Suspension polymerization phase inversion

Beaded acrylamide resins (28) are generally produced by w/o inverse-suspension polymerization. This involves the dispersion of an aqueous solution of the monomer and an initiator (e.g., ammonium peroxodisulfates) with a droplet stabilizer such as carboxymethylcellulose or cellulose acetate butyrate in an immiscible liquid (the oil phase), such as 1,2-dichloroethane, toluene, or a liquid paraffin. A polymerization catalyst, usually tetramethylethylenediamine, may also be added to the monomer mixture. The polymerization of beaded acrylamide resin is carried out at relatively low temperatures (20-50°C), and the polymerization is complete within a relatively short period (1-5 hr). The polymerization of most acrylamides proceeds at a substantially faster rate than that of styrene in o/w suspension polymerization. The problem with droplet coagulation during the synthesis of beaded polyacrylamide by w/o suspension polymerization is usually less critical than that with a styrene-based resin. 

An inverse suspension polymerization involves an organic solvent as the continuous phase with droplets of a water-soluble monomer (e.g., acrylamide), either neat or dissolved in water. Microsuspension polymerizations are suspension polymerizations in which the size of monomer droplets is about 1 pm. 

Moreover, a magnetic molecularly imprinted polymer of 4-divinylpyridine and EGDMA particles was synthesized by inverse suspension polymerization [119]. The reaction was carried out in silicon oil as a dispersion phase, and in the presence of 2,4-dichlorophenoxyacetic acid and MPTS. The advantages of the silicon oil as a continuous phase are a low polarity and immiscibility with the monomer mixture. The prepared particle average size is 20 pm and the magnetic content is very low, at around 1 wt%. 

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. 

When a water-miscible polymer is to be made via a suspension process, the continuous phase is a water-immiscible fluid, often a hydrocarbon. In such circumstances the adjective inverse is often used to identify the process [118]. The drop phase is often an aqueous monomer solution which contains a water-soluble initiator. Inverse processes that produce very small polymer particles are sometimes referred to as inverse emulsion polymerization but that is often a misnomer because the polymerization mechanism is not always analogous to conventional emulsion polymerization. A more accurate expression is either inverse microsuspension or inverse dispersion polymerization. Here, as with conventional suspension polymerization, the polymerization reaction occurs inside the monomer-containing drops. The drop stabilizers are initially dispersed in the continuous (nonaqueous phase). If particulate solids are used for drop stabilization, the surfaces of the small particles must be rendered hydrophobic. Inverse dispersion polymerization is used to make water-soluble polymers and copolymers from monomers such as acrylic acid, acylamide, and methacrylic acid. These polymers are used in water treatment and as thickening agents for textile applications. Beads of polysaccharides can also be made in inverse suspensions but, in those cases, the polymers are usually preformed before the suspension is created. Physical changes, rather than polymerization reactions, occur in the drops. Conventional stirred reactors are usually used for inverse suspension polymerization and the drop size distribution can be fairly wide. However, Ni et al. [119] found that good control of DSD and PSD could be achieved in the inverse-phase suspension polymerization of acrylamide by using an oscillatory baffled reactor. 

Commercially, suspension polymerization has been hmited to the free-radical addition of water-insoluble liquid monomers. With a volatile monomer such as vinyl chloride, moderate pressures are required to maintain it in the hquid state. It is possible, however, to perform inverse suspension polymerizations with a hydrophilic monomer or an aqueous solution of a water-soluble monomer suspended in a hydrophobic continuous phase. 

As with suspension polymerization, commercial emulsion polymerization has pretty much been restricted to the free-radical solution addition of water-insoluble, liquid monomers (with volatile monomers such as butadiene and vinyl chloride, moderate pressures are required to keep them in the liquid phase). Inverse emulsion polymerizations, with a hydrophilic monomer phase dispersed in a continuous hydrophobic base are, however, possible. 

PAA can be prepared using bulk polymerization, aqueous polymerization, nonaqueous polymerization, inverse phase emulsion and suspension polymerization. The precise structure of the resulting PAA chain is dependent upon many factors including the polymerization process and conditions. The tacticity of... 

