GB2244713A - Copolymer beads of a monovinyl aromatic monomer and monomer containing two active vinyl groups and process for its preparation - Google Patents

Abstract

In a process of preparing hard, crosslinked, discrete copolymer beads by free radical polymerization in an aqueous dispersion of a monomer mixture comprising a major proportion of a monovinyl aromatic monomer, and a minor proportion of a crosslinking monomer having at least two active vinyl groups, an improvement comprises conducting the polymerization reaction in the presence of a mercaptan.

Description

COPOLYMER BEADS OF A MONOVINYL AROMATIC MONOMER AND MONOMER CONTAINING TWO ACTIVE VINYL GROUPS AND PROCESS FOR ITS PREPARATION This invention relates to an improved process for the preparation of crosslinked vinyl copolymers as discrete copolymer beads in aqueous dispersions using certain reaction modifiers in the polymerization process. The invention also concerns the ion exchange resins having improved physical characteristics.

The techniques of preparing crosslinked vinyl copolymers in bead form (as precursors for conversion into ion exchange resins) by free-radical catalyzed polymerization of the monomer mixture in aqueous dispersion are well known. See Howell US Patent 4,375,519 and Howell US Patent 4,246,386.

The conventional conditions of polymerization used heretofore lead to crosslinked vinyl copolymers, which, when converted to ion exchange resins by attachment of functional groups thereto, have certain operational deficiencies resulting from physical weaknesses.

Prior to Howell US 4,246,386, it had generally been the practice to exclude reaction modifiers in the preparation of crosslinked vinyl polymers used as the base matrix copolymer for ion exchange resins because they have been regarded as detrimental to the properties of these copolymers. US Patent 2,241,770 of Driesbach et al teaches that styrene is stabilized for storage by treatment with a representative modifier, phenylacetylene, with the admonition to remove the phenylacetylene from the monomer prior to its polymerization. Polymerization of styrene in conjunction with phenyl-acetylene at temperatures between 100"C and 225"C to produce a linear, uncrosslinked polymer is disclosed in US Patent 2,290,547 of Driesbach et al.

There was no suggestion in the art that crosslinked copolymers useful for conversion into greatly improved ion exchange resins could be derived from polymerization systems incorporating modifiers such as phenylacetylene. British Patent 1,261,427 teaches that the gel effect in the polymerization thereof with styrene or vinyl acetate can be reduced or eliminated by the use of cyclic compounds such as 1,4-cyclohexadiene and terpinolene. Again, there was no suggestion in this art that such modifiers are useful in preparing improved crosslinked copolymers. US Patent No 3,976,629 discloses the use of various aliphatic compounds containing at least two polymerizable bonds, such as isoprene and cyclopentadiene, as crosslinkers in aliphatic monomer mixtures.Combinations of aromatic and polyunsaturated aliphatic crosslinkers including compounds used herein as rate modifiers, have also been generally known in the prior ion exchange art (see, (e.g.) US Patent 3,674,728 of Carbonnel et al). These polyunsaturated aliphatic materials have been employed previously at much higher levels than herein for different purposes and under different polymerization conditions.

Howell US Patent 4,246,386 discloses the use of a modifier in the preparation of crosslinked vinyl copolymers, the modifier being an organic compound containing acetylenic or allylic unsaturation. It is disclosed that the modifier must be capable of moderating the rate of polymerization. See also US Patent 4,375,519. It is disclosed that the use of the modifiers yield ion exchange resins in which the polymer beads have greater mechanical strength and increased resistance to swelling pressures which are produced within a bead during acid/base cycling (i.e., osmotic shock). The greater mechanical strength of the beads manifests itself in improved resistance to physical breakdown from external forces such as weight of the resin column bed, high fluid flows and back-washing.

Thus, the physically stronger ion exchange resins embodied therein are especially useful in treating fluid streams of high flow rates, for example, condensate polishing applications in which resins of lesser quality are prone to mechanical breakdown and short life spans.

In the preparation of copolymer beads using the described process a significant number of the beads, obtained after activation to sulfonic resin are of poor quality, i.e. have poor optical appearance, diminution of perfect beads. Imperfect beads can create problems in use.

What is needed is a process for preparing copolymer beads with a monovinyl aromatic monomer and a monomer containing at least two active vinyl groups which increase the number of perfect beads of the sulfonic resin obtained after activation of the copolymer.

