Abstract: A process for forming polyesters that comprises the steps of esterifying a terephthalic acid and an ethylene glycol to produce an esterification product containing a bis (betahydroxyethyl) terephthalate, and continuously polycondensing the obtained esterification product to form a polyester. In both the esterifying step and the polycondensing step, a reaction catalyst consisting of a compound of antimony and a compound of titanium is used. The weight ratio of the compound of antimony to the compound of titanium is 0.01 to 100. The process of the present invention can considerably reduce both the esterification time and the polycondensation time and provides good color, reduced diethylene glycol content and reduced condensation of terminal carboxyl groups in the resulting polyesters.

Abstract: In situ formation of a melt processable liquid crystalline polyester composition capable of forming an anisotropic melt phase at a temperature of below approximately 400.degree. C. having an improved heat distortion temperature under a load, the composition being formed in the presence of from approximately 0.05 to approximately 1.0 weight percent of a divalent metal salt of terephthalic acid as a nucleating agent. Films, fibers and molded articles produced from the composition exhibit improved thermal properties.

Abstract: An unsaturated alkyd having a number-average molecular weight of at least 5000 obtained by esterification and then deglycolization reaction of (a) at least 10 mol % of an .alpha., .beta.-unsaturated polybasic acid or an anhydride thereof relative to 100 mol % of the total polybasic acid including any saturated or non-.alpha., .beta.-unsaturated polybasic acid (or anhydride thereof) and (b) at least 50 mol % of a polyhydroxy alcohol having a boiling point of 300.degree. C. or less at 760 mmHg relative to 100 mol % of the total polyhydroxy alcohol summed up with another polyhydroxy alcohol not defined above. The polyester resin can be widely used in a various applications by employing the excellent physical properties thereof.

Abstract: High molecular weight polymethylene adipates useful as melt adhesives and sealants, including adipates with molecular weights in excess of 30,000 are disclosed. Also provided is a process for preparing high molecular weight polymethylene adipates by polymerization of an acid and one or more diols with a minute molar excess of diol with respect to acid.

Abstract: A process for preparing aromatic polyesters having excellent physico-mechanical and solvent-resistance properties, consisting in reacting at least a dialkyl ester of an aromatic diacid with an aromatic polycarbonate and/or an aromatic dialkyldicarbonate in the presence of an esterification or transesterification catalyst.

Abstract: This invention relates to processes for the preparation of polybutylene terephthalate based polymer, which processes produce polymer having excellent color, a small amount of side-product and a high degree of polymerization within a short time. Complex catalyst which is employed comprises antimony compounds and titanium compounds, optionally together with phosphorous based compounds and hindered phenolic, and the catalyst decreases the phenomena that conventional catalysts of tin and titanium based compounds have of making the polymer colored by declining hydrolyzation and heat-stability. The catalyst also decreases the amount of side-product, THF sharply. As a result, improved polybutyleneterephthalate can be prepared.

Abstract: Disclosed is a water-dissipatable or dispersible polyester containing sulfonate groups that has improved stability having a substantial number of capped carboxyl groups attached to the sulfomonomer moieties. This polyester can be prepared by capping the carboxyl end groups of the polyester with oxirane compounds or by polymerizing the reactants wherein one reactant is a diol that is a diester adduct of a glycol and a dicarboxylic acid sulfomonomer.

Abstract: Process for continuous production of polyesters by esterification of terephthalic acid with 1,4-bis(hydroxymethyl)cyclohexane with a split feed in the presence of a titanium catalyst at 265.degree. to 305.degree. C. and 0.8 to 1.5 bar up to a conversion of 98.0% to 99.0%, then performing precondensation at 290.degree. to 305.degree. C. and 0.01 to 1.5 bar up to a conversion of 99.4% to 99.7% and finally performing polycondensation at 295.degree. to 310.degree. C. and less than 1.5 mbar up to a conversion corresponding to 50% to 95% of the maximum possible viscosity after adding more titanium catalyst.

Abstract: A high molecular weight unsaturated polyester having a number-average molecular weight of 5,000 or greater, the polyester being represented by a general formula: ##STR1## wherein G denotes a glycol residue, m denotes 1 to 10, n denotes 1, and M denotes a number corresponding to a number-average molecular weight greater than 5,000. The high molecular unsaturated polyester of the present invention is an unsaturated polyester having a large molecular weight unattainable by conventional methods, and can find wide application by utilizing its improved physical properties.

