Synthesis and studies on forward and reverse reactions of phenol-blocked polyisocyanates: an insight into blocked isocyanates

Abstract

Blocked isocyanates are an important class of raw materials used in the polyurethane industry. In this study, the synthesis and kinetics of blocking and deblocking reactions of a series of phenol-blocked polyisocyanates have been studied in detail using a hot-stage FT-IR spectrophotometer adapting to neat conditions. The results were compared with an aim to resolve complex questions on the relationship between the forward and reverse reaction parameters. As a result, double Arrhenius plots for thermally reversible reactions were proposed for the first time. Using these plots, the most probable equilibrium temperatures for the forward and reverse reactions and the equilibrium rate constants of these reactions were assessed. It was found that the trend present in the rate constants of the forward reaction, reverse reactions and equilibrium were uniform i.e., the rate of these reactions decreased or increased with respect to the acidity of the blocking agent. A phenol with more acidity and less nucleophilicity, e.g., 2-chlorophenol, was found to be a better blocking agent; it blocks the isocyanate quickly, and at the same time, it cleaved off easily. The most probable temperatures assessed using the double Arrhenius plots were found in accordance with deblocking temperatures. The data, such as time required for conversion into product, the equilibrium temperature range for forward and reverse reactions and most probable equilibrium temperature in combination with deblocking temperature, reported in this work are very attractive from the manufacturing and application points of view.

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Introduction

The extreme reactivity of isocyanates with hydroxyl compounds makes them attractive curing agents for making polyurethanes in different forms such as elastomers, foams, coatings for many purposes and adhesives. In some of these applications, it is necessary to delay the desired isocyanate–hydroxyl reactions until the final stages of the process. To achieve this objective, blocked isocyanates are used to generate the isocyanate functionality at the final stage. Thus, blocked isocyanates are used to obtain the performance of two package (2K) polyurethane in one package (1K) thermally curable systems. In addition, blocked isocyanates have several advantages like a marked reduction of moisture sensitivity and elimination of toxicity associated with free isocyanates. Blocked isocyanates have a prolonged work life, which is particularly important for all heat-curable polyurethane products. Blocked isocyanates are used in a wide range of coating applications such as coil coatings, electronic coatings, powder coatings and eco-friendly water-borne coatings. These coatings exhibit good adhesion, high weather resistance and good mechanical properties.¹ The industrial importance of blocked isocyanates can be clearly seen from the number of patents issued on this kind of material. A very recent Scifinder search on “blocked isocyanate” returned 7627 patents and this is more than 85% of the related overall literature. This shows the industrial importance of blocked isocyanates.

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 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 a urethane or urea with thermodynamically more stable bonds. The overall reaction in a typical polyurethane heat-curable system is described in Fig. 1.

Fig. 1 Concept of blocked isocyanate.

The deblocking temperature of a blocked isocyanate is one of the limiting factors 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 heat-curable systems such as organic powder coatings.² 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 materials).³

Blocked isocyanates have been the subject of thorough and well-updated reviews.^4–9 Several compounds, namely phenols,^10–13 oximes,^14,15 amides,¹⁶ imides,¹⁶ imidazoles,^17,18 amidines,¹⁹ lactams,^20–23 mercaptans,²⁴ amines,^25–27 alcohols,^26–30 sodium bisulphite,^31–34 pyrazoles,^35–37 1,2,4-triazoles,³⁸ hydroxamic acid esters³⁹ and formyl acrylate,⁴⁰ have been reported as blocking agents. Among these blocking agents, phenols are widely used and the most studied blocking agent because of the possibilities of introducing a number of substituents on the benzene ring: substituents that can seriously influence the synthesis process and deblocking temperature of blocked isocyanates.

The field of blocked isocyanates now has developed well in terms of the development of new blocking agents and blocked isocyanate equivalents,^41–43 products that are stable and have good market potential. However, the complex question on the relationship between kinetic and activation parameters of the forward and reverse reactions have not been resolved yet. These parameters are essential to optimize the production process of blocked polyisocyanates and the curing process with co-reactants. With this background, we decided to study real systems of phenol-blocked polyisocyanates in detail. In this paper, we report the synthesis, deblocking temperatures, kinetic and activation parameters of the blocking and deblocking reactions of a series of phenol-blocked polyisocyanates carried out without solvent. Also, we report the equilibrium temperature ranges, and most probable equilibrium temperatures, assessed using double Arrhenius plots for these thermally reversible reactions for the first time. All of the data reported in this paper will make the design and synthesis of phenol-blocked polyisocyanates, and their end applications, easier.

