Polymerization of vinyl ethers

The observation in 1949 (4) that isobutyl vinyl ether (IBVE) can be polymerized with stereoregularity ushered in the stereochemical study of polymers, eventually leading to the development of stereoregular polypropylene. In fact, vinyl ethers were key monomers in the early polymer Hterature. Eor example, ethyl vinyl ether (EVE) was first polymerized in the presence of iodine in 1878 and the overall polymerization was systematically studied during the 1920s (5). There has been much academic interest in living cationic polymerization of vinyl ethers and in the unusual compatibiUty of poly(MVE) with polystyrene. 

Several mechanisms for the polymerization of vinyl ether and epoxies have been suggested [20,22,23,25,27,28,33-35]. On irradiation with gamma rays or electrons, pure epoxies polymerize via a cationic mechanism [35]. However, this cationic polymerization is inhibited by just traces of moisture, as shown below for cyclohexene oxide in reaction 5. 

Lapin [33] also suggests the following cationic mechanism for the polymerization of vinyl ethers... 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

The initiation reaction in the polymerization of vinyl ethers by BF3R20 (R20 = various dialkyl ethers and tetrahydrofuran) was shown by Eley to involve an alkyl ion from the dialkyl ether, which therefore acts as a (necessary) co-catalyst [35, 67]. This initiation by an alkyl ion from a BF3-ether complex means that the alkyl vinyl ethers are so much more basic than the mono-olefins, that they can abstract alkylium ions from the boron fluoride etherate. This difference in basicity is also illustrated by the observations that triethoxonium fluoroborate, Et30+BF4", will not polymerise isobutene [68] but polymerises w-butyl vinyl ether instantaneously [69]. It was also shown [67] that in an extremely dry system boron fluoride will not catalyse the polymerization of alkyl vinyl ethers in hydrocarbons thus, an earlier suggestion that an alkyl vinyl ether might act as its own co-catalyst [30] was shown to be invalid, at least under these conditions. 

Some early polymerizations reported as Ziegler-Natta polymerizations were conventional free-radical, cationic, or anionic polymerizations proceeding with low stereoselectivity. Some Ziegler-Natta initiators contain components that are capable of initiating conventional ionic polymerizations of certain monomers, such as anionic polymerization of methacrylates by alkyllithium and cationic polymerization of vinyl ethers by TiCLt-... 

There are some isolated reports of metallocenes and phenoxy-imines as effective initiators for the polymerization of vinyl ethers, but the reactions do not proceed in a stereoselective manner [Baird, 2000 Kawaguchi et al., 2002]. 

Ketley, A. D., Stereospecific Polymerization of Vinyl Ethers, Chap. 2 in The Stereochemistry of Macromolecules, Vol. 2, A. D. Ketley ed., Marcel Dekker, New York, 1967a. 

Recently, Spange et al. (19,20) have successfully achieved cationic graft polymerizations of vinyl ethers, vinyl furan, and cyclopentadiene onto silica, initiated by a stable ion pair formed from silanol and aiylmethyl halide, such as di(p-methoxy-phenyl)methyl chloride. The grafting of the polymer onto silica is proposed to take place via the propagation based on olefin insertion to a cation center being in a rapid equilibrium with the ion pair, as shown in Scheme 12.1.3. 

The Stereorcgiilar Polymerization of Vinyl Ethers with Transition Metal Catalysts. J. Polymer Sci. C 7, 207 (1963). 

Chiral induction was observed in the cyclopolymerization of optically active dimethacrylate monomer 42 [88], Free-radical polymerization of 42 proceeds via a cycliza-tion mechanism, and the resulting polymer can be converted to PMMA. The PMMA exhibits optical activity ([ct]405 -4.3°) and the tacticity of the polymer (mm/mr/rr =12/49 / 39) is different from that of free-radical polymerization products of MMA. Free-radical polymerization of vinyl ethers with a chiral binaphthyl structure also involved chiral induction [91,92]. Optically active PMMA was also synthesized through the polymerization of methacrylic acid complexed with chitosan and conversion of the resulting polymer into methyl ester [93,94]. 

Most of the reported polyfvinyl ether) macromonomers have been prepared with a methacrylate end group which can be radically polymerized and which is non-reactive under cationic polymerization conditions [71-73]. Generally, the synthesis was based on the use of the functional initiator 30, which contains a methacrylate ester group and a function able to initiate the cationic polymerization of vinyl ethers. Such initiator can be obtained by the reaction of HI and the corresponding vinyl ether. With initiator 30 the polymerization of ethyl vinyl ether (EVE) was performed using I2 as an activator in toluene at -40 °C. The MW increased in direct proportion with conversion, and narrow MWD (Mw/Mn= 1.05-1.15) was obtained. The chain length could be controlled by the monomer to initiator feed ratio. Three poly(EVE) macromonomers of different length were prepared by this method Mn=1200,5400, and 9700 g mol-1. After complete... 

The transient existence of the silylenium ion has been considered in other reactions of chlorosilanes in the presence of Lewis acids. Guyot (147) successfully used the silyl chloride-silver salt system for initiation of the cationic polymerization of vinyl ethers [Eq. (35)]. Gel permeation... 

