Polyarylate formation by ester interchange reaction
FIELD OF THE INVENTION 
     The invention pertains to a method for preparing automatic polyesters (polyarylates). In a further aspect, the invention relates to aromatic polyesters with suitable inherent viscosity and color. 
     BACKGROUND OF THE INVENTION 
     Aromatic polyesters (polyarylates) are a class of polyesters generally made from a bisphenol and an aromatic diacid. Heretofore, the prior art has taught that polyarylates preferably are made in a two-step process starting with the bisphenol and reacting with such as acetic acid or acetic anhydride to prepare a diacetate of the bisphenol, followed by reaction with an aromatic diacid. In general, the prior art has taught that satisfactory polymer cannot be made from such as Bisphenol-A diacetate and aromatic diacids in the absence of a solvent, which, of course, adds another reactant to control, later separate, and in a commercial process recover for recycle. 
     In addition to the cumbersome nature of a multi-step process, the polyarylates in the past have frequently been unduly brittle, or had low inherent viscosities, or have been dark colored which is undesirable, or some combinations of these undesirable features. 
     Needed is a simple, effective, useful process to produce polyarylates conveniently, with a minimum of reagents, and yet result in a product of good moldability, suitably high inherent viscosity, and low color. 
     BRIEF DESCRIPTION OF THE INVENTION 
     I have discovered a process for preparing aromatic polyesters (polyarylates) from a bisphenol, an aromatic diacid, in admixture with a carboxylic acid anhydride, by conducting the process reaction in four phases: 
     (1) heating of an admixture of the reactants at a first moderately elevated temperature, not to exceed about 160° C., in an inert gas atmosphere for a first suitable time interval; 
     (2) gradually elevating the temperature of the reaction admixture, in the inert gas atmosphere, to a higher second temperature over a second suitable time interval while maintaining a flow of inert gas to assist in removing volatiles; 
     (3) reducing the pressure of the reaction admixture gradually to a vacuum condition during a third time interval of at least 60 minutes while substantially maintaining the second higher temperature; and 
     (4) maintaining the reaction admixture at substantially the higher second temperature at the vacuum conditions for a suitable fourth time interval. 
     The result of my process is a light-colored, tough, polyarylate polymer. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Bisphenol 
     The bisphenols suitable for use in the context of my invention can be represented by the structural formula: 
     
         HO--Ar--OH 
    
     wherein Ar is a divalent aromatic unit. Examples of such divalent aromatic units Ar include: ##STR1## and the like. In the above formulae, each R can be the same or different, and each is independently selected from and represents H or a lower alkyl or cycloalkyl groups of 1 to 8 carbon atoms per group, presently preferably H or the methyl group. R 2  is an alkylene or alkylidene radical having 1 to 8 carbon atoms, such as --CH 2  --, --CH 2  --CH 2  --, --CH(CH 3 )--, --C(CH 3 ) 2  --, and the like. 
     Examples of bisphenols include: 
     hydroquinone, 
     resorcinol, 
     bis(4-hydroxyphenyl)methane, 
     1,1-bis(4-hydroxyphenyl)ethane, 
     1,2-bis(4-hydroxyphenyl)ethane, 
     2,2-bis(4-hydroxyphenyl)propane (Bisphenol A), 
     2,2-bis(2,6-dimethyl-4-hydroxyphenyl)propane, 
     2,2-bis(2,4-dioctyl-4-hydroxyphenyl)propane, 
     2,2-bis(2-t-butyl-4-hydroxyphenyl)pentane, 
     3,3-bis(2-isoamyl-4-hydroxyphenyl)heptane, 
     2,4-bis(2-isoamyl-4-hydroxyphenyl)heptane, 
     bis(4-hydroxyphenyl)ether, 
     bis(4-hydroxyphenyl)sulfide, 
     bis(4-hydroxyphenyl)sulfoxide 
     bis(4-hydroxyphenyl)sulfone, 
     bis(4-hydroxybenzophenone), and 
     2,6-dihydroxynaphthalene. 
     The presently preferred bisphenol is Bisphenol-A. 
