Emulsion polymerization monomer-starved conditions

Sajjadi, S. Particle formation under monomer-starved conditions in the semibatch emulsion polymerization of styrene. I. Experimental. J. Polym. Sci. Pt. A Polym. Chem. 2001, 39, 3940-3952. 

In order to take full advantage of semi-batch emulsion polymerization, it is necessary to use monomer-starved conditions. The following section is devoted to the reasons why these conditions are so important. Except where indicated otherwise, statements such as addition of monomer and monomer added simply mean... 

According to Hergeth and coworkers [55], a minimum surface of the inorganic particles is needed to prevent secondary nucleation. To estimate this amount, a formula was derived for seeded emulsion polymerization with spherical particles and a water-soluble initiator [55]. This formula was based on the observation that primary particles are produced by a coUapse and micellization process of oligomeric chains. An upper limit for the particle size was estimated to be 100 nm for the encapsulation of silica with polyvinyl acetate. A relatively water-soluble monomer is applied here for more hydrophobic monomers this upper limit will be higher. Because the surface area needed to prevent secondary nucleation is proportional to the monomer conversion per unit of time, the encapsulation efficiency can be improved by using monomer-starved conditions. So far, mainly submicrometer particles have been encapsulated with this method. The encapsulation of the larger filler particles... 

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

The core-shell particles were prepared by seeded semi-continuous emulsion polymerization under monomer-starved conditions. A detailed experimental procedure for similar latices is given by Winnik et al. [20] and Kruger et al. [21]. The seed was prepared by batch emulsion polymerization. After synthesizing the PBMA core a highly crosslinked PnBA-shell with MAA comonomer... 

Sorvik et al. [81, 82] carried out the emulsion polymerization of VC at subsaturated pressures. They reported that the reaction order with respect to monomer concentration is ca. 1.0. The polymerization proceeds under monomer-starved conditions in which the rate of polymerization is proportional to the monomer concentration at the reaction loci. The rate of polymerization linearly decreases with decreasing pressure (without any maximum). The dependence of polymerization rate on the equilibrium monomer concentration, Rp oc [M]cq can be expressed as the dependence on the monomer concentration in water because this dependence is a function of the saturation degree X and, therefore, we can write Rp oc [M]e,w or Rp oc [8.8X], respectively. 

Conversion curves for the systems with high BA concentrations are similar to those for the conventional emulsion polymerization (Fig. 9). In systems with high concentration of VC, no acceleration of polymerization is observed. The specific rate of butyl acrylate in the emulsion copolymerization of BA and VC increases with increasing BA concentration (Rp oc [BA] ). The rate of emulsion polymerization of BA under the same reaction conditions is proportional to [BA] °. The reaction order 1.0 is twice that obtained for the fractional rate of BA in the presence of VC. The high reaction order is due to the fall in the monomer concentration in particles with decreasing initial monomer concentration. Thus, at the low initial monomer concentration polymerization proceeds under the monomer-starved conditions and at high monomer concentration under monomer-saturated conditions. In contrast, the specific rate of VC... 

In summary, formation of particle nuclei from emulsified monomer droplets is almost certain to occur in any emulsion polymerization system in which these droplets are present. As mentioned earlier, however, monomer droplets containing polymer will primarily serve as reservoirs to provide monomer to the much more numerous and smaller latex particles formed by other particle nucleation mechanisms. Polymerization in monomer droplets can be eliminated or at least minimized by using seed polymer particles and slowly adding monomer (neat or as an emulsion) to supply the growing seed particles (i.e., seeded semibatch emulsion polymerization under the monomer-starved condition). 

Unzueta and Forcada [24] carried out semibatch emulsion copolymerizations of methyl methacrylate and n-butyl acrylate stabilized by mixed anionic and nonionic surfactants (sodium dodecyl sulfate and dodecyl polyethoxyl-ate) under the monomer-starved condition. The polymerization system stabilized by nonionic surfactant alone results in a slower rate of polymerization... 

Chern [42] developed a mechanistic model based on diffusion-controlled reaction mechanisms to predict the kinetics of the semibatch emulsion polymerization of styrene. Reasonable agreement between the model predictions and experimental data available in the literature was achieved. Computer simulation results showed that the polymerization system approaches Smith-Ewart Case 2 kinetics (n = 0.5) when the concentration of monomer in the latex particles is close to the saturation value. By contrast, the polymerization system under the monomer-starved condition is characterized by the diffusion-con-trolled reaction mechanisms (n > 0.5). The author also developed a model to predict the effect of desorption of free radicals out of the latex particles on the kinetics of the semibatch emulsion polymerization of methyl acrylate [43]. The validity of the kinetic model was confirmed by the experimental data for a wide range of monomer feed rates. The desorption rate constant for methyl acrylate at 50°C was determined to be 4 x 10 cm s ... 

Sajjadi [47] developed two mechanistic models for the particle nucleation process involved in the semibatch emulsion polymerization of styrene under the monomer-starved condition. In the first model, Smith-Ewart theory was extended to take into account the particle nucleation under the monomer-starved condition. The number of latex particles per unit volume of water is proportional to the surfactant concentration, the rate of initiator decomposition, and the rate of monomer addition, respectively, to the 1.0,2/3, and -2/3 powers. The second model considers the aqueous phase polymerization kinetics and its effect on the efficiency of free radical capture by the monomer-swollen micelles. This model is capable of predicting some features of the particle nucleation process. 

Materials A series of core-shell PBMA latex particles containing different amount of MAA groups in their shell were prepared by three-stage emulsion polymerization under monomer starved condition (Iml/hr) at 80 xhe latex dispersions were purified using a mixed... 

Bauer et al. describe the use of a noncontact probe coupled by fiber optics to an FT-Raman system to measure the percentage of dry extractibles and styrene monomer in a styrene/butadiene latex emulsion polymerization reaction using PLS models [201]. Elizalde et al. have examined the use of Raman spectroscopy to monitor the emulsion polymerization of n-butyl acrylate with methyl methacrylate under starved, or low monomer [202], and with high soUds-content [203] conditions. In both cases, models could be built to predict multiple properties, including solids content, residual monomer, and cumulative copolymer composition. Another study compared reaction calorimetry and Raman spectroscopy for monitoring n-butyl acrylate/methyl methacrylate and for vinyl acetate/butyl acrylate, under conditions of normal and instantaneous conversion [204], Both techniques performed well for normal conversion conditions and for overall conversion estimate, but Raman spectroscopy was better at estimating free monomer concentration and instantaneous conversion rate. However, the authors also point out that in certain situations, alternative techniques such as calorimetry can be cheaper, faster, and often easier to maintain accurate models for than Raman spectroscopy, hi a subsequent article, Elizalde et al. found that updating calibration models after... 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

Monteiro et al. have used a RATF Transurf in the ab initio emulsion polymerization of methyl methacrylate at 70° C. The Transurf was synthesized by esterifying a methyl methacrylate dimer with 1,10 decandiol followed by sulfonation. The authors found that only a small amount of Transurf was incorporated and suggested that, in order to increase the Transurf incorporation, the ratio of monomer to Transurf should be kept as low as possible, as achieved, e.g. in starved-feed conditions [12]. 

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