Alkyl vinyl ethers polymerization

The second part of the theory, which is a logical consequence of the first, is that monomers that have more than one basic site, e.g., an aromatic ring or an oxygen atom, can form more than one type of complex with the carbenium ion this idea was first proposed by Plesch (1990) in the context of chemically initiated polymerizations. It helps to explain why aryl alkenes and alkyl vinyl ethers polymerize more slowly than isobutene and cyclopentadiene. The reason is that all the complexes formed by the alkyl alkenes are propagators, whereas for the aryl alkenes and vinyl ethers only a fraction of the population of complexes can propagate. 

Scheme 1 shows the model and the corresponding table the definition of the symbols. This model is comprehensive as it encompasses two formally different but fundamentally similar mechanisms polymerizations induced by a purposely-added initiator (RX) shown on the left side of Scheme 1 and polymerizations induced by adventitious protic impurities HX on the right side of Scheme 1. The two sides of Scheme 1 are connected by two routes due to chain transfer to monomer these will be discussed below. The model is valid for Friedel-Crafts acid (MtX ) coinitiated polymerizations, including all kinds of conventional and living olefin and alkyl vinyl ether polymerizations.

While alkyl vinyl ethers polymerize readily by eationie meehanism, the films that they form often laek good physieal properties. Aromatie analogs of the aliphatie vinyl ethers, on the other hand, were reported by Crivello and Ramdas to yield thermally stable materials with improved properties. In addition, they photopolymerize readily in the presenee of diaryliodonium salts. The syntheses of the monomers was earried out aeeording to the following seheme... 

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). 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

Quasi-living Carbocationic Polymerization of Alkyl Vinyl Ethers and Block Copolymer Synthesis... 

Fluorinated polymers, especially polytetrafluoroethylene (PTFE) and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene (HFP) and perfluorinated alkyl vinyl ethers (PFAVE) as well as other fluorine-containing polymers are well known as materials with unique inertness. However, fluorinated polymers with functional groups are of much more interest because they combine the merits of pefluorinated materials and functional polymers (the terms functional monomer/ polymer will be used in this chapter to mean monomer/polymer containing functional groups, respectively). Such materials can be used, e.g., as ion exchange membranes for chlorine-alkali and fuel cells, gas separation membranes, solid polymeric superacid catalysts and polymeric reagents for various organic reactions, and chemical sensors. Of course, fully fluorinated materials are exceptionally inert, but at the same time are the most complicated to produce. 

During polymerization, a polymeric radical with a perfluoro(alkyl vinyl ether)-derived active center can have one of two fates it can cross-propagate to tetrafluoroethylene or it can undergo P-scission to yield an acid-fluoride-terminated polymer chain and generate a peduoroalkyl radical capable of initiating further polymerization (ie., chain transfer to monomer). These scenarios are illustrated in Scheme 3. 

For the polymerization of alkyl vinyl ethers there is no direct evidence for or against the chain-carriers being ionic. Some aspects of this question are discussed in sub-section 4.4. 

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. 

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] ... 

Thus the reaction site, the double bond, participates in the charge and polymerization can take place. That the vinyl group conjugated with the oxygen atom is more basic than an ordinary ether oxygen is shown by the fact that alkyl vinyl ethers can be polymerised readily in dialkyl ethers as solvents. 

In many systems k2 = ki [C] or k4 [C] [Cocat] P, where a and /3 may equal 1 or may be fractional. The polymerizations of alkyl vinyl ethers by iodine and some other catalysts obey these kinetics, with a = 1, /3 = 0 (see reference 35 and the preceding papers of that series). 

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 theory developed for the hydrocarbons was transferred virtually without change to the polymerization of alkyl vinyl ethers (Ueno et al. 1967), and it was not realized that these monomers might behave rather differently, for several reasons. 

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 relative reactivities of alkyl vinyl ethers have been assessed in a number of chemical reactions and structure/activity correlations made via several NMR studies [for bibliography see Ref. (80)]. From the polymerisation data it appears that steric interaction between the incoming polymeric electrophile and monomer is the major factor controlling reactivity, rather than electronic effects. Vinyl ethers are known to exist in either a planar s-cis or a planar s-trans (or gauche) conformation. Infra-red absorption spectroscopy shows that methyl vinyl ether almost certainly exists largely in the s-cis form at room temperature (106), and it seems most likely that /(-chloroethyl vinyl ether also has an energetically favourable planar s-cis form as a result of a favourable gauche interaction of Cl and O atoms. The other alkyl vinyl ethers studied exist predominantly in either planar... 

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. 

From patent literature, it appears that Vandenberg (353) was the first to polymerize vinyl ethers to crystalline polymers using modified Ziegler catalyst preparations. A series of papers have been published by Vandenberg and his associates which describe the polymerization of simple alkyl vinyl ethers (29), ROCH=CHCHs (354), CH30CH=CH--CH=CH2 (355), and epoxides (356). 

Since coordination of the ether oxygen is involved in the stereoregulating step, any factor which weakens this will decrease stereospecificity. This explains why the more hindered, higher alkyl vinyl ethers give less stereoregular polymerization than vinyl methyl ether. 

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