PROCESS FOR REMOVAL OF LOW MOLECULAR WEIGHT FRACTIONS FROM VINYL PYRIDINE POLYMERS AND COPOLYMERS

Abstract: Disclosed is a process for separating high molecular weight polymers of alkenyl pyridine compounds having a pKa of about 2-10 from mixtures thereof with monomers and low molecular weight fractions comprising dispersing the mixture in a solution of an acid having a pka of between about -10 and about 6 at a temperature of between about 50°C and about 70°C, to thereby dissolve the monomers and low molecular weight fractions while maintaining the high molecular weight polymers as a solid, and subsequently separating the solid high molecular weight polymer from the solution of monomers and low molecular weight fractions in aqueous acid.

Description: Description Process for Removal of Low Molecular Weight Fractions From Vinyl Pyridine Polymers and Copolymers Technical Field This invention relates to a process for separating high molecular weight polymers of alkenyl pyridine compounds from mixtures thereof with monomers and oligomers. Background of the Invention For many end-use applications, it is desirable to process high molecular weight polymers having low residual monomer concentrations and low levels of oligomers. The conventional methods used for removal of unpolymerized monomer from polymer dispersions, latexes, or suspensions generally depend on steam distillation, degasification, or vacuum stripping processes. Examples of these conventional methods are described in literature references such as: (a) Ger. Offen. 2, 635, 478; (b) Fr. Demande 2, 302, 306; (c) B.P. 1468490; and (d) Chem. Eng. Prog., Aug., 1981, pp 55-59. Although these methods are generally effective for removal of volatile monomers, they are generally ineffective for removal of the nonvolatile low molecular weight components from the polymer. The present invention is concerned with a method for the concurrent removal of residual monomer and low molecular weight components from vinyl pyridine polymers and copolymers. The increased desirability and use of polymers for pharmaceutical, biomedical, and agricultural applications has resulted in the need for efficient and satisfactory methods for removing low molecular weight fractions and unpolymerized monomers from the polymer mass. Polymers that have been adequately separated from residual monomers and low molecular weight components are expected to provide greater product uniformity, more consistent physical and chemical properties, and improved storage and shelf-life capability. This invention demonstrates a novel process for the simultaneous removal of residual monomers and low molecular weight components from a vinyl pyridine polymer or copolymer by continuous extraction using dilute aqueous acids. Disclosure of Invent ion According to the present invention, there is provided a process for separating high molecular weight polymers and copolymers of alkenyl pyridine compounds from mixtures thereof with monomers and low molecular weight fractions of the compounds comprising dispersing the mixture in a solution of an acid at a temperature of between about 500C. and about 700C., to thereby dissolve the monomers and low molecular weight fractions while maintaining the high molecular weight polymers as a solid. Subsequently, the solid high molecular weight polymer is separated from the solution of monomers and low molecular weight fractions in aqueous acid. The alkenyl pyridine compounds which may be used in accordance with this invention have a pKa of about 2-10 and have the general formula EMI2.1 wherein R1 is hydrogen or alkyl having 1-4 carbon atoms, and R2 is hydrogen, alkyl having 1-4 carbon atoms, cycloalkyl having up to 8 carbon atoms, phenyl or alkyl substituted phenyl having up to 12 carbon atoms, or a halogen. The polymer and copolymers of alkenyl pyridine compounds contain at least 50 weight % of polymer units derived from at least one monomer having the structure shown above. The polymer, however, may contain up to about 50 weight percent of comonomer derived from other copolymerizable monomers such as, for example, styrene, vinyl acetate or methyl methacrylate. Polymers made by emulsion polymerization techniques normally contain small amounts of monomer and low molecular weight polymers. Normally, these monomers and low molecular weight polymers account for less than about 3% of the total weight of polymers and monomers. In practicing the process, a mixture of the monomer and polymer (high and low molecular