HK1084379A - Process for the preparation of polyamines of the diphenylmethane series at a low degree of protonation - Google Patents

Description

Method for preparing polyamine of diphenylmethane series under low protonation degree

Technical Field

The invention relates to a process for the preparation of polyamines of the diphenylmethane series.

Background

Polyamines of the diphenylmethane series are a class of compounds or mixtures of such compounds represented by the following general formula:

wherein x represents a number from 2 to n, and

n represents a natural number greater than 2.

The preparation of polyamines of the diphenylmethane series, continuous, discontinuous or semicontinuous (also referred to as MDA in the following description), is described in numerous patents and publications. (see, e.g., H.T.Twitchtt, chem.Soc.Rev.3(2), 209(1974), M.V.Moore in; Kirk-Othmer encycle. chem.Technol, 3rd ed., New York, 2, 338-348 (1978)). The preparation is usually carried out by reacting aniline and formaldehyde in the presence of an acid catalyst, HCl being usually used as the acid catalyst. According to the prior art, the acid catalyst is used up by neutralizing it at the end of the process and before the work-up step (e.g. removal of excess aniline by distillation) by adding a base.

In general, the acid condensation of aromatic amines with formaldehyde to form polyamines of the diphenylmethane series can be carried out by several reactions. The following exemplary equation explains this process:

condensation:

formaldehyde is first condensed with aniline in the absence of an acid catalyst to form the so-called aminated product (aminal) and water. The rearrangement reaction to produce MDA takes place under acid catalysis, the first step producing the para-and ortho-aminobenzylanilines. Aminobenzylaniline was rearranged again in a second step to produce MDA. The main products of the acid-catalyzed reaction of formaldehyde and aniline are diamine 4, 4 ' -MDA, its positional isomers 2, 4 ' -MDA and 2, 2 ' -MDA, and the analogous higher polyamines of the diphenylmethane series.

Aminobenzylanilines are formed directly from aniline and formaldehyde in the presence of an acid catalyst. They then react further to form MDA containing two rings and MDA analogs containing more than two rings.

Polyisocyanates of the diphenylmethane series, also referred to herein as MDI, are prepared by phosgenation of the corresponding polyamines. Polyisocyanates of the diphenylmethane series prepared in this manner include a wide variety of isocyanate isomers and higher analogs thereof in relative amounts comparable to the relative amounts of the polyamines used to prepare them.

In the acid-catalyzed reaction of aniline and formaldehyde, the control parameters that influence the isomer distribution are, in addition to the molar ratio of aniline to formaldehyde, the amount of acid catalyst used in the process. In this connection, the degree of acidification of a simple molecular acid (e.g. HCl) refers to the molar ratio of the acid catalyst used and the number of moles of amino functions in the reaction mixture. In order to be able to prepare MDA having the desired isomer distribution, it is sometimes necessary to add a considerable amount of acid catalyst (corresponding to a high degree of protonation) and correspondingly to have to add a considerable amount of base to neutralize the acid catalyst.

Thus, DE-A-1643449 describes a process for preparing high contents of 4, 4' -MDA by reacting aniline first with an acid and then with formaldehyde, the degree of protonation being at least 25%, more suitably at least 50%, even more suitably 75-100%. In this connection, it is necessary to use an acid catalyst in such an amount that the reaction proceeds in a homogeneous phase. The water of reaction and water from the starting reactants remain in the reaction mixture at all times.

DE-A-10111337 describes a process for preparing MDA with a degree of protonation of < 20%. However, in this way the content of 2, 4' -MDA in MDA increases. This process also shows that water from the reaction and water from the starting reactants remain in the reaction mixture at all times.

It is further known that the distribution of isomers and homologues of MDA can be controlled within limits by modifying the retention time and temperature. As described in EP-A-1053222, elevated temperatures are advantageous if the preparation of 2, 4 '-MDA and 2, 2' -MDA is preferred.

According to the prior art, the increased amount of 2, 4' -MDA formed at low protonation degrees can be at least partially offset by lowering the temperature during the rearrangement. However, a low degree of protonation means a lower consumption of acid catalyst and at the same time a lower consumption of base for neutralizing the acid catalyst after the condensation reaction. The process of low protonation degree is therefore clearly economical.

