Polymerization, suspension

Suspension Polymerized Particulate Resin Supports -Structural and Morphological Variants 

Since cross-linked polymers caruiot be re-formed or re-shaped it is necessary to synthesize them in the final physical form appropriate for each particular application. Particles in the size range 50-1000 pm are suitable for laboratory scale chemistry, while larger particles have advantages in large scale continuous processes. Irregularly shaped particles are susceptible to mechanical attrition and breakdown to fines , whereas the process of suspension polymerization [13] yields uniform spherical cross-linked polymer particles often referred to as beads, pearls or resins. These are much more mechanically robust and are widely exploited on both a small and large scale e. g. as the basis of ion exchange resins [14]. 

Suspension polymerization is designed to combine the advantages of both the bulk and solution polymerization techniques. It is one of the extensively employed techniques in the mass production of vinyl and related polymers. Suspension polymerization (also referred to as bead or pearl polymerization) is carried out by suspending the monomer as droplets by efficient agitation in a large mass (continuous phase) of nonsolvent, commonly referred to as the dispersion or. suspension medium. Water is invariably used as the suspension medium for all water insoluble monomers because of the many advantages that go with it. Styrene, methyl methacrylate, vinyl chloride, and vinyl acetate are polymerized by the suspension 

Heat and viscosity control in suspension polymerization is relatively easy compared to bulk polymerization. Another important advantage of the method is that the polymer product can be obtained directly in spherical bead form (which may subsequently be functionalized to make ion-exchange resins). 

Problem 6.47 A monomer is polymerized at 80°C (a) in benzene solution and (b) in aqueous suspension in two separate runs, both containing 60 g of the monomer (density 0.833 g/cm ) and 0.242 g of a peroxide initiator in a total volume of 1 liter. If the initial rate of polymerization for 1 liter of solution is 0.068 mol/h, what is the expected initial rate for 1 liter of suspension (Assume that rate constants and the initiator efficiency are same in both cases.) 

Suspension Polymerization is another synthetic polymerization method designed to deliver spherical particles with an appropriate size and shape in a single preparative step. 

The most widely used dispersing agent is water. However, the water can interrupt template-monomer bonding more especially if the non-covalentimprinting method is used. Perfluorocarbon liquid as a dispersing medium was used to produce MIPs with good recognition properties [Mayes and Mosbach, 1996 Hantash et al., 2006]. 

Mineral oil [liquid paraffin) was also used for the formation of pre-polymerization droplets [Kempe and Kempe, 2006). Being chemically inert in nature, these liquids do not affect the non-covalent interactions in template-monomer complex when used as a dispersing medium. There are some recent reports utilizing suspension polymerization for MIP synthesis for Staphylococcus aureus protein A-imprinted polyacrylamide [Pan et al., 2009), Stlgmasterol MIP microspheres [Han et al., 2008), MIP hydrogels for the peptide hepcidin [Abbate et al., 2010) and Promethazine based MIPs [Alizadeh et al., 2012). 

The major drawback of suspension polymerization method is formation of polydispersed MIP particles. Also, porogenic solvents such as chloroform and toluene cannot be used as these are miscible with mineral oil [Haginaka et al., 2008). 

Free-radical suspension polymerization, originally developed by Hoffman and Delbruch in 1909 [1] is commonly employed for producing a wide variety of commercially important polymers such as poly(vinyl chloride) (PVC), polystyrene (PS), expandable polystyrene (EPS), high-impact polystyrene (HIPS) and various styrene copolymers with acrylonitrile (SAN) and acrylonitrile-polybutadiene (ABS), poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), etc. [2], 

In general, the suspension polymerization can be distinguished into two types, namely, the bead and powder suspension polymerization [4]. In the former process, the polymer is soluble in its monomer and smooth spherical particles are produced. In the later process, the polymer is insoluble in its monomer and, thus, precipitates out leading to the formation of irregular grains or particles. The most important thermoplastic produced by the bead suspension polymerization process is PS. In the presence ofvolatile hydrocarbons (C4—C6), foamable beads, the so-called EPS, are produced. On the other hand, PVC, which is the second largest thermoplastic manufactured in the world, is an example of the powder type suspension polymerization. 

