GB2133406A - Cholesteric liquid crystal compounds useful for preparing polymeric films - Google Patents

Abstract

New compounds of formula IX are monomeric liquid crystals which are photopolymerizable to polymeric films having fixed optical characteristics <IMAGE> R1 is H or CH3; A is -R2-, -R3O-, or -R4O-; R2 is alkylene having 3-14 methylene or alkyl- substituted methylene groups; R3 is alkylene having 2-14 methylene or alkyl-substituted methylene groups; R4 is an alkylene ether, diether or triether having a total of 3-14 carbon atoms in the alkylene linkages, provided that the alkylene linkage adjacent the carbonate moiety has at least two carbon atoms; the two Hy's are both H or together form an additional 5,6-bond, with the proviso that when R1 is CH3 and A is -R2- then the Hy's are both H.

Description

SPECIFICATION Liquid crystalline materials useful to prepare polymeric films The present invention relates to liquid crystals and more particularly to monomeric liquid crystals which are useful to prepare polymeric liquid crystalline materials.

The existence of liquid crystalline materials has been recognized since the late 1800's. The terms "liquid crystal" and "mesogen" refer to a number of states of matter which lie between solid crystals and isotropic liquids, the latter being randomly ordered. Liquid crystalline materials possess some structural characteristics of crystals yet they may be viscous or quite mobile liquids.

The varying degrees of order which are possessed by liquid crystals give rise to three distinct types of structures called mesophases. A liquid crystal, when in the crystalline state, has a threedimensional uniform structure with orientational and positional order. As the crystal is heated, it may initially lose one dimension of its positional order. This is referred to as the smectic mesophase, a phase in which the liquid crystal retains the orientational order of the crystailine state, as well as twodirectional positional order.

With further heating, the liquid crystal can convert to the nematic mesophase. In this phase, the remaining positional order is lost and the liquid crystalline material retains only the one-directional orientational order of the crystalline state. The molecular order of nematic mesophases is characterized by orientation of the molecules along an axis which coincides with the long axis of the molecules. The centers of gravity of the molecules are arranged randomly so that no positional long-range order exists.

In the cholesteric mesophase, the molecular order is characterized by orientation of the molecules along an axis which coincides with the long molecular axis as in a nematic phase; however, the axis changes direction in a continuous manner along a second axis perpendicular to the first. For this reason, cholesteric mesophases are often referred to as twisted nematic mesophases. Optical activity is necessary for a mesogenic material to form a cholesteric mesophase.

The term "cholesteric" is primarily of historical significance because the first-discovered liquid crystalline material which exhibited a cholesteric mesophase was cholesteryl benzoate. It has long been recognized, however, that the presence of the cholesterol moiety is not required, and that noncholesterol derivatives may also exhibit a cholesteric mesophase.

Substantial interest has been shown in liquid crystalline materials which exhibit cholesteric mesophases because these materials exhibit unique optical properties such as selective reflection of visible light to produce iridescent colors, as well as circular dichroism. Thus, for example, U.S. Patent 3,720,623 discloses mixtures of cholesteric and nematic liquid crystals which are useful in temperature-sensitive visual displays; U.S. Patent 3,766,061 discloses decorative films comprising solid materials which are proportioned such that the composition demonstrates cholesteric properties; U.S. Patent 3,923,685 discloses cholesteric materials which convert to the nematic state upon exposure to an electric field; and U.S.Patent 3,931,041 discloses combinations of nematic and potentially cholesteric material which are useful in imaging and display devices.

Although the colored images produced using cholesteric material are quite useful, most such images are not permanent. Accordingly, there has been substantial interest in preparing cholesteric materials in which the color can be fixed. Thus, U.S. Patent 3,766,061, which was referred to above, discloses decorative films in which the color is fixed by cooling. In addition, U.S. Patent 4,239,435 discloses a polymeric liquid crystal in which the color is fixed by lowering the temperature of the polymer below the glass transition temperature, thereby fixing the polymer in the solid state.

