Free radical 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). 

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. 

Substituted olefins that are capable of forming secondary or tertiary carbo-nium ion intermediates polymerize well by cationic initiation, but are polymerized with difficulty or not at all free radically. In general, vinyl or /-alkenes that contain electron donating groups (alkyl, ether, etc) polymerize well via a carbo-cationic mechanism. 

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). 

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. 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

Alternating copolymers ( interpolymers ) of maleic anhydride and alkyl vinyl ethers are prepared by free radical polymerization in solvent, non-solvent or bulk media Products prepared from ethyl-, butyl-, hexyl-, and octyl-, and decyl- vinyl ethers are described in Table I. The decyl-copolymer was prepared by Dr A. W. Schultz. A methyl-copolymer ( Gantrez AN General Aniline and Film Corp.), not described in Table I, was included in some studies. Molecular weight estimates were... 

High-molecular-weight A-substituted maleimides have been prepared and used as polymeric food antioxidants which can achieve the desired gastrointestinal nonabsorption. A-(3,5-Di-t-Bu -hydroxyphenyl)maleimide was prepared in two steps (a) formation of 2,6-di-t-Bu-4-aminophenol either from 2,6-di-(-Bu-phenol by nitration followed by reduction, or from 4-aminophenol by alkylation, (b) amida-tion of maleic anhydride with the 2,6-di-t-Bu-4-aminophenol followed by dehydration. The nonabsorbable poly(A-(3,5-di-t-Bu-4-hydroxyphenyl)maleimide)s were prepared from the monomeric maleimides by free radical homo- and copolymerization with comonomers of alkyl vinyl ethers (Scheme 5.5) [43]. 

Copolymerization of TFE with perfluoroalkyl vinyl ethers proved to be quite facile, either by dispersion techniques similar to that employed with FEP [7] or by a newly developed process ploying a fluorocarbon solvent. However, from the earliest studies it was evident that some complications had to be overcome. The most significant of these was a tendency for the alkyl vinyl ethers to rearrange when exposed to free radicals. In the extreme case a chain reaction could be initiated which would result in incomplete rearrangement to the isomeric acid fluoride. During polymerization at temperatures low enough to prevent excessive reaction by this route, the process, nevertheless, competes effectively with free radical coupling as a termination mechanism. 

The efficiency of cationic photoinitiators can be enhanced by the use of free-radical sources such as benzoin alkyl ethers and alkoxyacetophenones. In the presence of THF, photo-active radical sources and diaryliodonium salts would be expected to yield cations as outlined in Scheme 1. The neral method of producing cations by the oxidation of radicals produced from any source has been demonstrated for the polymerization of vinyl ethers and THF. ... 

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. 

Both alkyl and aryl vinyl ethers and a variety of unsaturated cyclic ethers will undergo free-radical copolymerization with MA, under mild conditions, to give equimolar copolymers. DuPlessis and coworkers have shown that dialkyl maleates and alkyl vinyl ethers will also undergo equimolar copolymerization. This occurs even though all of the monomers are very sluggish to free-radical homopolymerization. In some cases, spontaneous or thermal copolymerization can even occur between vinyl ethers and MA, such as 1,2-dimethoxyethylene, " p-dioxene, and conjugated dihydro-anisole. It is also known that vinyl ethers will polymerize in the presence of amide-MA mixtures, with the amide-MA CTC playing the role of initiator. ... 

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... 

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 . ... 

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