Polymer phase transfer catalyst

Note 1 Polymer phase-transfer catalysts in the form of beads are often referred to as triphase catalysts because such catalysts form the third phase of the reaction system. 

Polymer phase-transfer catalysts (also referred to as triphase catalysts) are useful in bringing about reaction between a water-soluble reactant and a water-insoluble reactant [Akelah and Sherrington, 1983 Ford and Tomoi, 1984 Regen, 1979 Tomoi and Ford, 1988], Polymer phase transfer catalysts (usually insoluble) act as the meeting place for two immiscible reactants. For example, the reaction between sodium cyanide (aqueous phase) and 1-bromooctane (organic phase) proceeds at an accelerated rate in the presence of polymeric quaternary ammonium salts such as XXXIX [Regen, 1975, 1976]. Besides the ammonium salts, polymeric phosphonium salts, crown ethers and cryptates, polyethylene oxide), and quaternized polyethylenimine have been studied as phase-transfer catalysts [Hirao et al., 1978 Ishiwatari et al., 1980 Molinari et al., 1977 Tundo, 1978]. 

The terminal R groups can be aromatic or aliphatic. Typically, they are derivatives of monohydric phenoHc compounds including phenol and alkylated phenols, eg, /-butylphenol. In iaterfacial polymerization, bisphenol A and a monofunctional terminator are dissolved in aqueous caustic. Methylene chloride containing a phase-transfer catalyst is added. The two-phase system is stirred and phosgene is added. The bisphenol A salt reacts with the phosgene at the interface of the two solutions and the polymer "grows" into the methylene chloride. The sodium chloride by-product enters the aqueous phase. Chain length is controlled by the amount of monohydric terminator. The methylene chloride—polymer solution is separated from the aqueous brine-laden by-products. The facile separation of a pure polymer solution is the key to the interfacial process. The methylene chloride solvent is removed, and the polymer is isolated in the form of pellets, powder, or slurries. 

Tomoi and coworkers adopted a somewhat more direct approach in their synthesis of 16-crown-5 derivatives bearing a single alkenyl residue. They hoped to obtain precursors to polymers which could be used as phase transfer catalysts. In this approach I,I-bis-chloromethylethylene (a-chloromethallyl chloride) was allowed to react with the dianion of tetraethylene glycol (NaH/THF). By this method, methylene-16-crown-5 could be isolated in 66% yield after vacuum distillation. Ozonolysis led, in almost quantitative yield, to the formation of oxo-16-crown-5 as shown in Eq. (3.38). These authors prepared a number of other, closely related species by similar methods. 

A good deal of work has been done on polymeric crown ethers during the last decade. Hogen Esch and Smid have been major contributors from the point of view of cation binding properties, and Blasius and coworkers have been especially interested in the cation selectivity of such species. Montanari and coworkers have developed a number of polymer-anchored crowns for use as phase transfer catalysts. Manecke and Storck have recently published a review titled Polymeric Catalysts , which may be useful to the reader in gaining additional perspective. 

Baneijee et al. reported a number of soluble poly-imido [134], polyazomethine [135], and polyazoxy phos-phonates [136] by the two phase polycondensation method with or without any phase transfer catalyst. Resulting polymers exhibit high thermal stability and fire retardancy. 

Ford, W. T. and Tomoi, M. Polymer-Supported Phase Transfer Catalyst Reaction Mechanisms. Vol. 55, pp. 49—104. 

Dichloro monomers can also be polymerized with bisphenols in the presence of fluorides as promoting agents.78 The fluoride ions promote the displacement of the chloride sites to form more reactive fluoride sites, which react with phenolate anion to form high-molecular-weight polymers. Adding 5-10 mol % phase transfer catalysts such as A-alkyl-4-(dialkylamino)pyridium chlorides significantly increased the nucleophilicity and solubility of phenoxide anion and thus shortened the reaction time to one fifth of the uncatalyzed reaction to achieve the same molecular weight.79... 

Phase-separated polymers, 215 Phase separation, 217-222 Phase transfer catalysts, 288, 563-564 Phase-transfer-catalyzed alkaline hydrolysis of nylon-4,6, 570 of nylon-6,6, 569-570 PHB. See Poly(3-hydroxybutanoic acid) (PHB)... 

A useful application in the manufacture of ion-exchange resins may well be possible which avoids the use of carcinogenic chloromethyl ether. Here, a polymer of p-methyl styrene is chlorinated on the side chain with aqueous NaOCl and a phase-transfer catalyst. Sasson et al. (1986) have shown how stubborn . substituted aromatics like nitro/chlorotoluenes can be oxidized to the corresponding acids by using aqueous NaOCl containing Ru based catalyst. 