Experimental

Materials

Phenol (SRL, India), p-cresol (SRL, India), poly(tetrahydrofuran) (Terathane, M_n = 650) (Aldrich), toluene-2,4-diisocyanate (TDI) (Fluka), o-cresol (SRL, India), o-chlorophenol (SRL, India), o-methoxyphenol (Alfa-Aesar), p-methoxyphenol (Alfa-Aesar), methylsalicylate (Alfa-Aesar), p-hydroxy methylbenzoate (Alfa-Aesar), N,N-diethylcyclohexylamine (TCI) and p-chlorophenol (Spectrochem, India) 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 solvent, tetrahydrofuran (Merck) was 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.

General procedure for the synthesis of phenol-blocked polyisocyanates (2–10)

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 was taken in a three-neck round bottomed flask fitted with a mechanical stirrer and a nitrogen inlet. 5.0 g (7.69 mmol) of polyol, [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 the complete transfer. The reaction time was 5 hours; for the first 2 hours the temperature of 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 1.7194 × 10⁻² 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 –NCO absorption in the FT-IR spectrum. The structures of the blocked polyurethane prepolymers (blocked polyisocyanates) were confirmed by FTIR 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 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 they covered each other separated 0.5 mm by a lead spacer. Then, the NaCl windows with a lead spacer were placed in a heating 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 the –NCO group by phenol. The peak area under the absorption by the –NCO group in each spectrum was calculated using 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 have an equal initial concentration because it gave a linear fit.
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 T⁻¹) using the equation E_a = −2.303 × R × slope. The entropy of activation, ΔS^#, 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 a 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 a detectable absorption in the 2270 cm⁻¹ range, due to 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 on 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 120 °C, 130 °C and 140 °C. Here, the time set to reach the desired temperature from ambient was 7 minutes in the case of phenol-, o-cresol-, p-cresol- and p-methoxyphenol-blocked polyisocyanates, whereas the time set was only 4 minutes in the case of p-esterphenol-, o-chlorophenol and p-chlorophenol-blocked polyisocyanates. The program was made in the heating device in such a way that once the desired temperature was reached, the experiment proceeded isothermally. The spectrum was recorded for zero time immediately when the sample reached the desired temperature and was then recorded at regular time intervals. At the zero time (in all cases), 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 a first order rate equation since it gave a linear fit.

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

Results and discussion

Synthesis of phenol-blocked polyisocyanates (2–10)

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. 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 a constant ratio of –NCO group peak area to urethane carbonyl group formed was attained. The FT-IR spectrum of prepolymer is given in Fig. 2(a). The strong absorption band appearing at 2270 cm⁻¹ indicates the presence of free –NCO groups in the prepolymer. In the 2,4-TDI, the –NCO group adjacent to the –CH₃ group is less reactive due to the electron donating nature of the –CH₃ group, which reduces the positive charge on the carbon atom of the –NCO group. Moreover, this methyl group impedes the incoming nucleophile to the –NCO group.

Table 1 Synthesis of phenol-blocked polyisocyanates

Compd no.	Blocking agents	Time (min) required for 75% completion of blocking reaction at
		60 °C	50 °C	40 °C
2	Phenol	9	14	28
3	o-Cresol	23	36	45
4	p-Cresol	25	36	50
5	o-Methoxyphenol	37	44	50
6	p-Methoxyphenol	10	17	22
7	o-Chlorophenol	5	7	12
8	p-Chlorophenol	6	12	17
9	Methyl 2-hydroxybenzoate	Incomplete forever	Incomplete forever	Incomplete forever
10	Methyl 4-hydroxybenzoate	270 + 5 days at room temp.	270 + 5 days at room temp.	270 + 5 days at room temp.

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Scheme 1 Synthesis of phenol-blocked polyisocyanates (2–10).

