WO1993017993A1

WO1993017993A1 - Thin film oligomerization of hydroxycarboxylic acid compositions

Info

Publication number: WO1993017993A1
Application number: PCT/US1993/002300
Authority: WO
Inventors: Kamlessh Kumar Bhatia; Neville Everton Drysdale; Kang Lin; Robert Stephen Nash; Thomas Walter Stambaugh
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1992-03-13
Filing date: 1993-03-15
Publication date: 1993-09-16
Also published as: TW227561B

Abstract

A process for oligomerizing a thin film of a hydroxycarboxylic acid composition, such as lactic acid or an oligomer thereof, to an oligomer having a higher degree of polymerization. The process affords rapid and substantially complete conversion to the oligomer in high production rates and yields.

Description

TITLE

THIN FILM OLIGOMERIZATION

OF HYDROXYCARBOXYLIC ACID COMPOSITIONS

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Serial Number 07/734,977, filed July 25, 1991.

BACKGROUND OPF THE INVENTION 1. Field of the Invention:

This invention relates to the preparation of oligomers of alpha-hydroxycarboxylic acids and lower molecular weight oligomers thereof.

In particular, it relates to a continuous process for the condensation polymerization of an alpha-hydroxycarboxylic acid composition such as lactic acid (or a low molecular weight oligomer thereof) to an oligomer having a desired degree of polymerization by heating the hydroxycarboxylic material under thin film evaporation conditions. Such thin film oligomerization process provides for rapid transfer of heat into the reaction mass and for rapid transfer of volatile by-products of condensation out of the reaction mass, so that conversion to the oligomer occurs rapidly, making for high production rates and high yields.

2. Description of the Related Art:

Oligomers of such alpha-hydroxycarboxylic acids as glycolic and lactic acid, which are essentially linear polyesters, are useful intermediates to the corresponding dimeric cyclic esters. The dimeric cyclic esters, notably glycolide (l,4-dioxane-2,5-dione) from glycolic acid, and lactide (l,4-dioxane-3,6dimethyl-2,5-dione) from lactic acid are themselves well-known intermediates to high molecular weight poly (hydroxycarboxylic acids) useful in biomedical and other applications because of their ability to be degraded biologically and hydrolytically to physiologically and environmentally acceptable by-products (see, Bellis, U.S. Pat 4,727,163; and Bhatia U.S. Pat. 4,835,293).

The processes of the art for the preparation of oligomers of alpha-hydroxycarboxylic acids are generally conducted batchwise and require hours of reaction time at high temperatures. As such, they are not entirely satisfactory for commercial operation. For example, Bhatia in U.S. Patent 5,023,349 describes (see

Example 1) a typical preparation of an oligomer of L-lactic acid suitable for conversion into L-lactide, which involves gradually heating 754.1 grams of

88% L-lactic acid in the presence of stannous octoate as catalyst for a total of 25 hours at 100° to 176°C while removing free water and the water of condensation in a stream of nitrogen.

Muller, U.S. Pat. 5,053,522, describes the preparation of oligomers of LΛactic acid having an average molecular weight of approximately 400 to 2000, corresponding to a degree of polymerization, n, of from about 5.3 to 215, preferably 500 to 800 (n = 6.7 to 10.9) by heating in the presence of tin or a tin dicarboxylate at rising temperatures to approximately 150° to 170°C under reduced pressure, preferably at about 0.03 bar up to approximately 170°C. Examples 2 and 3 show that from about 3 to 4 hours of heating are required to convert about 800 gms of 90% lactic acid to lactic acid oligomers having an average degree of polymerization, n, of about 10 to 11 (mol. wt. 738 to 810) and over 9 hours to achieve n = about 19 (mol. wt. 1386).

Also, Dahlmann et al., East German Patent 261,362 discloses a two-step process for the production of polymerizable l,4-dioxane-2,5-diones by oligomerization of alpha-hydroxycarbo ylic acids followed by thermolysis of the oligomers. The oligomerization step is exemplified (Example 1) with 300 gms. of 90% D,L-lactic acid being heated in the presence of N2 at 130°C for about 4 hours until no more distills off, followed by heating at 160°C under reduced pressure to complete the oligomerization.