Surprisingly, however, the technically achievable lower limit of protonation is not marked by limits for the temperature and/or retention time to be adjusted. In contrast, in the processes described in the prior art, the acidic reaction mixture changes to multiphase or biphasic form when the degree of protonation is below 25% (see, for example, DE-A-19804918). This two-phase state causes problems in the operation reaction.

It is therefore an object of the present invention to provide a simple and economical process for preparing polyamines of the diphenylmethane series in which the reaction mixture does not separate into two-phase systems using acid catalysts which bring the protonation degree to < 15%. At the same time, polyamines having a lower isomer ratio of 2, 4 '-MDA than of 4, 4' -MDA can be prepared.

Summary of The Invention

The invention comprises a method for preparing polyamines of diphenylmethane series by acid catalysis. The method comprises the following steps:

a) aniline and formaldehyde are reacted in a molar ratio of 1.5: 1 to 6: 1 at a temperature of 20 ℃ to 100 ℃ (preferably 30 ℃ to 95 ℃, more preferably 40 ℃ to 90 ℃), wherein the acidic reaction mixture has a water content of < 20% by weight and a degree of protonation of < 15%.

b) When the weight content ratio of p-aminobenzylaniline and 4, 4' -MDA in the reaction mixture is less than 1.00, the reaction temperature is raised to 110 ℃ to 250 ℃.

In one embodiment of the invention, aniline is first mixed with an acid catalyst and then formaldehyde is added. However, the aniline, formaldehyde and acid catalyst may be mixed in another order, or the components may be mixed together at the same time. More suitably, the acid catalyst, aniline and/or formaldehyde are/is fed at a specific feed power (specific power input) of more than 10kW/m³Mixing under the condition of mixing volume, and the specific feeding power is more than 20kW/m³A mixing volume. In this context, the feed power, for example at a mixing orifice, depends on the pressure loss in the countercurrent of the pressure pump and on the volume of the mixing orifice.

The invention also relates to a process in which step a) aniline and formaldehyde are first reacted in the absence of an acid catalyst to produce an aminated product, then an acid catalyst is added to the aminated product, and the reaction is continued at a temperature of 20 ℃ to 100 ℃. In this way, an acidic reaction mixture is obtained which contains from 0 to 20% by weight of water (100% by weight based on the acidic reaction mixture).

The invention also relates to a process in which, after the reaction to give the aminated product of step a), at least a portion of the water is removed from the aminated product, so that the water content of the aminated product is between 0 and 5% by weight. Then, an acid catalyst is added to the aminated product and the reaction is continued at 20 ℃ to 100 ℃. In this way, an acidic reaction mixture is obtained which contains from 0 to 20% by weight of water.

Detailed Description

The process of the invention can be carried out continuously or semicontinuously or discontinuously. Conventional reaction equipment used in the prior art is also suitable for use in the present invention. Suitable equipment includes, for example, stirred reactors, tubular reactors or tubular reactors with baffles (e.g., perforated plates), which can affect the residence time properties of the reactor. Several reactor types may also be used in combination.

According to the process of the invention, the preparation of polyamines of the diphenylmethane series can be carried out with a degree of protonation of < 15%, preferably 4 to 14%, more preferably 5 to 13%. In this context, the degree of protonation of a monoprotic acid, for example hydrochloric acid, is the molar ratio of the acid catalyst used to the amino functions present in the reaction mixture. For dibasic or polybasic acids (n protons), the degree of protonation refers to the molar ratio of twice or n times the moles of acid catalyst used to the moles of amine functional groups present in the reaction mixture.

Suitable polyamine mixtures of the diphenylmethane series are typically prepared by condensation of aniline and formaldehyde in a molar ratio of from 1.5: 1 to 6: 1, more preferably from 1.8: 1 to 5: 1.

Formaldehyde is usually used industrially as a 30 to 50% strength by weight aqueous solution. However, other concentrations of aqueous formaldehyde or other compounds that provide methylene groups are also possible, such as polyoxymethylene glycol (polyoxymethylene glycol), polyoxymethylene, trioxane (trioxane).

Suitable acid catalysts have proven to be strong organic and inorganic acids, with inorganic acids being preferred in the present invention. Suitable acids include, for example, hydrochloric acid, sulfuric acid, phosphoric acid, and methanesulfonic acid. According to the invention, aqueous hydrochloric acid is preferably used in the process. The aqueous hydrochloric acid solution generally contains hydrogen chloride in a concentration of 25 to 36% by weight, based on the weight of the aqueous hydrochloric acid solution.

