Process for the preparation of polyisocyanates containing isocyanurate groups, and the use of the products of the process as isocyanate component in the production of polyurethanes

The present invention thus relates to a process for the preparation of polyisocyanates having isocyanurate groups by the trimerization of part of the isocyanate groups of organic polyisocyanates or of mixtures of di- and mono-isocyanates in the presence of basic compounds as trimerization catalysts, with termination of the trimerization reaction by the addition of a catalyst poison, characterized in that the trimerization catalysts used are 1:1-complexes of (i) basic sodium or potassium compounds and (ii) 1,4,7,10,13-pentaoxacylopentadecane or 1,4,7,10,13,16-hexaoxacyclooctadecane. This invention also relates to solutions suitable as catalyst components for this process, comprising 1:1-complexes of (i) basic sodium or potassium compounds and (ii) 1,4,7,10,13-pentaoxacyclopentadecane or 1,4,7,10,13,16-hexaoxacyclooctadecane dissolved in polar lacquer solvents and/or at least one compound within the molecular weight range of about 32 to 250 which has alcoholic hydroxyl groups and is liquid at room temperature. This invention also relates to the use of the products of the process according to the invention, optionally freed from monomeric starting polyisocyanate and/or optionally blocked with blocking agents for isocyanate groups, as an isocyanate component in the production of polyurethanes by the isocyanate polyaddition process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a new process for the preparation of 
polyisocyanates containing isocyanurate groups by the partial 
trimerization of the isocyanate groups of organic polyisocyanates or of 
mixtures of di- and mono-isocyanates, a catalyst solution suitable for 
carrying out this process, and the use of the products of the process as 
an isocyanate component in the production of polyurethanes by the 
isocyanate-polyaddition process. 
2. Description of the Prior Art 
Processes for the trimerization of organic isocyanates, in particular of 
polyisocyanates, are known in large numbers, (J. H. Saunders and K. C. 
Frisch, Polyurethanes Chemistry and Technology, page 94 et seq (1962)). 
Strong organic bases are suitable catalysts for this trimerization, e.g. 
those metal salts of carboxylic acids which are alkaline in reaction, 
metal alcoholates, metal phenolates, alkali metal carbonates, tert. 
amines, tert. phosphines and the "onium" compounds of nitrogen and 
phosphorus, and basic heterocyclic compounds of these elements. The 
catalysts are frequently used as combinations or together with other 
cocatalysts such as mono-N-substituted carbamic acid esters (A. Farkas and 
G. A. Mills, Advances in Catalysis, Vol. 13, 393 (1962)). 
Most processes for the preparation of high quality polyisocyanates 
containing isocyanurate groups use expensive catalyst systems since it is 
known that simple metal salts such as carboxylates or alcohols are only 
capable of effecting cyclotrimerization of isocyanates at relatively high 
concentrations and at a high temperature; the trimerization of 5 parts of 
phenylisocyanate, for example, requires 1 part of potassium acetate and 
heating for 3 hours at 100.degree. C. (Houben-Weyl, Vol. 8, page 244, 
publishers Thieme Verlag) (see also British Pat. No. 809,809, Example 6). 
If trimerization with metal salts is to be carried out in solvents, highly 
polar aprotic solvents such as dimethyl formamide or dimethylsulphoxide 
must be used because only these are capable of dissolving inorganic metal 
salts and metal salts with a small organic group (German 
Offenlegungsschrift No. 2,839,084). Even then, catalyst concentrations of 
as much as 0.1 to 0.5% by weight are required. This also applies when the 
solvents used are protic but form urethanes with the isocyanate, thereby 
lowering the isocyanate content, or form precipitates and cloudiness so 
that the reaction product must be filtered (British Pat. No. 920,080). 
Furthermore, the metal salts known in the art effect rapid trimerization 
only in the case of aromatic isocyanates while aliphatic mono- and 
polyisocyanates require a high catalyst concentration and temperatures 
above 50.degree. C., whereby the reaction frequently takes an uneven 
exothermic course and in the case of polyisocyanates results in highly 
viscous, strongly discolored products (see U.S. Pat. No. 3,330,828, 
Examples 1 to 4; British Pat. No. 952,931, Example 3; German 
Auslegeschrift No. 1,013,869, Example 3) or the formation of gel particles 
(British Pat. No. 966,338, Example 3), whereby the products become to a 
large extent unsuitable for use in polyurethane lacquers. Another major 
disadvantage of using metal salts as catalysts is that stopping the 
catalyst results in the formation of inorganic salts which are insoluble 
in the polyisocyanate and cause cloudiness. The more recent processes of 
the art therefore rarely use the simple and inexpensive basic metal salts 
such as potassium acetate but special organic bases, depending on the 
particular isocyanate, and these are used under quite specific reaction 
conditions. Thus, for example, the trimerization of aromatic 
polyisocyanates is carried out using Mannich bases (German 
Offenlegungsschrift No. 2,551,634 and German Offenlegungsschrift No. 
