Phosphonium salts, polymeric

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

A xylylene-fc/.v-phosphonium salt 11 gave films of PPV 1 upon clectropolymer-ization. The absorption and emission spectra of the resultant material were blue-shifted with respect to PPV produced by other routes, suggesting that the electro-polymerized material has a shorter effective conjugation length, possibly because of incomplete elimination of phosphonium groups [22]. 

Several other synthetic techniques have also been described. Redistribution polymerization was outlined in COMC II (1995) (chapter Organopolysilanes, p 99) and proceeds by phosphonium salt-catalyzed redistribution of chlorodisilanes.133 Disproportionation polymerization, which is a similar process, has been described for the formation of polymers by ethoxide-catalyzed disproportionation of alkoxydisilanes via silyl anion intermediates.134 These procedures give rise to network polymeric products of rather low molecular weight (see below, Section 3.11.7.1). 

The preparation of novel phase transfer catalysts and their application in solving synthetic problems are well documented(l). Compounds such as quaternary ammonium and phosphonium salts, phosphoramides, crown ethers, cryptands, and open-chain polyethers promote a variety of anionic reactions. These include alkylations(2), carbene reactions (3), ylide reactions(4), epoxidations(S), polymerizations(6), reductions(7), oxidations(8), eliminations(9), and displacement reactions(10) to name only a few. The unique activity of a particular catalyst rests in its ability to transport the ion across a phase boundary. This boundary is normally one which separates two immiscible liquids in a biphasic liquid-liquid reaction system. 

Reactions of butenylidene-bis-phosphonium salts with PCl3/triethyl-amine under similar conditions proceeded predominantly via base-induced fragmentation to triphenyl phosphine and polymeric products of unknown constitution (Scheme 2). Monocyclic bis-phosphonio-phospho-lides formed only as spectroscopically detectable but hardly isolable byproducts [18, 19]. 

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]. 

Investigation of template poly condensation kinetics has only been studied within a very narrow scope. Polymerization of dimethyl tartrate with hexamethylene diamine was found to be enhanced by using as a template poly(vinyl pyrrolidone), poly(2-vinyl pyridine), or polysaccharides and poly(vinyl alcohol), poly(4-vinyl pyridine). In this case, the template can be treated as a catalyst. No information exists on the influence of the template on the order of reaction. The increase in molecular weight of the polymerization product by the template can be induced by a shift of equilibrium or by an increase in the reaction rate. A similar increase in the reaction rate was observed when poly(4-vi-nyl pyridine) was used in the synthesis of poly(terephtalamides) activated by triphenyl phosphite.The authors suggested that a high molecular weight template was involved in the increase of the local concentration of the substrate (terephthalic acid) by adsorption and activation via N-phosphonium salt of poly(4- vinyl pyridine). 

The most straightforward way to obtain polymeric phosphonium salts involves introducing the phosphonio groups on to a suitable polymeric structure, for example by reacting tertiary phosphines with a poly(chloromethylstyrene) (reaction 99). The polymeric phosphonium salts obtained in this way are mostly used as polymer-supported phase-transfer catalysts for nucleophilic substitutions reactions under triphase conditions. 

Another way to prepare polymeric phosphonium salts involves the radical-initiated polymerization of allylphosphonium salts377 or their copolymerization with methacrylates378. 

In some cases, the anion exchange can also take place in solid-liquid biphasic conditions either for monomeric or dimeric phosphonium salts, in suspension or by using an anion-exchange resin, or for polymeric phosphonium salts, by washing with solutions of salt MX. Finally, it must be pointed out that the new anion Y can be generated in situ by a classical preparation of such anions (e.g. addition of cyanide anion to CS2 for NCCS2-527 or aromatic substitution with fluoride anion on substituted nitrobenzene derivatives for N02-528). Several recent examples of the various anions exchanges are shown in Table 10. 

Polymeric phosphonium salt-bound carboxylate, benzenesulphinate and phenoxide anions have been used in nucleophilic substitution reactions for the synthesis of carboxylic acid esters, sulphones and C/O alkylation of phenols from alkyl halides. The polymeric reagent seems to increase the nucleophilicity of the anions376 and the yields are higher than those for corresponding polymer phase-transfer catalysis (reaction 273). 

Vinylpolystyrene, a useful intermediate for the preparation of various functionalized supports for solid-phase synthesis [7,57-59], has been prepared by the polymerization of divinylbenzene [7], by Wittig reaction of a Merrifield resin derived phosphonium salt with formaldehyde [59-62], or, most conveniently, by treatment of Merrifield resin with trimethylsulfonium iodide and a base [63] (Figure 5.7). 

A Wittig style polymerization, shown in Scheme 33, is the result of condensation of dialdehyde monomers with bis(phosphonium) salts containing aromatic cores, and was reported for the first time in 1960 [134]. Unfortunately, due to low reactivity and conversion, the Wittig polymerization typically only affords materials with a DP of 10. Despite its limitation to forming low molecular... 

