WO2008075373A2

WO2008075373A2 - An efficient process for the production of polyester

Info

Publication number: WO2008075373A2
Application number: PCT/IN2007/000136
Authority: WO
Inventors: Vikas Madhusudan Nadkarni; Shreeram Ashok Wadekar; Dubey Rajan; Anand Anil Bopardikar
Original assignee: Reliance Industries Ltd.
Priority date: 2006-12-19
Filing date: 2007-03-30
Publication date: 2008-06-26
Also published as: WO2008075373A3

Abstract

The present invention discloses an efficient process for the production of a high molecular weight polyester resin having IV of about 0.4 dl/g to about 1.2 dl/g produced from a low molecular weight crystalline prepolymer having IV of about 0.1 dl/g to about 0.4 dl/g by adding oraganotin compound at any steps during the esterification or the melt polymerization but before a particle former process which reduces dust generation in the melt polymerization and also reduces dust generation and the oligomer volatilization in the solid state polymerization. The present invention also discloses a polyester prepolymer having IV of about 0.1 dl/g to about 0.4 dl/g which is used as a precursor for solid-state polymerization. The prepolymer particles are crystalline or amorphous in nature. The prepolymer particles are cylindrical, hemispherical or spherical in shape with no sharp edges, which also helps in reducing dust generation. The present invention also provides high molecular weight polyester resin having IV of about 0.4 dl/g to about 1.2 dl/g and is used as preforms, containers, fibers or filaments, films or sheets.

Description

TITLE An efficient process for the production of polyester

TECHNICAL FILED OF THE INVENTION:

The present invention relates to an efficient process for the production of polyester comprising adding organotin compound in an esterification or melt polymerization but before particle former process followed by solid state polymerization which reduces oligomer volatilization and dust formation.

BACKGROUND OF THE INVENTION:

Polyester resins such as polyethylene terephthalate (PET) are well known for making films, fibers and packaged container applications. A process for the production of high molecular weight polyesters comprises melt polymerization followed by solid-state polymerization (SSP).

In the melt polymerization process for producing PET, base prepolymer of Intrinsic Viscosity (IV) of about 0.4 to 0.65 dl/g is produced. The base prepolymer is either in the form of cylindrical chip or spherical chip and is amorphous in nature. The base chips are then subjected to solid-state polymerization after crystallizing it in a crystallizer so as to avoid sintering or lump formation in the solid-state polymerization reactor. Depending on the end application, polyester with different IV is produced by solid-state polymerization (ref: US 3,405,098, 3,544,525, 4,245,253, 4,238,593, and 5,408,035). Another polymerization process which is different than conventional polymerization process is disclosed in US 5,510,454, 5,532,333, 5,540,868, 5,714,262, 5,830,982, and 6,451 ,966 which are incorporated herein as reference in their entirety.

In this polymerization process, a crystalline hemispherical low molecular weight prepolymer having IV of about 0.1 to 0.4 dl/g is formed using particle former process. Such low IV prepolymer particles can also be produced with other techniques that are well known in the art and can be subjected to solid-state polymerization for producing higher molecular weight polyesters. Examples of such particle formation processes are given in US patents 4,123,207 and 7,008,203.

Prepolymer having IV from about 0.1 dl/g to about 0.4 dl/g produced by these various techniques are then used as a precursor for solid-state polymerization process to increase the IV from about 0.4 dl/g to about 1.2 dl/g.

Solid-state polymerization process involves three steps namely preheating of low molecular weight prepolymer in a preheater section, attaining intermediate IV (0.4 to 0.6 dl/g) in a reactor namely conditioning vessel and then achieving final desired IV (0.7 to 1 dl/g) in a solid-state polymerization reactor. Preheater is a fluidized section in which prepolymer is heated at a temperature in the range of 210⁰C to 25O⁰C under very high nitrogen gas flow.

In the melt polymerization process, hemispherical prepolymer particles are formed by dropping melt droplets on a preheated moving metal belt. Belt temperature is set between 100 to 200⁰C. Due to the very high temperature of melt droplet which is in the range of 260 to 300⁰C, quenching effect occurs when it falls on low temperature metal belt. As a result, bottom surface of hemispherical prepolymer particle exhibits different crystalline morphology as compared to the top dome surface. It is observed that for every hemispherical crystalline prepolymer particle, the bottom surface remains weaker in terms of mechanical strength as compared to top dome surface. Also the shape of the low molecular weight prepolymer is governed by the kinetics of crystallization from melt.

