Kinetics of water-isocyanate reaction in N,N-dimethylformamide☆

Zhirong Chen,Weitao Yang,Hong Yin*,Shenfeng Yuan

College of Chemical and Biological Engineering,Zhejiang University,Hangzhou 310027,China

1.Introduction

Like the polyol-isocyanate reaction,the reaction of isocyanate with water is one of the most important reactions in the production of polyurethane foam.The generally accepted theory[1-3]for waterisocyanate reaction assumes that carbamic acid is formed initially(Eq.(1)).

This intermediate almost instantaneously decomposes to yield an amine and carbon dioxide CO2(Eq.(2)),which is responsible for foam generation.

Then,the resulting amine reacts with the parent isocyanate,forming a substituted urea(RNH)2CO(Eq.(3)).

The kinetics of alcohol-isocyanate reaction has been extensively studied in various aspects including the structure of isocyanate[4,5]and alcohol[6-9],the type of catalyst[10,11]and solvent[12-14],catalytic mechanism[15-17],the autocatalysis of alcohol and urethane[18-21]and the reactivity of different groups in diisocyanate[22-24].However,the water-isocyanate reaction,in spite of its great theoretical interest and undoubted practical value,it has been studied fairly less than the alcohol-isocyanate reaction.

A detailed mechanism study of water-isocyanate reaction in dioxane at 353 K was conducted by Shkapenko and his coworkers[25]through monitoring the evolution of carbon dioxide.They found that only 50%-60%of the theoretical CO2is evolved.Therefore,a mechanism involving a carbamic acid anhydride intermediate and an ammonium salt intermediate both of which could decompose to liberate additional CO2was postulated.Unfortunately,there was no persuasive proofto defend the existence of these two intermediates in the reaction system.Ihms[26]and Borsus[27]also investigated the evolution of CO2in the reaction of water with diisocyanate under a variety of conditions and catalytic influences.Nevertheless,the overall objective of their work was to explore the factors that affected CO2evolution and offer guidance for industrial foaming process.No mechanism or in-depth kinetic study was performed.

In an early kinetic research,Farkas and Flynn[28]studied the reaction of phenyl isocyanate with water in dioxane and benzene in the absence and presence of amine catalysts.They noted that the reaction progress was in satisfactory agreement with second-order kinetics at initial stage.But positive deviation from linearity was observed after about half of the isocyanate was consumed,which might be due to the catalytic effect of the product ureide as they rationalized.Lately,Ni Hai and his coworkers[29]carried out a kinetic study of the reaction of water with hexyl isocyanate in 2-methoxyethyl ether.Their work showed that the second-order kinetics was operative fornon-catalyzed system.This was different from the conclusion of Farkas.So further study should be conducted to figure out whether the product ureide played a catalytic role in water-isocyanate reaction system.Besides,both of them ignored the concentration of the first product amine or assumed it to be steady-state without proving.Han et al.[30]developed a kinetic model that described second-order kinetics during the initial stage for water-isocyanate reaction but subsequently described thirdorder kinetics in consideration of the autocatalytic contribution of the urea group.The suggested model fitted the experimental data much better than second-order kinetic model from low to high isocyanate conversion.Yet,little attention was paid to the concentration change of the intermediate primary amine and no tests were made to make sure of the autocatalytic behavior of the newly formed urea linkage.

There are still some other studies[31-34]of the hydrolysis of isocyanates in organic solvents using a sufficient excess of water,throwing some light on the mechanism of the addition of water to isocyanate and the decarboxylation of carbamic acid.But these works are not instructive from a practical point of view.Recently,some new polyurea materials[35-37]have been preparedviathe reaction of isocyanates with water.These materials are expected to be multifunctional and attractive in many fields on account of their good mechanical and porous properties.Hence,a deep exploration of water-isocyanate reaction is still required for the purpose of extending its application to a larger scope.

