Radical desorption, emulsion

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

General. In this section, a mathematical dynamic model will be developed for emulsion homopolymerization processes. The model derivation will be general enough to easily apply to several Case I monomer systems (e.g. vinyl acetate, vinyl chloride), i.e. to emulsion systems characterized by significant radical desorption rates, and therefore an average number of radicals per particle much less than 1/2, and to a variety of different modes of reactor operation. 

Hayashi et al., 1989], involving the addition of monomer and initiator to a previously prepared emulsion of polymer particles, is especially useful for this purpose since it allows the variation of certain reaction parameters while holding N constant. Thus, h in seeded styrene polymerization drops from 0.5 to 0.2 when the initiator concentration decreases from 10-2 to 1CT5 M. At sufficiently low Ru the rate of radical absorption is not sufficiently high to counterbalance the rate of desorption. One also observes that above a particular initiation rate ([I] = lO-2 M in this case), the system maintains case 2 behavior with h constant at 0.5 and Rp independent of Ri. A change in Ri simply results in an increased rate of alternation of activity and inactivity in each polymer particle. Similar experiments show that h drops below 0.5 for styrene when the particle size becomes sufficiently small. The extent of radical desorption increases with decreasing particle size since the travel distance for radical diffusion from a particle decreases. 

We now report on some experiments using seeded emulsion polymerization of styrene in which conditions were carefully chosen to ensure that Smith-Ewart Case 2 kinetics (6) would obtain throughout, in the absence of chain transfer/radical desorption effects. Various hydrocarbons were investigated for their effects on kinetics of polymerization and equilibrium swelling of the latex particles. 

The desorption (exit) of free radicals from polymer particles into the aqueous phase is an important kinetic process in emulsion polymerization. Smith and Ewart [4] included the desorption rate terms into the balance equation for N particles, defining the rate of radical desorption from the polymer particles containing n free radicals in Eq. 3 as kftiN . However, they did not give any... 

As we discuss later in Section 3.3.3, Nomura et al. [45,47] first derived the rate coefficient for radical desorption in an emulsion copolymerization system by... 

As we discussed in Sect. 3.1.1, Hansen et al. [15] made significant improvements to the concept of the radical capture efficiency proposed by Nomura et al. [ 14]. Taking this concept into consideration, they examined the effect of radical desorption on micellar particle formation in emulsion polymerization [ 65 ]. Assuming that radical entry is proportional to the x power of the micelle radius and the polymer particle radius, they proposed the following general expression for the rate of particle formation ... 

After the nucleation period, three types of kinetic processes determine the kinetics of emulsion polymerization radical entry, radical desorption, and polymer chain formation in the polymer particles. The kinetics of emulsion polymerization are fully described by the following five dimensionless parameters ... 

To clarify the unique characteristics of the MWD of emulsion polymers, one of the simplest and most common cases [264], where neither chain transfer (Cf=0) nor radical desorption ((5=0) occurs, is considered here. The magnitude of bimolecular termination ( ) is changed with a constant radical entry frequency (c=2xl0 ). Figure 10 shows the calculated number- and weight-average chain lengths and the polydispersity index (PDI= as a func-... 

Fig. 15 Monte Carlo simulation results for emulsion polymerization that involves polymer transfer reactions, under the conditions Cm=5xl0" =5x10" and Cfp=5xl0" without radical desorption...

As far back as 1948, Smith and Ewart [ 13] included the effects of radical desorption in emulsion polymerization kinetics, and in 1965 Romatowski et al. [282-284] showed that radicals resulting from chain transfer to monomer indeed escape from the particles. 

In vinyl acetate emulsion polymerization radical desorption is important, i.e. with this monomer 0 and therefore m 0. Typical values of m and a lie in the intervals 10"1 - 10 3 and 10 3 - 10 6 respectively in the early stages of polymerization. 

Systems Where Radical Desorption is Negligible. Styrene and methyl methacrylate emulsion polymerization are examples of systems where radical desorption can be neglected. In Figures 4 and 5 are shown comparisons between experimental and theoretical conversion histories in methyl methacrylate and styrene polymerization. The solid curves represent the model, and it appears that there is excellent agreement between theory and experiment. The values of the rate constants used for the theoretical simulations are reported in previous publications (, 3). The dashed curves represent the corresponding theoretical curves in the calculation of which gel-effect has been neglected, that is, ktp is kept constant at a value for low viscosity solutions. It appears that neglecting gel-effect in the simulation of styrene... 

Desorption and Reabsorption of Free Radicals in Emulsion Polymerization ... 

Effect of Free-Radical Desorption on the Kinetics of Emulsion Polymerizaiion. 210... 

The kinetic behavior of emulsion polymerization is greatly affected by radical desorption from polymer particles. This has been shown by Dgelstad et al. (1969)> Litt et al. (1970), Harada et nl. (1971), Friis and Nyhagen (1973), and Nomura et al (1971). It is believed that the deviation of the kinetic behavior of the emulsion polymerization of water-soluble monomers such as vinyl acetate and vinyl chloride from the Smith and Ewart (1949) Case 2 kinetic theory is mainly due to dominant desorption of... 

Recently, Ugelstad et al. l969i proposed a semiempirtcal rate coefficient for radical desorption in vinyl chloride emulsion polymerization. On the other hand, Nomura et al. (1971, 1976) have derived a rate coefficient for radical desorption theoretically with both stochastic and deterministic approaches and have successfully applied it to vinyl acetate emulsion polymerization. They also pointed out that radical desorption from the particles and micelles played an important role in micellar particle formation, Fiiis et al. 1973 also derived the rate coefficient for radical desorption in a different way. Lift et al. (1981) discussed in more detail the chemical reactions incorporated in the physical process of radical desorption in the emulsion polymerization of vinyl acetate. 

In this chapter, the polymerization rate equations for emulsion polymerization will be reviewed briefly. Then, the rate coefficient for radical desorption from tbe panicles will be derived theoretically, and the effect of rathcal desorption on the rate of emulsirai polymerization and the micellar particle formation will he discussed. 

Garden (1968) has solved Eq. (12) numerically for the case of negligihle desotption of radicals from the particles without assuming the steady state, stating that the Stockmayer solution for n is incorrect because there is no steady state in principle and because Eq. (12) includes the time-dependent parameter Vp. However, the results of numerical calculation by Garden cohxnde almost completely with those predicted by the Stockmayer solution for no radical desorption from the particles. This also supports the validity of applying the steady-state hypothesis to the solution for Eq.(12) under normal conditions for emulsion polymerization. 

It is clear from the discussion so far that as long as the value of the rate coefficient for radical desorption from the particles kf cannot be estimated quantitatively, the rate of emulsion polymerization is impossible to predict. In the next section, therefore, the quantitative expression for kf will be derived. 

As shown in Fig. 1, the value of n becomes independent of the value of kf in the range of n 0.5. This means that tbe rate coefficient for radical desorption from the particles is important in Ibe range (n < 0.5) where the polymer particle contains at most one radical. For this reason, we consider an emulsion polymerization system where (i) the particles contain at most one radical and (ii) instantaneous termination takes place when another radical enters the particle that already contains a radical. 

The rate coefiicient for radical desorption in emulsion copolymerization was also derived in the same way as described in Section III, and it was successfully applied to explaining the rate of emulsion copolymerization of methyl methacrylate and styrene (Nomura et ai, 1978, 9T9). 

This corresponds to Smith-Ewart Case II kinetics and is applicable to styrene emulsion polymerization under normal conditions. On the other hand, when radical desorption from the particles is dominant (i.e., o = ) Eq.(IOS) lesdsto... 

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