The invention relates to a non-linear optically active polyurethane comprising a polymeric main chain and a donor-rr-acceptor sidegroup. Such non-linear optically active polyurethanes are known from EP 0350 112, which discloses non-linear optically active polyurethanes comprising a donor-rr-acceptor group of which the donor group comprises an oxygen atom or nitrogen atom coupled directly to a benzene ring of the rr-acceptor system, with the rr-system being a stilbene group.

When polymeric non-linear optically active material is poled, non¬ linear polarisation will be effected in it under the influence of an external field of force (such as an electric field of force). Non¬ linear electric polarisation may give rise to a number of optically non-linear phenomena, such as frequency doubling and Pockels effect. By utilising these phenomena it is possible to employ this material in optically active waveguiding structures such as optical switches, frequency doublers, etc. in the form of a poled film.

While the stability of poled films made of the present non-linear optically active polymers is excellent at room temperature, it leaves something to be desired at elevated temperature: relaxation results in lower values of the Pockels coefficients (>33 and ri3, in this description it is assumed that r33 = 3 x r- ) . The Pockels coefficient (1 ^* 33) is indicative of the non-linear optical behaviour of the film. The poor thermal stability of poled films of known non-linear optically active polymers gives rise to problems especially when the polymer is briefly heated to 200°-300°C during soldering. Neither are the present non-linear optically active polymers suitable for constant use at elevated operating temperatures in the range of 60° to 120°C. To enhance thermal stability efforts have been made, int. al., to

render non-linear optically active polyacrylates less flexible by omitting the conventional spacers between the main chain and the donor-rr-acceptor sidegroup. However, it was found that such polyacrylates could not be poled.

The present invention has for its object to obviate these drawbacks and provide a non-linear optically active polymer of which the poled film is thermally stable without the polability being negated. To this end the invention consists in that the sidegroup of the non-linear optically active polyurethane comprises a rigid donor group which is also part of the polymeric main chain.

Polyurethanes do not require spacers. When rigid donor groups are employed, these are in effect incorporated into the main chain, thus giving a rigid bond between the donor-rr-acceptor sidechain and the main chain. For further elucidation a schematic depiction is provided in Figure 1.

Figure 1

wherein D represents a donor group, π stands for a rr-system, A is an acceptor group, and H is the main chain of the polyurethane.

The result of this is a higher glass transition temperature (T _g above 170°C) and hence a higher thermal stability also. It was found that such polyurethanes could be poled. Since polable non-linear optically active polyurethanes of such a high T _g were hitherto unknown, the invention also relates to non-linear optically active polyurethanes having a Tg above 170°C. Among the many donor groups enumerated in EP-A2-0358476 is a rigid donor group which may be used in the donor- rr-acceptor sidegroups of a polymer, e.g., in the donor-rr-acceptor sidegroups of polyurethanes. However, this publication makes no mention of the fact that the use of such rigid donor groups will result in non-linear optically active polymers having high T _g s. In fact, this document fails to so much as mention T _g s.

Suitable rigid donor groups include alicyclic groups containing nitrogen or sulphur. These groups were found to render the bond between the donor-rr-acceptor sidegroup and the main chain so rigid as to give a polyurethane of high Tg without the polability of the polymeric material being negated. Examples of such donor groups are shown in formulae 1-5 below, in which FG represents a functional group. These functional groups may be the same or different.

formula 1 formula 2 formula 3

formula 4 formula 5

Pyrrolidine groups (according to formula 2) in which the nitrogen atom is coupled directly to the ---acceptor group and dithiafulvene groups (according to formula 1) in particular were found to be highly suitable for obtaining optically non-linear active polyurethanes of good thermal stability and polability.

It is possible in principle for any rr-acceptor group to be coupled to the donor groups according to the invention. As examples may be mentioned: substituted stilbene groups, such as nitrostilbene groups, cyanostilbene groups, sulphonate stilbene groups, and sulphonyl stilbene groups, substituted azo compounds, such as paranitro azobenzene, cyano azobenzene, sulphonyl azobenzene, and sulphonate azobenzene, substituted benzylidene aniline compounds, such as cyanobenzylidene aniline, nitrobenzylidene aniline, etc.