As a means to improve the rubber utilization, a bulk/suspension process evolved, whereby polybutadiene rubber was dissolved in styrene monomer and polymerized in bulk beyond phase inversion before being dropped into suspension. The HIPS produced this way had two distinct advantages over the compounded version styrene to rubber grafting and discrete rubber spheres or particles uniformly dispersed in a polystyrene matrix. This improved the impact strength dramatically per unit of rubber and gave better processing stability, because the rubber phase was dispersed instead of being co-continuous with the polystyrene. 

Today, HIPS is produced by two basic variants the batch process and the continuous process. Pre-polymerization, i.e. the polymerization phase up to completion of phase inversion, is identical in the two process variants. After completion of the pre-polymerization, the polymerization is continued in suspension in the batch process and in solution in the continuous process. The batch process is, therefore, also referred to as the bulk suspension process and the continuous variant as the solution process. The continuous process is a refinement of the original I.G. Farben process for standard polystyrene, which The Dow Chemical Company has adapted to the needs of rubber-containing styrene solutions. A number of modifications are now practiced. 

After completion of phase inversion, the polymer solution is dispersed in a 2-4-fold amount of water. Suspension aids used are water-soluble organic polymers, such as poly(vinyl alcohol) or polyvinylpyrrolidone, or inorganic compounds, such as Pickering systems. In order to achieve a final conversion of 99.5 %, initiator combinations with different decomposition times are used, and the polymerization follows a defined temperature-time profile. The suspension is then centrifuged, dried and compounded. 

The preparation of ASA in bulk or bulk-suspension polymerization processes has been described by McKee et al. [14-18]. The system is similar to that used in the preparation of high-impact polystyrene (HIPS) and in bulk-produced ABS. Thereby the rubber was prepared using free radical polymerization, dissolved in the SAN monomers which were then polymerized using free radicals. Phase separation between the rubber and PSAN occurred, followed by phase inversion. 

Inverse Emulsion Polymerization. Water-soluble monomers can be polymerized by emulsifying water solutions of these monomers in an organic continuous phase. This process, called inverse emulsion polymerization, yields a product comprised of a colloidal suspension of droplets of aqueous polymer solution. The original study of an inverse system by Vanderhoff et al. (20) involved the monomer sodium p-vinyIbenzene sulfonate, an organic phase of xylene. Span 60 as the emulsifier, and either benzoyl peroxide or potassium persulfate initiator. Later work by Kurenkov et al. (21) involved acrylamide in a toluene continuous phase, potassium persulfate, and Sentaraid-5 (emulsifier). DiStefano (22) examined three monomers acrylamide, dimethylaminoethyacrylate... 

Impact polystyrene is produced commercially in three steps solid polybutadiene rubber is cut up and dispersed as small particles in styrene monomer mass prepolymerization and completion of the polymerization either in mass or aqueous suspension. During the prepolymerization step, styrene starts to polymerize by itself by forming droplets of polystyrene upon phase separation. When equal phase volumes are attained, phase inversion occurs (15). The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. Impact strength increases with rubber particle size and concentration, whereas gloss and rigidity decrease. 

Microsuspension and Inverse-microsuspension. In suspension polymerizations, particle formation occurs through a droplet breakup-coalescence mechanism, with the diameter controlled by the temperature, interfacial tension, agitation intensity and conversion. Suspension polymerizations have typically been characterized by an initiator soluble in the monomer phase and particle diameters in the 50-1000 pm range [40]. Smaller particles (0.2-20 pm) have been produced at higher agitation speeds (lower interfadal tensions) [41] and in such cases a prefix micro has been added to the nomenclature (microsuspension) to reflect both the dominant synthesis conditions (suspension) and the nominal particle size (1 micron). Therefore, microsuspension polymerization has historically referred to a subdomain of suspension polymerization occurring at smaller particle sizes. Based on an analogy to this nomenclature, inverse-microsuspension polymerization has been proposed for similar water-in-oil... 

Inverse-suspension, inverse-emulsion and inverse-microemulsion polymerizations should be developed independently as has been the precedent for oil-inwater polymerizations. This includes explicitly considering the unique chemistry of various emulsifiers, organic phases, monomers and initiators. Furthermore, the chemical and colloidal models for each of the three water-in-oil polymerizations will be specific to a given type of organic phase and a restricted family of emulsifiers. 

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