JPH01247420A - Preparation of polyarylate - Google Patents

Abstract

PURPOSE:To obtain efficiently a polyarylate having excellent hue and physical properties, by melt-polymerizing an arom. dihydroxyl compd., diaryl carbonate and an arom. dicarboxylic acid as the raw material at a specified ratio to form a prepolymer and then polymerizing this prepolymer in a solid phase after crystallization thereof. CONSTITUTION:A diol (A) mainly contg. an arom. dihydroxyl compd., diaryl carbonate (B) and an arom. dicarboxylic acid (C) are used in such a way that relations of 0.7<=(A+C)/B<=1.1 and 0.9<=C/A<=1.1 based on the molar numbers are satisfied. These compd. are melt-polymerized together under heating to form a prepolymer and solid-phase polymn. of this prepolymer is performed after its crystallization. It is also possible to add an arom. hydroxy carboxylic acid (D), if necessary, in such a way that relations of 0.7<=(A+C+D)/B<=1.1, 0.9<=C/A<=1.1 and D/(A+C+D)<=0.7 are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a novel method for producing polyarylates. More specifically, the present invention makes polyarylate, an engineering resin with excellent properties such as heat resistance and little coloring, using inexpensive raw materials, without producing undesirable corrosive by-products, and The present invention relates to a method for efficiently producing the polymer at a high polymerization reaction rate.

BACKGROUND OF THE INVENTION In recent years, polyarylates have been used as engineering resins as materials for industrial parts in various fields because of their excellent heat resistance, chemical resistance, and mechanical properties.

As a method for producing polyarylates, for example, an interfacial polycondensation method (Japanese Patent Publication No. 10-1983) in which aromatic dicarboxylic acid chloride and aromatic dihydroxy compound are reacted has been used.
No. 59), a melt polymerization method in which an aromatic dihydroxy compound and a diphenyl ester of an aromatic dicarboxylic acid are reacted, or an aromatic dihydroxy compound, an aromatic dicarboxylic acid, and a diaryl carbonate are reacted (U.S. Pat. No. 4,680). , No. 372, Special Publication Sho H-2629
9), etc. are known.

However, the former method has the disadvantage that a large amount of solvent must be used, and the latter method has the disadvantage that the melt viscosity of the polymer becomes extremely high at the end of the polymerization, making it difficult to complete the polymerization. If an attempt is made to raise the polymerization temperature in order to lower the viscosity, there will be problems such as coloring of the polymer and deterioration of physical properties.

As a method for improving such drawbacks of the melt polymerization method, for example, a prepolymer obtained by melt polymerizing an acetate ester of an aromatic dihydroxy compound and an aromatic dicarboxylic acid is crystallized using a crystallization solvent. , solid phase polymerization method (JP-A-57-2331, U.S. Pat. No. 3,
No. 684,766, No. 3,780,148), after crystallizing a prepolymer obtained by melt polymerizing an aromatic dihydroquine compound and a diphenyl ester of an aromatic dicarboxylic acid, a solid phase Polymerization method (JP-A-53-35796, S-3-43797, Sho-53-51295, Sho-53-114895)
Publication No. 2), etc. have been proposed. However, in the former method, the performance of the obtained polymer is inferior, and
There are drawbacks such as acetic acid being produced as a by-product which causes corrosion of the equipment, and on the other hand, in the latter method, it is difficult to produce the aromatic dicarboxylic acid diphenyl ester as a starting material, and the cost is inevitably high.

Problems to be Solved by the Invention The present invention overcomes the drawbacks of the conventional polyarylate manufacturing method, and uses inexpensive raw materials to produce polyarylate with excellent color and physical properties without unfavorable corrosive properties. The purpose of this invention is to provide a method for efficiently producing polymers at a high polymerization reaction rate without producing any by-products.

Means for Solving the Problems The present inventors have conducted intensive research to achieve the above object, and have found that as raw materials, diols mainly consisting of aromatic dihydroxy compounds, diaryl carbonates, aromatic dicarboxylic acids, and as desired. It has been found that the purpose can be achieved by melt polymerizing hydroxy aromatic carboxylic acids in a specific ratio to form a prepolymer, then crystallizing the prepolymer, and then performing solid phase polymerization, Based on this knowledge, we have completed the present invention.