The use of temperature changes to fix the color is not always practical, however, and there has been interest in developing cholesteric materials whose color can be fixed by other means, such as by photopolymerization, whereby the resulting fixed color is temperature insensitive. The applicants are aware of only one such polymer. This was reported by a group of Japanese workers who disclosed that poly(gamma-butyl-L-glutamate) in trimethylene glycol dimethacrylate could be photopolymerized to fix the color such that it was temperature insensitive.

Accordingly, one objective of the present invention is to provide monomeric cholesteric liquid crystalline materials which are useful to prepare polymeric films having fixed, essentially temperatureinsensitive colors.

Yet another objective of the present invention is to provide monomeric compounds which may be used in combination with other mesogenic materials to provide compositions which exhibit variable optical responses over a variety of temperature ranges.

These and other objectives of the present invention will become apparent from the detailed description of preferred embodiments which follow.

The present invention concerns novel cholesteric liquid crystalline monomers which are useful to form polymeric materials having unique optical properties.

In one embodiment the present invention comprises photopolymerizable monomers which are useful to prepare polymeric films, said monomers having the structure

where R,=H or CH3, R2=alkylene chain having from 3-1 4 methylene or lower alky!-substituted methylene groups, and y=O or 1, provided that when R1=CH3, y=1.

In a second embodiment, the present invention comprises photopolymerizable monomers which are useful to prepare polymeric films, said monomers having the structure

where R1=H or CH3, A=-R3O- or-R4O-, R3=an alkylene chain having from 2-14 methylene or lower alkyl-substituted methylene groups, R4=an alkylene ether, diether or triether having a total of from 3-1 4 carbon atoms in the alkylene linkages, which may be branched or unbranched, any branch containing up to 4 carbon atoms, provided that the terminal alkylene linkage adjacent to the carbonate moiety comprises not less than two carbon atoms: and y=O or 1.

In a third embodiment, the present invention comprises compounds which are useful as intermediates to prepare photopolymerizable monomers, said compounds having the structure

where X,=Br, l or a sulfonic acid ester, R2=an alkylene chain having from 3-14 methylene or lower alkyl-substituted methylene groups, and y=O or 1.

The cholesterol derivatives which may be used to practice the present invention are cholesterol (where y=O) and 5,6-dihydrochlolesterol (where y=1). In addition, a number of options are available in the 3-position side chain. Thus, the polymerizable moiety of the side chain can comprise an acrylate or methacrylate moiety which is bridged to an ester or carbonate linkage. Where an ester linkage is present, the bridge will comprise an alkyl chain comprising from 3-14 methylene or lower alkylsubstituted methylene groups. Lower alkyl as used herein means an alkyl group comprising from 1-4 carbon atoms. The methacrylate esters, where R, and CH3 and n (where n represents the number of carbon atoms in the methylene chain)=5,10 and 14, have been reported in the Russian literature; however, these esters were prepared for use in solution polymerization reactions and there was no appreciation of their utility for preparing photopolymerized films as disclosed herein.

On the other hand, where a carbonate linkage is present, the bridge may be more complex. Thus, it may comprise from 2-14 methylene or lower alkyl-substituted methylene groups, or an alkylene ether, diether ortriether having a total of from 3-14 carbon atoms in the alkylene linkages, the alkylene linkages being branched or unbranched, any branch containing up to 4 carbon atoms, provided that the terminal alkylene linkage adjacent to the carbonate moiety comprises not less than two carbon atoms. Examples of ether moieties which may be utilized in practicing the present invention are those which are analogous to ethylene glycol, diethylene glycol, triethylene glycol, tetramethylene glycol. 3,3'-oxybis-l -propanol, 4,4'-oxybis-l -butanol, and 1-1 '-oxybis-2-propanol.

When in the pure state the above-described compounds are somewhat difficult to work with because they tend to crystallize inopportunely. Furthermore, it is difficult to obtain colored polymers from the pure monomers because the majority of them will show either no colored cholesteric mesophase, or a very narrow colored cholesteric mesophase. Therefore, the pure compounds of the present invention are limited in their ability to produce polymeric films having desirable optical responses.

