Vinyl alkyl ethers

With these monomers it is possible to achieve rapid and virtually quantitative polymerisations using both Ph3C and C7H7 salts (79-81). However, the initiation reaction does not appear to be as fast as that with N-vinylcarbazole and conversion/time curves show evidence of an initial acceleration to a maximum rate of polymerisation. Nevertheless, appropriate analysis of the kinetic data allows computation of rate constants for the propagation reaction kp, and the results obtained are summarised in Table 8. 

Alkyl vinyl ether Initiator (SbClg salts) Temp. (°C) io-3fcp (M 1 sec ) Ref 

Apart from some experiments with methyl and /i-chloroethyl vinyl ethers the initiator concentrations employed were such that the initiating cations, and presumably the propagating species, were essentially dissociated from the corresponding counterion. Once again therefore this data is a measure of the reactivity of the free polymeric cations derived from the various monomers. Isobutyl vinyl ether is the monomer most widely studied, and as would be anticipated for free cationic reactivities, the data varies little with the counterion employed (SbClg or BF4), or indeed with the carbocation used as initiator (C7H7 or Ph3C+) under similar experimental conditions. 

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 

Clearly s-cis conformers provide larger steric interference in forming the transition state for propagation (or reactions with electrophiles) than the other possible forms, and not surprisingly the methyl and /S-chloroethyl monomers yield data for the rate constant for propagation about one order of magnitude lower than the others, all of which exhibit comparable reactivity. 

---

The addition proceeds in three discrete steps and the intermediates can be isolated. Simple alkenes are less reactive than alkynes and do not undergo the addition to aHylic boranes, but electron-rich alkyl vinyl ethers react at moderate temperatures to give 1,4-dienes or dienyl alcohols (440). 

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropyhum hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationicaHy fairly reactive monomers (eg, A/-vinylcarbazole, -methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

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). 

Fig. 1. Effect of ion pairing on stereochemistry of propagation for alkyl vinyl ethers, where L is large substituent S, small substituent.

As shown in Table 3, the glass-transition temperatures of the amorphous straight-chain alkyl vinyl ether homopolymers decrease with increasing length of the side chain. Also, the melting points of the semicrystalline poly(alkyl vinyl ether)s increase with increasing side-chain branching. 

Poly(alkyl vinyl ether) CAS Registry Number T/C Mp, °C... 

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. 

Pyrazolin-5-ones with an exocyclic double bond at the 4-position (369 X = CMe2> react as heterodienes towards alkyl vinyl ethers (77G91). The kinetics of this Diels-Alder reaction giving pyrazolopyrans (370) have been studied. 

Synthesis of Phosphonic and Phosphinlc Acids and their Derivatives.-The phosphonium adducts from PCl and alkyl vinyl ethers are converted into the corresponding phosphonic dichlorides by hexamethyldisiloxane. Oxidative phosphorylation of the 1,3-butadienes (53) affords the phosphonic dichlorides (54). 

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

The preparation of quinoline and tetrahydroquinoline derivatives from metal carbonyl-catalyzed reactions of Schiff bases with alkyl vinyl ethers in... 

Table 1 Reaction of alkyl vinyl ether 57 with Cp 2Sm(THF)...

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. 

Table 13.2. Copolymerization of (S02F)-Functional Perfluorinated Alkyl Vinyl Ether Monomer with TFE...

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). 

From the point of view of the monomer what matters most is that the radical RCH2C R"R/" derivable from it should have a low ionisation potential. It may be that this is the reason for the eminent polymerisability of alkyl vinyl ethers and NVC. 

The formation of w-donor complexes (IV). This involves stronger forces than the previous two types because the lone pair of the hetero-atom is involved. It is clear that the polymerisations of some of the favourite monomers, such as the alkyl vinyl ethers, 4-MeO-styrene, and N-vinylcarbazole may be dominated by this phenomenon. A corollary of the complexation by an w-donor monomer is that the hetero-atoms in the corresponding polymers will also interact with the growing carbenium ions. The authors who have proposed this include Stannett (alkyl vinyl ethers) [11], Boelke (dimethoxyethene) [12], and Sauvet (4-MeO-St) [7]. 

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 ionization of the alkyl vinyl ethers generates radical cations which break up into radicals and cations. The question how an alkyl vinyl ether would split into a cation and an anion was considered by Hamann et al. (1949), and following their arguments it seems likely that the split would produce an alkyl cation R+ and a vinyloxy radical, ViO, thus ... 

The irradiation of alkyl vinyl ethers leads to dissociative electron capture. Yoshida et al. (1972) showed that dimethyl ether decomposes according to... 

---