That is, the present invention uses (A) a diol mainly composed of an aromatic dihydroxy compound, (B) a diaryl carbonate, and (C) an aromatic dicarboxylic acid in a ratio that satisfies the relationship of the formula based on the number of moles, and A method for producing a polyarylate, which comprises melt-polymerizing a prepolymer to form a prepolymer, crystallizing the prepolymer, and then solid-phase polymerizing the polyarylate, and components (A) and (B). and(
Component C) and (D) hydroquine aromatic carboxylic acid are used in a ratio that satisfies the relationship of formula % formula % () ... (IV) based on the number of moles, and are melt-polymerized under heating to form a prepolymer. The present invention provides a method for producing a polyarylate, which comprises forming a polyarylate, crystallizing the prepolymer, and then performing solid phase polymerization. Here, polyacrylate is a polyester in which both the dicarboxylic acid component and the diol component are composed of aromatic compounds.
It means a so-called wholly aromatic polyester.

The present invention will be explained in detail below.

In the method of the present invention, the raw material (A)! As the component, a diol mainly consisting of an aromatic dihydroxy compound is used. The aromatic dihydroxy compound is represented by the formula -maximum%%(), where A' represents a divalent aromatic residue of the aromatic dihydroxy compound, and such an aromatic group is For example, phenylene (various), naphthylene (various), biphenylene (various), pyridazine (
various types), and divalent aromatic groups represented by -maximum % formula %).

Here, Ar' and Ar2 are divalent aromatic groups which may be the same or different, such as phenylene (various), naphthylene (various), hyphenylene (various), pyridazine (various). represents a group such as Z is a simple bond or -〇-1-CO-1-S-1-SO□-1-
Represents a divalent group such as CO□-1-C0N(R')-1 (where R1, Q2. R3, R4 may be the same or different, and represents hydrogen, lower alkyl group, lower alkoxy group, cycloalkyl group, k is 3 to 11
).

Furthermore, such a divalent aromatic group (Ar or Ar1
．． Ar"), one or more hydrogen atoms can be substituted with other substituents that do not adversely affect the reaction, such as halogen atoms, lower alkyl groups, lower alkoxy groups, phenyl groups, phenoxy groups, vinyl groups, cyano groups, and esters. It may be substituted with a group, an amide group, a nitro group, or the like.

Examples of such aromatic dihydroxy compounds include dihydroxyphenols represented by: dihydroxybienyls represented by H3 CH1 CF.

(R6 and R6 in the formula are each a hydrogen atom, a halogen atom, a lower alkyl group having 1 to 4 carbon atoms, a lower alkoxy group having 1 to 4 carbon atoms, a cycloalkyl group, or a phenyl group, and these are the same) Even if there is one, tramethylene glycol is preferred, and one type of these may be used or two or more types may be used in combination.

In the method of the present invention, the diaryl carbonate used as component (B) of the raw material is represented by the formula -maximum%% () (Ar3 in the formula is an aromatic group), and Ar3 has no substituent. or other substituents in which one or more hydrogen atoms do not adversely affect the reaction, such as halogen atoms, lower alkyl groups, lower alkoxy groups, phenyl groups, phenoxy groups,
It may be substituted with a vinyl group, cyan group, ester group, nitro group, etc. Examples of such diaryl carbonates include diphenyl carbonate, dicresyl carbonate, di-β-7tyl carbonate, bis(2-chlorophenyl) carbonate, bis(cyanophenyl) carbonate, and his-trophenyl carbonate. m and n are i numbers from 1 to 4, and m is 2.
In the above cases, R5 may be different from each other, and when n is 2 or more, R6 may be different from each other.) Bisphenols such as these are preferably used.

Among these compounds, bisphenol A, which is 2,2-bis(4-hydroxyphenyl)propane, and substituted hisphenol A's are particularly preferred.

Further, these aromatic dihydroxy compounds may be used alone or in combination of two or more.

When two or more aromatic dihydroxy compounds are used, a polyarylate copolymer having the skeletons of two or more of these compounds is usually obtained.

The diol of component (A) is mainly the above-mentioned aromatic dihydroxy compound, but alkylene glycol may be mixed therein in an amount of 30% by weight or less. Examples of this alkylene glycol include ethylene glycol, 1,
Among these, diphenyl carbonate is preferred. These diaryl carbonates may be used alone or in combination of two or more.