These prepolymer particles are weak, and break easily during handling e.g. during transfer/conveying to SSP, during SSP and during packaging/transportation. This results in formation of smaller particles, refered hereafter as 'dust particles'. Dust generation is governed by the particle shape of crystalline prepolymer. Flatter the particle shape, sharper the particle edges and thus greater is the dust generation. Dust generation is lower for burger or dome shaped particles. More dust formation in the melt polymerization process causes increased waste generation and thereby reducing productivity.

In the solid-state polymerization process, due to the lower prepolymer IV, low molecular weight oligomers and unreacted PTA get liberated. These oligomers may be linear or cyclic in structures made up of the constituent units of the polyester. These oligomers can be low melting substances such as monohydroxy ethyl terephthalate (MHET), bishydroxyethyl terephthalate (BHET), etc. Due to very high nitrogen gas flow in preheater, oligomers get carried away along with dust particles into nitrogen circulation lines. This also leads to deposition of broken and dust particles on the vessel internals/walls, nitrogen heaters as well as the inert gas/vacuum line walls. It is also observed that broken chips, dust and oligomers stick to the preheater gas distribution plates, plenum and sidewalls. In addition, it is seen that, over a period of time, bridging of prepolymer, dust, broken particles or oligomers, hereafter referred as deposits, occurs in the preheater nitrogen gas distribution plates, plenum and sidewalls of preheater and nitrogen gas circulation lines. This results in the partial or complete blocking of holes of preheater nitrogen gas distribution plate and thereby disturbing nitrogen gas flow / velocity and also the fluidization of prepolymer in the preheater. This manifests as a rapid increase in the pressure drop across the preheater nitrogen distribution plates increases due to this bridging at a faster rate therefore demanding frequent shutdown of the plant to clean these deposits. Oligomers and dust particles also deposits on nitrogen heaters in the preheater loop resulting in fouling of heaters and thus increased pressure drop across heaters, demanding shutdown of the plant.

Similarly oligomer liberation also seen in conditioning vessel, which is on the downstream of preheater in the solid-state polymerization process. Oligomers get liberated to a large extent in conditioning vessel where IV increases up to 0.6 dl/g. Oligomers get carried away along with nitrogen gas and tends to deposit on the internals and the surface of the conditioning vessel. These deposits when dislodged, contaminates the product.

Increased oligomer liberation has also an impact on nitrogen purification unit (NPU) catalyst life. The life of catalyst gets reduced due to higher levels of liberated oligomers in the nitrogen stream entering into NPU. Therefore, more frequent replacement of catalyst is required. This increases the cost of polyester production.

Deposits also formed on the nitrogen gas circulation lines, filters and heaters thereby reducing their life too. These deposits get degraded over a period of time due to very high temperature and residence time. When these degraded deposit get dislodged during the process, contaminating the final product in the form of black specks. Thus the product obtained is of not commercial quality. Thus it is necessary to reduce both dust generation and oligomer liberation during solid-state polymerization process to overcome the problems associated with the process.

US patents 5,510,454, 5,532,333, 5,540,868, 5,714,262, 5,830,982, and 6,451 ,966 do not deal with the problems associated with the process. Also the polymerization process disclosed in these patents does not use tin catalysts for producing polyesters. These patents do not deal with the use of tin catalysts for reducing oligomers and dust content during solid-state polymerization process.

US patent 5,166,310 discloses the preparation of polyesters with organotin compounds. However, this patent deals with only melt polymerization process. This patent does not deal with solid-state polymerization process with tin compounds and the problems associated with oligomer volatilization during solid- state polymerization.

US patent 4,970, 288 discloses the process for the preparation of polyesters with non-toxic organotin catalysts for use as esterification catalysts. This patent does not deal with solid-state polymerization process with tin compounds and the problems associated with oligomer volatilization during solid-state polymerization.

