Cationic polymerization alkyl vinyl ethers

Cationic Polymerization. For decades cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition (see Elastomers, synthetic-BUTYLRUBBEr). More recently, development has led to the point where living cationic chains can be made, with many of the advantages described above for anionic polymerization (27,28). 

However, Bawn et al., take the view that when polymerization of an alkyl vinyl ether is initiated by a stable ion, such as tropylium, the initiation involves electron abstraction from the monomer with formation of a radical cation and a tropyl radical [52] ... 

This paper is about a reinterpretation of the cationic polymerizations of hydrocarbons (HC) and of alkyl vinyl ethers (VE) by ionizing radiations in bulk and in solution. It is shown first that for both classes of monomer, M, in bulk ([M] = niB) the propagation is unimolecular and not bimolecular as was believed previously. This view is in accord with the fact that for many systems the conversion, Y, depends rectilinearly on the reaction time up to high Y. The growth reaction is an isomerization of a 7t-complex, P +M, between the growing cation PB+ and the double bond of M. Therefore the polymerizations are of zero order with respect to m, with first-order rate constant k p]. The previously reported second-order rate constants kp+ are related to these by the equation... 

The polymerization of alkyl vinyl ethers is of some commercial importance. The homopolymers, which can be obtained only by cationic polymerization, are useful as plasticizers of other polymers, adhesives, and coatings. (The copolymerization of vinyl ethers with acrylates, vinyl acetate, maleic anhydride, and other monomers is achieved by radical polymerization but not the homopolymerizations of alkyl vinyl ethers.)... 

The cationic nature of the polymerization of various alkyl vinyl-ethers has been studied by Iwasaki, Fukutani and Nakano (JO). They conclude that magnesium salts, being weak Lewis acids, are mildly effective for the isotactic polymerization of alkyl vinylethers. 

Fig. 3. Cationic catalysts and their effectiveness for steric control in the polymerization of alkyl vinyl ethers...

Cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition. See also Elastomers and Rubber (Synthetic). 

POLYVINYL ALKYL ETHERS. These products have properties which range from sticky resins to elastic solids. They are obtained by the low-temperature cationic polymerization of alkyl vinyl ethers having the general formula ROCH=CH-. These monomers are prepared by the addition of die selected alkanol to acetylene in the presence of sodium alkoxide or mercury(ll) catalyst, As shown by the following equations, the latter yields an acetal which must be thermally decomposed to produce the alkyl vinyl ether. 

In 1982 Higashimura et al. [54] began studies focused on the development of living cationic polymerizations of vinyl monomers. They decided to use IBVE and related alkyl vinyl ethers as monomers because they form the alkoxy-stabilized growing carbocations, along with iodine as the initia-... 

Figure 16 Controlled/living cationic polymerizations of alkyl vinyl ethers with the HI/I2 initiating systems. (From Ref. 58.)...

The HB/MtX initiating systems with stronger Lewis acids than zinc halides induce very rapid or almost instantaneous polymerizations of alkyl vinyl ethers and are not suited for controlled/living cationic polymerizations (Section IV.B.2). These initiating systems include ... 

An important advantage of the use of such added nucleophiles is that it allows controlled/living cationic polymerization of alkyl vinyl ethers to proceed at +50 to +70°C [101,103], relatively high temperatures at which conventional cationic polymerizations fail to produce polymers but result in ill-defined oligomers only, due to frequent chain transfer and other side reactions. Recently, initiators with functionalized pendant groups [137] and multifunctional initiators [ 138—140] have been developed for the living cationic polymerizations with added nucleophiles. 

Since the development of living cationic polymerizations of alkyl vinyl ethers (Chapter 4, Sections IV and V.A), considerable efforts have been made to synthesize and polymerize vinyl ether derivatives carrying polar functional substituents, and thereby it is now possible to obtain a variety of pendant-functionalized poly(viny) ethers) of controlled molecular weights and narrow MWDs [1,2,13], Figure 3 lists typical examples of vinyl ethers carrying various pendant functionalities for which living cationic polymerizations are available [14-35]. These monomers are synthesized most conveniently from 2-chloroethyl vinyl ether, now commercially available,... 