In general, if condensation polymers are prepared with methylated aryl repeat units, free radical halogenatlon can be used to introduce halomethyl active sites and the limitations of electrophilic aromatic substitution can be avoided. The halogenatlon technique recently described by Ford11, involving the use of a mixture of hypohalite and phase transfer catalyst to chlorinate poly(vinyl toluene) can be applied to suitably substituted condensation polymers. 

The chloromethylated polymers are very reactive substrates for nucleophilic attach further elaboration can be accomplished under homogeneous conditions In aprotlc solvents, or under heterogeneous conditions In the presence of phase transfer catalysts. The following examples are representative of approaches to functionalized condensation polymers via chloromethylated Intermediates. 

Reduction of azides is a classical approach to primary amine synthesis. Treatment of 17 with sodium azide in DMF or in THF/H2O mixtures in the presence of phase transfer catalysts effects a quantitative conversion to the corresponding polymeric azide, 27. Recently the reduction of azides to primary amines via hydrolysis of iminophosphoranes produced by interaction of the azide with triethyl phosphite was reported.30 Application of this technique to the azidomethyl polymer, 27, as shown below, failed to produce a soluble polyamine. 

The design of functionalized polymers with a specific utilization is seen in new polysiloxanes used by Zeldin (p. 199) as phase transfer catalysts. Novel functional polyphosphazenes have been reported as well by Allcock (p. 250). The introduction of transition metal cyclopentadienyl, metal carbonyl and carborane moieties into polyphosphazene macromolecules is representative of truly novel chemistry achieved after careful model studies with corresponding molecular systems. 

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

Another palladium-catalyzed coupling reaction that has been successfully performed on soluble polymers is the Sonogashira coupling. Xia and Wang have presented an approach in which the PEG 4000 utilized simultaneously serves as polymeric support, solvent, and phase-transfer catalyst (PTC) in both the coupling and... 

With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

Phase-transfer techniques are widely used for the preparation of polymers. For example, potassium fluoride is used to produce poly(etherketone)s under phase-transfer conditions (Scheme 10.18). Use of this reagent allows the chloroaro-matics to be used as starting material as opposed to the more expensive flu-oroaromatics that are usually employed [23]. This method is suitable for the synthesis of high molecular weight semicrystalline poly(ether ketone)s, although the presence of excess potassium fluoride in the reaction mixture can lead to degradation reactions. The use of a phase transfer catalyst can allow the use of water-soluble radical initiators, such as potassium peroxomonosulfate used to promote the free-radical polymerization of acrylonitrile [24],... 

Long-chain alkyl 14B>146 and polymer-bound147 phosphonium salts have been used as phase-transfer catalysts. 

Quaternary ammonium and phosphonium halides were used as the phase transfer catalysts. For effective coupling, high-shear mixing and high concentrations of polymer and base were used. 

Cl /Cl 0 In the presence of polymer-supported phase transfer catalyst and redox mediator Cl /ClO, the oxidation of benzyl alcohol to benzaldehyde or benzoic acid was achieved [39-41]. 

Aside from the use of polymers as supports for phase transfer catalyst centers, much excellent work has been reported on the use of PTC in polymer chemistry for pol)rmerization methods (28), for the chemical modification of already formed polymers(29). for the modification of polymer surfaces without change of the bulk polvmerOO). and for the preparation and purification of monomers(31). 

For use of chiral phase transfer catalysts see the following references and references contained therein J. W. Verbicky Jr., and E. A. O Neil, J. Org. Chem.. 50, 1786 (1985) E. Chiellini, R. Solaro, and S. D Antone, Polvm. Sci. Technol. (Plenum), 24 (Crown Ethers Phase Transfer Catal. Polym. Sci.) 227 (1984). 

Although the lariat ethers (29-31) were conceived on principles related to biological activity, they are interesting candidates for study as either free phase transfer catalysts, or as polymer-bound catalysts. In the latter case, the sidearm could serve both a complexing function and as a mechanical link between macroring and polymer. Polymeric phase transfer catalyst systems have been prepared... 

For lariat ethers to be effective as polymer-bound phase transfer catalysts, sidearm and macroring cooperation must be intramolecular. It is unlikely that two lariat ethers will be close enough on a polymer backbone or other support for the ring of one compound to interact with the sidearm donors of another. The mechanical attributes of lariat ethers will be independent of spacing but for any advantage in cation binding and anion activation to be realized, the macroring and its attached sidearm must cooperate to envelop the cation, solvate it, and shield it from the counteranion. 

---