US patent 4,554,344 discloses the process for the preparation of polyesters with improved molecular weight from glycols containing a vicinal secondary hydroxy! group. Various tin compounds were used as catalysts in carrying out the process of this invention. This patent does not deal with solid-state polymerization process with tin compounds and the problems associated with oligomer volatilization during solid-state polymerization. According to our knowledge the use of tin compounds to reduce dust generation in the melt polymerization process and the volatilization of oligomers of polyester and dust in the solid-state polymerization process of the invention is not currently known in the art. However, when solid-state polymerization starts in some of the newer polyester technologies with unusually low molecular weight prepolymers, the generation of dust particles and volatilization of oligomers is substantially higher than conventional solid-state polymerization. These dust particles and oligomers condense on many vessel and piping surfaces and cause many process and product problems including black speck formation, plugging of gas distributors/filters, over burdening of the nitrogen purification unit, etc. The present invention addresses these issues.

OBJECTS OF THE INVENTION:

An object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation in the melt polymerization.

An object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation in the particle formation process. An object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation and oligomer liberation in the solid state polymerization.

Another object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation and oligomer liberation in the melt polymerization, particle former process or solid state polymerization thereby reducing deposit formation on preheater nitrogen gas distribution plates, plenum and sidewalls of preheater, nitrogen gas circulation lines, nitrogen gas heater and conditioning vessel, reactor internals and requiring low maintenance of process plant.

Another object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation and oligomer liberation in the melt polymerization, particle former process or solid state polymerization thereby reducing deposit formation on preheater nitrogen gas distribution plates, plenum and sidewalls of preheater, nitrogen gas circulation lines and conditioning vessel reactor internals and reducing shut down frequency for maintenance and increasing the productivity.

Another object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation and oligomer liberation in the melt polymerization, particle former process or solid state polymerization thereby reducing deposit formation on preheater nitrogen gas distribution plates, plenum and sidewalls of preheater, nitrogen gas circulation lines and conditioning vessel reactor internals and subsequent degradation thus maintaining commercial quality of the product and consistently.

Another object of the invention is to provide an efficient process for the production of polyester by using organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof at any steps during the esterification or the melt polymerization but before a particle former process to reduce the dust generation and oligomer liberation in the melt polymerization, particle former process or solid state polymerization where the process reduces the problem associated with the prior art.

Yet another object of the invention is to provide a low molecular weight polyester prepolymer of IV of about 0.1 to about 0.4 dl/g comprising, among other things, organotin compounds such as a organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof. Yet another object of the invention is to provide a high molecular weight polyester resins of IV of 0.4 dl/g to 1.2 dl/g from the low molecular weight crystalline polyester prepolymer of IV of about 0.1 to about 0.4 dl/g comprising, among other things, organotin compounds such as organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof.

Yet another object of the invention is to provide use of the above high molecular weight polyester resins of IV of 0.4 dl/g to 1.2 dl/g produced from a low molecular weight crystalline polyester prepolymer of IV of about 0.1 to about 0.4 dl/g comprising, among other things, organotin compounds such as organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, carboxylic acid derivatives of tin or alkyl tin sulfonates and mixtures thereof for the production of various packaging products such as performs, containers, films or sheets.

Detailed Description of the invention:

According to the invention there is provided an efficient process for the continuous production of a high molecular weight polyester resin having IV of about 0.4 dl/g to about 1.2 dl/g produced from a low molecular weight prepolymer having IV of about 0.1 dl/g to about 0.4 dl/g; the process comprising a) esterifying at least one dicarboxylic acid or mono-esters thereof or di-ester thereof and at least one diol or polyol at temperature in the range of 250⁰C to 290⁰C to obtain an esterified mixture; b) melt polymerizing the esterified mixture at temperature in the range of 260°C to 300°C to obtain polyester prepolymer having IV of about 0.1 to about 0.45 dl/g; c) producing prepolymer particles from the polyester obtained in step (b) by a particle former process at a temperature in the range of 110 to 160⁰C; d) adding organotin compound at any steps during the esterification or melt polymerization but before particle former process; e) solid state polymerizing the prepolymer particles to obtain a high molecular weight polyester resin having IV of about 0.4 dl/g to about 1.2 dl/g.

The term "polyester" as used herein means polyethylene terephthalate (PET), which intends to cover polymer and copolymers of polyethylene terephthalate.

The basic process comprising particle former process and solid-state polymerization is disclosed in US 5,510,454 (herein after referred as '454 patent), US 5,532,333, US 5,540,868, US 5,714,262, US 5,830,982, and US 6,451 ,966. However, none of these patents disclose the composition for the production of polyester resin by using organotin compounds such as organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, alkyl tin sulfonates or carboxylic acid derivatives of tin or any other tin compound and mixtures thereof for reducing dust generation in melt polymerization, dust generation and oligomer liberation during solid-state polymerization process.