Surprisingly, it has been discovered that these limitations may be overcome and that colored and uncolored films comprising compounds of the present invention can be prepared and photopolymerized in the presence of a suitable photoinitiator, thereby giving films having fixed optical characteristics. If the film is colored, the fixed co!or will preferably be substantially the same as the color of the unpolymerized film; however, in certain instances, it may be desirable to obtain a polymerized film having a fixed color which differs from that of the unpolymerized film. Thus, all such possibilities are contemplated by the present invention. Details relating to these polymeric films and their preparation are set forth in our copending application No. 83-33324, the contents of which are hereby incorporated by reference.

The cholesteryl ester derivatives of the present invention may be prepared in surprisingly good yield by reacting an w-substituted-alkyl acid halide (I), which preferably is a w-bromoalkyl acid chloride, with the desired cholesterol derivative (II) to produce a cholesteryl c)-substituted-alkyl ester (III). Reaction of this compound with an alkali metal acrylate or methacrylate (IV) in the presence of a radical inhibitor and an appropriate catalyst then yields the desired acrylate or methacrylate ester (V).

This sequence may be visualized by reference to the following general reaction:

In these and other reactions illustrated herein, R=H or CH3; R2=an alkylene chain having from 3-14 methylene or lower alkyl substituted methylene groups; y=O or 1; X 1=Br, I or a sulfonic acid ester; X2=Br or Cl; M=Na or K. Examples of suitable sulfonic acid esters are CH3SO-; p-CH3C6H4SO3-; C6H5SO-; and p-BrC6H4SO3-. Preferably Xa will be Br and X2 will be Cl. Further, for intermediates of type III, R2 will preferably have from 4 to 14 carbon atoms in the alkyiene chain.

The choiesteryl carbonate derivatives (IX) may be prepared in good yield by a somewhat different route. In this reaction sequence, a haloalcohol (Vl) is reacted with an appropriate cholesteryl haloformate (VII) to yield an co-haloalkylcarbonate derivative (VIII). This compound is then reacted with an alkali metal acrylate or methacrylate (IV) as previously described to yield the carbonate monomer (IX). Alternatively, compounds of type IX may be prepared by reacting a compound of type VII with a hydroxyalkyl acrylate or methacrylate (X).A general reaction sequence illustrating these alternatives is illustrated below:

where R1, X1, X2 and y are as described above and A=-R3O- or -R4O- where R3=an alkylene chain having from 2-1 4 methylene or lower alkyl-substituted methylene groups, and R4=an alkylene ether, diether or triether having a total of from 3-14 carbon atoms in the alkylene linkages, which may be branched or unbranched, any branch having up to 4 carbon atoms, provided that the terminal alkylene linkage adjacent to the carbonate moiety comprises not less than two carbon atoms. Examples of suitable ether moieties were referred to above; however, the restrictions on the length of one terminal linkage must be emphasized.The terminal linkage in question is the alkylene group adjacent to the carbonate moiety in product IX. Neither synthetic route illustrated herein is amenable to the use of an ether having single carbon atom adjacent this reaction site. Thus, compounds of type Vl and type X may not comprise a -O-CH2-OH moiety. Accordingly, the terminal alkylene linkage adjacent to carbonate moiety must comprise at least two carbon atoms.

The advantages and attributes of the present invention will become more apparent from the following examples which are intended to illustrate but not to limit the scope of the present invention.