The main advantages of suspension polymerization compared to the bulk process are the easier control of the reaction temperature due to the presence of the dispersion medium (e.g., water), the milder reaction conditions and the product homogeneity, especially for 

Pickering dispersant (tricalcium phosphate, 1 part (by m lss) barium sulfate, MgC03, etc.) 

100 parts (by mass) 100 150 parts (by mass) 5 10 parts (by mass) 0.2. 5 parts (by mass) 

This is another important industrial polymerization technique that is suitable for water-insoluble monomer and polymers (or indeed organic insoluble monomers for reverse systems) (Trijasson et aL, 1990) and is especially suited to the synthesis of large 10-1000 pm polymer 

A major advantage of suspension polymerization over bulk techniques is that the polymer separation is much more facile (Dawkins, 1989), however, the application of this technique is somewhat limited as few monomers are completely water insoluble. In addition, suspension polymerization is limited to radical polymerization systems and the nature of the agitation of the system is key, as is the viscosity in determining the size, stability and nature of the polymer particles formed. Of interest is the synthesis of porous particles via this technique that can be readily achieved by the inclusion of an inert porogen into the monomer phase, which can be readily removed after polymerization (Seidl et al, 1967). Suspension polymerization has a number of advantages including the ease of heat removal, low dispersion viscosity, low levels of impurities in the resultant particle, low separation costs and the final product is in particle form. On the other hand, the disadvantages of this technique are primarily wastewater problems, lower productivity compared to bulk and a continuous process has to date not been achieved industrially. 

Preparation of copoly(AOTcp-styrene) (8) by suspension polymerization is basically the same as that of commodity polymers such as polystyrene [57]. In fact, equimolar copolymerization of styrene with AOTcp proceeds at a considerably faster rate than homopolymerizadon of styrene (see above). As a result, complete mon ner conversion is accomplished during a correspondingly shorter period of time (ca. 5 h vs. 15 h). 

SEM micrographs of the surfoces and cross-sections of the activated resin samples 8a and 8c are shown in Fig. 9. These resin samples were obtained in the 

Activated resin Matrix type Crosslinking monomer (mol%) Monomer diluent (ml/g) Active ester content  

Scanning electron micrograph of activated resin sample 8c (see Table 6) 

Aminolysis of alkyl esters [58, 59] and activated esters [60, 61] is nerally known to be catalyzed by tertiary amines and basic solvents such as dimethyl-formamide (DMF). In addition, aminolysis of relatively less activated esters is catalyzed by the hydroxy components of the more reactive ones. This is especially true in the case of iV-hydroxysuccinimide (HOSu) and 1-hydroxy-benzotriazol (HOBt), and the latter compound is routinely used as acylation 

Tabk 6. Examples of activated resin intermediates (8) produced by suspension copolymerization of styrene with 2,4,S-trichlorophenyl acrylate and a crosslinking monomer [46] 

A sharp distinction must be drawn between suspension (or slurry) and emulsion polymerization processes. 

The term suspension polymerization refers to the polymerization of macroscopic droplets in an aqueous medium. The kinetics is essentially that of a bulk polymerization with the expected adjustments associated with carrying out a number of bulk polymerizations in small particles more or less simultaneously and in reasonably good contact with a heat exchanger (i.e., the reaction medium) to control the exothermic nature of the process. Usually, suspension polymerizations are characterized by the use of monomer-soluble initiators and the use of suspending agents. 

However, emulsion polymerizations involve the formation of colloidal polymer particles that are essentially permanently suspended in the reaction medium. The reaction mechanism involves the migration of monomer molecules from liquid monomer droplets to sites of polymerization that originate in micelles consisting of surface-active agent molecules surrounding monomer molecules. Emulsion polymerizations are usually characterized by the requirement of surfactants during the initiation of the process and by the use of water-soluble initiators. This process also permits good control of the exothermic nature of the polymerization. 

Polymerizations that are carried out in nonaqueous continuous phases instead of water are termed dispersion polymerizations regardless of whether the product consists of filterable particles or of a nonaqueous colloidal system. 

