Successful synthesis of blocked polyisocyanates, using easily cleavable phenols as blocking agents, and their deblocking and cure studies

Abstract

Phenols with electron withdrawing substituents at the 2,4-positions are important for use as blocking agents for isocyanates. Blocked polyisocyanates derived using such blocking agents are attractive for producing heat-cured polyurethane products at relatively low temperatures, i.e., below 160 °C. In this study, a series of blocked polyisocyanates were prepared using phenol, 2,4-dichlorophenol, 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol; their blocking and deblocking kinetics, deblocking temperatures, equilibrium temperatures, equilibrium rate constants and cure-times were studied using a hot-stage FT-IR spectrophotometer, adapting neat conditions. Double Arrhenius plots for these thermally reversible systems were reported with an aim to understanding the relationship between forward and reverse reactions. It was found that the rates of forward and reverse reactions and equilibrium rates increased with increasing the acidity of phenol, except in the case of 2-chloro-4-nitrophenol; correspondingly, the deblocking temperature and cure-time of blocked polyisocyanates decreased. Blocked polyisocyanates obtained using unsubstituted phenol showed equilibrium temperature as a range in the double Arrhenius plot, whereas, in the case of 2,4-dichlorophenol and 2-chloro-4-nitrophenol, the Arrhenius plots showed distinct equilibrium temperatures. The equilibrium temperature range or equilibrium temperature of 2-chloro-4-esterphenol-blocked polyisocyanate was not determined, as extrapolation of its plot was found to extend out of the temperature range studied. Importantly, as expected with strong background, all three di-substituted phenols were found to deblock remarkably below 55–70 °C, compared to unsubstituted phenol, which deblocks at 135 °C. More importantly, 2-chloro-4-nitrophenol deblocks at 65 °C and its blocked polyisocyanate cures with polyol within 25 minutes at 110 °C.

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Introduction

Blocked polyisocyanates are an important class of raw materials used to obtain the performance of two-component polyurethanes in one-component thermally curable systems. They have high market potential in the field of organic coatings, such as powder coatings,^1,2 coil coatings^3,4 and eco-friendly waterborne coatings.^5–7 They have also been used as heat-setting adhesive sealants,⁸ resorbable tissue adhesives for the repair of meniscus tears,⁹ and bioadhesives.¹⁰

A blocked isocyanate is an adduct containing a comparatively weak bond formed by the reaction between an isocyanate and a compound containing an active hydrogen atom. At elevated temperatures, the reaction tends to proceed in such a way as to regenerate the isocyanate and the blocking agent. The regenerated isocyanate can react with a co-reactant containing a nucleophile (alcohol or amine) to form urethane or urea with more thermally stable bonds. The overall reaction in a typical model polyurethane or polyurea heat-curable system can be written as follows (Fig. 1):

Fig. 1 Concept of a blocked isocyanate.

Where BH is the blocking agent. Phenols are one category of blocking agents used for aromatic isocyanates, since the urethane linkage formed from the aromatic reactants is unstable at elevated temperatures. Few reviews have been published on blocked isocyanates, in which a variety of blocking agents have been described.^11–16

The deblocking temperature of blocked isocyanates is a primary limiting factor in industrial applications and it should be as low as possible. For example, a blocked isocyanate that can be deblocked at temperatures below 160 °C is suitable for making heat-curable polyurethane products.¹⁷ There are certain application areas for which it is necessary to use a blocking agent that should be cleaved off at temperatures less than 80 °C (e.g., solid rocket propellant, which contains explosive material).¹⁸ As far as phenol-blocked isocyanates are concerned, lowering of the deblocking temperature can be achieved by means of introducing suitable substituents into the blocking agent and using catalysts and solvents. Solvents can decrease the deblocking temperature,^19–21 but are usually not preferred for an industrial process. While a catalyst is always used as an additive, researchers are interested in achieving low deblocking temperatures via suitable substituents.^19,22,23 We have studied phenol-blocked isocyanates extensively,^19,24–27 and very recently, we reported a detailed study on phenol-blocked polyisocyanate real systems.²⁸ Based on the review of all the reports dealing with phenol-blocked isocyanates, we concluded that a substituent at the 2-position reduces the deblocking temperature of phenol through exerting steric constraint.¹⁹ However, for this same reason, the blocking reaction becomes difficult to complete. Also, it was concluded that an electron withdrawing substituent at one of the 2,4-positions greatly reduces the deblocking temperature; the blocking reaction could not proceed to completion, as the substituent reduces the nucleophilicity of the phenol.²⁸ Interestingly, it has been found that the chloro substituent, whether it is present in the 2- or 4-position of the ring, favours both the forward, as well as reverse reactions of corresponding blocked isocyanates.²⁸ Based on these facts reported by us, we believe that some phenols could be chosen as possibly the best blocking agents for isocyanates. Such phenols should invariably have a chloro substituent at the 2-position and relatively bulky electron withdrawing ester or nitro substituent at the 4-position. In these phenols, the negative influence of ester or nitro substituents towards the blocking reaction will be compensated by the chloro substituent; ultimately, both substituents will favour the deblocking reaction.

