Benzene cationic polymerization

When the gas stream was changed from humidified air to dry air, the amount of carbon deposits, very probably attributable to the polymeric products, increased. In this stage, as the surface hydroxyl groups were consumed, the probability for the direct reaction of hole with benzene (formation of benzene cation radical) may be increased. The benzene cation radical formed on the solid surface may react with benzene, which is one of the main steps in the polymerization.83... 

Solvent polarity influences the dissociation of the intimate ion pair, while the fate of the ion-radicals depends mainly on the solvent basicity. In non-basic solvents (benzene or dichloromethane), cationic polymerization of VCZ takes place. In moderately basic solvents (acetone or acetonitrile), VCZ cyclodimerizes. Radical polymerization occurs along with cycloaddition in strongly basic solvents, such as DMF and DMSO, while only radical polymerization takes place in the extremely basic solvent hexamethylphosphoric triamide (HMPA). 

The alkenes most reactive to cationic polymerization contain electron-donating functional groups that can stabilize the carbocation intermediate. The reactivity order of substituents in cationic polymerization is similar to the reactivity order of substituted benzenes in electrophilic aromatic substitution reactions. 

It is well known that cyano derivatives of anthracene form charge transfer (CT) complexes with certain aromatic compounds. It was reported [67] that the radical cations formed upon irradiation of these complexes played an important role in initiation of cationic polymerization of cyclic ethers. Pyridinium salts were also found [68] to form CT complexes with hexamethyl benzene and trimethoxy benzene which result in the formation of a new absorption band at longer wavelengths where both donor and acceptor molecules have no absorption. This way the light sensitivity of the pyridinium salts may be extended towards the visible range. According to the results obtained from the... 

Bis(chlorodimethylsilyl)benzene-AgPF6 system was shown to act as a bifunctional initiator of substituted oxirane polymerization [42], Tri-methylsilyl iodide and triflate were used also as initiators of the cationic polymerization of oxazolines [43]. In this system, however, in contrast to typical initiation mechanism of oxazoline polymerization, O-silylation leads to initiation, because of the unfavorable charge distribution in N-siiyiated species ... 

The idea is to add a latent Friedel-Crafts catalyst to the polymerization feed that will be inert during the polymerization. Then at >200 °C it becomes activated and converts the residual styrene monomer to phenethyl carbocations. These carbocations can either initiate cationic polymerization of the styrene or can alkylate the benzene rings of PS to become attached. With this approach PS itself is the scavenger for residual styrene monomer in PS (Scheme 4.3). 

Cationic polymerization of thiiranes CMT (9-(thiiran-2-ylmethyl)-9//-carbazole) 217 and PMT (10-(thiiran-2ylmethyl)-1077-phenothiazine) 218 was studied by a Lithuanian group <2002JPH63>. Initiators were di-(/-butylphenyl)iodonium tetrafluoroborate (BPIT), diphenyliodonium tetrafluoroborate, cyclopropyldiphenylsulfonium tetrafluoroborate, and ( 7 -2,4-cyclopentadien-1 -yl) [1,2,3,4,5,6- )-( 1 -methylethyl)benzene]-iron(- -)-hexafluorophosphate(—1). The influences of temperature and initiator concentration on the polymerization rate and the conversion limit were determined. The values of initiator exponent and activation energy for the photopolymerization of CMT and PMT initiated with BPIT in 1,2-dichloroethane was established. 

Determination of propagation rate constants in cationic (and in anionic) systems is complicated by the simultaneous occurrence of different types of propagating sites. In olefin polymerizations, some portion of the active centers may exist as free ions and others as ion pairs of varying degrees of solvation. In the solvents in which cationic polymerizations are normally carried out, the polymerization is mainly due to free ions. In low dielectric constant media like benzene or hydrocarbon monomers, however, ion pairs will dominate the reaction. 

Diazonium Salts. The first use of a diazonium salt as an initiator for cationic polymerization of cyclic ethers was reported in 1965 by Dreyfuss and Dreyfuss (1 ), who announced the polymerization of tetrahydrofuran initiated by thermal decomposition of a "new catalyst" which they identified as benzene-diazonium hexafluorophosphate. In a later publication (13) this material was correctly identified as p-chlorobenzenediazomum hexaf1uorophosphate). 

