Abstract:
A high modulus polyester comprising the following structural units (I)  anr (II) and (III), the structural units (I) and (II) occupying 51-99 mol % of the whole and the structural unit (III) occupying 49-1 mol % of the whole, and a high modulus shaped article obtained from the polyester: ##STR1## wherein R represents one or more divalent radicals selected from ##STR2## and X represents chlorine or bromine atom.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a novel aromatic polyester capable of being melt-extruded and affording shaped articles having superior mechanical properties and optical anisotropy. 
     DESCRIPTION OF THE PRIOR ART 
     Recently, demand for high performance plastics has been increasing and a number of polymers having various novel performances have been developed and marketed. Above all, optically anisotropic liquid crystal polymers having a parallel arrangement of molecular chains have been attracting a special attention in that they have superior mechanical properties. 
     As such liquid crystal polymers, wholly aromatic polyesters are widely known. For example, homoand copolymers of p-hydroxybenzoic acid are now available commercially under the trade name &#34;EKONOL&#34;. However, these homo- and copolymers &#34;EKONOL&#34; are too high in melting point, so their melt fluidity is poor and their melt extrusion is impossible or difficult. In this connection, reference is made to the &#34;Modern Plastics&#34; (July 1975), p.62 in which there are described copolymers prepared by copolymerizing p-hydroxybenzoic acid with, for example, terephthalic acid and hydroquinone. These copolymers are extremely high in softening point ranging from about 427° to 482° C., so not only their melt extrusion is difficult but also their mechanical properties are not fully satisfactory. 
     As means for lowering the melting or softening points of such wholly aromatic polyesters to improve their melt extrudability and mechanical properties, there has been proposed, for example, such a method as disclosed in Japanese Patent Publication No. 482/1980 in which polycondensation reactions with various dicarboxylic acids are made using chlorohydroquinone or methylhydroquinone in place of hydroquinone. However, polyesters obtained using terephthalic acid as dicarboxylic acid have the drawback that their melting temperatures are higher than 500° C. On the other hand, as disclosed in Japanese Patent Laid Open No. 65421/1978, polyesters prepared using phenylhydroquinone and terephthalic acid are known to have relatively low melting points, not higher than 350° C., and afford a yarn having a high modulus of about 500 g/d after heat treatment. But, even this modulus is fairly lower than that of 1000 g/d of &#34;Kevlar-49&#34;  which is widely known as a wholly aromatic polyamide, and therefore the attainment of a higher modulus has been desired. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an aromatic polyester capable of being melt extruded and affording shaped articles having superior mechanical properties and optical anisotropy. 
     Having made extensive studies, the present inventors found out that polyesters of specific compositions of components selected from methylhydroquinone, chlorohydroquinone, phenylhydroquinone, 4,4&#39;-dihydroxybiphenyl, 2,6-dihydroxynaphthalene, 4,4&#39;-diphenyldicarboxylic acid, terephthalic acid, 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid, 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid and 1,2-bis(2-bromophenoxy)ethane-4,4&#39;-dicarboxylic acid could afford a novel aromatic polyester capable of achieving the above-mentioned object. 
     According to the present invention, therefore, there is provided a melt-extrudable aromatic polyester comprising the following structural units (I) and/or (II) and (III), the structural units (I) and (II) occupying 51 to 99 mol % of the whole and the structural unit (III) occupying 49 to 1 mol % of the whole: ##STR3## wherein R represents one or more divalent radicals selected from the group consisting of ##STR4## and X represents chlorine or bromine atom. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the aromatic polyester of the present invention, the structural unit (I) is of a polyester prepared using one or more diols selected from methylhydroquinone, chlorohydroquinone, phenylhydroquinone, 4,4&#39;-dihydroxybiphenyl and 2,6-dihydroxynaphthalene, and 4,4&#39;-diphenyldicarboxylic acid; the structural unit (II) is of a polyester prepared using the above aromatic diol component and terephthalic acid and/or 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid; and the structural unit (III) is of a polyester prepared using the above aromatic diol component and 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid and/or 1,2-bis(2-bromophenoxy)ethane-4,4&#39;-dicarboxylic acid. 
     The aromatic polyester of the present invention comprising such structural units usually melts at a temperature not higher than 400° C. and can afford shaped articles such as fibers, films and various molded products having superior mechanical properties and optical anisotropy by conventional melt extrusion techniques such as melt spinning and injection molding. In view of the fact that, for example, the melting point of polyethylene terephthalate is 256° C. and that of poly(ethylene-4,4&#39;-diphenyldicarboxylate) is 355° C., the polyester in the present invention prepared from 4,4&#39;-diphenyldicarboxylic acid which affords a polyester of a higher melting point as compared with terephthalic acid and a diol component such as methylhydroquinone is presumed to have an extremely high melting point. Actually, however, its melting point is not higher than 400° C. and thus relatively low. Besides, it has an extremely high modulus and a good heat stability in comparison with the foregoing prior art polyesters. These effects are quite unexpected. 