Abstract: Process for discontinuous production of polyesters by esterification of terephthalic acid with a definite excess of 1,4-bis(hydroxymethyl)cyclohexane in the presence of a titanium catalyst at 180.degree. to 300.degree. C. and 0.8 to 1.5 bar up to a conversion of 98.0% to 99.6%, then precondensation with the addition of more titanium catalyst at 270.degree. to 300.degree. C. and 500 to 5 mbar up to a conversion of 99.5% to 99.9% and polycondensation at 295.degree. to 310.degree. C. and less than 1.5 mbar up to a conversion corresponding to 40% to 95% of the maximum possible viscosity.

Abstract: Polyesters are prepared from dihydroxyl-containing compounds and di-carboxylic acids or anhydrides or lower alkyl esters thereof in the presence of a combination of tin catalysts comprising (1) at least one organotin salt of a carboxylic acid, and (2) either (a) at least one organotin oxide, or (b) at least one organostannoic acid, or (c) a combination of (a) and (b). The polyesters prepared by this process are useful in coating, adhesive, fiber and molding applications.

Abstract: A process for preparing a polyester having an inherent viscosity of at least 0.5 comprising(A) preparing an admixture of a molten polyester having an inherent viscosity of less than 0.5, a titanium alkoxide and a 13X zeolite, and(B) subjecting the admixture to a time and temperature sufficient to increase the inherent viscosity of the polyester to at least 0.5.

Abstract: Aliphatic polyesters of 3,4'-biphenyldicarboxylic acid and optionally 4,4'-biphenyldicarboxylic acid are prepared by contacting at least one aliphatic diol having from 2 to 12 carbon atoms with a lower alkyl diester or phenolic diester of 3,4'-biphenyldicarboxylic acid and, optionally, a lower alkyl diester or phenolic diester of 4,4'-biphenyldicarboxylic acid, in the presence of a catalyst at about 180.degree. to about 300.degree. C. These polyesters are suitable for use in making fibers, films, three dimensional molded articles, barrier containers and coatings.

Abstract: A copolyester is synthesized by reacting a dialkyl ester of an aromatic dicarboxylic acid (e.g., dimethyl terephthalate) with ethylene glycol, low molecular weight poly(ethylene glycol) and a salt of a sulfonic acid-substituted aromatic dicarboxylate (e.g., sodium sulfoisophthalic acid) simultaneously in a first stage reaction in the presence of a phosphite or phosphate stabilizer. The intermediate product of the first stage is further reacted in a second stage in the presence of a polycondensation catalyst to obtain a random copolyester of the desired molecular weight. The copolyester is extruded to form a film which, after orientation and heat setting, has excellent clarity and is useful as a photographic film support. Aqueous processing of the film relaxes any core set or curl.

Abstract: Macrocyclic polyarylate oligomers containing a mixture of isophthalate and terephthalate moieties are prepared by reaction of a mixture of isophthaloyl and terephthaloyl chloride with one or more bisphenols under specific reaction conditions. They are characterized by a reduced degree of crystallinity and therefore lower processing temperatures than the corresponding isophthalate oligomers.

Abstract: Disclosed is a process for preparing polyesters of dimethyl terephthalate with 1,4-cyclohexanedimethanol using a catalyst comprising a titanium alkoxide and an alkaline earth metal salt.

Abstract: Polyester polyols are prepared by reacting a multifunctional adduct (e.g., maleic anhydride (MA)) and tall oil fatty acid, (TOFA), and then esterifying/transesterifying the MA/TOFA reaction product with an excess of aromatic polyester polyols (preferably such polyols are produced from dimethyl terephthalate (DMT) process residue and polyhydric alcohols). The polyester polyols may be used in the manufacture of rigid polyurethane (PUR) and rigid urethane-modified polyisocyanurate (PUR/PIR) foams.

Abstract: Economic and efficient process for preparing an oligomer of poly(butylene terephthalate) or an oligomer of poly(butylene isophthalate) from a transesterification reaction mass by using a countercurrent column reactor system wherein the transesterification reaction mass is fed into the countercurrent column reactor and continuously flows downward while simultaneously a heated inert gas stream continuously flows upward to scrub out volatiles from the transesterification reaction mass, thereby yielding from the bottom of the countercurrent column reactor an oligomer of poly(butylene terephthalate) or an oligomer of poly(butylene isophthalate) suitable for use in a polycondensation reaction. Butanediol is recycled throughout the process through an absorber column.

