Polyacrylamide dispersion polymerization

In the polyacrylamide dispersion polymerization process, multivalent salts are used for the purpose of insolubilizing and depositing the polymer and are formulated to produce a poor solvent for the particular polyacrylamide being synthesized. At the onset of polymerization reaction, the formed polymer is soluble in the polymerization medium. As the reaction progresses, the polymer phase separates forming particles (0.1-10 pm) that are kept from agglomeration by the use of polymeric stabilizers. A mechanistic hypothesis of the formation of polyacrylamide dispersions was reported by Selvarajan [43]. 

Prepolymers of this type can also act as inisurfs for dispersion polymerizations in organic solvents by using prepolymers soluble in the particualr solvent. This can be determined by appropriate choice of the monomer in the polymerizations forming the prepolymer. So for instance the vinyl acetate prepolymer can be used to obtain stable polyacrylamide dispersions with a solid content of upto 50% by polymerization in methanol [47]. Another application of this type of polymeric azoinitiator mentioned very briefly is the preparation of graft and block copolymers [55,5Q. The chemical composition of the blocks as well as the polymerization technique employed can be matched over a wide range to obtain polymers with desired properties. 

Disporsion Poiymorization. Water-in-oil emulsions contain at least 30 wt% of a petroleum-based hydrocarbon that is a valuable natural resource. By using such formulations, oils are consumed mmecessarily and can enter the world s waterways as a source of secondary pollution. An aqueous polymer dispersion is one environmentally responsible formulation that contains no oil or surfactant, and near-zero amoimts of volatile organic compounds. Dispersion polymerization can be used to prepare cationic, anionic, and nonionic polyacrylamides. 

The most commonly used combination of chemicals to produce a polyacrylamide gel is acrylamide, bis acrylamide, buffer, ammonium persulfate, and tetramethylenediarnine (TEMED). TEMED and ammonium persulfate are catalysts to the polymerization reaction. The TEMED causes the persulfate to produce free radicals, causing polymerization. Because this is a free-radical driven reaction, the mixture of reagents must be degassed before it is used. The mixture polymerizes quickly after TEMED addition, so it should be poured into the gel-casting apparatus as quickly as possible. Once the gel is poured into a prepared form, a comb can be appHed to the top portion of the gel before polymerization occurs. This comb sets small indentations permanently into the top portion of the gel which can be used to load samples. If the comb is used, samples are then typically mixed with a heavier solution, such as glycerol, before the sample is appHed to the gel, to prevent the sample from dispersing into the reservoir buffer. 

Suspension polymerization of water-insoluble monomers (e.g., styrene and divinylbenzene) involves the formation of an oil droplet suspension of the monomer in water with direct conversions of individual monomer droplets into the corresponding polymer beads. Preparation of beaded polymers from water-soluble monomers (e.g., acrylamide) is similar, except that an aqueous solution of monomers is dispersed in oil to form a water-in-oil (w/o) droplet suspension. Subsequent polymerization of the monomer droplets produces the corresponding swollen hydrophilic polyacrylamide beads. These processes are often referred to as inverse suspension polymerization. 

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. 

By performing in situ the polymerization of acrylamide in water/AOT/toluene microemulsions, clear and stable inverse latexes of water-swollen polyacrylamide particles stabilized by AOT and dispersed in toluene have been found [192-194], It was shown that the final dispersions consist of two species of particles in equilibrium, surfactant-coated polymer particles (size about 400 A) with narrow size distribution and small AOT micelles (size about 30 A). 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

If first a concentrated emulsion of w/o in which the dispersed medium is an aqueous solution of polyethylene oxide) is prepared, followed by the addition of acrylamide and V,iV -methylenebisacrylamide and polymerization, a polyacrylamide shell that encapsulates the polyethylene oxide) can be generated [44]. This reservoir could be used in the controlled release of drugs. Indeed, if a... 

Finally, the preparation of nonaqueous polymer dispersions has been used as a technique for polymer production. Process advantages such as the use of lower temperatures for polyamide and polyester synthesis can be obtained [3.70]. Products such as synthetic elastomers can be obtained in powder form [3.104]. The fine particulate form of dispersions of water-soluble polymers (e.g., polyacrylamide) offers a convenient route to aqueous solutions of polymeric flocculants and thickeners [3.105]. 

Guha, S. Ray, B. Mandal, B.M. Anomalous solubility of polyacrylamide prepared by dispersion (precipitation) polymerization in aqueous tert-butyl alcohol. J. Polym. Sci. A 2001, 39 (19), 3434-3442. 

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). 

Liquid polyacrylamides are available as solutions, inverse (water-in-oil) emulsions, or dispersions. Dry polyacrylamides are available as powders from a dried gel or as beads from a water-in-oil suspension process. Polyacrylamides are formed from the radical chain polymerization of aCTylamide with cationic or aitioitic monomers, which is a highly exothermic... 

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