In general, optically non-linear active polyurethanes are obtained from polymerising a diisocyanate with a diol. The donor-rr-acceptor sidegroup may be present in either the diisocyanate or the diol. The functional groups in the formulae 1-5 will then stand for -N=C=0 and -(CH2) _n -0H (n=0 or 1), respectively. Since hydroxy-functionalised donor-rr-acceptorgroups are easy to prepare, their employment in combination with diisocyanates which do not contain donor-rr-acceptor groups is preferred.

The invention is also addressed to dihydroxy-functionalised donor-rr- acceptor groups comprising a 3,4-dihydroxy-pyrrolidine group. It was found that these diols, which have not been disclosed before, are easy to prepare, while their polymerisation with diisocyanates gives polyurethanes of outstanding non-linear optically active behaviour. Such diols satisfy formula 6 below:

formula 6

wherein X is -CR-=CR--, -N=N-, -CR ^* =N- or -N=CR--,

Y is -CN, -N0 ₂ , CR ^* =C(CN) ₂ , -CF ₃ , -CCN=C(CN) or R- is -halogen, -R ^* , -OR ^* , -COR ^* , COOR ^* , -CN or

-CF ₃ , R- is -H, or an alkyl group having 1-3 carbon atoms, R3 is an alkyl or aryl group having 1-8 carbon atoms, n is an integer from 0 to 4, and the X groups may be the same or different if n is greater than 1.

As suitable diisocyanates may be mentioned: isophorone diisocyanate (IPDI), methylene di (p-phenylene isocyanate) (MDI), methylene di (cyclohexylene-4-isocyanate) , toluene diisocyanate (TDI), hexamethylene diisocyanate (HDI), paraphenylene diisocyanate (PPDI),

and cyclohexylene diisocyanate. Alternatively, it is possible to employ diisocyanate mixtures in the polyurethane. It is preferred to utilise rigid diisocyanates, such as IPDI, MDI, and TDI, since these will give maximum T _g .

After being dissolved in an appropriate solvent the polyurethanes may be applied to a substrate by means of spincoating. Solvents that are suitable satisfy the following requirements: firstly, of course, the polyurethane must be soluble in the solvent. Further, the solvent should effect proper wetting of the substrate. The polymer solution formed must be filterable and, finally, the solvent should have a boiling point above 80°C to ensure that the solvent does not already evaporate during the spincoating process. Solvents satisfying these requirements for a silicon substrate or glass substrate include cyclopentanone and 2-methyl cyclohexanone. After evaporation of the solvent the thus formed film may be poled, for instance using the so-called DC-induced Pockels effect technique. In this process both an AC and a DC voltage are applied to the sample. The DC field orients the molecules and induces the Pockels effect, while the AC field serves to measure the Pockels coefficient. The strength of the DC field ranges from 10 to 30 V/μm.

The invention also relates to a non-linear optically active waveguide comprising a non-linear optically active polyurethane according to the invention.

The invention will be further illustrated with reference to several unlimitative examples, which are submitted solely for a better understanding of the invention.

EXAMPLES

General polymerising method

10 mmoles of diisocyanate (or diisocyanate mixture) were fed to 10 mmoles of diol with donor-rr-acceptor group in 20 ml of dry dimethyl formamide (DMF). The mixture was stirred under nitrogen at room temperature for 30 minutes. The temperature was then slowly increased to 90°C, and on conclusion of the reaction the reaction mixture was diluted with 10 ml of DMF and filtered. The clear solution was precipitated in 300 ml of ethanol . The precipitated polymer was filtered off, washed twice with 100 ml of ethanol being employed each time, and dried.

example 1: polyurethane of dimethylol dithiafulvene nitrostilbene and (IPDI) (polyurethane 1)

synthesis of dimethylol dithiafulvene nitrostilbene (diol 1): cf. diagram 1

Step 1:

To a solution of 7,6 g (100 mmoles) of CS ₂ , 20,8 g (100 mmoles) of compound 1, and 50 ml of Et ₂ 0 were added slowly and dropwise at 0°C 20,2 g of (n-Bu)3P. After cooling to -25°C 14,2 g (100 mmoles) of dimethyl acetylene dicarboxylate (DMAD) were added dropwise. Next, the whole was stirred at 0°C for one hour. After the addition of CH ₂ C1 ₂ the reaction product was filtered through Si0 ₂ , washed with CH ₂ C1 ₂ , and concentrated. The reaction product was purified by Flash column chromatography (Si0 ₂ : ethyl acetate/n-hexane (1/9)). Compound 2 was obtained in a 57 mole% yield.

