Emulsion Polymerization Method for Preparing Aqueous Dispersions of Organic Polymer Particles

This invention relates to a method for preparing aqueous dispersions of organic polymers, and more particularly, to an emulsion polymerization method carried out in the presence of a stabilizing emulsifier and a polymer of a sulfo ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid. A main embodiment of this invention relates to the preparation of microspheres having liquid centers and seamless rigid walls of an organic polymer.

Microspheres encompassing various kinds of liquids are quite useful in the plastic industry. For example, hollow thermoplastic microspheres are utilized as lightweight fillers for paints and paper coatings. Also porous foams are often prepared by heating thermoplastic microspheres having volatile liquid centers. These porous foams have many useful properties such as light weight, high insulating character and buoyancy. Such foams have been used in diverse fields such as packaging, refrigeration, flotation equipment, construction and the like. Various liquids are also encapsulated for ease of handling and the like, and later released from the capsule by diffusion, pressure rupture, heat, etc. Examples are insecticides, pharmaceuticals, inks, fire retardants and the like.

Conventionally microspheres having liquid centers are prepared by suspension polymerization processes. In such processes liquids are usually encapsulated in the microspheres by dispersing the liquid to be encapsulated in the monomer phase before polymerization. Known suspension polymerization procedures for encapsulating liquids comprise the steps of (1) suspending the monomeric components and the liquid to be encapsulated in water with the aid of a suspension stabilizer such as colloidal silica and (2) polymerizing the monomeric components by adding an oil-soluble free radical type catalyst and a small amount of heat. Unfortunately, suspension polymerization proceeds at a relatively slow rate. Also microspheres having diameters less than 2 microns which are often desired in certain coating compositions are not readily prepared by suspension polymerization. While solid polymer particles having diameters less than 2 microns are easily prepared by emulsion polymerization, microspheres having liquid centers cannot be satisfactorily prepared by ordinary emulsion polymerization methods. The difficulty in preparing microspheres by conventional emulsion polymerization is due to the nature of emulsion polymerization which involves the migration of unreacted monomer from the oil phase through the aqueous phase to a micelle where the monomer is then polymerized. As a result the liquid to be encapsulated appears as a separate oil phase.

In view of the potential utility of polymer particles having a uniform size, particularly small microspheres having liquid or hollow centers, and the lack of a suitable method for preparing them, it would be highly desirable to provide such a suitable method.

The present invention is a method for preparing aqueous dispersions of normally solid, organic polymeric particles, said method, as hereinafter described in detail, comprising the steps of (1) dispersing an oil phase comprising at least one emulsion polymerizable monomer in an aqueous phase containing from about 0.5 to about 4 weight percent based on total monomer of a stabilizing emulsifier and from about 0.2 to about 2 weight percent based on total monomer of a polymer of a sulfo ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid and (2) subjecting the dispersion to emulsion polymerization conditions. Polymeric particles of these aqueous dispersions generally have average diameters ranging from about 0.5 to about 20 microns, with some particles having diameters up to about 45 microns.

The essence of this invention resides in the use of a polymer of a sulfo ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid as a coalescing aid in an emulsion polymerization process.

The aqueous dispersions are useful in themselves as coating composition such as paints and are especially useful for the preparation of foam rubber. In addition, these aqueous dispersions may be concentrated to higher solids dispersions, or particles thereof may be filtered from the dispersions and used as fillers, etc.

Of particular interest in this invention is a method for preparing microspheres having liquid centers and seamless rigid walls of a normally solid, organic polymer. Accordingly, such microspheres are prepared by a method comprising the steps of (1) intimately admixing at least one non-polymerizable, water-insoluble liquid with at least one emulsion polymerizable monomer, said liquid being soluble in the monomer and insoluble in the resulting organic polymer; (2) dispersing the resulting oil mixture in an aqueous phase containing from about 0.5 to about 4 weight percent based on total monomer of a stabilizing emulsifier and from about 0.2 to about 2 weight percent based on total monomer of a polymer of a sulfo ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid; and (3) subjecting the dispersion to emulsion polymerization.

