The invention relates to substantially linear polyesters prepared from phosphoric acid and/or its oligomers, a method for their preparation and their use. Polyesters containing phosphorus in the backbone or side chains are known for example from biology in nucleic and teichoic acids. However these nucleic and teichoic acids are not substantially linear, are not prepared from phosphoric acid and/or its oligomers, are much more complex and comprise within their structure nitrogen-containing bases and sugars. Their technical application on a larger scale is not possible. Polyesters containing phosphorus in the backbone or side chains are described for example in Macromolecules 1993, 26, 2228-2233. However the phosphorus- containing polyester described there is highly branched and not substantially linear and is not prepared starting from the three-functional phosphoric acid. Phosphorus- containing polymers are also described in Makromol. Chem. 178, 2943-2947 (1977). However the polymers described here contain a phosphorus- hydrogen bond. The presence of the P-H bond makes the adjacent ester bonds very susceptible to hydrolysis. Additionally this polymer is prepared starting from a cyclic phosphor-compound and not from phosphoric acid. Substantially linear polyesters without nitrogen in its structure and prepared from a) phosphoric acid and/or its oligomers and b) at least one polyol are not yet known. The substantially linear polyesters prepared from phosphoric acid and/or its oligomers according to the invention can advantageously be represented by the following overall formula (I):

Formula I

Wherein: R is independently of each other the group -(CR1R2)n-Om-(CR1R2)q- or -(CR1R2J=(CR1R2)- R1 is independently of each other: -H, -OH1 -OROH, -OROP(O)Q2, or an alkyl group with 1-5 carbon atoms R2 is independently of each other: -H1 -OH, -OROH, -OROP(O)Q2, or an alkyl group with 1-5 carbon atoms n is a natural number 1-12 m is O or 1 q is a natural number 1-12 x represents the number of linear fragments and is a natural number 0-50 y represents the number of branched fragments and is a natural number 0-49 Q is independently of each other -OH or -OROH Z is independently of each other -OROH or -OROP(O)Q2. R1 R1 and R2 are the groups and atoms derived from the polyol that is used to prepare the polyester according to the invention. With "polyol" is meant here and hereinafter an alcohol with two or three hydroxyl-groups. Preferably R1 and R2 are independently of each other: -H, -OH, or an alkyl group with 1-5 carbon atoms, more preferably -H, or an alkyl group with 1-5 carbon atoms. R1 on the one carbon and R1 on the other carbon atom in the structural element -(CR1R2)n-Om-(CR1R2)q- or -(CR1R2)=(CR1R2)-, are also chosen independently from each other. Thus the structural element can be symmetrical but need not to be symmetrical. The same holds for R2. The polyester according to the invention as represented in Formula I, is a random copolymer containing a x number of linear fragments (-OROP(O)(OH)-) and a y number of branched fragments (-OROP(=O)Z-). The linear and branched fragments are randomly distributed along the polyester backbone. When reference is made to linear or branched, the distribution around the phosphorus atom is considered. With substantially linear polyester is here and hereinafter meant that the number of linear fragments exceeds the number of branched fragments, thus x being larger than y (x > y). Preferably the number of linear fragments exceeds the number of branched fragments with a factor of at least 2 (x ≥ 2y), more preferably the number of linear fragments exceeds the number of branched fragments with a factor of at least 5 (x ≥ 5y), even more preferably the number of linear fragments exceeds the number of branched fragments with a factor of at least 10 (x ≥ 1Oy). Most preferably the number of branched fragments is O. The number of linear and branched fragments can be determined by 31P-NMR. Depending on the reaction conditions the number of repeating linear units (x) and repeating branched units (y) can vary between within wide ranges. The numbers for linear and branched units are independent from each other. Generally x will vary between O and 50. Preferably x is between 1 -40, more preferably between 2- 30. Generally y will vary between 0 and 49. Preferably y is between 1-39, more preferably between 2-29. The substantially linear polyesters according to the invention are itself almost colourless, have an increased hydrolytic stability and are substantially linear. The polyesters according to the invention are advantageously used in various compositions, for example a composition comprising, in addition to the polyester according to the invention, also at least one other polymer that is structurally and/or chemically distinct from the polyester according to the invention, or a composition comprising, in addition to the polyester according to the invention, at least one crosslinker, or a composition comprising, in addition to the polyester according to the invention, at least one crosslinker and at least one additive and optionally one or more pigments and one or more fillers. The substantially linear polyesters according to the invention are advantageously used as a flame retardant or primer or adhesion improver. The invention also relates to a method of preparation of polyesters directly from a) phosphoric acid and/or its oligomers and b) at least one polyol. In the literature methods are known for synthesizing phosphorus- containing polymers. These methods are either based on the ring-opening polymerisation of cyclic phosphates or phosphites (Makromol. Chem. 178, 2943-2947 (1977)) or on the poly-transesterification of H-dialkylphosphonates (dialkylphosphites) (Macromolecules 1993, 26, 2228-2233 or Journal of Polymer Science, Part A: Polymer Chemistry, VoI 37, 1365-1381 (1999)). These methods result in the formation of poly(alkylene)-H-phosphonate that contains a P-H bond. Additionally the processes use rather expensive starting materials; at least materials that are not widely available on an economical scale. Therefore, although the methods are already known for some 25 years, it has never come to the production of these polymers on an economic scale. Additionally the methods known in literature do not lead to the production of stable, substantially linear polyesters. Adaptation of the known processes would lead to the development of processes that involve even more process steps, which make them even more expensive and thus undesirable. Therefore the need existed to develop a process for the preparation of phosphorus- containing polyesters from readily available starting materials. A readily available source of phosphor is phosphoric acid and/or its oligomers. Starting from this kind of phosphor source an alcohol source would be needed to arrive at the ester stage. However it is generally known to the man skilled in - A -

