Dispersion polymerization initiator concentration

The role of initiator concentration on dispersion polymerization can be summarized as follows. The in-... 

Ftgure 11 The electron micrographs of the final products and the variation of the monomer conversion with the polymerization time at different initiator concentrations in the dispersion polymerization of styrene. Initiator concentration (mol%) (a) 0.5, (b) 1.0, (c) 2.0. The original SEM photographs were taken with 2600 x, 2000 x, and 2600 x magnifications for (a), (b), and (c), respectively, and reduced at a proper ratio to place the figure. (From Ref. 93. Reproduced with permission from John Wiley Sons, Inc.)... 

The same PVP series were also tried for the dispersion polymerization of styrene in the ethanol medium by using AIBN as the initiator and aerosol OT as the costabilizer [84]. PVP K-15 usually yielded polymeric particles with a certain size distribution and some coagu-lum. The uniform products were obtained with PVP K-30 and PVP K-90 in the presence of the costabilizer. The tendencies for the variation of the final particle size with the stabilizer concentration and with the molecular weight of the stabilizer were consistent with those obtained for the dispersion polymerization of methyl methacrylate [84],... 

We have also examined the effect of stabilizer (i.e., polyacrylic acid) on the dispersion polymerization of styrene (20 ml) initiated with AIBN (0.14 g) in an isopropanol (180 ml)-water (20 ml) medium [93]. The polymerizations were carried out at 75 C for 24 h, with 150 rpm stirring rate by changing the stabilizer concentration between 0.5-2.0 g/dL (dispersion medium). The electron micrographs of the final particles and the variation of the monomer conversion with the polymerization time at different stabilizer concentrations are given in Fig. 12. The average particle size decreased and the polymerization rate increased by the increasing PAAc concentra-... 

We have studied the effect of monomer concentration in the dispersion polymerization of styrene carried out in alcohol-water mixtures as the dispersion media. We used AIBN and poly(acrylic acid) as the initiator and the stabilizer, respectively, and we tried isopropanol, 1-butanol, and 2-butanol as the alcohols [89]. The largest average particle size values were obtained with the highest monomer-dispersion medium volumetric ratios in 1-butanol-water medium having the alcohol-water volumetric ratio of 90 10. The SEM micrographs of these particles are given in Fig. 15. As seen here, a certain size distribution by the formation of small particles, possibly with a secondary nucleation, was observed in the poly-... 

Lu et al. [86] also studied the effect of initiator concentration on the dispersion polymerization of styrene in ethanol medium by using ACPA as the initiator. They observed that there was a period at the extended monomer conversion in which the polymerization rate was independent of the initiator concentration, although it was dependent on the initiator concentration at the initial stage of polymerization. We also had a similar observation, which was obtained by changing the AIBN concentration in the dispersion polymerization of styrene conducted in isopropanol-water medium. Lu et al. [86] proposed that the polymerization rate beyond 50% conversion could be explained by the usual heterogenous polymer kinetics described by the following equation ... 

The water solubilities of the functional comonomers are reasonably high since they are usually polar compounds. Therefore, the initiation in the water phase may be too rapid when the initiator or the comonomer concentration is high. In such a case, the particle growth stage cannot be suppressed by the diffusion capture mechanism and the solution or dispersion polymerization of the functional comonomer within water phase may accompany the emulsion copolymerization reaction. This leads to the formation of polymeric products in the form of particle, aggregate, or soluble polymer with different compositions and molecular weights. The yield for the incorporation of functional comonomer into the uniform polymeric particles may be low since some of the functional comonomer may polymerize by an undesired mechanism. 

Microspheres by solvent extraction method were obtained with rate of mixing equal 300 rev/s. Particles by spray drying were produced with spray dryer operated with an inlet temperature of 50°C and outlet temperature of 45°C. The air flow indicator was set at 700 and the aspirator at 5. The polymer solution (concentration 0.5% wt/v) was supplied at 10 mL/min. The concentrations of monomer, initiator, and surfactant in ring-opening dispersion polymerization leading to microspheres were as follows [Lc]o = 2.77 10 mol/L, [tin(II) 2-ethyUiexanoate]o = 4.9 10 mol/L, [poly(DA-CL)] = 1.6 g/L. 