The method of the invention may be carried out according to the following procedure: aniline, formaldehyde and aqueous hydrochloric acid are added to a stirred tank and mixed. While the reaction is proceeding, most of the water is separated off by distillation, so that the water content is from 0 to 20% by weight, based on 100% by weight of the acidic reaction mixture. In the discontinuous process, aniline, aqueous formaldehyde solution and aqueous hydrochloric acid solution can optionally also be added according to a time-dependent metering profile, and water is removed during or after the addition of the educts, for example by means of vacuum distillation. The mixing of aniline, aqueous formaldehyde solution and aqueous hydrochloric acid solution is preferably carried out at 20 to 60 ℃.

The invention also relates to a process in which aniline and formaldehyde are first reacted in the absence of an acid catalyst to form an aminated product, and then an acid catalyst is added to the reaction mixture. The mixture of amination products and acid catalyst is preferably carried out at a specific feed power of more than 10kW/m³Mixing volume (preferably, specific feed power greater than 20kW/m³Mixing volume) and preferably at a temperature of 20-60 ℃. The addition of the acid catalyst and the removal of water can be carried out, for example, by feeding an aqueous hydrochloric acid solution into a stirred tank already containing the amination product and separating off most of the water by distillation during the reaction, so that a concentrated product is obtained and has a moisture content of from 0 to 20% by weight, based on 100% by weight of the acidic reaction mixture.

In a preferred embodiment of the process, after the reaction to produce the aminated product, at least part of the water is first removed from the product so that the water content is 0-5% by weight of the aminated product. Then the amination product is reacted with an acidThe reactants are mixed, preferably at from 20 to 60 ℃ and the acidic mixture obtained in this way is then reacted at from 20 to 100 ℃ and a moisture content of from 0 to 20% by weight, based on 100% by weight of the acidic reaction mixture obtained, to give the condensation product. The amination product and the acid catalyst are preferably mixed at a specific feed power of more than 10kW/m³Mixing under the condition of mixing volume, and preferably specific feeding power is more than 20kW/m³A mixing volume.

In a preferred embodiment of the process, aniline and formaldehyde are first reacted in the absence of an acid catalyst at a temperature of from 20 ℃ to 100 ℃ (preferably from 40 ℃ to 100 ℃, more preferably from 60 ℃ to 95 ℃). Condensation products of aniline and formaldehyde (so-called amination products) are formed in this reaction. After the amination product has been formed, the water contained in the amination product is at least partially removed, for example by phase separation or other suitable means (e.g. distillation). This results in a water content in the product of from 0 to 5% by weight, preferably from 0.5 to 5% by weight, more preferably from 1 to 4% by weight, based on the weight of the aminated product. The moisture content can be determined, for example, by the method of Karl-Eischer, which is described by Jander, Jahr, Ma. beta. analysis, 15th ed.de Gruyter, Berlin (1989), page 289-292.

The aminated product is then mixed with an acid catalyst, the mixing preferably being carried out at a temperature of 20-60 ℃ and a specific feed power of more than 10kW/m³Mixing volume (more preferably specific feed power greater than 20kW/m³Mixing volume) was performed. The mixture is initially reacted in a holding device or a series of holding devices at a temperature of 20-100 c, preferably 30-95 c. The acidic reaction mixture obtained here has a moisture content of between 0 and 20% by weight, based on 100% by weight of the acidic reaction mixture. The amount of acid catalyst chosen here is such that the degree of protonation is < 15%, preferably 5 to 14%, more preferably 6 to 13%.

In step b) of the process of the invention, the temperature of the reaction mixture is raised in stages or continuously to 250 ℃ at 110-. According to the invention, the earliest time in which step b) is carried out is when the weight content ratio of p-aminobenzylaniline and 4, 4' -MDA in the acidic reaction mixture is less than 1.00, preferably less than 0.50, more preferably less than 0.25, most preferably less than 0.20.

In order to ensure complete conversion, the temperature in step b) is advantageously maintained in this temperature range for 20 to 300 minutes, preferably 40 to 200 minutes, more preferably 50 to 180 minutes. In accordance with these conversion/temperature constraints, it is possible to ensure that the weight ratio of the two isomers of 2, 4 '-MDA and 4, 4' -MDA in the product is between 0.05: 1.00 and 0.15: 1.00.