SUMMARY OF THE INVENTION The present invention provides an improved process for oligomerizing an oligomerizable hydroxycarboxylic acid composition having a first degree of polymerization, (including zero) to an oligomeric composition having a higher degree of polymerization, comprising:

(a) distributing a hydroxycarboxylic composition, characterized as having an initial or first degree of polymerization lower than the degree of polymerization of an oligomeric composition to be produced, as a thin liquid film on a heated evaporation surface of a reaction zone, said surface being heated to an oligomerizing temperature;

(b) heating the thin film at said oligomerization temperature and at a pressure and for a time effective to condensation-polymerize the hydroxycarboxylic composition, thus forming a liquid oligomer composition having the higher degree of polymerization and one or more vaporized by-products of the condensation; (c) removing the vaporized by-products from the reaction zone; and (d) recovering the oligomeric composition having the higher degree of polymerization.

In one particular embodiment of the present invention the process further comprises continuously feeding liquid hydroxycarboxylic composition to the heated surface of the reaction zone, wherein the hydroxycarboxylic composition is continuously distributed as a thin liquid film on the heated evaporation surface, the hydroxycarboxylic composition is continuously oligomerized, the vaporized by-products are continuously removed from the reaction zone, and the product oligomer is continuously recovered from the reaction zone. In another specific embodiment of the process, the vaporized by-product or products of the oligomerization reaction are condensed. Any alpha hydroxycarboxylic acid material or low molecular oligomer thereof recovered in this way is further processed, for example, concentrated, as by distillation if necessary, to remove water or other volatile hydroxylic by-product, and recycled to the oligomerization step.

Preferably, the thin film oligomerization process is carried out in a wiped film evaporator as more fully discussed hereinafter.

More particularly and preferably, the process is directed to the preparation of oligomers of lactic acid, including of L-lactic acid, as intermediates to lactide, including I_rlactide.

This invention is based on the discovery that conversion of an oligomerizable hydroxycarboxylic acid composition, such as lactic acid, to an oligomer thereof suitable for conversion to the corresponding dimeric cyclic ester, e.g., lactide, proceeds more rapidly than heretofore believed when the hydroxycarboxylic material is spread as a thin liquid/molten film on a surface heated to oligomerization temperatures and the temperature and pressure are such that the more volatile by-products of the condensation reaction, e.g., H2O, are allowed to distill away as formed.

Thus the above process according to the present invention further comprises:

(a) providing a vertically disposed reactor having: (i) a substantially tubular reaction zone for converting the hydroxycarboxylic feed material to an oligomer, the reaction zone comprising a substantially tubular evaporation surface having an upper region and a lower region, (ϋ) a hydroxycarboxylic material feed means communicating with the upper region of the evaporator surface, (ϋi) a wiping means contained within the tubular reaction zone adapted to spread the hydroxycarboxylic feed material into a film over the evaporation surface, (iv) a means for heating the evaporation surface to an effective oligomerization temperature, (v) a means for controlling the oligomerization pressure within the reaction zone, (vi) a means for removing volatilized by-products from the reactor • zone, and

(vii) an oligomer recovery means communicating with the lower region of the evaporation surface; (b) maintaining the evaporation surface at said effective oligomerization temperature and pressure; (c) flowing liquid hydroxycarboxylic feed material from said feed means along the evaporation surface while wiping the feed material with said wiping means so as to form a thin film over the evaporation surface and result in the formation of oligomer and volatile by-products; (d) removing the volatile by-products from the evaporation zone; and, (e) recovering the oligomer product of step (c) from the lower region of the evaporation surface.