     On the other hand, Japanese Patent Publication No. 482/1980 describes some polyesters prepared from terephthalic acid and/or 1,2-bis(phenoxy)-ethane-4,4&#39;-dicarboxylic acid and diols, e.g. methylhydroquinone. But, shaped articles obtained from those polyesters are poor in heat stability or low in modulus and thus the object of the present invention is not attainable. 
     Further, polyesters prepared from 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid and 1,2-bis(2-bromophenoxy)ethane-4,4&#39;-dicarboxylic acid and aromatic diols, e.g. methylhydroquinone are known from Japanese Patent Laid Open No. 41331/1984. But, there still remains unsatisfactoriness in point of modulus and heat stability, and the object of the present invention cannot be attained. 
     In the aromatic polyester of the present invention, the proportion of the structural units (I) and (II) is 51-99 mol %, preferably 55-95 mol %, of the whole and that of the structural unit (III) is 49-1 mol %, preferably 45-5 mol %, of the whole. Particularly preferably, the polyester contains the structural unit (I) in a proportion of 55-90 mol % of the whole, and most preferably it contains the structural unit (I) in a proportion of 65-90 mol % of the whole. If the proportion of the structural units (I) and (II) is in the range of 99 to 100 mol %, the resulting aromatic polyester will be too high in melting point or inferior in heat stability and mechanical properties, thus making it impossible to attain the object of the present invention. And if the proportion of the structural units (I) and (II) is in the range of 0 to 51 mol % of the whole, the heat stability and mechanical properties of the resulting polyester will be poor, thus making the object of the present invention unattainable. 
     As the diol component, methylhydroquinone or chlorohydroquinone is preferred, and chlorine atom is preferred as X in the structural formula (III). 
     Preferably, the aromatic polyester of the present invention has a melt viscosity in the range of 10 to 10,000 poise, more preferably 20 to 5,000 poise. The melt viscosity as referred to herein indicates a value determined using a Koka type flow tester at a temperature of melting point plus 40° to 100° C. and at a shear rate of 2,000 to 4,000 (1/sec). If the melt viscosity is lower than 10 poise, the resulting shaped article will be low in strength, and if the polyester has a melt viscosity higher than 10,000 poise, its moldability will be poor and the resulting shaped article will be inferior in mechanical properties. 
     The aromatic polyester of the present invention can be prepared according to conventional polycondensation techniques for polyester. No special restrictions are placed on its manufacturing method. For example, the following (1) to (3) are typical methods. (1) Preparation by polycondensation involving removal of monocarboxylic acid, from diester or diesters selected from methylhydroquinone diacetate, chlorohydroquinone diacetate, phenylhydroquinone diacetate, 4,4&#39;-diacetoxybiphenyl and 2,6-diacetoxynaphthalene, and aromatic dicarboxylic acids selected from 4,4&#39;-diphenyldicarboxylic acid, terephthalic acid, 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid, 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid and 1,2-bis(2-bromophenoxy)-ethane-4,4&#39;-dicarboxylic acid. (2) Preparation by polycondensation involving removal of phenol, from aromatic diol or diols selected from methylhydroquinone, chlorohydroquinone, phenylhydroquinone, 4,4&#39;-dihydroxybiphenyl and 2,6-dihydroxynaphthalene, and diphenyl esters of the aromatic dicarboxylic acids described in (1). (3) Preparation by reacting the aromatic dicarboxylic acids described in (1) with a desired amount of diphenyl carbonate, then adding the aromatic diol component described in (2) to the resulting diphenyl esters and performing polycondensation reaction involving removal of phenol. 
     Typical examples of catalyst used in the polycondensation reaction are metallic compounds such as stannous acetate, tetrabutyl titanate, lead acetate, antimony trioxide, sodium acetate and potassium acetate. These metallic compounds are effective especially in the polycondensation involving removal of phenol. 
     In the polycondensation reaction for preparation of the aromatic polyester of the present invention, aromatic dicarboxylic acids such as isophthalic acid, 3,3&#39;-diphenyldicarboxylic acid, 3,4&#39;-diphenyldicarboxylic acid, 2,2-diphenyldicarboxylic acid and 2,6-naphthalenedicarboxylic acid, alicyclic dicarboxylic acids such as hexahydroterephthalic acid, other aromatic diols such as 4,4&#39;-dioxydiphenyl ether and 2,7-dioxynaphthalene, and other aromatic hydroxycarboxylic acids such as p-hydroxybenzoic acid, may participate in the copolymerization in small amounts not impairing the object of the present invention, in addition to the constituents of the structural units (I)-(III). 
     The aromatic polyester of the present invention thus prepared, having a low melting point not higher than 400° C., can be subjected to conventional melt processings such as extrusion modling, injection molding, compression molding and blow molding, and formed into fibers, films, three-dimensional products, containers and hoses. 
     Additives such as reinforcing media, e.g. glass fibers, carbon fibers and asbestos, fillers, nucleating agents, pigments, antioxidants, stabilizers, plasticizers, lubricants, mold release agents and flame-retardants, as well as other thermoplastic resins, may be added to the aromatic polyester of the present invention at the time of molding to impart desired characteristics to the resulting shaped articles. 
     Shaped articles obtained from the novel aromatic polyester of the invention have a good optical anisotropy which is attributable to the parallel molecular array of the polyester, and are extremely superior in mechanical properties. For example, using the aromatic polyester of the invention there can be obtained fibers having a fineness of 0.5 mm in diameter and a high modulus not less than 50 GPa, as well as a shaped article having a thickness of 1/32 inch and a high modulus not less than 15 GPa. 
     The following examples are given to further illustrate the invention. 
    
