Styrene cationic copolymerization

In the cationic copolymerization of DOL with styrene considerable cleavage of polymer chains occurs if the styrene content is high hut a molecular weight as... 

Over 5.5 billion pounds of synthetic rubber is produced annually in the United States. The principle elastomer is the copolymer of butadiene (75%) and styrene (25) (SBR) produced at an annual rate of over 1 million tons by the emulsion polymerization of butadiene and styrene. The copolymer of butadiene and acrylonitrile (Buna-H, NBR) is also produced by the emulsion process at an annual rate of about 200 million pounds. Likewise, neoprene is produced by the emulsion polymerization of chloroprene at an annual rate of over 125,000 t. Butyl rubber is produced by the low-temperature cationic copolymerization of isobutylene (90%) and isoprene (10%) at an annual rate of about 150,000 t. Polybutadiene, polyisoprene, and EPDM are produced by the anionic polymerization of about 600,000, 100,000, and 350,000 t, respectively. Many other elastomers are also produced. 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

TABLE 6-11 Effects of Solvent and Counterion on Copolymer Composition in Styrene-p-Methylstyrene Cationic Copolymerization"... 

The general characteristics of anionic copolymerization are very similar to those of cationic copolymerization. There is a tendency toward ideal behavior in most anionic copolymerizations. Steric effects give rise to an alternating tendency for certain comonomer pairs. Thus the styrene-p-methylstyrene pair shows ideal behavior with t = 5.3, fy = 0.18, r fy = 0.95, while the styrene-a-methylstyrene pair shows a tendency toward alternation with t — 35, r% = 0.003, i ii 2 — 0.11 [Bhattacharyya et al., 1963 Shima et al., 1962]. The steric effect of the additional substituent in the a-position hinders the addition of a-methylstyrene to a-methylstyrene anion. The tendency toward alternation is essentially complete in the copolymerizations of the sterically hindered monomers 1,1-diphenylethylene and trans-, 2-diphe-nylethylene with 1,3-butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene [Yuki et al., 1964]. 

Monomer reactivity ratios and copolymer compositions in many anionic copolymerizations are altered by changes in the solvent or counterion. Table 6-12 shows data for styrene-isoprene copolymerization at 25°C by n-butyl lithium [Kelley and Tobolsky, 1959]. As in the case of cationic copolymerization, the effects of solvent and counterion cannot be considered independently of each other. For the tightly bound lithium counterion, there are large effects due to the solvent. In poor solvents the copolymer is rich in the less reactive (based on relative rates of homopolymerization) isoprene because isoprene is preferentially complexed by lithium ion. (The complexing of 1,3-dienes with lithium ion is discussed further in Sec. 8-6b). In good solvents preferential solvation by monomer is much less important and the inherent greater reactivity of styrene exerts itself. The quantitative effect of solvent on copolymer composition is less for the more loosely bound sodium counterion. 

Cationic copolymerization proceeds under the above conditions at such a rate that even at much iower initiator concentration almost half the monomer mixture is poiym-erized in less than 1 h.The reaction comes to an end after 50% conversion since with cationic initiators, the copolymer consists almost entirely of styrene units. 

The radical reaction mechanism was confirmed by polymerizing a mixture of styrene and methyl methacrylate. The ratio of the monomers in the copolymer (1.15) was nearly equal to the value (1.05) calculated from the reactivity ratio for radical copolymerization and differed considerably from the value of 10.5 for the cationic copolymerization and from the value 0.15 for anionic copolymerization (78). 

Sakurada, I., N. Isb, Y. Hayashi, and M. Nakao Cationic copolymerization of styrene with indene or a-methylstyrene catalyzed with boron trifluoride etherate under an electric field. Macromolecules 1, 265 (1968). 

Using Al(i-C4H9)3/TiCl4 catalyst for copolymerization of styrene with substituted styrenes, the reactivity ratios showed that cationic copolymerization occurred at Al/Ti< 1 and that stereospecific coordinated anionic copolymerization took place at Al/Ti > 2.5. 

Because of this difference in stability monomers which yield onium ions will not copolymerize cationically with olefins like isobutene or styrene. (A similar difference in radical stabilities accounts for the reluctance of styrene to copolymerize with vinyl chloride in free-radical reactions, Section 7.10.1.)... 

The most commonly used resins are gel type sulfonated cation exchangers or anion exchangers with a quaternary ammonium functional group. The exchange capacity of the resins used is generally higher than those used for ion chromatography. The resins may be either styrene-divinylbenzene copolymeric beads or polyacrylate beads. The diameter of the beads should be small and uniform. Resins with a 5 pm bead diameter are now available. 

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. 

The extruder can be used for a variety of polymerizations even if no preformed polymer is present.89 These include the continuous anionic polymerization of caprolactam to produce nylon 6,90 anionic polymerization of capro-lactone 91 anionic polymerization of styrene 92 cationic copolymerization of 1,3-dioxolane and methylal 93 free radical polymerization of methyl methacrylate 94 addition of ammonia to maleic anhydride to form poly(succin-imide) 95 and preparation of an acrylated polyurethane from polycaprolactone, 4,4 -methylenebis(phenyl isocyanate), and 2-hydroxyethyl acrylate.96 The technique of reaction injection molding to prepare molded parts is slightly different. Polyurethanes can be made this way by... 

Kern et al. found that, contrary to most heterocyclic monomers, TXN can be copolymerized with styrene (St) to random copolymers 149). Since that time the cationic copolymerization of TXN (and also DXL) with styrene has been studied by several groups but unfortunately GPC analysis was not commonly used and the only proof that copolymers were formed are based on solubility studies and on the presence of 4-phenyl-1,3-dioxane among the hydrolysis products of the products. Let us summarize the facts concerning TXN-styrene copolymerizations ... 

The copolymerization of styrene and vinyl acetate affords an example of the significant differences in the effects of the initiation system on the monomer composition. The copolymerization ratios for the three processes may be used as a guide to the effect under study. For M as styrene and M2 as vinyl acetate, in the cationic copolymerization, r is 8.25 and r2 is 0.1, while for the free-radical polymerization, r is 55 and ri is 0.01. 

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