The invention is a process of preparing hard, crosslinked, discrete copolymer beads by free radical polymerization in an aqueous dispersion of a monomer mixture comprising a major proportion of a monovinyl aromatic monomer, and a minor proportion of a crosslinking monomer having at least two active vinyl groups, the improvement which comprises conducting the polymerization reaction in the presence of a mercaptan.

In another aspect the invention is a hard crosslinked, discrete copolymer bead comprising the reaction product of a major portion of a monovinyl aromatic monomer and a minor portion of a crosslinking monomer having at least two active vinyl groups prepared by the process comprising the free radical polymerization in an aqueous dispersion of a monomer mixture comprising a major portion of a monovinyl aromatic monomer and a minor portion of a crosslinking monomer having at least two active vinyl groups, the improvement comprises conducting the polymerization reaction in the presence of a mercaptan.

Monovinyl aromatic monomer refers herein to any compound which has a reactive vinyl group bound to an aromatic ring. Such aromatic ring may be further substituted with one or more substituents which do not substantially interfere with the reactivity of the vinyl monomer. Examples of monovinyl aromatic monomers include styrene, vinyl toluene, vinyl naphtalene, ethyl vinyl benzene, vinyl chlorobenzene, chloromethyl styrene, and the like. The copolymers of this invention preferably comprise from 50 to 99.5 mole percent of the monovinyl aromatic monomer, and more preferably from 80 to 99 mole percent.

The polyvinyl compounds useful in this invention comprise compounds with two or more vinyl moieties which are reactive with the vinyl moieties of the monovinyl aromatic compound to form a crosslinked, insoluble, infusible copolymer. The polyvinyl compounds can be sustituted with other substituents which do not substantially interfere with the ability of the vinyl moieties to react. Examples of such polyvinyl compounds include divinyl benzene, trimethylolpropane trimethacrylate, ethylene glycol dimethacrylate, divinyl toluene, trivinyl benzene, divinyl chlorobenzene, diallyl phthalate, divinylpyridine, divinylnapthalene, ethylene glycol diacrylate, neopentyl glycol dimethacrylate, diethylene glycol divinyl ether, bisphenol-A-dimethacrylate, pentaerythritol tetra- and trimethacrylates, divinylxylene, divinylethylbenzene, divinyl sulfone, divinylketone, divinyl sulfide, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl succinate, diallyl carbonate, diallyl malonate, diallyl oxalate, diallyl adipate, diallyl sebacate, diallyl tartrate, diallyl silicate, triallyl tricarballylate, triallyl aconitate, triallyl citrate, triallyl phosphate, N,N'-methylene-diacrylamide, N,N'-methylene dimethacrylamide, N,N'-ethylene-diacrylamide, trivinyl naphthalene, polyvinyl anthracenes and the polyallyl and polyvinyl ethers of glycol, glycerol, pentaerythritol, resorcinol and the monothio and dithio derivates of glycols. The polyvinyl compound is preferably present in an amount of from 0.5 to 50 mole percent with from 1 to 20 mole percent being preferred.

The copolymer may also have incorporated therein polymerized units of up to about 5 mole percent of other vinyl monomers which do not affect the basic nature of the resin matrix, for example, acrylonitrile, butadiene, methacrylic acid and others known in the art.

In accordance with this invention, the vinyl aromatic monomer, polyvinyl monomer and other optional monomer or monomers, are polymerized in aqueous dispersion containing a free-radical initiator and a minor amount of modifier incorporated in the monomer mixture, said modifier being a mercaptan. A sufficient amount of modifier is used to improve the mechanical strength and optional character.

The mercaptan modifier of this invention may be any mercaptan which provides the advantages referred to herein. Preferably the mercaptan is an alkyl mercaptan, aryl mercaptan, alkaryl mercaptan, aralkyl mercaptan or the like. Such mercaptans may optionally contain other moieties which do not interfere in the copolymerization of monovinyl aromatic monomer and monomers containing two active vinyl groups. More preferred are Cl-16 alkyl mercaptans C6-18 aryl mercaptans C7-20 alkaryl mercaptans, or C7-20 aralkyl mercaptans; such mercaptans being optionally substituted with a non interfering moiety. Ever more preferred are the C8-16 alkyl mercaptans with the straight and branched chain dodecyl mercaptans being most preferred.

