Polymeric ylides

The effectiveness of ylides in the field of polymer science was first described in 1966 by George et al. [11] who felt that 3- and 4-(bromo acetyl) pyridines, which contain both the a-haloketone and the pyridine nucleus in a single molecule, could be quaternized to polymeric quaternary salts and finally to polymeric ylides Schemes 9 and 10 by treating these polymeric salts with a base. 

Similar results were obtained when 4,4 -bipyridyl was quaternized with p-bis(bromo-acetyl) benzene in refluxing benzene to give a 92% yield of the blue-green salt. Potassium carbonate treatment of their blue-green salt gave a greenish black polymeric ylide (Scheme 11), which was similar in appearance and properties to (Scheme 12). 

The structures of these ylide polymers were determined and confirmed by IR and NMR spectra. These were the first stable sulfonium ylide polymers reported in the literature. They are very important for such industrial uses as ion-exchange resins, polymer supports, peptide synthesis, polymeric reagent, and polyelectrolytes. Also in 1977, Hass and Moreau [60] found that when poly(4-vinylpyridine) was quaternized with bromomalonamide, two polymeric quaternary salts resulted. These polyelectrolyte products were subjected to thermal decyana-tion at 7200°C to give isocyanic acid or its isomer, cyanic acid. The addition of base to the solution of polyelectro-lyte in water gave a yellow polymeric ylide. 

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

We have also used j3-picolinium-p-chlorophenacylide ()3-PCPY) [15], a cycloimmonium ylide, as the radical initiator for the polymerization of MMA [18,19] at 60-70°C. (See also Table 1.) The kinetics of the system has been studied. 

In an attempt to broaden the use of nitrogen ylides in polymerization, Saini et al. [33] tried using imidazo-lium-p-chlorophenacylide (ICPY) for the polymerization of styrene [33] and MA [34]. Unlike /3-PCPY, ICPY... 

In contrast to /3-PCPY, ICPY did not initiate copolymerization of MMA with styrene [39] and AN with styrene [40]. However, it accelerated radical polymerization by increasing the rate of initiation in the former case and decreasing the rate of termination in the latter case. The studies on photocopolymerization of MMA with styrene in the presence of ICPY has also been reported [41], /8-PCPY also initiated radical copolymerization of 4-vinylpyridine with methyl methacrylate [42]. However, the ylide retarded the polymerization of N-vinylpyrrolidone, initiated by AIBN at 60°C in benzene [44]. (See also Table 2.)... 

The first important lead toward the application of ylides as an initiator came from the observations of Zweifel and Voelker [47] in 1972. In their experiment, these authors polymerized lactones or unsaturated compounds initiated by phosphorus ylides prepared directly from tertiary phosphines or similar compounds. 

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) ... 

If direct homolysis occurs in the case of tetraphenylphosphonium tetrafloroborate, triphenylphosphonium ylide was expected to function as a photoinitiator of radical polymerization because of its similar structure. Therefore, another milestone was reached by Kondo and colleagues [50] who investigated the use of triphenylphosphonium ethoxycarbonylmethylide (TPPY) (Scheme 22) as an effective photoinitiator for the polym-... 

The proposed mechanism is based on the basis of the fact that ylides (Scheme 23 and Scheme 24) undergo bond fission between the phosphorus atom and the phenyl group in TPPY as reported by Nagao et al. [51] and between the sulfur atom and the phenyl group in POSY as observed in triphenylsulfonium salts [52-55] when they are irradiated by a high-pressure mercury lamp. The phenyl radicals thus produced participate in the initiation of polymerization. 

Table 3 Effect of [p-ABTAY] on the Rate of Polymerization of Styrene Initiated by p-Acetylbenzylidene Triphenylarsonium ylide at 60 0.1 °C ...

Kondo maintained his interest in this area, and with his collaborators [62] he recently made detailed investigations on the polymerization and preparation of methyl-4-vinylphenyl-sulfonium bis-(methoxycarbonyl) meth-ylide (Scheme 27) as a new kind of stable vinyl monomer containing the sulfonium ylide structure. It was prepared by heating a solution of 4-methylthiostyrene, dimethyl-diazomalonate, and /-butyl catechol in chlorobenzene at 90°C for 10 h in the presence of anhydride cupric sulfate, and Scheme 27 was polymerized by using a, a -azobisi-sobutyronitrile (AIBN) as the initiator and dimethylsulf-oxide as the solvent at 60°C. The structure of the polymer was confirmed by IR and NMR spectra and elemental analysis. In addition, this monomeric ylide was copolymerized with vinyl monomers such as methyl methacrylate (MMA) and styrene. 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Recently, in connection with the use of sulfur ylides in polymerization, Kondo and his coworkers [64] attempted to use diphenylsulfonium bis(methoxycarbo-nyl)methylide (DPSY) (Schemes 27, 29) methylphen-ylsulfonium bis-(methoxycarbonyl)methylide (MPSY) (Scheme 30) and dimethylsulfonium bis (methoxycar-bonyl) methylide (DMSY) (Scheme 31) as photoinitiators for the polymerization of MMA and styrene. They concluded that DPSY and MPSY are effective photoini-... 

To examine potentiality of other ylides and their metal complex containing Sb, As, P, Bi, and Se as new novel initiator in polymer synthesis via living radical polymerization. 

Synthesis and polymerization of new monomers using ylides as starting materials. 

Kinetics and mechanism of polymerization of vinyl monomers initiated by ylides. 

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. 

Ni phosphorus-ylide complexes were reported by Ittel and co-workers to be active for the co-polymerization of ethylene and non-vinyl-functionalized monomers, yielding functionalized polyethylenes (PEs). These results demonstrate the high potential of late transition metal complexes for the production of co-polymers from hydrocarbon and polar monomers. 

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