A thermotropic chiral nematic liquid crystalline copolymer composition comprises ##STR1## wherein --NEM-- and --NEM'-- are each independently nematogenic units of formula ##STR2## wherein R is H or CH.sub.3, --Q-- is an alkylene radical having 1 to about 8 carbon atoms, PA1 --X-- is --O--, --S--, or --CH.sub.2 --, --Y-- is ##STR3## --Z is --CN, --NO.sub.2 or --N.dbd.C.dbd.S, q and r are each independently 0 or 1; wherein --CHI-- is a chiral unit of formula ##STR4## wherein R is H or CH.sub.3, --Q'-- is an alkylene radical having 1 to about 8 carbon atoms, PA1 --X'-- is --O--, --S--, or --CH.sub.2 --, PA1 --Z' is an alkoxy, aralkoxy, alkylamino, or aralkylamino radical having 4 to about 12 carbon atoms and containing at least one asymmetric carbon atom, PA1 q' and r' are each independently 0 or 1; PA1 and wherein x is the mole fraction of chiral units and (y+y') is the total mole fraction of nematogenic units in said copolymer composition, and the ratio of x to (y+y') is from about 1:50 to 1:1. This compolymer composition is employed to form an optical device.

FIELD OF THE INVENTION 
This invention relates to thermotropic chiral nematic liquid crystalline 
copolymer compositions, and more particularly to their use for forming 
optical devices that produce reflected light having broad band widths, and 
to processes for forming such devices. 
BACKGROUND OF THE INVENTION 
Liquid crystalline materials exhibiting the cholesteric mesophase have been 
proposed for use in a variety of optical applications, for example, notch 
filters, circular polarizing filters, selective reflectors, beam 
splitters, and beam apodizers. U.S. Pat. No. 3,711,181, for example, 
discloses an optical apparatus for modulating circular-polarized light 
that contains optically negative liquid crystal films. 
It has been recognized that low molecular weight liquid crystal materials 
suffer limitations as to durability, effective temperature, mesophase 
stability, and amenability to processes for device fabrication. Polymeric 
liquid crystalline compositions have been proposed as potentially useful 
for overcoming such limitations. U.S. Pat. No. 4,293,435 discloses a 
liquid-crystalline cholesteric polymer phase that consists essentially of 
a copolymer of particular nematogenic and chiral acrylic ester monomers. 
In U.S. Pat. No. 4,410,570 is disclosed a liquid crystalline phase that 
contains a cyclic organopolysiloxane to which is chemically bonded at 
least one mesogenic group. Thermotropic cholesteric liquid crystalline 
glutamate copolymers consisting of chiral glutamate ester repeating units 
are disclosed in U.S. Pat. No. 4,743,675. 
Many applications of chiral liquid crystalline polymeric materials in 
optical devices require polymers capable of forming both right- and 
left-handed helical structures. When a film of such a polymer is applied 
to a substrate, the helical structures must be capable of forming and 
maintaining the Grandjean texture, in which the helical axis is 
perpendicular to the substrate surface, to enable the selective reflection 
of circular-polarized light. An enantiomeric chiral pair of liquid 
crystalline polymers, whose individual structures are characterized as a 
right-handed and a left-handed helix, are thus capable of selectively 
reflecting fight-handed and left-handed circular-polarized light, 
respectively. 
Especially useful for optical information storage applications are chiral 
nematic liquid crystalline polymers which form clear, transparent films 
that absorb no light in the visible region but do selectively reflect 
visible circular-polarized light. It is especially desirable for the 
application of these films as high efficiency polarizers that the 
reflected light be characterized by a broad half band width (HBW), as 
defined by the width of the spectral band measured at one-half of its 
maximum height. In devising flat panel displays, for example, where 
electrical power requirements should be kept as low as possible, chiral 
nematic liquid crystalline copolymers that form films whose half band 
widths in the visible region are substantially broader than those known in 
the art would be extremely useful. 
In addition to the just described optical characteristics, it is necessary 
that the chiral nematic polymers be readily synthesized and that they have 
solubility properties which enable their processing into clear glassy thin 
films for use as optical devices. All of these requirements are met by the 
chiral nematic liquid crystalline copolymer compositions of the present 
invention. 
