Cationic 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. 

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-... 

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... 

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... 

Figure 20 Protonic acid/Lewis acid (HB/MtX ) initiating systems for living cationic polymerizations of vinyl ethers. See Section V.A.l for references.

For these initiating systems, externally added nucleophiles are necessary to induce controlled/living cationic polymerizations of vinyl ethers [36,64]. Table 2 A lists nucleophiles (Lewis bases) that are effective for such purposes and includes esters (carboxylates and carbonates) [100,101,130-133], ethers (linear and cyclic) [102-104,137-140], methyl-pyridines [140], and phosphines [21,141]. CF3S03H-initiated polymerizations, sulfides are also effective [37,38,134,135]. 

Almost all of the initiating systems discussed in this section can be applied to the living cationic polymerizations of vinyl ethers that carry a variety of functional pendant groups, in a general form [42,43,65,66,180] ... 

A class of end-functionalized polymers with polymerizable terminal groups are generally called macromonomers. By both functional initiator and terminator methods, a variety of macromonomers have been synthesized in living cationic polymerization of vinyl ethers, styrenes, and isobutene, as summarized in Table 3 [16,31,147,149-151,155,158-171]. Some of these macromonomers are used in the synthesis of graft polymers (Section VI.C). 

As summarized in Chapter 4, Section V.E.l, a variety of multifunctional initiators are currently available for the living cationic polymerizations of vinyl ethers [83,188,189], alkoxystyrenes [149,190], and isobutene [191-201], and up to tetraarmed polymers with controlled arm lengths are prepared by the use of these initiating systems. Scheme 9 exemplifies such a synthesis for vinyl ethers [188]. The details for the design of these initiating systems are found in Chapter 4. 

Such a synthesis is feasible in living cationic polymerization of vinyl ethers [208], For bifunctional vinyl ethers (20), a series of compounds are examined (Scheme 11), among which 20a turned out the best, probably due to the appropriate rigidity and length of the spacer. By adjusting reaction... 

If the theoretical aspects of the cationic polymerization of vinyl ethers are well documented, the literature remains particularly reserved concerning industrial syntheses of the commercial homopolymers. 

Figure 4, Photolysis of diaryliodonium salts having nonnucleophilic anions results in the formation of strong acids (HPFe). Nonnucleophilic acids such as HPFe can initiate cationic polymerization of vinyl ethers and cyclic ethers.

A key to the success of the living cationic polymerization of vinyl ethers is the stabilization of the unstable carbocations via suitable nucleophilic counterion. There are two ways to stabilize the carbocations (1) generation of suitable nucleophilic counterion resulted from the initiator and the catalyst, and (2) addition of nucleophilic agents to the polymerization media. In the first way, Bronsted acids such as hydrogen iodide are employed as the initiators, while Lewis acids such as zinc iodide are employed as the catalysts (Scheme 11.41) [140-143],... 

M. Kamigaito, M. Sawamoto, and T. Higashimura, Living cationic polymerization of vinyl ethers by electophile lewis acid initiating systems. 6. Living cationic polymerization of isobutyl vinyl ether by RCOOH/lewis acid initiating systems effects of carboxylate ions and lewis acid activators. Macromolecules 1991, 24(14), 3988-3992. 

In conclusion of this section it will be noted that one of the first examples of homogenous polymerization in which isotactic polymers with the content of isotactic triads higher than 80% have been obtained is the cationic polymerization of vinyl ethers [53, 54]. 

Even though the cationic polymerization of vinyl ethers already dates back to 1878 [43], the first report on a controlled polymerization only appeared in the late 1970s by Johnson and Young [44]. They discovered that the... 

Scheme 8.10 Mechanism for the hving cationic polymerization of vinyl ethers with hydrogen iodide and iodine as initiating system.

All previously discussed examples of living cationic polymerization of vinyl ethers were based on homogeneous polymerization media. In 2007, Oashima and coworkers demonstrated the living polymerization of isobutyl vinyl ether in the presence of iron(III) oxide as heterogeneous catalyst and ethyl acetate or dioxane as base [58]. The major advantage of this heterogeneous catalytic system is the easy removal of the metal oxide catalyst. In addition, it was demonstrated that the iron(III) oxide could be reused for at least five times without a decrease in activity. 

In 1982, Crlvello and co-workers published a report on the UV Initiated cationic polymerization of vinyl ether monomers using onlum salt catalysts(14). Vinyl ethers are among the most reactive monomers which polymerize by a cationic mechanism. The radiation Induced cationic curing of vinyl ethers occurs much faster than the cationic curing of epoxy coatings. In fact, cure rates that are at least as fast as the free radical polymerization of acrylates can be achleved(8,14). A recent report Indicates that the cationic polymerization of vinyl ethers can occur even in the presence of certain polar functional groups(15). 

High-Energy-Radiation-Induced Cationic Polymerization of Vinyl Ethers in the Presence of Onium Salt Initiators... 

Sugihara S, Matsuzono S-i, Sakai H, Ahe M, Aoshima S. Syntheses of amphiphilic block copolymers by living cationic polymerization of vinyl ethers and their selective solvent-induced physical gelation. J Polym Sci A 2001 39 3190-3197. 

Consider the low-temperature cationic polymerization of vinyl ethers and discuss the effect of solvent on polymer stereochemistry. 

With one exception [90], side-chain liquid crystalline binary copolymers have only been prepared by a living mechanism using cationic polymerizations of vinyl ether comonomers (Eq. 36). However, these... 

Fig. 4 Flow microreactor system for controlledAiving cationic polymerization of vinyl ether initiated by SnCL. M interdigital multilamination micromixer, R microtube reactor...

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