According to the present invention there is also provided prepolymer particles having IV of about 0.1 to about 0.45 dl/g, the prepolymer comprising at least one dicarboxylic acid or mono-esters thereof or di-esters thereof, at least one diol or polyol and organotin compound.

According to the invention there is also provided a high molecular weight polyester resin having IV of about 0.5 dl/g to about 1.2 dl/g prepared from low molecular weight prepolymer particles having IV of about 0.1 to about 0.45 dl/g, the resin comprising at least one dicarboxylic acid or mono-esters thereof or di- esters thereof, at least one diol or polyol and organotin compound.

The organotin compound is selected from organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, alkyl tin sulfonates or carboxylic acid derivatives of tin or any other tin compound or mixtures thereof. The organotin oxide is selected from monobutyl tin oxide, dibutyl tin oxide or dioctyl tin oxide or mixtures thereof. The organotin compound is added in the range of about 10ppm to IOOOppm as a metallic tin by weight of polyester. Preferably, the organotin compound is added in the range of about 50 ppm to about 200 ppm as a metallic tin by weight of polyester. The organotin compound is added as a powder, a liquid or a slurry in ethylene glycol at any stage of esterification or melt polymerization such as slurry mixing tank, oligomer line and column reactor particularly in the column reactor, but before the particle former process. The dicarboxylic acid or its mono-esters thereof or di-esters thereof is selected from terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid or 4,4'-biphenyl dicarboxylic or any suitable dicarboxylic acids or mono-esters thereof or di-esters thereof or combinations thereof. Preferably, the dicarboxylic acid is selected from isophthalic acid. The acid anhydride is optionally used instead of dicarboxylic acid or monoester or diester thereof and selected from phthalic anhydride or trimellitic anhydride or pyromellitic anhydride or any suitable anhydride. The term polyol is intended to cover any suitable alcohol containing two or more hydroxyl groups known to those skilled in the art. The monoalcohol, diol or polyol is selected from monoethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butylenes glycol or 1 ,4-cyclohexane diol, 2-methyl-2, 3-propane diol, neopentylglycol or any suitable polyol or combinations thereof. The ratio of dicarboxylic acid or mono-esters thereof or diesters thereof to diol to polyol ratio is in the range from about 1 :1 to about 1 : 3. The prepolymer particle is crystalline or amorphous in nature. The prepolymer particle is spherical, hemispherical or cylindrical with no sharp edges. These prepolymer particles are used as a precursor for solid-state polymerization for increasing the IV. The high molecular weight polyester resin may also comprise any suitable additives for the improvement of any performance of polyester article.

According to the present invention there is also provided a use of the resin for the production of polyester articles such as preforms, containers, fibers or filaments, films or sheets.

Thus the present invention provided a process for the production of polyester by using organotin compounds such as organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, alkyl tin sulfonates or carboxylic acid derivatives of tin or any other tin compound or mixtures thereof in the esterification or the melt polymerization step but before particle former process which has reduced substantially the dust generation in melt polymerization, reduced dust generation and oligomer liberation in the solid state polymerization. This reduces substantially deposit formation on preheater nitrogen gas distribution plates, plenum and sidewalls of preheater, nitrogen gas circulation lines and conditioning vessels. Thus the process plant using the above process requires low maintenance as well as reducing shut down frequency for maintenance and increasing the productivity. The reduction in deposit formation also substantially eliminates degradation thus maintaining commercial quality of the product consistently. Thus the present process is an efficient and cost- effective process.

The present invention is further exemplified by the following non-limiting examples. Example 1 :

Purified terephthalic acid was esterified with monoethylene glycol (MEG) in, 1 :2 ratio along with 2 wt % lsophthalic acid, 1.5 wt % diethylene glycol (DEG) and Dioctyl tin oxide (50ppm as a metallic tin) in the form of slurry at 28O⁰C. The oligomer obtained was further melt polymerized at 29O⁰C to obtain polyester prepolymer having IV up to 0.26 dl/g. About 290ppm of antimony was added as a catalyst and 10 ppm P was added as a thermal stabilizer to the prepolymer. The low IV prepolymer melt was then passed through the 1.5 mm diameter orifice to form droplets on a continuous moving steel belt of particle former. These droplets were crystallized on the particle former maintained at a temperature between 110 to 160⁰C to obtain crystalline prepolymer particles. The prepolymer particles were used as precursor for solid-state polymerization. The prepolymer throughput was 72 kg/ hour. The prepolymer particles are hemispherical in shape and have no sharp edges.