These initiators are primarily of academic interest because their initiation processes are fairly straightforward compared to the complicated equilibria that can exist in other systems (e.g.. Fig. 9-2). This simplifies the study of the kinetics of the propagation and other processes involved in cationic polymerization. However, since these cations are stable, their use is limited to the initiation of more reactive monomers like Af-vinylcarbazole and alkyl vinyl ethers. 

Molecular iodine was shown to be an effective initiator of the polymerization of active olefins (e.g. alkyl vinyl ethers) as long ago as 1878 [52]. The mechanism operative is now accepted as being a cationic one, and a number of kinetic investigations have been carried out from time to time 153—58]. In addition parallel ultraviolet/visible spectro-... 

A number of years ago triphenylmethyl cation, Ph3C, formed in situ by dissociation of triphenylmethyl chloride, was shown [73] to initiate the polymerization of 2-ethylhexyl vinyl ether in m-cresol solvent. More recently certain stable carbonium ion salts, notably hexachloroantimonate (SbCls) salts of cycloheptatrienyl (tropylium, C7H7) and triphenylmethyl cations have been shown [74, 50] to be very efficient initiators of the cationic polymerization of many reactive monomers [27, 29, 75]. Since the discovery of the effectiveness of the SbClg salt, triphenylmethyl salts with different anions have also been used [76—78]. The most detailed kinetic studies using these initiators have been carried out on alkyl vinyl ethers [27, 30] and A-vinylcarbazole [39] in homogeneous solution in methylene chloride. 

In the cationic-initiated polymerization of alkyl vinyl ethers it is possible to exercise fairly rigorous control of the configuration of the product by appropriate choice of the monomer and conditions. For example, isobutyl vinyl ether polymerized by BF3 etherate at 195 K in toluene can give isotactic polymer [15]. In this low polarity solvent, close association of the gegen ion with the cationic propagating center helps to block one mode of entry of fresh monomer (Eq. 22.45). 

Cationic polymerization shares several of the features discussed above for anionic polymerization, but the requirements in terms of the monomer structure will be opposite, i.e. the vinyl monomers should have an electron-donating side group since the propagating centre carries a positive charge. Alternatively, the presence of an aromatic ring can stabilize the cation by making available resonance structures. Examples of these are alkyl vinyl ethers, isobutylene, isoprene, styrene and a-methyl styrene (Sauvet and Sigwalt, 1989). 

SCHEME 11.33 General mechanism for photoinitiated cationic polymerization of alkyl vinyl ethers (a) and epoxides (b). 

In contrast to the direct cationic polymerizations of mesogenic vinyl ethers discussed in Sec. 2.3 of this chapter, poly[M-[(4 -cya-nophenyl-4"-phenoxy)alkyl] vinyl ether s... 

In (A), the reaction will predominate on the left-hand side. Exceptions appear to be olefins with strong electron-releasing substituents that confer thermodynamic stability to the newly formed cation, -CH2-CHR . This either results from a suitable charge delocalization over the 7r-electron system or from the presence of a heteroatom. Accordingly, only those olefins that possess relatively strong nucleophilic characteristics can be polymerized by stable carbon cations. Such olefins are alkyl vinyl ethers, A -vinyl carbazole, p-methoxystyrene, indene, and vinylnaphthalenes. Styrene and cf-methylstyrene, however, will not polymerize, because they are less reactive. 

Some radical sources will, in the presence of oxidizing agents, or light or heat energy, initiate cationic polymerizations of monomers, like n-butyl vinyl ether. Those that are most readily oxidized are carbon atom centered radicals that have substituents like benzyl, allyl, alkoxy, or structures with nitrogen or sulfur. Also, radicals that are formed by addition of other radicals to alkyl vinyl ethers are particularly reactive. 

One example is a combination of an alkyl vinyl ether (donor) with vinylidine cyanide (acceptor) that results in ionic polymerizations. The reaction actually contains the ingredients of both cationic-and anionic-type polymerizations ... 

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