Our target is to prepare phenol-blocked polyisocyanates with the lowest possible deblocking temperature and the best performance in the deblocking reaction. To achieve this objective in this work, we used disubstituted phenols, such as 2,4-dichlorophenol, 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol as the ultimate choices for blocking agents in this category for isocyanates and report for the first time, attractive results on synthesis, forward and reverse reaction parameters determined under neat conditions, deblocking temperature and cure-time of blocked polyisocyanates based on these phenols.

Experimental

Materials

Phenol (SRL, India), poly(tetrahydrofuran) (Terathane, M_n = 650) (Aldrich), toluene-2,4-diisocyanate (TDI) (Fluka), methyl-3-chloro-4-hydroxybenzoate (Alfa-Aesar), 2,4-dichlorophenol (TCI), 2-chloro-4-nitrophenol (TCI), N,N-diethylcyclohexylamine (TCI), and poly(polytetrahydrofuran carbonate)diol (M_n = 2000) (Aldrich) were used as received. The TDI used for the preparation of blocked polyisocyanates was a mixture of isomers containing 94% 2,4-isomer and 6% 2,6-isomer. The solvents, tetrahydrofuran (Merck) and toluene (Merck) were purified by standard procedures.

Measurements

FT-IR spectra of polyurethane prepolymer (polyisocyanate) and blocked polyisocyanates were recorded on a PerkinElmer Spectrum Two model (FT-IR C101375) spectrophotometer by the neat thin film method. ¹H-NMR spectra were recorded on a Bruker 500 MHz NMR spectrometer using CDCl₃ as a solvent. The relative viscosities (η_r) of the cured polymers were measured in toluene (0.6 g dL⁻¹) at 30 °C using an Ubbelohde viscometer.

General procedure for synthesis of phenol-blocked polyisocyanates (2–5)

Blocked polyisocyanates were prepared in a two-step process. In the first step, isocyanate-terminated polyurethane prepolymer 1 was prepared. Subsequently, in the second step, the terminal –NCO groups of the prepolymer were blocked with phenol. The following general procedure was adopted for the preparation of blocked polyisocyanates. In a typical synthesis, 2.68 g (15.38 mmol) of TDI were taken in a three-neck round bottomed flask fitted with a mechanical stirrer and a nitrogen inlet. Polyol (5.0 g, 7.69 mmol) [poly(tetrahydrofuran); M_n = 650] was added drop wise into the flask using an addition funnel, with stirring by mechanical means. The experimental setup was maintained in a temperature controlled oil bath. The addition rate was such that it took one hour for complete transfer. The reaction time was 5 hours; for the first 2 hours the temperature of the oil bath was maintained at 50 °C and for the next 3 hours it was maintained at 70 °C. After 5 hours, the reaction temperature was reduced to 40 °C. The –NCO terminated prepolymer was then blocked with an equivalent amount (i.e., 7.69 mmol) of phenol using 8.611 × 10⁻³ mmol of N,N-diethylcyclohexylamine as a catalyst. After thorough mixing, a very small quantity of sample was taken with the help of a glass-rod for FT-IR measurements towards kinetic analysis, and the reaction in the flask was allowed to continue until the complete disappearance of the –NCO absorption in the FT-IR spectrum. The structures of the blocked polyurethane prepolymers (blocked polyisocyanates) were confirmed by FT-IR and ¹H-NMR spectroscopic methods.