Example 2:

Purified terephthalic acid was esterified with monoethylene glycol in 1:2 ratio along with 1.5wt % lsophthalic acid, 1.5 wt % DEG and Dibutyl tin oxide (50ppm as a metallic tin) in the form of slurry at 280⁰C. The oligomer obtained was further polymerized at 290⁰C to obtain a polyester prepolymer having IV up to 0.26 dl/g. About 290ppm of antimony was added as a catalyst and 10 ppm P was added as a thermal stabilizer to the prepolymer melt. The low IV prepolymer melt was then passed through the 1.5 mm diameter orifice to form droplets on a continuous moving steel belt of particle former. These droplets were then crystallized on the particle former maintained at a temperature between 110 to 160⁰C to obtain crystalline prepolymer particles. The prepolymer particles were used as precursor for solid-state polymerization. The prepolymer throughput was 72 kg/ hour. The prepolymer particles are hemispherical in shape and have no sharp edges.

Example 3: Comparative

Purified terephthalic acid was esterified with MEG in 1 :2 ratio along with 1.5wt % lsophthalic acid and 1.5 wt % DEG at 28O⁰C. The oligomer obtained was further melt polymerized at 29O⁰C to obtain polyester prepolymer having IV up to 0.26 dl/g. About 290ppm of antimony was added as a catalyst and 10 ppm P was added as a thermal stabilizer to the prepolymer. The low IV prepolymer melt was then passed through the 1.5 mm diameter orifice to form droplets on a continuous moving steel belt of particle former. These droplets were then crystallized on the particle former maintained at a temperature between 110 to 16O⁰C to obtain crystalline prepolymer particles. The prepolymer particles were used as precursor for solid-state polymerization. The prepolymer throughput was 72 kg/ hour. This prepolymer was considered as "Control". The prepolymer particles are hemispherical in shape and have sharp edges.

Example 4:

Solid state polymerization

Crystalline prepolymer particles having IV of 0.26 dl/g obtained according to the examples 1 to 3, was solid-state polymerized at 23O⁰C under inert atmosphere to raise the IV up to 1 dl/g.

Oligomer volatilization experiment was carried out by heating prepolymer (4kg) prepared according to the examples 1 to 3 in a fixed bed reactor at 230⁰C for 4 hrs under nitrogen. The byproduct liberated were collected in a collection pot and weighed. The weight of the oligomers as % of the prepolymer weight was taken as the % oligomer volatilization. IV of prepolymer used for oligomer volatilization experiment was 0.26dl/g.

Reduction in dust generation and oligomer volatilization from low molecular weight crystalline prepolymer with and without organotin compounds is given in the table 1.

TABLE 1

As seen from the table -1, reduction in dust generation and oligomer volatilization was observed with the use of organotin compounds. e

16

According to the present invention, no adverse effect on the perform clarity was observed due to the addition of tin oxide compounds.

Bottles of 1.5 L volume were produced using SIDEL SB01 single cavity blow moulding machine. Performance of the bottles produced with tin oxide compounds was comparable to the bottles produced without tin oxide compounds.

Claims

We claim:

1. An efficient process for the production of a high molecular weight polyester resin having IV of about 0.4 dl/g to about 1.2 dl/g produced from a low molecular weight crystalline prepolymer having IV of about 0.1 dl/g to about 0.45 dl/g; the process comprising a) esterifying at least one dicarboxylic acid or mono-esters thereof or di-ester thereof and at least one diol or polyol at temperature in the range of 25O⁰C to 29O⁰C to obtain an esterified mixture; b) melt polymerizing the esterified mixture at temperature in the range of 260°C to 300⁰C to obtain a polyester having IV of about 0.1 to about 0.45 dl/g; c) producing prepolymer particles from the polyester obtained in step (b) by a particle former process at a temperature in the range of 110 to 160°C; d) adding organotin compound at any steps during the esterification or the melt polymerization but before a particle former process; e) solid state polymerizing the prepolymer particles to obtain a high molecular weight polyester resin having IV of about 0.5 dl/g to about 1.2 dl/g.