Abstract: In a process for the manufacture of shaped polyethylene terephthalate structures, such as film or fiber, a crystalline sodium aluminosilicate molecular sieve is used as the catalyst for the polymerization of di(2-hydroxyethyl phthalate) or higher oligomer intermediate. In film, especially for audio and video tapes, the molecular sieve serves both as the catalyst and as a slip additive. In addition to the fact that this process eliminates antimony trioxide, heretofore used as the polymerization catalyst, the polyethylene terephthalate film made by the process of this invention has considerably fewer defects such as microgel inclusions, black specks, and pimples. Further, the amount of diethylene glycol formed in the process is reduced. Finally, polyethylene terephthalate degradation, contributing to the exudation of lower molecular weight materials, also is reduced.

Abstract: A process for producing PET based polyester polymers from a lower dialkyl ester of a dicarboxylic acid and a glycol selected from the class of ethylene glycol, diethylene glycol, polyethylene glycol, blends of ethylene glycol and propane and/or butane diol, or mixtures of two or more of these, using the novel catalyst system and a small amount of dicarboxylic acid is described. The novel catalyst composition includes from about 10 ppm to about 150 ppm manganese; from about 50 ppm to about 250 ppm lithium; preferably from about 10 ppm to about 40 ppm cobalt; and from about 200 ppm to about 400 ppm antimony, all amounts being based upon the expected yield of the polyester based polymer.

Abstract: Disclosed is a polymer and process for producing the polymer using a complex of titanium alkoxide with an alkali or alkaline earth metal salt.

Abstract: A process for increasing the direct esterification reaction rate between a diacid and 1,4-butanediol and for directly esterifying greater than 95% of the acid ends of the diacid by mixing in any order the diacid, the 1,4-butanediol and a catalyst compound selected from organo-titanium, organo-tin, and organo-zirconium compounds, to form a reaction mixture wherein the ratio of diol:diacid is at least 2:1 and then reacting the reaction mixture at an average temperature ranging between 190.degree. C. to 200.degree. C. for a period of time no longer than sixty minutes. The same process may be followed substituting ethylene glycol for 1,4-butanediol, provided the catalyst compound is added neat, further provided that the average reaction temperature ranges from 190.degree. C. to 200.degree. C., and also provided that the reaction mixture be reacted for a period of time no longer than seventy minutes.

Abstract: The present invention provides a curable composition which is cured at a relatively low temperature and, when cured, exhibits excellent physical properties. The curable composition comprises;(A) a compound having a carboxylic anhydride group,(B) a polyfunctional amine compound, a polyfunctional hydroxyl group-containing compound, a compound having both an amino group and a hydroxyl group or a compound latently having an amino group and/or a hydroxyl group, and(C) a polyhydric metal chelate compound or a polyhydric metal alcoholate compound.

Abstract: A method of producing essentially haze-free isophthalic acid and polymethylolalkanoic acid-containing resins comprising reacting isophthalic acid, a polymethylolalkanoic acid, and a polyol in the presence of a catalytically effective amount of a catalyst selected from the group consisting of stannous oxalate, dibutyltin oxide, and tetrabutyl titanate at a temperature of between about 150.degree. C. and 230.degree. C. to produce haze-free resin.

Abstract: Isophthalic acid is commonly utilized as a comonomer in the preparation of copolyester resins which are utilized as solution adhesives. Unfortunately, solutions of such resins frequently become hazy due to the presence of cyclic oligomers. This invention is based upon the unexpected discovery that the presence of protonic acids during the synthesis of the copolyester resins containing isophthalic acid utilizing conventional metallic catalysts results in solutions of the resins in organic solvents being virtually haze free.

Abstract: Polyester amides and polyether thioether ester amides of the formula ##STR1## wherein A is a polyamide sequence,R.sub.1 and R.sub.2 can be the same or different and each represents a linear or branched C.sub.2 to C.sub.98 alkyl radical,m is an integer between 1 and 50, andn is one or greater; and the process of making such amides by reacting specific diols with a dicarboxylic polyamide having terminal acid functional groups.

Abstract: A composition of matter comprising the reaction product of a cyanuric acid reactant, an aliphatic vicinal diol reactant and a carboxyl reactant, a method for producing the same, a magnet wire coated therewith and an electromagnetic device utilizing the same.