SUMMARY OF THE INVENTION 
In accordance with the invention there is provided a thermotropic chiral 
nematic liquid crystalline copolymer composition 
##STR5## 
wherein --NEM-- and --NEM'-- are each independently nematogenic units of 
formula 
##STR6## 
wherein R is H or CH.sub.3, --Q-- is an alkylene radical having 1 to about 
8 carbon atoms, 
--X-- is --O--, --S--, or --CH.sub.2 --, --Y-- is 
##STR7## 
--Z is --CN, --NO.sub.2, or --N.dbd.C.dbd.S, q and r are each 
independently 0 or 1; 
wherein --CHI-- is a chiral unit of formula 
##STR8## 
wherein R is H or CH.sub.3, --Q'-- is an alkylene radical having 1 to 
about 8 carbon atoms, 
--X'-- is --O--, --S--, or --CH.sub.2 --, 
--Z' is an alkoxy, aralkoxy, alkylamino, or aralkylamino radical having 4 
to about 12 carbon atoms and containing at least one asymmetric carbon 
atom, 
q' and r' are each independently 0 or 1; 
and wherein x is the mole fraction of chiral units and (y+y') is the total 
mole fraction of nematogenic units in said copolymer composition, and the 
ratio of x to (y+y') is from about 1:50 to 1:1. 
The invention further provides an optical device formed from the above 
described copolymer composition.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, a thermotropic chiral nematic 
liquid crystalline copolymer composition is formed by the copolymerization 
of one or two monomeric nematogenic units and a monomeric chiral unit. The 
monomeric units are preferably alkyl acrylates or alkyl methacrylates, and 
the resulting copolymers have a weight average molecular weight of about 
1,500 to 50,000, preferably about 2,000 to 20,000. The molar ratio of 
chiral units to total nematogenic units in the copolymer compositions is 
from about 1:50 to 1:1, preferably from about 1:20 to 1:3, and most 
preferably from about 1:15 to 1:6. 
A monomeric nematogenic unit of formula (II) contains a mesogenic group, 
which imparts liquid crystalline characteristics to the unit and to 
copolymers formed from it. This mesogenic group, which typically has a 
rod-shaped molecular structure (Cf. H. Finkelmann, Angew. Chem. Int. Ed. 
Engl., 1987, vol. 26, pp. 816-824), is attached to the acrylic or 
methacrylic carbonyloxy moiety by a flexible link --Q--, which is an 
alkylene radical having either a straight or branched chain and containing 
one to about eight, preferably about two to six, carbon atoms. To obtain a 
copolymer composition with high optical birefringence, it is desirable 
that the --Y-- and --Z groups in the nematogenic units of formula (II) 
contain a multiplicity of conjugated unsaturated bonds; however, this 
conjugated unsaturation must not cause substantial absorption of light in 
the visible region by the copolymer composition. Useful mesogenic groups 
for the practice of the present invention include: biphenyl and 
diphenylacetylene moieties described in the aforementioned paper by 
Finkelmann and by Wu et al., J. Appl. Phys., 1990, vol. 68, pp. 78-85; 
terphenyl groups described by Gray et al., J. Chem Soc. Chem. Commun., 
1974, p. 431; and diphenylpyrimidines, as reported by Boller et al., Z. 
Natufforsch., 1978, vol. 33b, pp. 433-438. An acetylenic moiety, 
--C.tbd.C--, and a p-phenylene moiety are preferred as the --Y-- groups. 
Suitable --Z substituents are included in the aforementioned papers of 
Finkelmann and Wu et al.; a preferred --Z substituent is cyano. 
A monomeric chiral unit of formula (III) contains an optically active 
chiral group that includes at least one asymmetric carbon atom and is 
attached to the acrylic or methacrylic carbonyloxy moiety by a link 
--Q'--, which is an alkylene radical having either a straight or branched 
chain and containing one to about seven, preferably about two to six, 
carbon atoms. 