the art of polyester synthesis, that the direct synthesis of phosphoric acid esters from the acid and the alcohol is considered not possible. Both the overview literature (S. Gryglewicz, Wiadomosci chemiczne, 57, 1-2 (2003)) and the source bibliography (E. Cherbuliez, J-P. Leber, Helvetica Chimica Acta, 35, 644-664 (1952)) suggest that the direct synthesis of diesters would not be possible. This was explained in terms of the presence of the electron octet around the phosphorous atom in a transitional state. An attack against the phosphorous atom was considered to be impossible therefore. Another reason for the man skilled in the art to assume that the direct synthesis of phosphoric acid esters from the acid and the alcohol is not possible, is the fact that the known reaction of neopentyl alcohol with sulfuric acid leads only to 2- methyl butyral (Yvernault, Mazet, Bull. Soc. Chim. Fr., 2755 (1967)). Therefore the man skilled in the art would expect the reaction with phosphoric acid to result in the same. Additionally as phosphoric acid is tri-functional, that is, it has three acid groups available for reaction, it is expected to lead to excessive gelation under normal esterification conditions. For the above-mentioned reasons, it seemed that the attempts to search for a polyester synthesis directly from a) phosphoric acid and/or its oligomers and b) at least one polyol, were destined to be unsuccessful. Therefore it was very surprising and unexpected to find out that a direct synthesis was feasible. Therefore the invention also relates to a process for the preparation of a polyester based on phosphoric acid and/or its oligomers. Preferably the process for the preparation of the polyesters uses only these components a) and b). The process for the preparation of a polyester based on a) phosphoric acid and/or its oligomers and b) at least one polyol comprises at least the following steps: a. combining and mixing the phosphoric acid and/or its oligomers with the at least one polyol, b. reacting the phosphoric acid and/or its oligomers with the at least one polyol. It is possible in the preparation process according to the invention to use one kind of polyol, but it is also possible to use a combination of different polyols. With different polyols is both meant: two or more alcohols with the same functionality but with a different chemical structure and two or more alcohols with a different functionality. An example of the first combination of "different alcohols" is ethylene glycol with hexane diol (2-functional). An example of the second combination of "different alcohols" is ethylene glycol (2- functional) combined with trimethylolpropane (3-functional). With "functionality" in relation to the polyol is meant the number of hydroxyl- groups that is available for reaction. It has been found advantageous to use only 2-functional polyols (so-called "diols"). Examples of suitable polyols are substituted and unsubstituted aliphatic and cyclo-aliphatic polyols and polyetherpolyols. The polyols advantageously contain between 1-50 carbon atoms. Preferably polyols are used with 1-30 carbon atoms, more preferred between 1-20 carbon atoms. Examples of suitable polyols are ethylene glycol, propylene glycol, butane diol, heptane diol, cyclohexyldimethylol, 1 ,4- butene diol, diethylene glycol, trimethylolpropane, di(trimethylolpropane), glycerol, hexanetriol, trimethylol ethane and combinations of any of them. Preferably ethylene glycol, propylene glycol and/or diethylene glycol are used. Phosphoric acid and/or its oligomers are being used in the process according to the invention. Phosphoric acid is widely known and generally available. It is possible in the process according to the invention to use anhydrous phosphoric acid, however it is also possible to use phosphoric acid that contains a small amount of water. However anhydrous phosphoric acid is preferred as in that case less water needs to be removed from the production process. Suitable oligomers of phosphoric acid are triphosphoric acid and polyphosphoric acid. To obtain the desired substantially linear polyesters that can be prepared with the process according to the invention it is advantageous to use only the compounds a) phosphoric acid and/or its oligomers and b) at least one polyol, with the exclusion of other components. The man skilled in the art of polyester synthesis knows how the phosphoric acid and/or its oligomers and the polyol should be combined and can easily find the conditions where under reaction will take place. It has been found advantageous for obtaining higher yields on the di-ester compared to the tri-ester, to add the polyol to the phosphoric acid and/or its oligomers. The process of the invention can advantageously be used for the preparation of substantially linear polyesters. Substantially linear polyesters that can be prepared with the process according to the invention can be described by the following formula:

Formu|a |

Wherein: R is independently of each other the group -(CR1R2)n-Om-(CR1R2)q- or -(CR1R2HCR1R2)- R1 is independently of each other: -H, -OH, -OROH, -OROP(O)Q2, or an alkyl group with 1-5 carbon atoms R2 is independently of each other: -H, -OH, -OROH, -OROP(O)Q2, or an alkyl group with 1-5 carbon atoms n is a natural number 1-12 m is O or 1 q is a natural number 1-12 x represents the number of linear fragments and is a natural number 0-50 y represents the number of branched fragments and is a natural number 0-49 Q is independently of each other -OH, -OROH Z is independently of each other -OROH or -OROP(O)Q2.

R, R1and R2 are the groups and atoms derived from the polyol that is used to prepare the polyester according to the invention. Preferably R1 and R2 are independently of each other: -H, -OH, or an alkyl group with 1-5 carbon atoms, more preferably -H, or an alkyl group with 1-5 carbon atoms. The polyester according to the invention as represented in Formula I1 is a random copolymer containing a x number of linear fragments (-OROP(=O)(OH)-) and a y number of branched fragments (-OROP(=O)Z-). The linear and branched fragments are randomly distributed along the polyester backbone. It is advantageous in the process for the preparation according to the invention, to heat up the reactants. In that case the process will comprise at least the following steps: a. combining and mixing the phosphoric acid and/or its oligomers with the at least one polyol, b. reacting the phosphoric acid and/or its oligomers with the at least one polyol, c. and heating the phosphoric acid and/or its oligomers or heating the at least one polyol or heating both, whereby optional step c) can be performed in any order relative to step a) and b). The best reaction temperature depends on the chemical nature of the polyol. The man skilled in the art can easily determine the best temperature. For example, the reaction according to the invention of ethylene glycol with phosphoric acid can advantageously be performed between 50-2000C, preferably at a temperature between 75-140°C, more preferably at a temperature between 90-110°C. Surprisingly it was found that the addition of a catalyst during the esterification reaction can influence the reaction advantageously. It was namely found that the addition of certain catalysts leads to a higher production of di-esters compared to the production of mono-esters. Therefore it is advantageous to add the catalyst at the beginning of the process, thus preferably together with step a. Additionally it was found that side reactions that occasionally occur at higher reaction temperatures (for example higher than 15O0C) could almost completely be eliminated when a catalyst was present. With some catalysts the side reactions were not observed at all. In the case that a catalyst is used, the process will comprise at least the following steps: a. combining and mixing the phosphoric acid and/or its oligomers with the at least one polyol, b. reacting the phosphoric acid and/or its oligomers with the at least one polyol, c. heating the phosphoric acid and/or its oligomers or heating the at least one polyol or heating both, whereby optional step c) can be performed in any order relative to step a) and b), d. adding a catalyst, whereby the addition of the catalyst can be performed in any order relative to step a) b) and c). With certain catalysts the advantageous effect could be observed directly from the beginning of the preparation process. The catalysts that were found to be advantageous were metal salts. Preferably metal salt catalysts were used wherein the metal has a configuration of 3d- and 4s- outer electron shells, especially Stannum and the elements with number 21 to 30 in Mendeleyev's table (as presented on the inside cover of the Handbook of Chemistry and Physics, 65th edition, CRC Press Inc.) preferably the elements with number 21-28. The catalyst can be added to the reaction in its salt-form or it can be added in its metallic-form whereupon a salt is formed in-situ in the reaction mixture. It is possible to use one catalyst, however it is also possible to use a mixture of catalysts. Especially preferred catalysts are derivatives of Sc, Ni, Cr, Co and Sn. When the catalyst is directly applied in its salt form the counter ions to the metal ion can be chosen more or less freely. Preferred counter ions are acetate, trifluoromethanesulphonate, chloride, octanoate and 2-ethylhexanoate. The above-mentioned effect of enhanced production of di-esters over mono-esters could be observed over a broad range of molar ratios between the phosphoric acid and/or its oligomers on the one side and the polyol on the other side. Surprisingly this effect could even be observed when the molar ratio between the phosphoric acid and the polyol was 1 :1 , thus when 2 hydroxyl-groups were present per 3 acid groups. The molar ratio between the phosphoric acid and the polyol is not particularly critical and can be chosen between broad ranges, for example between 10:1 and 1 :10. A preferred ratio acid-to-polyol is chosen between 5:1 and 1 :5, more preferably the ratio is chosen between 1 :3 and 2:1 , most preferably the ratio acid-to- polyol is approximately 1 :1. The reaction between phosphoric acid and/or its oligomer and the polyol can be performed in the bulk without any solvent or in a solvent. Preferably the reaction is performed in the bulk. When the reaction takes place in the bulk preferably a dry, inert gas is applied. The polymerisation is preferably conducted in such a way as to enable continuous removal of water. To this effect a purge gas stream can be applied, for example nitrogen, argon or helium. It is also possible to use a solvent as reaction medium. In case a solvent is used it should be chosen so as to be inert towards the reactants, phosphoric acid and/or its oligomers and the polyols used. As solvents inert compounds can be used that form azeotropes with water. In such a case the reaction can be performed at the boiling temperature of the azeotrope. Examples of suitable solvents are benzene, toluene, xylene, heptane, cyclohexane, and methyl-cyclohexane and combinations of any of them. Preferably a combination of benzene and toluene is used. A preferred ratio between benzene and toluene is 1 ;1 ,5 (on volume basis). The polyesters prepared according to the invention are generally built on chains of poly (di-esters of phosphoric acid). The polyesters prepared may also contain fragments with higher orders of polyester, for example tri-esters of phosphoric acid. In the case of a tri-ester the group Z in formula I, is -OROP(O)Q2 . Preferably the amount of poly(tri-esters) formed is less than 33 mol% relative to the total of products formed. More preferred the amount of poly(tri-esters) formed is less than 17 mol%, even more preferred the amount of poly(tri-esters) formed is less than 9 mol% More preferably the prepared polyesters are only built on chains of poly (di-esters of phosphoric acid), thus the amount of poly(tri-esters) being 0 mol%. The end groups, Q, can be the same or different and are dependent on the reaction conditions and especially on the relative amounts of the components during the preparation. The end groups can therefore be a hydroxyl-group or a group represented by -OROH. When both groups at the same P that are represented in formula I by Q are hydroxyl-groups (-OH), the unity at the end of the polyester chain can be regarded as a mono-ester of phosphoric acid, when 1 Q-group is a hydroxyl- group and the other Q at the same P is represented by -OROH, the unity at the end of the polyester chain can be regarded as a di-ester of phosphoric acid. When both Q- groups at the same P are represented by -OROH, the unity at the end of the polyester chain can be regarded as a tri-ester of phosphoric acid. The polyester according to the invention will generally have acid groups available. These acid groups originate from the phosphoric acid or its oligomer. These acid groups are in Formula I available in the "linear fragment", (-OROP(=O)(OH)-) or in the group -P(O)Q2 when Q is -OH. Depending on the reaction conditions the polyester will also have hydroxyl-groups available. It is preferred to have a polyester with both acid- and hydroxyl- functionality. It is even more preferred to have a polyester with a higher acid value than hydroxyl-value. The final product from the preparation process can contain moieties with the same or with different structures. In the last case, where the structures are different, a mixture of molecules that differ structurally is formed. The final product can have a linear character, however it is also possible with the process according to the invention to obtain branched and/or star-shaped polyesters. Even cyclic structures are possible, for example as represented in Formula II:

Formula Il Wherein: i and h are natural numbers, independently chosen between 1-10. Preferably i and h are chosen so as to form a 5-, 6-, or 7- membered ring structure, k is a natural number between 1-99. Preferably 1-49, more preferably 1-20, even more preferably 2-10. The value for k can be regulated with the amount of triol that is used in the preparation of the polyester. The polyester according to the invention or the polyester prepared according the process of the invention can be used in industry as such or, for example, in a composition. The invention therefore also relates to the use of the polyesters according to the invention. The polyester according to the invention can for example be used as a flame retardant. The polyester can be used in a process for improving the flame retardancy of a composition that comprises at least a polymer B. With "polymer B" is meant a polymer that is chemically or structurally different from the polyester according to the invention. Polymer B is regarded to be the matrix material and can be chosen from any material that is able to suit the mechanical and/or aesthetical requirements of the part to be produced. The outcome of the process is a composition with improved flame retardancy. This result can be obtained by adding to the composition comprising at least polymer B, a polyester according to the invention. The polyester is described above in more detail. The composition can contain more than one polymer; it can also in addition to the at least one polymer contain usual additives and fillers. The amount of the flame-retardant polyester according to the invention is generally less than 50 % compared to the weight of polymer B. When polymer B comprises more than one polymer, the amount of flame-retardant polyester is related to the total amount of polymers making up polymer B. Preferably the amount of flame-retardant polyester according to the invention is less than 35%, more preferably less than 25%, most preferably less than 15%. Polymer B can be chosen from a wide range of polymers. Its choice is more or less determined by the structural or aesthetical properties of the object that is finally obtained from the flame-retardant composition. Examples of suitable polymers are polyolefin, polyamide, polystyrene, polyester, polycarbonate, polyurethane, polyepoxy, acrylic resin, phenol resin, polyphenylene and combinations of two or more of them. It is preferred to use polyolefins, as they are very suitable to make structural parts from. This is due to their intrinsic properties. Polyolefins are for example, polyethylene, polypropylene, polyisobutene, and ethylene-propylene rubber, as for example EPM and EPDM-rubber. More preferred is polypropylene. Polyamides are for example nylon-6, nylon-6,6, nylon-6,10, nylon-6,12, nylon-11 , and nylon-12. Polyesters are for example polyethylene terephthalate and polybutylene terephthalate. In another embodiment of the invention the polyester according to the invention can for example be used as a primer or adhesion improver of a composition on a substrate. The polyester can be used in a process for improving the adhesion of a composition on a substrate. The outcome of the process is a composition with improved adhesion. This result can be obtained by adding to the composition a polyester according to the invention. In again another embodiment of the invention the polyester according to the invention can for example be used as a dispersion agent. The dispersion agent can improve the dispersion of for example pigments or fillers in a composition, preferably a coating composition. The polyester can be used in a process for improving the dispersion of a component in a composition. The outcome of the process is a composition with improved dispersion of the component. Examples of components that benefit from an improved dispersion are pigments and fillers. The polyester is described above in more detail. The substrate is not particularly critical as it turned out that the polyester according to the invention improves the adhesion of all kinds of compositions on all kinds of substrates both of an organic and inorganic nature. It especially improved the adhesion of coating compositions. Examples of substrates are: metal, glass, ceramics, stone, concrete, paper, cardboard, wood, leather, cork and plastic. The invention also relates to a composition comprising at least a. one polyester based on a^ phosphoric acid and/or its oligomers and a2) at least one polyol and b. at least one crosslinker. Polyester a. is the polyester according to the invention and is as described above. By combining polyester a. with at least one crosslinker it is possible to obtain a crosslinked network. Such a crosslinked network is advantageously obtained on the surface of a substrate, thereby forming a coating on the surface. In principle any known crosslinker that is able to react with the functional