RELATED ART The present invention relates to a process for preparing a narrow molecular weight distribution (NMWD) poly(tetramethylene ether) glycol (PTMEG), and, more particularly, to an improved batchwise or continuous distillation removal of low molecular weight species in a short-path distillation system. Japanese Patent Application Tokukai 60-42421 (Mitsubishi) describes a process for preparing PTMEG of narrow molecular weight distribution in which a PTMEG having an average molecular weight of from 500 to 3000 is admixed with a mixed solvent comprising water and methanol. A much larger amount, i.e., greater than 1.2 times, of mixed solvent, having a water content in the range of 30-70% by weight based on total solvent, is used in proportion to PTMEG, in terms of weight ratio, to bring about separation of low molecular weight PTMEG. Separation is effected by the usual techniques for layer separation.

U.S. Patent 3,478,109 describes a method for removing the lower molecular weight fraction of PTMEG from a PTMEG starting material. The PTMEG starting material is dissolved in a cycloaliphatic (cyclohexane) and/or aromatic solvent, and in an aqueous methanol solution which is used as the extracting solvent for removal of the lower molecular weight fraction of PTMEG from the starting material. When the process is operated batchwise, the amount of cyclohexane present must be in excess, by weight, of the PTMEG starting material. Japanese Patent Application No. 215111 (1983), published as

Japanese Laid-Open No. 108424/1985 on June 13, 1985, describes a method for fractional precipitation of PTMEG having a sharp molecular weight distribution which comprises contacting a PTMEG starting material with water in the presence of methanol and/or ethanol. The amount of water needed is up to 1.7 times the amount of methanol, or up to 3.0 times the amount of ethanol, and the

precipitation of the desired PTMEG fraction can be controlled by the amount of water present.

U.S. Patent 4,762,951 (Mueller to BASF) describes a process for narrowing the molecular weight distribution of polytetrahydrofuran (PTHF = PTMEG) and of copolymers of tetrahydrofiiran and alkylene oxides in which the polymers are mixed with a solvent mixture consisting of an alkanol, a hydrocarbon, and water tailored to cause the mixture to separate into three phases upon standing, separating the three phases from one another, and isolating a polymer having a narrower molecular weight distribution from each of the two lower phases. The upper layer is said to contain essentially all of the oligomeric cyclic ethers. All the above-mentioned processes use a solvent mixture to fractionate PTMEG to produce narrow molecular weight distributed PTMEG. Commercial use of solvent extraction technology is associated with high investment cost and a large number of unit operations. U.S. Patent 3,925,484 (Baker to Du Pont) claims a process for producing PTMEG having a narrow molecular weight distribution of about 1.3 - 1.7 by partially depolymerizing the PTMEG at a temperature from about 120° - 150°C. The lower molecular weight fractions in this case are converted most rapidly to tetrahydrofiiran (THF) by the partial depolymerizing process. Even though the process produces narrow molecular weight distributed PTMEG, conversion of substantial amount of polymer to lower cost THF restricts the use of this technique.

U.S. Patent 4,585,592 (Mueller to BASF) describes a process for reducing the content of oligomeric cyclic ethers (OCE) in a polyoxybutylene/polyoxyalkylene glycol wherein a copolymer, obtained by copolymerization of tetrahydrofiiran with a 1,2-alkylene oxide in the presence of a compound containing reactive hydrogen under the catalytic action of a bleaching earth, is subjected to distillation at an elevated temperature and under reduced pressure, the improvement which comprises: treating the copolymer, prior to said distillation, with oxygen or an oxygen-containing gas at a temperature of from 20° to 110°C; and then carrying out the distillation under reduced pressure at a temperature above 200°C.

The product from a tetrahydrofuran/propylene oxide copolymer after removing residual monomers and treating with air at 80°C, was charged to a rotary evaporator and heated at 220°C and under 2 mbar pressure. Then distillate (7% by weight) consisted mainly (80%) of volatile oligomeric cyclic ethers), whose rings contained 1 to 3 oxypropyl or oxybutyl groups. Note that the PTMEG process

is run with precautions to exclude air. The reason for this is that terminal hydroxyl groups are lost by oxidation to carboxylic acid groups, and the customer would find these unacceptable for their polymer use. Column 3, lines 25-35 comments on the instability of commercial polytetramethylene ether glycols above 210°C. Therefore, we believe that the stability of PTMEG at temperatures as high as 350° to 400°C are not anticipated in the prior art.