Step 2:

The pH of a solution of 25 g (61,0 mmoles) of compound 2 in 450 ml of THF and 250 ml of water containing p-TosOH was set at 2, after which the solution was stirred for 16 hours. After 1 1 of water had been added the whole was extracted using ethyl acetate. The organic layer was washed with aHCθ3 solution and brine, dried on MgSO-j, filtered, and concentrated. Recrystallisation from MeOH gave compound 3 in a 75 mole% yield.

Step 3:

To a solution of 6,72 g (20 mmoles) of compound 3 in 7,24 g (40 mmoles) of nitrophenyl acetic acid and 60 ml of DMF were added slowly and dropwise 3,4 g (40 mmoles) of piperidine. After being stirred for 18 hours the reaction mixture was poured in water. The solid was filtered off, washed, and dried. Recrystallisation from CH3CN gave compound 4 in a 86 mole% yield.

Step 4:

To a mixture of 1,00 g (2,2 mmoles) of compound 4, 1,22 g (11,0 mmoles) of CaCl ₂ , 30 ml of THF, and 20 ml of EtOH were slowly added 0,42 g (11,0 mmoles) of NaBH/-.. At the end of the reaction H ₂ 0 was added, and the whole was extracted using ethyl acetate. The organic layer was washed with water and brine, dried on MgSO,-., filtered, and concentrated. Recrystallisation from CH3CN gave diol 1 in a 90 mole% yield.

Step 5:

A solution of 5 g (12,5 mmoles) of compound 5, 10 ml of THF, and 10 ml of AC ₂ 0 was stirred at 100°C for 2 hours. After cooling ethyl acetate was added, and the whole was washed with NH4CL solution and brine, dried on MgSO-., filtered, and concentrated. Recrystallisation from CH3CN gave compound 6 in a 40 mole% yield.

Step 6:

A solution of 15 g (31,1 mmoles) of compound 6, 300 ml of 10%-NaOH solution, and 1000 ml THF was kept at refluxing temperature for one hour. After cooling the layers were separated and the organic layer was washed with brine, dried on MgSO/-., filtered, and concentrated. After agitation with 400 ml of MeOH and filtering off diol 1 was obtained in a 83 mole% yield.

Polyurethane 1 was prepared with isophorone diisocyanate (IPDI) according to the general polymerising method disclosed hereinbefore. For the T _g reference is made to TABLE 1.

example 2: polyurethane of dihydroxypyrrolidine nitrostilbene and (IPDI) (polyurethane 2)

synthesis of dihydroxypyrrolidine nitrostilbene (diol 2):

For the preparation of compound 1 reference is made to J. Am. Chem. Soc. 76 (1954), 3584.

Step 1:

11,4 g (63,7 mmoles) of compound 1, 16,3 g (159,2 mmoles) of acetic anhydride, 60 ml of THF and 2 ml of triethyl amine were kept at refluxing temperature for 18 hours. After concentration by evaporation

200 ml of ethyl acetate were added, and the mixture was neutralised with a saturated NaHCθ3 solution. The layers were separated, and the organic layer was dried on MgSO-.. After filtration and concentration, recrystallisation from THF gave compound 2 in a 95 mole% yield.

Step 2:

8,83 g (57,7 mmoles) of P0C13 were added dropwise to 20 ml of DMF at a temperature below 10°C. Next, the whole was stirred at room temperature for one hour. Subsequently, a solution of 13,8 g (52,5 mmoles) of compound 2 and 12 ml of DMF were added dropwise, and the whole was stirred at 70°C for 2 hours. The reaction mixture, after being cooled down, was poured in ice/water and neutralised with 18,5 g (225 mmoles) of sodium acetate. Next, the whole was extracted with CH ₂ C1 ₂ , and the organic layer was washed with H ₂ 0 and brine, dried on MgSO--., filtered, and concentrated. The solid was recrystallised from MeOH and compound 3 was obtained in an 88 mole% yield.

