WO1997003105A1

WO1997003105A1 - Preparation of cyclic oligomers of substituted cyclic ethers

Info

Publication number: WO1997003105A1
Application number: PCT/GB1996/001599
Authority: WO
Inventors: Hemant Jashbhai Desai
Original assignee: The Secretary Of State For Defence
Priority date: 1995-07-10
Filing date: 1996-07-05
Publication date: 1997-01-30
Also published as: GB2303130A; GB9514018D0

Abstract

A method of producing cyclic oligomers of substituted oxiranes and oxetanes involves the slow addition to a solution of the ether in a relatively polar organic solvent such as dichloromethane or dichloroethane, of a stable protonic acid such as HBF4 or a cationogenic species for example triethyloxonium tetrafluoroborate or an alcohol in conjunction with a stable Lewis acid such as boron trifluoride etherate. The cyclic oligomer product is isolated from other reaction products, in particular the straight chain oligomers, by extraction with a non-polar solvent, hexane being preferred. The cyclic products are effective plasticisers for polyether polymers, particularly where the cyclic ether is related to or even the same as the ether in the polymer since compatability of the materials is then very likely and separation out of plasticiser unlikely to occur.

Description

PREPARATION OF CYCLIC OLIGOMERS OF SUBSTITUTED

CYCLIC ETHERS

The present invention relates to a process for the preparation of cyclic oligomers of substituted oxiranes and oxetanes and in particular, nitrato- substituted oxiranes and oxetanes, which process is susceptable to industrial application. By the term "cyclic oligomers" is meant cyclic polymers having upto 9 repeat units, though the preferred products are cyclic trimers and tetramers.

Various research workers have investigated the generation of cyclic oligomers in the cationic polymerisation of oxiranes and oxetanes in particular. Bucquoye and Goethals (Makromol. Chem. 179, 1681 (1978)), for example, using 3,3-dimethyloxetane, produced cyclic species from trimer to nonamer and found that the oligomer: polymer ratio was dependent on the initiating (catalyst) system used and also that the ratio increased with decreasing monomer concentration. Of various initiators tried, triethyloxonium tetrafluoroborate provided the highest yield of cyclic oligomer (17.2% of tetramer and 1.1% of pentamer with higher oligomers forming a minor proportion only). Similar results were obtained with unsubstituted oxetane. In each case the initiator and monomer were mixed together instantaneously and the concentrations of cyclic oligomers, higher polymers and unreacted monomer followed over periods of upto 2 hours.

Dale and Fredriksen (Acta Chemica Scandinavica 45 (1991) 82) examined cyclic oligomer formation with oxetane, 3-methoxymethyl-3-methyloxetane and 3-halomethyl-3- methyloxetane with a variety of catalysts and solvents. With oxetane as the monomer, they found that they were able to obtain yields of cyclics of upto 75% with a preponderance ofthe cyclic trimer by using BF₃ catalyst and by simultaneously adding a catalyst solution and a dilute (0.05M) solution of monomer to a large volume of dichloromethane. Using PF₅ as catalyst, the cyclic tetramer was produced in preference but with a rather lower overall yield ofthe cyclic species. Interestingly, when trifluoromethylsulphonic acid was used as the catalyst the yield of cyclics fell to under 15%.

With 3,3-dimethyloxetane as the monomer, however, the yields of cyclics were generally lower, more polymer being produced (with BF ₃) though a good yield (86%) of the cyclic tetramer was obtained using PF₅ as the catalyst. With 3-chloromethyl-3- methyloxetane, 3-bromomethyl-3-methyloxetane and 3-methoxymethyl-3-methyloxetane on the other hand , very low or almost non-existent yields of cyclic species were obtained with either catalyst. The polymerisation method with substituted oxetanes involved the addition of catalyst solution to the monomer dissolved in dichloromethane over periods of from 1 to 24 days. With oxetane as the monomer, a method with parallel addition of monomer solution and catalyst was used, the addition taking 11 hours followed by stirring until all ofthe monomer was consumed (24 hours).

