Desorption of radicals

Kiperman [31] also warns that detection of free radicals in the postcatalyst volume in itself cannot serve as concrete proof of their direct participation in the process. The relation also has to be revealed between the nature of these radicals formed in the volume and the intermediates of the true heterogeneous component of the reaction. Obviously, sophisticated analytical and characterization procedures are needed to elucidate the nature of the species reacting on and desorbing from a catalytic surface. A powerful tool to study adsorption and desorption of radicals from surface is laser-induced fluorescence, applied to hydroxyl and oxygen radicals by a number of researchers cf. Ref 37. Such techniques will continue to aid in the elucidation of heterogeneous-homogeneous mechanisms. 

The value of n in Eq. (6.230) is of critical importance in determining the rate of polymerization in stage II. Three cases — designated 1, 2, 3 — corresponding, respectively, to n < 0.5, n = 0.5, and n > 0.5 can be distinguished based on the work of Smith and Ewart [69] and others [70-74]. The kinetic treatment given above conforms to Case 2 (n = 0.5), which is the predominant behavior for emulsion polymerizations. It occurs when desorption of radicals does not occur or is negligible compared to the... 

Case 2 h = 0.5. If the rate of chain transfer-desorption of radicals is negligible compared to the rate of free-radical capture in particles, and the mutual termination rate of two radicals is so high that only particles with zero and one radicals can exist, then approximately one-half of the particles contain a single free radical. Small particle sizes and high rates of free-radical capture in particles favor this scenario. 

The exit (desorption) of radicals has been established as of importance in the kinetics of emulsion polymerization involving small particles, even for a relatively insoluble species such as styrene. The simplest demonstration of this comes from y-radiolysis relaxation studies, where polymerization in a seeded emulsion... 

Figure S3 The basic mechanism for exit (desorption) of radical activity from a particle...

Since both oil-soluble initiator and monomer are expected to partition predominantly in the seed particle, the contribution of surface-related phenomena (e.g. the adsorption and desorption of radicals) to the radical distribution is likely to be responsible for the difference between the formation of surface and internal... 

In the first situation (Case 1), where the particles are small or the monomer is substantially water-soluble, and desorption of radicals from the particle is likely, n is very low, and polymerisation is slow. In the second situation. Case 11, radical exit is negligible. When a radical enters a particle, polymerisation occurs until a second radical enters, and both are instantaneously terminated (zero-one kinetics). Under these conditions, n is equal to V2. In the third situation. Case III, the particles are large enough that two or more radicals may coexist within the same particle without... 

We begin by describing the current understanding of the kinetics of polymerization of vinyl chloride with a high water-solubility. In particular, we note the importance of desorption of radicals and the formation of less reactive radicals for vinyl chloride. These concepts will then be drawn upon as we try to understand the behavior of a monomer insoluble in its polymer in polymerization. 

The kinetics of emulsion polymerization of vinyl chloride is described by kinetic parameters such as rate constants of the initiation, kj, propagation, kp, termination, k and desorption of radicals, kd s. Concerning the kp value, one may admit that a value of 1 x 10" dm mol s at 50°C is generally accepted for vinyl chloride, at least in processes of saturation with monomer [142], Variation of the other kinetic parameters was more complicated. The value k depends on the nature and concentration of emulsifier and co-emulsiher utilized and also on monomer [92,143,144]. 

Desorption of radicals from polymer particles refers to the radicals formed in the chain-transfer to monomer. Vinyl chloride monomer is known to be an active chain-transfer agent. The high-water solubility of VC monomer favors desorption of radicals from particle. Exit of radicals from polymer particles... 

The foregoing results indicate that the emulsion polymerization of VC is influenced in a complex way by the high water solubility of the monomer and the limited miscibility of polymer and monomer and deviates strongly from the conventional emulsion polymerization. This is discussed in terms of the water-phase polymerization of VC, precipitation polymerization of VC, polymerization in the emulsifier layer zone (a water/particie interphase), desorption of radicals from particles, chain transfer to monomer and polymer, formation of unsaturated and branching structures of PVC, formation of occluded radicals and diffusion of more volatile radical species to the vapor phase. 

The monomer-saturated emulsifier layer (or the shell of polymer particlesX the high chain transfer constant to VC and the polymerization in the interphase should promote desorption of radicals from particles to the aqueous phase. Desorbed radicals may take part in initiation and termination or re-enter the particles. In both phases, desorbed or re-entered radicals are more eflicient in termination. In addition, the mobile radicals irreversibly diffuse to the particle core in which they are immobilized by the polymer phase and/or occluded by propagation. 

At subsaturation conditions, the rate is proportional to the 0.4th power of the initiator concentration which agrees well with the micellar predictions. This behavior indicates that the monomolecular termination is more suppressed while the bimolecular termination is more favoured. Thus, the reaction order 0.4 should result from a strong contribution of primary radical termination and/or desorption of radicals. 

The rate of polymerization is only slightly affected by the particle concentration. The reaction order reaches a maximum value of 0.2 [164]. This behavior favors the assumption that polymerization of VC takes place in the emulsifier layer zone in which the polymerization seems to be governed by the kinetics of the homogeneous polymerization. Desorption of radicals from this layer to the aqueous phase is expected to be operative. 

The application of these comprehensive models to the prediction of the emulsion polymer molecular weight distribution requires a fundamental understanding of the very conaplex reaction mechanisms and knowledge of various kinetic parameters (e.g., the rate coefficients for the absorption of free radicals by the latex particles, the desorption of radicals out of the particles, and the bimolecular termination reaction). However, these mathematical models in combination with the polymer molecular weight distribution data may serve as a useful tool for estimating the values of the kinetic parameters involved in emulsion polymerization. 

These equations take into consideration the concentrations of radicals bonded with the template (denoted by [A ] and [B ]) and nonbonded ([A°] and [B°]). The equations were formulated with the assumption that the process sorption-desorption of radicals onto template is reversible with equilibrium constants and K ... 

Stockmayer also presented solutions for the case that takes into account desorption of radicals. This solution, however, is wrong for the most important range in desorption rates. But Sto ckmayer s solution(s) lead the way for the possibility of exact mathematical solution of emulsion polymerisation kinetics at a time when digital computers were not yet very important in chemical computations. The general solution when desorption is taken into account was presented by O Toole (1965). He applied a modified form of the Smith-Ewart recursion equation that gave the solution... 

Consider the seed polymerization in which seeds of polymer particles (each of volume r) are each charged to the reaction mass having monomer and initiator. When desorption of radicals and water-phase termination of radicals are neglected, the average number of growing polymer radicals n can be derived ... 

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