It is an object of this invention to provide a more efficient process for preparing low molecular polymers (oligomers) of oligomerizable hydroxycarboxylic acid compositions, in particular oligomers of lactic acid. Another object is to provide such a process as above which enables the reaction time required to achieve a particular degree of polymerization to be materially shortened. DETAILED DESCRIPTION OF THE INVENTION

The oligomerizable hydroxycarboxylic acid composition may be an alpha-hydroxycarboxylic acid, an ester of an alpha-hydroxycarboxylic acid or a heat-dissociable nitrogen base salt thereof. Accordingly, the invention includes converting an oligomerizable alpha-hydroxycarboxylic acid composition having the formula I, below, to an oligomer having the formula π, below:

n HOCR1R2CO2X —*- H(OCRιR2CO)_nOX +(n-l) HOX

I II

where: n is an integer of 2 to 30; X is independently H, R, or a cationic group HA; R_j, R2, and R, are independently H or a C^ to Cg hydrocarbyl radical; and A is a volatile nitrogen base. Preferably, R-,, R-2 and R,, when other than H in the above are H or methyl, as

R2= methyl). Preferably, the cationic group is' derived from a nitrogen base such as ammonia or alkyl amine, and preferably is ammonia or a tertiary amine such as trimethylamine, triethylamine, methyldiethylamine, tripropylamine, tributylamine or the like. Preferably, n is not greater than about 20, more preferably not greater than about 10. The value of n defines the average number of monomeric units in the oligomer, the value n-1 the average degree of polymerization. It should be further appreciated for purposes of this invention that low molecular weight oligomers having formula II may be used (either alone or incombination with compounds of formula I) as starting material and in which case the average value of n will increase as the condensation polymerization proceeds with formation of a mole of HOX for every new oligomer molecule produced, e.g.:

2H(OCR₁R₂CO)₂OH ~^ H(OCR₁R₂CO)₄OH + HOH

HOCR^O^H + H(OCR₁R₂CO)₂OH →» H(OCR₁R₂CO)₃OH + HOH

It is to be understood that when starting material I is a heat-dissociable amine salt, i.e., X=HA, then oligomer II would have the formula I-^OCR_jR_jCO) OH, i.e., X=H, and the by-products would correspond to EUO and the nitrogen base, A. Correspondingly, when the starting material I is an alkyl ester, X in oligomer II is an alkyl group, and the by-product, HOX, is the alcohol, ROH. Preferably, X is H throughout, for reasons of economy, and by-product HOX is water. Whatever the starting material, X is so constituted that condensation by-product, HOX, whether water, alcohol or a nitrogen base, is more volatile than the oligomer, and distills away from it during the course of the reaction.

In general, the process is conducted by introducing an oligomerizable hydroxycarboxylic acid material as a thin molten film into a reaction zone maintained at a pressure and temperature such that the hydroxycarboxylic material condenses to an oligomer thereof, as defined by "n" in above formula π, with formation of volatile by-products, removing the volatile by-products and recovering the oligomer from the reaction zone substantially free of the volatile by-products.

The process is advantageously carried out in a continuous manner using a thin film evaporator characterized in that the hydroxycarboxylic material is continuously fed to and disposed or distributed on a heated surface as a thin liquid film, preferably so as to have a relatively large surface area and a uniform thickness. Desirably, the film thickness is as low as can be practicably attained. Practically speaking, the film thickness is in the range of from about 0.05 to about 5mm, more usually from about 0.1 to 3mm, and preferably from about 0.3 to 1mm; such thicknesses are readily attainable in conventional thin film evaporators, exemplified by the falling film type and, in particular, by the wiped-film embodiment thereof (as more fully described and taught in copending and commonly assigned U.S. Patent application serial number 07/734.977, herein incorporated by reference for such purposes).

In such thin film evaporator process, the molten/liquified feed material flows downward along heated walls of the evaporator. The quality of the film that forms depends primarily on the force of gravity, the feed material's viscosity and its flow rate to the heating surface. Preferred are thin film evaporators of the wiped-film type which are equipped with an internal means for spreading the incoming feed material horizontally and vertically on the heating surface so as to create a thin film having a substantially uniform thickness and a relatively large surface area. The ratio of surface area to film thickness is not critical, but is desirably high since for a given thickness a greater ratio should increase the total heat transfer from the heating means to a greater quantity of the material of the film, enhance the mass transfer of the oligomerization by-products through and out of the film into the space above the film as vapor, and result in a greater quantity of oligomer product being formed in a given time period.

