Segmented Poly(urethane-urea)s Synthesized Directly from Isocyanate-Terminated Prepolymers and Masked Diamines II. Kinetic and Mechanistic Study on Masking and Demasking Reaction

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.