Example 1 The following example will illustrate the preparation of the cholesteryl w-substituted-alkyl esters of type III. A 0.1 mol quantity of cholesterol or 5,6-dihydro-cholesterol, 0.12 mol of pyridine and 0.12 mol of ov-bromoalkanoyl chloride is dissolved in 200-300 ml of a suitable solvent such as ethanolfree choroform or ether/dichloromethane. The mixture is stirred at OOC for 2 hours and at ambient temperature for 1 6 hours, and is then diluted with 300 ml of solvent. The organic phase is washed with two 200-ml portions of 1 N hydrochloric acid and then with water, after which it is dried over magnesium sulphate.Upon concentration the dried solution, the esters (lit) are obtained and purified by recrystallization from a suitable solvent such as 1:1 ether-ethanol. Representative compounds are illustrated in the following table:

Compound n y Yield {%) MP ( CJ Illa 10 0 89 99-100 Illb 5 0 86 120-121 Illc 3 0 86 87-90 solid W 3 1 91 92-93 Ille 10 1 87 65-67 Example 2 This example will illustrate the preparation of cholesteryl ester monomers of type V.A biphasic solution comprising 0.15 mol of potassium acrylate or methacrylate, 0.05 mol of ester 111, 0.01 mol of N,N,N,N,-tetra-n-butylammonium bromide and 0.0034 mol of 2,6-di-t-butylcresol radical inhibitor is prepared in a mixture of water (30 ml) and chloroform (1 5 ml). The biphasic solution is heated and stirred magnetically for 40 hours in an oil bath maintained at 1 10--1 1 50C. After cooling the mixture is diluted with 500 ml of a 4:1 solution of ether and dich!oromethane, and the organic phase is separated and washed twice with water. After being dried over magnesium sulphate, the organic phase is concentrated to yield the acrylate or methacrylate monomer (V) which is recrystallized from a suitable solvent, such as ether-ethanol or acetone-ethanol. Representative monomers are illustrated in the following table:

Yield Melting or mesophase Compound R1 n y {%) range (OC/ Va H 10 0 83 *54.571.5 Vb CH3 10 0 88 *58 -64 Ve H 5 0 87 *45.568.5 Vd' CH3 5 0 90 *48 -57.5 Ve H 3 0 83 68.5-70.5 (67.5) Vf CH3 3 0 75 73 -74 (56.0) Vg H 3 1 58 41 -43 (35.5) Vh CH3 3 1 71 43 -45 (Below RT) Vi H 10 1 70 62.5-64.5 (58.0) Vj CH3 10 1 56 *33.749.0 As used in this example and hereafter, the temperature ranges are melting ranges unless otherwise indicated by an asterisk (*) or by parentheses.An asterisk signifies that the range is a mesophase range whereas parentheses indicated that the range is a monotropic mesophase range, the latter being measured as the temperature is decreased. With materials that have ascertainable melting points, the monotropid mesophase range is often below the melting range.

Example 3 This example will illustrate the preparation of cholesteryl ester monomers of type V using a polar aprotic solvent system rather than phase transfer catalyzed conditions as set forth in Example 2.A mixture comprising 0.05 mol of bromine-substituted ester 111,0.1 mol of potassium acrylate, and 0.0034 mol of 2,6-di-t-butylcresol radical inhibitor is prepared in dimethylformaide (50 ml). The mixture is heated with stirring for 3 hours in an oil bath maintained at 70--800C. After cooling, the mixture is diluted with water (250 ml) and extracted with ether (4x 1 50 ml). The organic phase is washed with brine, dried over magnesium sulfate, and concentrated to yield the acrylate monomer V which is recrystallized from ether-ethanol. Iodine-substituted esters of type Ill will also give suitable results under these conditions.

Example 4 This example illustrates the preparation of cholesteryl co-haloalkyl carbonates of type VI II. To a solution of 6-bromohexanol (O.'O75 mol), and pyridine (0.055 mol) in dichloromethane (50 ml) is added a solution of commercially available cholesteryl chloroformate (0.05 mol) in 50 ml of dichloromethane.

The addition is achieved at room temperature and the resulting mixture is stirred for 18 hours, after which it is diluted with 200 ml of dichloromethane, washed twice with 75-ml portions of 1N hydrochloric acid and then withy water. The organic phase is dried over magnesium sulfate and concentrated to give a solid w-bromoalkyl carbonate of type VIII which is purified by recrystallization from ether-ethanol. The resulting compound (Vllla), which is obtained in 78% yield, has a melting point of 87-88.50C.