Use of polymeric catalysts for polymerization of tetrahydrofuran are also reported. U.S. Patent 4,163,115 (Heinsohn, et al. to Du Pont) disclosed that the molecular weight of PTMEG, when using a catalyst which is a fluorinated resin containing sulfonic acid end groups, can be controlled by adding an acylium ion precursor to the reaction medium. Typically, the acylium ion precursors are anhydrides of carboxylic acids. Acetic anhydride is commonly used for this purpose. Several other solid non-hydrolyzable and recyclable catalysts are also reported. The reaction product is an ester of capped PTMEG which is conveniently reacted with an alkanol (methanol) to provide the final product PTMEG and yielding methyl acetate as a by-product. The molecular weight distribution parameters of PTMEG produced in this manner is similar to the unwashed PTMEG produced by using the fluorosulfonic acid (FSA) catalyzed polymerization process, namely: Dispersity = 2.0 - 2.1

MWR = 2.3 - 2.6 Even though the low molecular weight (LMW) fraction can easily be washed out of the polymer, the environmental and energy costs associated with a wash step are prohibitive. In addition, narrow molecular weight distribution (NMWD) PTMEG cannot be produced in this manner.

U.S. Patent 4,510,333 (Pruckmayr to Du Pont) claims a process for preparing poly(tetramethylene ether) glycols of narrow molecular weight distribution by bringing the tetrahydrofuran monomer and a cationic initiator together in a mole ratio such that the degree of polymerization will be at about a minimum, and at a temperature which will promote about a maximum number of tertiary oxonium ions, preferably 40° to 60°C, then quickly cooling the reaction mass to -25° to +25°C, and then adding enough tetrahydrofuran to complete the polymerization. When the desired molecular weight and molecular weight distribution have been reached, the polymerization is quenched and the poly(tetramethylene ether) glycol is isolated.

Poly(tetramethylene ether) glycols having molecular weight distributions (MWD) of 1.1 to 1.4 by this process are claimed.

SUMMARY OF THE INVENTION The present invention is an improved process for preparing PTMEG having a molecular weight between about 400 and about 4000, and having a dispersit between about 1.20 and 1.8. The process utilizes a starting material comprising PTMEG having an average molecular weight between about 400 and about 3000, and having a dispersity between about 1.6 and 2.3.

BRIEF DESCRIPTION OF THE DRAWING The Figure is a simplified process flow diagram which illustrates the major components of the process of the invention and their interconnection. DETAILED DESCRIPTION OF THE DRAWING The Figure depicts a two stage short path evaporation system. The feed PTMEG is fed at 11 to a first stage short path evaporator depicted generally at 12. The PTMEG feed drips onto product distributor plate 13 rotated by shaft 14 connected to gear motor 15. The PTMEG feed is flung by centrifugal force against the interior wall of heater jacket 16 and flows downwardly along the heater jacket 16. The interior wall of heater jacket 16 is continuously wiped with roller wipers depicted at 17 and 18. Generally three sets of rollers are used each set mounted on a shaft driven in an annular path by product distribution plate 13. The rollers serve to insure that a thin film of the desired thickness is maintained on the interior wall of heater jacket 16. Heater cycle fluid is feed at 20 to the interior of heater jacket 16 and removed at 21 to maintain the desired temperature on the interior wall of heater jacket 16. An exhaust vacuum is drawn at 22, by means not shown, to maintain the desired pressure inside the first stage of the short path evaporator 12.

Internal condenser 23 is fed with cooling water at 24 and the cooling water is discharged at 25 to maintain internal condenser 23 at the desired temperature. The distillate which condenses on internal condenser 23 is removed at 26. The PTMEG residue which flows down the interior wall of heater jacket 16 is collected and discharged at 27.