weight) is first dispersed in an aqueous solution of an acid at a temperature between about 500C. and 700C., preferably about 55-65"C. The concentration is adjusted such that the solids content (monomer and polymer) is between about 10% and about 50% by weight. The monomer and low molecular weight polymer dissolves in this acid solution, leaving the higher molecular weight polymers as the only solid. This solid polymer is easily separated from the solution by conventional techniques well known to those skilled in the art, such as for example, by filtering. The acids useful for the purpose of this invention include both organic acids and inorganic acids. Acids suitable are those with pKa values between -10 and 6. The preferred acids have pKa values between -3 and about 5. Suitable acids include the following: Acid pKa at 250C. Hydrobromic acid -9 Sulfuric acid -3 Nitric acid -1.64 Pyrophosphoric acid 1.0 Ortho-phosphorous acid 1.8 Ortho-phosphoric acid 2.1 Chloroacetic acid 2.87 Formic acid 3.75 Acetic acid 4.75 Butyric acid 4.82 Carbonic acid 6.37 Boric acid 9.14 The pKa is most frequently used to express the extent of dissociation, or the strength of acids, and bases. pKa is defined as: pKa = -log10 Ka where Ka is the thermodynamic equilibrium acidity constant. Clearly, the stronger the acid, the smaller the value of pKa, conversely, for bases, the larger the pKa, the stronger is the base. Unless otherwise specified, pKa as used herein is determined at 250C. Typically, high molecular weight copolymers of pyridinium vinyl monomers and selected vinyl comonomers are prepared either by batch or continuous emulsion polymerization at 650C. using a water soluble free radical initiator such as potassium persulfate. The polymer is coagulated by addition of saturated sodium chloride solution, heated to 700C., and collected by filtration. The polymer is redispersed in a fresh portion of water, heated to 700C., and again filtered. This process is repeated two additional times. The polymer filter cake is then transferred to an extraction vessel and extracted with 1% aqueous acid while maintaining the extraction temperature at 50-600C. The polymer mass may be maintained in a fixed-bed and extracted or, more desirably, be dispersed as a fluid-bed and continuously extracted. Analysis of the polymer before and after extraction shows a remarkable reduction in the concentration of residual, unpolymerized monomer, and also the low molecular weight fraction of the polymer. Recovery of purified, high molecular weight polymer from the extraction equipment at the conclusion of the purification step is about 97%. The following examples are submitted for a better understanding of the invention. EXAMPLE 1 This example demonstrates the effect of extraction time on the removal of residual vinyl monomer and low molecular weight fractions from a high molecular weight copolymer of 5-vinyl-2-picoline. The extraction is performed using a fixed-bed continuous extractor charged with 1% aqueous acetic acid. The extraction temperature is maintained at 60-650C. Residual Monomers Low Mol. Wt. Styrene, 5-Vinyl-2-picoline, Extraction Time (hr.) Fraction,* ppmÚ ppbê ppbê 0 (starting material) 2,700 503 1,030 1.5 690 96 197 3 836 92 202 6 641 99 122 12 759 87 213 24 554 78 102 *Having mol. wt. #2,000 g/mol 1 - Parts per million 2 - Parts per billion EXAMPLE 2 This example illustrates the important effect of extraction temperature on the efficient removal of residual, unpolymerized monomers, and low molecular weight products from a high molecular weight copolymer of 5-vinyl-2-picoline. The continuous extraction is performed using a fixed-fed extractor for 24 hrs. using 1% aqueous acetic acid at the selected temperatures shown below. Residual Monomers Extraction Low Mol. Wt. Styrene, 5-Vinyl-2-picoline, Temperature Fraction,* ppm ppb ppb - (starting material) 4,000 > 50,000 > 200,000 25 C. 1,500 < 100 379 50 C. 690 < 100 370 50 -60 C. 443 < 100 338 70 -80 C. 438 133 476 *Mol. wt. #2,000 g/mole EXAMPLE 3 This example shows the effect of extractant volume on the removal of residual monomer and low molecular weight components from a high molecular weight polymer of 5-vinyl-2-picoline. One-hundred grams of the polymer is placed in a fluid-bed extraction vessel. The polymer is extracted with 1.258 aqueous acetic acid at 700-800C. by continuous counter-current washing. The following data show the rapid removal of residual monomer and low molecular weight components from the polymer as a