    
     EXAMPLE 1 
     104.5 g. (5×10 -1  mol) of methylhydroquinone diacetate, 84.7 g. (3.5×10 -1  mol) of 4,4&#39;-diphenyldicarboxylic acid, 12.5 g. (0.75×10 -1  mol) of terephthalic acid and 27.8 g. (0.75×10 -1  mol) of 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid were charged into a test tube for polymerization and reacted at 250-320° C. in a nitrogen gas atmosphere for hours, then the pressure was reduced to a vacuum degree of 0.7 mmHg and heating was continued at 320° C. for 1.5 hours to allow polycondensation reaction to proceed. As a result, 58 g. of acetic acid corresponding to 97% of a theoretical amount was distilled out to obtain a highly fibrillated brown polymer. The polymer was of the following theoretical structural formula, and elementary analysis values of the polyester well coincided with theoretical values as set out in Table 1. As a result of infrared spectroscopic analysis, the polyester proved to have characteristic absorption at 1495, 1610 and 1725 cm -1 . ##STR5## (l/m/n mole ratio=70/15/15) 
     
                       TABLE 1______________________________________  Measured Value             Theoretical Value  (wt %)     (wt %)______________________________________C        72.9         72.4H        4.7          4.1Cl       2.9          3.2O        19.5         20.3______________________________________ Note The oxygen content (%) was calculated as (100%  C %  H %  Cl %). 
    