Suspension polymerizations are among the most convenient laboratory procedures as well as plant procedures for the preparation of polymers. The advantages of this method include wide applicability (it may be used with most water-insoluble or partially water-soluble monomers), rapid reaction, ease of temperature control, ease of preparing copolymers, ease of handling the final product, and control of particle size. 

FIGURE 5.13 Basic stirred, jacketed batch reactor. For solution polymerizations, solvent, monomer, and intiator are charged. For suspension polymerization, water and a protective colloid such as poly(vinyl alcohol) form one phase, whereas the monomer and initiator form a second phase. In emulsion polymerization, a water-soluble initiator such as potassium persulfate is used together with a surfactant such as sodium stearate. 

A simple suspension polymerization can be carried out in a glass flask with agitation. 

Under the discussion of bulk polymerization, it was mentioned that one of the ways of facilitating heat removal was to keep one dimension of the reaction mass small. This is carried to its logical extreme in suspension polymerization by suspending the monomer in the form of droplets 0.01 to 1 mm in diameter in an inert, nonsolvent liquid (almost always water). In this way, each droplet becomes an individual bulk reactor with dimensions small enough that heat removal is 

An important characteristic of these systems is that the suspensions are thermodynamccdly unstable, and must be maintained with agitation and suspending agents. A typical charge might consist of 

Chain-transfer agent (monomer soluble) J Water 

Two types of suspending agent are used. A protective colloid is a water-soluble polymer whose function is to increase the viscosity of the continuous water phase. This hydrodynamicaUy hinders coalescence of monomer drops, but is inert with regard to the polymerizatioiL A finely divided insoluble inorganic salt such as MgCOs may also be used. It collects at the droplet-water interface by surface tension, and prevents coalescence of the drops upon collision. A pH buffer is sometimes also used to help stability. 

The monomer phase is suspended in the water at about a to i mono-mer/water volume ratio. The reactor is purged with nitrogen and heated to start the reaction. Once underway, temperature control in the reactor is facilitated by the added heat capacity of the water and the low viscosity of the reaction mass—essentially that of the continuous phase—allowing easy heat removal through a jacket. 

Through agitating or stirring with the aid of a dispersion agent, water-insoluble monomers can be dispersed in water as fine droplets of 1-10 cm in diameter. Oil-soluble radical initiators start polymerization in the droplets. After polymerization, the droplets will have been converted into beads or pearls, and because of this the method is also called pearl polymerization. Because of the suspension, the reaction space available for the bulk polymerization is, so to speak, divided into many small regions. Therefore the heat of polymerization can be better dissipated. Purely mechanistically, this method corresponds to a water-cooled bulk polymerization. 

It is only possible to polymerize by the suspension method those mon- 

Some polymers are insoluble in their own monomers and therefore are precipitated during polymerization. Examples are poly(vinyl chloride) and poly(acrylonitrile). Polymerization continues in the precipitated product, but its rate is determined by the diffusion of the monomer to the free radicals and consequently by the physical structure of the coagulate. Factors such as the rate of agitation can thus affect the rate of polymerization quite considerably. The advantage of precipitation polymerization is that the polymers immediately assume solid form. For this reason, polymerizations are often carried out with the addition of substances that precipitate the polymer but that are also solvents for the monomer. 

Just as any added solvent, however, the precipitation media can affect the polymerization in other ways. The solvent acts as a diluting medium. 

Solution polymerizations have a technological disadvantage in that the solvent is difficult to remove from the product after polymerization. For this reason, solution polymerizations are only carried out when the polymer can be marketed as a solution in the solvent, e.g., lacquer resins. 

Two types of stabilizers are used, one of which is basically the type of water-soluble polymers (often in the presence of an electrolyte or a buffer) and the other is a type of water-insoluble inorganic compounds. The former type includes polyvinyl alcohol (PVA), hydroxypropyl cellulose, sodium poly(styrene sulfonate), and sodium salt of acrylic acid-acrylate ester copolymer. The latter type includes magnesium silicate hydroxide (TALC), hydroxyapatite, barium sulfate, kaolin, magnesium carbonate and hydroxide. 

Besides the normal suspension pol)unerization, the inverse-suspension polymerization is also employed in large-scaled production, which is mainly hmited to the water-soluble monomer, such as the acrylamide and soluble acrylates and the solutions of the monomer and initiator are suspended in an oil phase. 