The residue discharge line 27 is connected to a second short path evaporator depicted generally at 30. The PTMEG residue drips onto product distributor place 31 rotated by shaft 32 connected to gear motor 33. The residue PTMEG is flung by centrifugal force against the interior wall of heater jacket 34 and flows downwardly along the heater jacket 34. The interior wall of heater jacket 34 is continuously wiped with roller wipers depicted at 35 and 36. Again three sets

of rollers are generally used with each shaft driven in an annular path by product distribution plate 31. Again the rollers serve to insure that a thin film of the desired thickness is maintained on the interior wall of heater jacket 34. Heater cycle fluid is fed at 37 to the interior of heater jacket 34 and removed at 38 to maintain the desired temperature on the interior wall of heater jacket 34. An exhaust vacuum is drawn at 40, by means not shown, to maintain the desired pressure inside the second stage of the short path evaporator 30. Internal condenser 41 is fed with cooling water at 42 and the cooling water is discharged at 43 to maintain internal condenser 41 at the desired temperature. The distillate which condenser on internal condenser 41 is removed at 44. The PTMEG residue (Product) which flows down the interior wall of heater jacket 34 is collected and discharged at 45.

The type of short path distillation apparatus depicted in the drawings enables the use of very low pressures which are essential to the success of the process of the present invention. In a single stage short path distillation apparatus evaporative pressures of 0.001 to 1.0 mbar were used. In the preferred two stage system using two short path distillation apparatus connected in series evaporative pressures in the final short path distillation apparatus is preferably in the range of 0.01 mbar which provides a narrow number average weight distribution corresponding to a dispersity in the range of 1.2 to 1.8 and preferably 1.25 to 1.40.

Generally the temperatures used in the short path distillation apparatus will be from 140° to 400°C with 170° to 350°C being the preferred range.

DETAILED DESCRIPTION OF THE INVENTION Certain terms, which are typical molecular weight distribution parameters and whose meanings are set out below, are used herein to describe the invention: Number Average Molecular Weight (Mn)

Mn - Sum of (Wi) for All i Values...(l) Sum of (Wi/Mi) where: Wi is the weight of the "i" the polymer species/oligomer, and

Mi is the molecular weight of the "i" the polymer species/oligomer. Mn is determined by end group analysis by titration. Weight Average Molecular Weight (Mw)

Mw = Sum of f fWiVMi) for All i Values Sum of (Wi) for All i Values

Mw is determined by gel permeation Chromatography or liquid chromatography. Dispersity /Polydispersity (Mw/Mn)

Dispersity or polydispersity, used herein interchangeably, is a universally accepted measure of molecular weight distribution, or MWD. The lower the value of dispersity, the narrower is the molecular weight distribution for the PTMEG sample under consideration. Molecular Weight Ratio f MWR

Molecular weight ratio (MWR) is another measure of broadness of molecular weight distribution and is related to the melt viscosity of the polymer as follows:

MWR = J 160}in}{0 _i 493)...(2) Mn where: n is melt viscosity in poise, and Mn is number average molecular weight as defined in equation (1), and is determined by end group analysis by titration.

PTMEG is made commercially by polymerizing anhydrous THF in the presence of strong acid catalysts. Most commercial plants use fluorosulfonic acid (FSA) as the catalyst. When using FSA as the catalyst, the polymer produced in the polymerization reactor is beheved to be the sulf ate ester of PTMEG which is hydrolyzed with water to obtain higher, more economic yields of the polyol product. Unreacted THF is removed from the resultant aqueous polymer dispersion by conventional steam stripping. The acidic aqueous dispersion of impure PTMEG is then subjected to washing with water. The purpose of the washing is two-fold; (1) to remove the sulfuric acid and hydrofluoric acid from the polymeric dispersion and (2) to remove the low molecular weight PTMEG fraction from the polymer by taking advantage of the high solubility of the low molecular weight species in water. Typically, the molecular weight distribution parameters of the polymer produced in the polymerization step are: Dispersity = 2.0 - 2.1