     The polyester was put on a sample stand of a polarizing microscope, then the temperature was raised and a check was made on optical anisotropy with shear. As a result, the polyester exhibited a good optical anisotropy at temperatures not lower than 223° C. Further, the polyester was measured for thermal characteristics by means of a differential scanning calorimeter (PERKIN-ELMER DSC-2C.) to obtain the following results: glass transition temperature 111° C., melting point 242° C. 
     Moreover, the polyester was charged to a Koka type flow tester and spun through a spinneret 0.3 mm in diameter at a spinneret temperature of 280° C. to obtain a spun yarn 0.05 mm in diameter. The melt viscosity of the polyester was 1,700 poise at a shear rate of 3,100 (1/sec). 
     Further, the thus-spun yarn was measured for modulus at a frequency of 110 Hz, a heating rate of 2° C./min and an interchuck distance of 40 mm by means of RHEOVIBRON DDV-II-EA (a product of Toyo Baldwin Co., Ltd.); as a result, it was found to have a modulus as high as 132 GPa. 
     Moreover, the polymer was injection-molded at 280° C. (mold 30° C.) using Sumitomo NESTAL injection molding machine (0.5 ounce) to obtain a molded product (plaques) having a thickness of 1/32&#34;. The molded product was measured for bending modulus using TENSILON UTM-4 (a product of Toyo Baldwin Co., Ltd.) in accordance with ASTM D 790; as a result, it was found to have a bending modulus as high as 29 GPa. 
     COMPARATIVE EXAMPLE 1 
     114.8 g. (5×10 -1  mol) of chlorohydroquinone diacetate, 36.3 g. (1.5×10 -1  mol) of 4,4&#39;-diphenyldicarboxylic acid and 67.2 g. (3.5×10 -1  mol) of terephthalic acid (a known composition as described in Japanese Patent Publication No. 482/1980) were charged into a test tube for polymerization and polycondensed in the same way as in Example 1 to obtain an optically anisotropic polyester having a melting point of 290° C. 
     Using the polyester, spinning was performed in the same manner as in Example 1 to obtain a spun yarn 0.9 mm in diameter. The spun yarn thus obtained was measured for modulus; as a result, it was found to have a modulus of 44 GPa at 30° C., lower than that in Example 1. 
     Further, the polyester was injection-molded in the same manner as in Example 1 to obtain a molded product having a thickness of 1/32&#34;. The molded product was measured for bending modulus, which was found to be 14 GPa, lower than that in Example 1. Example 2-6 and Comparative Examples 2-7 
     5×10 -2  mol of diacetate selected from methylhydroquinone diacetate (I) and chlorohydroquinone diacetate (II), and 5×10 -2  mol of dicarboxylic acid or dicarboxylic acids selected from 4,4&#39;-diphenyldicarboxylic acid (III), terephthalic acid (IV), 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid (V), 1,2-bis(2-bromophenoxy)ethane-4,4&#39;-dicarboxylic acid (VI) and 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid (VII), as shown in Table 2, were charged into a test tube for polymerization and polycondensed in the same way as in Example 1. 
     The resultant polyesters were measured for liquid crystal initiation temperature and melting point; as a result, they proved to be liquid crystal polyesters except the polyesters prepared in Comparative Examples 4 and 5. 
     Spinning was performed using those polyesters, but the polyesters obtained in Comparative Examples 2 and 3 were too high in melt viscosity to melt-spin and the polyesters in Comparative Examples 4 and 5 were also incapable of being spun. Then, the spun yarns thus obtained were measured for modulus using RHEOVIBRON in the same manner as in Example 1; as a result, the spun yarns obtained from the polyesters of Examples 2-6 were found to have high values of modulus, not lower than 50 GPa, while the spun yarns obtained using the polyesters of Comparative Examples 6 and 7 were low in modulus, not higher than 50 GPa. 
     
                                           TABLE 2__________________________________________________________________________                                        Liquid  Hydroquinone          Dicarboxylic Acid             Crystal                                               Melting  Component          Component       (III):(IV):(V + VI + VII)                                        Initiation                                               Point  (I) (II)          (III)             (IV)                (V)                   (VI)                      (VII)                          (mol ratio)   Temp. (°C.)                                               (°C.)__________________________________________________________________________Example 2  100 --  80 10 10 -- --  80/10/10      248    298Example 3  100 --  85 7.5                7.5                   -- --   85/7.5/7.5   268    269Example 4  --  100 80 10 10 -- --  80/10/10      310    315Example 5  --  100 70 15 15 -- --  70/15/15      296    313Example 6  100 --  70 15 -- 15 --  70/15/15      304    320Comparative  100 --  100             -- -- -- --  100/0/0       360    372Example 2Comparative  --  100 100             -- -- -- --  100/0/0       357    354Example 3Comparative  100 --  -- 100                -- -- --  0/100/0       &gt;500   --Example 4Comparative  100 --  -- 100                -- -- --  0/100/0       &gt;500   --Example 5Comparative  100 --  30 70 -- -- --  30/70/0       272    321Example 6Comparative  100 --  -- 70 30 -- --   0/70/30      257    314Example 7__________________________________________________________________________                                      Fine-                                 Spinning                                      ness    Melt.sup.(1)                                 Temp.                                      (mm                                         Modulus                                              Viscosity                                 (°C.)                                      dia.)                                         (GPa)                                              (poise)__________________________________________________________________________                          Example 2                                 320  0.08                                         52    400                          Example 3                                 330  0.08                                         70    70                          Example 4                                 360  0.20                                         51   1000                          Example 5                                 360  0.34                                         52   1100                          Example 6                                 350  0.27                                         56   1200                          Comparative                                 Spinning was above                          Example 2                                 impossible.  10,000                                              poise                                              even at                                              400° C.                          Comparative                                 Spinning was above                          Example 3                                 impossible.  10,000                                              poise                                              even at                                              400° C.                          Comparative                                 Spinning was Measure-                          Example 4                                 impossible.  ment was                                              impossible                          Comparative                                 Spinning was Measure-                          Example 5                                 impossible.  ment was                                              impossible                          Comparative                                 310  0.15                                         31    300                          Example 6                          Comparative                                 360  0.07                                         49    400                          Example 7__________________________________________________________________________ .sup.(1) Measured at a spinning temperature of melting point plus 40° to 100° C. and at a shear rate of 2,000 to 4,000 (1/sec). 
    
