Uncatalyzed urethane forming reaction of 1,3-xylylene diisocyanate with aliphatic alcohols of varying chain lengths and polyols

Abstract

In this article, we report a detailed study on the kinetics of the reaction of 1,3-xylylene diisocyanate (1,3-XDI) with alcohols of varying alkyl chain lengths and with low molecular weight polyols such as monomethoxylated polyethylene glycol (mPEG) and polytetrahydrofuran (PTHF) in toluene, in the absence of a catalyst. It was found that the pseudo-firstorder rate coefficient increased with the increasing alkyl chain length (k 1,app was found to be 0.0166 and 0.0341 min-1 for 1-propanol and 1-hexanol, respectively) and the reactivities of the 1,3-xylyene diisocyanate towards the alcohols and polyols are between those of the aromatic and alkyl isocyanates. The activation energies were also determined and were found to vary from 25.6 to 38.6 kJ/mol (from 1-propanol to 1-hexanol). According to further kinetic studies, the rate coefficients k 1,app and k 2,app for the reaction of 1,3-XDI with PTHF were determined to be (1.13±0.01)•10-2 and (1.28±0.01)•10-2 min-1 , respectively. It was also found that the rate constants did not depend significantly on the length of the polymer chain. However, significantly lower rate coefficients were obtained for the 1,3-XDI-mPEG reaction (k 1,app = (2.64±0.03)•10-3 min-1 and k 2,app = (2.45±0.03)•10-3 min-1). Besides, the dependence of the pseudo-first-order rate coefficient on the concentration of the alcohols was also studied, and based on these findings a reaction mechanism is proposed.

Reaction Mechanisms and Rate Constants of Auto‐Catalytic Urethane Formation and Cleavage Reactions

“…as well as Oberth and Bruenner [34] studied the influence of different solvents on urethane formation kinetics and found a strong acceleration of the reaction in non‐polar compared to polar reaction media. This finding is in accordance with experimentally determined activation energies of urethane formation in different reaction solvents [16,17,21,24,26–28,31,32,35] . Values of activation energies around 30 kJ mol −1 were found in very non‐polar solvents and values of up to 50 kJ mol −1 for reaction in excess alcohol.…”

“…This finding is in accordance with experimentally determined activation energies of urethane formation in different reaction solvents. [16,17,21,24,[26][27][28]31,32,35] Values of activation energies around 30 kJ mol À 1 were found in very non-polar solvents and values of up to 50 kJ mol À 1 for reaction in excess alcohol.…”

“…While we are not aware of any elaborated studies of urethane cleavage without added catalyst, urethane formation has been studied extensively [8] . In particular, urethane formation has been investigated experimentally for decades, studying the reaction kinetics of selected urethane reactions in liquid phase [9–17] and comparing the reactivity of different isocyanates [18–21] and alcohols [22–32] . The reactivity of isocyanates seems to be strongly influenced by different substituents.…”

“…[8] In particular, urethane formation has been investigated experimentally for decades, studying the reaction kinetics of selected urethane reactions in liquid phase [9][10][11][12][13][14][15][16][17] and comparing the reactivity of different isocyanates [18][19][20][21] and alcohols. [22][23][24][25][26][27][28][29][30][31][32] The reactivity of isocyanates seems to be strongly influenced by different substituents. Alcohols show different reactivity depending on the position of the hydroxy group, while the reactivity of different primary alcohols seems to be very similar.…”

“…Based on experimental results, different reaction mechanisms have been suggested for urethane formation reactions. [3,8,32,36,37] A first group of mechanisms do not involve any catalysis: * Non-catalytic 1-step mechanisms:…”