In the method of the present invention, the aromatic dicarboxylic acid used as component (C) is represented by the formula % (), where Ar' is a divalent aromatic residue, and the -max (V ) can be exemplified. Examples of such aromatic dicarboxylic acids include terephthalic acid, inphthalic acid, naphthalene dicarboxylic acid,
Biphenyl dicarboxylic acid, diphenyl ether-4,4
'-Dicarboxylic acids and the like can be mentioned, among which terephthalic acid, isophthalic acid and mixtures thereof are preferred. One type of the aromatic dicarboxylic acid may be used, or two or more types may be used in combination.

In the method of the present invention, the above (A), (B) and (C)
The raw material components are based on the number of moles and (C) component 0.9≦□≦1.1...(I[)(
A) It is necessary to use them in proportions that satisfy the relationship between the components.

If the ratio of the total number of moles of component (A) and component (C) to the number of moles of component (B) deviates from the range of formula (I), it will be difficult to obtain the desired degree of polymerization. The preferable molar ratio is selected within the range of 0.8 to 1.05. Also,
If the ratio of the number of moles of component (C) to the number of moles of component (A) is less than 0.9, the resulting polyarylate may have poor heat resistance, and if it exceeds 1.1, a product with a high degree of polymerization may be obtained. There is a tendency to be less likely to be exposed.

In the method of the present invention, as raw material components, the above (A),
In addition to components (B) and (C), component (D), ie, hydroquine aromatic carboxylic acid, can be used.

This hydroxy aromatic carboxylic acid is represented by the formula %(), where Ar'' is a divalent aromatic residue, and is the same as explained by Ar in the general formula (V). Specific examples of such hydroquino aromatic carboxylic acids include p-hydroxybenzoic acid and 2-hydroquinonaphthalene-6-carboxylic acid. You can also use 2
You may use combinations of more than one species.

When such component (D) is used, the proportion of each component used needs to satisfy the following relationship based on the number of moles:

If the ratio of the total number of moles of component (A), component (C), and component (D) to the number of moles of component (B) deviates from the range of formula (Iff), the desired degree of polymerization may not be obtained. Hateful. The preferable molar ratio is selected within the range of 0.8 to 1.05. Furthermore, if the ratio of the number of moles of component (C) to the number of moles of component (A) is less than 0.9, the heat resistance of the resulting polyarylate may be poor, and if it exceeds 1.1, the resulting polyarylate may have a high degree of polymerization. is obtained, and a tendency to increase by 1 occurs. Furthermore, if the ratio of the number of moles of component (D) to the total number of moles of component (A), component (C), and component CD) exceeds 0.7, the resulting polyarylate tends to have reduced processability. .

Next, to explain the embodiments of the present invention, the method of the present invention includes three components (A), (B), and (C) or (
D) A first step in which the four components to which the components are added are reacted to form a prepolymer, and a second step in which the prepolymer is crystallized.
and a third step of solid-phase polymerizing - with the crystallized prepolymer to a desired degree of polymerization.First, in the first step, three components (A), (B) and (C) are
A prepolymer is formed by using the required amounts of each component or four components including component (D) and reacting them with heat. There are no particular restrictions on the order in which each component is added, and various methods can be used. For example, in the case of three components, (1
) A method in which the three components are simultaneously prepared and heated to polymerize; (2) a method in which the (A) component and (B) component are prepared in advance and heated to react, and then the (C) component is added and heated to polymerize; (3)
) A method in which components (B) and (C) are prepared in advance and reacted by heating, and then component (A) is added and polymerized by heating;
A method can be used in which component (C) is added little by little to a mixture of component (4XA) and component (B). Further, in the case of four components, a similar preparation method can be used.

The reaction temperature is preferably lower in order to prevent coloring of the polymer, but it is usually selected in the range of 150 to 330°C, taking into consideration the melt viscosity of the prepolymer and the reaction rate. Further, it is desirable to carry out the polymerization while removing carbon dioxide and phenols produced during the reaction from the system, and for this purpose, it is preferable to carry out the reaction while circulating an inert gas or under reduced pressure.

