Polymerization particle formation

Fig. 6 Mechanism of large porous polymeric particle formation by pressure quench method using SC CO2. (From RefP. )...

Microsuspension and Inverse-microsuspension. In suspension polymerizations, particle formation occurs through a droplet breakup-coalescence mechanism, with the diameter controlled by the temperature, interfacial tension, agitation intensity and conversion. Suspension polymerizations have typically been characterized by an initiator soluble in the monomer phase and particle diameters in the 50-1000 pm range [40]. Smaller particles (0.2-20 pm) have been produced at higher agitation speeds (lower interfadal tensions) [41] and in such cases a prefix micro has been added to the nomenclature (microsuspension) to reflect both the dominant synthesis conditions (suspension) and the nominal particle size (1 micron). Therefore, microsuspension polymerization has historically referred to a subdomain of suspension polymerization occurring at smaller particle sizes. Based on an analogy to this nomenclature, inverse-microsuspension polymerization has been proposed for similar water-in-oil... 

When the emulsion polymerization is conducted in the absence of an emulsifier, this process is termed emulsifier free or soapless emulsion polymerization [68-73]. In this case, the particle formation occurs by the precipitation of growing macroradicals within the continuous... 

Figure 9 The schematical representation of dispersion polymerization process, (a) initially homogeneous dispersion medium (b) particle formation and stabilizer adsorption onto the nucleated macroradicals (c) capturing of radicals generated in the continuous medium by the forming particles and monomer diffusion to the forming particles (d) polymerization within the monomer swollen latex particles, (e) latex particle stabilized by steric stabilizer and graft copolymer molecules (f) list of symbols.

Stable particles in sufficient number, all the oligo-radi-cals and nuclei generated in the continuous phase are captured by the mature particles, no more particles form, and the particle formation stage is completed. The primary particles formed by the nucleation process are swollen by the unconverted monomer and/or polymerization medium. The polymerization taking place within the individual particles leads to resultant uniform microspheres in the size range of 0.1-10 jjLvn. Various dispersion polymerization systems are summarized in Table 4. 

The water solubilities of the functional comonomers are reasonably high since they are usually polar compounds. Therefore, the initiation in the water phase may be too rapid when the initiator or the comonomer concentration is high. In such a case, the particle growth stage cannot be suppressed by the diffusion capture mechanism and the solution or dispersion polymerization of the functional comonomer within water phase may accompany the emulsion copolymerization reaction. This leads to the formation of polymeric products in the form of particle, aggregate, or soluble polymer with different compositions and molecular weights. The yield for the incorporation of functional comonomer into the uniform polymeric particles may be low since some of the functional comonomer may polymerize by an undesired mechanism. 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

Emulsion polymerizations most often involve the use of water-soluble initiators (e.g. persulfate see 33.2.6.1) and polymer chains are initiated in the aqueous phase. A number of mechanisms for particle formation and entry have been described, however, a full discussion of these is beyond the scope of this book. Readers are referred to recent texts on emulsion polymerization by Gilbert4 and Lovell and El-Aasser43 for a more comprehensive treatment. 

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

However, since virtually no published data are available on the mechanism of continuous particle formation and growth, particlarly under the influence of mechanical agitation, continuous precipitation polymerization remains a vast engineering wilderness waiting for experimental and theoretical explorations. 

On the other hand, very little is known about the mechanism of continuous precipitation polymerization. In particular, the mechanism of continuous particle formation and growth and the effects of starvation feeding on reaction rate and copolymer composition are areas of particular interest. 

The precipitation polymerization literature is reviewed with particular attention to the influence of particle formation and growth, autoaccelerating polymerization rates, and copolymer composition drift on polymer reactor design. 

The physical picture of emulsion polymerization is complex due to the presence of multiple phases, multiple monomers, radical species, and other ingredients, an extensive reaction and particle formation mechanism, and the possibility of many modes of reactor operation. 

We begin the discussion of EPM by elaborating on this physical picture. Figure 1 shows a typical emulsion CSTR reactor and polymerization recipe. The magnified portion of the latex shows the various phases and the major species involved. The latex consists of monomers, water, surfactant, initiator, chain transfer agent, and added electrolyte. We used the mechanism for particle formation as described in Feeney et al. (8-9) and Hansen and Ugelstad (2). We have not found it necessary to invoke the micellar entry theory 2, 2/ 6./ 11/ 12/ 14. 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

The surfactant is an important component of this process and acts to stabilize the growing polymeric particles by surface adsorption. Phase separation and the formation of solid particles occur before or after termination of the polymerization process [42]. Polymerization can occur in some systems without the presence of surfactants [40]. Various particulate systems have been prepared by this method, including poly(styrene) [43], poly(vinylpyridine) [44, 45], poly(acrolein) [46, 47], and poly(glutaraldehyde) [48-50],... 

The polymerization is an anionic mechanism initiated by hydroxide ions or any bases present [42], The reaction scheme can be seen in Figure 5. The polymerization rate is regulated by hydroxyl ion concentration and hence is carried out at pH values below 3.5. Above this pH, the reaction rate is too rapid to allow discrete particle formation [55, 56]. 

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