     Diacids 
     Diacids employable in the process of my invention are the aromatic dicarboxylic acids. These include such as the presently preferred terephthalic acid, isophthalic acid, and mixtures thereof, as well as naphthalene dicarboxylic acids, and alkyl-substituted homologs of such dicarboxylic acids wherein the alkyl group contains 1 to 4 carbon atoms. 
     Mixtures of any ratio of isophthalic acid and terephthalic acids can be used. In these mixtures, the isophthalic acid:terephthalic acid mole or weight ratio can range from such as about 95:5 to 5:95. Preferably, the ratio in such a mixture is in the range of about 90:10 to 30:70 for more amorphous properties, with a presently more preferred ratio of about 70:30 to 30:70 to produce the presently preferred non-crystalline polyarylates. Outside of the broad range, crystalline characteristics of the resulting polyarylates become predominant as is apparent by examination by differential scanning calorimetry. 
     Other suitable and typical aromatic diacids include those mentioned and: 
     5-methylisophthalic acid, 
     2-methylisophthalic acid, 
     2-methyl-5-ethylisophthalic acid, 
     2-methylterephthalic acid, 
     2,6-dimethylterephthalic acid, 
     2-propylterephthalic acid, 
     2,6-naphthalene dicarboxylic acid, and mixtures thereof. 
     Optionally, hydroxyaromatic monocarboxylic acids, such as m- or p-hydroxybenzoic acid, can be used in part, such as in a mole ratio relative to the diacid of about 1:100 to 1:2, preferably about 1:100 to 1:10, to vary the properties of the product. 
     Acid Anhydrides 
     The acid anhydride employed is derived from a hydrocarbon organic dicarboxylic acid, preferably of 2 to 8 carbon atoms per molecule. The presently preferred acid anhydride is acetic anhydride for convenience and availability. Other useful acid anhydrides include such as propionic anhydride, butyric anhydride, valeric anhydride, caproic anhydride, and caprylic anhydride. Mixtures can be employed. 
     The anhydride(s) used preferably should be normally liquid, or, if normally solid, should have a melting point not exceeding about 40° C. so as to be liquid under convenient reaction temperatures. 
     REACTION CONDITIONS 
     Summary 
     In accordance with my invention, at least one bisphenol, at least one aromatic diacid, and at least one acid anhydride, are admixed together to form a reaction admixture, and the reaction then proceeds in a programmed sequence of four phases: 
     (1) heating the reactants at a first moderately elevated temperature, preferably not to exceed about 160° C., in an inert gas atmosphere, for a suitable time; 
     (2) elevating the temperature of the reaction admixture, in the inert gas atmosphere, to a higher second temperature during a second time interval while maintaining a flow of inert gas into and through the reactants to assist in removing volatiles; 
     (3) reducing the pressure gradually over a third time interval of at least about 60 minutes to a vacuum condition while maintaining the higher second temperature; 
     (4) heating the reaction mixture at substantially the second temperature at vacuum condition for a suitable fourth further time interval; and recovering the product. 
     Reactant Ratios 
     The molar ratio of bisphenol:aromatic diacid(s) should be such as to avoid large excesses of either reactant, though an excess of the bisphenol generally is considered less desirable. The presently preferred molar ratio of bisphenol:aromatic diacid(s) should be in the range of about 0.8:1 to 1.2:1, presently more preferably very close to stoichiometric, about 0.95:1 to 1:05:1 mole ratio. 
     Further employed is the hydrocarbon organic acid anhydride. The amount employed can range widely as convenient. Presently suggested is a molar ratio of anhydride:aromatic diacid(s) in the range of about 1:1 to 4:1, preferably about 2:1 to 2.3:1. 
     REACTION STEPS 
     First step 
     In the first reaction step, the reactants as a reaction admixture are placed in a reactor means which is filled with an inert gas blanket. 
     The reaction admixture is heated up to a first reaction temperature not to exceed about 160° C., preferably in the range of about 120° to 160° C., more preferably about 130° to 140° C., under an inert gas atmosphere during a first heating time interval of about 0.25 to 2 hours, preferably about 0.5 to 0.75 hour. The reaction admixture is maintained at substantially the first reaction temperature for an effective time, preferably about 1 to 3 hours, more preferably about 1.3 to 1.7 hours. 