Dale (Tetrahedron. 49, 39, 8707 (1993)) has discussed the contrasting behaviour of oxirane and oxetane in cationic cyclooligomerisation and polymerisation. For oxirane, typically, using BF₅, PF₅ or SbF₅ as the catalyst, a mixture of cyclic oligomers alone can be obtained though the major component is dioxane (40%). with around 15% ofthe tetramer. Using BF₃ etherate as catalyst, on the other hand, led to a mixture of cyclic and non-cyclic species, while other Lewis acids (A1C1₃, FeCl_3> p-toluenesulphonic acid, etc.) either did not work at all or gave other, non-oligomeric, products.

Dale has pointed out that the differing behaviour of oxirane and oxetane may, at least in part, be dependent on the differences in basicity such that the order of basicity is: oxetane > tetrahydofuran > tetrahydropyran » oxirane, with the basicity ofthe ether oxygen in open chains and larger rings being close to that of tetrahydropyran. Consequently, the polymeric or macrocyclic products formed from oxirane will tend to intercept the growing chain and stop chain growth, allowing other reactions, such as degradation to dioxane, to proceed. By contrast, the products formed from oxetane will not be able to compete with the more basic monomer and chain growth and/or cyclisation will continue. It will be appreciated that the effect of electron- withdrawing substituents such as nitro- and nitrato- groups will be to reduce the basicity ofthe cyclic ether concerned which would be expected to result in generally lower yields of cyclic oligomers.

It is an object ofthe present invention therefore, to provide a method for the production of cyclic oligomers (particularly trimers and tetramers) derived from substituted oxiranes and oxetanes, wherein substantially high yields are obtained but without the use of excessively large volumes of organic solvent which would be inappropriate to an industrial production process. It is a further object ofthe invention to avoid the use of those toxic and/or volatile catalysts (such as BF₃ and PF5 ) which have been used in the previous preparative methods giving the most advantageous yields of cyclic oligomers.

In a first aspect ofthe invention, therefore, there is provided a process for the production of cyclic oligomers of substituted oxiranes and oxetanes by cationic polymerisation thereof, comprising the steps of:

a) slowly adding a solution of a substituted oxirane or oxetane dissolved in a relatively polar, aprotic organic solvent to a solution in a solvent ofthe same description of a polymerisation initiating catalyst selected from a stable protonic acid or a cationogen optionally in combination with a stable Lewis acid;

b) allowing the oligomerisation reaction to proceed substantially to completion;

c) neutralising, as required, any acid remaining in the reaction mixture;

d) separating the organic fraction and evaporating off the organic solvent(s) therefrom; and e) extracting the cyclic oligomeric products from the remnant organic fraction with a non-polar solvent.

The period of addition ofthe cyclic ether is conveniently from 1 to 24 hours, preferably at least 10 hours. Following the addition ofthe cyclic ether to the catalyst, the reaction mixture should be allowed to stand for a period of several, typically 2 to 6 hours, to enable the oligomerisation reaction to proceed to substantial completion.

The solution ofthe cyclic ether is preferably a relatively dilute solution in order to encourage the production of cyclic oligomers rather than the corresponding straight chain species. Typically, the concentration ofthe monomer solution should be ofthe order of from 0.5 to 10% (w/v) and preferably from 1 to 5% (w/v).

Where it is intended that the oligomeric products ofthe process ofthe present invention are to be used as plasticisers, it is advantageous that the degree of polymerisation ofthe cyclic ether starting material should be between 2 and 6, most preferably 4. To achieve such a degree of polymerisation, a ratio of 4 moles of monomer to 1 mole of catalyst should be used in the reaction process.