MWR = 2.3 - 2.6 The molecular weight distribution of commercially salable regular PTMEG, however, are narrower than the polymer produced in the reactor: Dispersity = 1.5 - 1.8 MWR = 1.95 - 2.05

The molecular weight distribution parameters of narrow molecular weight distributed (NMWD) PTMEG, which is desired for certain specific applications, are extremely stringent:

Dispersity = 1.25 - 1.40 MWR = 1.5 - 1.7

In a conventional FSA catalyzed THF polymerization system, the low molecular weight (LMW) PTMEG fraction is washed out of the polymer. Generally a substantial amount of aqueous acidic effluent results from the PTMEG washing. U.S. Patent 4,115,408 (Baker to Du Pont) provides a process for recovering the dissolved LMW PTMEG by converting it to tetrahydrofuran by a high temperature depolymerization process.

STEPS OF THE PROCESS The process for narrowing the dispersity begins with a PTMEG product of broad molecular weight distribution resulting from the typical commercial manufacturing process described above using fluorosulfonic acid as polymerization catalyst. The process can also be used to narrow the MWD of PTMEG from any other similar process including blends of different molecular weight PTMEG lots.

This process for narrowing the molecular weight of PTMEG was first carried out in a laboratory single-stage glass short-path distillation unit similar to one of the stages as shown in the drawing. The unit "dosing chamber' was the feed reservoir for the PTMEG feed material. The desired temperature for distillation was maintained by the hot oil system at either 150° or 210°C, and the entire distillation path was maintained by the vacuum pump at 1.0 to 0.1 mbar. The PTMEG was distributed uniformly across a heated vertical surface into a thin film in the distillation column by means of a mechanical arm fitted with rollers or wipers turning at about 100 rmp. The evaporated low boiling fractions were condensed on the surface of an internal condenser and the condensate collected in the low boiler flask. The higher molecular weight PTMEG fractions from the heated film were collected in the high boiler flask. The polymer hold-up time in this unit was ten minutes. The PTMEG feed rate was approximately 1 mL/minute. The results of these tests are reported in Table 1.

Terathane (R) 1,000 is a commercial PTMEG having a nominal number average molecular weight of 1,000 and a Mw/Mn of 1.75. Terathane (R) 2,000 is a commercial PTMEG having a nominal number average molecular weight

of 2,000 and a Mw/Mn of 1,85. PTMEG 250 is a commercially available PTMEG having a nominal number average molecular weight of 250 and a low Mw/Mn.

Table 1 Short-Path Distillation of PTMEG EXAMPLE 1

Distillation of TERATHANE (R) 1,000

Calculated Pressure Hot Oil Mw/Mn Mn mbar °C

1.74 0.1 150

1.70 0.1 150

6.5% of PTMEG 250 blended with TERATHANE (R) 1,000

PTMEG Feed 863 1.91 830 Distillate 268 0.3 150

Residue 974 1.77 0.3 150

EXAMPLE 3

1.56% of PTMEG 250 blended with TERATHANE (R) 2,000

PTMEG Feed 1802 2.075 Distillate 261 0.3 150

Residue 2099 1.82 0.3 150

EXAMPLE 4

Distillation of TERATHANE (R) 1,000

PTMEG Feed 1000 1.75 Distillate 361 0.1 210

Residue 1165 1.66 0.1 210

The second series of short-path distillation of PTMEG samples was carried out using the apparatus shown in the Drawing, a pilot unit designed to produce higher feed rates. An excellent description of "Molecular and Short-path Distillation" by Klaus J. Erdweg appeared in Chemistry and Industry (London), 2

May 1983, pages 342-345. In these units, PTMEG feed rate is usually controlled at

8-10 Kg/hour. A sketch of the pilot unit used is displayed in the Drawing. The system used was a two-stage stainless steel (316) unit and the evaporator surface was 0.1 square meter/evaporator. The PTMEG feed was pumped into the first short-path evaporator where it was distributed into a thin film by means of a highly efficient self-cleaning roller wiper system available from UIC Inc., Joliet, Illinois.

