The use and reactions of isocyanates are well documented and they form the basis of polyurethane polymers which have a wide variety of uses. On page 391 of"Comprehensive Polymer Science-The Synthesis, Characterisation, Reactions and Applications of Polymers", Volume 5,1989, details are given of the polymerisation of isocyanates.

However, it will be noted that the reactions are all with protic nucleophiles.

Such reactions involve an R-NH-COOR group. On heating, this breaks down to give off HCN. This, as is well known, is a deadly poison and has, in recent times, caused laws to be passed in certain countries regarding the non-use of such polyurethane foams in, for example, furniture.

Attention has therefore been given to overcoming this problem. As far as we are aware, this has primarily been concerned with the alkylation of the hydrogen atom in the amide group. This is described, for example, in Eur. Polyol J. VoL 32, No. 12,1996, published by Elsevier Science Ltd. at pages 1377-1380. This document further suggests first forming an amide and then alkylating such compound. In practice, this is difficult to achieve.

The present invention seeks to provide new compounds which overcome the above- described problems and a method of making such compounds.-, In a preferred aspect, the present invention seeks to provide new polymeric materials which can be used for a variety of purposes and which, by suitably selecting the appropriate starting material, can be tailored to provide desired properties.

According to the present invention, there are provided new azaboroxines of the formula :- in which R and R', which may be the same or different, each represent a substituted or unsubstituted alkyl aryl, cycloalkyl group, a heterocyclic group or, in the case of R a silyl group and, in the case of R', a silyloxy group and X represents either an oxygen atom or a sulphur atom. Preferably, R represents a benzyl group. Advantageously, R'represents a methyl group.

Also according to the present invention, there is provided a method of making azaboroxine compounds of the formula:- comprising reacting a boroxine of the formula:- wherein R'represents a substituted or unsubstituted alkyl group, aryl group, cycloalkyl group, heterocyclic group or silyloxy group with a monoisocyanate or monoisothiocyanate of the formula R-N = C = X wherein R represents a substituted or unsubstituted alkyl group, aryl group, cycloalkyl group, heterocyclic group or a silyl group and X represents either an oxygen atom or a sulphur atom.

In a preferred aspect of the present invention, the azaboroxine so formed is heated to drive off the aromatic residues therefrom to leave a compound of the formula:- the resultant material containing N-B-O bonds which can then pyrolytically form a ceramic material Such pyrolised material may have a variety of uses such as being used as a strengthening or blocking layer deposited on glass or metaL The isocyanate or thioisocyanate may be a mono-compound. However, it is preferred if it either a diisocyanate (or thioisocyanate) or even a triisocyanate (or trithioisocyanate).

In such a case, the present invention provides polymeric azaboroxines having the following repeating unit:- in which R is a substituted or unsubstituted alkyl group, an aryl group, a cycloalkyl group, a heterocyclic group, a silyl group or a metal, R'is a substituted or unsubstituted alkyl group, aryl group, cycloalkyl group, heterocyclic group or a silyloxy group and X represents an oxygen atom or a sulphur atom.

Preferably, R'represents a methyl group. Desirably, R represents a benzyl group.

In accordance with a preferred aspect of the present invention, these compounds are prepared by reacting a di-or triisocyanate (or thioisocyanate) with a substituted or unsubstituted boroxine of the formula:- wherein R'represents an unsubstituted alkyl, aryl, cycloalkyl, heterocyclic or silyl group or one of such groups substituted by at least one halogen atom.

In a preferred embodiment, the reaction is carried out in the presence of an aprotic dilution agent. Preferably, in such a case, the dilution agent is tetrahydrofuran or an ester.

We have surprisingly found that the reaction of a di- (or tri-) isocyanate or thioisocyanate with aprotic compounds such as the boroxines leads to monomeric or polymeric products which have a wide range of uses, in dependence upon the nature of the R-group, without the disadvantages associated with known polyurethanes. For example, because no protons are present, there is no possibility of hydrogen cyanide being produced if the product is subjected to pyrolysis.

Since the boroxine has no labile proton, it would not be expected to react with an isocyanate or an isothiocyanate. The resulting polymeric structure has a high inorganic content and a low organic content which is unusual for"polyurethane-type"polymers.

Such high inorganic content gives rise to properties which are highly desirable. For example, such compounds have a low flammability.

Moreover, the structure lends itself to the use of such polymers as ceramic precursors.

Hitherto, it has generally been laborious to make ceramic materials since this involves the high temperature heating of metal oxides followed by sintering and the like. One then finishes with a ceramic material which is a mixture of metal oxides, carbides and nitrides.

The present preparation of the polymer precursor is generally effected at or below room temperature.

Normally, the reaction to produce such polymers will be carried out with equimolar quantities of the reactants. However, in an advantageous aspect of the present invention, the reaction is effected with an excess of the boroxine. By so doing, free alkoxy, aryloxy or cycloalkoxy groups (depending on the starting material employed) are produced. These can then be reacted with, for example, tetraethylorthosilicate (TEOS) in a manner which is described and claimed in our prior United Kingdom Patent Application No. 9705764.0 (International Application published under No. WO 98/42627 corresponds), thereby producing a ceramic material which may have a variety of uses.

The invention will be further described, by way of illustration only, with reference to the following non-limitative Examples.

EXAMPLE 1 Benzyl isocyanate (11.6g) was introduced into a reaction flask immersed in an ice bath.

Dry trimethoxyboroxine (Sg) was added dropwise to the dry isocyanate with stirring, the stirring being terminated once the addition of the trimethoxyboroxine had been completed.

The reaction flask was maintained in the ice/water bath until it reached ambient temperature.

The reaction product was a relatively free-flowing material whilst the temperature of the ice-bath remained below 0°C. As the temperature increased, so did the viscosity of the system. At ambient temperatures, the reaction product was a water-white, bubble-free, solid material Physical examination of the material indicated it to be relatively hard with no residual liquor being present.

The material thus obtained was ascertained to be:- EXAMPLE 2 The procedure adopted in Example 1 was followed with the exception that the benzyl isocyanate was replaced with 4,4'-Methylenebis- (cyclohexylisocyanate) hereinafter referred to as HMDI.

After the completion of the addition of the trimethoxyboroxine, the reaction product was fairly mobile although some rheological enhancement was observed. The product was water-white. Within ten minutes, and although the ice bath still contained predominantly ice, the viscosity of the reaction product increased rapidly with the product being just capable of flow. After being left at room temperature for a period of 48 hours, the material had solidified. Whilst still a clear transparent material, the amount of"bubble"was substantiaL The product thus obtained was identified to be:- The gel from Example 2 was pyrolysed at 750°C and a mixture, approximately equal in proportions, of black and red grains was produced. Upon spectroscopic analysis, the red product showed the presence of organo-nitrogen species such as CN and CNO, borate clusters BO and B02 and a variety of clusters of the form (BN) XBO2. This is strong evidence to prove the existence of boron-nitrogen containing bonds in the material. Analysis of the black grains showed similar results but in these the borate clusters are more prevalent and the boron-nitrogen clusters were less so. This suggests that the boron-nitrogen containing clusters were more prevalent in the ash at the bottom of the crucible in which the material was pyrolysed. This may be due to the higher temperature in such region.

The reaction of the isocyanate (or thioisocyanate) with the boroxine may be carried out in the presence of an aprotic dilution agent such as tetrahydrofuran. The dilution agent does not take part in the reaction itself but does modify the reaction rate. By so doing, the processability of the polymer obtained can also be modified. Esters are also suitable dilution agents.