groups on polyester a., can be used. Examples of suitable crosslinkers are: • crosslinkers of the epoxy type, for example triglycidyl isocyanurate, • polyisocyanates for example caprolactam blocked IPDI (isophorone diisocyanate) derivatives, uretidione of IPDI, TDI (toluene diisocyanate) derivatives, TMXDI (tetramethylxylene diisocyanate) derivatives, or trimers thereof, • polyphenols, for example polyphenols of the resol or novolac type, • and amino resins, for example alkylated melamine or benzoguanimine resins. An example of an alkylated melamine is hexamethoxymethylmelamine (HMMM). Preferably hexamethoxymethylmelamine or an epoxy resin crosslinker is used. The amount of crosslinker is not particularly critical, however depending on the nature of the crosslinker and the polyester and the functionality of both, the amount of crosslinker is optimised so that the obtained coating after curing is able to fulfil the chemical, mechanical and other requirements. Generally the crosslinker is present in an amount of at least 0,1 w% relative to the sum of polyester and crosslinker, preferably at least 1 w%, more preferred at least 5 w%, most preferred at least 7 w%. Generally the crosslinker is present in an amount less than 50 w% relative to the sum of polyester and crosslinker. Preferably the crosslinker is present in an amount of not more than 30 w%, more preferable not more than 20 w%, most preferable not more than 15 w%. Next to the polyester according to the invention the composition can optionally contain other polymers that take part in the formation of a network. With other polymer is meant a polymer that is structurally and/or chemically different from the polyester according to the invention. Examples of suitable optional other polymers are polyolefin, polyamide, polystyrene, saturated or unsaturated polyester, polycarbonate, polyurethane, polyepoxy, acrylic resin, phenol resin, polyphenylene. To take part in the formation of a network suitable functional groups that are reactive to the crosslinker used need to be present in or on the polymer. Preferably the optional polymer is a polyester. The composition containing the polyester and the crosslinker is generally referred to as the binder. A coating composition generally comprises next to the binder of the composition according to the invention, one or more customary additives, optionally one or more fillers and optionally one or more pigments. The invention also relates to such a coating composition. With additive is generally meant a substance that is added in a small quantity and which has a particular chemical or technological effect. Examples of additives are: degassing agents, dispersing agents, flow control agents, flow- promoting agents, rheology-influencing agents, anti- or de-foaming agents, (light)stabilizers, thickeners, wetting agents, anti-skinning agents, anti-sedimentation agents, anti-flocculation agents, adhesion promoters, structural additives, gloss- enhancing additives, catalysts, and flatting agents. Examples of stabilizers are: primary and/or secondary antioxidants, UV stabilizers for example quinones, (sterically hindered) phenolic compounds, phosphonites, phosphites, thioethers and HALS compounds (hindered amine light stabilizers). Examples of degassing agents are benzoin and cyclohexane dimethanol bisbenzoate. Examples of flow agents include polyalkylacrylat.es, fluorohydrocarbons and silicone fluids. Other suitable additives are for example additives for improving tribocharging, for example sterically hindered tertiary amines that are described in EP-B-0.371.528. With filler is generally meant a compound that gives the coating material a greater volume (body). Examples of fillers are: chalk, talcum, metal oxides, silicates, carbonates and sulphates. With pigment is generally meant a finely divided colouring compound. The pigment can be of an organic or inorganic nature. Examples are titanium dioxide, carbon black, pearlescent pigments, zinc phosphate, zinc sulphide, iron oxide and chromium oxide. A suitable example of an organic compound is the group of azo compounds. Sometimes a difference is made between a pigment and a dye, whereby the dye is regarded as a colouring compound that dissolves in the composition. Here and hereinafter dyes are incorporated within the term pigments. The coating composition according to the invention can next to the binder and the at least one additive and optional filler and pigments, contain one or more solvents and/or dispersants. With solvent is meant a liquid or blend of liquids that is able to dissolve the binder. Examples include butyl acetate, butyl glycol, white spirit and water. Dispersants are liquids that do not dissolve the binder but instead hold them in a dispersion or emulsion. Examples are water and in non-aqueous dispersions hydrocarbons. The invention will be further illustrated in the following, non-limitting, examples.

