Tetrahydrofuran carbocation polymerization

The polymeric carbocation initiates polymerization of isobutylene, styrene, tetrahydrofuran, and other monomers [Cai and Yan, 1987 Cameron and Sarmouk, 1990 Kennedy and Ivan, 1992 Martinez et al., 2001 Pi and Kennedy, 2001]. 

Carboanionic Intermediates.—Whereas characterization of the carbocations active in cationic polymerization has been difficult, identification and quantification of the carboanionic centres in anionic propagations have been somewhat easier, largely as a result of a considerably reduced tendency to rearrange and isomerize. Nevertheless, detailed investigations continue in particular areas. Thus Bywater and Worsfold ° have used n.m.r. spectroscopy to probe the structure of various model compounds which closely resemble those presumed to be responsible for anionic propagations. Whereas in tetrahydrofuran solvent 2,5-diphenyl-2,5-dipotassiohexane and 2-lithio-4,4-dimethyl-2-phenyl-pentane display coupling constants consistent with the carbon atoms... 

With some cationic initiators, such as the triphenyl methyl (trityl) salts of the general formula PhsC " X , where X is a stable anion such as CIO4, SbClg, and PFg, the initiation does not involve direct addition to monomer. The carbocation abstracts a hydride (H ) ion from the a-carbon of the monomer and the resulting new carbocation initiates polymerization. Thus the reaction of trityl hexafluo-rophosphate and tetrahydrofuran has been shown to proceed as follows (Cowie, 1991) ... 

Carbocations generated in this way can add directly to appropriate monomers (e.g., tetrahydrofuran, cyclohexene oxide, n-butyl vinyl ether) or can form Bronsted acids by abstracting hydrogen from surrounding molecules. This method, which is commonly referred to as free-radical-promoted cationic polymerization, is quite versatile, because the user may rely on a large variety of radical sources. Some of them are compiled in Table 10.9. 

In the simplest case, cationic polymerizations are propagated by repeated addition of monomer molecules on to carbocations and onium ions. The growing macrocation is stabilized by positive charge delocalization. Examples are macrocations of isobutylene (I), nitrobenzene (II), vinyl methyl ether (III), acetaldehyde (IV), and tetrahydrofuran (V) ... 

The most synthetic polymers are prepared by a free radical chain-growth polymerization (active centers are radicals). Other active centers are carbocations (cationic chain-growth polymerization, e.g. isobutylene, vinylether, tetrahydrofuran) and carboanions (anioific chain growth polymerization, e.g. butadiene, ethylenoxide, caprolactam). Also organometallic complexes are used as chain carriers (coordination polymerization, e.g. ethylene, propylene, butadiene). [08IUP2]... 

Other monomers that are suitable for cationic polymerization include cyclic ethers (like tetrahydrofuran), cyclic acetals (like Irioxane), vinyl ethers, and N-vinyl carbazole. In these cases the hetero atom is bonded directly to the electron deficient carbon atom, and the respective carboxonium ion (9-13) and immonium ion (9-14) are more stable than the corresponding carbocations. 

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