Emulsion polymerization reaction intervals

Apart from intrinsic interest, the theoiy of compartmentalized free-radical polymerization reactions is of importance primarily because it is believed that most of the polymer which is form in the course of an emulsion polymerization reaction is formed via reactions of this type. The general sl pe of the conversion-time curve for many emulsion polymerization reactions suggests (see Fig. I) that the reaction occurs in three more-or-less distinct stages or intervals. The first of these, the so-called Interval I, is interpreted as the stage of polymerization in which the discrete reaction loci are formed. In the second and third stages—Intervals II and III—the polymerization is believed to occur essentially by compartmentalized free-radical polymerization within the loci which were formed during Interval I. 

The colloidal nature of the reaction media has a significant influence on the course of an emulsion polymerization reaction. A number of distinct phases exist during different intervals of a batch reaction. Chemical and physical phenomena within these phases and at the interfaces can be important in determining reaction kinetics and the properties of the latex product. 

Interval III Particle Growth in the Absence of Monomer Droplets.—James and Sundberg have published the results of an experimental study of ideal and non-ideal behaviour in the seeded emulsion polymerization of styrene. Unlike the experiments on seeded emulsion polymerization reported in papers referred to above, the amounts of monomer added to the seed latices were less than those required to saturate the particles and form a separate monomer droplet phase. The reaction systems were therefore the seed analogues of Interval III of a conventional emulsion polymerization reaction. The results are found to be in good agreement with the predictions of the Stockmayer-O Toole theory, provided that allowance is made for the effect of monomer/polymer ratio at the reaction locus upon the rate coefficient for bimolecular mutual termination. A paper by Hamielec and Marten is concerned with the effects of chain entanglements and the rubber-glass transition... 

It has been common practice for many years to regard the course of a conventional emulsion polymerization reaction as being divided into the following three more-or-less distinct intervals ... 

A typical batch emulsion polymerization reaction contains three distinct intervals, labeled Interval I, II and HI, such as reported in a pioneering paper by Harkins [277]. 

Fig. 1.24 The three intervals of a typical emulsion polymerization reaction, showing surfactant molecules ( "), large monomer droplets, micelle (indicated by clusters of surfactant molecules within Interval 1), radicals (R ), initiator (1) and surfactant-stabilized latex particles (Reprinted from Thickett and Gilbert [274]. Copyright 2009, with permission from Elsevier)...

In ordinary batch macroemulsion polymerization reactions, monomer macroemulsion droplets (tf = 1 -10 p.m) are in equilibrium with excess surfactant in the form of micelles d 0.01 xm). The emulsion polymerization reaction can be divided into three intervals (Figure 9.14). In Interval I, nucleation of particles takes place by invasion of radicals from the aqueous phase into micelles or by precipitation of oligomer particles in the aqueous phase outside the particles. Macroemulsion droplets play little role in Interval I owing to the fact that they are of a large size... 

A variety of behaviors are observed for the polymerization rate versus conversion depending on the relative rates of initiation, propagation, and termination, which are in turn dependent on the monomer and reaction conditions (Fig. 4-2). Irrespective of the particular behavior observed, three intervals (I, II, III) can be discerned in all emulsion polymerizations based on the particle number N (the concentration of polymer particles in units of number of... 

In Fig. 8 the calorimetric curve of a typical miniemulsion polymerization for 100-nm droplets consisting of styrene as monomer and hexadecane as hydrophobe with initiation from the water phase is shown. Three distinguished intervals can be identified throughout the course of miniemulsion polymerization. According to Harkins definition for emulsion polymerization [59-61], only intervals I and III are found in the miniemulsion process. Additionally, interval IV describes a pronounced gel effect, the occurrence of which depends on the particle size. Similarly to microemulsions and some emulsion polymerization recipes [62], there is no interval II of constant reaction rate. This points to the fact that diffusion of monomer is in no phase of the reaction the rate-determining step. 