Example 5 This example illustrates the preparation of cholesteryl acryloyl- or methacryloyloxyalkyl carbonates of type IX from carbonates of type Ill. A solution of 0.02 mol of the product of Example 4 (VIII) and 0.06 mol of potassium methacrylate is heated for 40 hours as described in Example 2. Upon recrystallization of the solid product from a 1:1.5 solution of acetone-ethanol, a 68% yield of methacrylate monomer is obtained which melts at 58.5--600C.

Example 6 This example will illustrate the alternative method of preparing compounds of type IX by reacting a hydroxyalkyl acrylate or methacrylate of type X with the cholesteryl haloformate or type VII. To a solution of hydroxyalkyl acrylate or methacrylate (0.06 mol) and pyridine (0.044 mol) in 40 ml of dichloromethane is added drop-wise a solution of cholesteryl haloformate (0.04 mol) dissolved in 40 ml dichloromethane. The addition is achieved at OOC, after which the mixture is allowed to warm to ambient temperature and stirred for six hours. The resulting product mixture is diluted with 250 ml of dichloromethane, washed with 60 ml of 1N hydrochloric acid and then with water.The organic phase is dried over magnesium sulfate and concentrated to give a solid which is recrystallized from a suitable solvent such as acetone-ethanol. Representative products are as follows:

Yield Melting or mesophase Compound R, A y (%) range ( C) IXa CH3 (CH2)6O 0 68 48.5--60 (51.0) IXb CH3 (OH2)2O 0 82 80 -81(40.1) IXc H (CH2)2O 0 81 85.5-87 (56.0) IXd H (OH2)6O 0 48** *5262 ** prepared by treating compound Vllla as described in Example 2.

Example 7 This example will illustrate the preparation of alkylene ethers and diethers of type IX where A=(CH2CH2O)2- and -(CH2CH2O)3-. Starting compounds of type X were prepared by means described in the chemical literature and were treated as described in Example 6 to give the following products: Yield Melting mesophase Compound R1 A y (%) range ( C) IXe CH3 (OH2CH2O)2 0 60 48.5-52.9(33.1) IXf CH3 '(CH2CH2O)3 0 62 no m. pt. (6.5) Example 8 This example sets forth the color ranges of various monomeric esters V described above, measured with a Leitz optical microscope using transmitted light through cross-polars at 250X magnification. A Mettler FP5 temperature regulator and a Mettler FP52 hot stage is used to controi the temperature, cooling being obtained by passirig a nitrogen stream through a dry-ice cooled copper coil and, subsequently, the FP52 hot stage.

Compound Color range (0C) Va 57.8-59.2 Vb (55.8-55.3) Vc (48.5-33.0) Vd (51.0--26.5) Ve No color Vf No color

Claims (8)

Claims

1. A compound of the formula

where R,=H or CH3, R2=an alkylene chain having from 3-14 methylene or lower alkyl-substituted methylene groups, and y=O or 1, provided that when Rt=CH3, y=1.

2. A compound of the formula

where Rt=H or CH3,A=-R3O- or --R,OO-, R3=an alkylene chain having from 2-1 4 methylene or lower alkylene-substituted methylene groups, R4=an alkylene ether, diether or triether having a total of from 3-1 4 carbon atoms in the alkylene linkages, provided that the terminal alkylene linkage adjacent to the carbonate moiety comprises not less than two carbon atoms, and y=O or 1.

3. A compound as claimed in claim 2, which contains an R4 group and wherein R4 contains at least one branched alkylene chain, the or each branch containing up to 4 carbon atoms.

4. A compound of the formula

where R1=Br, I, or a sulfonic acid ester, R2=an alkylene chain having from 3-14 methylene or lower alkyl-substituted methylene groups, and y=O or 1.

5. A compound as claimed in claim 4 wherein R2 comprises from 4 to 14 methylene or lower alkyl-substituted methylene groups.

6. A compound as claimed in claim 4 or claim 5 wherein X1=Br.

7. A compound as claimed in any one of claims 1,2 or 4 specifically mentioned herein.

8. Any new compound, material, composition or process hereinbefore described.