The roller wiper system consisted of a wiper basket with an upper holding plate and stabilization rings, which were interconnected by the holders of the guide rods for the pory(tetrafhιoroethylene) (PTFE) rollers. The PTMEG feed was flung by centrifugal force from the upper rotating distribution plate against the upper part of the evaporator surface. Follower rollers, made of glass-fiber-reinforced PTFE immediately spread the product to a film of uniform thickness. Rollers, supported on the guide rods with an extremely large clearance, were also pressed into the product film by centrifugal force. No product residue remained inside the rollers. Even small amounts of material were readily pressed out of the roller's interior by centrifugal force. Hence there were no product residues to cause thermal decomposition. The evaporated low molecular weight PTMEG condensed on the surface of an internal condenser without any noticeable decomposition and was collected in a condensate flask. An external cooling trap condensed any residual vapor that may have been present. Finally, any non-condensable constituents were aspirated by the three vacuum pumps - a vane pump, a Roots blower and a diffusion pump. Any one or all three pumps were used depending upon the vacuum desired. The high molecular weight species next flowed via a pressure barrier into the second short-path evaporator, and was there separated into distillate II, which was collected in a distillate receiver and residue. Results are reported in Table 2.

Table 2 Pilot Unit Distillation of PTMEG Samples Feed PTMEG consisted of a blend of 6.5% (Mn = 250) with 93.5% (Mn = 982, MWR = 2.06) to five PTMEG (Mn = 818, MWR = 2.40, viscosity = 2.90 poise @ 40°C).

EXAMPLES 5, 6, 7 and 8 used a single stage KD-10 unit from UIC, Joliet, Illinois. The vacuum pump set for these examples consisted of a D16A vane pump on each stage. U-0094

Evap.2 Temp.Out Condenser 1 Condenser2 Residue Temp. Trap 1 Temp. Trap 2 Temp. Pressure 1 Pressure 2 Distillate 1 Distillate 2 Residue Total Mass Cut Time Feed Time

PTMEG FEED Mn PTMEG FEED MW Distillate Mn Distillate MW Residue: Mn MWR Dispersity Water, ppm OCE, ρpm

The feed and residues for this series of short path distillations were found to have the following linear ohgomeric (2, 3, 4, 5) and cyclic ohgomeric (C3, C4, and C5) ether contents by gas chromatography (GC). The oligomer content of commercial PTMEG 'TERATHANE" Mn 1000 is given for comparison. These data are reported as area percent relative to an internal standard here and for the other EXAMPLES.

Oligomer PTMEG Mn 1000

2 0.20

C3 0.03 3 1.72

C4 0.39

The feed material for EXAMPLES 9, 10 and 11 was the same as for EXAMPLES 5-8 above.

The data for these Examples were obtained on a single stage KD-10 unit.

The vacuum pump set for EXAMPLES 9, 10 AND 11 consisted of a WA-150 Roots Pump/D16A.

The PTMEG feed for EXAMPLES 16 through 21 was a blend of 1.5% PTMEG 250 in 98.5% PTMEG 1800 to give PTMEG of Mn 1605 with a MWR of 2.19.

The vacuum pump set for EXAMPLES 16 through 18 consisted of a D16A vane pump on each stage.

The vacuum pump set for EXAMPLES 19 through 21 consisted of WA-150 Roots Pump/D16A on the single stage KD-10.

EXAMPLE 16 EXAMPLE 17 EXAMPLE 18

Residue Mn 1806 1804 1787

Dispersity 1.83 1.77 1.83 MWR 1.92 2.00 2.01

Water, ppm <20 EXAMPLE 19 EXAMPLE 20 EXAMPLE 21

Feed Heat 49°C 49°C 49°C Feed Temp. 38°C 38°C 38°C Evap. 1 In — not applicable Evap 1 Out — not applicable Siphon Lock — not applicable Evap. 2 In 205°C 215°C 225°C Evap. 2 Out 203°C 214°C 223°C Condenser 1 — not applicable Condenser 2 41°C 40°C 40°C Residue 71°C 76°C 80°C Trap l — not applicable Trap 2 -39°C -40°C -39°C Pressure 1 — not applicable Pressure 2 0.15 mbar 0.09 0.09 Distillate 1 — not applicable Distillate 2 2.06% Residue 97.94% Total Mass 912.00 g Cut Time 11 min. Feed Rate 5.0 kg/hr PTMEG FEED Mn PTMEG FEED MWR Distillate: Mn