     EXAMPLE 7 
     114.8 g. (5×10 -1  mol) of chlorohydroquinone diacetate, 84.7 g. (3.5×10 -1  mol) of 4,4&#39;-diphenyldicarboxylic acid and 55.7 g. (1.5×10 -1  mol) of 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid were charged into a test tube for polymerization and subjected to polycondensation reaction involving removal of acetic acid in the following manner. First, reaction was allowed to take place at 250-310° C. in a nitrogen gas atmosphere for 3 hours, then the temperature was raised to 330° C. at a heating rate of 0.5 hour and at the same time the pressure was reduced to 0.6 mmHg, and heating was further continued for about 1 hour to complete polycondensation. As a result, 60 g. of acetic acid corresponding to 98% of a theoretical amount was distilled out to obtain a highly fibrillated brown polymer. 
     The polymer was of the following theoretical structural formula, and elementary analysis values of the polyester well coincided with theoretical values as set forth in Table 3. As a result of infrared spectroscopic analysis, the polyester proved to have characteristic absorption at 1485, 1600 and 1735 cm -1 . ##STR6## 
     
                       TABLE 3______________________________________  Measured Value             Theoretical Value  (wt %)     (wt %)______________________________________C        67.9         67.2H        2.7          3.2Cl       10.2         9.6O        19.2         20.0______________________________________ Note The oxygen content (%) was calculated from (100%  C %  H %  Cl %). 
    
     The polyester was put on a sample stand of a polarizing microscope, then the temperature was raised and a check was made on optical anisotropy. As a result, the polyester exhibited a good optical anisotropy at temperatures not lower than 289° C. 
     The polyester was charged to a Koka type flow tester and spun through a spinneret 0.3 mm in diameter at a spinneret temperature of 350° C. to obtain a spun yarn 0.09 mm in diameter. The melt viscosity of the polyester was 220 poise at a shear rate of 3,000 (1/sec). 
     The spun yarn was measured for modulus at a sample length of 50 mm and at a pulling rate of 10 mm/min by means of TENSILON; as a result, it was found to have a modulus as high as 75 GPa. 
     Further, the polymer was injection-molded at 350° C. (mold 30° C.) in the same manner as in Example 1 to obtain a molded product having a thickness of 1/32&#34;. The molded product was measured for bending modulus, which was found to be as high as 28 GPa. 
     Moreover, the polymer was measured for thermal characteristics by means of a differential scanning calorimeter; as a result, its glass transition temperature and melting point were 98° C. and 307° C., respectively. 
     EXAMPLE 8 
     10.45 g. (5×10 -2  mol) of methylhydroquinone diacetate, 8.47 g. (3.5×10 -2  mol) of 4,4&#39;-diphenyldicarboxylic acid and 5.57 g. (1.5×10 -2  mol) of 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid were charged into a test tube for polymerization and subjected to polycondensation reaction involving removal of acetic acid in the following manner. First, reaction was allowed to take place at 250-310° C. in a nitrogen atmosphere for 3 hours, then the temperature was raised to 330° C. at a heating rate of 0.5 hour and at the same time the pressure was reduced to 0.6 mmHg, and heating was further continued for 1 hour to complete polycondensation. As a result, 6.01 g. corresponding to 98% of a theoretical amount was distilled out to obtain a highly fibrillated brown polymer. 
     The polymer was of the following theoretical structural formula, and elementary analysis values of the polyester well coincided with theoretical values as set forth in Table 4. As a result of infrared spectroscopic analysis, the polyester proved to have characteristic absorption at 1500, 1600 and 1720 cm -1 . ##STR7## 
     