FIGURE 12.6 Interfacial pol3fmerization of a nylon. The upper (organic) phase contains sebacoyl chloride (AA monomer) while the lower (aqueous) phase contains hexamethylene diamine (BB monomer). The reaction occurs where the two phases meet. 

The monomer phase is suspended in the water at about a V2 to 74 monomer/water 

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Polymerization occurs in particles whose dimensions are in the nanometer size range, perhaps 10 times smaller than the particles in suspension polymerization. 

Azobisnittiles are efficient sources of free radicals for vinyl polymerizations and chain reactions, eg, chlorinations (see Initiators). These compounds decompose in a variety of solvents at nearly first-order rates to give free radicals with no evidence of induced chain decomposition. They can be used in bulk, solution, and suspension polymerizations, and because no oxygenated residues are produced, they are suitable for use in pigmented or dyed systems that may be susceptible to oxidative degradation. 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

Suspension Polymerization. Suspension polymerisation yields polymer in the form of tiny beads, which ate primarily used as mol ding powders and ion-exchange resins. Most suspension polymers prepared as mol ding powders are poly(methyl methacrylate) copolymers containing up to 20% acrylate for reduced btittieness and improved processibiUty are also common. 

Initiators of suspension polymerization are organic peroxides or azo compounds that are soluble in the monomer phase but insoluble in the water phase. The amount of initiator influences both the polymerization rate and the molecular weight of the product (95). 

Because the polymerization occurs totally within the monomer droplets without any substantial transfer of materials between individual droplets or between the droplets and the aqueous phase, the course of the polymerization is expected to be similar to bulk polymerization. Accounts of the quantitative aspects of the suspension polymerization of methyl methacrylate generally support this model (95,111,112). Developments in suspension polymerization, including acryUc suspension polymers, have been reviewed (113,114). 

The discovery of PTFE (1) in 1938 opened the commercial field of perfluoropolymers. Initial production of PTFE was directed toward the World War II effort, and commercial production was delayed by Du Pont until 1947. Commercial PTFE is manufactured by two different polymerization techniques that result in two different types of chemically identical polymer. Suspension polymerization produces a granular resin, and emulsion polymerization produces the coagulated dispersion that is often referred to as a fine powder or PTFE dispersion. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

There are two principal PVC resins for producing vinyl foams suspension resin and dispersion resin. The suspension resin is prepared by suspension polymerization with a relatively large particle size in the 30—250 p.m range and the dispersion resin is prepared by emulsion polymerization with a fine particle size in the 0.2—2 p.m range (245). The latter is used in the manufacture of vinyl plastisols which can be fused without the appHcation of pressure. In addition, plastisol blending resins, which are fine particle size suspension resins, can be used as a partial replacement for the dispersion resin in a plastisol system to reduce the resin costs. 

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

A Gaussian distribution of particle size is the result of copolymer manufactured by suspension polymerization. A jetting process produces beads with more uniform particle size. The uniformity coefficient is a numerical method of indicating closeness of all beads to the same size. 

Emulsion—Suspension Polymerized Pigment Ink. Polymerization of a polar prepolymer as the internal phase in an oil-based external phase (24) gives a fluorescent ink base in which spherical fluorescent particles are dispersed. This base is suitable for Htho and letterpress inks (qv). An... 

In a suspension polymerization, monomer is suspended ia water as 0.1—5 mm droplets, stabilized by protective coUoids or suspending agents. Polymerization is initiated by a monomer-soluble initiator and takes place within the monomer droplets. The water serves as both the dispersion medium and a heat-transfer agent. Particle size is controlled primarily by the rate of agitation and the concentration and type of suspending aids. The polymer is obtained as small beads of about 0.1—5 mm in diameter, which are isolated by filtration or centrifugation. 

The criterion of maintaining equal power per unit volume has been commonly used for dupHcating dispersion qualities on the two scales of mixing. However, this criterion would be conservative if only dispersion homogeneity is desired. The scale-up criterion based on laminar shear mechanism (9) consists of constant > typical for suspension polymerization. The turbulence model gives constant tip speed %ND for scale-up. 

