Amine‐blocked polyisocyanates. I. Synthesis of novel N‐methylaniline‐blocked polyisocyanates and deblocking studies using hot‐stage fourier transform infrared spectroscopy

Abstract

A series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, 1H NMR, and 13C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature 1H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007

Synthesis and Deblocking Studies of Low Temperature Heat-Curable Blocked Polymeric Methylene Diphenyl Diisocyanates

“…It was demonstrated before that electron withdrawing substituents positioned at para to the N-methyl aniline had positive effects on the deblocking temperature (15). However, the results obtained in this work contrary to that previous study.…”

“…The low deblocking temperature of BI-1 is attributed to the four centered hydrogen bonding structure, which is illustrated in Figure 6. The formation of this type of intra molecular four centered hydrogen bonding structure was shown previously using the variable temperature 1 H-NMR spectra (15). Deblocking reaction can be explained as transfer of hydrogen atom from -NH of isocyanate moiety to -NCH 3 moiety of the blocking agent.…”

“…Although several substituted N-methyl aniline blocked isocyanates has been reported, blocked polymeric methylene diisocyanates to be used in this paper are not among those already reported (15,17). Blocked isocyanates were prepared using aromatic, as well as aliphatic diisocyanates.…”

“…It has been shown that structures of blocking agents and isocyanates equally have a strong influence on deblocking temperature (1,(15)(16)(17). Several compounds, having active hydrogen atom, have been reported as blocking agents, including phenols, oximes, amides, imides, imidazoles, amidines, pyrazoles, N-methyl anilines, and active methylene compounds (1,2,(15)(16)(17)(18)(19)(20)(21). It has been reported that substituent present in the blocking agent has a very strong influence on the deblocking temperature.…”

On the Versatility of Urethane/Urea Bonds: Reversibility, Blocked Isocyanate, and Non-isocyanate Polyurethane

“…However, in the case of aromatic amines (e.g., aniline), an autocatalytic effect is observed; the tertiary nitrogen atom of the blocking molecule forms a four-centered complex through an intramolecular hydrogen bond with the urea hydrogen (Scheme 37). This transition state, proven by NMR, 173 weakens the bond between the carbonyl carbon and the nitrogen atom of the blocking molecule. The N−H breaking is thus the rate-determining step of the deblocking reaction.…”