The thin film evaporator design is not critical. It may, for example, comprise a heated cylindrical or tapered tubular reactor which includes means for distributing the oligomer over the heated inner surface of the reactor. In one aspect of the invention, the tubular reactor includes a series of rotatable wiper elements that maintain a close clearance from the wall or ride upon a film of liquid on the wall. Typically, the rotating wiper elements can be operated at various speeds of rotation and can be adjusted to provide different spacings between the heated evaporator wall and the wiper blades. The wiper blade spacing determines film thickness, while wiper rotation rate and oligomer feed rate determine the rate of film formation.

Suitable wiper spacing (e.g., for appropriate film thickness) and speed of rotation are readily determined by trial for any particular hydroxycarboxylic feed composition, its viscosity and other process conditions (e.g., temperature, pressure, etc.). Typically, too, the thin film evaporator is equipped with means for feeding the hydroxycarboxylic feed component to the reaction zone of the evaporator (reactor), means for removing volatilized by-products, e.g., water and other volatile materials from the reaction zone, and means for recovering the resulting oligomer from the reaction zone. One preferred embodiment of the invention process comprises:

(a) providing a vertically disposed reactor having

(i) a substantially tubular reaction zone for converting the hydroxycarboxylic material to an ohgomer, the reaction zone comprising a substantially tubular evaporation surface having an upper region and a lower region; «

(ii) hydroxycarboxylic material feed means communicating with the upper region of the evaporator surface; (ϋi) a wiping means contained within the tubular reaction zone adapted to spread the hydroxycarboxylic feed material into a film over the evaporation surface;

(iv) a means for heating the evaporation surface to an effective polymerization temperature; (v) a means, e.g., a condensing means, for removing volatilized by-products from the evaporation surface and reaction zone; (vi) an oligomer recovery means communicating with the lower region of the evaporation surface; and, (vii) a means for controlling the pressure within the reaction zone;

(b) maintaining the evaporation surface at said effective oligomerizing temperature and pressure;

(c) flowing liquid hydroxycarboxylic feed material from said feed means along the evaporation surface while wiping the feed material with said wiping means so as to form a thin film over the evaporation surface, and result in the formation of oligomer and volatile by-products;

(d) removing the volatile by-products from the evaporation zone; and,

(e) recovering the oligomer product from step (c) from the lower region of the evaporation surface. The process is preferably conducted in a continuous manner by which it is meant to include pulsed or intermittent feed of the hydroxycarboxylic acid material to the evaporation surface.

Oligomer product having a particular average degree of polymerization, as defined by the value of "n" above and readily determined by end-group titration by standard methods known to the art, can be recycled if desired to the thin film evaporator to have its degree of polymerization further increased. Alternatively, the oligomer product can be forwarded to any of the processes known to the art for converting it to a dimeric cyclic ester, the oligomer may also be passed to a thin film evaporator adapted to convert oligomers of the alpha-hydroxycarboxylic acid compositions to dimeric cyclic esters as described in U.S. Patent application serial number 07/734,977.

The invention process is generally conducted in the presence of a catalyst, which may be included in the hydroxycarboxylic reactant before it is fed to the evaporator. The catalyst can be any catalyst which is suitable for promoting the oligomerization reaction. Suitable catalysts include metals or compounds of metals of groups IV, V and Vm of the Periodic Table. Preferred are metals of groups IV, notably Sn as the metal (powdered), oxide, halogenide or carboxylate, or V, notably Sb, usually as the oxide Sb2θ3. Most preferred are Sn(II) carboxylates, especially those that are soluble in the feed material and the oligomer aa exemplified by stannous bis(2-ethylhexanoate), commonly referred to as stannous octoate. The catalyst will be employed in catalytically-effective amounts, which can vary widely depending upon the particular feed material employed and the reaction conditions. The optimum catalytically-effective amounts for any particular system can readily be determined through trial runs. For example, with stannous octoate as the catalyst, the quantity of catalyst will generally be such that the reaction mass contains from about 0.01 to about 5% by weight, usually from about 0.3 to 3% and for best results, at least about 1%. Higher catalyst loadings are more desirable because residence time decreases with increases in the initial catalyst concentration, thereby improving the oligomer production rate.