2,641,380) or tertiary phosphines, in which case uretdiones are first 
formed which are converted to the isocyanurate only in a second phase of 
the reaction (German Offenlegungsschrift No. 1,201,992). Organic bases 
having a betaine structure, such as quaternary ammonium hydroxides 
(European Offenlegungsschrift No. 010,589 and European Offenlegungsschrift 
No. 009,694), aminimides (J. E. Kresta, R. J. Chang, S. Kathiriya and K. 
C. Frisch, Makromol. Chem. 180, 1081 (1979)) and azirine derivatives in 
combination with tert. amines (German Auslegeschrift No. 2,325,826) are 
frequently used for the trimerization of (cyclo)aliphatic diisocyanates. 
One disadvantage of all these catalyst systems is that quite specific 
temperature intervals must be observed and the process may in part only be 
carried out solvent-free and in part only in selected solvents, and in 
particular trimerization can only be carried out on aromatic 
polyisocyanates alone or aliphatic polyisocyanates alone. 
It is an object of the present invention to provide a process by which 
colorless aromatic and aliphatic polyisocyanates containing isocyanurate 
groups may be obtained by a technically simple procedure either in 
solvents or solvent-free and without elaborate temperature control, using 
one and the same catalyst. 
It was surprisingly found that this problem could be solved by carrying out 
the trimerization using 1:1-complexes of basic sodium or potassium 
compounds with 1,4,7,10,13-pentaoxacyclopentadecane ("15-crown-5") or 
1,4,7,10,13,16-hexaoxacyclooctadecane ("18-crown-6") as trimerization 
catalysts. 
Although C. J. Pedersen already recognized that crown-ether-complexed 
alkali metal salts are fundamentally suitable as trimerization catalysts 
for aromatic isocyanates (J. Am. Chem. Soc. 89, 7017 (1967) or U.S. Pat. 
No. 3,686,225), the crown ethers containing condensed benzene or 
cyclohexane rings described in these prior publications and their 
complexes with basic sodium or potassium compounds are not suitable for 
large scale technical production of high quality polyisocyanates 
containing isocyanurate groups, firstly because the crown ethers with 
condensed cyclohexane rings have only an extremely slight complex forming 
action on basic alkali metal compounds and, secondly, because 1:1 
complexes based on crown ethers having condensed benzene rings have very 
poor solubility in organic media so that, for example, concentrated 
catalyst solutions in physiologically harmless solvents cannot be prepared 
using these complexes. By contrast, the 1:1 complexes described below 
which are to be used in the process according to the invention do not have 
such disadvantages. The crown ethers underlying the 1:1 complexes which 
are an essential feature of this invention are eminently suitable for the 
preparation of stable complexes with basic sodium and potassium compounds 
and at the same time these complexes are readily soluble both in the 
polyisocyanates to be trimerized and in the auxiliary solvents which will 
be described below. They may therefore be added as relatively concentrated 
solutions to the trimerizing polyisocyanate, a factor which is important 
for the large scale technical production of polyisocyanates having 
isocyanurate groups. 
SUMMARY OF THE INVENTION 
The present invention thus relates to a process for the preparation of 
polyisocyanates having isocyanurate groups by the trimerization of part of 
the isocyanate groups of organic polyisocyanates or of mixtures of di- and 
mono-isocyanates in the presence of basic compounds as trimerization 
catalysts, with termination of the trimerization reaction by the addition 
of a catalyst poison, characterized in that the trimerization catalysts 
used are 1:1-complexes of (i) basic sodium or potassium compounds and (ii) 
1,4,7,10,13-pentaoxacyclopentadecane or 
1,4,7,10,13,16-hexaoxacyclooctadecane. 