Experimental:

31P-NMR The progress of the esterification of H3PO4 with polyols was monitored by 31P(1HJ-NMR spectroscopy. Esters, pyrophosphates and unreacted H3PO4 were expected in the reaction mixture. Esters of orthophophoric acid are classified according to the number of ester groups in the phosphate group: mono-ester, di-ester and tri-ester. To obtain separate signals from the P-atoms in the different phosphate (or pyrophosphate) groups, all foreign cations were removed from the reation mixture on a cation exchange resin, and the solution (in D2O) was adjusted to pH=12 with NaOH. Assignments of the signals found in the 31P-NMR spectra are as follows: H3PO4 (PO43" anion) at about 6.2 ppm, Mono-esters (ROPO32" anion) in the range from 4.5 to 5.0 ppm, di-esters ((RO)2PO2" anion) in the range from 0.5 to 1.5 ppm, tri-esters ((RO)3PO) in the range from -0.5 to 0.5 ppm, pyrophosphate anion in the range from -2.0 to -10.0 ppm, at the range of chemical shifts -2.0 to -4.0 ppm, there are signals from cyclic six- membered derivatives of phosphoric acid. The 31P NMR spectra were measured at 24,2 MHz on a JeolFX60 apparatus, with 85 wt% H3PO4 as an external standard.