In one aspect of the invention, it may be desirable to admix a solvent for both the feed material and the oligomer with the feed material introduced into the evaporator. A suitable solvent must be substantially inert under the polymerization process conditions and be generally equal to in volatility or less volatile than the oligomer to be produced. A solvent can improve the fluidity of the oligomer product and maintain the cleanliness of the heating wall within the wiped-film evaporator. The hydroxycarboxylic feed material is preferably preheated at or close to the operating temperature before it is fed to the reaction zone of the evaporator. The flow of the hydroxycarboxylic feed material to the heated wall of the wiped-film evaporator can vary widely depending in its composition, viscosity and temperature (viscosity decreasing with increasing temperature); also, on the oligomerization temperature and pressure.

The flow of feed material into the evaporator should be sufficient for the feed material to form a film under gravitational flow onto the surface of the heated wall, but it should not be so fast as to flood the evaporator. The flow rate should be coordinated with the oligomerization temperature so that a thin film is established and maintained throughout the run and oligomerization proceeds such that most, if not substantially all the feed material, is polymerized to the desired degree of polymerization. Further, the flow rate of the feed should also be coordinated with the rotation rate of the wiper blades so as to cause the feed material to form a uniform film along the heated surface of the evaporator. For example, a relatively high flow rate should be accompanied with a rapid wiper blade rotation rate to ensure effective polymerization to oligomer to the desired degree.

Suitably effective temperatures in the evaporator can vary widely. Normally, the temperature is in the range of from about 110° to 350°C The optimum temperature range for any particular hydroxycarboxylic feed composition-to-oligomer conversion will vary with the composition of the hydroxycarboxylic feed composition. For example, for the production of L- or D-lactic acid oligomer, the temperature is usually in the range from about 110° through about 350°C and, for glycolic acid oligomer from about 110° through 300°C.

The surface of the evaporator's heating zone can be heated by any expedient means. The heating zone is advantageously constructed of thermally conductive material so that heat can be supplied from an outside source through the wall of the heating zone to its internal evaporation surface and thus heat the hydroxycarboxylic film to polymerization temperatures. Heat may be supplied, for example, electrically by wrapping the outside of the heating zone with heating tape or by jacketing the zone with an electrically-heated mantle. Alternatively, the heating zone may be jacketed such that hot oil may be circulated through it and in this way bring the internal heating surface to the desired temperature. If desired, thermocouples can be placed at the external surface of the evaporator's heating zone to monitor and record the apparent oligomerization temperature. The vapor by-product stream produced on oligomerizing the feed material to volatile by-products in the evaporator, is preferably contacted with one or more condensing surfaces, each maintained at a temperature such that the by-product stream condenses as liquid, which is allowed to drain into a receiver. The condensing surfaces may be internal or external condensers or a combination of the two types. For example, the wiped film evaporator may include a heated surface which surrounds and is spaced apart from an internal condenser. The evaporator may also be connected to one or more external condensers which function as substitutes for, or as a supplement to, the internal condenser. The surface area and temperature of each condensing surface may be modified to control the manner in which the by-product stream is condensed.