A liquid crystalline copolymer composition of the present invention that 
includes a chiral unit as described above exhibits selective reflection of 
visible circular-polarized light of wavelength .lambda..sub.R. The 
selective reflection wavelength .lambda..sub.R can be varied by changes in 
the structure and concentration of the chiral unit in the liquid 
crystalline copolymer composition. To achieve liquid crystalline 
copolymers whose selective reflection wavelengths are in the visible 
region, it is necessary that the copolymers exhibit adequate helical 
twisting power. The helical twisting power of a chiral liquid crystalline 
copolymer can be determined from the slope of the plot of the reciprocal 
of the selective reflection wavelength 1/.lambda..sub.R vs the mole 
fraction of the chiral component x as x.fwdarw.0 (cf. S. Krishnamurthy and 
S. H. Chen, Macromolecules, 1991, vol. 24, pp. 3481-3484; 1992, vol. 25, 
pp. 4485-4489). Helical twisting power of chiral nematic liquid 
crystalline copolymers depends not only on the structure of the chiral 
units but also on the structure of the nematogenic units, in particular, 
the extent of the conjugated unsaturation and the length of the flexible 
link in said units (cf. S. Chen and M. L. Tsai, Macromolecules, 1990, vol. 
23, pp. 5055-5058). 
Many applications of the chiral copolymer compositions of the present 
invention require a pair of structurally similar polymers capable of 
forming a right-handed and a left-handed helical structure, which enables 
them to selectively reflect right-handed and left-handed 
circular-polarized light, respectively. Using an enantiomeric pair of 
compounds to form two monomeric chiral units of opposite chirality, which 
are then copolymerized with one or two common monomeric nematogenic units, 
provides a pair of copolymers capable of forming right- and left-handed 
helices. For example, chiral nematic liquid crystalline copolymers 
containing chiral units prepared from R-(+)- and S-(-)-1-phenylethylamine 
form helical structures of opposite handedness (cf. M. L. Tsai and S. H. 
Chen, Macromolecules, 1990, vol. 23, pp. 1908-1911). 
In accordance with the present invention, optically active compounds 
preferred for preparing chiral units of formula (III) include the 
enantiomers of 1-phenylethanol, 1-phenylpropanol, 
2-methoxy-2-phenylethanol, mandelic acid methyl ester, .alpha.-tetralol, 
1-phenylethylamine, 1-cyclohexylethylamine, and 
3-amino-.epsilon.-caprolactam, Especially preferred are the enantiomeric 
pairs of 1-phenylethanol and 1-phenylethylamine. 
As discussed above, the chiral nematic liquid crystalline polymers of the 
present invention absorb no light in the visible region but do selectively 
reflect visible circular-polarized fight. They exhibit a helical twisting 
power sufficient to produce selective reflection wavelengths in the 
visible region and are capable of forming both right- and left-handed 
helical structures. Their selectively reflected light is characterized as 
having a broad half band width (HBW), substantially broader than that 
obtained with liquid crystalline polymers of the prior art. 
In addition to the above-described properties, the chiral nematic liquid 
crystalline copolymer compositions of the invention exhibit glass 
transition temperatures T.sub.g, at which the copolymers are converted to 
the liquid crystalline phase, of about 30.degree. C. to 120.degree. C., 
preferably about 40.degree. C. to 100.degree. C. These compositions are 
further characterized as having clearing temperatures T.sub.c, at which 
liquid crystalline phases lose long-range order and become isotropic, of 
about 80.degree. C. to 250.degree. C., preferably about 100.degree. C. to 
200.degree. C. 
In forming an optical device in accordance with the present invention, a 
film of a chiral nematic liquid crystalline copolymer composition is 
applied to a transparent substrate such as glass or fused quartz. The film 
can be formed from a melt of the copolymer, or it can be formed by 
applying a solution of the copolymer in a volatile organic solvent to the 
substrate, followed by drying to remove the solvent. To utilize the latter 
method for film formation, it is preferred that the copolymer composition 
be soluble in organic solvents such as methylene chloride, chloroform, 
tetrahydrofuran, and the like. 
The film of chiral nematic liquid crystalline copolymer on the substrate 
has a thickness of about 0.5 .mu.m to 50 .mu.m, preferably about 5 .mu.m 
in to 20 .mu.m. Furthermore, the film of copolymer in the liquid 
crystalline phase is capable of forming and maintaining the Grandjean 
texture, in which the helical structure of the polymer is perpendicular to 
the surface of the substrate. 