This invention also relates to solutions suitable as catalyst components 
for this process, comprising 1:1-complexes of (i) basic sodium or 
potassium compounds and (ii) 1,4,7,10,13-pentaoxacyclopentadecane or 
1,4,7,10,13,16-hexaoxacyclooctadecane dissolved in polar lacquer solvents 
and/or at least one compound within the molecular weight range of about 32 
to 250 which has alcoholic hydroxyl groups and is liquid at room 
temperature. 
This invention also relates to the use of the products of the process 
according to the invention, optionally freed from monomeric starting 
polyisocyanate and/or optionally blocked with blocking agents for 
isocyanate groups, as an isocyanate component in the production of 
polyurethanes by the isocyanate polyaddition process. 
DETAILED DESCRIPTION OF THE INVENTION 
In the context of this invention, the term "1:1-complexes" is used to 
denote complexes of equimolar quantities of a basic sodium or potassium 
compound with 15-crown-5 or 18-crown-6. Complex formation of the sodium 
compound is carried out using the first mentioned cyclic polyether while 
complex formation of the potassium compound is carried out with the last 
mentioned cyclic polyether. 
The basic sodium or potassium compounds used according to the invention may 
be any compounds of the aforesaid alkali metals whose aqueous solution at 
a 1 molar concentration has a pH of at least about 7.5. Suitable basic 
compounds are, for example, sodium or potassium carboxylates preferably 
having 1-12 carbon atoms, alcoholates preferably having 1-8 carbon atoms, 
phenolates preferably having 6-10 carbon atoms, carbonates, hydroxides, 
cyanates, enolates or cyanides. Suitable basic sodium or potassium 
compounds are, for example, the formates, acetates, propionates, 
2-ethylhexanoates, n-dodecanoates, caprylates, methylates, ethylates, 
butylates, hexylates, phenolates, tert.-butylphenolates, carbonates, 
hydroxides, cyanates, thiocyanates or cyanides of the above-mentioned 
metals or also, for example, sodium- or potassium-N-methylacetamide. 
Included among the preferred basic compounds are the aforementioned 
carboxylates, alcoholates, phenolates, carbonates, hydroxides, cyanates 
and cyanides. Particularly preferred are simple carboxylates of the 
above-mentioned alkali metals having 1-4 carbon atoms, in particular 
potassium acetate. 
The cyclic polyethers used for complex formation are known compounds. Their 
preparation may be carried out, for example, according to G. Johns, C. J. 
Ransom, C. B. Reese, Synthesis (1976), page 515. 
The preparation of the 1:1-complexes to be used in the process according to 
the invention may be carried out, for example, according to one of the 
following methods: 
1. Preparation is carried out using the polyisocyanate which is to be 
trimerized or its solution in a suitable solvent which may also serve as 
reaction medium for carrying out the process according to the invention, 
so that the cyclic polyether is dissolved in the polyisocyanate or its 
solution, and the alkali metal salt is stirred in as a solid, with complex 
formation and solution. 
2. The cyclic polyether is dissolved in a suitable solvent, and the alkali 
metal salt is then added with complex formation and solution. Any 
cloudiness is removed by filtration. 
3. The procedure is as described under 2. but using a relatively volatile 
solvent which is drawn off after complex formation so that the complex 
precipitates as solid residue which is subsequently dissolved in another 
solvent and/or in the polyisocyanate to be trimerized. 
When preparing the 1:1-complexes, components (i) and (ii) are preferably 
used in equimolar quantities. It would, of course, be possible to operate 
with different quantitative proportions, but either the basic alkali metal 
compound or the cyclic polyether would then be present in excess. As will 
readily be seen, such a procedure would hardly be suitable since the 
excess would have little or no catalytic activity. When preparing 
solutions of 1:1-complexes, components (i) and (ii) are generally used in 
quantities such that the complexes are present as about 0.4 to 40% by 
weight, preferably about 0.8 to 20% by weight solutions. It is precisely 
one of the main advantages of the catalysts which are essential to this 
invention that they are soluble at such comparatively high concentrations 
in the solvents mentioned as examples below. 