Microspheres prepared in accordance with the method of this invention have average diameters ranging from about 0.5 to about 20 microns, with some microspheres having diameters up to about 45 microns. These microspheres can be utilized as expandable spheres in the fabrication of porous foams. Thermoplastic microspheres encapsulating liquids are also useful in the production of insecticides, pharmaceuticals, inks, fire retardants and the like. Additionally it is possible to remove the liquid by evaporation or a similar means, thus leaving a hollow microsphere which serves as an excellent lightweight filler for paints or paper coatings. Microspheres having polymeric liquid centers are useful in the production of multipurpose plastic materials.

The method of this invention requires, as starting materials, (1) at least one emulsion polymerizable monomer, (2) a polymer of a sulfo ester of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid, said polymer acting as a coalescing aid, and (3) a stabilizing emulsifier. In the preparation of microspheres having liquid centers by the method of this invention, a non-polymerizable, essentially water-insoluble liquid is employed in addition to the aforementioned starting materials.

Monomers suitable for preparing organic polymeric particles according to the method of this invention polymerize under emulsion polymerization conditions to form water-insoluble polymers. Representative of such monomers are ethylenically unsaturated monomers such as the monovinylidene aromatic compounds, the acyclic conjugated dienes, the esters of .alpha.,.beta.-ethylenically unsaturated carboxylic acids, and other ethylenically unsaturated monomers.

The monovinylidene aromatic compounds are represented by styrene, substituted styrenes (e.g., styrene having halogen, alkoxy, cyano or alkyl substituents), vinyl naphthalene and the like. Specific examples are styrene, .alpha.-methylstyrene, ar-methylstyrene, ar-ethylstyrene, .alpha.,ar-dimethylstyrene, ar,ar-dimethylstyrene, ar-t-butylstyrene, methoxystyrene, ar,ar-diethylstyrene, acetylstyrene, monochlorostyrene, dichlorostyrene, other halostyrenes and vinylnaphthalene.

In this specification and claims, by the word "monovinylidene" in the term "monovinylidene aromatic" monomer or compound is meant that to an aromatic ring in each molecule of the monomer or compound is attached one radical of the formula, ##STR1## wherein R is hydrogen or a lower alkyl such as an alkyl having from 1 to 4 carbon atoms, e.g., methyl, ethyl, n-propyl, n-butyl and isobutyl.

The acyclic conjugated dienes operable in the practice of this invention include butadiene and substituted butadiene and other acyclic compounds having at least two sites of ethylenic unsaturation separated from each other by a single carbon-to-carbon bond. Specific examples are 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, piperylene, 2-neopentyl-1,3-butadiene, and other hydrocarbon homologs of 1,3-butadiene, and, in addition, the substituted dienes, such as 2-chloro-1,3-butadiene, 2-cyano-1,3-butadiene, the substituted straight-chain conjugated pentadienes, the straight and branch-chain hexadienes, and others.

Suitable esters of .alpha.,.beta.-ethylenically unsaturated carboxylic acids include the alkyl acrylates in which the alkyl portion has from 1 to 12 carbon atoms, e.g., methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, n-octyl acrylate, n-octyl methacrylate, n-decyl methacrylate, and dodecyl methacrylate; the esters of .alpha.,.beta.-ethylenically unsaturated dicarboxylic acids, e.g., methyl maleate, ethyl fumarate, propyl citraconate, and butyl itaconate; and the like.

Other suitable ethylenically unsaturated monomers include chloro-substituted aliphatic monoethylenically unsaturated monomers such as vinyl chloride and vinylidene chloride; the acrylonitriles such as acrylonitrile and methacrylonitrile; ethylenically unsaturated esters of saturated carboxylic acids such as vinyl acetate and vinyl propionate; the ethylenically unsaturated ketones such as methyl isopropenyl ketone, and the like.

