A novel cyclopolymer containing residues of essential amino acid methionine: synthesis and application

Abstract: Ethyl ester hydrochloride of essential amino acid methionine upon treatment with allyl bromide affords N,N-diallyl methionine ethyl ester (I) which after treatment with HCl leads to a novel monomer N,N-diallyl methionine ethyl ester hydrochloride (II). Monomer II is cyclocopolymerized with sulfur dioxide in acetone or ethanol to obtain the corresponding water-insoluble alternate cocyclopolymer III (i.e., II-alt-SO₂), while quite surprisingly, the polymerization carried out in dimethyl sulfoxide (DMSO) affords a water-soluble terpolymer IV: (II{in sulfide form}-alt-SO₂)-ran-(II{in sulfoxide form}-alt-SO₂) in which half of the sulfide groups has been oxidized to sulfoxide groups via oxygen exchange with solvent DMSO. Polymer IV containing the residues of methionine demonstrates superior inhibition of mild steel corrosion in 1 M HCl at 60 °C; inhibition efficiency (IE) of 99 % is achieved at a concentration of 25 ppm. At a meager concentration of 1.25 ppm, the polymer leads to an astonishing IE of 93 %, while its corresponding monomer II achieves an IE of 31 % at the same concentration. The adsorption of polymer IV on the metal surface was found to obey the Langmuir adsorption isotherm. The high negative ΔG_ads^∘ value of −57.7 kJ mol⁻¹ ensured the favorability of the adsorption process and indicated the prevalence of both electrostatic adsorption and chemisorption. The polymer having multiple adsorption sites gave superior protection of mild steel surface than methionine or its ester derivative I or the monomeric counterpart II. Graphical abstract: The essential amino acid methionine has been converted into diallylamine salt which undergoes cyclocopolymerization with SO₂ to give a novel polymer in which 50 % of the sulfide group is oxidized to sulfoxide via oxygen exchange with dimethyl sulfoxide. The polymer (at a concentration of 1.25 ppm) demonstrates a superb inhibition efficiency (IE) of 93 % of mild steel corrosion in 1 M HCl at 60 °C, while at 100 ppm it achieves an IE of 99 %. [Figure not available: see fulltext.]