Preferably, from about 0.005 to about 0.3 weight percent of modifier is used to obtain the benefits of the invention, with a more preferred amount being from 0.01 to 0.1 weight percent, most preferably 0.01 to 0.03 weight percent.

The polymerization is preferably carried out at temperatures ranging from about 30 to 950C, more preferably 450C to 850C, and most preferably from 70 to 80"C. It is desirable to employ lower temperatures of reaction in the initial stages of the polymerization, that is until at least about 50 % of the monomers in the dispersion are reacted, preferably 75 % or more. The free radical initiator used in the process of the invention is one capable of catalyzing polymerization at the aforesaid temperatures, which are in general somewhat lower, e.g., from 15 to 350C lower, than those normally used heretofore in suspension polymerization for similar products.Representative initiators are di(4-t-butyl-cyclohexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate, di-(sec-butyl)peroxydicarbonate, di (2-ethylhexyl) peroxy dicarbonate, dibenzyl peroxydicarbonate, diisopropyl peroxydicarbonate, azobis (isobutyronitrile), azobis (2,4-dimethyl-valeronitrile), t-butyl peroxypivalate, lauroyl peroxide, benzoyl peroxide, t-butyl peroctoate, t-butyl peroxyisobutyrate, and the like. The amount of initiator employed is normally from 0.1 to 2 percent, based on monomer weight, preferably 0.3 to 1 %.It also may be advantageous when using catalysts which are active at relatively low temperatures, such as 30-60"., to employ a second socalled "chaser catalyst" which is active at higher temperatures, e.g., 75-100 C, in order to achieve higher yields of crosslinked vinyl polymer, for example, from 0.05 to 0.1 % based om monomer weight of such initiators as benzoyl peroxide, t-butyl peroctoate, t-butyl peroxyisobutyrate, and the like.

The aqueous media in which the polymerization is conducted in dispersion form may contain minor amounts of the conventional suspension additives, that is, dispersants such as xanthan gum (biosynthetic polysaccharide), poly(diallyl dimethyl ammonium chloride), polyacrylic acid (and salts), polyacrylamide, magnesium silicate, and hydrolyzed poly(styrene-maleic anhydride); protective colloids such as carboxymethyl cellulose, polyvinyl alcohol, gelatin, and alginates; buffering aids such as phosphate and borate salts; and pH control chemicals such as sodium hydroxide and sodium carbonate.

The crosslinked, high-molecular weight copolymers are recovered from the reactor as hard, discrete beads of particle size within the range of 0.02 to 2 mm, average particle size being on the order of 0.2 to 1 mm. These copolymers are converted to ion exchange resins by attachment of functional groups thereto by conventional means, such functional groups including sulfonamide, trialkylamino, tetraalkyl ammonium carboxyl, carboxylate, sulfonic, sulfonate, hydroxyalkyl ammonium, iminodiacetate, amine oxide, phosphonate, and others known in the art.Functionalizing reactions which may be performed on vinyl aromatic copolymers to produce ion exchange resins are exemplified by sulfonation with concentrated sulfuric acid, chlorosulfonation with chlorosulfonic acid followed by amination, reaction with sulfuryl chloride or thionyl chloride followed by amination, and chloromethylation followed by amination.

Typical functionalizing reactions on (vinyl)acrylic copolymers include hydrolysis to acrylic acid resins, amidolysis, transesterification and the like. Ion exchange resins may be further delineated by the types: strong acid cation, i.e., containing the groupings sulfonic (-S03H) or sulfonate (-S03M, where M is usually an alkali methyl ion); weak acid cation, i.e., containing the groupings carboxyl (-CO2H) or carboxylate (-COM, where M is usually an alkali metal ion); strong base anion, i.e., containing the tetraalkyl ammonium groupings -NR3X, where R is an alkyl or hydroxy alkyl group and X is usually chloride or hydroxide; and weak base anion, i.e., containing a trialkylamino group, NR2, where R is an alkyl or hydroxyalkyl group. Less of the C8-16 alkyl mercaptan compounds are needed to achieve the desired results than is used of known additives to the polymerization, in some circumstances as little as 20 to 25 percent as compared to known additives.

