Polymerization with alkyl vinyl ethers

Results obtained in the many copolymerizations of carbazole-containing monomers with different chiral comonomers may be summarized as follows i) real copolymer macromolecules are obtained in the cationically and free radically initiated polymerization with alkyl vinyl ethers, acrylic and methacrylic derivatives, and butene-dioic acid diesters ii) homopolymer mixtures are obtained in copolymerization runs with a-olefms in the presence of Ziegler-Natta catalysts, indicating that the conventional anionic coordinate mechanism is not effective in the polymerization of carbazole monomers... 

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). 

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.)... 

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

For example, the polymerization of alkyl vinyl ethers using an HC1/ SnCU (or adduct S/SnCl4) initiating system in methylene chloride is very fast even at - 15° C to give polymers with broad and often bimodal MWDs (Figure 17D) [105], Similar effects of solvent polarity are found in the polymerizations of p-alkoxystyrenes [107], styrene [25], and iV-vinylcar-bazole [108],... 

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. 

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). 

Other monomers that copolymerize with alkyl vinyl ethers are vinyl ketones [47], acrolein diacetate [48], acrylamide [49], alkoxy 1,3-butadienes [50], butadiene [51], chloroprene [52], chlorotrifluoroethylene [53], tri-and tetrafluoroethylene [54], cyclopentadiene [55], dimethylaminoethyl acrylate [56], fluoroacrylates [57], fluoroacrylamides [58], A-vinyl car-bazole [59,60], triallyl cyanurate [59,60], vinyl chloroacetate [61,62], N-vinyl lactams [63], A-vinyl succinimide [63], vinylidene cyanide [64, 65], and others. Copolymerization is especially suitable for monomers having electron-withdrawing groups. Solution, emulsion, and suspension techniques can be used. However, in aqueous systems the pH should be buffered at about pH 8 or above to prevent hydrolysis of the vinyl ether to acetaldehyde. Charge-transfer complexes have been suggested to form between vinyl ethers and maleic anhydride, and these participate in the copolymerization [66]. Examples of the free-radical polymerization of selected vinyl ethers are shown in Table IV. 

One of the examples illustrating the living cationic polymerization process is the polymerization of alkyl-vinyl ethers initiated by the mixture of hydrogen iodide with iodine. In this process the system is stabilized by a suitably strong interaction of carbocation with counterion ... 

Kennedy et al., inspired by Olah s successful stabilization of carbocations with superacids , attempted living polymerization of isobutene using superacids as initiators . Indeed, no termination of polymerization occurred but, unfortunately, the reaction suffeied from frequent chain transfer. With Ph3C SbCI as initiator, Stannett et al. prepared AB-type block polymers by sequential polymerization of alkyl vinyl ethers and NVC in a polar solvent (Table 9). Althou the blocking efficiency was as low as 40 to 55 %, this work is of significance as an example showing... 

Whereas free radically initiated polymerization yields polymers characterized by an essentially random distribution of monomeric units, block-like polymeric products are obtained in the cationic copolymerization of NVC with alkyl vinyl ethers. The possible formation of preferential aggregates of the vinyl aromatic monomer or a possible control of the homopropagation process connected with the bulkiness of the monomers has been suggested as responsible for such polymer structures . A random distribution of monomeric units is conceivable in copolymers based on the spaced-carbazole-containing monomers 11-14, whilst a quasi alternating distribution is observed in copolymers from NVC and fumaric acid diesters . ... 

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). 

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. 

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. 

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 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. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

In the early 1980s, Kennedy and his co-workers reported quasiliving polymerizations, which are phenomenologically akin to living polymerizations [57]. These processes involved slow and continuous monomer addition to a stirred initiator solution kept at a relatively low temperature. The monomers used therein included a-methylstyrene, isobutene, styrene, and alkyl vinyl ethers. In most cases, the number-average molecular weights steadily increased with the weight of the added monomer and the formed polymers had relatively narrow MWDs. 

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-... 

---