Polycondensation of activated L‐valine and L‐leucine esters with various lewis acids |
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Authors: | Tadamasa Nemoto Daisuke Ando Kensuke Naka Yoshiki Chujo |
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Institution: | 1. Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo‐Ku, Kyoto 615‐8510, Japan;2. Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo‐Ku, Kyoto 615‐8510, JapanDepartment of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo‐Ku, Kyoto 615‐8510, Japan |
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Abstract: | To develop a novel polycondensation method for the preparation of poly (amino acid)s, we screened a transition metal or a rare‐earth triflate as a Lewis acid for the polycondensation of activated amino acid esters in N,N‐dimethylformamide solutions at room temperature. The polymerizations of 4‐nitrophenyl L ‐leucinate ( 1a ) and 4‐nitrophenyl L ‐valinate ( 1b ) scarcely proceeded without any Lewis acid at room temperature. In the presence of 5 mol % metal triflates, especially scandium(III) trifluoromethanesulfonate, the polymerizations of both monomers were promoted effectively. The products, which were collected by the reaction mixture being poured into water, were recognized as poly(L ‐valine)s by Fourier transform infrared spectroscopy, gel permeation chromatography analysis, and 1H NMR spectroscopy. These results showed that a metal triflate as a Lewis acid could coordinate to a carbonyl oxygen of activated L ‐valinate and L ‐leucinate even in a highly polar solvent, such as N,N‐dimethylformamide; therefore, the polymerizations of activated L ‐valinate and L ‐leucinate were promoted. Because steric hindrance derived from the isobutyl group in 1b was less than that of the isopropyl unit in 1a , the effect of the metals was not as sensitive for the polymerization of 1b . © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 543–547, 2007 |
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Keywords: | α ‐amino acid ester catalysis Lewis acid oligomers polycondensation poly(α ‐amino acid)s |
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