The reaction time required to form the prepolymer varies depending on the reaction conditions, but usually about 1 to 8 hours is sufficient. Furthermore, the reaction may be carried out in the absence or presence of a catalyst; however, although the reaction rate becomes faster when a catalyst is used, it may cause coloring of the polymer or adversely affect physical properties such as the hydrolysis resistance of the polymer. It is desirable to carry out the process without a catalyst because the

In addition, when carrying out the reaction using a catalyst, as a catalyst,
Alkali such as alkali metal salts of bisphenol and various transesterification catalysts can be used. Examples of the catalyst include hydroxides of alkali metals and alkaline earth metals such as lithium hydroxide, sodium hydroxide, potassium hydroxide, and calcium hydroxide, such as lithium hydride, sodium hydride, calcium hydride, etc. Alkali metal and alkaline earth metal hydrides; alkali metal salts, alkaline earth metal salts, and quaternary hydrides of boron and aluminum such as lithium aluminum hydride, sodium borohydride, and tetramethylammonium borohydride. Ammonium salts; alkoxides of alkali metals and alkaline earth metals such as lithium methoxide, sodium ethoxide, calcium methoxide; lithium phenoxide, sodium phenoxide, red sea bream, sium phenoxide, Lio-Ar-0Li +, -NxO
Aryloxides of alkali metals and alkaline earth metals such as -Ar-ONa (Ar is an aryl group); Organic acid salts of alkali metals and alkaline earth metals such as lithium acetate, calcium acetate, and sodium benzoate; zinc oxide, acetic acid Zinc, zinc compounds such as zinc phenoxide - boron compounds such as boron oxide, boric acid, sodium borate, trimethyl borate, tributyl borate, triphenyl borate; silicon oxide, sodium silicate, tetraalkyl silicon , tetraaryl silicon, diphenyl-
Compounds of silicon such as ethyl ethoxysilicon; compounds of germanium such as germanium oxide, germanium tetrachloride, germanium ethoxide, germanium phenoxide - tin oxide, dialkyl tin oxide, diaryl tin oxide, dialkyl tin carboxylate, tin acetate , tin compounds bonded with alkoxy or aryloxy groups such as ethyltin triptoxide, and organic tin compounds: lead oxide, lead acetate, lead carbonate, basic lead carbonate, alkoxides of lead and organic lead; Lead compounds such as aryl oxides: Onium compounds such as quaternary ammonium salts, quaternary phosphonium salts, quaternary arsonium salts - Antimony compounds such as antimony oxide and antimony acetate: manganese acetate, manganese carbonate, boron Catalysts such as manganese compounds such as manganese acid; titanium compounds such as titanium oxide, titanium alkoxide or aryloxide; zirconium compounds such as zirconium acetate, zirconium oxide, zirconium alkoxide or aryloxide, and zirconium acetylacetone; Can be done.

When using a catalyst, these catalysts may be used alone or in combination of two or more. Also,
The amount of these catalysts used is usually 1 to 10,000 to 1,000,000 per diol mainly composed of aromatic dihydroxy compounds as raw materials.
000 ppm.

The prepolymer formed has an intrinsic viscosity of 0.1 to
Those having a molecular weight in the range of 0.4 are preferred. If the intrinsic viscosity is less than 0.1, the solid phase polymerization time will be too long, and if it is more than 0.4, the melt viscosity will be high and melt polymerization will be difficult, which is not preferred.

When only an aromatic dihydroxy compound is used as the diol of the raw material (A>component), the terminal groups of the resulting prepolymer are -Ar-011(2), -Ar-0-C-0-A
There are four types represented by r'(2') (Ar and A- in the formula have the same meanings as above), and it is desirable that their content ratios satisfy the formula on a molar basis.

(Y) ten (Y')/(Z)+(Z
') outside the above range is not preferred because the molecular weight will be difficult to increase. Also, (Y)+( in the formula (,l)
When Z)/(y')+(Z') is less than 0.5, the reaction rate is slow, and when it exceeds 1.1, the thermal stability of the obtained polyarylate tends to decrease, which is not preferable.

Conditions for producing a prepolymer having terminal groups that satisfy the above formulas (X) and (n) depend on the type of raw materials used, but in general, in order to satisfy formula (X), component (A) The molar ratio of the diol of the diol and the aromatic dicarboxylic acid of the component (C) may be appropriately selected within the range of the formula (I[), and in order to satisfy the formula (II), the molar ratio of the diol of the formula (I ) or (I[l), the amount of diaryl carbonate used as component (B) may be appropriately selected.