2. A process as claimed in claim 1 , wherein the organotin compound is selected from organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, alkyl tin sulfonates or carboxylic acid derivatives of tin or mixtures thereof.

3. A process as claimed in claim 1 , wherein the organotin oxide is selected from monobutyl tin oxide, dibutyl tin oxide or dioctyl tin oxide or their mixtures thereof.

4. A process as claimed in claim 1 , wherein the organotin compound is added in the range of about 10 ppm to 5000 ppm as metallic tin by weight of polyester.

5. A process as claimed in claim 1 , wherein the organotin compound is added as a powder, a liquid or a slurry by mixing it in ethylene glycol at any stage of esterification or melt polymerization such as slurry mixing tank, oligomer line and column reactor particularly in the slurry mix tank, but before the particle former process.

6. A process as claimed in claim 1 , wherein the diol or polyol is selected from monoethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butylenes glycol or 1 ,4-cyclohexane diol.

7. A process as claimed in claim 1 , wherein the dicarboxylic acid or its mono- esters thereof or di-ester thereof is selected from terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid or 4,4'-biphenyl dicarboxylic acid.

8. A prepolymer having IV of about 0.1 to about 0.45 dl/g, the prepolymer comprising at least one dicarboxylic acid or mono-esters thereof or di- esters thereof, at least one diol or polyol and an organotin compound.

9. A prepolymer as claimed in claim 8, wherein the organotin compound is selected from organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, alkyl tin sulfonates or carboxylic acid derivatives of tin or mixtures thereof.

10. A prepolymer as claimed in claim 8, wherein the organotin oxide is selected from monobutyl tin oxide, dibutyl tin oxide or dioctyl tin oxide or their mixtures thereof.

11. A prepolymer as claimed in claim 8, wherein the organotin compound is added in the range of about 10 ppm to 5000 ppm as metallic tin by weight of polyester.

12. A prepolymer as claimed in claim 8, wherein the diol or polyol is selected from monoethylene glycol, diethylene glycol, ■ triethylene glycol, propylene glycol, dipropylene glycol, butylenes glycol or 1 ,4-cyclohexane diol, 2- methyl-2, 3-propane diol, neopentylglycol or any suitable polyol or combinations thereof.

13. A prepolymer as claimed in claim 8, wherein the dicarboxylic acid or its mono-esters thereof or di-ester thereof is selected from terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid or 4,4'-biphenyl dicarboxylic or any suitable dicarboxylic acids or mono-esters thereof or di-esters thereof, or combinations thereof.

14. The prepolymer as claimed in claim 8, wherein the prepolymer is crystalline, semi-crystalline or amorphous in nature.

15. The prepolymer as claimed in claim 8, wherein the prepolymer is spherical, hemispherical or cylindrical with no sharp edges.

16. A high molecular weight polyester resin having IV of about 0.5 dl/g to about 1.2 dl/g prepared from a low molecular weight prepolymer having IV of about 0.1 to about 0.45 dl/g, the resin comprising at least one dicarboxylic acid or mono-esters thereof or di-esters thereof, at least one diol or polyol and organotin compound.

17. A resin as claimed in claim 16, wherein the organotin compound is selected from organotin oxides, organotin hydroxides, organotin alkoxides, organostannoic acids, alkyl tin sulfonates or carboxylic acid derivatives of tin or mixtures thereof.

18. A resin as claimed in claim 16, wherein the organotin oxide is selected from monobutyl tin oxide, dibutyl tin oxide or dioctyl tin oxide or their mixtures thereof.

19. A resin as claimed in claim 16, wherein the organotin compound is added in the range of about 10 ppm to 5000 ppm as metallic tin.

20. A resin as claimed in claim 16, wherein the diol or polyol is selected from monoethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butylenes glycol or 1 ,4-cyclohexane diol, 2- methyl-2,3-propane diol, neopentylglycol or any suitable polyol or combinations thereof.

21. A resin as claimed in claim 16, wherein the dicarboxylic acid or its mono- esters thereof or di-ester thereof is selected from terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid or 4,4'-biphenyl dicarboxylic or any suitable dicarboxylic acids or mono-esters thereof or di-esters thereof, or combinations thereof.

22. High molecular weight polyester resin as claimed in claim 16, the resin is used for the production of polyester articles such as preforms, containers, fibers or filaments, films or sheets.