                       TABLE 4______________________________________  Measured Value             Theoretical Value  (wt %)     (wt %)______________________________________C        70.7         70.3H        3.6          4.0Cl       6.2          5.7O        19.5         20.0______________________________________ Note The oxygen content (%) was calculated as (100%  C %  H %  Cl %). 
    
     The polyester was put on a sample stand of a polarizing microscope, then the temperature was raised and a check was made on optical anisotropy. As a result, the polyester exhibited a good optical anisotropy at temperatures not lower than 238° C. Moreover, the polyester was measured for thermal characteristics by means of a differential scanning calorimeter; as a result, its glass transition temperature and melting point were 125° C. and 251° C., respectively. 
     Further, the polyester was charged to a Koka type flow tester and spun through a spinneret 0.3 mm in diameter at a spinneret temperature of 310° C. to obtain a spun yarn 0.14 mm in diameter. The melt viscosity of the polyester was 800 poise at a shear rate of 3,000 (1/sec). 
     The thus-spun yarn was measured for modulus at a sample length of 50 mm and at a pulling rate of 10 mm/min by means of TENSILON; as a result, it was found to have a modulus as high as 58 GPa. Moreover, when measured using VIBRON, the spun yarn proved to have a modulus as high as 73 GPa at 30° C. 
     COMPARATIVE EXAMPLE 8 
     10.45 g. (5×10 -2  mol) of methylhydroquinone diacetate, 3.63 g. (1.5×10 -2  mol) of 4,4&#39;-diphenyldicarboxylic acid and 10.57 g. (3.5×10 -2  mol) of 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid (a known composition as described in U.S. Pat. No. 3,991,013) were charged into a test tube for polymerization and polycondensed in the same way as in Example 1 to obtain an optically anisotropic polyester having a melting point of 258° C. 
     The polyester was spun through a spinneret 0.3 mm in diameter at a spinneret temperature of 300° C. to obtain a spun yarn 0.085 mm in diameter. The yarn was measured for modulus using RHEOVIBRON in the same way as in Example 1; as a result, its modulus wass 41 GPa, lower than that of 58 GPa in Example 1. 
     EXAMPLES 9-13 AND COMPARATIVE EXAMPLE 9 
     5×10 -2  mol of diacetate selected from methylhydroquinone diacetate (I) and chlorohydroquinone diacetate (II), and 5×10 -2  mol of dicarboxylic acids selected from 4,4&#39;-diphenyldicarboxylic acid (III), 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid (IV), 1,2-bis(2-bromophenoxy)ethane-4,4&#39;-dicarboxylic acid (V) and 1,2-bis(phenoxy)ehtane-4,4&#39;-dicarboxylic acid (VI), as shown in Table 5, were polycondensed in a test tube for polymerization. 
     The resultant polyesters were measured for liquid crystal initiation temperature and melting point, using a differential scanning calorimeter for the measurement of melting point, results of which are as set forth in Table 5. 
     Further, spun yarns were obtained by spinning of those polyesters and measured for modulus using RHEOVIBRON. As is apparent from the results shown in Table 5, the spun yarns obtained from the polyesters of Examples 9-13 have high values of modulus ranging from 61 to 103 GPa, while the spun yarn obtained from the polyester of Comparative Example 9 has a lower modulus of 40 GPa. 
     
