Abstract
An attempt was made to clarify the mechanism of the reactions of acetone with propylenediamine (PDA) or ethylenediamine (EDA) for direct synthesis of poly(urethane-urea) (DSPUU reaction) by using the reaction products (masked diamines) and an isocyanate-terminated prepolymer. In the masking reaction of acetone with PDA at 40°C, 1-(N-isopropylidene)propanediamine (1), 2-(N-isopropylidene)propanediamine (1′), 2,2-dimethyl-4-methylimidazolidine (3) as main product, N,N′-diisopropylidenepropylenediamine (2) were formed in this order due to the dominant rate (k30=3.0 kg mol−1min−1) of the water-catalyzed reaction to form (3) in addition to the slow rate (k20=3.2×10−3 kg mol−1min−1) of (2). The concentration of (1′) remained lower than (1) because of the steric hindrance of the methyl group of the former. The formation reactions of (1) and (2) were more acid-catalyzed than the reverse reactions, leading to shorter equilibrated time and larger molar fractions of these compounds. The reaction profiles for acetone-EDA were analogous except for lower molar fraction of 2,2-dimethylimidazolidine (6) and larger N,N′-diisopropylideneethylenediamine. The rates of demasking reactions which are of importance in DSPUU reactions using masked EDAs were as follows; (i) the water- and acid-catalyzed ring opening reaction rate of (6), k−30 and k−3=1.43 and 0.26 kg mol−1min−1 and (ii) the acid-catalyzed hydrolysis reaction rates of the ketimine groups, k−1=5.09 and k−2=3.13 kg2mol−2min−1, respectively.
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By ab initio MO method (Gaussian 92 Rev. G.3 19-May-1994) enthalpies of (i) a mixture of EDA and acetone, (ii) an intermediate, N-(1-hydroxy-1-methyl ethyl)ethanediamine, (iii) a mixture of N-isopropylideneethylenediamine and water were calculated on 3–61G* level. The enthalpy difference between (i) and (ii) is −0.14 kcal mol−1 and that between (ii) and (iii) 12.6 kcal mol−1, clearly suggests that the rate limiting step is the dehydration reaction. For the reverse reaction the hydrolysis reaction from a ketimine group must be an another limiting step.
G. D. Christian, “Analytical Chemistry,” Part I, John Wiley & Sons, Inc., New York, N.Y., 1986.
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Hanahata, H., Yamazaki, E. & Kitahama, Y. Segmented Poly(urethane-urea)s Synthesized Directly from Isocyanate-Terminated Prepolymers and Masked Diamines II. Kinetic and Mechanistic Study on Masking and Demasking Reaction. Polym J 29, 818–825 (1997). https://doi.org/10.1295/polymj.29.818
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DOI: https://doi.org/10.1295/polymj.29.818