Abstract: Isophthalic acid is commonly utilized as a comonomer in the preparation of copolyester resins which are utilized as solution adhesives. Unfortunately, solutions of such resins frequently become hazy due to the presence of cyclic oligomers. This invention is based upon the unexpected discovery that the presence of protonic acids during the synthesis of the copolyester resins containing isophthalic acid utilizing conventional metallic catalysts results in solutions of the resins in organic solvents being virtually haze free.

Abstract: A process is disclosed for the preparation of polymeric butanediol ester of terephthalic acid by means of ester interchange process having improved rates of polymerization to produce polyesters having improved color. The process employs the neat addition of terephthalic acid on completion of the ester interchange reaction and maintains the ratio of glycol to dimethyl terephthalate in the ratio of not more than 1.3 to 1. Preferably the process is conducted with titanium containing catalysts and has a polymerization rate greater than 700 deciliters per hour.

Abstract: A process for producing a linear polyester comprising a main structural unit of an aromatic dicarboxylic acid ester of an alkylene glycol, particularly, comprising a polyethylene terephthalate or polytetramethylene terephthalate unit, is provided. In the process of the invention, the polycondensation reaction is conducted in the presence of a niobic acid catalyst that is obtained by heat-treating a hydrated niobium compound at the temperature range between 80.degree. C. and 400.degree. C. The process of the invention is also effecitvely applied to the manufacture of an elastomeric polyester comprising a polyalkylene terephthalate as a hard segment and a polyalkylene glycol as a soft segment.

Abstract: A method for stabilizing an unsaturated organosilicone is disclosed wherein a stabilizing amount of a hydroxylamine compound is added to the organosilicone to prevent thermal polymerization of the latter. The organosilicone may be an alkoxysilane or an organopolysiloxane having in its molecule a reactive organic group bearing at least one radical selected from acryloxy, methacryloxy, acrylamide, methacrylamide, styryl or vinylbenzyl radicals. The hydroxylamine compound may be added either to the organosilicone or during the preparation of the alkoxysilane from its respective reactants.

Abstract: A soil release promoting copolymer of polyethylene terephthalate and polyoxyethylene terephthalate (PET-POET) having a weight average molecular weight in the range of 15,000 to 35,000, and a PET:POET molar ratio of about 2:1 to 6:1, is prepared by reacting polyethylene terephthalate with polyethylene glycol in the presence of a catalyst system including both antimony trioxide and calcium acetate or both titanium isopropoxide and sodium isopropoxide. The PET-POET copolymer provides improved soil release properties for oily and greasy stains from polyester fabrics at high and low wash temperatures and is stable even at high alkalinity levels. The starting PET can advantageously be obtained from scrap bottles.

Abstract: Poly(butylene terephthalate) is prepared in a two-stage process wherein terephthalic acid and 1,4-butanediol are esterified in the presence of a catalytic amount of a catalyst having an organo-metallic component which is an organo-titanate compound, an organo-tin compound or a combination thereof and a salt component which is an organic acid salt or an inorganic salt. In a second step, the esterification product is subject to polycondensation. By following the foregoing procedure, the formation of tetrahydrofuran is significantly suppressed and the percent conversion of esterification products increased.

Abstract: Disclosed are polyester polyols prepared by esterifying tall oil with an excess of aromatic polyester polyols. The polyester polyols that are produced by this process are compatible with a trichlorofluoromethane blowing agent and are suitable for use in the manufacture of polyurethane and urethane-modified polyisocyanurate foams.

Abstract: Cyclic polyester and especially polyarylate oligomers, typically having degrees of polymerization from 2 to about 7, are prepared by reacting a bisphenol such as bisphenol A or 6,6'-dihydroxy-3,3,3',3'-tetramethylspiro(bis)indane with a dicarboxylic acid chloride such as isophthaloyl or terephthaloyl chloride in the presence of a catalyst, under conditions of low concentration. The cyclic oligomers may be polymerized to linear polyesters by contact with a transesterification catalyst at a temperature in the range of about 200.degree.-300.degree. C.

Abstract: Adhesion promoters for metal substrates for paints cross-linkable with U.V. or E.B. radiations or with peroxides of general formula: ##STR1## (wherein R, m and n have the meaning which we will specify in the disclosure), obtained by means of the reaction of glycidyl acrylate and/or glycidyl methacrylate with dihydrophosphonoacetic acid.