To determine the weight content ratio between p-aminobenzylaniline and 4, 4' -MDA in the acidic reaction mixture, the weight content of these components can be determined, for example, by means of a reversed phase HPLC column using a gradient eluent of a mixed solvent of methanol, acetonitrile and water. The components are preferably measured by an ultraviolet detector at a wavelength of 254 nm. One procedure is as follows: for example, the preliminary reaction is carried out at temperatures of from 20 ℃ to 100 ℃, samples are taken from the acidic reaction mixture and these samples are used for the analytical determination of the weight ratio of p-ABA and 4, 4' -MDA. It must be ensured that the composition of the acidic reaction mixture does not change between the removal and analysis of the sample. This can be achieved, for example, by cooling the acidic reaction mixture vigorously, for example below 0 ℃, so that the reaction rate becomes so slow that no significant change in composition occurs. It is also possible to add a base to the acidic reaction mixture and to remove the acid catalyst from the reaction, thereby stopping the reaction. In a discontinuous procedure, for example, samples may be removed from the reaction cell and analyzed in any advantageous temporal sequence, or at regular intervals of 60 minutes (preferably 30 minutes). When the ratio of p-aminobenzylaniline to 4, 4' -MDA is less than 1.00, the mixture can be heated to greater than 110 ℃ or equal to 110 ℃. In a continuous procedure, for example, the preliminary reaction is carried out at 20 to 100 ℃ in at least one reaction apparatus or a series of reaction apparatuses, a sample may be taken from the outlet of the last reaction vessel and then analyzed by the aforementioned method.

If the ratio of p-ABA and 4, 4' -MDA exceeds 1.00, the course of the reaction can be controlled in suitable measures so that the ratio values are below 1.00. For example, the amount of acidic reaction solution produced can be reduced, so that the conversion of the preliminary reaction at the outlet of the last reaction tank is high due to the increased retention time, thereby making the ratio of p-ABA to 4, 4' -MDA lower than 1.00 at present. However, it is also possible to adjust the temperature or the temperature profile in the reaction tank used for the preliminary reaction such that the ratio of p-ABA and 4, 4' -MDA at the outlet of the last reaction tank is less than 1.00, as long as the temperature or the temperature profile remains within the limits of 20 to 100 ℃.

In the presence of an acid catalyst, aniline and formaldehyde react to form polyamines of the diphenylmethane series, which can also be carried out in the presence of other substances, such as salts or organic and inorganic acids.

According to the process of the invention, no demixing occurs in the reaction, although the degree of protonation is < 15%. This is clearly an advantage, since at low protonation levels and high moisture contents the acidic reaction mixture separates into an aqueous phase and an organic phase, the majority of the acid catalyst passing unrestricted into the aqueous phase and the organic phase no longer having acid catalyst, thus resulting in the acid catalyst being removed from the reaction. This leads on the one hand to a low reaction rate, so that the residence time required for the reaction becomes longer, which, however, impairs the profitability of the process. On the other hand, this leads to an unfavorable product composition, since, for example, more unfavorable 2, 4' -MDA is formed.

Also, according to the process of the present invention, MDA having a weight ratio of two isomers of 2, 4 '-MDA and 4, 4' -MDA of between 0.05: 1.00 and 0.15: 1.00 can be obtained. The ratio of p-ABA and 4, 4' -MDA is a measure of the conversion achieved. In the reaction of p-aminobenzylaniline and other aminobenzylaniline isomers as well as those of more polycyclic compounds, the high temperature promotes the formation of 2, 2 '-and 2, 4' -MDA isomers. This means that if high conversions are obtained at higher temperatures (e.g.above 110 ℃), 2, 4' -MDA is formed in large amounts. Conversely, if high conversions are obtained at lower temperatures, the formation of 2, 2 ' -and 2, 4 ' -MDA is reduced, whereas 4, 4 ' -MDA is formed in large amounts. Thus, according to the present invention, the formation of 4, 4 ' -MDA can be promoted, and the formation of 2, 2 ' -MDA and 2, 4 ' -MDA can be hindered. This is due to the fact that in the process according to the invention, the temperature of the reaction mixture is only raised to 110-250 ℃ when the ratio of the weight contents of p-aminobenzylaniline and 4, 4' -MDA is less than 1.00. As a result, the weight ratio of the two isomers of 2, 4 '-MDA and 4, 4' -MDA in the product is between 0.05: 1.00 and 0.15: 1.00.

