Cationic polymerization technique

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Structures of the same type have also been prepared by cationic polymerization techniques, as can be seen in Scheme 16. Vinyl ethers having isobutyl-, ac-etoxy ethyl-, and malonate ethyl- pendant groups have been used. Hydrolysis of... 

In the early 1980s, Moad et al initially attempted to control free-radical polymerization using stable free radicals such as nitroxides. Following developmental work by Georges et al. at Xerox, the approach has led to the realization of a diverse range of polymer architectures that were previously unobtainable using traditional anionic or cationic polymerization techniques. 

Living cationic polymerization techniques are also capable of producing well defined star-block copolymers. An approach similar to the DVB method described above for the case of anionic polymerization was employed in order to prepare amphiphilic star-block copolymers [20]. In one case, living diblock copolymers of vinyl ethers and ester-containing vinyl ethers, prepared by the initiating system Hl/Znh in toluene, were reacted with a small amount of a difunctional vinyl ether to produce star shaped block copolymers (Scheme 5). 

Triblock copolymers using NVK, 4-(l-pyrenyl)butyl vinyl ether and 2-chloroethyl vinyl ether have been synthesized in an sequential cationic polymerization technique." The block copolymers were further functionalized with 2-(4-hydrox)q)henyl)-5-phenyl-1,3,4-oxadiazole, by reaction of... 

Penczek and Kubisa developed a new cationic polymerization technique for cyclic monomers in which the chain propagation involves the reaction of a protonated (activated) monomer molecule with a nucleophilic site in the neutral growing macromolecule. This so-called activated monomer (AM) polymerization is depicted mechanistically in Scheme 58. According to this mechanism, when the polymerization of an oxirane (a cyclic ether) is carried out in the presence of... 

Access to the living cationic polymerization technique mentioned earlier has allowed the synthesis of a range of block copolymers, either phosphazene-phosphazene blocks, phosphazene-organic polymer blocks, or phosphazene-polyorganosiloxane blocks.Both di- and triblock copolymers have been produced, and a summary of these structures is shown in Scheme 7.7. 

Rempp et al. synthesized polytetrahydrofuran macromers by cationic polymerization techniques, using methacryloyl or propionyl hexafluoroantimonate as initiator and then terminating with alcoholates containing the vinyl... 

There are many such polymers derived from alkenes, prepared by both radical polymerization and cationic polymerization techniques. Some examples... 

Cationic polymerization has also been used for the synthesis of macromonomers, especially after the development of living cationic polymerization techniques (73). Macromonomers were prepared by the cationic ringopening poljnnerization of tetrahydrofuran (THF) using methyltriflnoromethane sulfonate, followed by termination with 3-sodio-propyloxydimethylvinylsilane to give a macromonomer with vinyl silane end groups (74) (eq. 18). 

FIGURE 1.9 Preparation of a polyphosphazene with one fluorine and one phenyl group on every phosphorus atom using the living cationic polymerization technique. 

More recently there has been much interest in the development of living cationic polymerization techniques useful for the synthesis of defined polymer structures. In order to obtain this one must eliminate chain transfer and termination steps. For example Higashimura reported two types of systems for the polymerization of vinyl ethers [85, 86]. In one case an a-iod. ether-ended polymer is activated for monomer insertion by iodine or Znl2. 

PIB-toughened poly(methyl methacrylate) (PMMA) networks have been reported in which the PIB domains are covalently bound to a PMMA matrix [26]. The synthesis involved a macromonomer approach where a combination of radical and cationic polymerization techniques were used. Methacrylate tri-telechelic PIBs were prepared by living cationic polymerization followed by end functionalization and used in free-radical solution copol)mierization with various amounts of MMA. Scheme 14 represents the general reaction pathways for the synthesis of network. 

---