Solvents which, as indicated above, may also be used as reaction media for 
the preparation of the complexes are in particular the usual polar, 
physiologically substantially harmless solvents used in polyurethane 
lacquer technology, which have a boiling point from about 50.degree. C. to 
350.degree. C. at normal pressure, or compounds containing alcoholic 
hydroxyl groups which are liquid at room temperature and have a molecular 
weight from about 32 to 250, preferably from about 46 to 162. Any mixtures 
of such solvents may, of course, also be used. Examples of suitable 
lacquer solvents of the above-mentioned type are: ethyl acetate, butyl 
acetate, ethyl glycol acetate, acetone, methyl ethyl ketone, methyl 
isobutyl ketone, cyclohexanone, methoxyhexanone or also chlorinated 
hydrocarbons such as, for example, chloroform or chlorobenzene. With 
diluents such as toluene, xylene and higher aromatic compounds, for 
example, there is only limited solubility. Larger additions of such 
solvents may lead to cloudiness and precipitation. Examples of compounds 
with alcoholic hydroxyl groups which are suitable as solvents are: 
methanol, ethanol, isopropanol, ethylene glycol acetate, ethylene glycol, 
diethylene glycol, ethylene glycol monoethylether, glycerol or 
trimethylolpropane. Since the 1:1 -complexes are generally very readily 
soluble in such compounds having hydroxyl groups so that these alcoholic 
solvents need only be used in very small quantities, their presence does 
not interfere with carrying out the process according to the invention. 
In addition to the aforesaid solvents, however, higher boiling solvents may 
also be used, e.g. the usual plasticizers such as dibutylphthalate, 
butylbenzylphthalate or phosphoric acid esters such as tricresylphosphate. 
Polyisocyanates suitable for carrying out the process according to the 
invention are in principle any organic polyisocyanates having 
aliphatically, cycloaliphatically, araliphatically, heterocyclically 
and/or aromatically bound isocyanate groups of the molecular weight range 
of about 140 to 300, such as e.g. tetramethylene diisocyanate, 
hexamethylene diisocyanate, 
1-isocyanato-3,3,5-trimethyl-5-isocyanato-methyl-cyclohexane (isophorone 
diisocyanate, abbreviated: IPDI), the isomeric xylylene diisocyanates, 
2,4- and/or 2,6-diisocyanato-toluene, or 2,4- and 
4,4'-diisocyanato-diphenylmethane, and any mixtures of such 
polyisocyanates. Included among the preferred starting materials for the 
process according to the invention are 2,4-diisocyanato-toluene, 
2,6-diisocyanato-toluene, any mixtures of these isomers, 
hexamethylenediisocyanate, IPDI, and any mixtures of these preferred 
diisocyanates. Particularly suitable are also mixtures of the 
last-mentioned diisocyanates having cycloaliphatically or aliphatically 
bound isocyanate groups with aromatic diisocyanates of the last-mentioned 
type in proportions by weight of about 1:3 to 3:1. 
Isocyanate prepolymers having isocyanate groups, such as are obtained by 
the reaction of excess quantities of the diisocyanates exemplified above 
with compounds having isocyanate reactive groups, or also higher 
functional polyisocyanates, for example polyisocyanate mixtures such as 
are formed from the phosgenation of aniline/formaldehyde condensates, may 
in principle also be used as starting materials in the process according 
to the invention, although the use of such modified or higher functional 
polyisocyanates is less preferred. Mixtures of diisocyanates and 
monoisocyanates may in principle also be used as starting materials in the 
process according to the invention to result in interesting 
polyisocyanates having isocyanurate groups with the isocyanate 
functionality reduced by a controlled amount. In this case, the di- and 
mono-isocyanates are generally used in a molar ratio 
diisocyanate:monoisocyanate of about 1.5:1 to 2.5:1. Suitable 
monoisocyanates are, for example, aliphatic monoisocyanates having 1 to 
18, preferably 4 to 8 carbon atoms, such as methyl isocyanate, 
n-butylisocyanate, n-octylisocyanate or stearylisocyanate or aromatic 
monoisocyanates such as in particular phenylisocyanate. 
When carrying out the process according to the invention, only a portion of 
the isocyanate groups of the starting polyisocyanate is trimerized. This 
means that the trimerization reaction is stopped at a degree of 
trimerization (degree of trimerization=percentage of trimerized isocyanate 
groups, based on the total number of isocyanate groups originally present) 
of about 10 to 70%. If the process according to the invention is carried 
out in the presence of solvents so that the process products according to 
the invention are directly obtained as solutions which are used, as such, 
e.g. to serve as polyisocyanate components in lacquers, the trimerization 
reaction is preferably stopped at a trimerization degree of from about 50 
to 70% in order to keep the proportion of completely unreacted starting 
isocyanate in the solutions as low as possible. If the process is carried 
out in the absence of solvents, in particular if the unreacted starting 
diisocyanate is to be removed after termination of the trimerization 
reaction, for example by thin layer distillation, the trimerization 
reaction is generally stopped at a degree of trimerization of about 10 to 
50%, preferably about 20 to 40%. 