After the film of chiral nematic liquid crystalline copolymer has been 
applied to a transparent substrate, it is annealed by first heating at a 
temperature that is above the glass transition temperature but below the 
clearing temperature of the copolymer, then cooling rapidly to a 
temperature below the glass transition temperature of the copolymer. In 
the annealing step, the Grandjean texture is formed in the chiral liquid 
crystalline polymeric film and is maintained when the film is rapidly 
cooled below the glass transition temperature. Annealing is suitably 
performed at a temperature that is about 0.86 to 0.98, preferably about 
0.90 to 0.95, of the clearing temperature for a period of about 1 hour to 
2 days, preferably about 2 hours to 16 hours. 
Procedures for the synthesis of monomeric nematogenic and chiral units and 
the corresponding chiral liquid crystalline copolymers are described in 
the previously mentioned paper, Macromolecules, 1990, vol. 23, pp. 
1908-1911, and in the previously mentioned U.S. Pat. No. 4,293,435, which 
is incorporated herein by reference. 
In Table 1 are shown the formulas of representative chiral nematic liquid 
crystalline copolymer compositions of the present invention. 
TABLE 1 
__________________________________________________________________________ 
(1) 
##STR9## 
##STR10## 
(2) 
##STR11## 
##STR12## 
(3) 
##STR13## 
##STR14## 
##STR15## 
(4) 
##STR16## 
##STR17## 
##STR18## 
(5) 
##STR19## 
##STR20## 
(6) 
##STR21## 
(7) 
##STR22## 
##STR23## 
(8) 
##STR24## 
##STR25## 
(9) 
##STR26## 
(10) 
##STR27## 
##STR28## 
__________________________________________________________________________ 
The following examples further illustrate the invention. 
EXAMPLE 1 
Preparation of Copolymer (1),x=0.08 
The nematogenic acrylate monomer 
##STR29## 
was synthesized by the following reaction scheme: 
##STR30## 
Intermediate (1-1)--A solution of 4-iodophenol (74.4 g) and dihydropyran, 
DHP, (84.1 g) in 800 ml of dry methylene chloride containing pyridinium 
p-toluenesulfonate, PTTS, (17.5 g) was stirred at room temperature for 5 
hr. Then the solution was diluted with ether and washed several times with 
half-saturated brine to remove the catalyst. After evaporation of the 
solvent, the residue was recrystallized to yield white chunky crystals of 
the tetrahydropyranyl (THP) ether (1-1) (90 g, 85%). 
Intermediate (1-2)--To a mixture of (1-1) (25 g) and 
trimethylsilylacetylene (12 g) in 300 ml of triethylamine were added 
bis(triphenylphosphine) palladium dichloride (1 g) and copper (I) iodide 
(0.133 g). The reaction mixture was stirred under nitrogen at room 
temperature for 3 hr. before the solvent was removed under reduced 
pressure. The residue was extracted with 500 ml of petroleum ether; the 
extract was filtered and washed with water and then dried over anhydrous 
MgSO.sub.4. After evaporation of the solvent, the brown crude product was 
purified by flash chromatography on silica gel, using 1:15 diethyl 
ether/petroleum ether as the eluent. The pale yellowish product was 
recrystallized from ethanol to yield white chunky crystals (1-2) (15 g, 
66%). 
Intermediate (1-3)--5.0 g (1-2) and anhydrous potassium carbonate (1.0 g) 
were dissolved in 100 ml of methanol and stirred for 2 hr. The solvent was 
then evaporated, and the residue was dissolved in 200 ml of petroleum 
ether. The solution was washed with water, dried over anhydrous 
MgSO.sub.4, and evaporated. Recrystallization of the residue from ethanol 
gave white chunky crystals (1-3) (3.62 g, 98%). 
Intermediate (1-4)--To a solution of (1-3) (3.4 g) in 10 ml of anhydrous 
tetrahydrofuran, THF, at 0.degree. C. was added n-butylithium (0.078 g) in 
hexane (2M). The solution was stirred for 5 min. prior to addition of 
anhydrous zinc chloride (2.29 g) dissolved in anhydrous THF (20 ml). The 
mixture was stirred at room temperature for an additional 15 min. 
