Rate constant emulsion polymerization

Because of its overriding importance in determining the rate of emulsion polymerization, the factor discussed first is the number of particles formed. The mechanism of particle formation described above has been treated quantitatively by Smith and Ewart (58) on the basis of some simplifying assumptions. The most important of these is that a soap molecule occupies the same interfacial area whether it forms part of a micelle or is adsorbed on the polymer-water interface— i.e., during the period of particle formation, the total surface area of micelles plus polymer particles is constant. 

If Rpp is constant and the number of particles per unit volume, Np, is constant, then the overall rate of emulsion polymerization per unit volume. 

Problem 6.50 The experimental value of dynamic concentration of styrene in polymer latex particles under the conditions of constant rate in emulsion polymerization has been found to be 5.2 mol/liter. Assuming this value to be applicable, calculate the rate of polymerization per liter of aqueous phase in stage II of the reaction of the emulsion polymerization recipe given in Problem... 

The rate of emulsion polymerization of styrene at 60°C during the constant rate period (stage II) is 5.6x10 mol/cm -min and the number of M/P particles is 1.40x10 per cm . Taking kp from Table 6.7, calculate the dynamic concentration of monomer in particles under these conditions. 

Smith and Ewart (13a. 13b) quantified the Harkins theory by the equation R = k MpN/2 where Rp is the rate of propagation, kp is the rate constant for propagation, M is the monomer concentration in growing chain particles, and N the number of polymer particles per unit volume. If M is the constant, this equation is reduced to R = k N. Thus, the rate of emulsion polymerization should solely be a function of the number of polymer particles. In actuality, the reaction rate increases up to 20-25% conversion because of the increase in the number of growing radical chains then the rate steadies as does the number of polymer particles up to 70-80% conversion. Beyond this point, the rate drops off because of low monomer concentration. Thus, as Talamini (13c. 13d) has noted, available evidence indicates that emulsion polymerization of vinyl chloride does not resemble true emulsion polymerization as described by Smith and Ewart, but shows the general behavior of heterogeneous polymerization. 

When conversion is determined gravimetrically. Interval I may be completed before the first sample is taken. When rates of emulsion polymerization are discussed it is the constant rate fi-equently observed during Interval II which is meant. Most commonly, practically all the emulsifier is adsorbed on the surface of the latex particles at the end of Interval I then the surface concentration of adsorbed emulsifier decreases as the latex particles grow. Soap titration [67], in which the volume of a standard emulsifier solution required to reduce the surface tension of the latex to the value characterizing the presence of micellar soap is determined can be used to determine the final surface concentration of emulsifier. The surface-average particle size can be calculated from a knowledge of the amount of surfactant adsorbed at this point and the area occupied at the interface by a surfactant molecule in a saturated monolayer. This area should... 

Because the monomer concentration [M] in the aqueous phase is constant (saturated solution) and practically there is no influence on its concentration in micelles, the rate of emulsion polymerization depends only on the number of grains of produced pol5mier. However, the number of grains of polymer depends on emulsifier used and on initiator concentrations. Greater is the number of free radicals of initiator greater is the number of primary spores of polymerization. 

Emulsion polymerization of vinyl chloride is initiated by a water-soluble initiator such as potassium persulfate. Initially in the reactor, monomer droplets are dispersed in the aqueous phase (continuous phase) containing initiator and surfactant (emulsifier). As the reactor content is heated, the initiator decomposes into free radicals. When the surfactant concentration exceeds the critical micelle concentration (CMC), micelles are formed. Free radicals or oligomers formed in the aqueous phase are then captured by these micelles. Vinyl chloride monomer is slightly soluble in water. As the monomer dissolved in water diffuses into micelles containing radicals, polymerization occurs. With an increase in monomer conversion in the polymer particles, separate monomer droplets become smaller and eventually they disappear. The monomer concentration in polymer particles is constant as long as liquid monomer droplets exist. The rate of emulsion polymerization is represented by... 

The rate of an ideal emulsion polymerization is given by Eqn (4). In this expression [/] is the initiator concentration, [ ] is the emulsifier concentration, and [M] is the concentration of monomer within the forming latex particles. This value is constant for a long reaction period until all the monomer droplets disappear within the water phase. 

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

Propagation constants for butadiene and isoprene were determined from rate of polymerization per particle in emulsion polymerization. 

Since the same propagation rate constant applies to both bulk and emulsion polymerization, comparable rates of polymerization R must obtain when the number of emulsion particles is twice the number of radicals at the steady state in the bulk polymerization. An increase in the bulk rate at the given temperature can only be realized by an increase in the rate of initiation and, thus, an increase in the... 

Quantitatively compare the rate and degree of polymerization of styrene polymerized in bulk at 60°C with an emulsion polymerization (case 2 behavior h — 0.5) containing 1.0 x 1015 polymer particles per milhhter. Assume that [M] = 5.0 molar, R, = 5.0 x 1012 radicals per milliliter per second, and all rate constants are the same for both systems. For each polymerization system, indicate the various ways (if any) by which the polymerization rate can be affected without affecting the degree of polymerization. 

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