function of extraction volume. Residual Monomers Extraction Low Mol. Wt. 5-Vinyl-2-picoline, Volume (mL) Fraction,* ppm ppb 0 (starting material) 1,200 4,000 1500 427 96 3000 424 56 4500 375 63 6000 185 < 50 7200 57 73 *Mol. wt. #2,000 g/mole EXAMPLE 4 The extraction process described in Example 3 is repeated using another polymer sample having different starting levels of low molecular weight components and residual monomer. The following data show the effect of extract ant volume on removal of residual monomer and low molecular weight fraction from the polymer. Residual Monomers Extraction Low Mol. Wt. 5-Vinyl-2-picoline, Volume (mL) Fraction,* ppm ppb 0 (starting material) 1,000 2,790 1500 386 160 3000 620 105 4500 618 66 6000 69 < 83 7200 186 80 *Mol. wt. #2,000 g/mole EXAMPLE 5 This example demonstrates the utility of using dilute aqueous sulfuric acid as an extractant for removal of residual monomer and low molecular weight fraction from a copolymer of 5-vinyl-2-picoline (5V2P) and styrene (ST). One-hundred grams of the copolymer is extracted using the fluid-bed continuous extractor. The extractant is 0.01% sulfuric acid at 700-800C. The extractant feed pump rate is maintained at 21 mL/min. Residual Monomers Extraction Low Mol. Wt. 5-Vinyl-2-picoline, Volume (mL) Fraction,* ppm ppb ppb 0 (starting material) 1,700 319 10,900 1500 1,300 54 1,000 3000 372 50 332 4500 523 < 50 303 7200 280 < 50 89 *Mol. wt. #2,000 g/mole EXAMPLE 6 This example compares the effect of extraction of the vinyl pyridine copolymer using 0.01% sulfuric acid and also with an equal volume of distilled water. The polymer is extracted using the same equipment and procedure described in Example Nos. 3 and 5. The following results show that extraction of the polymer with distilled water is useful for reducing the residual monomer level, but is not effective for removing the low mol. wt. fractions from the polymer. Residual Monomers Extraction Low Mol. Wt. 5-Vinyl-2-picoline, Volume (mL) Fraction,* ppm ppb ppb Starting Material 1,700 319 10,900 7200 mL Distilled Water 1,300 < 50 413 7200 mL 0.01% H2SO4 280 < 50 89 *Mol wt. #2,000 g/mole Unless otherwise specified, all parts, percentages, ratios, etc., are by weight, and molecular weight is on a weight average basis. The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims: CLAIMS We Claim: 1. Process for separating high molecular weight polymers of alkenyl pyridine compounds having a pKa of about 2-10 and of the formula EMI18.1 wherein R1 is hydrogen or alkyl having 1-4 carbon atoms, and R2 is hydrogen, alkyl having 1-4 carbon atoms. cycloalkyl having up to 8 carbon atoms, phenyl or alkyl sub sty'muted phenyl having up to 12 carbon atoms, or halogen, from mixtures thereof with monomers and low molecular weight fractions of said compounds characterized in that said mixture is dispersed in a solution of an acid having a pKa of between about-l0 and about 6 at a concentration of between about 10% and about 50% solids and at a temperature of between about 500C and about 700C, to thereby dissolve said monomers and low molecular weight fractions while maintaining said high molecular weight polymers as a solid, and subsequently said solid high molecular weight polymer is separated from the solution of monomers and low molecular weight fractions in aqueous acid. 2. Process according to Claim 1 wherein said compounds are dispersed in said solution of an acid at a temperature of about 55-650C. 3. Process according to Claim 1 wherein said acids have a pKa of about -3 and 5. 4. Process according to Claim 1 wherein said alkenyl pyridine compound is a polymer or copolymer of vinyl pyridine. 5. Process for separating high molecular weight polymers or copolymers of vinyl pyridine from mixtures thereof with monomers and low molecular weight fractions thereof characterized in that said mixture is dispersed in a solution of an acid having a pKa of between about -3 and about 5 at a concentration of between about 108 and about 50% solids and at a temperature of between about 550C and about 650C, to thereby dissolve said monomers and low molecular weight fractions while maintaining said high molecular weight polymers as a solid, and subsequently said solid high molecular weight polymer is separated from the solution of monomers and low molecular weight fractions in aqueous acid.

Inventors: DOMBROSKI, JOHN RICHARD, HAWKINS, JAMES MAYNARD
Applicants: EASTMAN KODAK COMPANY
Application year: 1984
Main classification: C08F6
IPC codes: C08F6