Examples:

Example I In a round bottom reaction flask, that was equipped with a magnetic stirrer, were placed: 5.7 g (0.0786 mole) of anhydrous H3PO4, 7.76 g (0.125 mole) of ethylene glycol and 0.1 g (5.66-10"4 mole) of a Ni(CH3COO)2 -catalyst (Ni(II) acetate). The reaction mixture comprising the indicated components was heated at 150 0C in an oil bath for 17.5 hours in a stream of slowly flowing argon. After certain time intervals samples were taken from the reaction mixture and its composition was determined by 31P-NMR. In this way the proportions of mono-, di-, and triesters were established. The following results were obtained. After 0.5 hours 7.7% of monoesters and 7.0% of diesters (The rest of the 100% are the still unreacted starting components), after 3.5 hours 39.1% of monoesters and 22.2% of diesters, after 11 hours 48% of monoesters and 30.4% of diesters, and after 17.5 hours 35.3% of monoesters, 50.4% of diesters and 8.7% of triesters. On the basis of the analysis of the 1H and 31P NMR spectra, as well as MALDl-TOF (Matrix-assisted laser desorption/ionisation-time of flight mass spectrometry)- and FAB (Fast Atom Bombardment)- mass spectrometry, it was established that the obtained reaction product is a mixture of products of the polymerization degree ≤8, with the following end-groups: either acid at both ends or alcohol at both ends as well as the acid group on one side and the alcohol group on the other side. These products contain both in repeating units and in the end-groups, segments of di- and triethylene glycols. GLC analysis of the hydrolysis products revealed, that these units do not exceed triethylene glycol. The hydrolysis products were obtained by decomposition of the polyesters at elevated temperature, in the presence of an aqueous solution of p-toluenesulfonic acid.

Example Il A mixture of 1.48g (0.015 mole) of anhydrous H3PO4, 1.47g (0.0237 mole) of ethylene glycol and 0.015g (3.05-10"5 mole) of a scandium trifluoromethanesulfonate -catalyst was heated in a round bottom reaction flask that was equipped with a magnetic stirrer at 100 0C on an oil bath and in a stream of slowly flowing argon for 105 hours. After certain time intervals samples were taken from the reaction mixture and its composition was determined by 31P-NMR. The established proportions were as follows: after 10 hrs 3.3% of monoesters and 8.4% of diesters, after 30.5 hrs 8.4% of monoesters and 27.6% of diesters. After 51.5 hours: 18.4% of monoesters and 35% of diesters. After 105 hrs 30.6% of monoesters and 56.7% of diesters. It was found that when the Sc(III) catalyst was used the proportion of diesters in the reaction mixture was right from the beginning higher than the proportion of monoesters compared to the situation with no catalyst used. On the basis of the analysis of 1H- and 31P- NMR spectra, as well as MALDI-TOF-mass spectrometry it was established that the product is a mixture of macromolecules of the polymerization degree ≤12 with the following end-groups: either acid at both ends or alcohol at both ends as well as the acid group on one side and the alcohol group on the other side. These products contain both in repeating units and in the end-groups, segments of di- or (less often) triethylene glycol. GLC analysis of the hydrolysis products revealed, that these segments do not exceed triethylene glycol.

Example III A mixture of 5.36g (0.0547 mole) of anhydrous H3PO4, 51.05g (0.823 mole) of ethylene glycol (EG) and 0.15g (6.3- 10"4 mole) of a CoCI2-6H20-catalyst, was heated in a round bottom reaction flask that was equipped with a magnetic stirrer at 150 0C on an oil bath and in a stream of slowly flowing argon for 130 hours. After certain time intervals samples were taken from the reaction mixture and instantaneously its composition was determined by 31P-NMR. The following results were obtained: after 11 hours 11.8% of monoesters and 17.8% of diesters, after 30 hours 19.2% of monoesters and 39.5% of diesters and, finally after 130 hours 20.8% of monoesters and 72.5% of diesters. No triesters were detected in the reaction mixture, with this ratio of reagents [EGy[H3PO4] =15. On the basis of 1H and 31P NMR and MALDI-TOF mass spectrometry it was established that the product is a mixture of molecules with hydroxylic end-groups on both ends being higher than 50%.