Example 1 To a stirred four liter vessel is added a solution of 1540 g of D.I. Water and 2.3 g of methylhydroxyethyl cellulose.

To said solution is added a mixture of: - 1278 g of styrene - 166 g of 63 % technical divinylbenzene - 4.3 g of 75 % benzoylperoxide .29 g of normal-dodecyl mercaptan.

The vessel is stirred in order to promote a proper monomer droplet size, the same stirred speed is maintained until the polymerization is complete. The polymerization is complete after five hours at 75"C and two hours at 90"C. The copolymer is washed with DI water and dried two hours at 750C. The copolymer activation to sulfonic resin is performed by swelling 100 g of copolymer using 35 g 1.2 dichloroethane; adding the swollen copolymer to 800 g of 98 % H2S04 in a stirred glass vessel; and heating the mixture from room temperature to 1050C in 90 minutes and keeping it at 105"C for two hours.

After the hydration, made using H2SO4 of decreasing concentration and the final washing with DI water, a sulfonic strong acid cation exchanger is obtained having 2.05 meq/ml, WRC = 50 % and density of 0.8 g/ml. The percentage of perfect beads is 99 t.

The average physical strength of the beads is roughly 1500 bead.

Example 2 The same procedure as in example 1 with the exception that tert. dodecyl-mercaptan is used in place of N-dodecyl-mercaptan, a resin is prepared having 1100 g/bead physical strength and 96 % of perfect beads respectively.

Example 3 - not an example of the invention A copolymer is made using the same raw materials and the same proc edure as in Example 1 but substituting phenylacethylene for n-dodecyl mercaptan.

The resin prepared has a 900 g/bead physical strength and 93 % perfect beads.

Example 4 - not an example of the invention Applying the same conditions as in example 1 but using .1 % alpha-methylstyrene dimer in place of ndodecyl mercaptan a resin having 1100 g/bead physical strength and 95 % perfect beads is prepared.

Claims (10)

PATENT CLAIMS:

1. A process of preparing hard, crosslinked, discrete copolymer beads by free radical polymerization in an aqueous dispersion of a monomer mixture comprising a major proportion of a monovinyl aromatic monomer and a minor proportion of cross linking monomer having at least two active vinyl groups, the improvement which comprises conducting the polymerization reaction in the presence of a mercaptan.

2. The process of Claim 1 wherein the mercaptan is a C8-16 alkyl mercaptan.

3. The process of Claim 1 or 2 wherein the mercaptan is present in an amount of from 0.005 to .3 percent by weight of the monomer mixture.

4. The process of one of Claims 1 to 3 wherein the reaction is performed at 70 to 800C for from 3 to 8 hours.

5. The process of one of Claims 1 to 4 which further comprises attaching functional groups selected from the class consisting of sulfonamide, sulfonic acid, sulfonate, carboxyl, carboxylate, trialkyl amino, tetraalkyl ammonium, iminodiacetate, amine oxide and phosphonate to the crosslinked copolymer product.

6. A hard crosslinked, discrete copolymer bead comprising the reaction product of a major portion of a monovinyl aromatic monomer and a minor portion of a crosslinking monomer having at least two active vinyl groups prepared by the process comprising the free radical polymerization in an aqueous dispersion of a monomer mixture comprising a major portion of a monovinyl aromatic monomer and a minor portion of a crosslinking monomer having at least two active vinyl groups, the improvement comprises conducting the polymerization reaction in the presence of a mercaptan.

7. The copolymer bead of Claim 6 wherein the mercaptan is a C8-16 alkyl mercaptan and is present in the process in an amount of from 0.005 to 0.3 percent by weight of the monomer mixture.

8. The copolymer bead of Claims 6 or 7 wherein functional groups are attached to the copolymer bead, the functional groups selected from the class consisting of sulfonamide, sulfonic acid, sulfonate, carboxyl, carboxylate, trialkyl amino, tetraalkyl ammonium, hydroxyalkyl ammonium, iminodeacetate, amine oxide and phosphonate.

9. The copolymer bead of Claim 6, 7 or 8 wherein the monovinyl aromatic monomer is styrene, and the crosslinking monomer is divinylbenzene.

10. The copolymer bead of Claims 6, 7, 8 or 9 wherein the mercaptan is a straight or branched chain dodecyl mercaptan.