Copolymerization, cationic

Cationic copolymerization can be treated in identical manner to anionic copolymerization with the mechanistic scheme for propagation obtained simply by replacing the negative signs in Eqs. (8.92) to (8.95) by positive signs. The copolymerization equation derived for free-radical initiation [Eqs. (7.11) and (7.17)] may also be applied to cationic polymerization to determine ratios, several of which are listed in Table 8.5. The situation is complicated, however, due to counterion effects (Ham, 1964 Kennedy and Marechal, 1982). Thus, unlike in free radical copolymerizatiqn, different initiators used in cationic polymerization can 

The most important of the commercial cationic copolymers is butyl rubber prepared from isobutene and isoprene. Because of its very Ioav air permeability, butyl rubber finds extensive use in the inner tubes and protective clothing. It is manufactured by low-temperature (-100°C) copolymerization of about 97% isobutene and 3% isoprene in chlorocarbon solvents with AICI3 as initiator (see Table 8.5). More recently, an ozone-resistant copolymer of isobutylene and cy-clopentadiene has been marketed. 

Allcock, H. R. and Lampe, F. W., Contemporary Polymer Chemistry , Prentice Hall, Englewood Cliffs, New Jersey, 1990. 

Polymers Chemistry and Physics of Modem Materials , Blackie, Glasgow, 1991. 

Fontanaille, M., Carbanionic Polymerization General Aspects and Initiation, pp 365-386 and Carbanionic Polymerization Termination and Functionalization, pp 425-432 in Comprehensive Polymer Science (Eastmond, G. C., Ledwith, A., Russo, S., and Sigwalt, R, eds.), vol, 3, Pergamon Press, London, 1989. 

The effect of a substituent on the reactivity of a monomer in cationic copolymerization depends on the extent to which it increases the electron density on the double bond and on its ability to resonance stabilize the carbocation that is formed. However, the order of monomer reactivities in cationic copolymerization (as in anionic copolymerization) is not nearly as well defined as in radical copolymerization. Reactivity is often influenced to a larger degree by the reaction conditions (solvent, counterion, temperature) than by the structure of the monomer. There are relatively few reports in the literature in which monomer reactivity has been studied for a wide range of different monomers under conditions of the same solvent, counterion, and reaction temperature. 

Among the most extensive studies of monomer reactivity have been those involving the copolymerization of various meta- and para-substituted styrenes with other styrene monomers (styrene, a-methylstyrene, and p-chlorostyrene) as the reference monomer [Kennedy and Marechal, 1983], The relative reactivities of the various substituted styrenes have been correlated by the Hammett sigma-rho relationship  

TABLE 6-9 Steric Effects in Cationic Copolymerization of a- and (i-Methylstyrenes (Mx) with p-Chlorostyrene (M2)a b 

Although the Hammett-type approach is most useful for the quantitative correlation of monomer reactivity with structure, it is applicable only to substituted styrenes. One is, however, usually more interested in the relative reactivities of the commonly encountered monomers such as isoprene, acrylonitrile, and isobutylene. The appropriate quantitative data are relatively sparse for these monomers. The generally observed order of monomer reactivity is 

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. 

Billmeyer, Jr., F. W., Textbook of Polymer Science , John Wiley, New York, 1994. 

---

Cationic Copolymerization. Dlustiative cationic copolymeiizations are shown in equations 26 and 27 (34,35). 

The only known instance of ring-opening polymerization with these compounds is also the only report on the successful polymerization of 2,5-dihydrofuran74 in which this compound was cationically copolymerized with epichlorhydrin (rx 0, r2 0), propylene oxide (r, 0, r2 0) and 3,3-bischloromethyl oxacyclobutane (/ ] 0, r2 = 1.6). It was shown that all the copolymers obtained possessed a certain degree of unsaturation which was attributed to the presence of open units from 2,5-dihydrofuran. Thus, for example the alternating copolymer with epichlorhydrin had the following structure (IR spectra, Cl content. C=C analysis) ... 

Cationic copolymerization of cyclic ethers, formals, esters and anhydrides has been thoroughly studied in recent years and sufficient information about it is now available. The propagating species involved in the cationic copolymerization of these oxacyclic monomers are believed to be the oxonium ions in most cases, but their detailed nature is dependent on monomer structure. From their copolymerization behavior, these monomers can be arranged in the following order of increasing car-bocationic character of the propagating species ... 

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... 

The effect of a catalyst is important in cationic copolymerizations. Epoxides and /3-lactones form random copolymers only with trialkyl aluminum catalysts. Unusual sequence distributions were observed in the cationic copolymerization of epoxides or lactones using Lewis acids175-177) have been attributed to the di-... 

Cationic copolymerization of other cyclic monomers has been studied less extensively. Such copolymerization between substituted 2-oxazolines has been reported198. ... 

The relative rate of cationic homopolymerization is governed by three factors, ie. the concentration of the propagating species, the ring-opening reactivity of the growing species and the nucleophilic reactivity of the monomer. From kinetic studies196 197 of the polymerization of oxazolines and oxazines it was found that the second factor was the most important. On the other hand, the relative reactivity in the cationic copolymerization is mainly determined by the nucleophilicity of the monomer and for 2-substituted 2-oxazolines this is in the order of benzyl > methyl > > isopropyl > H > phenyl195. ... 

Cationic copolymerization of sulfur dioxide and propylene oxide was studied and the product was identified as polysulfite ethers180,2S3 ... 

Cationic copolymerization of other monomers which do not polymerize by themselves often yields alternating copolymers. Some examples are254,2SS) ... 

The following example shows that the influence of statistical thermodynamical calculations on qualitative assertions is often insignificant. The reactions (3) <5) describe three of the first propagation steps of the cationic copolymerization of ethene and isobutene. 

Furthermore, quantum chemical results can be used to confirm assertions from kinetic and non-kinetic methods. For instance, with these calculations experimental results concerning donor influence on the cationic copolymerization could be confirmed and supplemented 82). 

Cationic carbamoyl polymers, 1 313-314 Cationic catalysts, 16 95-97 Cationic copolymerization, 7 626-627 Cationic coupling, 10 708 Cationic dyes, 9 217 azo, 9 421-424... 

Among the monomers given in Scheme 1, several possess sufficient nucleophilicity to undergo cationic polymerization (7,8), namely 2-vinyltetrahydrofiiran 1, the alkenylfiirans 2, the furfurylidene ketones 3, 2-furyl oxiranes 4 and the furfuryl vinyl ethers 6, Moreover, furfural and its 5-methyl derivative can act as comonomers in certain cationic copolymerizations. 

Another stable polyacetal (POM Celcon) is produced by the cationic copolymerization of a mixture of trioxane and dioxolane (structure 5.23). 

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. 

Basko M, Kubisa P (2006) Cationic copolymerization of E-caprolactone and L,L-lactide by an activated monomer mechanism. J Polym Sci A Polym Chem 44 7071-7081... 

TABLE 6-10 Effect of Solvent and Initiator on r Values in Cationic Copolymerization... 

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

Monomer reactivities in anionic copolymerization are the opposite of those in cationic copolymerization. Reactivity is enhanced by electron-withdrawing substituents that decrease the electron density on the double bond and resonance stabilize the carbanion formed. Although the available data are rather limited [Bywater, 1976 Morton, 1983 Szwarc, 1968], reactivity is generally increased by substituents in the order... 

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]. 

---