Determination of blocking kinetic parameters

The blocking kinetics of –NCO terminated prepolymer with a series of phenol was followed by using a hot-stage FT-IR spectrophotometer. The experiments were carried out isothermally at 40 °C, 50 °C, and 60 °C. In a typical experiment, as soon as the catalyst was added, a thin film of the reaction mixture of phenol and –NCO terminated prepolymer was cast on two NaCl discs and covered with each other, with separation by a 0.5 mm lead spacer. Then, the NaCl windows with the lead spacer were placed in a heated transmission cell (HT-32, model 0019-200, Thermo Electron Corp., Madison, WI, USA.), connected to a microprocessor-based programmable temperature controller (Omega CT-3251). A program was set in the temperature controller in such a way that once the desired temperature was reached dynamically, the experiment proceeded isothermally. Eight scans at a resolution of 4 cm⁻¹ were co-added to produce a single spectrum in a single run. The spectrum at time zero was recorded within two minutes from the time of catalyst addition to the reaction mixture and then the spectra were recorded at regular time intervals. As the time increased, the absorption by the –NCO group of isocyanate-terminated polyurethane prepolymer at 2270 cm⁻¹ decreased, due to the blocking of –NCO group by phenol. The peak area under the absorption by the –NCO group in each spectrum was calculated with PerkinElmer Spectrum software (Version 10.4.4) and considered as equivalent to the concentration of the polyisocyanate at time t. The obtained data were treated according to a second-order rate equation in which the two reactants had an equal initial concentration, because it gave a linear fit.

Second order rate equation:

where ‘a’ is the initial concentration of the polyisocyanate at time t = 0 and (a − x) is the concentration of the polyisocyanate at time t. The activation energy E_a, for the catalyzed blocking reaction of polyisocyanate with phenol was calculated from the slope of the Arrhenius plot (log k versus 1/T) using the equation E_a = −2.303 × R × slope. The entropy of activation, ΔS^# at 50 °C was calculated from the well-known Eyring equation,
where k_B is the Boltzmann constant (1.3806 × 10⁻²³ J K⁻¹), h is Planck's constant (6.626 × 10⁻³⁴ J s), R is the gas constant (8.314 J K⁻¹ mol⁻¹), T is the reaction temperature in Kelvin, k is the rate constant at T and ΔH^# is the enthalpy of activation. The ΔH^# in the above equation was obtained from the energy of activation using the relation ΔH^# = E_a − RT.

Determination of deblocking temperatures of blocked polyisocyanates

In a typical experiment, one drop of a solution of a blocked polyisocyanate in THF was placed at the center of a NaCl disc. The solvent was evaporated under the IR lamp and then the discs were assembled with the heating accessory. The sample was heated dynamically; the temperature was increased from ambient to 170 °C with a heating rate of 5 °C min⁻¹. Eight scans at a resolution of 4 cm⁻¹ were co-added to produce a single spectrum in a single run. The lowest temperature at which there was a detectable absorption in the 2270 cm⁻¹ range, due to the regeneration of the –NCO group, was noted as the minimum deblocking temperature.

Determination of deblocking kinetic parameters

In a typical quantitative experiment, 7.5 μl of 5.0 × 10⁻⁴ molar solution of un-blocked polyisocyanate (i.e., –NCO terminated prepolymer) in dry THF was placed at the center of one of the NaCl discs. The solvent was evaporated under the IR lamp and then the discs were assembled and fitted with a heating accessory. The spectrum was recorded immediately. The peak area under the –NCO absorption was calculated and considered as “a” appearing in the first order equation. Then, the solution of the same concentration of a blocked polyisocyanate was used for deblocking kinetic studies. The deblocking experiments were carried out isothermally at 110 °C, 120 °C and 130 °C for phenol- and 2,4-dichlorophenol-blocked polyisocyanates, whereas for 2-chloro-4-esterphenol- and 2-chloro-4-nitrophenol-blocked polyisocyanates, the experiments were carried out at 90 °C, 100 °C and 110 °C. The time set to reach the desired temperature from ambient was 7 minutes in the case of unsubstituted phenol-blocked polyisocyanate, and was only 4 minutes for all the disubstituted phenol-blocked polyisocyanates. The program was set in the heating device in such a way that once the desired temperature was reached, the experiment proceeded isothermally. A spectrum was recorded at zero time, immediately as the sample reached the desired temperature, and subsequently recorded at regular time intervals. At zero time, there was no isocyanate absorption at 2270 cm⁻¹ and as the time increased, the absorption by the –NCO group at 2270 cm⁻¹ increased, due to the deblocking of blocked polyisocyanates. The peak area of the –NCO group in each spectrum was calculated using the PerkinElmer Spectrum software and considered as ‘x’ appearing in the first order rate equation. From the ‘a’ and ‘x’, a − x was calculated and the data obtained were treated according to the first order rate equation, since it gave a linear fit.

