Polymerization carbocationic intermediates

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Polymeric hydrocarbon by-products accompany the products of the latter two reactions. The structures of the products are clear evidence of the occurrence of 1,2-alkyl shifts leading to more stable carbocationic intermediates. ... 

AU four of the elementary reactions in a cationic polymerization involve electrophilic or cationic intermediates. Thus, initiation, propagation, transfer, and termination may be classified as either nucleophilic substitution, electrophilic addition, elimination, rearrangement, or possibly as a pericyclic reaction. Initiation occurs in alkene polymerizations by either addition of acid to the alkene, or by ionization of a covalent initiator followed by addition of the resulting carbocationic intermediate to an olefin s double bond. Although initiation is an electrophilic addition (AdE) reaction in... 

Attempts to perform the cationic polymerization of vinylcyclohexane have been reported. While coordination-type polymerization of vinylcyclohexane monomer gives isotactic PCHE, polymerization under conditions that lead to the formation of carbocationic intermediates leads to polymers with a differentiated backbone structure. Instead of propagating via the vinyl carbons, the cationic polymerization proceeds via hydride shift to a tertiary carbocation, which then propagates to provide the polymer shown in Scheme 23.3 [32]. Conditions for these polymerizations typically involved the use of aluminum halide catalysts in halogenated hydrocarbon solvents at low temperatures [32,38]. For the most part, molecular weights are relatively low. 

Much effort has been expended to realize the efficient polymerization of isocyanides using promoters or catalysts. Such effort can be classified using three criteria those where the polymerization occurs via radical intermediates (Scheme 2), via anionic intermediates (Scheme 3), and via carbocationic intermediates (Scheme 4). Only the third criterion has been fruitful, and has led to the findings of the remarkable catalytic activity of homogenous and... 

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]. 

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. 

Recently, it has been demonstrated that good control of molecular weight and molecular weight distribution can be attained by using microreactor systems without stabilizing the carbocationic intermediates. The concept of this new technology (flow-microreactor-system-controlled polymerization) is described in the following... 

Cationic polymerization is one of the most fundamental methods for synthesizing polymers [19, 20]. Although there are several types of cationic polymerization, the most important one is cationic polymerization of vinyl monomers having a cation stabilizing group (Y) (Scheme 14.1). The initiation usually involves the addition of a cationic species (A ) to a vinyl monomer to produce a carbocationic intermediate associated with a counter anion (X ), which is derived from the initiator. In general, proton acids or carbocations generated from their precursors by acid-promoted ionization reactions [21-23] are used as initiators. 

Namely, the P-proton of the carbocationic intermediate is inherently acidic because of the positive charge on the carbon. On the other hand, monomers used in cationic polymerization are inherently nucleophilic or basic. Therefore, proton abstraction from the carbocationic intermediate by the monomer is inevitable and is very difficult to suppress (Scheme 14.2). 

Cationic Polymerization Involving Carbocationic Intermediates Using Micrqflow Systems 231 monomer + RCI. ... 

In both methods, the positive charge of the carbocationic intermediate 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 [27]. 

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). 

In general, cationic vinyl polymerization is initiated by addition of an initiator A B to a monomer, and the resulting carbocationic intermediate (2) propagates through electrophilic addition onto... 

Most carbocationic and cationic ring-opening polymerizations are chain processes proceeding with carbocations and/or onium ions as the active species. Nonchain processes which occur via cationic and electrophilic intermediates will be discussed in Chapter 7. 

Figure 5.13. Reactions involved in cationic addition polymerization. Shown are (a) generation of a carbo-cation intermediate from a Lewis acid initiator, (b) propagation of the polymer chain through the combination of the carbocationic polymer chain and additional monomers, and (c) termination of the polymer growth through either proton abstraction (i) or anionic attachment (ii) routes.

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. 

In the propagation step, the carbocationic active polymer end adds to the monomer to give the next carbocationic polymer intermediate, which adds to another monomer. The polymerization can be terminated by the addition of a nucleophile or a base to trap an active carbocationic polymer end to obtain a stable dead polymer chain. 

Although Friedel-Crafts or Lewis acid catalysts are often used to initiate carbocationic polymerizations and are very important from an industrial viewpoint, very little is known about the active intermediates involved. Such information is important because, in general for ionic polymerization reactions, small changes in the structure of the active center can result in large changes in molecular weight, molecular weight distribution (MWD),... 

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