Intermediates (1-5) and (1-6)--To the solution containing (1-4) cooled to 
0.degree. C. were added sequentially a solution of 4-bromobenzonitrile 
(3.07 g) in anhydrous THF (20 ml) and a solution of 
tetrakis(triphenylphosphine) palladium (0.5 g) in anhydrous THF (20 ml), 
both at 0.degree. C., to produce (1-5), which was not isolated. Instead, 
30 ml of 1N HCl and 10 g of ammonium chloride were added to form two 
clearly separated layers, which were stirred at room temperature for 
another 3 hr. The two layers were separated after shaking with 50 ml of 
added petroleum ether, and the aqueous portion was extracted with 
petroleum ether. The organic portions were combined, washed with saturated 
aqueous sodium bicarbonate solution, and dried over anhydrous MgSO.sub.4. 
After the solvent was evaporated under reduced pressure, the brown residue 
was purified by flash chromatography on silica gel with methylene chloride 
as the eluent. The yellowish product was recrystallized from chloroform to 
give white flakes (1-6) (2.35 g, 64%). 
Nematogenic monomer (1-7)--Diethyl azodicarboxylate, DEAD, (1.91 g) in 10 
ml of anhydrous THF was added slowly to a mixture containing (1-6) (2.0 
g), 4-hydroxybutyl acrylate (1.56 g), triphenylphosphine (2.87 g), and 20 
ml of anhydrous THF. The reaction mixture was stirred at room temperature 
for 5 hr. and then concentrated under reduced pressure. The residue was 
purified by flash chromatography on silica gel with methylene as the 
eluent. The crude product was recrystallized from ethanol to give white 
flakes (1-7) (2.0 g, 64% ). 
Chiral monomer (1-8)--The chiral methacrylate monomer (1-8) was synthesized 
from (S)-(-)-1-phenylethanol by the procedure reported in the previously 
mentioned Macromolecules, 1991, Vol. 24, pp. 3481-3484. 
##STR31## 
Copolymer (1), x=0.08--The nematogenic monomer (1-7) (0.433 g), the chiral 
monomer (1-8) (0.0530 g), and 2,2'-azobis(isobutyronitrile), AIBN, (1 mg) 
were dissolved in anhydrous THF (1.5 ml). The reaction mixture was stirred 
under nitrogen at 60.degree. C. for 2 days. The copolymer product was 
isolated and purified by repeated dissolution-precipitation cycles, using 
methanol as the nonsolvent, and dried under vacuum. The copolymer (1) had 
a weight-avenge molecular weight of 13,600, with a polydispersity index of 
2.0. The chiral mole fraction, x, was determined to be 0.08 by proton NMR 
spectroscopy. The differential scanning calorimetry (DSC) thermogram 
showed a glass transition temperature, T.sub.g, at 44.degree. C. and a 
cholesteric to isotropic transition, or clearing temperature, T.sub.c, at 
106.degree. C., the cholesteric mesophase being identified by polarized 
optical microscopy. 
EXAMPLE 2 
Preparation of Copolymer (2), x=0.10 
The nematogenic methacrylate monomer 
##STR32## 
was synthesized by the following scheme: 
##STR33## 
Intermediate (2-1 )--4-Hydroxy-4-cyanobiphenyl (10 g) and 
4-dimethylaminopyridine (20 g) were dissolved in anhydrous methylene 
chloride (200 ml); the solution was kept at 0.degree. C. while triflic 
anhydride (15 g) was added dropwise over a period of 1/2 hr. The mixture 
was then shaken twice with 1N HCl (250 ml each) in a separatory funnel. 
The organic layer was separated and evaporated, and the residue was then 
flash chromatographed on silica gel with methylene chloride as the eluent 
to obtain a white crystalline powder (2-1) (15.5 g, 92%). 
Intermediate (2-2)--Tetrakis(triphenylphosphine) palladium (1.79 g) and 
(2-1) (10.55 g) were mixed with 200 ml of benzene and 100 ml of 2M aqueous 
sodium carbonate. The mixture was stirred vigorously, and 
4-methoxybenzeneboronic acid (5.64 g) in 20 ml of methanol and 40 ml of 
benzene was added. The mixture was heated under reflux for 2.5 hr.; after 
cooling to room temperature, 0.5 ml of a 30% aqueous solution of H.sub.2 
O.sub.2 was added. The rust colored solid was collected by filtration, and 
the filtrate was shaken with an aqueous solution of sodium carbonate (2M). 
The organic layer was dried over anhydrous Na.sub.2 SO.sub.4, and the 
solvent was evaporated under reduced pressure to give a dark red tar, 
which was combined with the solid obtained above and subjected to flash 
chromatography on silica gel with methylene chloride as the eluent. 