First order rate equation:

The energy of activation (E_a), enthalpy of activation, (ΔH^#) and entropy of activation (ΔS^#) at 110 °C for the amine-catalyzed deblocking reactions were calculated as described in the preceding section.

Cure-time studies

Poly(polytetrahydrofuran carbonate)diol (5.06 g, 2.5 mmol) was taken separately in four beakers, each of 30 mm diameter. To each beaker, an equivalent amount (2.5 mmol) of blocked polyisocyanate was added. To this mixture, 9.568 × 10⁻³ mmol of N,N-diethylcyclohexylamine catalyst was added and mixed thoroughly. The beakers were then placed in an air circulated oven maintained at 80 °C or 110 °C and were inverted at regular time intervals to observe the flow behaviour of the mixture. The time at which the mixture ceased to flow was taken as the cure-time. A duplicate experiment was conducted for each blocked polyisocyanate to ensure the accuracy of the data collected.

Results and discussion

Synthesis of phenol-blocked polyisocyanates (2–5)

The polyisocyanate was prepared separately for each entry given in Table 1 by the reaction of two equivalents of TDI with one equivalent of poly(tetrahydrofuran)diol (Scheme 1). The reaction was monitored using a FT-IR spectrophotometer. First, to ensure the completion of the prepolymer formation, the spectrum of the reaction mixture was recorded at regular time intervals after 3 hours from the start of the reaction until constant ratio of the –NCO group peak area to the urethane carbonyl group formed was attained. The FT-IR spectrum of the prepolymer is given in Fig. 2a. The strong absorption band appearing at 2270 cm⁻¹ indicates the presence of free –NCO groups in the prepolymer. Then, to block the terminal –NCO group of the prepolymer, phenol and three disubstituted phenols were used as blocking agents.

Table 1 Synthesis of phenol-blocked polyisocyanates

Compd no.	Blocking agents	Equivalent used	Time (min) required for 50% completion of blocking reaction in IR cell at
			40 °C	50 °C	60 °C
a The excess amount of blocking agent was added after stopping the kinetic study.
2	Phenol	1	75	32	9
3	2,4-Dichlorophenol	1	25	11	6
4	Methyl-3-chloro-4-hydroxybenzoate	1.2^a	14	11	7
5	2-Chloro-4-nitrophenol	1.2^a	126	64	29

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Scheme 1 Synthesis of phenol-blocked polyisocyanates (2–5).

Fig. 2 FT-IR spectrum of (a) isocyanate-terminated polyurethane prepolymer and (b) 2,4-dichlorophenol-blocked polyisocyanate.

Diethylcyclohexylamine was used as a catalyst because its boiling point is high enough for the reverse reaction to be studied and it was also found to mix thoroughly with the reaction mixture. The concentration of the catalyst was reduced to half the value of that in our previous work, because the blocking agents chosen for this work are more acidic, and are expected to be more reactive than the mono substituted phenols already studied.²⁸ The blocking reaction was stopped when the –NCO absorption at 2270 cm⁻¹ in the FT-IR spectrum completely disappeared. The times required to complete 50% of the reaction at three different temperatures are given in Table 1. The blocking reactions of polyisocyanates with phenol and 2,4-dichlorophenol were complete at 40–60 °C within 12 h. However, the reaction with 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol were found to be incomplete forever at 60 °C, and this is due to the competitive deblocking (reverse) reaction, which starts at this temperature. Thus, for these two later cases, 20% excess blocking agent was added after stopping the kinetics analysis at 60 °C, and then the reaction was maintained at 40 °C until the complete disappearance of the –NCO group in the FT-IR spectrum.

The FT-IR spectra of all the phenol-blocked polyisocyanates were identical, and show the urethane –NH stretching absorption at around 3300 cm⁻¹, urethane –NH bending absorption at around 1535 cm⁻¹, urethane –C=O stretching absorption at 1730 cm⁻¹, C–O stretching absorption of the C–O–C group in polyol at around 1100 cm⁻¹ and the stretching vibration of the C=O group of the urethane combined with N–H at around 1200–1240 cm⁻¹. The absence of an absorption at 2270 cm⁻¹ indicated that the isocyanate groups were completely blocked with phenol. A typical example spectrum of 2,4-dichlorophenol- blocked polyisocyanate is given in Fig. 2b. Like the FT-IR spectra, the ¹H-NMR spectra of phenol-blocked polyisocyanates were also identical. All the ¹H-NMR spectra recorded in CDCl₃ showed the urethane proton attached to polyol at 6.45–6.55 ppm, the urethane proton attached to the phenol ring at 6.80–6.95 ppm, aromatic methyl protons at 2.15–2.35 ppm, inner –CH₂ of polyol at 1.61 ppm, –OCH₂ of polyol at 3.4 ppm, –CH₂ of polyol attached to the urethane group at 4.14 ppm, in addition to aromatic protons at 7.10–8.30 ppm. These characterizations confirm the formation of the phenol-blocked polyisocyanates and their structures without any ambiguity.