Exponents 0.6-0.8 obtained for the dependence of the rate of dispersion (co)polymerization or molecular weight on initiator concentration were discussed in terms of depressed termination (the first-order radical loss process) and variation of the surface activity of the formed graft copolymer with its molecular weight. The higher the surface activity of graft copolymer (or lower its molecular weight) the higher the particle number. 

Equation 4 was foimd to explain particle size data fairly well, with reasonable kinetic and coverage parameter values (k s and Sent), in the dispersion polymerization of styrene in ethanol with PVP dispersant [24]. Many other dispersion polymerization systems with homopolymer dispersants appear to be explained by Eq. 4, except for the frequently observed direct particle size dependence on initiator concentration [27]. 

Polymerization Results. Preliminary polymerization runs were conducted to evaluate the effect of Initiator concentration, temperature, and continuous-phase density on the rate of reaction as well as the ultimate molecular weight of the polymer. Continuous-phase density could be varied in two ways 1) by varying the pressure at constant temperature and ethane/propane ratio, and 2) by varying the ethane/propane ratio at constant temperature and pressure. In all of these polymerizations, the acrylamide ratio was 1.0, water was 3.5, and the total dispersed-phase volume fraction was 0.16. 

Polymer Synthesis. Copolymers of alkylacrylamide (R) and acrylamide (AM), which we called RAM, were prepared with a micellar polymerization technique (4). A micellar surfactant solution was used to disperse the hydrophobic alkylacrylamide monomer into an aqueous phase that contained acrylamide. The monomers were polymerized with a standard free-radical initiator (e.g., potassium persulfate) or a redox initiator to yield the desired random copolymer. Varied temperature and initiator concentrations were used to provide polymers of different molecular weights. Polymerizations were taken to essentially complete conversion. Compositions, in terms of hydrophobe level reported in this chapter, were based on amounts charged to the reactor. Further details on the synthesis and structure of these RAM polymers... 

Among the observable facts it was found that there is no significant effect of the concentration of emulsifier on this system. Therefore, the implication is that the polymerization initially takes place exclusively in the aqueous phase [136]. The resulting polymer particle precipitates as it forms [134]. In this case we may assume, that only a microscopic phase-separation takes place. The polymer particles which form adsorb emulsifier fiom the aqueous environment and remain dispersed. Then the particles may absorb more monomer somewhat in the manner called for by the Smith-Ewart theory. Of course, other dissolved vinyl acetate monomer molecules may continue to be polymerized in aqueous solution, thus accounting for the increase in the number of particles as the polymerization proceeds to high conversion. The classical Smith-Ewart treatment states that the number of particles is determined by the surfactant to monomer ratio and, in effect remains constant throughout the process. 

When water-soluble initiators are used, most of the authors concluded that acrylamide polymerization proceeds within the monomer droplets, irrespective of the nature of the organic phase (aromatic or aliphatic) [28,30-34], Both monomer and initiator reside in the dispersed droplets and each particle acts as a small batch reactor. The process is essentially a suspension polymerization and therefore the kinetics resemble those for solution polymerization. Note that a prefix micro has been added in some cases to this type of polymerization (microsuspension) to emphasize the smallness of the reactor (d 1 pm) and the possibility of interfacial reactions [33]. A square root dependence of the polymerization rate, / p, on initiator concentration, [I] was often observed, in good accord with solution polymerization [28,32-34]. Higher orders were also found which were attributed to chain transfer to the emulsifier [30]. The reaction order with respect to monomer was found to vary from 1 [2832] to 1.7 [3031]> Orders higher than 1 are common for acrylamide polymerization in homogeneous aqueous solution and are explained by the occurrence of a cage effect [35]. 

The effects of initiator in a dispersion polymerization have not yet been adequately described by theory [35], but in practice an increase in the concentration of initians- leads to an increase in particle size in almost eveiy system so far studied [3]. Tuncel [26] et al. found that the increase occurred when styrene was polymerized using azobis(isobutyronitrile) (AIBN) as initiator in aqueous mixtures of propan-2-ol, butan-l-ol or butan-2-ol, although the magnitude of the effect varied with solvent ratio. Figure 22.5 shows the effect of increased initiator concentration in the styrene/aqueous ethanol system for a varieQr of initiators and stabilizers. 

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