Small amounts, i.e., up to about 10 weight percent based on the polymer of other ethylenically unsaturated monomers, may be copolymerized with the above prescribed monomers for use in the practice of this invention. Typical such other monomers are the .alpha.,.beta.-ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, fumaric acid and the like; crosslinking monomers such as divinyl benzene; and others such as 2-hydroxyethyl acrylate, 2-sulfoethyl methacrylate, and the like.

Of the suitable monomers, particularly advantageous monomers include styrene and methyl methacrylate. Of the monomers used in small quantities, it is found that the use of divinylbenzene as a cross-linking agent in the monomeric mixture substantially increases the percent of liquid which can be encapsulated. However, the use of divinylbenzene is limited to concentrations up to about 3.0 percent by weight based on the monomer weight in order to avoid formation of large amounts of coagulum during polymerization.

Liquids which are encapsulated by the process of this invention are substantially inert, i.e., non-reactive with the monomer or with the polymer formed. Liquids suitable for this purpose are non-polymerizable and do not inhibit the emulsion polymerization of the monomeric components. Such liquids are generally soluble in the monomeric phase and insoluble in the resulting polymer.

While particular liquids which are suitable depend on the particular monomer phase and resulting polymer, the following liquids are generally suitable in the systems contemplated by this invention.

Examples of such suitable liquids include the inert hydrocarbons and halo-hydrocarbons, e.g., hexane, neopentane, carbon tetrachloride and the like; the aromatic hydrocarbons and halogen-substituted aromatic hydrocarbons, e.g., benzene, toluene, chlorobenzene and the like; the chlorofluorocarbons, e.g., trichlorofluoromethane, difluorodichloromethane, trifluorochloromethane, etc; and the tetraalkyl silanes, e.g., tetramethyl silane, trimethyl ethyl silane, trimethyl isopropyl silane and the like. Also included are liquid polymers having the aforementioned properties, for example, low molecular weight polymers of the .alpha.-olefins, e.g., ethylene, propylene, butene-1 and the like; of conjugated diolefins, e.g., butadiene and isoprene; of the monovinylidene aromatic compounds, e.g., styrene, .alpha.-methylstyrene, t-butylstyrene, ar-methylstyrene and the like; of other ethylenically unsaturated monomers, e.g., vinyl chloride, acrylonitrile, acrylamide and the like. In accordance with this invention, oil solutions of various solids such as the normally solid, organic polymers and the like may also be encapsulated. Of the more volatile liquids, neohexane and neopentane are preferred, and a preferred liquid polymer is low molecular weight polybutadiene.

Coalescing aids utilized in the method of this invention are polymers of sulfo esters of .alpha.,.beta.-ethylenically unsaturated carboxylic acids, said esters being completely described in U.S. Pat. Nos. 3,024,221 and 3,147,301 which are incorporated herein in their entirety. Polymers of the sulfo esters used in this invention are characterized as having hydrophobic and hydrophilic groups in the polymer molecules. Suitable coalescing aids are the homopolymers and copolymers of one or more of the sulfo esters and copolymers of the sulfo esters and one or more ethylenically unsaturated monomers.

Coalescing aids which are particularly suited for the purposes of this invention are copolymers of a sulfo ester and one or more ethylenically unsaturated monomers copolymerizable therewith. Preferred copolymers, which act as especially effective coalescing aids, have concentrations of sulfo ester monomers of at least 20 weight percent based on the copolymer. Other copolymers which act as effective coalescing aids usually contain sulfo ester monomers in concentrations of 15 weight percent or more.

