Emulsion polymerization compartmentalization

Even though the chemical reactions are the same (i.e. combination, disproportionation), the effects of compartmentalization are such that, in emulsion polymerization, particle phase termination rates can be substantially different to those observed in corresponding solution or bulk polymerizations. A critical parameter is n, the average number of propagating species per particle. The value of h depends on the particle size and the rates of entry and exit. 

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 . 

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

Bimolecular Termination Alone. The effect of compartmentalization in an emulsion polymerization is to broaden significantly the MWD of the polymer produced if termination is dominated by bimolecular events. This was clearly established by Katz,... 

Shinnar and Saidel (2) for termination by combination but also holds for disproportionation. We note in passing that one theory (6) of emulsion polymerization claims that compartmentalization decreases the polydispersity of the polymer produced at any iistant there is, however, no sound theoretical basis for this claim. 

Considerations of radical compartmentalization and higher polymer concentration effects are not sufficient to describe the processes that build branched polymer molecules in emulsion polymerization, and the effects of limited space must be properly taken into account [266-269]. 

Friis and Hamielec (1975) have used GPC to study the MWD development in vinyl acetate and vinyl chloride emulsion polymerizations. For these monomers, the main chain-stopping mechanism is thought to ha transfer, and so the compartmentalize nature of the system is relatively unimportant. These workers found that the MWDs produced at early times, where branching reactions are unimportant, have a P value dose to 2, as expected for transfer-dominated reactions. 

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 theory also has relevance to the so-called seeded " emulsion polymerization reactioas- In these reactions, polymerization is initial in the presence of a seed latex under conditions such that new particles are unlikely to form. The loci for the compartmentalized free-radical polymerization that occurs are therefore provided principally by the particles of the initial seed latex. Such reactions are of interest for the preparation of latices whose particles have, for instance, a core-shell" structure. They are also of great interest for investigating the fondamentals of compartmentalized free-radical polymerization processes. In this latter connection it is important to note that, in principle, measurements of conversion as a function of time during nonsteady-state polymerizations in seeded systems offer the possibility of access to certain fundamental properties of reaction systems not otherwise available. As in the case of free-radical polymerization reactions that occur in homogeneous media, investigation of the reaction during the nonsteady state can provide information of a fundamental nature not available through measurements made on the same reaction system in the steady state. 

The term zero-one designates that all latex particles contain either zero or one active free radical. The entry of a radical in a particle that already contains a free radical will instantaneously cause termination. Thus, the maximum value of the average number of radicals per particle, n, is 0.5. In a zero-one system, compartmentalization plays a crucial role in the kinetic events of emulsion polymerization processes. In fact, a radical in one particle will have no access to a radical in another particle without the intervention of a phase transfer event. Two radicals in proximity will terminate rapidly however, the rate of termination will be reduced in the process because of compartmentalization, as the radicals are isolated as separate particles. Consequently, the propagation rate is higher and the molecular weight of the polymer formed is larger than in the corresponding bulk systems. Which model is more appropriate depends primarily on the particle size. Small particles tend to satisfy the zero-one model, as termination is likely to be instantaneous. ... 

The dependence of the maximal Rp on [KPS] is quite similar for both the MMA mini-emulsion polymerization with HD (x=0.4) and the conventional emulsion polymerization (x=0.39) but different on [SDS] (y=0.16,ME) and (y= 0.24, CE) [ 108]. The reaction orders x and y are a complex function of the radical entry (particle nucleation) and the extent of compartmentalization of radicals. The radical entry or particle nucleation increases Rp. Np increases with increasing [KPS] and the degree of increase is more pronounced for the MMA emulsion polymerization (Np°c[KPS]x, x =0.28) as compared with that for the MMA mini-emulsion polymerization (x =0.11) (Table 1). The radical entry events are restricted due to the close-packed droplet surface layer, but the pseudo-bulk ki-... 

S-E cases 1 and 2 correspond to what is known as zero-one systems, in which the radicals grow in isolated compartments, reaching very high molecular weights hence, this characteristic feature of emulsion polymerization is known as compartmentalization. In case 3, this characteristic is relaxed so that radicals in a given particle grow in the presence of other radicals. As more radicals coexist within the particles, the system approaches the behavior of a bulk polymerization (or pseudobulk system). 

Water is a chief ingredient in both suspension and emulsion polymerization. As the continuous phase, although inert, it acts to maintain a low viscosity and provides for good heat transfer. In addition, it serves to isolate the polymerization loci. Termed compartmentalization, this is a particular advantage in emulsion polymerization as will be described later in terms of rates of polymerization and molar masses. The water also acts as the medium of transfer of monomer from... 

In an emulsion polymerization system, radicals are distributed among the polymer particles. The size of these particles is so small that there are only a small number of radicals per particle, as an average less than one radical per particle in many cases of practical interest. The compartmentalization of radicals among the particles is the most distinctive kinetic... 

Microemulsion polymerization [114] involves the polymerization of oil-in-water and water-in-oil monomer microemulsions. Microemulsions are thermodynamically stable and isotropic dispersions, whose stability is due to the very low interfacial tension achieved using appropriate emulsifiers. Particle nucleation occurs upon entry of a radical into a microemulsion droplet. Microemulsion polymerization allows the production of particles smaller than those obtained by emulsion polymerization. This leads to a higher number of polymer particles, which results in a more compartmentalized system. Under these conditions, the life-time of the polymer chains increases leading to ultra-high molecular weights. Inverse microemulsion polymerization is used to produce highly efficient flocculants. 

The kinetic behavior in each segregated entity can be different in view of die random nature of exit and entry phenomena and the nanometer scale of these identities, that is, a deviation from bulk kinetics ( one big droplet ) is to be expected. Hence, for emulsion polymerization, it is crucial to track the number of low-abundant (radical) species per segregated entity, as compartmentalization of radical species may influence the overall kinetics and thus the development of the polymer microstructure. If u radical types are present, this implies the calculation of the number of segregated entities characterized by u indices, with each index reflecting the discrete presence of one radical type. 

Compartmentalized Free-radical Polymerization.—Considerable interest has been shown in recent years in the solution of the differential difference equations which are obtained when the theory of Smith and Ewart is applied to reaction systems which contain a fixed number of reaction loci, but in which a steady state for the various locus populations has yet to be established. An example of such a reaction system would be a seeded emulsion polymerization system within whose external phase new radicals suddenly begin to be generated, and which does not contain sufficient surfactant to permit the nucleation of new particles. The theory which has been developed is concerned with the question of the nature of the approach to the steady-state distribution of locus populations, and with what might be learned from accurate measurements made during the approach to the steady state. 

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