It is clear from Eq. 1 that the monomer concentration in a polymer particle is one of the three key factors that control the particle growth rate, and accordingly, the rate of polymerization. In emulsion polymerization, the course of emulsion polymerization is usually divided into three stages, namely. Intervals I, II and III. In Intervals I and II of emulsion homopolymerization, the monomer concentration in the polymer particles is assumed to be approximately constant. In Interval III, it decreases with reaction time. Two methods are now used to predict the monomer concentration in the polymer particles in emulsion homopolymerization empirical and thermodynamic methods. 

In contrast to emulsion polymerization, the reaction kinetics of microemulsion polymerization is characterized by two polymerization rate intervals the interval of constant rate characteristic of emulsion polymerization is missing [42,49,53], as shown in Fig. 2. Polymer particles are generated continuously during the reaction by both micellar and homogeneous mechanisms. As the solubility of the monomer in the continuous domain increases, homogeneous... 

The foregoing mechanism is amenable to mathematical analysis, with the salient results that during ideal interval II polymerization, the rate of reaction is proportional to and while DP depends on and [1] . (Here [I] is the initiator molar concentration and S is the weight concentration of surfactant.) In conventional solution, suspension, or low-conversion bulk free-radical reactions, the rate of polymerization depends on [1] / while DP is proportional to [I]". In these cases DPp cannot be increased at given [M] without decreasing Rp. In emulsion polymerization, however, both Rp and DP can be changed in parallel by controlling the soap concentration. 

A particular emulsion polymerization yields polymer with Mp = 500,000. Show quantitatively how you would adjust the operation of a semibatch emulsion process to produce polymer with = 250,000 in interval II without changing the rate of polymerization, reaction temperature, or particle concentration. 

Analytic solutions for Eq. (5) provide the most direct path of the prediction of PSD evolution. For batch polymerizations in Interval II, however, analytic solutions have only been achieved for the so-called zero-one system (Lichti et al., 1981). These are systems wherein negligibly few particles contain two or more free radicals because of the rapidity of the bimolecular termination reaction (e.g., in styrene emulsion polymerizations with small latex particles). In this case, Eq. (5) may be written as follows ... 

PSD in Interval II hatch reactions, the results of a sample calculation are provided (see Fig. 1). In this calculation for a zero-one system, it was assumed that p = 1, fe = 0. K = I and c > p, the units being aibitrarily, but appropriately, chosen. The initial condition selected, which is typical of a s cd emulsion polymerization system, was that... 

Semicontinuous emulsion polymerizations are characterized by the continued addition of monomer to the reaction vessel. This permits the production of latexes with high weight percentage solids while allowing the initial burst of nucleation to be achieved in substantially aqueous surroundings. The theory for semicontinuous systems is substantially that set forth for Interval III of batch polymerizations, except that the materials balance equations [Eq. (17)] must be modified to include the flow of new material into the reactor. The effect of the monomer input is twofold first, the mass of material present in the system is increased and seccmd, the concentration of other reagents may be reduced. 

Trommsdorff et al. (38) and Baxendale et d. (7, 8) observed a similar behavior in the catalytic emulsion polymerization of MM A the reaction was not studied dilatometrically, however. The conversion-time functions of Baxendale and coworkers do not show the acceleration effect as obviously as the curves of Trommsdorff or ours, probably because of long intervals between sampling. 

If monomer, initiator, surfactant and water are the only reactants (so-called ab initio emulsion polymerization), it is possible for particle nucleation and particle growth to proceed simultaneously for a significant period of the reaction, especially if the initiator concentration is low and the surfactant concentration is high. However, in most instances, the particle nucleation period in an nb initio batch process is short, thereby giving rise to distinct nucleation and growth periods. In such situations the conversion-time curves take on the classic shape showing the three intervals of emulsion polymerization (see Section 2.4. and Section 4.3 and Figure 4.3). 

---