Phosphonium salts anionic polymerization

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) ... 

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. 

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). 

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). 

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. 

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]. 

Anionic polymerization of methyl methacrylate (MMA) has been carried out with the phosphonium salt P[N=P(Nme2)3]4 [n-C5HnCPh2] as initiator. The polymerization process is very fast and follows first-order kinetics, yielding polymers with a narrow molecular weight distribution. No conclusive explanation can be given for the low initiator efficiency. Other polymerization pro-... 

The anionic polymerization of acrylonitrile initiated by triphenylphosphine was reinvestigated by Jaacks et al.238). The absence of the phosphine in the polymer and the isolation of 99% of the initiator in the form of the phosphonium salt, Ph3PCH2CH2CN, demonstrates that macro-zwitter-ions are not formed by this initiation, a conclusion contrary to the claims of other workers, e.g. Enikolopyan et al.244). The previously discussed proton-transfer was established therefore,... 

Also, it was reported [218] that quaternary onium salts coupled wit bulky organoaluminum diphenolates initiate controlled (living) coordinate anion polymerizations of oxetane to give narrow molecular weight distribution polyethers The catalyst system consists of onium salts, such as quatemal ammonium phosphonium halides that are combined with sterically hindered methylaluminum diphenolates [47]... 

Some polymeric phosphonium salts have been reported to have advantages in reaction rate or ease of separation relative to monomeric phosphonium salts as catalysts for nucleophilic reactions where the large cation favors nucleophilic reactivity of the anion (36). 

However, in the presence of a suitable Lewis base the polymerization becomes living, due to the nucleophihc stabilization of the growing cation generated by the added base. (3) Initiator, strong Lewis acid and onium salt as additive The previous method cannot be easily applied in polar media. In this case the living cationic polymerization is promoted by the addition of salts with nucleophihc anions, such as ammonium and phosphonium derivatives. 

Zagala et al. investigated the polymerization of methacrylates in the presence of tetraphenylphosphonium (TPP) ion at ambient temperature. The polymerization appears to have living character [228]. In case of MMA number average molecular masses increase linearly with conversion and molecular mass distributions are narrow (< 1.30). Results of H, and P NMR studies indicated the presenee of phosphorylides formed by the addition of the PMMA enolate anion to one of the phenyls of the TPP cation. Muller et al. managed to synthesize another metal-free initiator, namely the salt of the tetrakis[tris(dimethylamino)-phosphoranylideneamino]phosphonium (P5) cation with the... 

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