The vapor by-product stream normally may comprise water, an alcohol, and/or an amine, i.e., HOX above, and/or other volatiles, including open-chain hydroxycarboxylic acids (e.g., lactic acid, lactoyllactic acid) and dimeric cyclic ester (e.g., lactide). The condensed vapor product can be readily separated into its constituents using conventional methods. For example, the normal boiling points of water, lactic acid and lactoyllactic acid (n=2 in formula II above) are 100°, 217° and 258°C, respectively. Also, at 180°C, for example, a typical oligomerization temperature, the vapor pressures of these constituents are, respectively, greater than atmospher for water, about 210 mm of Hg for lactic acid and about 1 mm of Hg for lactoyllactic acid. Thus, fractionation of a vapor by-product stream containing these constituents is attainable by selective condensation using a series of condensers, each cooled to a different temperature. Also, by controlling the oligomerization temperature of the evaporator surface and the pressure within the evaporator/reaction zone, so that at least the HOX by-products distill out of the oligomerizing mass, one can control both the average degree of polymerization (n-1 in formula II above) of the oligomer product and the molecular weight distribution within the oligomer for any degree of polymerization. The oligomerization process can be carried out under sub-atmospheric to atmospheric pressures consistent with the vapor pressures of the volatile by-products and the oligomers being recovered at the operating (polymerizing) temperature. The pressure can vary from about atmospheric down to as low as pratical to attain. Normally the pressure is between about 1 and 760 mm of Hg. Preferably, the pressure is in the range of about 5mm to 760mm of Hg.

An important aspect of the invention is that one or more of the following process conditions, such as the hydroxycarboxylic feed composition, feed viscosity, feed flow rate, speed of film formation, (i.e., rotation rate of the wiper blades), film thickness, temperature, pressure and catalyst loading, can be coordinated such that substantially all the hydroxycarboxylic material being fed is converted to an oligomer and a vapor by-product stream rapidly and substantially completely. Control of these process conditions makes feasible a short residence time continuous process affording high conversion of monomers and high yields of the desired oligomers with little or no occurrence of racemization, decomposition or other undesirable side reactions. Residence times can be extremely short, i.e., generally are a matter of minutes.

The following examples are provided to illustrate, not limit, the scope of the appended claims. Unless specified otherwise, commercially available materials were used and a UIC Industries (Joliet, Illinois, USA) Model KDL-4 wiped film evaporator was used in each of the following examples. The evaporator included a vertically arranged cylindrical housing which possessed about 0.043 square meters of evaporator surface. The evaporator was modified to include an external condenser downstream from its internal condenser. The evaporator was jacketed for external heating of its evaporator surface with circulating hot oil and was also equipped with (a) a jacketed feed funnel, also heatable with circulating hot oil, (b) a feed means for metering liquid hydroxycarboxylic material into the evaporator at controlled rates, the feed being provided at the top and allowed to flow down the interior cylindrical wall of the evaporator, (c) glass-reinforced "TEFLON"/polytetrafluoroethylene rollers serving as rotating wiper blades for mechanically spreading the oligomer as a thin film both horizontally and vertically over the heated wall. The wiper blades were rotated by a Janke-Kunkel model no. RW20 drive mechanism which had a setting of 2.5. The wiper blade spacing was set to provide a film thickness of about 0.5mm. The unit included means for evacuating and maintaining the evaporator under reduced pressure.

The average chain length, n, where given, was determined by titration with methanolic sodium methoxide and phenolphthalein as indicator.

Example 1 This example illustrates the oligomerization of lactic acid and lactic acid oligomer under thin film evaporation conditions. A lactic acid oligomer composition having an average chain length of 1.45, i.e., n= 1.45 in formula II above, and containing 2.3 weight % of stannous octoate was preheated to 105°C and fed to the top of the wiped film evaporator heated with 190°C oil circulating through its jacket and maintained at 760mm of Hg pressure. The oligomer was fed at a rate of 1022g/hr over a 20-minute period. During this time low-boilers (vaporized by-products) distilled out of the evaporation zone and product oligomer was removed from the lower end of the evaporator at a rate of 459 g/hrs. The product oligomer had an average chain length, n, equal to 2.2.

Example 2

The procedure of Example 1 was repeated except that the feed material was a lactic acid oligomer having an average chain length of 10. It contained 0.65 weight % of stannous octoate, was preheated to 150°C and was fed to the top of the evaporator at a rate of 172 g/hr. The evaporator was heated with 250°C oil circulated through its jacket and evacuated to and maintained at 0.3 to 0.8 mm of Hg. Low-boilers were collected at a rate of 52.8 g/hrs., the oligomer product at a rate of 97.5 g/hr.

The oligomer product had an average chain length of 19.