The distribution of oligomeric species obtained by the present process may also be influenced by the introduction into the initiator solution of a metal salt such as lithium or sodium tetrafluoroborate or lithium, sodium or potassium chloride. The essential requirement for the metal salt is that it should give rise to stable cations which are capable of forming complexes with the cyclic species: satisfactory salts for this purpose (in addition to those indicated above) will be readily apparent to the skilled reader.

The reaction may be carried out at any suitable temperature having regard to the stabilities ofthe various reactants; temperatures in the range of 0 to 70°C, preferably from 10 to 50°C are convenient. At lower temperatures proportionately greater amounts ofthe higher oligomers will be produced and at higher temperatures the stability ofthe reactants and/or oligomeric products may be adversely affected.

Conveniently, the organic solvents used for the solutions ofthe cyclic ether monomer and ofthe initiating catalyst will be the same solvent. Such solvents are exemplified by dichloromethane, dichloroethane and dioxane; more strongly polar solvents such as dimethylformamide, dimethylsulphoxide, methanol or ethanol are not suitable for the present process.

Suitable non-polar solvents for use in extracting the cyclic species from the other reaction products, which may principally include linear oligomers and possibly higher polymeric species, include carbon tetrachloride, cyclohexane, methylcyclohexane and diethyl ether, but the preferred solvent on account of its cheapness and relative volatility which enables its easy removal from the oligomer product after the extraction stage, is hexane.

The initiator (catalyst) used may be a stable protonic acid such as hydrogen tetrafluoroborate (or the etherate thereof) or hydrogen hexafluoroantimonate. Suitable cationogens include trimethyloxonium or triethyloxonium tetrafluoroborate, triethyloxonium hexafluorophosphate, trimethyloxonium or triethyloxonium hexachloroantimonate or trifluoromethyl sulphonic acid. Alternatively, the initiator may comprise a combination of a cationogen such as an alcohol, thiol or an alkyl or aryl halide, with a stable Lewis acid, for example, borontrifluoride etherate, antimony pentafluoride or antimony pentachloride.

The substituted cyclic ether monomer is a substituted oxirane or oxetane such as 3,3-dimethyloxetane, 3,3-bisazidomethyloxetane or 3-azidomethyl-3-methyloxetane, and particularly a nitrato-substituted oxirane or oxetane such as glycidyl nitrate or 3- nitratomethyl-3-methyloxetane. According to a further aspect ofthe invention there are provided novel cyclic oligomers of substituted oxiranes and oxetanes having the formula:

m

wherein n is 0 or 1, m is an integer having a value from 2 to 9 and, where n = 0, Ri is hydrogen and R₂ is nitratomethyl and where n = 1, Ri and R₂ comprise any combination of methyl, halomethyl, azidomethyl or nitratomethyl except that one of Ri and R₂ is azidomethyl or nitratomethyl . Particularly useful oligomers are those according to the above formula in which m is 3 or 4.

The cyclic oligomers ofthe present invention are generally useful as plasticisers, in particular for related polymers such as poly(glycidyl nitrate) and poly(3-nitratomethyl-3- methyloxetane) which may be produced as described in UK patents nos. 2 249 314 and 2 248 623 respectively, both in the name ofthe present applicant. Polyethers in general are an important class of polymers having applications as detergents, absorbents, disinfectants and elastomer prepolymers, for example. The cyclic oligomers provided according to the present invention are especially useful in the formulation of elastomeric materials where, on account of their close chemical similarity to the polymeric constituents of those materials, a high degree of compatibility therewith is to be expected. In such cases the oligomers will provide a strong plasticising effect and thereby obviate or at least greatly reduce the tendency of chemically unrelated species to separate out from the polymer with time. According to a further aspect ofthe invention therefore, there is provided a polyether polymeric material which contains, in a minor proportion, a cyclic oligomer of a substituted cyclic ether having the formula:

wherein Ri, R₂, n and m are as previously defined. The minor proportion ofthe cyclic oligomer will typically be ofthe order of 5 to 50% by weight ofthe composition, depending upon the degree of polymerisation required.