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

As a result, the isocyanate group at the 2-position of TDI is approximately four times less reactive than the isocyanate group present at the 4-position.⁴⁴ Thus, the –NCO group present at the 4-position of TDI was converted to urethane during the formation of polyisocyanate. To block the terminal –NCO group of the prepolymer, phenol and several substituted phenols were used as blocking agents. The relative reactivity of the phenolic hydroxyl towards isocyanate is 1000 times less than that of an aliphatic primary hydroxyl group⁹ because phenols are poor nucleophiles for isocyanates due to its size and resonance-stabilized planar configuration. Thus, all the blocking reactions were catalyzed using a tertiary amine. Diethylcyclohexylamine was chosen as a catalyst because its boiling point was high enough for the reverse reaction to be studied and also it was found to mix thoroughly with the reaction mixture. The blocking reaction was stopped when the –NCO absorption at 2270 cm⁻¹ in the FT-IR spectrum completed disappeared. The time required to complete 75% of the reaction at three different temperatures are given in Table 1.

The FT-IR spectra of all the phenol-blocked polyisocyanates were identical and show the urethane –NH stretching absorption around 3300 cm⁻¹, urethane –NH bending absorption around 1535 cm⁻¹, urethane –C=O stretching absorption at 1730 cm⁻¹, C–O stretching absorption of the C–O–C group in the polyol around 1100 cm⁻¹ and the stretching vibration of the C=O group of the urethane combined with N–H around 1200–1240 cm⁻¹. The absence of the absorption at 2270 cm⁻¹ indicated that the isocyanate groups were completely blocked with phenol. A typical example spectrum of phenol-blocked polyisocyanate is given in Fig. 2(b). Like the FT-IR spectra, the ¹H-NMR spectra of phenol-blocked polyisocyanates were also identical. All the ¹H-NMR spectra recorded in CDCl₃ show the urethane proton attached to the polyol at 6.82–6.84 ppm, the urethane proton attached to the phenol ring at 6.90–6.94 ppm and aromatic methyl protons at 2.25–2.34 ppm in addition to aromatic protons at 7.10–7.82 ppm. All the ¹H-NMR spectra show a distinct singlet at 7.77–7.82 ppm, which is due to the aromatic ring proton present in between the two urethane groups. This observation is in agreement with D₂O exchange studies reported by Mohanty and Krishnamurti.⁴⁵ These characterizations confirm the formation of phenol-blocked polyisocyanates and their structures without any ambiguity.

Blocking kinetics

Hot-stage FT-IR spectrophotometry is the best analytical tool for studying the neat blocking kinetics of –NCO terminated polyurethane prepolymer with phenol as this tool directly measures the disappearance of the isocyanate functional group of the polyisocyanate during the course of the blocking reaction.^25,27 With the intention of calculating the kinetic and activation parameters for the neat blocking reaction of polyisocyanates, isothermal experiments were carried out 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 decease 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, 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 at isothermal conditions for the blocking reaction of polyisocyanate with phenol: (a) 40 °C, (b) 50 °C and (c) 60 °C.

The plots of x/a (a − x) versus time were found to be linear, passes through the origin and confirm 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, including phenol, reactions that follow second order kinetics have been well studied.⁹ Typical second order rate plots for the tertiary amine-catalyzed blocking reaction of polyisocyanate with phenol is 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 phenol.

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	1.001	1.782	3.019	47.81	45.12	96.34	−158.55
3	o-Cresol	0.49 × 10^−10	0.468	0.703	0.904	28.60	25.92	98.84	−225.76
4	p-Cresol	0.52 × 10^−10	0.513	0.800	1.035	30.48	27.80	98.49	−218.87
5	o-Methoxyphenol	1.17 × 10^−10	0.394	0.525	0.935	37.31	34.64	99.62	−201.21
6	p-Methoxyphenol	0.62 × 10^−10	1.033	1.321	1.910	26.56	23.88	97.14	−226.82
7	o-Chlorophenol	20.0 × 10^−10	1.645	3.110	4.334	42.08	39.39	94.84	−171.67
8	p-Chlorophenol	4.17 × 10^−10	1.424	2.705	3.934	44.12	41.43	95.22	−166.51
9	Methyl 2-hydroxybenzoate	1.34 × 10^−10	0.0415	0.115	0.259	79.36	76.67	103.70	−83.69
10	Methyl 4-hydroxybenzoate	30.0 × 10^−10	0.484	0.625	0.758	19.45	16.76	99.15	−225.08

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In order to know more about the mechanistic aspects of the blocking reaction, we followed the kinetics of the uncatalyzed reaction at 50 °C. Since the rate of the uncatalyzed blocking reaction was too slow, we could follow the kinetics only up to 10% conversion and the rate constants are given in Table 3.

