Pearl polymerization

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. 

Water insoluble monomers such as vinyl chloride may be polymerized as suspended droplets (10-1000 nm in diameter) in a process called suspension (pearl) polymerizations. Coalescence of droplets is prevented by the use of small amounts of water-soluble polymers, such as PVA. The suspension process is characterized by good heat control and ease of removal of the discrete polymer particles. 

Polymerization in suspension (bead or pearl polymerization) under normal pressure in the range from 60 to 80°C operates with a suspension of globules of an oil-soluble monomer in water and uses a monomer soluble catalyst. Substantial quantities of polystyrene and poly vinyl acetate are made by this method. 

Suspension Polymerization. The suspension or pearl polymerization process has been used to prepare polymers for adhesive and coaling applications and for conversion to poly(vinyl alcohol). Suspension polymerization are carried out with monomer-soiubie initiators predominantly, with low levels of stabilizers Continuous tubular polymerization of vinyl acetate in suspension yields stable dispersions of beads with narrow particle size distributions at high yields. 

The monomer can be polymerized either directly, that is undiluted (block or substance polymerization), or in the presence of a non-polymerizable solvent (solvent polymerization). In the first case there is a problem with dissipating the localized heat of reaction (traces of decomposition products result from overheating) and in the second case the solvent must be completely removed. Other possibilities are to suspend the monomers in dispersions, e.g. in water (suspension or pearl polymerization), or eventually using an emulsifier (emulsion polymerization). Emulsifiers and dispersants are considered to be processing aids for the production of polymers. 

There are difficulties involving coalescence of particles, however, so a variety of additives (protective colloids, etc,) are used to stabilize the droplets. The beads or monomer droplets in such a suspension polymerization (sometimes also called bead or pearl polymerization) are usually abont 5 mm in diameter and require the mechanical energy of stirring to maintain their integrity. If the stirring is stopped, a gross phase separation into two layers occurs. 

The cation-exchange resins obtained from SDVB copolymers prepared by conventional pearl polymerization technique mostly have the gel structure. The conventional SDVB copolymer has been modified by using a solvent or a linear.polymer in the monomer mixture during the polymerization to make porous SDVB copolymers. 

The copolymerization of acrylic acid or methacrylic acid and a crosslinking agent such as DVB or ethylene dimethacrylate gives weak acid cation-exchange resins. The pearl polymerization technique can be used if esters instead of water soluble acids are copolymerized... 

For values of a between 1 and 10, the character of the polymerization kinetics is intermediate between that of emulsion and that of solution polymerization. This is the region of suspension or pearl polymerization, where the rate and degree of polymerization are somewhat higher than for reaction in solution. The value... 

An interesting method for producing plastic microspheres is disclosed by Bayer AG (13). The microspheres consist of vinyl chloride-ethylene copolymers. This method involves the use of the difference of monomer reactivity between vinyl chloride and ethylene. Upon radical polymerization of the two monomers at 50°C, vinyl chloride reacts faster than does ethylene, and unreacted monomeric ethylene then remains in the resulting copolymer jn-oduced by the pearl polymerization. The monomer can then act as the blowing agent. 

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... 

The catalysts found to be most effective for pearl polymerizations are of the peroxidic type and are soluble in the monomer. This fact, together with data on the energy of activation for the initiation reaction (found to be approximately 26,000 cal per mole) and the relationship of the over-all consumption of monomer and the average molecular weight of the polymer to the square root of the catalyst concentration c, points to a bulk... 

Pearl polymerization is industrially carried out in closed kettles, provided with a condenser and with an efficient stirring mechanism. The finished product is separated from the liquid by sieving procedures, then washed to remove the suspension stabilizer, and finally dried. 

Copolymers containing mostly aciylonitrile are made in much the same manner as described for the homopoiymer. If acrylonitrile is present as a minor component, the copol3U]ier may be soluble in the monomer mixture. This is the case in preparation of aciylonitrile-styiene molding compounds containing 25-30 per cent acrylonitrile, and in this event, it may be advantageous to copoiymerize the monomers in bulk or as beads. As the amount of acrylonitrile in a copolymer is reduced, it usually becomes easier to carry out typical emulsion or suspension (pearl] polymerization. 

The styrene-DVB copolymer beads are prepared by suspension (pearl) polymerization technique. The monomers are mixed and a polymerization catalyst such as benzoyl peroxide is added. The mixture is then dispersed into small droplets in a thoroughly agitated aqueous solution that is kept at a temperature required for polymerization (usually 85°C-100°C). A suspension stabilizer (gelatin, polyvinyl alcohol, sodium oleate, magnesium silicate, etc.) in the aqueous phase prevents agglomeration of the droplets. The size of the droplets depends chiefly on the stabilizer, the viscosity of the solution, and the agitation, and it can be varied within wide limits. As polymerization takes place, the droplets are transformed into polymer beads. For most purposes, a bead size of 0.1-0.5 mm is preferred, but beads from 1 pm to 2 mm in diameter can be prepared without much difficulty. 

Weak-acid, cation exchange resins are prepared by copolymerization of an organic acid or add anhydride and a cross-linking agent. As a rule, acrylic or methacrylic acid is used in combination with divinyl benzene, ethylene dimethacrylate, or similar compounds with at least two vinyl groups. The pearl polymerization technique described above can be used if esters instead of the water-soluble acids are polymerized. The esters are hydrolyzed after polymerization. The final products have ionic contents of 9-10 eq/kg (dry). Resins of this type are Amberlite IRC-50, Duolite CS-101, Permutit H-70, and Wofatit CP-300. 

Through agitating or stirring with the aid of a dispersing agent, water-insoluble monomers can be dispersed in water as fine droplets of 0.001-1 cm diameter, which, together, would make up the reaction space available to a bulk polymerization. Because of dispersion in water, the heat of polymerization can be more readily dissipated. Oil-soluble free radical initiators start polymerization in the droplets. The polymerization proceeds mechanistically like a bulk polymerization, but behaves, as it were, as a water-cooled bulk polymerization. After polymerization, the droplets will have been converted into beads or pearls, and, because of this, the method is also called pearl polymerization. 

Pearl polymerization See granular polymerization and suspension and bead polymerization. 

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