Representative of the sulfo esters which polymerize to form suitable coalescing aids include the sulfo esters of .alpha.-methylene carboxylic acids, as described in U.S. Pat. No. 3,024,221, the mono- and di-sulfo esters of ethylenically unsaturated vicinal dicarboxylic acids, as described in U.S. Pat. No. 3,147,301, and the sulfonate salts of the above sulfo esters. For the purposes of illustrations and not limitation, the following examples are given: the sulfo esters of .alpha.-methylene carboxylic acids, e.g., 2-sulfoethyl acrylate, 2-sulfoethyl methacrylate, 2-sulfoethyl .alpha.-ethylacrylate, 2-sulfoethyl .alpha.-propylacrylate, 2-sulfoethyl .alpha.-butylacrylate, 2-sulfoethyl .alpha.-hexylacrylate, 2-sulfoethyl .alpha.-cyclohexylacrylate, 2-sulfoethyl .alpha.-chloroacrylate, 2-sulfo-1-propyl acrylate, 2-sulfo-1-propyl methacrylate, 1-sulfo-2-propyl acrylate and methacrylate; and the sulfo esters of ethylenically unsaturated vicinal dicarboxylic acids, e.g., bis(2-sulfoethyl) maleate, bis(2-sulfopropyl) fumarate, bis(2-sulfopropyl) itaconate, bis(3-sulfopropyl) itaconate, bis(2-sulfobutyl) fumarate, bis-(1-sulfobutyl) maleate, bis(3-sulfobutyl) itaconate, and the like. Of the sulfo esters, those of the methylene carboxylic acids are preferred, especially 2-sulfoethyl methacrylate.

Representative of the ethylenically unsaturated monomers which are suitably copolymerized with the suitable sulfo esters include the monovinylidene aromatic compounds, e.g., the styrene compounds; the ethylenically unsaturated carboxylic acids and derivatives thereof, e.g., the acrylic acids and salts, the acrylic esters, the acrylic nitriles, the acrylic amides, the acrylic anhydrides, maleic esters, maleic acid polyesters, unsaturated alcohol esters; unsaturated ketones; unsaturated ethers; and other compounds containing one or more ethylenic linkages capable of addition polymerization. Specific examples of such ethylenically unsaturated monomers are styrene, .alpha.-methylstyrene, ar-methylstyrene, ar-ethylstyrene, .alpha.,ar-dimethylstyrene, ar,ar-dimethylstyrene, divinylbenzene, vinylnaphthalene, divinylnaphthalene, vinylbenzenesulfonic acid, divinylbenzenesulfonic acid, hydroxystyrene, methoxystyrene, aminostyrene, cyanostyrene, acetylstyrene, monochlorostyrene, dichlorostyrene and other halostyrenes, acrylic acid and salts, methacrylic acids and salts, methyl methacrylate, ethyl acrylate, butyl acrylate, glycol diacrylate, hexyl acrylate, lauryl methacrylate, phenyl acrylate, allyl acrylate, acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, ethyl .alpha.-chloroacrylate, ethyl maleate, maleic anhydride, polyglycol maleate, diallyl fumarate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl benzoate, vinyl chloride, vinyl bromide, vinylidene chloride, vinylidene bromide, vinylidene cyanide, vinyl methyl ketone, methyl isopropenyl ketone, vinyl carbazole, vinyl ethyl ether, divinyl ether, isobutylene, 1,3-butadiene, isoprene, and the like. Of the above monomers, the monomers which are characterized by the fact that homopolymers thereof are water-insoluble are preferred, with the alkyl acrylates being especially preferred. For the purposes of this invention the term "alkyl acrylate" includes the alkyl esters of acrylic acid, methacrylic acid and other .alpha.-substituted acrylic acids. Such alkyl acrylates are further characterized by having alkyl moieties of 1 to 12 carbon atoms.