Table 3 Second-order rate constants for the uncatalyzed neat blocking reactions of polyisocyanate at 50 °C

Compd no.	Blocking agents	Ka of blocking agent [×10^10]	Second order rate constant k × 10^5 (eq.^−1 min^−1)
2	Phenol	1.05	1.007
3	o-Cresol	0.49	0.394
4	p-Cresol	0.52	0.599
5	o-Methoxyphenol	1.17	1.059
6	p-Methoxyphenol	0.62	0.436
7	o-Chlorophenol	20.0	14.008
8	p-Chlorophenol	4.17	1.827
9	Methyl 2-hydroxybenzoate	1.34	3.011
10	Methyl 4-hydroxybenzoate	30.0	4.153

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The rate constant values for the uncatalyzed blocking reaction were found to be increase with increased acidity (dissociation constant, K_a) of the phenolic compounds. This observation is opposite of what one would expect. Because in general, electron releasing substituents in the blocking agent increases the nucleophilicity of the blocking agent, and hence, increases the reactivity. Similarly, electron withdrawing substituents will decrease the nucleophilicity of the blocking agent, and hence, reduce the reactivity with an isocyanate. But in this complete study, it was found that the electron releasing methyl and methoxy substituents decrease the rate and the electron withdrawing substituents, like chloro and ester groups, increased the rate of the uncatalysed blocking reaction. Moreover, the rate of the o-chlorophenol was higher than that of its para isomer; the acidity of the ortho isomer is high. These observations ensure that the blocking rate of the uncatalyzed phenol–isocyanate reaction depends only on the acidity of blocking agents, i.e., the polarizability of hydroxyl group of phenolic compounds. If the acidity of a particular phenolic compound is high, the more polarized oxygen atom of hydroxyl group will easily attack the partially positive carbon atom of the –NCO group, thereby increasing the rate of the blocking reaction.

The second-order rate constants for the amine-catalyzed blocking reactions were also found to be in accordance with the acidity of the blocking agents, except the cases of phenols with an ester substituent. Because the phenol is acidic, it can associate easily with the tertiary amine base catalyst to form an association complex. Then, this complex will again associate with isocyanate group to form a four centered active intermediate complex during the blocking reaction (Fig. 5). The polarization of the phenolic hydroxyl group by the tertiary amine catalyst is responsible for the multi order (∼100 times) increase in the reactivity compared to the un-catalyzed reaction.⁴⁶ The low reactivity of blocking agents with ester substituents can be explained with this mechanism. Owing to the high acidity of these blocking agents, they can easily associate with the basic catalyst. But, the phenolate group becomes a poor nucleophilic due to the high electron withdrawing power of ester substituent present in the ring. Thus, the formation of an active intermediate complex with the –NCO group becomes difficult resulting in a low reaction rate. Though the rate of the reaction involving o-chlorophenol was relatively higher than that of phenol, it was not proportional to its acidity value compared to unsubstituted phenol. Similarly, the rate of o-methoxyphenol was found to be less even though its acidity value is slightly higher than that of phenol. In these cases, steric factors play a role. The time required for 75% conversion into product, given in Table 1, also supports this discussion. The ΔG^# of all the blocking reactions of isocyanate reported in this work were found to be almost identical, and were between 95 kJ mol⁻¹ to 103 kJ mol⁻¹. This clearly indicates that all the reactions follow the same mechanism. This was further verified by both the isokinetic plot and Exner plot; the linearity of ΔH^# vs. ΔS^# and log k_obs (50 °C) vs. log k_obs (60 °C), respectively, confirm that all the reactions follow a common blocking mechanism.⁴⁷ The higher negative entropy of activation (ΔS^#) supports the formation of a rigid complex in the transition state.

Fig. 5 Possible mechanism for the tertiary amine-catalyzed blocking reaction of polyisocyanate with phenol.