     The inert gas and the reaction admixture is maintained at a pressure of about 0 to 120 psig, preferably about 0 to 20 psig. More preferably, the first step is conducted while a stream of inert gas passes through the reaction admixture. 
     The inert gas can be selected from the presently preferred nitrogen, as well as from such as helium, argon, and neon. 
     Second Step 
     In the second step, the reactor means is vented within a short time period, of such as about 10 minutes, when an elevated inert gas pressure (i.e., &gt;0 psig) has been used in the first step. The temperature of the reaction admixture is raised to a higher second reaction temperature, in the range of about 250° C. to 350° C., preferably 280° C. to 330° C., most preferably 290°-310° C., and maintained thereat over a second time interval of about 1 to 4 hours, presently preferred about 1 to 2 hours. 
     Third Step 
     Thereafter, the pressure on the reaction admixture is gradually reduced to a vacuum condition of about 1 to 30 mm Hg, preferably about 1 to 5 mm Hg, over a third time interval of at least about 1 hour, preferably over about 1 to 1.5 hours, all while substantially maintaining the aforesaid second reaction temperature. Carboxylic acid, water, and any excess reactant, are removed by evaporation during this step. 
     A prepolymer is formed in the sequence of the first three steps. This relatively low molecular weight prepolymer preferably has an inherent viscosity of about 0.2 to 0.3. 
     Fourth Step 
     The prepolymer then is maintained at substantially the second reaction temperature, preferably at about 280° to 330° C., under the vacuum condition for a fourth time interval, of at least about 2 hours, preferably about 2 to 4 hours, most preferably, about 2 to 3 hours, to complete the polymerization so as to prepare a polymer preferably having an inherent viscosity of about 0.5 to 1. 
     Product Recovery 
     All four process steps of this invention can be carried out as a one-pot process in the same reactor. The cooled product is a hardened polymer product which can be removed from the reactor by breaking pieces of polymer off by means of such as chisel and hammer. Obviously, this recovery method is not very feasible for large-scale, commercial processes. 
     In the preferred recovery process, the prepolymer is prepared by the first, second and third steps in an autoclave or similar reactor means, and then extruded by means of elevated pressure under an inert gas into a ventable extruder such as a Polycon® twin extruder, where the fourth step is carried out. The final polymer then is preferably expelled (extruded) through one or more dies, cooled (e.g., in a cold water bath), and chopped into pellets. 
    
    
     EXAMPLES 
     Examples are provided in an effort to assist one skilled in the art to a further understanding of the invention, and yet not be unduly limitative of the reasonable scope of the invention. The particular reactants, conditions, ratios, and the like, are all intended to be illustrative of my invention, and not limitative of the reasonable and suitable scope thereof. 
     Example 
     In the runs of this example, results are shown of numerous experimental runs for preparing polyarylates from Bisphenol-A (abbreviated BPA; UHP-grade) supplied by Union Carbide Corp., Danburg, Conn.), terephthalic acid (abbreviated TPA; fiber grade; supplied by Amoco Chem. Corp., Chicago, Ill.), isophthalic acid (abbreviated IPA; ultra-pure grade IPA 220; supplied by Amoco Chem. Corp.), and acetic anhydride (abbreviated AA; 99+% purity; supplied by Aldrich Chem. Co., Milwaukee, Wisc.). These reactants were charged to a 2-liter, stirred autoclave made of 316 stainless steel. The autoclave was equipped with valves and pipes for pressurizing, venting, evacuating, and distilling. 
     The objective of the polymerization runs, the most pertinent process parameters of which are summarized in Table IA, was, was to make amorphous polyarylates of light color and high inherent viscosity (a measure of molecular weight), without excessive foaming and plugging problems during the vacuum distillation stage. The amounts of reactants used were about 1 mole of BPA, about 0.5 mole of TPA, about 0.5 mole of IPA, and about 2.15-2.2 moles of AA. 