Preferred coalescing aids are the copolymers of the preferred ethylenically unsaturated monomers and the sulfo esters of .alpha.-methylene carboxylic acids, for example, copolymers of n-butyl acrylate and 2-sulfoethyl methacrylate, copolymers of styrene and 2-sulfoethyl methacrylate and copolymers of acrylonitrile and 2-sulfoethyl methacrylate. Especially preferred coalescing aids are the copolymers of the alkyl acrylates having alkyl moieties from 1 to 12 carbon atoms and the sulfo esters of .alpha.-methylene carboxylic acids, and in particular, a copolymer of 70 weight percent of n-butyl acrylate and 30 weight percent of 2-sulfoethyl methacrylate. Other preferred and suitable copolymers and methods for the preparation thereof are described in U.S. Pat. No. 3,033,833 which is incorporated herein in its entirety.

Stabilizing emulsifiers suitable for the purposes of this invention are the water soluble anionic and non-ionic surfactants, with anionic or mixtures of anionic and non-ionic being preferred. Non-ionic surfactants which are suitable include the polyethenoxy agents, e.g., the ethylene glycol polyethers, the ethylene nonyl phenol polyethers, and the like; the fatty acid esters of polyhydric alcohols, e.g., propylene glycol fatty acid ester; and others set forth in Becher, Emulsions: Theory and Practice, 2nd ed., Reinhold Publishing Corporation, New York, 221-225 (1965). Anionic surfactants, which are generally preferred over suitable non-ionic surfactants, include water soluble soaps of soap-forming monocarboxylic acids, e.g., alkali metal salts of linoleic acid dimer; and sulfated and sulfonated compounds having the general formula R - O SO.sub.3 M and R - SO.sub.3 M, wherein R represents an organic radical having from 9 to 23 carbon atoms and M represents an alkali metal, an ammonium or amine group. Examples of the sulfonate and sulfate emulsifiers include sodium dodecyl benzene sulfonate, sodium oleyl sulfate, ammonium dodecyl benzene sulfonate, potassium lauryl sulfate, sodium dodecyl diphenyl oxide disulfonate, dioctyl potassium sulfosuccinate, dihexyl sodium sulfosuccinate, and the aryl sulfonate-formaldehyde condensation products. Choice of emulsifiers varies with the emulsion system, particle size desired, and so forth. In systems where a particle size ranging from about 4 to 5 microns is desired; sodium dodecyl benzene sulfonate is preferred.

In some instances it is preferable to include a small amount, i.e., up to about 1 weight percent based on total monomer, of an electrolyte in the polymerization recipe in order to keep the formation of coagulum at a minimum and to increase the degree of liquid encapsulation. However, the use of an electrolyte is not a critical requirement of this invention. Examples of electrolytes which may be used, if desirable, include the alkali metal and ammonium salts such as sodium sulfate, potassium bisulfate, sodium nitrate, ammonium sulfate and the like.

The process of this invention is emulsion polymerization of one or more suitable monomers carried out in the presence of from about 0.5 weight percent to about 4 weight percent of a suitable stabilizing emulsifier and from about 0.2 weight percent to about 2 weight percent of a suitable polymer of a sulfo ester, both percentages being based on total monomer.

Concentration of emulsifier is critical in the process of this invention in that in the absence of emulsifier the polymer coagulates completely and in the presence of excess emulsifier the desired partial coalescence is not effected by the copolymer of the sulfo ester. In preferred practices the concentration of emulsifier is within the range of from about 1 to about 3 weight percent based on total monomer.

Concentration of the polymer of sulfo ester is likewise critical since no encapsulation is observed in its absence and only very little encapsulation and a substantial amount of coagulum are observed in its excess. Different concentrations of the polymer of sulfo ester within the operable range of from about 0.2 to about 2 weight percent based on total monomer affect the size of the resulting microspheres. Larger amounts, e.g., about 2 weight percent, of the polymer of sulfo ester generally have the effect of decreasing the size of the microsphere, usually an average diameter from about 0.5 to about 3 microns. Preferred concentrations of the polymer of sulfo ester range from about 0.25 weight percent to about 1 weight percent, especially for preparing microspheres having an average diameter ranging from about 1 to about 5 microns.