Lal (352) appears to have been first to note that the polymerization of vinyl ethers by Ziegler catalysts is cationic in nature. His evidence was that AlR3/TiCl4 polymerized vinyl ally ether through the vinyl group in the same manner as BFS etherate. The allyl unsaturation was expected to polymerize if the catalyst were anionic. Also, both catalysts gave the same crystalline polymer of vinyl isobutyl ether. 

The polymerization of vinyl ethers follows much the same mechanism, using the oxonium ion as an intermediate instead of the tertiary carbocation. Termination might again be by loss of a proton or by picking up a nucleophile at the oxonium ion centre. 

A very interesting variant of this kind of initiation, leading to living cationic polymerizations of vinyl ethers, is the dissociation [233]... 

In a series of papers (see for example, ref. 250), Higashimura and Sawamo-to have described the living cationic polymerization of vinyl ethers at low... 

Spontaneous ionization requires both good leaving groups and that the resulting carbenium ions are sufficiently stable. For example, although primary triflates are very stable covalent species which do not self-ionize, secondary triflates with phenyl substitents are very reactive and spontaneously ionize. The ionization equilibrium of styryl triflate could not be established because of side reactions such as Friedel-Crafts alkylation [56], On the other hand, methoxymethylium triflate is partially ionized with equilibrium constants Kj = 5-10 4 at 10° C and Kt = 210 4 at -70° C in S02 [57]. In this system, ionization is endothermic. Secondary triflates with alkoxy substituents, such as those in polymerizations of vinyl ethers, are apparently more strongly ionized than their primary counterparts [58,59],... 

Thus, the terms initiator and coinitiator, as well as catalyst and cocatalyst, must be clearly distinguished. As proposed earlier [68], an initiator is consumed in the initiation process whereas a catalyst remains unchanged during the polymerization. In the polymerization of alkenes initiated directly by Lewis acids (e.g., iodine initiated polymerization of vinyl ethers) [69], the Lewis acid plays both roles. Nevertheless, Lewis acids usually act only as catalysts rather than as initiators, with protonogenic compounds such as adventitious moisture being the initiator. 

Polymerizations of vinyl ethers initiated by HI/I2 in hexane are zero order in monomer, but first order in monomer in more polar solvents. Several groups [177-179] have proposed that this zero-order dependence is due to monomer complexing with iodine. Alternatively, polymerization will be zero order in monomer if ionization is the rate-determining step, followed by faster addition of monomer to carbenium ions [180]. In this case, the concentration of monomer will not affect the polymerization rate. 

The last twelve kp values reported in Table 14 are lower than those from the Paris group. They are less reliable because they were calculated assuming that initiation of polymerization of vinyl ethers, Al-vinyl carba-zole and p-methoxystyrene with trityl or tropylium salts were quantitative and instantaneous, whereas kt is 103-105 times smaller than kp. 

There are some measurements of the rates of polymerization in systems with reversible formation of covalent species. The equilibrium constants of ionization can be calculated from these kinetic data according to the procedure outlined subsequently in Section IV.D.2.a. The ionization constant depends on the strength of the Lewis acid. For example, the propagating species are almost completely ionized in polymerizations of vinyl ethers with SbCL-, BCLt-, and SnCl5- counteranions, but only partially ionized when the counteranions are I3- or Zn3-. 

Excess nucleophile is often needed in polymerization of more nucleophilic monomers. For example, esters, ethers, and amines afe used in large excess over aluminum halides and alkylaluminum halides to control polymerization of vinyl ethers [269]. The original Lewis acid is no longer available and covalent species are activated by the Lewis acid/nucleophile complex. Carbenium ions are additionally deactivated by excess nucleophile. 

The common ion effect does not influence the kinetics of collapse of the ion pairs to dormant covalent species since it is a unimolecular reaction. In this case, however, deactivation of the ion pair can be increased by using less stable and more nucleophilic counteranions. The nucleophi-licity of both pure halides and complex anions with halide ligands increases in the order FOther examples of polymerizations which are well behaved because the equilibrium is favorable due to nucleophilic counteranions include Hl/I2 initiated polymerizations of vinyl ethers and polymerizations of isobutene and styrene using acetate-based initiators in the presence of BC13. 

Phosphine trapping and 31P NMR has also been used to monitor polymerizations of vinyl ethers and follow the polymer tacticity [208], However, less nucleophilic phosphines, such as tris(p-chlorophenyl)phos-phine, do not terminate the growing chains efficiently, and polymerization continues slowly [66]. Nevertheless, trapping with less nucleophilic phosphines provides information on microstructure and also on kinetics in slow polymerizations. 

These salt effects are schematically depicted in Scheme 8. As we will discuss later more in detail (Sections Vl.B.3 and VII.E.3), mechanistically, salts may act in two different ways. In polar solvents they will suppress the free ions and considerably reduce their lifetime. This often converts bimodal MWD to monomodal MWD and provides controlled polymers. However, in polymerization of vinyl ethers initiated by strong Lewis acids such as SnCl4, where only ion pairs are present after addition of a few percent of salts or in nonpolar toluene, control is still very poor (Fig. 17B). Controlled polymers can be obtained only after addition of a more than equimolar amount of tetra-n-butylammonium halides. This implies that the salts change the weakly nucleophilic counterion SnCIs-to the more nucleophilic SnCl62 , which faster converts growing carbo-cations to covalent species. Another effect of added salts is related to... 

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