Dispersity Residue: Mn MWR

EXAMPLE 19 EXAMPLE 20 EXAMPLE 21 Dispersity 1.76 1.76 Water, ppm

The content of low molecular weight linear and cyclic oligomers in the feed and the residues of EXAMPLE 20 and EXAMPLE 21 foUow.

The following Short Path Distillation runs were carried out in a KD- 10 unit which had an evaporator of 0.1 square meter. The feed rate was 10 lbs./hour, and the hold-up time in the short path distillation unit was 35 seconds.

EXAMPLE 22 EXAMPLE 23 EXAMPLE 24 EXAMPLE 25 Distilled 18.6 24.2 30.3 35.0

Fluoride 315 ppm Residue Anal.

Mn by GPC 1374. 1435. 1525 1627

Dispersity 1.32 1.28 1.25 1.20 Mw by end- group titration 1365. 1450. 1580. 1681.

Viscosity 4.01 4.25 4.53 4.80

MWR 1.69 1.63 1.55 1.50

Ash <5. <5. <5. <5.

Calcium 0 0 0 0

Iron 0 0 0 0

Fluoride 95 ppm

Carbonyl Ratio 1.77 3.16 7.3 11.8

Stabilizer 0.022% 0.033% 0.002% 0.001%

EXAMPLES 26 - 28

STARTING MATERIAL

Mn 1314.

Viscosity 6.53

MWR 2.23

Dispersity 1.92

Ash, ppm 0

Calcium, ppm 0

Iron, ppm 0

Carbonyl Ratio 1.8

Stabilizer, % 0.04

Fluoride, ppm 128.

Feed Rate, lb/hr 10.

Hold-up Time, sec 35.

The merits offered by commercial applications of the short-path distillation emphasize the flexibility to product different grades of polytetramethylene ether) glycols tailored or optimized for specific polymer end- uses. This cannot be done by existing continuous plants because of a hold-up time of 20-24 hours in the process equipment. For example the change from producing polytetramethylene ether glycol having Mn 1,000 to polytetramethylene ether glycol having Mn 1,800 in a commercial plant under steady state conditions results in a year's time several hundred thousand pounds of transition material with a molecular weight of 1200-1300 and extremely high polydispersity. The transition products cannot usually be blended with regular grades because of the adverse affect on performance in critical applications. Proper disposal of the unusable transition material is costly regardless of how the disposal is carried out. Short- path distillation equipment, however, allows fractionation of the transition material into useful or blendable material. For example, Mn 1200 to Mn 1300 transition material by use of short-path distillation can be converted into a Mn 1000 grade distillate suitable for

use and Mn 1600 residue that can be blended satisfactorily with Mn 1800 residue.

Or the same Mn 1200-1300 transition material can be converted to a Mn 650 grade distillate for sale and a Mn 1700 residue for blending with a Mn 1800. Or the Mn

1600 residue from the first example by short-path distillation can yield Mn 250 as a distillate and excellent Mn 1800 residue. Another variation is to make Mn 800 material in the reactor and use short-path distillation to give a Mn 250 distillate and a residue of Mn 1000 grade.

PTMEG is very hygroscopic. Water in PTMEG is a source of constant concern to polyurethane manufacturers since water reacts with isocyanates in competition with the polyols thus altering the molecular weight and stoichiometry of the polyurethane. The water content of short-path distillation of

PTMEG has result ed in water contents of <20 ppm (anhydrous) consistently. Commercial crude PTMEG is washed with water to remove low molecular weight linear ohgomeric glycols. At the same time some very low molecular weight ohgomeric cychc ethers may be partially removed. This water wash step is taken also to lower the polydispersity of the product. The water wash is capital-intensive since it results in an aqueous stream that requires distilling water off the low molecular weight fraction before discharge.