If the method of this invention is carried out in the presence of an electrolyte in addition to an emulsifier and a polymer of a sulfo ester, it is desirable that said electrolyte be present in concentrations from about 0.2 weight percent to about 1 weight percent based on total monomer, preferably from about 0.25 to about 0.5 weight percent.

In preparing microspheres having liquid centers by the method of this invention it is necessary to use concentrations of the liquid to be encapsulated less than about 50 volume percent based on the oil phase of liquid and monomer, with the best results being attained at concentrations from about 15 to about 20 volume percent.

In addition, the ratio of the volume of the oil phase to the volume of the aqueous phase also has a decided effect on the amount of liquid which is encapsulated in the microspheres. For example, with 27 volume percent of neopentane, based on the oil phase, the greatest degree of encapsulation is observed in a system having a total oil concentration of 44 volume percent based on the total aqueous and oil phases. In this system, increasing the total oil phase above 44 volume percent results in decreased encapsulation and increased formation of coagulum. In another example where from 27 to 40 volume percent of neohexane is used, the largest amount of encapsulation is observed in a system having a total oil phase of 55 volume percent based on the total aqueous and oil phases. Thus it appears that operable limits of total oil phase vary somewhat with the liquid to be encapsulated and in all likelihood with the particular monomer or monomer used. However, as a general rule, total oil phase concentrations from about 33 volume percent to about 55 volume percent based on the total oil and aqueous phases are operable, with concentrations from about 44 to about 55 volume percent being preferred.

The method of this invention is carried out by first dispersing an oil phase containing at least one emulsion polymerizable monomer in an aqueous phase containing a stabilizing emulsifier and a suitable coalescing aid. The dispersion is usually achieved by subjecting the two phases to high shear mixing conditions, for example, a homogenizing device, a Waring Blendor and the like. In the preparation of microspheres, encapsulation of the liquid is not accomplished unless a dispersion of the phases is achieved.

The resulting dispersion of the oil and aqueous phases is charged to a suitable reaction vessel and subjected to conditions of emulsion polymerization, i.e., initiation by a free radical type catalyst such as potassium persulfate at a temperature from about 60.degree. C. to about 100.degree. C. and agitation of the dispersion during the polymerization period. The emulsion polymerization is usually carried out in the presence of from about 0.01 to about 3 weight percent based on the monomer weight of the free radical producing catalyst. Suitable catalysts conveniently are the peroxygen compounds, especially the inorganic persulfate compounds such as sodium persulfate, potassium persulfate, ammonium persulfate; the peroxides such as hydrogen peroxide; the organic hydroperoxides, for example, cumene hydroperoxide and t-butyl hydroperoxide; the organic peroxides, for example, benzoyl peroxide, acetyl peroxide, lauroyl peroxide, peracetic acid, and perbenzoic acid-- sometimes activated by a water-soluble reducing agent such as a ferrous compound or sodium bisulfite--and other free-radical producing materials such as 2,2'-azobisisobutyronitrile.

The following examples are given to illustrate the invention, but are not to be read as limiting its scope. In the following specification and claims all parts and percentages are by weight unless otherwise indicated.

A 1.25 gram-portion of 2-hydroxyethyl acrylate and 98.75 grams of methyl methacrylate were mixed in 25 grams of neohexane. This oil mixture was mixed with an aqueous solution of 125 grams of deionized water, 0.5 gram of sodium sulfate, 0.5 gram of potassium persulfate, 1.0 gram of sodium dodecyl benzene sulfonate and 1.0 gram of a copolymer of 30 percent of 2-sulfoethyl methacrylate and 70 percent of butyl acrylate. The mixture was passed through a hand homogenizer to obtain intimate mixing of the oil and aqueous media. The intimate mixture was poured into a citrate bottle and then was agitated by tumbling the bottle end-over-end at about 4.5 rpm for a period of 16 hours in a water bath at a temperature of 70.degree. C. At the end of the sixteen hour period 94 percent of the charge was recovered in addition to a small amount of coagulum. Under an optical microscope the reaction product was shown to consist of microspheres having a uniform size of about 1 micron with only a few particles as large as 5 microns. In order to show the presence of a liquid inclusion, a film containing the microspheres was cast upon a flat surface and allowed to dry at room temperature overnight. By heating this film in a vacuum oven at 150.degree. C. and measuring the loss of weight, it was found that the dry film contained 12 percent neohexane. Inclusion of liquid in the microspheres was further confirmed by observation of the Brownian motion of a bubble on the surface of the encapsulated liquid.