The second crystallization step in the method of the present invention is a step of preparing a powdered prepolymer crystallized from a molten prepolymer, and this preparation method includes, for example, (1)
A method of cooling and pulverizing a molten prepolymer and then crystallizing it;
(2) A method in which the molten prepolymer is crystallized into chips or pellets and then pulverized; (3) A method in which crystallization and pulverization are carried out simultaneously, such as pulverizing the molten prepolymer while directly contacting it with a crystallization solvent. etc. are used.

In addition, as a crystallization method, for example, (1) a crystallization solvent such as acetone or toluene (U.S. Pat. No. 3,684,7
66, a method using steam,
(2) Crystallization by heating using a crystal nucleating agent, such as highly crystalline polyarylate, diphenyl carbonate, aromatic carboxylates such as sodium 0-chlorobenzoate, various ferulates, aromatic sulfonates, etc. Method, (3
) A method of crystallizing by preheating at a temperature of 100 to 220° C. for about 1 to 20 hours, or a combination of these methods is used.

The ease of pulverization and crystallization (speed) largely depend on the primary structure and molecular weight of the polymer. Low molecular weight polyarylates obtained from aromatic dicarboxylic acid mixtures with 20 mol% acid and bisphenol A crystallize easily by either method.

On the other hand, a low molecular weight polyarylate obtained from an equimolar mixture of terephthalic acid and isophthalic acid and bisphenol A is difficult to crystallize, but can be crystallized using a crystallization solvent such as acetone. At this time, in order to accelerate crystallization, the contact temperature with the crystallization solvent may be raised to a high temperature, or the prepolymer may be slightly swollen with a variable length solvent and then brought into contact with the crystallization solvent. It is also effective. It is also effective to further advance crystallization by preheating after crystallizing in a solvent.

The degree of crystallinity of the crystallized prepolymer thus obtained is preferably at least 5% by X-ray diffraction. If the degree of crystallinity is less than 5%, particles tend to fuse together during solid phase polymerization, which is not preferable. Further, it is preferable that the crystallinity of the surface is high because it is difficult to fuse.

In the solid phase polymerization step of the third step, the crystallized prepolymer powder obtained in the second step is preferably
Polymerization is carried out by heating at a temperature in the range of 300°C. In this in-phase polymerization, in order to remove the condensation products carbon dioxide, phenols, and a small amount of diaryl carbonate from the system, polymerization is carried out under inert gas flow, reduced pressure, or a combination of these conditions. This is desirable.

In addition, when a preheating method is used in the second step of crystallization, it is advantageous to reduce the pressure of the system at that temperature and enter the continuous in-phase polymerization step once crystallization has sufficiently progressed. .

The average particle size of the crystallized prepolymer powder is desirably in the range of 10 μm to 10 mm, preferably 1 Opm to IIIm. This particle size can be measured using a knot or various particle size distribution measuring devices (for example, ZM type analyzer manufactured by Coulter Co.). Although pulverization can be performed using various mechanical pulverizers for plastics, porous crystallized prepolymers are particularly brittle and can be sufficiently pulverized with simple pulverizers.

The shape of the crystallized prepolymer powder is preferably porous with a large surface area, and this can be produced by selecting an appropriate crystallization solvent.

In the solid phase polymerization of the present invention, it is desirable to carry out the polymerization at a temperature as high as possible within a range where the particles of the crystallized prepolymer do not fuse with each other, in order to increase the polymerization rate. Furthermore, when the melting point of the prepolymer increases as the polymerization progresses, it is also advantageous to increase the polymerization temperature accordingly. Generally, polymerization is carried out at a temperature at least 5°C below the melt onset temperature. Furthermore, the resulting polymer has
-ArCOOH group or -Ar which is thermally unstable as a terminal group
It is desirable to have substantially no -OH groups, and if these end groups are present, it is desirable to continue eliminating the polymerization.

Boria Relay 1. thus obtained has little coloring and is excellent in heat resistance, hot water resistance, mechanical properties, etc.

