Propagation-controlled entry

Maxwell et al. [ 11 ] proposed a radical entry model for the initiator-derived radicals on the basis of the following scheme and assumptions. The major assumptions made in this model are as follows An aqueous-phase free radical will irreversibly enter a polymer particle only when it adds a critical number z of monomer units. The entrance rate is so rapid that the z-mer radicals can survive the termination reaction with any other free radicals in the aqueous phase, and so the generation of z-mer radicals from (z-l)-mer radicals by the propagation reaction is the rate-controlling step for radical entry. Therefore, based on the generation rate of z-mer radicals from (z-l)-mer radicals by propagation reaction in the aqueous phase, they considered that the radical entry rate per polymer particle, p p=pJNp) is given by 

There has been discussion on the value of z. Maxwell et al. [11] proposed a semi-empirical thermodynamic model to predict the value of z for persulfate-derived oligomeric radicals, which is given by 

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One of the most important parameters in the S-E theory is the rate coefficient for radical entry. When a water-soluble initiator such as potassium persulfate (KPS) is used in emulsion polymerization, the initiating free radicals are generated entirely in the aqueous phase. Since the polymerization proceeds exclusively inside the polymer particles, the free radical activity must be transferred from the aqueous phase into the interiors of the polymer particles, which are the major loci of polymerization. Radical entry is defined as the transfer of free radical activity from the aqueous phase into the interiors of the polymer particles, whatever the mechanism is. It is beheved that the radical entry event consists of several chemical and physical steps. In order for an initiator-derived radical to enter a particle, it must first become hydrophobic by the addition of several monomer units in the aqueous phase. The hydrophobic ohgomer radical produced in this way arrives at the surface of a polymer particle by molecular diffusion. It can then diffuse (enter) into the polymer particle, or its radical activity can be transferred into the polymer particle via a propagation reaction at its penetrated active site with monomer in the particle surface layer, while it stays adsorbed on the particle surface. A number of entry models have been proposed (1) the surfactant displacement model (2) the colhsional model (3) the diffusion-controlled model (4) the colloidal entry model, and (5) the propagation-controlled model. The dependence of each entry model on particle diameter is shown in Table 1 [12]. 

Two major entry models - the diffusion-controlled and propagation-controlled models - are widely used at present. However, Liotta et al. [28] claim that the collision entry is more probable. They developed a dynamic competitive growth model to understand the particle growth process and used it to simulate the growth of two monodisperse polystyrene populations (bidisperse system) at 50 °C. Validation of the model with on-line density and on-line particle diameter measurements demonstrated that radical entry into polymer particles is more likely to occur by a collision mechanism than by either a propagation or diffusion mechanism. 

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

In the cationic-initiated polymerization of alkyl vinyl ethers it is possible to exercise fairly rigorous control of the configuration of the product by appropriate choice of the monomer and conditions. For example, isobutyl vinyl ether polymerized by BF3 etherate at 195 K in toluene can give isotactic polymer [15]. In this low polarity solvent, close association of the gegen ion with the cationic propagating center helps to block one mode of entry of fresh monomer (Eq. 22.45). 

The kinetics of emulsion polymerization is complex, involving a large number of species and at least two phases. The first quantitative approach to emulsion polymerization kinetics led to extensions by many others.The important events to consider are 1) the free-radical reactions of chain formation initiation, propagation, chain transfer, and termination and 2) the phase transfer events that control particle formation radical entry into particles from the aqueous phase, radical exit into the aqueous phase, radical entry into micelles, and the aqueous phase coil-globule transition. In free-radical emulsion polymerization, the fundamental steps are shown schematically in Fig. 1... 

At low monomer concentration (that is, at high monomer conversion), entry, propagation, and termination become diffusion-controlled processes. 

Ejfect of Hypothyroidism on Expression of Ion Channels and Pumps The effect of hypothyroidism on the expression of sarcolemmal proteins involved in action potential propagation and Ca entry in skeletal muscle has been incompletely studied. For example, hypothyroid muscles have lower levels of Na —ion pump activity (Kjeldsen et al., 1986) and expression (Everts and Clausen, 1988 Everts, 1996). Moreover, plasma K -ion concentrations increase to a greater extent during exercise in hypothyroid dogs compared to controls, consistent with hypothyroid-induced reductions in Na —ATPase pump activity... 

The data in Table 2.4 provide evidence that the slow rates and low molecular weights obtained in homogeneous free radical polymerization of these dienes are not due to a low rate constant for propagation but rather must be caused by a high rate constant for termination (as indicated in Table 2.1) (Matheson et al., 1949,1951 Morton and Gibbs, 1963). Hence, under the special conditions of emulsion polymerizations, where the termination rate is controlled by the rate of entry of radicals into particles, it becomes possible to attain both faster rates and higher molecular weights. It is this phenomenon which led to the rise of the emulsion polymerization system for the production of diene-based synthetic rubbers. 

In this situation, molecular simulations such as BD as a type of numerical experiment are advantageous, as the situation can be easily controlled. Capture rate coefficients can be determined under predetermined conditions based on well-established mechanistic equations (e.g., molecular Brownian motion). This has been used recently to study the kinetics of radical entry, without the interference of competitive events such as radical desorption, propagation, and termination [38, 39]. 

Chmura et al. also prepared air and moisture resistant chiral imino phenoxide complexes of zirconium and titanium, 14 [16]. They envisioned to study the effect of supporting ligand chirality on the stereoselectivity of LA ROP reaction. But at the end, they did not gain acceptable evidence enable to support any relationship. They showed that all isolated polymers had similar and moderate heterotactic microstructure which implied simple chain end control mechanism and resulted to the selective racemic enchainment during the propagation process. First, they investigate polymerization in toluene at 80°C and ambient temperature in which titanium complexes were absolutely inactive and zirconium coxmterparts showed moderate activity after 2 and 24 hours, respectively. Then they checked out solvent free conditions at 130°C and received almost complete conversion after 30 minutes for both titanium and zirconium alkoxide complexes (Table 7.2, entry 33-36). In this condition, titanium coxmterpart, in contrast to zirconium, resulted to full atactic polymer. Their investigation also showed that zirconium complex retained its activity in moisture or with lactic acid impurity in crude monomer which is deleterious for most metal alkoxide catalysts. 

Calcium channels are responsible for the genesis of APs in cardiac pacemaker cells (diastolic depolarisation) and the propagation of slow APs in sino-atrial and atrioventricular node cells and are equally important in the control of depolarisatiOTi-induced Ca entry responsible for the plateau (phase 2) of the AP (see Fig. 1). 

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