     
                                           TABLE IA                                
__________________________________________________________________________
Phase I                           Phase II                                
1st Peak  Peak   Heating Time (minutes)                                   
                                  2nd Peak                                
                                         Heating Time (Minutes)           
Run                                                                       
   Temp. (°C.)                                                     
          Press. (psig)                                                   
                 up to Peak Temp.                                         
                          at Peak Temp.                                   
                                  Temp. (°C.)                      
                                         During Venting                   
                                                 During N.sub.2           
__________________________________________________________________________
                                                 Purge                    
1  200-230                                                                
          30-55  45       50      295    10      120                      
2  240-260                                                                
          150    60       30      285    90      --                       
3  200-205                                                                
          100    35       85      291    20      48                       
4  200-205                                                                
          100    30       90      296    20      55                       
5  200-201                                                                
          110    50       90      285    30      45                       
6  206-210                                                                
          170    65       90      272    55      10                       
7  200-210                                                                
          170-230                                                         
                 35       120     295    70      25                       
8  200-204                                                                
          100-105                                                         
                 40       120     304    60      55                       
9  200-202                                                                
          170    35       120     304    60      45                       
10 200-201                                                                
          120-160                                                         
                 50       90      302    30      55                       
11 200-205                                                                
          105    32       90      293    30      118                      
12 200-204                                                                
           92-110                                                         
                 43       90      306    49      119                      
13 200-206                                                                
          118    40       120     305    30      83                       
14 199-202                                                                
          110    30       120     299    28      88                       
15 200    130-135                                                         
                 42       120     300    42      75                       
16 198-200                                                                
          50-60  35       125     280    20      70                       
17 200-205                                                                
          160    39       120     300    30      65                       
18 200    90     25       120     299    25      98                       
19 200-240                                                                
          260-280                                                         
                 105      45      290    60      40                       
20 194-200                                                                
          100-120                                                         
                 35       120     295    25      70                       
21 140-150                                                                
          85     35       90      285     5      90                       
22 150    50-80  35       90      300    10      95                       
23 150-151                                                                
          40-50  25       60      310    18      255                      
24 150-160                                                                
          40     31       90      305    40      160                      
__________________________________________________________________________
 
    
     
                                           TABLE IB                                
__________________________________________________________________________
Phase III                                                                 
                Heating Time (minutes)                                    
Final Press.                                                              
         Final  During                                                    
                      at    Polymer                                       
Run                                                                       
   (mm Hg)                                                                
         Temp. (°C.)                                               
                Evacuation                                                
                      Final Press.                                        
                            Color   IV Observation                        
__________________________________________________________________________
1  0.1-0.4                                                                
         300-320                                                          
                55     70   dark amber                                    
                                    0.25                                  
                                       --                                 
2  5     298-299                                                          
                100    85   medium amber                                  
                                    0.37                                  
                                       no foaming                         
3  5     301-311                                                          
                47    115   --      0.66                                  
                                       foaming problems                   
4  5     298-310                                                          
                95    145   --      0.50                                  
                                       no foaming                         
5  5     300-319                                                          
                85    120   amber   0.61                                  
                                       --                                 
6  4-5   300-319                                                          
                70    145   dark brown                                    
                                    -- some plugging                      
7  3-5   305-322                                                          
                75    140   dark    0.52                                  
                                       --                                 
8  5     304-310                                                          
                80          medium dark                                   
                                    0.60                                  
                                       --                                 
9  8     300-330                                                          
                75    195   medium amber                                  
                                    0.35                                  
                                       --                                 
10 5     310-324                                                          
                70    150   dark amber                                    
                                    0.44                                  
                                       --                                 
11 5     310    40    --    --      -- lines plugged                      
12 5     313-329                                                          
                85    125   medium amber                                  
                                    0.52                                  
                                       --                                 
13 1-5   317-320                                                          
                70    120   good color                                    
                                    0.69                                  
                                       --                                 
14 1-5   299-318                                                          
                65    120   --      0.32                                  
                                       plugging problems                  
15 1-5   300-317                                                          
                66     65   medium dark                                   
                                    0.66                                  
                                       no plugging                        
16 1     300    60    120   good color                                    
                                    0.60                                  
                                       --                                 
17 5     310-315                                                          
                65    120   dark    0.74                                  
                                       --                                 
18 5     311-319                                                          
                67     75   medium amber                                  
                                    0.40                                  
                                       --                                 
19 2-5   280    60    120   dark    0.53                                  
                                       --                                 
20 1-5   290-310                                                          
                60    120   dark amber                                    
                                    0.72                                  
                                       --                                 
21 1-5   300-310                                                          
                55    120   good color                                    
                                    0.32                                  
                                       --                                 
22 1-5   310-315                                                          
                115   120   light color                                   
                                    0.59                                  
                                       --                                 
23 5     311    54     58   good color                                    
                                    0.27                                  
                                       partial plugging                   
24 1-5   310    60    170   medium amber                                  
                                    0.50                                  
                                       some sublimation.sup.(2)           
__________________________________________________________________________
 .sup.(1) venting was done at the 1st peak temperature                    
 .sup.(2) several grams of sublimate accumulated on the upper reactor     
 walls; observed also in various other runs not listed in this table.     