Deblocking temperatures of phenol-blocked polyisocyanates

As mentioned in the preceding section, hot-stage FT-IR spectrophotometry is the best analytical tool to study the deblocking reaction of blocked isocyanates too as this tool directly measures the regeneration of the –NCO functional group and the disappearance of the carbonyl group of the blocked –NCO group. All the blocked polyisocyanates, except the one based on phenol with ortho ester substituent (9), were subjected to FT-IR analyses under dynamic condition for the purpose of assessment of deblocking temperatures. The blocked polyisocyanate (9) was not included in the deblocking study since its blocking reaction was found to be incomplete after a long reaction time and it was cured to a hard and insoluble mass after two weeks. For this reason, it may be given exclusively focused attention. The spectra recorded from room temperature to 170 °C for a typical phenol-blocked polyisocyanate are given in Fig. 6, 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 4.

Fig. 6 FT-IR spectra of phenol-blocked polyisocyanate recorded at (a) different temperatures. (b) Zoomed range of isocyanate absorption region.

Table 4 Deblocking temperatures of phenol-blocked polyisocyanates determined using hot-stage FT-IR spectrophotometry under dynamic conditions

Compd no.	Blocking agents	Deblocking temperature (°C)
2	Phenol	130
3	o-Cresol	135
4	p-Cresol	135
5	o-Methoxyphenol	135
6	p-Methoxyphenol	135
7	o-Chlorophenol	85
8	p-Chlorophenol	120
10	Methyl 4-hydroxybenzoate	95

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The ease with which a blocked isocyanate will dissociate into isocyanate and the blocking agent will depend upon the magnitude of charge separation between the carbonyl carbon and the oxygen atom of the blocking agent.

Thus, electron-donating substituents in the blocking agent will strengthen the labile bond; and electron-withdrawing substituents in the blocking agent will weaken the bond, resulting in a low deblocking temperature. In accordance with this argument, the deblocking temperature obtained was consistent with the electronic effects as well as with the acidity values of the blocking agents. If the acidity (dissociation constant) of blocking agent is high, it means that its phenolate anion is loosely bonded with carbonyl carbon. So, the blocking agent with electron-withdrawing substituents, like chlorine and ester groups, deblock at lower temperatures whereas the blocking agent with electron-releasing substituents, like methyl and methoxy groups, deblock at higher temperatures compared to phenol (Table 4). Within the series of phenols with electron-releasing substituents, the effect of acidity on deblocking temperatures was not reflected well, but it was reflected well in kinetic results.

Deblocking kinetics (a comparison with the blocking reaction)

Based on the minimum deblocking temperature determined, isothermal amine-catalyzed deblocking kinetics of substituted phenol-blocked polyisocyanates at 120 °C, 130 °C and 140 °C under neat conditions as blocking kinetics were performed. Three changes were observed in the FT-IR spectrum of each blocked polyisocyanate. The first change observed was an increase in the intensity of the –NCO absorption at 2270 cm⁻¹ with respect to time, upto 30% conversion in comparison with quantitatively recorded spectrum of unblocked polyisocyanate, and then it becomes constant for some time, after which it decreased slowly due to the reaction between regenerated isocyanate and the –NH group of remaining urethane moieties leading to the formation of allophanate groups. This side reaction was observed by the appearance of a shoulder in the urethane N–H stretching absorption peak at 3300 cm⁻¹. The second change observed was due to the absorption by the urethane carbonyl at 1732 cm⁻¹ that was decreased due to the cleavage of blocked isocyanate groups. But the change was not as much as in the case of –NCO absorption because the other urethane carbonyl derived from polyol and isocyanate also absorbs in the same region. Moreover, the absorption by C=O was not as strong as the –NCO group. The third change observed was the decrease in absorption intensity by the urethane N–H at 3300 cm⁻¹ due to the cleavage of blocked isocyanate groups. Here also, the two different urethane N–H groups present in the blocked polyisocyanate absorb in the same region. Hence, 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. 7. 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. 8. The first order rate constants and activation parameters calculated for all the blocked polyisocyanates, except for blocked polyisocyanate 9, are given in Table 5.

Fig. 7 FT-IR spectra recorded for different time intervals under isothermal conditions for the deblocking reaction of phenol-blocked polyisocyanate: (a) 120 °C, (b) 130 °C and (c) 140 °C.