Abstract: A coating composition containing a film forming binder and a carrier for the binder; wherein the binder consists essentially of(1) a reactive polyurethane of the formula ##STR1## where R is a divalent or trivalent aliphatic or cycloaliphatic group,R.sup.1 is an alkyl group having 1-4 carbon atoms, n is 2-3;(2) a hydroxy containing polymer having a hydroxyl number of 50-200 and having pendent ester groups;(3) an organo metallic alkylate catalyst selected from the group of an aluminum alkylate, zirconium alkylate, titanium alkylate or mixtures thereof.The composition can be applied by conventional techniques such as spraying and can be cured at relatively low baking temperatures.

Abstract: Hydrolysis-stable, thermoplastic block polyetheresteramides are produced by a catalyzed polycondensation of linear, aliphatic polyamides having carboxyl end groups with .alpha.,.omega.-dihydroxypolyethers and additionally, a linear or branched, aliphatic or cycloaliphatic diol having 2-8 carbon atoms in an amount of 3-30 mol %, relative to the sum of the moles of both other starting materials. The ratio of the moles of the polyamide components to the sum of the moles of the dihydroxypolyether and diol components is 0.81 to 0.985.

Abstract: A coating composition useful as an exterior for automobiles and trucks and as a refinish composition containing a film forming binder and a carrier for the binder; wherein the binder contains(1) a reactive component of the formula ##STR1## where R is a divalent or trivalent aliphatic or cycloaliphatic group,R.sup.1 is an alkyl group having 1-4 carbon atomsn is 2-3(2) a hydroxy containing acrylic polymer having drying oil fatty acid moieties and a hydroxyl number of 20-500 and(3) an organo metallic alkylate selected from the group of an aluminum, titanium, zirconium alkylate, or mixtures thereof.The composition can be applied by conventional techniques such as spraying and cures at ambient temperatures or can be baked at relatively low temperatures.

Abstract: Aliphatic polyesters containing sulphur, which contain the sulphur as structural units in the form of sulphide or disulphide groups in both the acid component and the diol component, a process for the preparation of these polyesters, and also polymers containing these polyesters.

Abstract: A five-component copolyester not described in the prior art literature which comprises at least five components, i.e. (a) two aromatic dicarboxylic acid components composed of a major amount of an isophthalic acid component and a minor amount of another dicarboxylic acid component, (b) a diol component composed of an ethylene glycol component and optionally another diol component, (c) an aromatic monohydroxy-monocarboxylic acid component and (d) a polyfunctional compound component having at least three functional groups. The copolyester has excellent melt moldability, mechanical strength, transparency and gas-barrier property in a well balanced combination, and shows improved properties over conventional copolyesters containing a major amount of isophthalic acid as a dicarboxylic acid component.

Abstract: A process for the manufacture of thermoplastic block polyetheresteramides by polycondensation of linear polyamides having carboxyl end groups with approximately equimolar quantitites of .alpha.,107 -dihydroxypolyethers in the presence of organic or inorganic stannous compounds as esterification catalysts. Preferred polyamide-forming materials include the monomers of PA-6, PA-11, PA-12, PA-6.6, PA-6.9, PA-6.10, PA-6.12, PA-6.13, or mixtures thereof and linear and branched aliphatic, aromatic, and cycloaliphatic dicarboxylic acids. The molecular weight of the polyamides used is 700-15,000, and that of the dihydroxypolyethers is 400-4,000. Polycondensation takes place at atmospheric or reduced pressure and at temperatures of 210.degree.-280.degree. C.

Abstract: A process for continuous production of high molecular polybutyleneterephthalate by direct esterification of terephthalic acid and 1,4 butanediol in the presence of tin- or titanium-containing catalysts at preferred temperatures of 235.degree. to 250.degree. C., and preferred pressures of 0.3 to 0.6 bar during the first stage--with minimal formation of tetrahydrofurane--followed by precondensation and polycondensation at reduced pressure.

Abstract: Polymeric materials are repolymerized by intensely admixing the polymer with an additive having the formula: ##STR1## wherein M is titanium or zirconium, R, R.sup.1 and R.sup.2 are each a monovalent alkyl, alkenyl, alkynyl, aralkyl, aryl or alkaryl group having up to 20 carbon atoms or a halogen or ether substituted derivative thereof, and, in addition, R.sup.2 may also be an oxy derivative or an ether substituted oxy derivative of said groups; A, B, and C are each a monovalent aroxy, thioaroxy, diester phosphate, diester pyrophosphate, oxyalkylamino, sulfonyl or carboxyl containing up to 30 carbon atoms; and a+b+c=3. The repolymerized polymers have improved physical properties and higher heat distortion temperatures, experience less thermal degradation and show greater solvolysis resistance than the polymers prior to the repolymerization.

