaction intermediates. 

Recent review articles on the following topics were published the controversy concerning the cationic ring-opening polymerization of cyclic acetals (213), photoinitiators for cationic polymerization (21A), living polymerization and selective dimerization (215). raacroraonomers (216), and functional polymers and sequential copolymers by carbocationic polymerization (217). 

This chapter will first discuss the living carbocationic polymerization of the three most important monomer classes isobutene, vinyl ethers, and styrenics. The second part of the chapter will focus on living cationic ROP of cyclic ethers, cyclic imines, and cyclic imino ethers. For more detailed discussions on carbocationic polymerizations [8-14] and cationic ROPs [15-18] in general, the readers are referred to previous literature [19]. 

Initially, Gandini and Plesch proposed that the perchloric acid-initiated low temperature polymerization of styrene is based on monomer insertion on the nonionic perchlorate chain ends, which was based on the observation that the polymerization mixture was not conductive [68, 69]. These nonionic polymerizations were referred to as pseudo-cationic polymerizations. However, more detailed investigations by stopped-flow UV-vis spectroscopy revealed the presence of short-lived carbocations indicating that these are the propagating species in the cationic polymerization of styrene [70, 71]. This was also confirmed for the polymerization of styrene with trifiic acid for which Matyjaszewski and Sigwalt showed that the covalent triflic ester adduct was unstable even at -78 °C leading to carbocationic propagating species [72]. 

Living cationic ring-opening polymerization (CROP) techniques represent important methods for the polymerization of a wide variety of heterocyclic monomers, such as cyclic ethers, cyclic amines, and cyclic imino ethers [7, 84-87]. The main differences between carbocationic polymerization of vinyl monomers and CROP of heterocyclic monomers arise from the nucleophilic heteroatoms... 

Potentially, there are greater numbers of monomers that are suitable for cationic polymerization than for anionic, but the cationic method is less successful in block copolymer synthesis because, in many systems, the existence of a living carbocationic species is doubtful. Consequently, the involvement of carbocations in block copolymer synthesis tends to be limited to mixed reactions, e.g., the couphng of poly(tetrahydro-furan) cations with polystyryl anions to give an (A - B) diblock (Equation 5.19). 

The preparation of telechelics by carbocationic polymerization (qv) of vinyl monomers (196,197) has received little attention because of the high reactivity and low selectivity of growing species. The synthesis of useful telechelics requires the control of molecular weight and end groups this was not possible imtil the discovery of living cationic poljunerization. 

Normally, molecular weight is difficult to control in cationic polymerization of styrene. This is not only because of transfer to polymer and solvent but also of transfer to monomer. Friedel-Crafts reactions during growth with aromatic solvents significantly decrease the molecular weight [100]. A living carbocationic polymerization of styrene has been described [101] ... 

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