Following the procedure of Example 1, except using varying amounts of sodium sulfate, sodium dodecyl benzene sulfonate and the copolymer of 2-sulfoethyl methacrylate and butyl acrylate, numerous examples were carried out to determine the effect of concentration of these components. Results of these examples, in addition to Example 1, are shown in the Table I for the purposes of comparison.

Several polymerization recipes were prepared and polymerized according to Example 1 except that various emulsifiers were substituted for sodium dodecyl benzene sulfonate and the polymerization was carried out at 90.degree. C. in 4 hours. Results of these examples are shown in Table II.

An oil phase consisting of 50 parts of styrene, 1.0 part of 2-hydroxyethyl acrylate and 50 parts of low molecular weight liquid polybutadiene and an aqueous phase consisting of 0.5 part of potassium persulfate, 0.5 part of sodium sulfate, 1.0 part of copolymer of 30 percent of 2-sulfoethyl methacrylate and 70 percent of butyl acrylate, and 100.0 parts of deionized water were intimately mixed by using a hand homogenizer. The intimate mixture was poured into a citrate bottle and then was agitated by tumbling the bottle end-over-end at about 4.5 rpm for a period of 16 hours in a water bath at a temperature of 70.degree. C. The particles formed were of two separate phases. Microscopic observation indicated that a shell of polystyrene encompassed the low molecular weight polybutadiene. Similar recipes except containing different coalescing aids were prepared and polymerized according to the above procedure. The results of these examples are shown in Table III.

As indicated by the preceding table, copolymers of the sulfo ester and butyl acrylate were the most effective coalescing aids, particularly the coalescing aids used in Examples 19-21.

An oil phase consisting of 20 parts of low molecular weight liquid polybutadiene, 80 parts of styrene and 1.00 part of 2-hydroxyethyl acrylate and an aqueous phase according to Example 19 were intimately mixed by the use of a hand homogenizer. The intimate mixture was poured into a citrate bottle and polymerized according to the procedure of Example 19. The results are shown in Table IV. For the purposes of comparison several polymerization recipes containing the same amount of the various coalescing aids used in Examples 20-27 were polymerized in a similar manner and the results thereof are also shown in Table IV.

Again the copolymers of the sulfo ester and n-butyl acrylate were observed to be the best coalescing aids.

An oil phase containing 10 parts of acrylonitrile, 45 parts styrene, and 45 parts of liquid polybutadiene and the aqueous phase of Example 19 were mixed and polymerized according to procedure of Example 19. Results of 97 percent recovery, 96 percent conversion and microspheres ranging in size from less than one to twenty microns were observed.

The following ingredients were placed into a 12-ounce citrate bottle:

The bottle and contents were cooled in ice and 50 grams of liquid butadiene was added to the contents. The bottle was capped and tumbled in a 70.degree. C. water bath for 24 hours after which the bottle was opened and the contents filtered. The contents consisted of 20 grams of coagulum and 235 grams of an aqueous dispersion having 38.7 percent polymer solids. The particles of the dispersion were examined under an optical microscope and most were found to have diameters ranging from about 6 to about 18 microns, with some particles having diameters as small as 1 to 2 microns. The aqueous dispersion was readily concentrated to high solids by evaporation.