For working up the acidic reaction mixture, the reaction mixture can be neutralized with a base according to the prior art. According to the prior art, neutralization is usually carried out at from 90 to 100 ℃ without further substances (H.J.Twitchett, chem.Soc.Rev.3(2), 223 (1974)). However, the neutralization can also be carried out at another temperature, for example in order to accelerate the decomposition of undesired by-products. Suitable bases include, for example, hydroxides of alkali and alkaline earth elements. Aqueous NaOH is preferably used.

The amount of base used in the neutralization reaction is greater than 100% (preferably 105-120%) calculated on the stoichiometric amount required to neutralize the acid catalyst

After the neutralization reaction, the organic phase is separated from the aqueous phase in a separating vessel by customary methods. After the aqueous phase has been separated off, the remaining organic phase containing the product is subjected to further work-up (e.g. washing) and the product is then liberated from the mixture of excess aniline and other substances (e.g. solvents) by suitable methods, for example distillation, extraction or crystallization.

The polyamines or polyamine mixtures of the diphenylmethane series obtained in this way can be reacted with phosgene in inert organic solvents in a known manner to give the corresponding di-or polyisocyanates of the diphenylmethane series, for example MDI. The crude MDA and phosgene are chosen in such a way that the molar ratio is chosen such that 1 to 10mol, preferably 1.3 to 4mol, of phosgene are present per mole of NH2 group in the reaction mixture. Chlorinated aromatic hydrocarbons, such as monochlorobenzene, dichlorobenzene, trichlorobenzene, the corresponding toluene and xylenes and chloroethylbenzene, are suitable inert solvents. Monochlorobenzene, dichlorobenzene or mixtures of these chlorobenzenes are particularly suitable as inert organic solvents. The amount of solvent is chosen such that the isocyanate content of the reaction mixture is from 2 to 40% by weight, preferably from 5 to 20% by weight, based on the total weight of the reaction mixture. When the phosgenation is complete, excess phosgene, inert organic solvent or mixtures thereof may be removed from the reaction mixture by distillation.

A series of products of polymeric MDI containing di-or polyisocyanates of the diphenylmethane series having two or more rings, a series of products of mono-MDI containing dimeric isocyanates of the diphenylmethane series having two rings (in particular in the form of polymeric technical-grade pure 4, 4 '-MDI and/or technical-grade pure 2, 4' -MDI and mixtures thereof having a high viscosity of from 80 to 3000mPas at 25 ℃) can be prepared from the crude MDI obtained. These products are all known in the prior art. According to the prior art, these products can be separated from the crude MDI by methods known in the art, for example by distillation. These products are suitable as starting materials for the reaction with polyols to prepare polyurethanes in the form of polymers and prepolymers.

The invention will be explained in more detail with the aid of the following examples.

The following examples further illustrate details of the process of the present invention. The invention, which has been set forth in the foregoing description, is not to be limited either in spirit or scope by these examples. Those skilled in the art will appreciate that known variations of the conditions in the following procedures may be made. Unless otherwise noted, all temperatures are degrees Celsius and all percentages are percentages by weight.

Example (b):

for the determination of the components of the polyamine mixtures, the separation was carried out by using a C18 reverse phase column of HPLC 1050 from Hewlett Packard, eluting with a gradient eluent of a ternary mixed solvent of methanol, acetonitrile and water, followed by detection with a UV monitor at a wavelength of 254 nm. The separate independent signals of the components p-ABA and 4, 4' -MDA can be clearly seen. The sample from the acidic reaction mixture was neutralized prior to analysis.

Example 1 (not according to the invention, two-phase reaction mixture is formed)

310g of a 31.7% by weight aqueous formaldehyde solution are added dropwise to 670g of aniline at 80 ℃ over the course of 20 minutes while stirring. After the addition was complete, the mixture was stirred at 80 ℃ for a further 5 minutes and then cooled to 35 ℃. 83.0g of 31.9% aqueous hydrochloric acid are added to the reaction mixture at this temperature with vigorous stirring. The water content in the acidic reaction mixture was 30.7% by weight. Even when heated to 110 c, the acidic reaction mixture is clearly biphasic, i.e. without the action of a stirrer, the acidic reaction mixture separates into an aqueous phase and an organic phase.

