Polymerization continued seeded

Deionized water (720 g), sodium lauryl sulfate (4.3 g), dioctanoyl peroxide (40 g), and acetone (133 g) were emulsified using an ultrasonic probe for 10 minutes. The step 1 polystyrene seed (48.0 g seed, 578 g latex) was added to the emulsion together with lauryl sulfate (0.8 g) and acetone (29.6 g). The mixture was transferred to a flask and left to agitate at approximately 25°C for 48 hours. Acetone was then removed and the solution added to a 5-liter double-walled glass reactor. The temperature was increased to 40°C while styrene (336 g) and divinyl benzene (0.88 g) were added drop-wise over approximately 60 minutes. After 4 hours the mixture was treated with deionized water (1200 g), potassium iodide (1.28 g), and polyvinyl pyrrolidone (18.48 g) with the temperature increased to 70°C. The polymerization continued for 6 hours at 70°C and 1 hour at 90°C. Styrene-based oligomer particles with a diameter of 1.7 pm and with a narrow size distribution were obtained. 

Core-shell polymerization is a seed particle polymerization variation of emulsion polymerization. The seed particles are suspended in the continuous phase. The pre-polymerization mixture of monomer, cross-linker, template and initiator is added to the particle suspension as an emulsion prepared in the continuous phase. The mixture is stirred until the polymerization has completed. The addition of pre-polymerization mixture is repeated several times until the spheres reach the desired size range. The beads formed are composed of a core (i.e. the seed particle) and a shell ofMIP [98, 99],... 

Composite latex particles of poly(n-butyl acrylate) ly(benzyl methacrylate) (PBA/PBM) [67] prepared by semi-continuous seeded onulsion polymerization in the presence of a chain transfer agent (isooctyl mercrptoproprionate) (lOMP) exhibited a hemispherical morphology or else laiger domains of PBM were... 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

The rate of polymerization with styrene-type monomers is directly proportional to the number of particles formed. In batch reactors most of the particles are nucleated early in the reaction and the number formed depends on the emulsifier available to stabilize these small particles. In a CSTR operating at steady-state the rate of nucleation of new particles depends on the concentration of free emulsifier, i.e. the emulsifier not adsorbed on other surfaces. Since the average particle size in a CSTR is larger than the average size at the end of the batch nucleation period, fewer particles are formed in a CSTR than if the same recipe were used in a batch reactor. Since rate is proportional to the number of particles for styrene-type monomers, the rate per unit volume in a CSTR will be less than the interval-two rate in a batch reactor. In fact, the maximum CSTR rate will be about 60 to 70 percent the batch rate for such monomers. Monomers for which the rate is not as strongly dependent on the number of particles will display less of a difference between batch and continuous reactors. Also, continuous reactors with a particle seed in the feed may be capable of higher rates. 

Mass transfer of monomer from the suspended drops through the aqueous phase to the seeded particles continues throughout the polymerization. 

There are many variations on this theme. Fed-batch and continuous emulsion polymerizations are common. Continuous polymerization in a CSTR is dynamically unstable when free emulsifier is present. Oscillations with periods of several hours will result, but these can be avoided by feeding the CSTR with seed particles made in a batch or tubular reactor. 

Preparation of latex Samples. Two-stage latex samples were prepared by emulsion polymerization of the second-stage monomer mix in the presence of the first-stage polymer latex. The first-stage latexes were either in-situ or separately made using an externally prepared polystyrene latex seed. The mode of polymerization was a semi-continuous process for both stages. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

This study of the continuous, tubular, seeded emulsion polymerization of vinyl acetate has led to the following conclusions ... 

Synthesis. A series of latexes was prepared by semicontinuous emulsion polymerization of methyl methacrylate. A dialkyl ester of sodium sulfosuccinic acid surfactant yielded the narrow particle size distribution required. An ammonium persulfate/sodium metabisulfate/ferrous sulfate initiator system was used. The initiator was fed over the polymerization time, allowing better control of the polymerization rate. For the smaller size latexes (200 to 450 nm), a seed latex was prepared in situ by polymerizing 10% of the monomer in the presence of the ammonium persulfate. Particle size was adjusted by varying the level of surfactant during the heel reaction. As the exotherm of this reaction subsided, the monomer and the sodium metabisulfate/ferrous sulfate feeds were started and continued over approximately one hour. The... 

Specific turbidity histories are also plotted vs. dimensionless time for a continuous emulsion polymerization run the samples were withdrawn from the second reactor of a continuous train where the first reactor is a small seeding reactor. Part A of Figure 3 shows the particle size behaviour during start up all monomer, water, initiator and soap feedrates were kept constant until the process reached a steady state. In part B, the soap concentration in the seed reactor was increased a decrease in the particle size was expected and it is clearly shown from the specific turbidity measurements. 

The process usually starts with the polymerization of a small proportion of the reagents at a very low monomer to water ratio (the seed stage), followed by the feeding of the remaining monomer (which may take several hours) and of other materials (if needed) once the conversion in the reactor has reached 70% or more. The in-reactor conversion will then depend upon the rate of polymerization compared to the rate of feed. If the reaction is continued under the so-called monomer-starved conditions, the in-reactor conversion is kept at a high 80-90%, which reduces the polymerization rate. To compensate, temperature is raised however, then the initiator depletes faster and more has to be added during the reaction. 

The dynamic swelling method (DSM) [10] has also been described for the preparation of crosshnked microspheres with free vinyl groups [78]. Therefore, polystyrene seed particles (1.9 pm) prepared by dispersion polymerization are dispersed in ethanol-water (7/3, w/w) containing divinylbenzene (DVB), benzoyl peroxide, and poly(vinyl alcohol) (PVA). The slow drop-wise addition of water to the mixture causes the DVB phase to separate, and it is continuously imbibed by seed particles to produce relatively large swollen particles (4.3 pm), which are then polymerized to afford the respective PS-PDVB composite particles with free vinyl groups. DSM has recently been developed in order to prepare hohow microspheres and various oddly-shaped polymer particles, including a rugby ball, red blood cells, or snowman structures [79-83]. 

Latexes. Latexes were made in a monomer addition recipe described earlier (10). This is a seeded continuous monomer addition recipe using t-butylhydro-peroxide/hydroxylamine hydrochloride redox couple as initiator. Polymerizations were carried out in stirred glass reactors at 50°C. The only variation in the original recipe was in the surfactants. In the present procedvire, < 1/3 of the soap (- 1.5% based on total monomer) was used in the seed and the remainder fed to the reactor during polymerization. The monomer feed contained styrene and butylacrylate in a 40/60 ratio. This composition was selected because it is readily filmforming and is not affected chemically by the electrodeposition process. The polymer remains soluble and... 

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