The benzene skeleton stretching of compounds 5 and 6, seven-membered ring model compounds, occurred at 1580 cm l in TR, while that of six-membered ring model compounds 3 and 4 absorbed at 1600 cm-1 in IR. The IR of polymer 1 from cationic polymerization had a peak at 1580 cm but none at 1600 cm- , while that of polymer from radical polymerization had a peak at 1600 cm-1 but none at 1580 cnrl. 

G. Hizal, Y. Yagci, and W. Schnabel, Charge-transfer complexes of pyridinium ions and methyl-substituted and methoxy-substituted benzenes as photoinitiators for the cationic polymerization of cyclohexene oxide and related-compounds. Polymer 1994, 35(11), 2428-2431. 

Cationic polymerizations of allyl esters require further investigation. Schild-knecht [7] mentions the polymerization of allyl acetate and of allyl formate in benzene solution upon heating the solution with 6% of benzenediazonium fluoroborate under nitrogen at 200°C in sealed tubes. The solid products melt at about 190°C (d) and are soluble in benzene and insoluble in methanol. 

Crivello and Lee have described the synthesis and characterization of a series of (4-alkoxyphenyl)phenyliodonium salts 7, which are excellent photo- and thermal-initiators for the cationic polymerization of vinyl and heterocyclic monomers [17]. Iodonium salts 7 are conveniently prepared by the reaction of alkoxyphenols 6 with [hydroxy(tosyloxy)iodo]benzene followed by anion exchange with sodium hexafluoroantimonate (Scheme 7.2). Products 7 have very good solubility and photoresponse characteristics, which make them especially attractive for use in UV curing applications. Compounds 7 with alkoxy chains of eight carbons and longer are essentially nontoxic, compared to diphenyliodonium hexafluoroantimonate, which has an oral LD50 of 40 mg kg (rats) [17]. 

The preparation and properties of (9-oxo-9//-fluoren-2-yl)phenyliodonium hexafluoroantimonate (14) as a new photoinitiator for the cationic polymerization of epoxides have been reported [37]. Compound 14 was prepared by the reaction of (diacetoxyiodo)benzene with fluorenone followed by treatment with sodium hexafluoroantimonate (Scheme 7.3). Photoinitiator 14 has the advantage of intramolecular photosensitization and it is a more effective initiator than the conventional iodonium salts [37]. 

It is accepted that piperylene oligomerization in the presence of electrophilic catalysts (TiCU, AICI3 OR2, SnCU and so on) proceeds according to traditional for cationic polymerization scheme. However, it was revealed in [240], that piperylene oligomers received in toluene medium contain 3-5 phenyl groups on 100 main piperylene units, i.e. toluene taken as a solvent take part in chain transfer reaction. This leads to molecular mass reduction under piperylene polymerization in benzene or toluene in comparison with paraffin hydrocarbons as solvents (900-1200 against 1260-1430). Mechanism of chain transfer (TiCU-HaO catalyst) can be presented by following scheme... 

Dittmer T, GmberF, Nuyken O (1989) Cationic polymerization of bis(l-alkylvinyl)benzenes and related monomtas - structure elucidation of 1,1,3-trimetnyl substituted polyindane. Makromol Chem 190 1755-1770... 

Solvations of the charged species accelerate the transfer of electrons and the ionizations are enhanced by polar solvents.Charge transfer reaction studies with tetracyanoethylene, an acceptor, and A -vinyl carbazole, a donor, in benzene solution demonstrated that both cation radicals and anion radicals form. This can be used in a subsequent cationic polymerizations ... 

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. 

Benzene can be oxidative-cationically polymerized to brown or black products with iron chloride, aluminum trichloride/copper dichloride, etc. at mild temperatures. The insoluble material can be compressed at high pressures into mold inserts. 

Typical cationic polymerization solvents are methylene chloride, benzene, and nitrobenzene. 

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