In the present work,concentrations of all species occurring during water-isocyanate reaction are determined by means of HPLC.This is in contrast with the classical dibutylamine back-titration method[28,29]which follows only the unreacted isocyanate concentration and results in a great loss of details of the chemical process.DMF is chosen to study the effect of amide on the reaction because it has no active hydrogen on the nitrogen atom.The results help us to develop a new understanding of the catalysis of amide and the mechanism of waterisocyanate reaction.

2.Experimental

2.1.Materials

Thep-tolyl isocyanate(p-TI,≥98%)was purchased from Aladdin and used as received.DMF,dioxane and triglyme were dried over type 3A molecular sieves for at least 24 h and then distilled prior to use.Karl Fischer titration showed the water content in the purified solvents to be less than 0.02 wt%.Water was twice distilled in a glass apparatus.Analytical reagent dibutylamine and p-toluidine and chromatographic grade acetonitrile were used as supplied.1HNMR measurements were performed with a 500 MHz Bruker spectrometer.HPLC was carried out with a Prominence LC-20A diode array detector SPD-M20A.

2.2.Model synthesis

The symmetrical di-p-tolyl urea was obtained by the reaction ofp-TI withp-toluidine.DMF solution(5 ml)ofp-TI(1.33 g,0.01 mol)was added slowly into the beaker containingp-toluidine(1.07 g,0.01 mol)dissolved in DMF(5 ml)at ambient temperature.After the addition,the mixture was stirred for 1 h to ensure that the reaction was completed.The productdi-p-tolyl urea was then precipitated by adding the solution dropwise to water(500 ml)under vigorous agitation.The white precipitate was collected by filtration and dried under vacuum at 373 K.Di-p-tolyl urea was characterized with1H-NMR,as described in a previous publication[38].1H-NMR(500 MHz,DMSO-d6,δ:)2.24(s,6H,CH3),7.07-7.08(d,J=5 Hz,4H;Ar H),7.32-7.34(d,J=10 Hz,4H;Ar H),8.50(s,2H,NH).

The di-n-butyl-p-tolyl urea was prepared by mixing acetonitrile(15 ml)solutions ofp-tolyl isocyanate(1.33 g,0.01 mol)and dibutylamine(1.29 g,0.01 mol).After stirring the mixture for a period of time,abundant crystal would precipitate.Heat the solution until all the crystal disappeared and kept stirring for 0.5 h.Cool down the solution at room temperature and the product di-n-butyl-p-tolyl urea would recrystallize.The crystal was gathered through vacuum filtration and dried at 353 K.Similarly,the structure of di-n-butyl-p-tolyl urea was also confirmed with1H-NMR.1H-NMR(500 MHz,DMSO-d6,δ:)0.88-0.91(t,J=7.5 Hz,6H;CH3),1.24-1.31(m,4H;CH2),1.44-1.50(m,4H;CH2),2.22(s,1H;CH3),3.25-3.28(t,J=7.5 Hz,4H;CH2),7.00-7.02(d,J=10 Hz,2H;Ar H)7.31-7.33(d,J=10 Hz,2H;Ar H),8.00(s,1H;NH).

Di-p-tolyl urea and di-n-butyl-p-tolyl urea were used to obtain calibration curves for quantitative HPLC analysis.

2.3.Kinetic arrangements

The experiments were conducted in a 100-ml,round-bottom,fournecked flask equipped with a thermometer,a condenser,an inlet for dry nitrogen and a rubber stopper with a stainless steel puncture needle for samplingviasyringes.Thep-tolyl isocyanate dissolved in a DMF solution(30 ml)was preheated to a certain temperature(293,303,313,or 323 K)in the four-necked flask under magnetic stirring.Afterwards,desired amount of water was added to the flask at the initial time(t=0).Aliquots were withdrawn from the flask at regular time intervals and quenched with excess dibutylamine.All the materials were weighed and recorded before the next step.