The invention will now be further described with reference to the following examples and to the accompanying drawings in which:

Figures 1 to 6 represent size exclusion chromatograms for the reaction products prepared in Examples 1 to 6, respectively, and

Figure 7 is a graph demonstrating the relationship between the glass transition temperature for poly (3-nitratomethyl-3-methyl oxetane) to which a plasticising amount of cyclic oligomer has been added.

Example 1

A 2% (w/v) solution of 3 -nitratomethyl-3 -methyl oxetane (NIMMO, lOg, 0.068 mol) in 450 cm³ of dichloromethane was added continuously to a jacketed vessel containing tetrafluoroboric acid-diethyl ether complex (2.8g 9 0.017 mol) in 50 cm³ of distilled dichloromethane over a period of 16 hours. The reaction was stirred throughout the addition period and for a further four hours thereafter. The reaction mixture was kept at a temperature of 20°C and under a dry N₂ atmosphere. The mixture was neutralised using 5% aqueous sodium bicarbonate, the organic layer collected and the solvent evaporated to leave the oligomers. The size exclusion chromatogram ofthe product at that stage (labelled HDN117CR) is shown in Figure 1 and clearly shows the presence of the main oligomer peak at a retention volume of 31.0 cm³ but also small amounts of oligomers at retention volumes 33.8 cm³ and 34.2 cm³, probably corresponding tb trimer and dimer repectively. Also noteworthy is that there is a higher molecular weight fraction which begins to elute at a retention volume of 27cm³.

To effect separation ofthe cyclic oligomers from the mixture obtained in the reaction process, the mixture (which was a pale yellow liquid) was added dropwise to a beaker containing 500 cm³ of rapidly stirring n-hexane. The stirring is continued for another hour after addition. The mixture is then allowed to stand for 2 hours after which the hexane is decanted into a round bottom flask leaving the insoluble fraction ofthe linear oligomers in the beaker.

Evaporation ofthe hexane yields 3g of cyclic species. This product was subject to size exclusion chromatography and the results are shown in Figure 1 (sample labelled HDN117C1). The main oligomer appears again at a retention volume of 31.0 cm³ but it will be seen that the higher molecular weight fraction has now been much reduced.

Analysis by ^!H and ¹³C n.m.r indicate the presence of cyclic tetramer along with some linear oligomer. This is confirmed by mass spectroscopy which shows a molecular ion peak at a mass of 589 corresponding to the cyclic tetramer,

Interestingly this sample, upon standing over a period of some months, solidified and preliminary x-ray diffraction results suggest that the solid is crystalline. It is probably formed by gradual crystallisation out ofthe liquor consisting of other oligomers, and residual solvent, probably upon evaporation of volatile species.

Example 2

The procedure as in Example 1 was carried out but using triethyloxonium tetrafluoroborate (17cm³, 0.017mol, in 50cm³ dichloromethane) as the initiator. 450cm³ of 2% (w/v) solution of NIMMO was added over a period of 16 hours at a temperature of 20°C, followed by stirring for a further 4 hours to complete the reaction. lOg of crude product was recovered which, after hexane extraction, yielded 3.3g (33%) of cyclic oligomer.

The crude product before hexane extraction was subject to size exclusion chromatography with the result shown at Figure 2 (trace labelled HDN115CR). It will be seen that, as with the crude product of Example 1, there is a peak corresponding to a cyclic oligomer product showing that the use of triethyloxonium tetrafluoroborate as the cationogen also leads to enhanced formation of this product. The initiatory species is in this case the ethyl carbocation which suggests that the process is amenable to other electrophilic initiators. The presence of a substantial shoulder representing a higher molecular weight fraction may be noted as well as some peaks (partly hidden) corresponding to smaller cyclic oligomers.

