Cationic polymerization practical systems

Cationic polymerization of tetrahydrofuran is one of the few systems in cationic ring polymerization in which chain transfer to polymer may be practically avoided. The reasons for that are of purely kinetic nature. 

Therefore, all requirements for living polymerization seem to be satisfied, at least in a practical sense, in the microflow system controlled cationic polymerization. The livingness strongly depends on the reaction time. In a very short period of time highly reactive intermediates, in this case reactive propagating polymer ends, can survive and they can be utilized for subsequent reactions when different monomers or a terminating reagent are added. This concept is quite similar to that discussed in Section 6.3. 

The simple copolymer equation [Eq. (7.11)] has been experimentally verified in innumerable comonomer systems. The equation is equally applicable to radical, cationic, and anionic chain copolymerizations, although the and T2 values for any particular monomer pair can be drastically different in the three types of chain copolymerization. For example, for the monomer pair of styrene (Mx) and methyl methacrylate (M2) the ri and T2 values are 0.52 and 0.46 in radical copolymerization, 10 and 0.1 in cationic polymerization, and 0.1 and 6 in anionic copolyraerization. Methyl methacrylate as expected has higher reactivity in anionic copolymerization and lower reactivity in cationic copolymerization, while the opposite is the case for styrene. Thus the copolymer obtained from an equimolar styrene-methyl methacrylate feed is approximately a 1 1 copolymer in the radical case but is essentially a homopolymer of styrene in cationic copolyraerization and a homopolymer of methyl methacrylate in anionic copolymerization. This high selectivity of ionic copolymerization limits its practical use. Since, moreover, only a small number of monomers undergo ionic copolyraerization (see Chapter 8), the range of copolymer products that can be obtained is limited. On the other hand, almost all monomers undergo radical copolymerization and thus a wide range of copolymers can be synthesized. 

Poly(e-caprolactone) is another practically important polyester formed by ionic polymerization of the cyclic ester. Cationic polymerization requires relatively high temperatures this enhances proton transfer and decreases the molecular weight, whereas anionic polymerization provides living systems. 

Reiser reports that in order not to terminate the reaction and hence inhibit propagation, the counter anion must have very low nucleophilicity, since strong nucleophiles or bases will terminate the reaction immediately. Nevertheless, the polymerization can tolerate a small amount of water (1-2%), which is important for the practical usefulness of the system. Oxygen, which acts as a biradical, shows no effect on cationic polymerization—quite an important practical advantage. Characteristically, the epoxy polymers that are the result of the curing process tend to have excellent mechanical properties, including thermal and dimensional stability, nontoxicity, and chemical inertness. 

In actual practice, there are, of course, many complexities superimposed on this basic mechanism of cationic polymerization. Many of the catalyst systems consist of two components. In some systems, for example, BF3 is an effective catalyst only in the presence of some water. It may be that the active catalyst is the adduct, which serves as a proton donor. Alkyl halides in combination with Lewis acids can also... 

Cationic polymeri2ation is of great theoretical and practical importance. Worldwide production of polymers by cationic vinyl polymerization is estimated at 2.5 million metric tons per year [258]. Since the discovery of living cationic systems, cationic polymerization has progressed to a new stage where the synthesis of designed materials is now possible. 

It was not until the invention of iodonium and sulfonium salts as photo-initiators by Crivello (1975) that cationic photo-polymerization became practical (see Crivello et al., 1977,1990,2000). Upon irradiation of these Crivello salts, acids are generated. Another significant difference between free radical and cationic polymerizations is the latter process is a living polymerization— once the acid species is formed, it remains active even after the irradiation is stopped. In contrast to this behavior free radicals die soon after irradiation is stopped. Also, unhke free radical polymerizations cationic reactions are not inhibited by oxygen. Quite often the dark reaction following irradiation can play an important role in enabhng a cationic system to develop its full properties and this leads manufacturers of commercial cationic photopolymers to often recommend a thermal (dark) postcure after carrying out the photo-irradiation process. 

Knowledge of the order of basicities of cyclic and linear ethers is important for understanding certain phenomena in cyclic ether polymerization. As indicated earlier, chain transfer to polymer is a general feature of the cationic polymerization of cyclic ethers because the nucleophilic site of the monomer molecule (oxygen atom) is transferred to the polymer unit. To what extent chain transfer to polymer competes with propagation depends on the relative nucleophilicity of monomer and polymer unit. Thus, for five-membered THF, the polymer unit is a weaker base than the monomer. This makes the polymer less reactive than the monomer in nucleophilic substitution type reactions. Consequently, for this monomer, chain transfer to polymer is slow as compared to propagation. In contrast, in the polymerization of three-membered EO, the polymer unit is more basic than monomer. Therefore, reactions involving the polymer chain are important in this system. Practical consequences will be discussed in the subsequent sections devoted to polymerization of different classes of cyclic ethers. 

Both types of copolymer can be synthesized by means of photochemical methods based on free radical or cationic mechanisms [33-46], although for practical applications cationic polymerizations are less attractive than free-radical polymerizations. In the latter case, light absorption induces the dissociation of the photolabile groups into pairs of free radicals capable of initiating the polymerization of a monomer present in the system (Scheme 3.11). 

The radiation-induced cationic polymerization in the presence of onium salts has attained practical importance for the EB curing of systems containing epoxides or vinyl ethers [22,23]. The chemical structures of typical compounds were presented in Table 3.23. THE does not play a role in this context, because of its very low propagation rate constant (kp 4 x 10 lmol s ). A reaction mechanism for the polymerization of vinyl ethers in the presence of an iodonium salt, as proposed by Crivello [22], is shown in Scheme 5.8. 

Three classes of molecules are found to be valuable for practical use here diazonium salts, onium salts and organometallic complexes, about which detailed discussion have been pubUshed [4, 105]. Compared to radical type reactions, cationic polymerizations feature (i) low curing speed, (ii) lower viscosity, (iii) small shrinkage after polymerization, and (iv) severe post-irradiation dark polymerization. Sometimes extra thermal processing is needed to increase the conversion of monomers [106]. The above general information is instructive for choosing a suitable material system for laser fabrication. 

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