2.4.HPLC analysis

0.2 g quenched reaction mixture was diluted with acetonitrile to 2 g for HPLC analysis.Prominence LC-20A chromatograph fitted with a diode array detector SPD-M20A was used.The reversed-phase method was adopted with a column:Hypersil ODS2,4.6 mm × 150 mm,5 μm pore size,column temperature 313 K.An eluent composition of acetonitrile/water(volume ratio 40/60)at a flow rate of 1.0 ml·min-1was applied.The analytes were detected at 220 nm.In view of minimizing the effect of experimental error,the determinations of each sample were repeated no fewer than three times.

3.Results and Discussion

Fig.1.Consumption rate of isocyanate in different solvents(303 K,[NCO]/[H2O]=2/1).

The effect of DMF on alcohol-isocyanate reaction has been reported before[14]and the result shows that the reaction in DMF has a higher second order reaction constant than other aprotic polar solvents.In this paper,the effect of DMF on water-isocyanate reaction was also examined in comparison with dioxane and triglyme,as shown in Fig.1.Under the same conditions,the concentration of p-TI in dioxane kept almost constant after stirring for 23 h and in triglyme about 9%ofp-TI was consumed after 12 h.But in DMF,86%ofp-TI was consumed in 2.5 h.It is clearthatamide has some catalytic effecton water-isocyanate reaction.If the second order kinetics is valid during the initial stage of the reaction,the apparent reaction constant of DMF(kobs=131.3 g·mol-1·min-1)is about 188 times larger than that of triglyme(kobs=0.696 g·mol-1·min-1).

According to Baker and Gaunt[7],the following mechanism is suggested for the DMF catalyzed reaction of isocyanate with water: first,the DMF molecule forms an intermediate complex with the isocyanate;then this complex can react with water to yield a carbamic acid,which undergoes instantaneous decomposition to give an amine and CO2;in the end,substituted urea is formed by the reaction of intermediate complex with amine.

All of the following experiments were carried out in DMF in order to investigate how amide would affect the reaction.Despite all of the works mentioned before,it was not clear(1)whether there were carbamic acid anhydride and ammonium salt in water-isocyanate reaction system;(2)what caused the shortage of CO2;(3)whether the concentration of intermediate amine was so low that it could be taken as steady-state;(4)whether there was reaction between isocyanate and the product urea to yield a biuret under the ordinary experimental conditions in the present work.By the use of HPLC it was possible to answer these questions.

A thorough HPLC analysis of the reaction mixture separated four chromatographic peaks at 0.96,1.96,6.36 and 20.58 min.According to the retention time of standard substances under the same working conditions,these peaks were assigned to,in order,DMF,p-toluidine,di-p-tolyl urea and di-n-butyl-p-tolyl urea.Water and excess dibutylamine had no chromatographic peaks under the present analysis conditions.Concentrations ofp-tolyl isocyanate,p-toluidine and di-p-tolyl ureaversustime are shown in Fig.2,as determined by quantitative analysis.

Fig.2 shows that the consumption of isocyanate is rapid at the initial stage and slows down during the later period.It is most likely due to the consumption of the reactants.The concentration of intermediate ptoluidine experiences an apparent increase first and then decreases gradually.Also it is noticeable that at the very initial stage,the majority of consumed isocyanate turns into amine and there are only a few urea in the reaction mixture.The maximum content of ptoluidine we detect is 6.31%of the initial concentration ofp-tolyl isocyanate and the minimum is 2.36%.Obviously,the amount of intermediatep-toluidine in the reaction system is not as small as many studies[25,30]assume.This is consistent with a previous study by Ekberg and Nilsson[39],which shows that the reaction of phenyl isocyanate in dioxane-water mixtures gives phenylamine and diphenylurea at the same time.So,the application of steady-state hypothesis to the concentration of amine is unreasonable.It is necessary to take intermediate amine into consideration when kinetic equation of water-isocyanate reaction is established.Moreover,existence of amine will interfere the result of back titration in the dibutylamine back-titration method,leading to a larger acid consumption and thus a lower isocyanate concentration.The higher the content of amine,the greater the error.So dibutylamine back-titration method is not advisable in water-isocyanate reaction study.