Suitable catalyst poisons are, for example, acid chlorides such as benzoyl 
chloride, acetyl chloride, oxalyl chloride, succinic acid dichloride, 
terephthalic acid dichloride, 2,5-dichlorobenzyl acid chloride, phosphorus 
trichloride or thionyl chloride or strong acids such as p-toluene 
sulphonic acid, nonafluorobutanesulphonic acid or phosphoric acid as well 
as carbamic acid chlorides such as may be formed by the addition of HCl to 
isocyanates, which inactivate the essential catalysts of the invention 
with neutralization of the basic alkali metal compounds. 
Preparation of the polyisocyanates containing isocyanate groups may be 
carried out solvent-free or in the presence of suitable solvents. Suitable 
solvents are in particular the lacquer solvents already mentioned above as 
examples, which have no isocyanate reactive groups. 
The process according to the invention is generally carried out in the 
temperature range of about 0.degree. to 80.degree. C., preferably about 
20.degree. to 60.degree. C. The catalyst quantity to be used depends on 
the nature of the starting polyisocyanate and is generally at about 0.001 
to 1.0% by weight, preferably about 0.003 to 0.5% by weight, based on the 
weight of the 1:1-complex and the weight of the starting polyisocyanate. 
The particularly preferred range in the case of aromatic starting 
polyisocyanates is about 0.003 to 0.05% by weight and in the case of 
aliphatic starting polyisocyanates is about 0.03 to 0.5% by weight. 
The method of adding the catalyst may be carried out by various methods as 
may the preparation of the catalyst already described above: 
1. When the catalyst is prepared in the starting polyisocyanates as already 
mentioned under 1. above, the trimerization reaction starts spontaneously 
after formation of the complex. In this variation, the individual 
components of the complex are therefore added separately to the starting 
polyisocyanate. 
2. The catalyst may in principle also be incorporated in a solid form with 
the starting polyisocyanate. 
3. The catalyst is preferably added in the form of the above described 
solutions in the lacquer solvents exemplified above and/or in the 
compounds with alcoholic hydroxyl groups exemplified above. Such solutions 
of the 1:1-complexes which are particularly suitable for the process 
according to the invention generally have a solid content of about 0.4 to 
40, preferably about 0.8 to 20% by weight. 
If the process according to the invention is carried out in the presence of 
one of the lacquer solvents exemplified above, it may often be advisable 
to dispense with separation of the solvent and use the solution of the 
process products in the above-mentioned solvents directly for the 
preparation of polyurethane products, for example for the preparation of 
polyurethane lacquers. In such a case, the same solvent is preferably used 
both for the 1:1-complex which is essential to the invention and for the 
reaction medium, the quantity of which solvent is calculated so that 
solutions of the process products according to the invention have a solid 
content of about 20 to 80, preferably about 40 to 60% by weight. 
After addition of the catalyst, for example at room temperature, the 
trimerization reaction starts up spontaneously, both in the case of 
starting isocyanates having aliphatic isocyanate groups and those having 
aromatic isocyanate groups, and both in the presence of solvents and in 
their absence. When using the optimally active quantity of the essential 
catalyst of the invention, which can be determined by a simple 
investigative preliminary test, the reaction temperature generally rises 
to about 30.degree.-60.degree. C. so that as a result of the careful 
trimerization reaction, a colorless, clear reaction product is obtained 
after a period of about 1 to 8 hours. When the maximum temperature has 
been reached, no additional heating need be applied to the reaction 
mixture but it may be advantageous to maintain the reaction temperature 
within the range of about 40.degree. to 50.degree. C. by heating or 
cooling in order to observe the optimum reaction times. 
When the desired degree of trimerization has been reached, the reaction is 
stopped by the addition of one of the catalyst poisons exemplified above. 