Fig. 8 Amine-catalyzed first-order kinetic plots of the deblocking reaction of phenol-blocked polyisocyanate.

Table 5 First-order rate constants and activation parameters for amine-catalyzed deblocking reactions of blocked polyisocyanates

Compd no.	Blocking agents	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)
			120 °C	130 °C	140 °C
2	Phenol	1.05 × 10^−10	6.483	9.002	13.612	50.05	46.70	115.51	−170.74
3	o-Cresol	0.49 × 10^−10	1.810	4.130	8.400	103.81	100.46	118.12	−43.83
4	p-Cresol	0.52 × 10^−10	2.900	4.928	8.433	72.15	68.80	117.53	−120.92
5	o-Methoxyphenol	1.17 × 10^−10	2.690	4.730	6.330	57.98	54.63	117.67	−156.41
6	p-Methoxyphenol	0.62 × 10^−10	1.105	1.740	3.230	72.41	69.05	121.02	−128.94
7	o-Chlorophenol	20.0 × 10^−10	13.080	18.400	26.700	48.23	44.88	113.12	−169.32
8	p-Chlorophenol	4.17 × 10^−10	6.631	9.691	13.885	49.40	46.05	115.26	−171.75
10	Methyl 4-hydroxybenzoate	30.0 × 10^−10	12.510	15.270	18.940	28.02	24.67	113.74	−221.02

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When compared to the deblocking temperature, the role of blocking agent acidity on the deblocking rate is clearly seen. The deblocking rate decreases with decreasing phenol acidity, and the reverse is also found to be true. However, careful analysis of the rate constants (k) and energy of activation (E_a) clearly reveals that, unlike the blocking reactions, electronic effects mainly determine the rate of the reaction in the deblocking reactions, as discussed for deblocking temperatures. This could be realized when considering the rate of both chloro and ester substituted phenols. That is, when comparing the rate of blocking and deblocking reactions involving blocking agent with chloro and ester substituents, unlike in the case of blocking reaction, here both substituents increase the rate.

Also, it was found that, unlike the case of the blocking reactions, here the rate constants (k), energy of activation (E_a) and enthalpy of activation (ΔH^#) are consistent with each other. These results lead to the conclusion that deblocking in the presence of a basic catalyst proceeds through the deprotonation of the urethane N–H by the base to form a conjugate base of urethane. The dissociation then proceeds by eliminating phenolate anion from the negatively charged conjugate base of urethane, which then accepts a proton from the protonated base. According to this mechanistic pathway, and a comparison with blocking kinetics, a phenol with more acidity and less nucleophilicity will be a better blocking agent. At the same time, it will be a better leaving group, which leads to easy cleavage. The narrow free energy of activation (ΔG^#) obtained confirm that all the deblocking reactions uniformly follow this mechanism.

Analysis of Arrhenius plots of the forward and reverse reactions

Arrhenius plots of the reversible blocking and deblocking reactions of blocked polyisocyanates are given in Fig. 9(a)–(h). The upper temperature of the blocking reaction was extrapolated to determine the maximum temperature upto which the forward reaction will take place. Likewise, the lower temperature of the deblocking reaction was extrapolated to determine the temperature below which the reverse reaction will never take place. We believed that extrapolation of such double Arrhenius plots will give new information about thermally reversible reactions. A typical example of such a plot is given in Fig. 10. Interestingly, all such plots, except the one fitted for phenol with the ester substituent, show a temperature range (equilibrium temperature range, ETR). Within this ETR, both the forward and reverse reactions are found to coexist and proceed with different rates. The ETR was found relatively narrow (the difference was only 10 °C) for p-chlorophenol-blocked polyisocyanate and it was found relatively broad (the difference was 47 °C) for o-methoxyphenol-blocked polyisocyanate. Two important conclusions could be derived from this analysis: (i) the forward reaction should be carried out below the lower limit of the ETR and the reverse reaction should be carried out at substantially above the upper limit of the ETR. (ii) The mid-point of the ETR is the most probable equilibrium temperature, in principle, below or above which only the forward or reverse reaction, respectively, will take place.