Fig.2.Reaction of p-TI with water(303 K,[NCO]0/[H2O]0=2/1).

The dashed line in Fig.2 represents the initial concentration ofp-TI([NCO]0).[NCO],[NH2]and[Urea]represent the concentration ofp-TI,p-toluidine and di-p-tolyl urea at timet,respectively.As shown in Fig.2,the sum of[NCO],[NH2]and 2[Urea]is almost identical with[NCO]0throughout the reaction.The same results are obtained under other conditions in this study.It can be concluded thereof thatp-TI is merely converted top-toluidine and di-p-tolyl urea by the reaction with water.No carbamic acid anhydride or ammonium salt is formed in the reaction process.There is no biuret in the reaction system even when an excess of isocyanate is used.Actually,only at a high temperature or under a harsh chemical condition can the isocyanate react with urea to produce biuret[40,41].

According to the above conclusions,the theoretical amount of CO2evolved during the reaction progress should slightly exceed 1/2([NCO]0-[NCO])owing to the small quantity of amine remaining in the reaction mixture.However,experiments[25-27]have assuredly demonstrated that the total amount of CO2evolved in these experiments never reaches 100%of theoretical.The work is repeated in the current system and the result turns out to be similar as previous reports.However,CO2has a high solubility in organic solvents[42,43],especially in dioxane(6.73 ml CO2per ml dioxane,293 K,101.3 kPa).It is plausible to suspect that the dissolution of CO2in solvent leads to the evolved CO2falling short of theoretical amount.

The effect of temperature on the reaction rate and amine content is shown in Figs.3 and 4.The consumption rate of isocyanate increases with the increasing of temperature as expected.But neither the maximum of amine content nor the minimum changes that much when the temperature varies from 293 K to 323 K.It can be concluded that the formation and the consumption of-NH2accelerate at the same time when temperature rises.Also it indicates that temperature does not significantly influence the theoretical amount of CO2.

Figs.5 and 6 depict the reaction progresses under different waterconcentrations while [NCO]0remains unchanged (2.0× 10-4mol·g-1).The overall reaction rate increases as the concentration of water gets higher.It will be noted that when the molar ratio[NCO]0/[H2O]0changes from 4/2 to 4/3,the observed reaction rate experiences a larger increase than it does when[NCO]0/[H2O]0further grows from 4/3 to 4/4.It implies that the reaction rate is sensitive to the molar ratio of[NCO]0to[H2O]0when it is around the stoichiometric ratio 2/1.

Fig.3.Effect of temperature on the consumption of isocyanate([NCO]0/[H2O]0=2/1).

Fig.4.Effect of temperature on the NH2 content([NCO]0/[H2O]0=2/1).

Fig.5.Effect of water concentration on the consumption of isocyanate(303 K).

Fig.6.Effect of water concentration on-NH2 content(303 K).

An increase of amine content is observed with the increasing water concentration(Fig.6).The maximum content of amine is 5.43%of the initial concentration ofp-tolyl isocyanate when[NCO]0/[H2O]0is 4/1 and 8.51%when[NCO]0/[H2O]0is 4/4.This can be interpreted as a result of acceleration of carbamic acid formation.High water concentration promotes the first step more than the second step of the water-isocyanate reaction to produce more amine.These phenomena confirm that the concentration of intermediate amine is not steady-state.Meanwhile,it indicates that the theoretical quantity of CO2will increase if the water percentage in the reaction system rises.

Also,a series of experiments were carried out under various isocyanate concentrations while the concentration of water was kept constant(1.0 × 10-4mol·g-1).The results are shown in Figs.7 and 8.For the sake of comparison,the consumption of isocyanate is expressed as conversion and the content of amine is represented by a percentage of initial concentration of isocyanate.The conversion-time curves under different isocyanate concentrations almost coincide with each other till the last stage of the reaction(Fig.7).It indicates that more isocyanate is consumed within the same period of time when the concentration of isocyanate increases.