Example IV A mixture of 5g (0.037 mole) trimethylolpropane, 2.91g (0.030 mole) H3PO4 and 0.1g (2.47- 10"4 mole) tin (II) 2-ethylhexanoate (catalyst) was placed in a round bottom reaction flask and heated with stirring under reduced pressure (~1mm Hg ) at 150 0C for 30 hours. After certain time intervals samples were taken from the reaction mixture and instantaneously its composition was determined by 31P-NMR. The following results were obtained: after 2 hours 15.5% of monoesters and 1.3% of "cyclic structures", after 10 hours 29.5% of monoesters 13.7% of diesters and 15.6% of "cyclic structures" and after 30 hours 23.3% of monoesters, 32.2% of diesters and 36.7% of "cyclic structures". On the basis of the 31P NMR spectra recorded at basic, neutral and acidic conditions and 31P- and 1H- NMR spectra it was established, that "cyclic structures" are six-membered triesters.

Example V A mixture of 5.49 g (0.056 mole) H3PO4, 5.52g (0.089 mole) ethylene glycol and 0.0126g (2.56-10"5 mole) scandium trifluoromethanesulfonate (catalyst) was placed in a round bottom flask equipped with a magnetic stirrer, condenser and azeotropic head and refluxed under normal pressure for 140 hours under a separate phase of benzene/toluene (1 :1.5 v/v) mixture boiling at 100 0C. At the chosen times samples of the reaction mixture were removed and the proportions of mono-, di-, and triesters were determined with 31P- NMR method. The following results were obtained: after 20 hours 7.5% of diesters, after 40 hours 2.5% of monoesters and 22.5% of diesters, after 70 hours 11.9% of monoesters and 56.5% of diesters and after 140 hours 25.0% of monoesters, 58.5% of diesters and 11.5% of triesters. On the basis of the analysis of the 1H- and 31P- NMR spectra, as well as MALDI-TOF-mass spectrometry it was established, that this product is a mixture of products of the polymerization degree n~12 with the following end- groups: either acid at both ends or alcohol at both ends as well as the acid group on one side and the alcohol group on the other side. These products contain in both repeating units and in the end-groups segments of some di- and triethylene glycols. GLC analysis of the hydrolysis products revealed, that these units do not exceed triethylene glycol.

Example Vl 6.97 g (0.071 mole) of H3PO4 and 6.77 g (0.109 mole) of ethylene glycol (EG) (the initial ratio of reagents ([EG]0/ [H3P04]o =1.54) were put into a two-neck flask equipped with a magnetic stirrer and the mixture was heated for 300 hours at a temperature of 1000C on an oil bath in a stream of slowly flowing argon. Samples were taken at certain points in time and their ester- content was determined with the 31P- NMR method as described above. The following results were obtained: after 50 hours: 15.8% of monoesters and 13.8% of diesters. After 100 hours: 25.4% of monoesters and 29.4% of diesters. After 200 hours: 34.1% of monoesters and 39.0% of diesters. After 300 hours: 38.0% of monoesters and 44.0% of diesters. It was determined on the basis of the mass spectrometry MALDI-TOF and FAB that the mixture contains compounds of the polymerisation level < 5. The compounds are bilaterally ended with acid groups, alcohol groups, as well as the acid group at one side and the alcohol group at the other side.

Example VII 1.54 g (0.0157 mole) of H3PO4 and 14.68 g (0.2368 mole) of ethylene glycol (EG) (the initial ratio of reagents ([EG]0/ [H3P04]o =15.08) were put into a two- neck flask equipped with a magnetic stirrer and the mixture was heated for 110 hours at a temperature of 15O0C on an oil bath in a stream of slowly flowing argon. Samples were taken at certain points in time and their ester- content was determined with the 31P- NMR method as described above. The following results were obtained: after 6 hours: 2.7% of monoesters and 5.5% of diesters. After 24 hours: 18.2% of monoesters and 39.2% of diesters. After 50 hours: 28.3% of monoesters and 49.0% of diesters. After 110 hours: 32.2% of monoesters and 57.4% of diesters. It was determined on the basis of the mass spectrometry MALDI-TOF and FAB mass spectrums and 1H-NMR spectrums that the mixture contains compounds of a low polymerization level < 3. The compounds are bilaterally ended with acid groups, alcohol groups, as well as the acid group at one side and the alcohol group at the other side, however the majority of the end-groups being alcohol groups.