Kanazawa A, Ikeda T, Endo T (1993) Polymeric phosphonium salts as a novel class of cationic biocides. III. Immobilization of phosphonium salts by surface photografting and antibacterial activity of the surface-treated polymer films. J Poly Sci A Poly Chem 31 1467-1472... 

The synthesis of poly-dibenzylidene-l,3-dithietane 201 is based on the Wittig reaction of/ -xylcnc-bis(triphcnyl-phosphonium) chloride 199 with carbon disulfide <2001MM346, 2002MM3806>. The phosphonium salt 199 was converted to the ylide 200, which reacted with carbon disulfide, yielding, after methanolysis, a thioketene. The latter was stirred at room temperature for 12 h to provide the polymeric compound 201, bearing 1,3-dithietane moieties in 54% yield (Scheme 25) <2001MM346, 2002MM3806>. 

Under these reaction conditions, the use of ammonium and related onium salts with nucleophilic anions has been found effective at converting the HCl/SnCU-initiated, uncontrolled polymerizations into controlled/ living processes [105], Similar results are reported for TiCl4-based polymerizations [174,175], Effective salts include tetraalkylammonium and phosphonium salts R4N+Y and R4P+Y (Y = I, Br, Cl, CH3COO R = CH3, C2Hs, 71-C4H9, etc.). As added nucleophiles do in nonpolar solvents, the added salts retard the polymerization, narrow the MWD of the polymers, and render their M values directly proportional to conversion and close to the calculated values (one living chain per initiator molecule). 

Phosphonium salts, formed in the presence of phosphines, are more stable than sulfonium salts. With triphenylphosphine no polymerization of IBVE by triflic acid was observed [250]. Polymerization proceeds slowly only with the most weakly nucleophilic phosphines such as tris(p-chlorophenyl)phosphine. The equilibrium constant is much higher than for sulfides, Kt /p = 10 ° mol/L [141]. In that case, Eq. (29) should be modified to Eq. (33), because only a small excess of phosphine can be used ... 

The theoretical treatment above is based on the assumption that ion pairing is intramolecular. Hence, it is only valid for low conversions of monomer to polymer. If low molecular weight polymer or a phosphonium salt were introduced the polymerization rate fell by two orders of magnitude. At higher polymer concentrations it seems that intermolecular contact ion pairs form. Added water also decreases the rate, perhaps partly because termination reduces the average molecular weight of the polymer at a given conversion. 

A possible strategy to improve control of the anionic polymerization of methacrylates relies on the substitution of metal-free cations for metal cations. Ammonium and phosphonium salts were investigated as discussed hereafter. 

Hogen-Esch stressed that the MMA polymerization initiated by PhsCPPlu is very fast (0.3 s < half life < 1 s) with a very short induction period (0.05 to 0.2 s). Therefore, it is mandatory that the polymerization medium be very rapidly homogenized for the polymerization to be controlled. The dropwise addition of a MMA solution in THE to a solution of the phosphonium salt in the same solvent was recommended. ... 

A possible correlation between the strucmre of the phosphonium salt and polymerization control was investigated. Based on kinetic analysis and NMR data , it was proposed that a fast equilibrium is established between the propagating enolate (46) and a dormant ylide (47) in the polymerization initiated by Ph3CPPh4 (equations 44 and 45). 

Through steric hindrance and conjugative effects, these ionic phosphonium salts are very stable to hydrolysis. This, coupled with the lipophilic nature of the cation, results in a very soft, loosely bound ion pair, making materials of this type suitable for use as catalysts in anionic polymerization [8 - 13]. Phosphazene bases have been found to be suitable catalysts for the anionic polymerization of cyclic siloxanes, with very fast polymerization rates observed. In many cases, both thermodynamic and kinetic equilibrium can be achieved in minutes, several orders of magnitude faster than that seen with traditional catalysts used in cyclosiloxane polymerization. Exploiting catalysts of this type on an industrial scale for siloxane polymerization processes has been prevented because of the cost and availability of the pho hazene bases. This p r describes a facile route to materials of this type and their applicability to siloxane synthesis [14]. 

Despite the early use of phosphonium salt melts as reaction media [12, 18, 25], the use of standard ionic liquids of type 1 and 2 as solvents for homogeneous transition metal catalysts was described for the first time in the case of chloroaluminate melts for the Ni-catalyzed dimerization of propene [5] and for the titanium-catalyzed polymerization of ethylene [6]. These inherently Lewis-acidic systems were also used for Friedel-Crafts chemistry with no added catalyst in homogeneous [7] as well as heterogeneous fashion [8], but ionic liquids which exhibit an enhanced stability toward hydrolysis, i. e., most non-chloroaluminate systems, have been shown to be of advantage in handling and for many homogeneously catalyzed reactions [la]. The Friedel-Crafts alkylation is possible in the latter media if Sc(OTf)3 is added as the catalyst [9]. 

Derivation By a modified Grignard synthesis. Use Synthesis of organic compounds, phosphonium salts, other phosphorus compounds, polymerization initiator. 

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