 
    
     Data in Table IA and IB show the following critical parameters: 
     (1) the peak temperature during the first heating stage under elevated pressure should not exceed about 160° C. to attain a light colored polyarylate; 
     (2) the time for reducing the pressure from about 760 to about 1-5 mm Hg should be at least 60 minutes so as to avoid foaming and vacuum line plugging problems; 
     (3) the heating time at the final temperature of 300°-320° C. under vacuum conditions should be about 2-3 hours to give a polymer inherent viscosity of at least about 0.5 
     Exemplary Summary 
     In this summary, the optimal lab-scale process conditions for making amorphous polyarylates having light (yellow) color and desirably high inherent viscosity (at least about 0.5) are summarized. One mole of BPA, about 1 mole of a mixture of TPA and IPA (molar ratio 7:3 to 3:7, preferably 1:1), and 2.2 moles of AA are placed in a closed steel autoclave (about 2 liter capacity). The cold autoclave is pressurized three times with about 150 psi N 2  and vented to atmospheric pressure. The reactor is heated to about 130°-140° C. within a time period of about 30-40 minutes while a continuous stream of 0.4 SCFM N 2  is passed through the autoclave. Then the reaction mixture is kept at about 130°-140° C. in a nitrogen atmosphere (optionally under 60-120 psig pressure) for about 1.5 hours. 
     During the second phase, the autoclave, if under elevated pressure, is vented at about 130°-140° C. within about 10 minutes to about atmospheric pressure. Then the temperature is gradually raised to about 300° C. within a time period of about 1.5 hours, while a continuous stream of N 2  is passed through the reactor (autoclave) and volatiles (mainly acetic acid) are distilled off. To avoid the accumulation of variable amounts of sublimate on the upper inside walls of the reactor, the entire autoclave including its head and its exit lines are heated. 
     During the third phase of the process, the reactor is gradually evacuated at about 300° C. to a final pressure of about 1-5 mm Hg during a time period of at least 60 minutes. Then the reactor content is heated for at least two additional hours, preferably about 3 hours, at a temperature of about 300° to 320° C. and a pressure of 1 to 5 mm Hg. 
     In a lab scale process, the hard final polymer is removed from the cold steel reactor with tools such as chisel and hammer. In a pilot or commercial process (employing larger amounts of BPA, TPA/IP and AA and a proportionately larger autoclave than indicated above, yet retaining the above-cited molar ratios), the intermediate polymer (&#34;prepolymer&#34;) from Phase II is expelled into a heated extruder, which can be evacuated to about 1-5 mm Hg, and then polymerized under Phase III conditions. The finished polymer is extruded through one or more dies, cooled (e.g., in a water bath) and chopped into pellets. 
     The disclosure, including data, has illustrated the value and effectiveness of my invention. The Examples, the knowledge and background of the field of the invention and the general principles of chemistry and of other applicable sciences have formed the bases from which the broad descriptions of my invention including the ranges of conditions and the generic groups of operant components have been developed, and formed the bases for my claims here appended.