Slurry (Suspension) Polymerization. This polymerization technology is the oldest used for HDPE production and is widely employed because of process engineering refinement and flexibHity. In a slurry process, catalyst and polymer particles are suspended in an inert solvent, ie, a light or a... 

Hydroxyhydroquinone and pyrogaHol can be used for lining reactors for vinyl chloride suspension polymerization to prevent formation of polymer deposits on the reactor walls (98). Hydroxyhydroquinone and certain of its derivatives are useful as auxiUary developers for silver haUde emulsions in photographic material their action is based on the dye diffusion-transfer process. The transferred picture has good contrast and stain-free highlights (99). 5-Acylhydroxyhydroquinones are useful as stabilizer components for poly(alkylene oxide)s (100). 

Suspension Polymerization. In this process the organic reaction mass is dispersed in the form of droplets 0.01—1 mm in diameter in a continuous aqueous phase. Each droplet is a tiny bulk reactor. Heat is readily transferred from the droplets to the water, which has a large heat capacity and a low viscosity, faciUtating heat removal through a cooling jacket. 

The product of a successful suspension polymerization is small, uniform polymer spheres. For certain appHcations, they are used directly, eg, as the precursors for ion-exchange resins or bead foams. For others, they may be extmded and chopped to form larger, more easily handled mol ding pellets. 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

The original wartime process was mn batchwise in reactors similar to those used for suspension polymerization. Since then, in many plants, the reactors have been hooked together as a series of continuous stirred tanks. 

PS Foams. The eady history of foamed PS is available (244), as are discussions of the theory of plastic foams (245). Foamable PS beads were developed in the 1950s by BASF under the trademark of STYROPOR (246—248). These beads, made by suspension polymerization in the presence of blowing agents such as pentane or hexane, or by post-pressurization with the same blowing agents, have had an almost explosive growth, with 200,000 metric tons used in 1980. Some typical physical properties of PS foams are Hsted in Table 10 (see Foamed plastics). 

Processes that are essentially modifications of laboratory methods and that allow operation on a larger scale are used for commercial preparation of vinyhdene chloride polymers. The intended use dictates the polymer characteristics and, to some extent, the method of manufacture. Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stabiUty of the polymer. 

Suspension Polymerization. At very low levels of stabilizer, eg, 0.1 wt %, the polymer does not form a creamy dispersion that stays indefinitely suspended in the aqueous phase but forms small beads that setde and may be easily separated by filtration (qv) (69). This suspension or pearl polymerization process has been used to prepare polymers for adhesive and coating appHcations and for conversion to poly(vinyl alcohol). Products in bead form are available from several commercial suppHers of PVAc resins. Suspension polymerizations are carried out with monomer-soluble initiators predominantly, with low levels of stabilizers. Suspension copolymerization processes for the production of vinyl acetate—ethylene bead products have been described and the properties of the copolymers determined (70). Continuous tubular polymerization of vinyl acetate in suspension (71,72) yields stable dispersions of beads with narrow particle size distributions at high yields. 

In the suspension polymerization of PVC, droplets of monomer 30—150 p.m in diameter are dispersed in water by agitation. A thin membrane is formed at the water—monomer interface by dispersants such as poly(vinyl alcohol) or methyl cellulose. This membrane, isolated by dissolving the PVC in tetrahydrofuran and measured at 0.01—0.02-p.m thick, has been found to be a graft copolymer of polyvinyl chloride and poly(vinyl alcohol) (4,5). Early in the polymerization, particles of PVC deposit onto the membrane from both the monomer and the water sides, forming a skin 0.5—5-p.m thick that can be observed on grains sectioned after polymerization (4,6). Primary particles, 1 p.m in diameter, deposit onto the membrane from the monomer side (Pig. 1), whereas water-phase polymer, 0.1 p.m in diameter, deposits onto the skin from the water side of the membrane (Pig. 2) (4). These domain-sized water-phase particles may be one source of the observed domain stmcture (7). 

Mass-polymerized PVC also has a skin of compacted PVC primary particles very similar in thickness and appearance to the suspension-polymerized PVC skin, compared in Figure 3. However, mass PVC does not contain the thin-block copolymer membrane (7). 

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