Blocking kinetics

The hot-stage FT-IR spectrophotometer is the best analytical tool for studying the blocked isocyanates, as this tool directly measures the disappearance or appearance of the isocyanate functional group during the course of the blocking or deblocking reaction.^29,30 With the intention of calculating the kinetic and activation parameters for the neat blocking reaction of polyisocyanates, isothermal experiments were carried out using a hot-stage FT-IR spectrophotometer at 40 °C, 50 °C and 60 °C. The distinct changes observed in the FT-IR spectrum of each blocking reaction mixture were the steady intensity decrease of the absorption peak at 2270 cm⁻¹ corresponding to –NCO groups, and the steady increase at 1730 cm⁻¹ corresponding to the urethane carbonyl group formed with respect to time. We followed the disappearance of the isocyanate absorption at 2270 cm⁻¹ for the blocking kinetics. Typical FT-IR spectra recorded for different time intervals at three different temperatures for the blocking reaction are given in Fig. 3.

Fig. 3 FT-IR spectra recorded for different time intervals under isothermal conditions for the blocking reaction of polyisocyanate with 2,4-dichlorophenol: (a) 40 °C (b) 50 °C and (c) 60 °C.

The plots of x/a (a − x) versus time were found to be linear and pass through the origin, confirming that the blocking reaction of polyisocyanates with phenol follows second order kinetics of the type A + B → products, in which the two reactants have the same initial concentration. These isocyanate–alcohol reactions, including phenol, which follow second order kinetics have been well studied.¹⁶ Typical second order rate plots for the tertiary amine-catalyzed blocking reaction of polyisocyanate with 2,4-dichlorophenol are given in Fig. 4 and the second order rate constants for 50% conversion and activation parameters are given in Table 2.

Fig. 4 Amine-catalyzed second-order kinetic plots of the blocking reaction of polyisocyanate with 2,4-dichlorophenol.

Table 2 Second-order rate constants and activation parameters for amine-catalyzed, neat blocking reactions of polyisocyanate

Compd no.	Blocking agents	Ka of blocking agent	Second order rate constant k × 10^3 (eq.^−1 min^−1)			Ea (kJ mol^−1)	ΔH^# (kJ mol^−1)	ΔG^# (kJ mol^−1)	ΔS^# (J K^−1 mol^−1)
			40 °C	50 °C	60 °C
2	Phenol	1.05 × 10^−10	0.203	0.391	1.139	74.57	71.89	101.47	−91.60
3	2,4-Dichlorophenol	1.20 × 10^−8	0.340	0.804	1.418	61.91	59.22	99.54	−124.81
4	Methyl-3-chloro-4-hydroxybenzoate	1.90 × 10^−8	0.593	0.815	1.254	32.34	29.65	99.29	−215.60
5	2-Chloro-4-nitrophenol	3.80 × 10^−6	0.0783	0.154	0.354	65.0	62.31	102.91	−125.70

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The second-order rate constants for the amine-catalyzed blocking reactions increased with the increasing acidity of the blocking agents, except with 2-chloro-4-nitrophenol. In accordance with this observation, the activation energy (E_a) of the reactions decreased. Because the phenol is acidic, it can associate easily with a tertiary amine based catalyst to form an association complex. This complex can again associate with the isocyanate group to form a four centered active intermediate complex during the blocking reaction. Owing to the high acidity of 2-chloro-4-nitrophenol, it can easily associate with basic catalysts; however, its phenolate is poorly nucleophilic, due to the high electron withdrawing power of the nitro substituent present in the ring. The formation of the active intermediate with the –NCO group becomes difficult and therefore, it reacts with a low rate. It was stated in the discussion pertaining to synthesis that the blocking reactions of polyisocyanate with 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol were not completed at 60 °C, due to the competitive deblocking reaction starting at this temperature. However, this is not reflected in the kinetic results because we followed the kinetics up to only 50% conversion; moreover, the rate of this deblocking reaction may be too low at this temperature. The ΔG^# of all the blocking reactions of isocyanate reported in this work were found to be almost identical and were between 98 kJ mol⁻¹ and 103 kJ mol⁻¹; this clearly indicates that all the reactions follow the same mechanism. The higher negative entropy of activation (ΔS^#) indicates the formation of a rigid complex in the transition state.