For complete termination of the trimerization reaction, it is generally 
sufficient to add an at least equimolar quantity of the catalyst poison, 
based on the 1:1-complex. After termination of the trimerization reaction, 
the reaction mixture may, if desired, be worked up by distillation. Thus, 
for example, the amount of excess starting polyisocyanate in the process 
products may be reduced to below about 3% by weight, preferably below 
about 0.7% by weight, by its removal in a thin layer evaporator. 
The process according to the invention has the following advantages over 
the known art: 
1. The catalyst system which is essential to the invention is suitable for 
the trimerization of both aromatic and aliphatic polyisocyanates, and the 
reaction may be carried out as a slightly exothermic reaction at a low 
reaction temperature so that the risk of impairing the quality of the 
process products by high temperatures can be virtually excluded. 
2. The quantity of catalyst is invariably less than in the known processes 
of the state of the art, in particular in the case of aromatic starting 
polyisocyanates. 
3. The trimerization velocity is so high that the reaction may generally be 
carried out without external supply of energy. 
4. The catalysts may also be spontaneously inactivated at the low 
temperature range of about 20.degree. to 60.degree. C., whereby 
discolorations of the process products such as can be observed in case of 
thermal inactivation are eliminated. 
5. Due to the inactivation of the catalyst, the formation of insoluble 
inorganic salts which cause cloudiness, as is normally the case when using 
metal compounds does not occur, and the neutral salts formed remain in 
solution due to the complex forming effect of the crown ethers, so that 
clear, colorless end products are formed. 
6. The process products according to the invention are stable in storage 
and by-products such as uretdiones or carbodiimides are not formed. 
The process products according to the invention are valuable raw materials 
for the production of polyurethanes by the isocyanate polyaddition 
process. They are suitable in particular as isocyanate components in 
two-component polyurethane lacquers. For this purpose they may also be 
used in a masked form, masked with blocking agents for isocyanate groups. 
Another important field of application for the process products according 
to the invention is their use as cross-linking agents for adhesives based 
on at least one high molecular weight compound containing isocyanate 
reactive hydrogen atoms(s).

The percentages mentioned in the following examples are percentages by 
weight. 
EXAMPLE 1 
0.05 ml of a 0.2 molar solution of potassium acetate/18-crown-6 in 
diethylene glycol monomethylether (0.0036% total catalyst concentration) 
are added to 100 g of a mixture of 65% by weight 2,4- and 35% by weight 
2,6-tolylene diisocyanate at 40.degree. C. Trimerization sets in at once 
with heating, a maximum temperature of 56.degree. C. being reached. After 
15 minutes stirring, the isocyanate content is 38.8%. The reaction is 
stopped by the addition of 0.05 ml of a 0.2 molar solution of benzoyl 
chloride in diethylene glycol dimethylether and the mixture is stirred for 
a further 30 minutes at 40.degree. C. 
EXAMPLE 2 
100 g of 2,4-tolylenediisocyanate are dissolved in 100 g of anhydrous butyl 
acetate, and 0.35 ml of a 0.1 molar solution of potassium 
acetate/18-crown-6 in 2-ethylhexanol (0.013% total catalyst concentration) 
are added with stirring at 40.degree. C. Trimerization sets in at once, 
the temperature rising only minimally (42.degree. C.). After 16 hours, 35 
minutes stirring at 40.degree. C., the isocyanate content has fallen to 
7.9%. The reaction is stopped by the addition of 0.01 ml of 
nonafluorobutanesulphonic acid and the mixture is stirred for a further 15 
minutes at 60.degree. C. The clear, colorless solution contains 0.53% of 
free tolylene diisocyanate (based on the solid content) and has a 
viscosity of 3054 mPas (25.degree. C.). 
EXAMPLE 3 
100 g of 2,4-tolylene diisocyanate are dissolved in 100 g of anhydrous 
butyl acetate, and 5 ml of a 0.01 molar solution of potassium 
acetate/18-crown-6 in butyl acetate (0.018% total catalyst concentration) 
are added with stirring at room temperature. Trimerization sets in at once 
with heating, a maximum temperature of 57.degree. C. being reached. After 
8 hours, 20 minutes stirring without additional heating, the isocyanate 
content has fallen to 8.9%. The reaction is stopped by the addition of 
0.007 g of benzoyl chloride and the mixture is stirred for a further 10 
minutes. The clear, colorless solution contains 0.043% of free tolylene 
diisocyanate (based on a 100% product) and has a viscosity of 2390 mPas 
(25.degree. C.). 