Example 2 (complete conversion above 110 ℃ C., not according to the invention)

For the preparation of the amination product 334g of 32.1% by weight aqueous formaldehyde solution are added dropwise to 931g of aniline over 20 minutes with stirring and at a temperature of 80 ℃. After the addition, the mixture is stirred at 80 ℃ for a further 5 minutes and the phases are separated at 70 ℃ to 80 ℃. 900g of the organic phase, i.e.the aminated product, were mixed with 137g of 32.0% aqueous hydrochloric acid at a temperature of 115 ℃ over 10 minutes with vigorous stirring. After addition of hydrochloric acid, the water content was 12.0% by weight and the ratio of p-ABA and 4, 4' -MDA was 0.27. After stirring at 115 ℃ for a further 3 hours, all aminobenzylanilines containing two rings were reacted. The ratio of 2, 4 '-MDA to 4, 4' -MDA was 0.38. This example shows the effect on product composition when conversion is complete at elevated temperatures greater than 110 ℃.

Example 3 (Process according to the invention, conversion for the most part at 20-100 ℃ C.)

The aminated product was formed according to the procedure described in example 2 above. 900g of the aminated product were mixed with 137g of 32.0% strength by weight aqueous hydrochloric acid at 45 ℃ over 10 minutes with vigorous stirring. After the addition of hydrochloric acid, the content of water was 12.0% by weight, and the reaction mixture obtained showed no sign of separation into an aqueous phase and an organic phase if the action of the stirrer was not present. After stirring the mixture at 45 ℃ for 10 minutes, it was heated to 80 ℃ and stirred at 80 ℃ for a further 60 minutes. The ratio of p-ABA/4, 4' -MDA was 0.05: 1.00. The mixture was then heated to 115 ℃ and stirred at this temperature for a further 3 hours. Aminobenzylaniline containing two rings was no longer detected and the ratio of 2, 4 '-MDA to 4, 4' -MDA was 0.14: 1.00.

Compared to example 2, it can be seen that the ratio of 2, 4 '-MDA and 4, 4' -MDA is significantly reduced, since most of the conversion is achieved below 110 ℃.

Although the foregoing invention has been described in some detail for purposes of illustration. It is to be understood, however, that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims. The scope of the invention is limited by the claims.

Claims (9)

1. An acid-catalyzed process for preparing polyamines of the diphenylmethane series comprising:

a) reacting aniline and formaldehyde in a molar ratio of 1.5: 1 to 6: 1 at a temperature of 20 ℃ to 100 ℃, wherein the acidic reaction mixture has a water content of < 20% by weight and a protonation degree of < 15%; and

b) when the weight content ratio of p-aminobenzylaniline and 4, 4' -MDA in the reaction mixture is less than 1.00, the reaction temperature is raised to 110 ℃ to 250 ℃.

2. The method of claim 1, wherein step a) comprises:

(1) firstly, aniline and formaldehyde react to generate amination products in the absence of an acid catalyst;

(2) adding an acid catalyst to the amination product; and

(3) the reaction is continued at a temperature of 20-100 ℃ and a moisture content of 0-20% by weight.

3. The process of claim 2 wherein in step a), after (1) reacting aniline and formaldehyde to form an aminated product, and before (2) adding an acid catalyst to the aminated product and (3) continuing the reaction at a temperature of 20-100 ℃ and a moisture content of 0-20% by weight, at least a portion of the moisture is removed from the aminated product so that the moisture content in the aminated product is 0-5% by weight.

4. The process according to claim 1, wherein in step a) the reaction of aniline and formaldehyde is carried out at 30 to 95 ℃.

5. The method of claim 1, wherein in step a) the established degree of protonation is 5-14%.

6. The method as claimed in claim 1, wherein in step b) the temperature is raised to 110-180 ℃.

7. The method as claimed in claim 1, wherein in step b) the temperature is raised to 110-160 ℃.

8. A process for the preparation of polyisocyanates of the diphenylmethane series, comprising the steps of: (A) phosgenating polyamines of the diphenylmethane series, wherein said polyamines of the diphenylmethane series are prepared by the process of claim 1.

9. A method of preparing a polyurethane, comprising the steps of: reacting a polyisocyanate of the diphenylmethane series prepared by the process of claim 8 with one or more polyether polyols and/or polyester polyols to form a polyurethane.