Example 3

An identical procedure to that of Example 1 was followed but using a mixture of 1,4-butanediol (0.94g , O.OlOmol) and boron trifluoride etherate (3.17g , 0.022mol) as the initiator. These were respectively added to 50 cm³ of distilled dichloromethane and a 2% (w/v) solution of NIMMO in 450cm³ of dichloromethane was then added over a period of 16 hours. The mixture was then stirred for a further 4 hours, neutralised with 5% aqueous sodium bicarbonate and the organic layer separated off. After evaporating off the solvent 9.39g of crude oligomer was obtained which after hexane extraction, gave 3. OOg (32%) of cyclic oligomer.

A size exclusion chromatogram for the crude sample (designated HDN118CR) is shown in Figure 3. This demonstrates essentially the same phenomena as the previous examples ie. that there is again preferential formation ofthe cyclic species with these reagents and under dilute monomer and slow feed rate conditions. The alcohol is in this case the source ofthe initiating cations.

Example 4

100cm³ of a 20% (w/v) solution of NTMMO was added to a solution of HBF₄ OEt₂ (7.01g, 0.043mol) in 20cm⁵ of dichloromethane over a period of 16 hours. The mixture was held at a temperature of 20°C and was stirred for a further 4 hours after the addition was completed. After neutralisation with 5% aqueous sodium bicarbonate, separation ofthe organic layer and evaporation ofthe solvent, 19.5g of crude oligomer was obtained.

A size exclusion chromatogram for the crude sample (HDN11 ICR) is shown at Figure 4 from which it will be seen that the yield of tetramer is less than previously, with a more undifferentiated mixture of products resulting. This may be attributed to the use of a higher concentration ofthe monomer solution.

Example 5

This example illustrates the effect of a templating salt on the molecular weight distribution. The same method of preparation as used in Example 1 was employed, except that lithium tetrafluoroborate (3.2 g, 0.034 mol) was added to the initiator mixture prior to commencement of monomer feeding. Figure 5 (sample HDN120CR) shows the s.e.c. for this crude sample and the trace is very similar to that of Example 1 (Fig. 1) except that the peak at retention volume 34.2 cm³ (dimer) has been enhanced relative to the peak at 33.8 cm³ (trimer) and both of these are enhanced relative to the main (tetramer) peak. Also, the small "hump" observed at the retention volume of 32.0 cm³ in Example 1 has disappeared altogether. Thus although LiBF₄ does not appear to have a significant effect in enhancing cyclic(s) formation relative to linear oligomer formation, possibly due to the poor solubility ofthe salt in dichloromethane, it does have some effect on the distribution ofthe oligomeric species relative to each other. Specifically, it appears to enhance the amounts ofthe lower oligomer (dimer or trimer) which are produced. The small amount of Li ion which may have dissolved is probably causing the enhancement ofthe peak at the retention volume of 34.2 cm³.

Example 6

The procedure of Example 5 was repeated except that 100cm³of a 20% (w/v) solution of NTMMO was used, the quantity of lithium tetrafluoroborate used was increased to 6.374g (0.068 mol) and the amount of HBF etherate used was 7.41g (0.045 mol). Also the reaction temperature was maintained at 30°C.

The yield of crude product was 12.3g. After a single hexane wash 4. lg of oligomeric product was isolated. The size exclusion chromatogram ofthe crude product is shown in Figure 6 (referenced HDN108CR) and as with Example 4 the product is more of a mixture of linear and cyclic oligomers due to the use of higher monomer concentrations as well as, probably, the use of a higher temperature. Example 7

Using polyNIMMO prepared by the method described in applicant's UK Patent No.2 248 623 as the homopolymer, about 0.59 to lg of polymer was placed in each of several vials. Various amounts ofthe cyclic oligomer product of Example 1 were added to the vials and mixed using a teat pipette. The vials were allowed to stand overnight and the mixtures then analysed by differential scanning calorimetry (DSC).

A graph of glass transition values against percentage of cyclic species by weight was plotted (Figure 7). This shows that increasing the amount of cyclic species reduces the T_g and clearly demonstrates that the cyclic oligomer is acting as a plasticiser for the polyether polymer.