Example 3

The procedure of Example 1 was repeated except that (a) llOOg of 88% -Hactic acid containing stannous octoate as the catalyst was the feed material, (b) the lactic acid feed was preheated to 90°C and (c) its feed rate to wiped film evaporator was 39.7 g/min over a 7 minute period. During this time 5.1g/min of low-boilers and 34.6g/min of high-boilers were collected. The high-boilers were Hactic acid oligomers having an average of 1.49 lactoyl units, -OCH(CH3)CO-, in the molecule.

Having thus described and exemplified the invention with a certain degree of particularity, it should be appreciated that the following claims are not to be so limited but are to be afforded a scope commensurate with the wording of each element of the claim and equivalents thereof.

Claims

WHAT IS CLAIMED

1. A process for oligomerizing an oligomerizable hydroxycarboxylic acid composition comprising:

(b) heating the thin film at said oligomerization temperature and at a pressure and for a time effective to condensation-polymerize the hydroxycarboxylic composition, thus forming a liquid oligomer composition having the higher degree of polymerization and one or more vaporized by-products of the condensation;

(c) removing the vaporized by-products from the reaction zone; and (d) recovering the oligomeric composition having the higher degree of polymerization.

2. The process of Claim 1 further comprising contimiosly feeding liquid hydroxycarboxylic composition to the heated surface of the reaction zone, wherein the hydroxycarboxylic composition is continuously distributed as a thin liquid film on the heated evaporation surface, the hydroxycarboxylic composition is continuously oligomerized, the vaporized by-products are continuously removed from the reaction zone, and the product oligomer is continuously recovered from the reaction zone.

3. The process of Claim 2 wherein the feed rate of the hydroxycarboxylic composition to the reaction zone, the rate of distributing as a thin film of the composition on the evaporation surface, the thickness of the film, the temperature and the pressure are coordinated and controlled such that the oligomerization proceeds substantially completely to form the product oligomer substantially free of the by-products.

4. The process of Claim 1 wherein the heating of the thin film is effected in a thin film evaporator.

5. The process of Claim 4 wherein the evaporator is a wiped-film evaporator.

6. A process of Claims 1 wherein the oligomerizable hydroxycarboxylic acid composition is lactic acid or an oligomerizable lactic acid ester or nitrogen based salt thereof.

7. The process of Claim 6 wherein the hydroxycarboxylic composition is lactic acid and the product oligomer is an oligomer of lactic acid.

8. The process of Claim 1 wherein the evaporation surface of the heated reaction zone comprises the inner wall of a tubular reactor and the oligomer is spread as a thin film over the inner surface of the reactor.

9. The process of Claim 8 wherein the tubular reactor is a wiped film evaporator and the hydroxycarboxylic composition is wiped so that it forms a thin film having a substantially uniform thickness over the inner wall of the evaporator.

10. The process of Claim 1 further comprising:

(a) providing a vertically disposed reactor having: (i) a substantially tubular reaction zone for converting the hydroxycarboxylic feed material to an oligomer, the reaction zone comprising a substantially tubular evaporation surface having an upper region and a lower region,

(ii) a hydroxycarboxylic material feed means communicating with the upper region of the evaporator surface,

(ϋi) a wiping means contained within the tubular reaction zone adapted to spread the hydroxycarboxylic feed material into a film over the evaporation surface, (iv) a means for heating the evaporation surface to an effective oligomerization temperature,

(v) a means for controlling the oligomerization pressure within the reaction zone, (vi) a means for removing volatilized by-products from the reactor zone, and (vii) an oligomer recovery means communicating with the lower region of the evaporation surface;

(b) maintaining the evaporation surface at said effective oligomerization temperature and pressure;

(c) flowing Uquid hydroxycarboxylic feed material from said feed means along the evaporation surface while wiping the feed material with said wiping means so as to form a thin film over the evaporation surface and result in the formation of oligomer and volatile by-products;

(d) removing the volatile by-products from the evaporation zone; and,

(e) recovering the oligomer product of step (c) from the lower region of the evaporation surface.