Deblocking temperatures of phenol-blocked polyisocyanates

The deblocking temperature of a blocked isocyanate is a very important parameter that decides the viability in industrial use; a low deblocking temperature is always attractive. All the blocked isocyanates designed towards low deblocking temperatures were subjected to FT-IR analyses under dynamic conditions for the purpose of the assessment of deblocking temperatures. The spectra recorded from room temperature to 170 °C for a typical phenol-blocked polyisocyanate are given in Fig. 5, in which some spectra have been removed for the sake of clarity and the minimum deblocking temperature determined from these spectra are given in Table 3.

Fig. 5 FT-IR spectra of 2,4-dichlorophenol-blocked polyisocyanate recorded at (a) different temperatures. (b) Zoomed range of the isocyanate absorption region.

Table 3 Deblocking temperatures of phenol-blocked polyisocyanates determined using a hot-stage FT-IR spectrophotometer under dynamic conditions

Compd no.	Blocking agents	Deblocking temperature (°C)
2	Phenol	135
3	2,4-Dichlorophenol	80
4	Methyl-3-chloro-4-hydroxybenzoate	70
5	2-Chloro-4-nitrophenol	65

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The bond formed between the isocyanate and blocking agent (phenol) is indeed labile, and the strength of this bond is dependent on the magnitude of polarization present in the –COO– moiety of the –NHCOO– group. Electron donating substituents in the blocking agent will increase the charge difference and hence strengthen the labile bond. Similarly, electron withdrawing substituents in the blocking agent will reduce the charge difference and hence weaken the bond, resulting in a low deblocking temperature. In accordance with this argument, the deblocking temperature obtained for the blocked polyisocyanates containing the electron withdrawing chloro, ester or nitro substituent, are consistent with the electronic effect, as well as with the acidity values of blocking agents. If the acidity (dissociation constant) of the blocking agent is high, it means that its phenolate anion is loosely bonded with the carbonyl carbon. In this study, all three disubstituted phenols used are highly acidic and contain electron withdrawing substituents at the ortho and para positions, thereby deblocking remarkably below 55–70 °C, compared to the unsubstituted phenol that deblocks at 135 °C (Table 3).

Deblocking kinetics

Based on the minimum deblocking temperature determined, the isothermal, amine-catalyzed, deblocking kinetics of phenol-blocked polyisocyanates were performed at 90–130 °C under neat conditions. Evidence of side reactions that lead to the formation of the allophanate group and decrease the intensity of the C=O absorption, due to cleavage of the urethane group, in addition to the regeneration of the –NCO group, were observed in the FT-IR spectrum during the course of the deblocking experiment. However, the absorption peak of the –NCO group at 2270 cm⁻¹ was conveniently followed for deblocking kinetics, as it absorbs strongly and exclusively in that region. Typical FT-IR spectra recorded for different time intervals at different temperatures for the deblocking kinetic analysis are given in Fig. 6. The plots of log a/(a − x) vs. time were found to be linear, and confirm that the amine-catalyzed deblocking of all the phenol-blocked polyisocyanates follow first order kinetics. Typical example plots for three different temperatures are given in Fig. 7. The first order rate constants and activation parameters calculated are given in Table 4.

Fig. 6 FT-IR spectra recorded for different time intervals under isothermal conditions for the deblocking reaction of 2,4-dichlorophenol -blocked polyisocyanate: (a) 110 °C (b) 120 °C and (c) 130 °C.

Fig. 7 Amine-catalyzed first-order kinetic plots of the deblocking reaction of 2,4-dichlorophenol-blocked polyisocyanate.