Effects of the Invention The method for producing polyarylates of the present invention does not require the use of a large amount of solvent unlike the interfacial polycondensation method, and can easily produce high molecular weight polymers with less coloring than the conventional melt polymerization method. Compared to conventional solid-phase polymerization methods, this method uses cheaper raw materials, has higher polymerization reactivity, and can easily produce high-molecular-weight polymers by controlling the end groups of the polymer. In addition, it can provide polyarylates with excellent hue, heat resistance, mechanical properties, etc., and can be said to be a method of extremely high industrial value.

Examples Next, the present invention will be explained in more detail with reference to examples.
The present invention is not limited in any way by these examples.

The reduced viscosity of the polymer is measured at a temperature of 25° in a mixed solvent of phenol/tetrachloroethane (weight ratio 60/40).
0.5 at C! ? /100m(l).

Example 1 Terephthalic acid 5 s,tg (o,a 5 mol), isophthalic acid 24.99 (0.15 mol), bisphenol AI 18.69 (0.52 mol) and diphenyl carbonate 256.8 g (1, 2 mol) was charged into a polymerization vessel, and while stirring under a nitrogen atmosphere, the produced carbon dioxide and phenol were distilled out of the system, and then the temperature was raised from room temperature to 250 °C over 2 hours, and then 2
It took some time to raise the temperature to 280'O, then gradually reduce the pressure, and finally the pressure was reduced to 5 mmHg for 1 hour. The reduced viscosity of the obtained prepolymer was 0.024.

This prepolymer was pulverized and then immersed in acetone at room temperature for one day to obtain a porous crystallized prepolymer. The crystallinity of this product was found to be 12% by X-ray diffraction.

This crystallized prepolymer powder was placed in a rotary evaporator and heated to remove acetone.
000 for 2 hours, 200°C for 2 hours, 220°C for 2 hours
Time, 2 hours at 240℃, 3 hours at 2700C, 2-5
A small amount of dry nitrogen under reduced pressure of mm'fh.

Solid phase polymerization was carried out without introducing .

The obtained polymer had a glass transition temperature of 192° C1 and a reduced viscosity of 0.63, and the injection molded molded pieces were strong with little coloring.

Note that the terminal hydroxyl group and carpoquine group of the polymer were not detected by colorimetric analysis or NMR analysis.

Example 2 A prepolymer was produced in the same manner as in Example 1, except that 0.25 mol of terephthal M and 0.25 mol of isophthalic acid were used instead of 60.35 mol of terephthal and 60.15 mol of isophthalic acid, respectively. . The reduced viscosity of the ground prepolymer was 0.21.

After crushing this prepolymer to an average particle size of about 100 μ, it was placed in an autoclave with acetone and stirred at 100° C. for 2 days. After drying the porous crystallized prepolymer powder with a crystallinity of 8% thus obtained,
2 hours at 150'C, 2 hours at 180C, 4 hours at 200C, 3 hours at 240C, 5 hours at 270C, 2~
Solid phase polymerization was carried out under reduced pressure of 5 mrnHg without introducing a small amount of dry nitrogen.

This polyarylate had a reduced viscosity of 0.70 and a glass transition temperature of 193°C. Furthermore, the injection-molded molded piece was colorless, transparent, and strong.

Examples 3 to 5 Polymerization was carried out by the method of Example 1 or Example 2 using the raw materials shown in the following table. The results are also shown in the table.

Claims (1)

[Scope of Claims] 1 (A) a diol mainly composed of an aromatic dihydroxy compound, (B) a diaryl carbonate, and (C) an aromatic dicarboxylic acid, based on the number of moles, according to the formula 0.7≦{(A) component + (C) component}/{(B) component}≦1.1 and 0.9≦{(C) component}/{(A) component}≦1.1, and under heating. 1. A method for producing polyarylate, which comprises melt-polymerizing to form a prepolymer, crystallizing the prepolymer, and then solid-phase polymerizing the prepolymer. 2 (A) diol mainly composed of aromatic dihydroxy compound, (B) diaryl carbonate, (C) aromatic dicarboxylic acid, and (D) hydroxy aromatic carboxylic acid, based on the number of moles, formula 0.7≦{ (A) component + (C) component + (D) component}/{
(B) component}≦1.10.9≦{(C) component}/{(A
) component}≦1.1 and {(D) component}/{(A) component + (C) component + (D) component}≦0.7, and melt polymerized under heating. A method for producing polyarylate, which comprises forming a prepolymer, crystallizing the prepolymer, and then performing solid phase polymerization.