                                           TABLE 5__________________________________________________________________________                                 Liquid                                 Crystal                                 Initia-                                     Melt-                                         Spin-                                             Fine-  Hydroquinone          Dicarboxylic Acid      tion                                     ing ning                                             ness    Melt.sup.(1)  Component          Component   (III):(IV + V + VI)                                 Temp.                                     Point                                         Temp.                                             (mm                                                Modulus                                                     Viscosity  (I) (II)          (III)             (IV)                (V)                   (VI)                      mol ratio  (°C.)                                     (°C.)                                         (°C.)                                             dia.)                                                (GPa)                                                     (poise)__________________________________________________________________________Example 9  100 --  85 15 -- -- 85/15      275 280 330 0.06                                                83    20Example 10  100 --  85 7.5                7.5                   -- 85/15      266 268 320 0.24                                                61   600Example 11  100 --  85 7.5                -- 7.5                      85/15      267 269 340 0.12                                                64   500Example 12  --  100 85 7.5                -- 7.5                      85/15      312 303 360 0.09                                                103  900Example 13  --  100 70 15 -- 15 70/30      286 301 360 0.05                                                85   1500Comparative  --  100 30 -- -- 70 30/70      231 263 280 0.10                                                40   400Example 9__________________________________________________________________________ .sup.(1) Measured at a spinning temperature of melting point plus 40-100° C. and at a shear rate of 2,000 to 4,000 (1/sec). 
    
     EXAMPLE 14 
     11.48 g. (5×10 -2  mol) of chlorohydroquinone diacetate, 5.57 g. (1.5×10 -2  mol) of 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid and 5.81 g. (3.5×10 -2  mol) of terephthalic acid were charged into a test tube for polymerization and subjected to polycondensation reaction involving removal of acetic acid in the following manner. First, reaction was allowed to take place at 250-330° C. in a nitrogen gas atmosphere for 2.5 hours, then the pressure was reduced to 0.5 mmHg and heating was continued for another 1 hour to complete polycondensation. As a result, 5.7 g. of acetic acid corresponding to 96% of a theoretical amount was distilled out to obtain a silver black polyester. 
     The polyester was of the following theoretical structural formula, and elementary analysis values thereof well coincided with theoretical values as set forth in Table 6. In infrared spectroscopic analysis, the polyester exhibited characteristic absorption at 1400, 1480, 1590 and 1735 cm -1 . ##STR8## 
     
                       TABLE 6______________________________________  Measured Value             Theoretical Value  (wt %)     (wt %)______________________________________C        54.2         55.3H        2.4          2.5Cl       14.8         16.0O        28.6         26.2______________________________________ Note The oxygen content (%) was calculated as (100%  C %  H %  Cl %). 
    
     The polyester was put on a sample stand of a polarizing microscope, then the temperature was raised and a check was made on optical anisotropy. As a result, the polyester exhibited a good optical anisotropy at temperatures not lower than 262° C. 
     Moreover, the polyester was measured for thermal characteristics by means of a differential scanning calorimeter; as a result, its glass transition temperature and melting point were 117° C. and 297° C., respectively. 
     Further, the polyester was charged to a Koka type flow tester and spun through a spinneret 0.3 mm in diameter at a spinneret temperature of 360° C. to obtain a spun yarn 0.036 mm in diameter. The melt viscosity of the polyester was 300 poise at 360° C. and at a shear rate of 3,100 (1/sec). 
     The spun yarn was measured for modulus using RHEOVIBRON; as a result, it proved to have a modulus as high as 110 GPa at 30° C. 
     COMPARITIVE EXAMPLE 10 
     Phenylhydroquinone and terephthalic acid were polymerized in accordance with Japanese Patent Publication No. 40978/1983, and yarn was obtained by spinning of the resultant polyester and it was measured for modulus, which was found to be as low as 20 GPa at 30° C. Moreover, chlorohydroquinone and 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid were polymerized in accordance with Japanese Patent Publication No. 482/1980, and yarn was obtained by spinning of the resultant polyester and it was measured for modulus, which was found to be as low as 12 GPa at 30° C. 
     EXAMPLE 15 
     10.45 g. (5×10 -2  mol) of methylhydroquinone diacetate as a substitute for the 11.48 g. (5×10 -2  mol) chlorohydroquinone diacetate used in Example 15, 5.57 g. (1.5×10 -2  mol) of 1,2-bis(2-chlorophenoxy)-ethane-4,4&#39;-dicarboxylic acid and 5.81 g. (3.5×10 -2  mol) of terephthalic acid were reacted at 250-320° C. for 3 hours as in Example 1, thereafter the pressure was reduced to 0.5 mmHg, at which pressure reaction was allowed to proceed for another 1 hour. As a result, 5.8 g. of acetic acid corresponding to 97% of a theoretical amount was distilled out to obtain a brown polyester. 
     The polyester was of the following theoretical structural formula, and elementary analysis values thereof well coincided with theoretical values as shown in Table 7. In infrared spectroscopic analysis, the polyester exhibited characteristic absorption at 1500, 1600 and 1740 cm -1 . ##STR9## 
     
