Bead polymerization

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 [Lm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reaciors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCh solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

Beaded polymeric support, whether polystyrene-divinylbenzene, polymethacrylate, or polyvinyl alcohol, is conventionally produced by different variations of a two-phase suspension polymerization process, in which liquid microdroplets are converted to the corresponding solid microbeads (1). 

Beaded polymeric supports are produced by a two-phase suspension polymerization in which microdrops of a monomer solution are directly converted to the corresponding microbeads. The size of a microdroplet is usually determined by a number of interrelated manufacturing parameters, which include the reactor design, the rate of stirring, the ratio of the monomer phase to water, the viscosity of both phases, and the type and concentration of the droplet stabilizer. 

It will be apparent that if the particle is larger in volume by several orders of magnitude than the size considered above, it may then accommodate two or more radicals without immediate termination. The typical characteristics of emulsion polymerization vanish, and we have what is often referred to as bead polymerization, which resembles ordinary bulk polymerization in its kinetics. 

Uses. Comonomer for preparation of cross-linked polymers in production of ion exchange beads and gel permeation chromatography polystyrene beads polymerization monomer for synthetic rubber, drying oils, and casting resins... 

Polymerization of Vinyl Acetate in Suspension (Bead Polymerization)... 

As described later, liquid-liquid reactors are mechanically agitated in order to achieve a good dispersion and large interfacial area between two immiscible liquids. The increase in interfacial area due to stirring enhances the reaction rate (e.g., saponification, bead polymerization, etc.). It should be noted that the interfacial area is also increased by the addition of surfactants. This process is called emulsification and is governed by completely different principles than the ones described here. 

Tg taken as midpoint in baseline transition at a heating rate of 10°C/min. b Inherent viscosity, (lnijr i)/c, where c = 0.25 g/dL, in benzene. c Rohm and poly (methyl methacrylate). Plexiglas type V-811. d Prepared by radical suspension bead polymerization by J. L. Tucker. Dissolved in benzene, precipitated, and freeze dried from benzene. Numbers in parentheses are the percentages of styrene determined by NMR. 

Returning to Vollmert s patent (18), we apply the more complete spectrum of operations to his example 1. In this case butyl acrylate and acrylic acid are dissolved in acetone with azoisobutyronitrile (initiator) and dodecyl mercaptan (chain transfer agent), polymerized, and the acetone is evaporated to form polymer 1. Separately, styrene and 1,4-butanediol monoacrylate are bead polymerized with benzoyl peroxide to form polymer 2. Polymers 1 and 2 are mechanically blended with the simultaneous addition of 1,4-butanediol followed by heating to promote grafting and crosslinking. These steps can be represented in some detail by ... 

Segregated Flow A real example is bead polymerization of styrene and some other materials. The reactant is in the form of individual small beads suspended in a fluid and retarded from agglomeration by colloids on their surfaces. Accordingly, they go through the reactor as independent bodies and attain conversions under batch conditions with their individual residence times. This is called segregated flow. With a particular RTD, conversion is a maximum with this flow pattern. The mean conversion of all the segregated elements then is given by... 

Suspension polymerizalion is also known as pearl or bead polymerization. Ki-netically, suspension polymerizations are water-cooled bulk reactions. Monomer droplets with dissolved initiator are dispersed in water. As the polymerization proceeds the droplets become transformed into sticky, viscous monomer-swollen particles. Eventually, they become rigid particles with diameters in the range of about (50-500) 10 cm. The final reaction mixture typically contains 25-50% of polymer dispersed in water. The viscosity of the system remains fairly constant during the reaction and is determined mainly by the continuous water phase. 

Microporous (gel-type) substrates are prepared by bead polymerization. In this process, the two monomeric compounds, styrene and divinylbenzene, are suspended as small droplets in water by rapid, even stirring. The polymerization is initiated by appropriate catalysts and leads to uniform particles, which have a microporous structure and a size distribution depending on the stirring speed. In general, a decrease of the average particle size is observed with increasing stirring speed. 

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