Recrystallization from 1:1 acetone/methylene chloride was accomplished to 
give a granular white solid (2-2)(7.51 g, 82%). 
Intermediate (2-3)--(2-2) (6.8 g) was dissolved in 200 ml of anhydrous 
methylene chloride, and the clear solution was kept at -78.degree. C. 
while 50 ml of a solution of boron tribromide in methylene chloride (1M) 
was added, which resulted in a tan slurry. The reaction mixture was 
allowed to warm to room temperature before a small amount of fluffy 
precipitate was removed by filtration. The filtrate was cooled in an ice 
bath, and 200 ml water was added to produce a solid, which was collected 
by filtration and further washed with water. The solid product was dried, 
then stirred in boiling 20:1 methylene chloride/acetone mixture for a few 
minutes. The mixture was allowed to cool, and the solid was again 
collected and dried, yielding a tan powder (2-3) (6 g, 93%). 
Nematogenic monomer (2-4)--Triphenylphosphine (2.42 g) and (2-3) (2.42 g) 
were mixed with 100 ml of anhydrous THF to form a milky slurry, to which 
2-hydroxypropyl methacrylate (1.30 g) in 50 ml of anhydrous THF was added. 
Diethyl azodicarboxylate, DEAD, (1.63 g) in 40 ml of anhydrous THF was 
slowly added to the slurry to produce a clear solution; the reaction was 
then left to proceed overnight. The volume of the solution was reduced via 
evaporation, and the concentrate was shaken with a methylene chloride and 
water mixture (100 ml each). The organic layer was washed with water 
(3.times.100 ml), and the aqueous layer was extracted with methylene 
chloride (2.times.25 ml). The combined organic portions were dried over 
anhydrous Na.sub.2 SO.sub.4, and the crude product was isolated by 
evaporating the solvent. Flash chromatography on silica gel with methylene 
chloride as the eluent gave monomer (2-4) as a white solid (0.98 g, 28%). 
Chiral monomer (2-5)--The chiral methacrylate monomer (2-5) was synthesized 
from (S)-(-)-1-phenylethylamine by the procedure reported in the 
previously mentioned Macromolecules, 1990, vol. 23, pp. 1908-1911. 
##STR34## 
Copolymer (2), x=0.10--The nematogenic monomer (2-4) (0.25 g), the chiral 
monomer (2-5) (0.03 g), and 2,2'-azobisisobutyronitrile, AIBN, (1 mg) were 
dissolved in anhydrous THF (10 ml). The reaction mixture was stirred at 
60.degree. C. under nitrogen for 3 days. The product was isolated by 
precipitation in methanol and purified with repeated 
dissolution-precipitation cycles to yield copolymer (2) (0.12 g, 40%), 
x=0.10. The weight-average molecular weight was determined to be 3,960, 
with a polydispersity index of 1.7. A vacuum dried sample showed a T.sub.g 
at 69.degree. C. and a T.sub.c at 144.degree. C., the cholesteric 
mesophase being identified by polarized optical microscopy. 
EXAMPLE 3 
Preparation of Copolymer (3), x=0.15, y=0.50, y'=0.35 
Nematogenic monomer (3-1)--The terphenylsubstituted nematogenic acrylate 
monomer 
##STR35## 
was prepared from the cyanohydroxyterphenyl compound (intermediate (2-3) 
of Example 2) (2.45 g), 4-hydroxybutyl acrylate (1.10 g), 
triphenylphosphine (2.37 g), and diethyl azodicarboxylate (1.62 g) in 
tetrahydrofuran, following the procedure used to make monomer (2-4) of 
Example 2. The crude product was purified by flash chromatography, then 
recrystallized from 3:1 methanol:acetone to give monomer (3-1) as a white 
solid (2.04 g, 67%). 
Nematogenic monomer (3-2)--The biphenylsubstituted nematogenic acrylate 
monomer 
##STR36## 
was prepared by the following reaction scheme: 
##STR37## 
Intermediate (3-3)--A solution of 4-hydroxy -4'-cyanobiphenyl (5.03 g) and 
potassium hydroxide (1.73 g) in 30 ml of methanol was heated to reflux, 
and a solution of 6-bromohexanol (5.15 g) in 11 ml of methanol was added. 