EXAMPLE 4 
1044 g (6 mol) of 2,4-tolylene diisocyanate are dissolved in 1000 g of 
anhydrous butyl acetate, and 60 ml of a 0.01 molar solution of potassium 
acetate/18-crown-6 in butyl acetate (0.021% total catalyst concentration) 
are added with stirring at room temperature. Trimerization sets in at once 
with heating, a maximum temperature of 60.degree. C. being reached after 2 
hours. After a further 3 hours stirring without additional heating, the 
isocyanate content is 9.0%. The viscous solution is then divided into 10 
samples of about 208 g each, and these samples are stopped with 0.06 mMol 
of, respectively, benzoyl chloride, phosphoric acid, thionyl chloride, 
phosphorus trichloride, acetyl chloride, oxalyl chloride, succinic acid 
dichloride, terephthalic acid dichloride, 2,5-dichlorobenzoic acid 
chloride (Sample 10 without stopper). After 2 months, the isocyanate 
content of all nine samples is 9.0%. In Sample 10 (without stopper), the 
isocyanate content has fallen to 7.3% NCO within the same length of time. 
EXAMPLE 5 
3 ml of a 0.01 molar solution of potassium acetate/18-crown-6 in butyl 
acetate (0.015% total catalyst concentration) are added to 100 g of a 
mixture of 65% by weight 2,4- and 35% by weight 2,6-tolylene diisocyanate 
at room temperature. Trimerization sets in at once with heating, a maximum 
temperature of 79.degree. C. being reached. After 25 minutes stirring, the 
isocyanate content is 32.7%. The reaction is stopped by the addition of 
0.0035 ml benzoyl chloride and the mixture is stirred for a further 10 
minutes. 
EXAMPLE 6 
0.46 g (0.091%) of a preformulated, crystalline complex of potassium 
acetate and 18-crown-6 are added at room temperature to 504 g (3 mol) of 
hexamethylene diisocyanate with stirring. The weakly exothermic 
trimerization (maximum temperature 37.degree. C.) sets in at once. After 3 
hours, 35 minutes, the isocyanate content is 42%. The reaction is stopped 
by the addition of 0.15 ml benzoyl chloride and the mixture is stirred for 
a further 15 minutes. The slightly yellowish solution is filtered (part of 
the complex remains insoluble in HDI). After thin layer distillation, 125 
g of a pale yellowish product is obtained. Isocyanate content: 22.3%, 
viscosity (25.degree. C.): 1949 mPas, monomeric HDI content: &lt;0.1%. 
The results of gel chromatography are: 63% monoisocyanurate and 35% higher 
molecular weight polyisocyanates with isocyanurate structure. 
EXAMPLE 7 
1.53 ml of a 0.5 molar solution of potassium acetate/18-crown-6 in 
diethylene glycol monomethylether (0.055% total catalyst concentration) 
are added to 504 g (3 mol) of hexamethylene diisocyanate with stirring at 
room temperature. The exothermic trimerization (maximum temperature 
58.degree. C.) sets in at once. After 45 minutes. The isocyanate content 
of the solution is 40.8%. The reaction is stopped by the addition of 5 mg 
of nonafluorobutanesulphonic acid and the mixture is stirred for a further 
15 minutes. After thin layer distillation without previous filtration, 140 
g of a clear, almost colorless product is obtained. Isocyanate content: 
23%, viscosity (25.degree. C.): 2375 mPas, free monomeric HDI content: 
&lt;0.1%. 
EXAMPLE 8 
5 ml of a 0.1 molar solution of potassium acetate/18-crown-6 in 
2-ethylhexanol (0.036% total catalyst system concentration) are added to 
504 g (3 mol) of hexamethylene diisocyanate with stirring at room 
temperature. The weakly exothermic trimerization sets in at once (maximum 
temperature 36.degree. C.). After 4 hours 30 minutes, the isocyanate 
content of the solution is 40.6%. The reaction is stopped by the addition 
of 0.012 ml benzoyl chloride and the mixture stirred for a further 15 
minutes. Thin layer distillation of the clear solution yields 180 g of a 
clear, slightly yellowish product. Isocyanate content: 22.9%, viscosity 
(25.degree. C.): 2653 mPas, free monomeric HDI content: &lt;0.1%. 
Although the invention has been described in detail in the foregoing for 
the purposes of illustration, it is to be understood 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.