Abstract: A method for producing of a polybutylene terephthalate by reacting a dicarboxylic acid component containing terephthalic acid as a main constituent directly with a glycol component containing 1,4-butanediol as a main constituent to form an esterification reaction product and subjecting the esterification reaction product to a polycondensation reaction, characterized in that 0.4 to 1.3 mole of 1,4-butanediol per mole of terephthalic acid is added in the presence of an esterification catalyst which is at least one member selected from the group consisting of titanium compounds and tin compounds, to start said esterification reaction and thereafter a required amount of 1,4-butanediol is supplied additionally to complete the esterification reaction.

Abstract: A process for preparing a high-molecular weight polyester comprising additionally polymerizing a polyester obtained from a dicarboxylic acid component composed mainly of an aromatic dicarboxylic acid and/or an ester-forming derivative thereof and a glycol component composed mainly of ethylene glycol and/or an ester-forming derivative thereof and having an intrinsic viscosity of at least 0.8, at a temperature above the melting point of the polyester and under a reduced pressure, in the presence of a compound (A) successively generating a substance which is gaseous under the polymerization conditions and does not substantially reduce the molecular weight of the polyester, to form a polyester having an intrinsic viscosity of at least 1.0.

Abstract: A process for producing an aromatic polyester having an aromatic dicarboxylic acid as a main acid component and trimethylene glycol or tetramethylene glycol as a main glycol component from a starting material composed mainly of an aromatic dicarboxylic acid or its functional derivative and trimethylene glycol or tetramethylene glycol; wherein at least one organometallic catalyst selected from the group consisting of organotitanium compounds and organotin compounds and at least one promoter selected from the group consisting of organic sulfonic acids and aliphatic carboxylic acids and being capable of increasing the catalyst activity of the organometallic catalyst are added to the reaction system so that the promoter increases the catalytic activity of the organometallic catalyst, and an aromatic polyester having an increased degree of polymerization is formed at an increased rate of polymerization.

Abstract: A polyester having an initial accumulation electric charge of not less than 2.9 .mu.c/mm.sup.2 when determined in a melt state according to the method as hereinabove defined and a heat resistance of not more than 0.210 when determined according to the method as hereinabove defined, which comprises at least one magnesium compound and at least one phosphorus compound solubilized therein in amounts as satisfying the following formulas:30.ltoreq.[Mg].ltoreq.400 (1)0.8.ltoreq.Mg/P.ltoreq.3 (2)wherein [Mg] is the content (ppm) of the magnesium compound in terms of magnesium atom to the polyester and Mg/P is the atomic ratio of the magnesium atom in the magnesium compound to the phosphorus atom in the phosphorus compound.

Abstract: A process for manufacturing a colorless polyetheresteramide, which comprises the step of reacting (A) a polyamide-forming compound selected from aminocarboxylic acids and lactams each having at least 10 carbon atoms, (B) a dicarboxylic acid having from 4 to 20 carbon atoms and (D) a polyoxyalkylene glycol having an average molecular weight of from 300 to 6,000, in the molten state, in the presence of 0.0001 to 0.00999 percent by weight, based on the total weight of the above mentioned reactants, of a tetraalkyl titanate having a straight or branched alkyl having 1 to 24 carbon atoms, at a pressure of at the highest 5 mmHg at a temperature of 200.degree. to 300.degree. C.

Abstract: A polyester fiber having a tenacity of at least 4.0 g/d is formed from a copolyester having a degree of polymerization of from 80 to 100 of which copolyester 1.0 to 2.0 mol % of its acid component has a metal sulfoisophthalic group and 0.5 to 1.9 weight % of which copolyester is provided by a glycol component provided by glycol having a molecular weight of from 400 to 6000. The polyester fiber is readily dyed with basic dyes.The polyester fiber can be prepared by spinning the copolyester at a shear rate of .ltoreq.10.sup.4 sec.sup.-1, a spinning draft ratio of .ltoreq.250, a back pressure of the spinneret of .gtoreq.40 kg/cm.sup.2 and a shear stress of .ltoreq.(Q+0.7).times.10.sup.7 dyne/cm.sup.2, where Q is the mass output rate per single hole (g/min).