Table 4 First-order rate constants and activation parameters for the amine-catalysed deblocking reaction of phenol-blocked polyisocyanates

Compd no.	Blocking agent	Ka of blocking agent	First order rate constant k × 10^3 (min^−1)					Ea (kJ mol^−1)	ΔH^# (kJ mol^−1)	ΔG^# (kJ mol^−1)	ΔS^# (J K^−1 mol^−1)
			90 °C	100 °C	110 °C	120 °C	130 °C
2	Phenol	1.05 × 10^−10	—	—	1.384	3.015	7.462	107.67	104.48	115.58	−28.98
3	2,4-Dichlorophenol	1.20 × 10^−8	—	—	13.140	17.850	25.210	41.64	38.46	108.41	−182.64
4	Methyl-3-chloro-4-hydroxybenzoate	1.90 × 10^−8	13.594	18.028	21.627	—	—	26.90	23.72	106.82	−216.99
5	2-Chloro-4-nitrophenol	3.80 × 10^−6	7.009	13.645	26.762	—	—	77.52	74.34	106.15	-83.04

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Phenol- and 2,4-dichlorophenol-blocked polyisocyanates were subjected to deblocking kinetics at 110 °C, 120 °C and 130 °C; whereas, blocked polyisocyanates with ester and nitro substituents were studied at 90 °C, 100 °C and 110 °C, because they deblock with high rates above 110 °C, which we could not follow. When compared to the blocking kinetics, the role of blocking agent acidity on the deblocking rate is clearly seen; the deblocking rate increases with increasing the acidity of the phenol, without exemption. This observation is also consistent with the electronic effect of the substituents as discussed for deblocking temperature. The mechanism of deblocking in the presence of a basic catalyst may involve deprotonation of the urethane N–H by the base to form a conjugate base of urethane. The dissociation then proceeds by eliminating the phenolate anion from the negatively charged conjugate base of urethane, which then accepts a proton from the protonated base (Fig. 8). Based on this mechanistic pathway and a comparison of deblocking kinetics data with blocking kinetics, it can be concluded that all three disubstituted phenols used in this study are good blocking agents. They are also better leaving groups and therefore cleave off easily, substantially below the deblocking temperature of the unsubstituted phenol. The activation energy (E_a) of deblocking reactions involving disubstituted phenols are also found to be low, compared to that involving unsubstituted phenols. The narrow range of free energy of activation (ΔG^#) obtained confirms that all the deblocking reactions uniformly follow this mechanism.

Fig. 8 Mechanism of amine-catalysed deblocking of a phenol-blocked polyisocyanate.

Double Arrhenius plots of the forward and reverse reactions

Very recently, we reported double Arrhenius plots and a method for the determination of equilibrium temperature (or range) and equilibrium rate constants from such plots for thermally reversible reactions, for the first time.²⁸ Since that approach paved the way to look at the blocked isocyanate equilibrium region, it was extended to the present work and the results are given in Fig. 9 and Table 5. Extrapolations of Arrhenius plots of forward and reverse reactions of blocked polyisocyanate obtained using unsubstituted phenol were found to not meet each other and thus, showed equilibrium temperature as a range (equilibrium temperature range, ETR) (Fig. 9a); the mid-point of the ETR is the most probable equilibrium temperature in this case. Within this ETR, both the forward and reverse reactions are found to coexist and proceed with different rates; the equilibrium rate of this reaction was determined from the log k at which both the reactions meet each other.²⁸

Fig. 9 Double Arrhenius plots of forward and reverse reactions of blocked polyisocyanates; blocking agent (a) phenol, (b) 2,4-dichlorophenol, (c) methyl-3-chloro-4-hydroxybenzoate and (d) 2-chloro-4-nitrophenol.

Table 5 Thermal equilibrium data for phenol-blocked polyisocyanates determined using Arrhenius plots for forward and reverse reactions

Compd no.	Blocking agents	Equilibrium temperature range for forward and reverse reactions (°C)	Most probable equilibrium temperature for forward and reverse reactions (°C)	Equilibrium rate constant k × 10^3
a This is the actual value, not a probable value.
2	Phenol	82–93	87	1.213
3	2,4-Dichlorophenol	Not observed	76^a	4.011
4	Methyl-3-chloro-4-hydroxybenzoate	Found outside of the temperature range studied	—	—
5	2-Chloro-4-nitrophenol	Not observed	56^a	0.282