Step 3:

To 2,12 g (52,8 mmoles) of NaH (60% in oil) a solution of 13,0 g (44,7 mmoles) of compound 3, 12,0 g (44,0 mmoles) of compound 4, and 150 ml of DMF was added slowly and dropwise, with stirring. The mixture was stirred for 18 hours and the poured in 1,51 of water. After being stirred for one hour the mixture was filtered and washed with H ₂ 0. The solid was dissolved in ethyl acetate, washed with a saturated aHCθ3 solution and brine, dried on MgSO-., filtered, and concentrated. After purification by column chromatography (Si0 ₂ /CH ₂ C1 ₂ ) compound 5 was obtained in an 80 mole% yield.

Step 4:

A solution of 14,0 g (36,3 mmoles) of compound 5 in 200 ml of THF was added dropwise to a suspension of 4,32 g (80 mmoles) of NaOMe and 50 ml of MeOH. After one hour of stirring 100 ml of MeOH were added and the reaction mixture was poured in H ₂ 0. The solid was dissolved in ethyl acetate, washed with saturated NaHCθ3 solution and brine, dried on MgSO/-., filtered, and concentrated. After purification by Flash chromatography (Si0 : CH ₂ Cl ₂ /n-hexane (95/5)) diol 2 was obtained in a 90 mole% yield.

Polyurethane 2 was prepared according to the above-described general polymerising method using isophorone diisocyanate (IPDI). The Tg was measured by DSC and was 200°C.

comparison example: polyurethane of 4-di-(2-hydroxyethyl)amino-4'-nitrostilbene and (IPDI)

For the preparation of 4-di-(2-hydroxyethyl)amino-4 ¹ -nitrostilbene reference is made to EP-A1-0350 112. Polyurethane 3* was prepared by means of the above-described conventional polymerising method using this diol and isophorone diisocyanate.

Test samples were made of the prepared optically non-linear active polymers for use in Fabry-Perot experiments (r- _^ 3) and crossed polariser experiments (^-r- ) in transmission. To this end a polyurethane layer provided between two planeparallel , semi- transparent metal electrodes was applied to a glass substrate. Prior to being metallised in a Balzers evaporation chamber, the substrate was cleaned in situ with a glow discharge. The polyurethane was dissolved in cyclopentanone and filtered through a miHipore ^® filter having a pore size of 0,45 μm. The films were prepared by spincoating. After the spincoating process the samples were placed on a hot stage and heated slightly above Tg for 0.5-3 hours.

The polyurethane film 1 and 2 were poled using the DC-induced Pockels effect technique as described hereinbefore at a temperature above the polyurethane's T _g (180°C and 200°C, respectively) for 15 minutes. The freezing in efficiency 1 ₇ (i.e., the ratio of the Pockels coefficient (ri3) for a switched off DC-field after freezing in at room temperature to the Pockels coefficient for the switched on DC-field, ^r 13 field off/ ^r 13 field on) ^ shown in TABLE 1, which also lists the Tg (measured by DSC) and the Pockels coefficients ^33), it being assumed that r33 = 3 x r- . It proved possible to pole the film of polyurethane 3* at 140°C in one minute.

The relaxation measurements were carried out at various temperatures. The temperatures employed for polyurethane 1 and 2 were in the range of 100°to 175°C, for polyurethane 3* they were in the range of 100° to 135°C. These values were plotted in an Arrhenius plot and from them the values of the half-life, i.e., the time which signifies the loss of 50% of the Pockels coefficient, could be calculated. The half-life (t ) is indicative of the film's stability. The results are compiled in TABLE 1.

TABLE I

The data shows that fi lms prepared from the opti cal ly non-l i near active polyurethanes accordi ng to the i nvention have a higher Tg than

the already known optically non-linear active polyurethanes, and can be poled. In addition, the thermal stability of poled films containing polyurethanes according to the invention was found to be much higher at elevated temperatures such as 140°-160°C than that of poled films of already known optically non-linear active polyurethanes. This means that the optically non-linear active polyurethanes according to the invention are highly resistant to momentary heating such as occurs in soldering.

Diagram 1

COOMe

COOMe

SUBSTITUTE SHEET

Diagram 2

SUBSTITUTE SHEET