Ionic type monomers copolymerization

Table 4-7= Copolymerization parameters of Sn(IV)-containing ionic-type monomers. 

Mention has been made of the fact that the polar character of polymer surfaces is strongly affected by the ionic polymer end groups that are residues of initiator-derived ion radicals, when persulfates are used in emulsion polymerizations. Variation of the initiator type between those that yield ionic and nonionic end groups is an effective way to control particle stability and avoid complications due to migration of surfactant from one polymer surface to another [25]. This method can also be supplemented by copolymerization with polar monomers to affect surface hydrophilicity. 

Copolymerization of ionic-type MCMs, alkali and alkaline earth metal salts, with various comonomers has been studied quite well. A detailed review of the basic studies has been given in a monograph [4] and they will not be analyzed in this book. Studies of the mechanism of copolymerization of such types of MCM were carried out in the case of organotin and -lead monomers. The widest recognition among these MCMs used as comonomers has been gained by trimethyl-, tributyl-, and triphenyltin methacrylates (Table 4-7) [4 and references therein]. 

Polymer chains containing more than one type of monomer can be synthesised by selecting suitable monomers A and B and initiating the polymerization process using free radical or ionic initiators. The process is called copolymerization. The copolymer can exhibit better qualities than the parent homopolymer. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

There are no essential differences in experimental technique required for ionic copolymerizations, as compared with ionic homopolymerizations. However, the type of initiator and the solvent have a potential influence on the course of ionic copolymerizations as well as on the composition of the copolymers so that the optimum conditions for each monomer pair must be individually determined. 

The solution thus consists of different particles denoted as contact ion pairs, solvent-separated ion pairs and free ions. The fraction of the individual particles depends on the type of salt, type of solvent, polymerization system, temperature, and salt concentration. The catalytic effect of these particles may be very different as is evident in anionic polymerization of vinyl monomers. For instance, free polystyryl anion is 800times more reactive than its ion pair with the sodium counterion 60 . From this fact it follows that, although the portion of free ions is small in the reaction system, they may play an important role. On the other hand, anionic polymerization and copolymerization of heterocycles proceeds mostly via ion pairs. This is due to a strong localization of the negative charge on the chain-end heteroatom which strongly stabilizes the ion pair itself62. Ionic dissociation constants and ion contributions to the reaction kinetics are usually low. This means that for heterocycles the difference between the catalytic effect of ion pairs and free ions is much weaker than for the polymerization of unsaturated compounds. This is well documented by the copolymerization of anhydrides with epoxides where the substi-... 

Ionomers have been prepared by two general routes (1) copolymerization of a low level of functionalized monomer with an oleflnlcally unsaturated comonomer or (2) direct functionalization of a preformed polymer. Almost all ionomers of practical Interest have contained either carboxylate or sulfonate groups as the ionic species. Other salts, such as phosphonates, sulfates, thloglycolates, ammonium, and pyridinium salts have been studied, but nowhere to the extent of the carboxylate and sulfonate anlonomers. (An anlomer is defined as an lonomer In which the anion is bonded to the polymer. Conversely, ionomers that have the cation bonded to the polymer are termed cationomers). Relatively little information is available on the structure and properties of these types of ionomers. 

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. 

Equation (7.13) means that k /ki2 and k2i/k22 wiU be simultaneously either greater or less than unity or in other words, that both radicals prefer to react with the same monomer. Ail copolymers whose Vir2 product equals 1 are therefore called ideal copolymers or random copolymers. Most ionic copolymerizations are characterized by the ideal type of behavior. 

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