Vinyl ethers, polymerisation

Van der Waals radii, 8 Vibrational modes, 342 Vinyl ethers, polymerisation, 189 Vinyl polymerisation, 320 branching in, 321 chain length in, 321 coordination, 322 induction period in, 321... 

Vinyl ethers and a,P unsaturated carbonyl compounds cyclize in a hetero-Diels-Alder reaction when heated together in an autoclave with small amounts of hydroquinone added to inhibit polymerisation. Acrolein gives 3,4-dihydro-2-methoxy-2JT-pyran (234,235), which can easily be hydrolysed to glutaraldehyde (236) or hydrogenated to 1,5-pentanediol (237). With 2-meth5lene-l,3-dicarbonyl compounds the reaction is nearly quantitative (238). 

It is not possible to polymerise vinyl ethers by free-radical-initiated methods but, as with isobutylene polymers, it is possible to make polymers using Friedel-Crafts type catalysts. 

The vinyl alkyl ethers polymerise violently in the presence of small quantities of inorganic acids. 

Methylpropene can be made to continue the process to yield high polymers—cationic polymerisation—but most simple alkenes will go no further than di- or tri-meric structures. The main alkene monomers used on the large scale are 2-methyIpropene (— butyl rubber ), and vinyl ethers, ROCH=CH2 (— adhesives). Cationic polymerisation is often initiated by Lewis acid catalysts, e.g. BF3, plus a source of initial protons, the co-catalyst, e.g. traces of HzO etc. polymerisation occurs readily at low temperatures and is usually very rapid. Many more alkenes are polymerised by a radical induced pathway, however (p. 320). 

Methyl vinyl ether is rapidly hydrolysed by contact with dilute acids to form acetaldehyde, which is more reactive and has wider flammability limits than the ether [1], Presence of base is essential during storage or distillation of the ether to prevent rapid acid-catalysed homopolymerisation, which is not prevented by antioxidants. Even mildly acidic solids (calcium chloride or some ceramics) will initiate exothermic polymerisation [2],... 

Methanesulfonic acid is too powerful a catalyst for (9-alkylation with the vinyl ether, causing explosive polymerisation of the latter on multimolar scale. Dichloroacetic acid is a satisfactory catalyst on the 3 g mol scale. 

Typical monomers which polymerise through cationic mechanisms are isobutene, styrene, a-methylstyrene, vinyl ethers and vinyl carbazoles. At present, about 100 vinyl monomers are known that can be polymerised by the cationic initiators. 

P. H. Plesch, S. H. Shamlian, The Propagation Rate-Constants of the Cationic Polymerisation of Alkenes Part III. Indene, two Vinyl Ethers, and General Discussion, Europ. Polym. J., 1990, 26, 1113. 

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. 

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. 

Table 2 Propagation rate constant for the polymerisation of isobutyl vinyl ether ...

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. 

Suggestions Concerning the Ionic Polymerisation of Vinyl Ethers (1950)... 

Eley and Pepper [1] and Eley and Richards [2] have recently studied the kinetics of the catalysed polymerisation of vinyl ethers. The very simple formal scheme which accounts adequately for the kinetics of these reactions does not provide a definite picture of the mechanism of the reaction. Nor does the oxonium theory of Shostakovskii [3] appear adequate to explain the observations. Our suggested explanation of the observed facts arises from the following considerations. 

It is difficult to see how the presence of two double bonds in each polymer molecule (reported by Eley and Richards for the polymerisation of 2-ethyl hexyl vinyl ether) can be explained without assuming that the chain is started by an unsaturated entity, and that the second double bond is formed in the termination process. Since the chain growth is almost certainly a carbonium ion process the initiating entity must be a positive ion of some sort. We assume therefore that the ether is split into two ions under the influence of the catalyst. This may obviously occur in two different ways, but energetic considerations can show which of these will in fact take place. 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

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]. 

The only studies on olefin polymerisations in methylene dichloride in which kp was deduced directly from the rate of reaction were carried out by Ledwith and his collaborators [9, 13] with extremely low concentrations of monomer and catalyst. They polymerised isobutyl vinyl ether and N-vinyl carbazole in a Biddulph-Plesch calorimeter with trityl or tropylium salts and obtained the first-order rate constants k1 from the conversion curves. Since different catalysts gave the same ratio of kx c they concluded that for each of them Xxr = c0 and hence identified with kp which must in fact be k p, as explained above. It seems unlikely that if several initiators give the same value of kp, they do so because they are all equally inefficient, and the inference that they do so because they are all 100% efficient, i.e., that for all of them x = c0, seems plausible - but it would be useful to have a direct check of this. 

We report on the measurement of the propagation rate constants kp of styrene, indene, phenyl vinyl ether (PhViE) and 2-chloroethyl vinyl ether (CEViE) in nitrobenzene at (mostly) 298 K with 4-ClC6H4CO+SbF 6 as initiator. The dependence of the conductivity on the [4-ClC6H4CO+SbF"g] = c0 helped to establish that [Pn+] = c0 and thus to validate the foundation of this work. It is shown that most probably the propagating species are the uncomplexed, unpaired, solvated carbenium ions. Some new enthalpies of polymerisation have been found. 

This is the third report on attempts to measure the propagation rate constant, kp+, for the cationic polymerisation of various monomers in nitrobenzene by reaction calorimetry. The first two were concerned with acenaphthylene (ACN) [1, 2] and styrene [2]. The present work is concerned with attempts to extend the method to more rapidly polymerising monomers. With these we were working at the limits of the calorimetric technique [3] and therefore consistent kinetic results could be obtained only for indene and for phenyl vinyl ether (PhViE), the slowest of the vinyl ethers 2-chloroethyl vinyl ether (CEViE) proved to be so reactive that only a rough estimate of kp+ could be obtained. Most of our results were obtained with 4-chlorobenzoyl hexafluoroantimonate (1), and some with tris-(4-chlorophenyl)methyl hexafluorophosphate (2). A general discussion of the significance of all the kp values obtained in this work is presented. 

Some exploratory experiments with ethyl vinyl ether showed that it polymerised too rapidly for our method to be useful. The polymerisation of 2-methyl butene-2 had a manageable rate but the results showed inconsistencies which we did not have time to resolve, as this was the last monomer to be studied. 

Table 3 The polymerisation of phenyl vinyl ether by (1) in PhN02 at 293 K ... 

CEViE was the first vinyl ether whose polymerisation was attempted in this work. With the initiator (2) dissolved in PhN02, manageable reaction rates and reasonably consistent values of kJcQ, were obtained (Table 4). However, when we attempted to check these results by means of the initiator (1), the reactions were uncontrollably fast and, as is seen from the last two entries in Table 4, k /c() is ca ten times as great as with the other initiator, so that we can only say that kp+ is of the order of 104dm3 inol 1s 1. Work with this monomer was abandoned. The inefficiency of the initiator (2) is noteworthy. 

The present author wanted to determine the propagation rate-constant, kp, for the cationic polymerisation of various alkenes (this term here includes vinyl ethers, VE) under such conditions that the interpretation of the measurements should be as unambiguous as possible. If there are no complications from the complexation of the propagating species with constituents of the reaction mixture, the rate of such polymerisations is given generally by equation (1) ... 

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