Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2000-10-17
2002-03-12
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S619000
Reexamination Certificate
active
06355846
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for narrowing the dispersity and molecular weight distribution of poly (tetramethylene ether) glycol by stripping out lower weight fractions or oligomers at a low pressure and elevated temperature using an inert solvent
2. Description of Related Art
Poly (tetramethylene ether) glycol (PTMEG) is a commodity in the chemical industry, widely used to form segmented copolymers with polyfunctional urethanes and polyesters. It is typically made by the polymerization of tetrahydrofuran (THF) using a strong acid catalyst such as fluorosulfonic acid and then quenching the product with water. The resulting PTMEG polymer contains molecules of varying chain lengths, with a molecular weight distribution that is quite broad, non-Gaussian and skewed toward higher molecular weight fractions. If the chain length variation is too great, and in particular if the polymer contains too high a percentage of short chain molecules or oligomers, it can have a harmful effect on the properties of the end product.
Most commercial plants use fluorosulfonic acid as the catalyst. When using this catalyst, the polymer produced in the polymerization reaction is believed to be the sulfate ester, 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 twofold: (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.
PTMEG copolymers are typically made by a similar process in which anhydrous THF is copolymerized with alkylene oxides, for example ethylene oxide or propylene oxide. In certain copolymers, up to about 20% by weight of the THF can be replaced by 3-methyl tetrahydrofuran. Additionally, small amounts of other monomers, i.e., below about 10% by weight of the total monomers, can be present to modify product characteristics.
In a conventional fluorosulfonic acid-catalyzed THF polymerization system, the low molecular weight PTMEG fraction is washed out of the polymer. Generally, a substantial amount of aqueous acidic effluent results from the PTMEG washing. U.S. Pat. No. 4,115,408 provides a process for recovering the dissolved PTMEG fractions by converting it to THF by a high temperature depolymerization process.
U.S. Pat. No. 3,925,484 to Baker (assigned to E.I. du Pont de Nemours and Company, hereafter DuPont) discloses a process for producing PTMEG having a narrow molecular weight distribution of about 1.3 to 1.7 by partially depolymerizing the PTMEG at a temperature from about 120 to 150 degrees C. The lower molecular weight fractions in this case are converted most rapidly to tetrahydrofuran (THF) by the partial depolymerizing process and so removed from the polymer. Even though the process produces PTMEG with a narrow molecular weight distribution, the conversion of substantial amounts of polymer to a lower value THF restricts the use of this technique.
U.S. Pat. No. 4,933,503 to Mueller discloses a process for narrowing the molecular weight distribution of PTMEG and of copolymers of PTMEG and alkylene oxides by distilling off the low molecular weight fractions at pressures of less than 0.3 mbar at 200 to 260 degrees C, and then mixing the distillation residue with a specified three-component solvent system which forms three separate phases each containing a narrow weight distribution polymer.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for narrowing the molecular weight distribution and dispersity of poly(tetramethylene ether) glycol or copolymer comprising the steps of:
feeding poly(tetramethylene ether) glycol or copolymers thereof having a molecular weight ratio of 2.3 to 2.6 and a dispersity of 1.9 to 2.3 and an effective amount of an inert solvent to a stripping apparatus which has one or more separation stages wherein the stripping is conducted at a temperature of 150 to 220 degrees C and a pressure of 0.5 to 5 mm of mercury;
recovering a vaporized overhead mixture of inert solvent and a low molecular weight fraction of the poly(tetramethylene ether) glycol polymer or copolymer; and
recovering from the bottom of the stripping apparatus an unevaporated poly(tetramethylene ether) glycol polymer or copolymer having a reduced molecular weight ratio of 1.9 to 2.07 and a dispersity of 1.2 to 1.8.
REFERENCES:
patent: 3925484 (1975-12-01), Baker
patent: 4115408 (1978-09-01), Baker
patent: 4933503 (1990-06-01), Mueller
patent: 5282929 (1994-02-01), Dorai et al.
Dorai Suri Narayan
Ernst Richard Edward
Padmanabhan Sreeni
Witherspoon Sikarl A.
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