                       TABLE 7______________________________________  Measured Value             Theoretical Value  (wt %)     (wt %)______________________________________C        65.4         66.2H        3.9          3.7Cl       7.5          6.8O        23.2         23.3______________________________________ Note- The oxygen content (%) was calculated as (100%  C %  H %  Cl %). 
    
     Using a polarizing microscope, the polyester was checked for optical anisotropy; as a result, it exhibited a good optical anisotropy at temperature not lower than 245° C. 
     Further, the polyester was measured for thermal characteristics by means of a differential scanning calorimeter; as a result, its glass transition temperature, melting point and temperature of crystallization on cooling were 128° C., 287° C. and 215° C., respectively. 
     A 0.045 mm dia. yarn was obtained by spinning of the polyester at 305° C. as in Example 1. It proved to have a modulus as high as 68 GPa at 30° C. when measured using RHEOVIBRON. 
     EXAMPLES 16-20 AND COMPARATIVE EXAMPLES 11-14 
     The following aromatic diols (I)-(V) and aromatic dicarboxylic acids (VI)-(IX) were combined as shown in Table 8, charged into test tubes for polymerization so that in each combination the amount of aromatic diol component and that of aromatic dicarboxylic acid component were each 0.5×10 -2  mol, and polycondensed therein: 
     (I) chlorohydroquinone diacetate 
     (II) methylhydroquinone diacetate 
     (III) phenylhydroquinone diacetate 
     (IV) 4,4&#39;-diacetoxybiphenyl 
     (V) 2,6-diacetoxynaphthalene 
     (VI) 1,2-bis(2-chlorophenoxy)ethane-4,4&#39;-dicarboxylic acid 
     (VII) 1,2-bis(2-bromophenoxy)ehtane-4,4&#39;-dicarboxylic acid 
     (VIII) 1,2-bis(phenoxy)ethane-4,4&#39;-dicarboxylic acid 
     (IX) terephthalic acid 
     The polyesters thus obtained were checked for optical anisotropy, results of which are as set forth in Table 8. Reference to Table 8 shows that all of the polyesters obtained in Examples 16-20 exhibit good optical anisotropy and fluidity, while the polyesters obtained in Comparative Examples 11-14 are all high in melt viscosity and poor in fluidity. 
     
                                           TABLE 8__________________________________________________________________________                                        Liquid Melt-  Diol Component Dicarboxylic Acid                               [(VIII) + (IX)]/                                        Crystal                                               ing Melt.sup.(1)  (mol %)        Component (mol %)                               [(VI) + (VII)]                                        Initiation                                               Point                                                   Viscosity  (I)     (II)        (III)           (IV)              (V)                 (VI)                    (VII)                        (VIII)                            (IX)                               (mol ratio)                                        Temp. (°C.)                                               (°C.)                                                   (poise)__________________________________________________________________________Example 16  100     -- -- -- -- 30 --  70  -- 70/30    226    245 300Example 17  -- 100        -- -- -- -- 30  70  -- 70/30    201    234 300Example 18  -- -- 100           -- -- 30 --  --   70                               70/30    197-339                                               not 200                                               clearExample 19  -- --    100              -- 30 --  70  -- 70/30    303    327 300Example 20  -- -- -- -- 100                 30  0  70   0 70/30    308    315 300Comparative  -- -- -- 100              -- -- --  --  100                               100/0    &gt;500   --  Measure-Example 11                                              ment wasComparative  -- -- -- -- 100                 -- --  --  100                               100/0    &gt;500   --  impossible.Example 12Comparative  -- -- -- 100              -- -- --  100 -- 100/0    370     395,                                                   more thanExample 13                                          410 10,000Comparative  -- -- -- -- 100                 -- --  100 -- 100/0    378    395 poiseExample 14__________________________________________________________________________ .sup.(1) Measured at a temperature of melting point plus 40-100° C and at a shear rate of 2,000 to 4,000 (1/sec.)