Fig.7.Effect of isocyanate concentration on the consumption of isocyanate(303 K).

Though the amountofp-toluidine is increasing as the[NCO]0/[H2O]0gets larger,its percentage of initial concentration of isocyanate is decreasing(Fig.8).That means that high isocyanate concentration,contrary to water,promotes the second step more than the first step of the water-isocyanate reaction to consume more amine.Besides,it will be noted that more isocyanate is transformed into amine when initial concentration of isocyanate decreases and water concentration remains unchanged.

Fig.8.Effect of isocyanate concentration on the NH2 content(303 K).

If intermediate amine is assumed to be steady-state according to previous literatures,then Eqs.(1)-(3)can be combined into one general expression,as shown in Eq.(8).

According to the second order kinetics,the reaction rate equation can be written as follows:

wherekis the apparent second-order rate constant.

When[NCO]0/[H2O]0is 2/1,the plot of 1/[NCO]versustime should give a straight line.However,positive deviation from linearity during the later stage is observed(Fig.9).The result suggests that the kinetics of water-isocyanate reaction is not simple second order.

Instead of interpreting the deviation as a result of the autocatalytic effect of the newly formed urea[28,30],we propose a multistep mechanism(Eqs.(10)-(11))on the basis of our research findings to take into account the concentration of intermediate amine.

Fig.9.Second order plot for the reaction of p-tolyl isocyanate with water(303 K,[NCO]0/[H2O]0=2/1).

If second order kinetics is valid for the addition of water to isocyanate as well as the reaction of isocyanate with amine,these expressions result(stationary state condition is applied to carbamic acid):

Since it has been proved that DMF itself is an active catalyst in water-isocyanate reaction,it is reasonable to assume that DMFoverpowers the weakly basic carbanilide in performing the catalytic function for the reaction.Thereforek1andk2here represent the apparent reaction rate constants under the catalysis of DMF.Applying the suggested kinetic model of Eqs.(12)-(14)to the experimental results,Fig.10 presents an example of fitting results.Clearly,there is a rather good agreement between experimental and calculated values.The rate constants are calculated using MATLAB based on Runge-Kutta method and the results are listed in Table1.As expected,bothk1andk2increase with rising temperature.Using Arrhenius equation ofk=Aexp(-Ea/RT),the activation energy and frequency factor for each step of the reaction were determined,as shown in Fig.11.The activation energyEa2is 9.37 kJ·mol-1lower thanEa1.So it can be inferred that the first step of water-isocyanate reaction is more sensitive to temperature than the second step.

Fig.10.Comparison between experimental and calculated values(303 K,[NCO]/[H2O]=2/1).

Table1Kinetic data for the reactions of isocyanate with water

Fig.11.Arrhenius plots for the water-isocyanate reaction.

4.Conclusions

The non-catalyzed reaction ofp-tolyl isocyanate with water was monitored by HPLC method which proved to be a more accurate technique than dibutylamine back-titration for the study of waterisocyanate reaction.DMF showed an efficient catalysis for the reaction.That confirmed the autocatalysis of product urea which also had amide structure.The quantitative analysis of the reaction mixture showed that isocyanate was only converted to primary amine and substituted urea.No carbamic acid anhydride,ammonium salt or biuret was formed in the process.A considerable amount ofp-toluidine was found in the reaction system,which indicated that the hypothesis of steady state should not be applied to intermediate amine.Moreover,amine content would increase if the concentration of water went up and decrease when the concentration of isocyanate rose.

The second order kinetics which was used in other papers could not be applied to the spontaneous reaction of isocyanate with water in DMF.The new kinetic equations we proposed to take into account the effect of amine fitted the experimental data well all through the reaction.Kinetic parameters were obtained by Runge-Kutta method.The activation energy 42.39 kJ·mol-1for the addition of water to isocyanate and 33.02 kJ·mol-1for the reaction of isocyanate with amine indicated that the first step of the water-isocyanate reaction was more sensitive to temperature than the second step.

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