Claims

1. A process for the production of cyclic oligomers of substituted oxiranes and oxetanes by cationic polymerisation thereof, comprising the steps of:

a) slowly adding a solution of a substituted oxirane or oxetane monomer dissolved in a relatively polar, aprotic organic solvent to a solution-in a solvent ofthe same description of a polymerisation initiating catalyst selected from a stable protonic acid or a cationogen, optionally in combination with a stable Lewis acid;

c) neutralising, as required, any acid remaining in the reaction mixture;

d) separating the organic fraction and evaporating off the organic solvent(s) therefrom; and

e) extracting the cyclic oligomeric products from the remnant organic fraction with a non-polar solvent.

2. A process according to claim 1 characterised in that the time of addition ofthe cyclic ether in step (a) is from 1 to 24 hours.

3. A process according to claim 2 characterised in that the time of addition is at least hours.

4. A process according to any of claims 1 to 3 characterised in that the concentration ofthe cyclic ether monomer solution is from 0.5 to 10% w/v.

5. A process according to claim 4 characterised in that the concentration ofthe monomer is from 1 to 5% w/v.

6. A process according to any ofthe preceding claims characterised in that it is carried out at a temperature in the range of from 0 to 70°C.

7. A process according to any ofthe preceding claims characterised in that the relatively polar, aprotic solvent is selected from dichloromethane, dichloroethane and dioxane.

8. A process according to any ofthe preceding claims characterised in that the non¬ polar solvent used in step (e) is selected from hexane, cyclohexane, methylcyclohexane, carbon tetrachloride and diethyl ether.

9. A process according to any ofthe preceding claims characterised in that the stable protonic acid catalyst is hydrogen tetrafluoroborate or hydrogen hexafluoroantimonate.

10. A process according to any of claims 1 to 9 characterised in that the cationogenic catalyst when used alone is selected from trimethyloxonium tetrafluoroborate, triethyloxonium tetrafluoroborate, trimethyloxonium hexachloroantimoate, triethyloxonium hexachloroantimonate, triethyloxonium hexafluorophosphate or trifluoromethyl sulphonic acid.

11. A process according to any of claims 1 to 9 characterised in that the cationogenic catalyst when used in conjunction with a stable Lewis acid is selected from an alcohol, a thiol or an alkyl or aryl halide.

12. A process according to claim 11 characterised in that the stable Lewis acid is selected from borontrifluoride etherate, antimony pentafluoride or antimony pentachloride.

13. A process according to any ofthe preceding claims characterised in that the substituted oxirane or oxetane is a nitrato-substituted oxirane or oxetane.

14. A process as hereinbefore described and with particular reference to the Examples.

15. A cyclic oligomer derived from a substituted oxirane or oxetane monomer and having the formula:

wherein n is 0 or 1, m is an integer having a value of from 1 to 9 and where n = 0, Ri is hydrogen and R₂ is nitratomethyl and where n = 1, Ri and R₂ are selected from any combination of methyl, halomethyl, azidomethyl and nitratomethyl except that one of Ri and R₂ is azidomethyl or nitratomethyl.

16. A cyclic oligomer as claimed in claim 15 wherein m is 3 or 4.

17. A polyether polymeric material containing from 5 to 50% by weight of a cyclic oligomer according to claim 15 or claim 16 as a plasticiser.

18. Poly (3-nitratomethyl-3-rnethyloxetane) containing from 5 to 50% by weight of a cyclic oligomer according to claim 15 or claim 16 in which n is 1, Ri is methyl and R₂ is nitratomethyl, as a plasticiser.

19. Poly (glycidyl nitrate) containing from 5 to 50% by weight of a cyclic oligomer according to claim 15 or claim 16 in which n is 0, Ri is hydrogen and R₂ is nitratomethyl, as a plasticiser.