Water-soluble imidazole polymers

Catalysis by Water-Soluble Imidazole-Containing Polymers... 

References concerning research studies of enyzme-like polymers conducted during the period of 1978-1982, catalytic properties of water-soluble imidazole-containing polymers during ester hydrolysis, as well as the latest achievements in the manufacture and application of high-molecular weight catalysts containing thiocrown-ethers or crown-sulfides, can be found in previous reviews [199-201]. 

ESTEROLYTIC ACTION OF WATER-SOLUBLE IMIDAZOLE CONTAINING POLYMERS... 

This review article summarizes our more recent investigations of the esterolytic action of water soluble imidazole containing polymers. In our previous work we have explained increased reactivity of polymeric reactants in terms of electrostatic and cooperative effects. Recently, we have directed our efforts in emphasizing the apolar interaction as it has reflected dramatic rate enhancements and proven a predominant factor in determining the maximum catalytic efficiency. Polymeric catalysts which are included in this review are poly[l-alkyl-4(5)-vinyl-imidazoles] and their copolymers, hydro-phobic terpolymers of 4(5)-vinylimidazole, copoly[vinylamine/4(5)-vinylimidazole], and poly(ethylenimine-g L-histidine). All of these catalysts share the common property of water solubility and the capability of attaining high catalytic efficiencies which is attributed to apolar interactions. 

Research by Overberger [13], Kabanov [14,15], Klotz [16], Kunitake [17] and others [18, 19] has contributed much to development of the catalysis by water-soluble polymers. Among numerous synthetic polymer catalysts, imidazole derivatives are cited in particular because they are constituents of hydrolytic enzymes [1, 20-22]. Polyvinylimidazole (PVI) is several times more catalytically effective than imidazole because of its polyfunctional nature. Let us consider three possible mechanisms of cooperative catalysis (see p. 2). 

Radical polymerization of (14) and the subsequent removal of the benzyl group produces water-soluble polymers containing imidazole-4-carbohydroxamic acid residues. The hydrolysis of 4-nitrophenyl acetate catalysed by this polymer proceeds faster than that catalysed by imidazoles or carbohydroxamic acid but slower than that catalysed by the monomeric analogue. " The hydrolysis of 4-nitrophenyl acetate is catalysed by the bifunctional hydroxamic acid (IS) and... 

In most of the examples considered so far, the enhancement of catalytic activity by synthetic polymers over that of low molecular weight analogs is usually modest (a factor of 100 at the most). However Klotz and his collaborators could show that, in some special cases, an impressive enhancement can be obtained [62]. These authors prepared a highly branched water soluble polyethylenimine (PEI) to which they covalently attached methyl-imidazole residues and dodecyl residues (XXa). They postulate that the branched PEI provides a globular-like matrix having a high local density of catalytic and binding (hydro-phobic) residues. 

The most important physical properties of poly[l-alkyl-4(5)-vinylimidazoles] is their water solubility. The alkylation of the imidazole ring reduces intramolecular H-bonding which causes the insolubility of the polymer in aqueous media. The apolar character of the poly[1-alky1-4(5)-vinylimidazoles] is easily varied by the nature of the alkyl substituent. The preparation of 1-alky1-5-vinyl-imidazole monomer and polymer is shown in Scheme I. 

Water soluble, hydrophobic terpolymers of 4(5)-vinylimidazole were prepared in order to investigate hydrophobic effects attributed to random, covalently bound pendant apolar side chains in the hydrolysis of the charged apolar ester substrates (S ). The rationale for the preparation of these terpolymers is provided in the analogy to the poly[4(5)-vinylimidazole] catalysts. The apolar nature of the catalyst poly[4(5)-vinylimidazole] is derived from its hydrocarbon backbone and formation of the transient and hydrophobic N-acylimidazole intermediate. Accumulation of long-chain acylated imidazole residues, which increases hydrolysis rates dramatically, inspired the concept of utilizing hydrophobic polymer side chains. 

The reaction of polyethylenimine with p-nitrophenyl ester 44 was carried out in DMSO at 25 °C in the presence of imidazole. The obtained polymer, A-PEI (45), was soluble in water below pH 2, acidic solvents and DMSO and insoluble in concentrated hydrochloric add, DMF, or other commonly used organic solvents. The synthetic way to the graft polymer containing thymine (T-PEI) (47) is the same as that used for adenine. The results are compiled in Table 17. 

The polymer shares the following properties with hemoglobin and myoglobin (a) good solubility in water in order to achieve high concentration of O2 (b) hindrance of irreversible oxidation of the oxygen complex by the functionalized polymer and (c) imitation of a distal imidazole. 

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