Fig. 9 Arrhenius plots of the forward and reverse reactions of blocked polyisocyanates. Blocking agent: (a) phenol, (b) o-cresol, (c) p-cresol, (d) o-methoxyphenol, (e) p-methoxyphenol, (f) o-chlorophenol, (g) p-chlorophenol and (h) methyl 4-hydroxybenzoate.

Fig. 10 Arrhenius plots of the forward and reverse reactions of phenol-blocked polyisocyante showing the ETR and most probable equilibrium temperature.

To determine the equilibrium rate constant in a typical case, the temperature was shifted 0.5 °C simultaneously above and below the upper and lower temperature of the forward and reverse reactions, respectively, in the double Arrhenius plot. The log k was identical at one particular shift. From that log k, the equilibrium rate constant was calculated and given in Table 6. Interestingly, the trend observed in the equilibrium rate constants is similar to that observed in the forward and reverse reactions. It is worth mentioning here that, depending upon the temperatures at which blocking and deblocking reactions are carried out, the apparent ETR may be wide or narrow, but the most probable equilibrium temperature will not change. There was a surprising observation from this analysis that the p-methoxyphenol–polyisocyanate system was found not to pass through the equilibrium state; however, it was found to be a thermally reversible system (Fig. 11). Finally it should be mentioned that the extrapolation of the Arrhenius plots of the ester substituted phenol–polyisocyanate system was found to be out of the temperature range studied. As already mentioned, this needs separate investigation.

Table 6 Thermal equilibrium data of phenol-blocked polyisocyanates determined using Arrhenius plots for the forward and reverse reactions

Compound number	Blocking agents	Equilibrium temperature range for forward and reverse reaction (°C)	Most probable equilibrium temperature for forward and reverse reaction (°C)	Equilibrium rate constant k × 10^3
a Coexistence of forward and reverse reaction not found in this case.
2	Phenol	80–98	90	4.898
3	o-Cresol	102–142	121	1.072
4	p-Cresol	89–122	105	1.549
5	o-Methoxyphenol	75–122	98	1.622
6	p-Methoxyphenol	85–119^a	101	—
7	o-Chlorophenol	64–102	82	8.709
8	p-Chlorophenol	80–90	85	5.623
10	Methyl 4-hydroxybenzoate	—	—

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Fig. 11 Arrhenius plots of forward and reverse reactions of p-methoxyphenol-blocked polyisocyante not showing equilibrium.

Conclusion

In this article, we report: (i) the synthesis of a series of blocked polyisocyanates using phenol with methyl, methoxy, chloro and ester substituents at 2- and 4-position as blocking agents. (ii) Both forward and reverse reactions of these blocked polyisocyanates were studied in detail without using solvent. (iii) The rates of the uncatalyzed forward reactions were found to be proportional to the acidity of the blocking agent. (iv) The identical trend was observed in the time required for 75% conversion into products and the rate of the catalyzed forward reaction. (v) The rate of the amine-catalyzed forward and reverse reactions were found to be proportional to the acidity of the blocking agent, except for the phenol with electron withdrawing ester substituents at 4-position. (vi) Chlorophenols were found to block the isocyanate quickly and, at the same time, they cleaved off easily compared to all other phenols studied. (vii) Phenol with an ester substituent was found to block the isocyanate group slowly and deblock quickly. (viii) There was an expected trend between the rate and activation parameters only in the deblocking reactions; no correlation was found between activation parameters of the forward and reverse reactions. (ix) The most probable equilibrium temperatures were assessed using double Arrhenius plots, for the first time, and these values, and the trend present in these values, were found in agreement with the deblocking temperatures, kinetic parameters and activation parameters of the reverse reactions. (x) The trend observed in the equilibrium rate constants is similar to that observed in the forward and reverse reactions. (xi) The p-methoxyphenol–polyisocyanate system was found not passes through the equilibrium state; however, it was a thermally reversible system.

Acknowledgements

One of the authors (S. K.) thank 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 also thank CSIR for providing FT-IR with hot-stage accessories under a scheme No. 02(0100)/12/EMR-II. Dt. 31.10.2012.

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Footnote

† Electronic supplementary information (ESI) available: ¹H-NMR spectra of phenol-blocked polyisocyanates (2–10), 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 and Arrhenius plots of blocking and deblocking reaction of blocked polyisocyanates. See DOI: 10.1039/c6ra15643f

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