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In the case of blocked polyisocyanates obtained using 2,4-dichlorophenol and 2-chloro-4-nitrophenol, the extrapolations of the Arrhenius plots of the blocking and deblocking reactions intersect each other and show equilibrium temperatures distinctly, and not as a range (Fig. 9b and d and Table 5). In principle, below or above this temperature, only the forward or reverse reaction will predominantly take place. It was interesting to note that in the case of 2,4-dichlorophenol-blocked polyisocyanate, the equilibrium temperature (76 °C) was well above and well below the experimental upper and lower temperatures of the blocking and deblocking reactions, respectively; whereas, in the case of 2-chloro-4-nitrophenol-blocked polyisocyanate, the equilibrium temperature (56 °C) was found to be within the blocking temperature range studied (Fig. 9d). This observation verifies the prediction of deblocking below 60 °C in this case and supports the preceding discussion for synthesis and blocking kinetics. From the log k value corresponding to the intersection point, the equilibrium rate constants were calculated and are given in Table 5. Interestingly, the trend observed in the equilibrium rate constants is similar to that observed in the forward and reverse reactions. Extrapolations of Arrhenius plots of the blocking and deblocking reactions of 2-chloro-4-esterphenol-blocked polyisocyanate resulted in slightly different patterns, in which the extrapolations neither intersect each other nor intersect the temperature scale studied; instead, they extend to out of the temperature range studied (Fig. 9c). From our very recent studies²⁸ and from the present study, we understand that the activation energy (i.e., slope of the line) of both the forward and reverse reactions should be sufficiently high, or the difference between the slopes of the lines should be sufficiently high, so that the lines will meet each other or intersect in the temperature range; otherwise, the equilibrium temperature or its range cannot be determined from the double Arrhenius plot. Thus, we were not able to determine the equilibrium parameters for 2-chloro-4-esterphenol-blocked polyisocyanate.

Gel-time studies

To study the structure–cure reaction relationship of the blocked polyisocyanates synthesized, they were reacted with poly(polytetrahydrofuran carbonate)diol. The use of blocked isocyanates instead of isocyanate along with polyol, increases the pot life; long pot life is a must for single component systems to be used for practical applications. At elevated temperatures, the isocyanate functionality of the blocked polyisocyanate was regenerated and the reaction mixture was cured, as described in Scheme 2. During the course of the reaction, the viscosity was increased and at once the flow of the reaction mixture was arrested. The times required to cure the poly(polytetrahydrofuran carbonate)diol with blocked polyisocyanates are given in Table 6. The trend observed with cure-time was found to be similar to that found for deblocking temperature. Also, the effect of the substituents of blocked polyisocyanates on cure reaction was consistent with the rate of the deblocking reaction. The relative viscosities (η_r) of the cured polymers were determined using an Ubbelohde viscometer to ensure the constancy of the cure reaction. It was found that the values of η_r are identical, confirming a uniform cure reaction in all the experiments.

Scheme 2 Cure-reaction of phenol-blocked polyisocyanate with poly(polytetrahydrofuran carbonate)diol.

Table 6 Gel-time and relative viscosity data for cured polymers using blocked polyisocyanates

Blocked polyisocyanate	Gel-time (min) at		ηr
	80 °C	110 °C
2	>600	600	1.1920
3	285	150	1.1969
4	145	35	1.2045
5	215	25	1.2121

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Conclusions

In this article, we report (i) the synthesis of a series of blocked polyisocyanates using disubstituted phenols, such as 2,4-dichlorophenol, 2-chloro-4-esterphenol and 2-chloro-4-nitrophenol, as ultimate choices for blocking agents in this category for isocyanate. (ii) Blocking, deblocking and cure reactions of these blocked polyisocyanates were studied in detail without using solvent. (iii) The rates of both forward and reverse reactions were found to be proportional to the acidity of the blocking agent, except for 2-chloro-4-nitrophenol, which blocked the isocyanate with a low rate, even though its acidity is high. (iv) The trend observed in the equilibrium temperature was similar to that observed with the deblocking temperature and cure-time. Similarly, the trend observed for equilibrium rate constants was similar to that observed with the forward and reverse reaction rates. (v) All three disubstituted phenols were found to deblock remarkably below 55–70 °C, compared to unsubstituted phenol, which deblocks at 135 °C. They are therefore attractive for industrial use and for organic facile synthesis involving the protection–deprotection of isocyanates.

Acknowledgements

One of the authors (S.·K.) thanks University Grants Commission, India for the award of Teacher Fellowship under Faculty Development Programme of UGC-XII Plan to carry out this work. The authors thank CSIR for providing FT-IR with hot-stage accessories under a scheme no. 02(0100)/12/EMR-II. Dt.31.10.2012. The authors also thank Prof. V. Kannapan for valuable discussion.

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Footnote

† Electronic supplementary information (ESI) available: ¹H-NMR spectra of phenol-blocked polyisocyanates (2–5), FT-IR spectra of blocked polyisocyanates recorded at dynamic condition, FT-IR spectra of blocked polyisocyanates record at isothermal condition for blocking and deblocking reaction. See DOI: 10.1039/c6ra24409b

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