Cationic mechanism block copolymers

Tn copolymerization by a radical mechanism, random copolymers are obtained in almost every case, but true copolymers are not obtained in copolymerization by a cationic mechanism. Usually copolymers with considerable block character are obtained, or some homopolymer is formed together with the copolymer. 

Styrene—butadiene block copolymers are made with anionic chain carriers, and low molecular weight PS is made by a cationic mechanism (110). Analytical standards are available for PS prepared by all four mechanisms (see Initiators). 

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. 

Polyethers are prepared by the ring opening polymerization of three, four, five, seven, and higher member cyclic ethers. Polyalkylene oxides from ethylene or propylene oxide and from epichlorohydrin are the most common commercial materials. They seem to be the most reactive alkylene oxides and can be polymerized by cationic, anionic, and coordinated nucleophilic mechanisms. For example, ethylene oxide is polymerized by an alkaline catalyst to generate a living polymer in Figure 1.1. Upon addition of a second alkylene oxide monomer, it is possible to produce a block copolymer (Fig. 1.2). 

Keywords Living cationic polymerization Polyisobutylene Block copolymers Macromolecular architecture Combination of polymerization mechanism... 

To obtain a high molecular weight block or random copolymer of the oxonium ion type monomer and carbonium ion type monomer, experimental conditions must be such that termination or transfer reactions are minimized. The living nature of the cationic polymerization of THF (7) is well established, but it has been difficult to obtain a high polymer of styrene or DOL by cationic mechanism. In this paper we demonstrate the living nature of the polymerization of DOL and the high polymer of St-DOL copolymer. Using this technique, we were able to obtain a block copolymer of vinyl monomer and cyclic monomer. 

Cationic polymerizations induced by thermally and photochemically latent N-benzyl and IV-alkoxy pyridinium salts, respectively, are reviewed. IV-Benzyl pyridinium salts with a wide range of substituents of phenyl, benzylic carbon and pyridine moiety act as thermally latent catalysts to initiate the cationic polymerization of various monomers. Their initiation activities were evaluated with the emphasis on the structure-activity relationship. The mechanisms of photoinitiation by direct and indirect sensitization of IV-alkoxy pyridinium salts are presented. The indirect action can be based on electron transfer reactions between pyridinium salt and (a) photochemically generated free radicals, (b) photoexcited sensitizer, and (c) electron rich compounds in the photoexcited charge transfer complexes. IV-Alkoxy pyridinium salts also participate in ascorbate assisted redox reactions to generate reactive species capable of initiating cationic polymerization. The application of pyridinium salts to the synthesis of block copolymers of monomers polymerizable with different mechanisms are described. 

A number of synthetic methods have been successfully developed for the synthesis of block copolymers. They include polycondensation, anionic, cationic, coordinative and free-radical polymerizations and also mechano-chemical synthesis. Despite the exceptional amount of attention paid to the prospects of various catalytic systems, radical polymerization has not lost any of its importance, particularly in this area. Its competitiveness with other methods of conducting polymerization are attributable to the simplicity of the mechanism and good reproducibility. Actually, the extensive use of free radical polymerization in practice is well understood when considering the ease of the process, the soft processable conditions of vacuum and temperature, the fact that reactants do not need to be highly pure and the absence of residual catalyst in the final product. Thus, it can be easily understood that more than 50% of all plastics have been produced industrially via radical polymerization. 

The number of copolymer types accessible in this way is only limited by the number of monomers able to polymerize by the living anionic mechanism. Similarly block copolymers can also be formed by living cationic polymerization [274-276],... 

This characteristic feature of cationic polymerization of THF allows the important synthetic application of this process for preparation of oli-godiols used in polyurethane technology and in manufacturing of block copolymers with polyesters and polyamides (cf., Section IV.A). On the other hand, the cationic polymerization of THF not affected by contribution of chain transfer to polymer is a suitable model system for studying the mechanism and kinetics of cationic ring-opening polymerization. 

Acrylate monomers do not generally polymerize by a cationic mechanism. However, the anionic polymerization of acrylic monomers to stereoregular or block copolymers is well known. These polymerizations are conducted in organic solvents, primarily using organometallic compounds as initiators. 

Eventually, the anion will spontaneously terminate by mechanisms that are apparently not yet completely established. There are a lot more interesting things about anionic polymerization—the effect of polar groups, the fact that not all monomers can be used to make block copolymers, the ability to make certain polymers with very narrow molecular weight distributions, and so on—but these topics are for more advanced treatments, so now we will turn our attention to cationic polymerization. 

The synthesis of block copolymers from a chlorine-terminated pdiymer, Et2AlCl and a cationically sensitive moncmer was achieved by Kennedy and collaborators and apparently constitutes a strong indication for the validity of their initiation mechanism. The blocking of styrene on pdyisobutene with chlorine end poups and of isobutene on polystyrene with chlorine end poups was hi y efficient This is apparently in contradiction with air interpretation, because if we assume the occurrence of the exchange reaction... 

The use of polymeric initiators or coinitiators to induce the polymerisation of a second monomer by a cationic mechanism is a particularly attractive possibility, since it would permit the synthesis of block and graft copolymers. The search for adequate systems in this context has been intensive, but only very recently has it met with some success, and this is far from being as spectacular as the achievements obtained in the same area with anionic systems. The main difficulties to be surmcwntedhave been discussed in the general introduction to this review (see Chap. I), and have to do with the ubiquity of transfer and termination reactions in cationic polymerisation. Nevertheless, the advances of the last few years seem encouraging and one would expect that the near future will provide considerable progress, both quantitative and qualitative. 

The subjects Include fundamental and applied research on the polymerization of cyclic ethers, slloxanes, N-carboxy anhydrides, lactones, heterocycllcs, azlrldlnes, phosphorous containing monomers, cycloalkenes, and acetals. Block copolymers are also discussed where one of the constituents is a ring opening monomer. Important new discussions of catalysis via not only the traditional anionic, cationic and coordination methods, but related UV Initiated reactions and novel free radical mechanisms for ring opening polymerization are also Included. 

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