The mixture was allowed to cool to room temperature, then treated with 100 
ml of chloroform and 100 ml of water. The chloroform layer was separated, 
and the aqueous phase was extracted with chloroform. The chloroform 
solutions were combined, dried over anhydrous Na.sub.2 SO.sub.4, and 
evaporated. The residue was recrystallized from acetone and further 
purified by flash chromatography on silica gel using 10:1 methylene 
chloride:methanol to give (3-3) (4.17 g, 55%). 
Nematogenic monomer (3-2)--To a solution of (3-3) (4.94 g) and 
triethylamine (2.02 g) in 15 ml of dry THF was added dropwise a solution 
of acryloyl chloride (1.65 g) in 10 ml of dry THF. To the mixture was 
added 75 ml each of methylene chloride and water. The organic layer was 
dried over anhydrous Na.sub.2 SO.sub.4 and evaporated to dryness. The 
residue was purified by flash chromatography on silica gel with methylene 
chloride, followed by recrystallization to yield the monomer (3-2) as 
shiny white crystals (4.11 g, 70%). 
EQU Copolymer (3), x=0.15, y=0.50, y'=0.35 
The nematogenic monomer (3-1) (0.200 g), the nematogenic monomer (3-2) 
(0.124 g), and the chiral monomer (2-5 of Example 2) (0.72 g), and 
2,2'-azobisisobutyronitrile (1 mg) were dissolved in 4 ml of anhydrous 
THF. The mixture was stirred at 60.degree. C. under nitrogen for 3 days. 
The product was then isolated by precipitation in methanol and purified 
with repeated dissolution--precipitation cycles to yield copolymer (3) 
(0.355 g, 90%). The dried copolymer had a weight-average molecular weight 
of 7,270, with a polydispersity index of 2.0. The DSC thermogram showed a 
glass transition temperature, T.sub.g, at 48.degree. C. and a clearing 
temperature, T.sub.c, at 163.degree. C., the mesophase being identified by 
polarized optical microscopy. 
EXAMPLE 4 
Formation of Optical Devices 
Optical devices were prepared from copolymers (1), (2), and (3) of the 
invention and from the prior art copolymer (C-1) of the previously 
mentioned U.S. Pat. No. 4,293,435 and Macromolecules, 1991, vol. 24, pp. 
3481-3484. 
##STR38## 
Approximately 20 mg of each copolymer was placed between a pair of soda 
lime glass substrates (50 mm in diameter and 1.6 mm thick) separated by 13 
.mu.m-thick Kapton.RTM. (from DuPont) spacers. Each element was annealed 
for approximately 12 hours at a temperature that was about 0.95 of the 
clearing temperature of the copolymer, then rapidly cooled to room 
temperature. The selective reflection wavelength .lambda..sub.R and half 
band width HBW, as previously defined, were determined for each of the 
elements, using a Perkin-Elmer Lambda 9 UV-visible-near IR 
spectrophotometer. Half band width values (HBW).sub.588 corresponding to 
an .lambda..sub.R value of 588 nm were also calculated for several of the 
elements, using the procedure described in J. L. Fergason, Mol. Cryst., 
1966, Vol. 1, pp. 293-307; P. V. Adomenas et al., Opt. Spectroscopy 
(USSR), 1983, Vol. 54, pp. 179-182. The results are shown in Table 2. 
TABLE 2 
______________________________________ 
Element Copolymer .lambda..sub.R 
HBW (HBW).sub.588 
______________________________________ 
1 control 
(C-1) 543 nm 105 nm 114 nm 
2 invention 
(1) 588 nm 140 nm 140 nm 
3 invention 
(2) 644 nm 166 nm 152 nm 
4 invention 
(3) 583 nm 143 nm 144 nm 
______________________________________ 
The data of Table 2 clearly demonstrate the desirable broadening of the 
spectral reflection bands, as manifested in the HBW and (HBW).sub.588 
values, for the chiral nematic liquid crystalline copolymers (1), (2), and 
(3) of the present invention compared with the prior art copolymer (C-1). 
Although the invention has been described in detail for the purpose of 
illustration, it is understood that such detail is solely for that 
purpose, and variations can be made therein by those skilled in the art 
without departing from the spirit and scope of the invention that is 
defined by the following claims.