Particle solution polymerization

The rate of termination reaction is slower than that observed in the homogenous bulk or solution polymerization since the limited number of free radicals exists in the polymerization loci having a reasonably small volume (i.e., monomer swollen forming latex particle). Higher degree of polymerizations can be achieved in an emulsion system relative to the homogenous polymerization due to the existence of this limitation. 

For very high melting polymers (Tm > 300°C), a solution polymerization is normally employed. If this is started from the reactive acid chloride, the reaction temperature can be low. Polymers from acid chlorides can also be prepared by the interfacial method. Semicrystalline PA can be postcondensed in the solid state to higher molecular weights. To do this, the polymer powder/particles are heated for many hours below their melting temperature in an inert atmosphere. 

When a monomer such as acrylonitrile is polymerized in a poor solvent, macroradicals precipitate as they are formed. Since these are living polymers, polymerization continues as more acrylonitrile diffuses into the precipitated particles. This heterogeneous solution polymerization has been called precipitation polymerization. 

Compartmentalization (or segregation) of a propagating radical in a polymer particle isolates it from other propagating radicals and allows growth to higher molecular weight than might be achieved in solution polymerization, provided that the rate of entry of another radical from the aqueous phase is sufficiently slow. 

In Figure 11.2.1A the process of dispersion polymerization is shown focusing on the formation process of particles. Dispersion polymerization starts from a homogeneous solution, and when oligomeric radicals and polymer, formed in the monomer solution, do not have affinity for the medium, they become insoluble and precipitate. The precipitate is unstable and homoaggregates to become primary particles. Primary particles homoaggregate further until they become stable secondary particles. The mechanism to keep the particles stable depends on what type of stabilizer is used. The propagation processes from nuclei to primaiy particles and from primary to secondary ones does not have to be considered as discontinuous steps. However, it... 

Heterogeneous Copolymerization. When copolymer is prepared in a homogeneous solution, kineiic expressions can be used to predict copolymer composition Bulk and dispersion polymerization are somewhat different since the reaction medium is heterogeneous and polymeri/aiion occurs simultaneously in separate loci. In bulk polymerization, for example, the monomer swollen polymer particles support polymerization within the particle core us well as on the particle surface, lit aqueous dispersion or emulsion polymeri/aiion the monomer is actually dispersed in two or three distinct phases a continuous aqueous phase, a monomer droplet phase, and a phase consisting of polymer particles swollen at Ihe surface with monomer. This affect the ultimate polymer composition because llie monomers are partitioned such that the monomer mixture in the aqueous phase is richer in the more water-soluble monomers than the two organic phases. 

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. 

The rate of the dispersion copolymerization of PEO-MA with styrene increased with increasing temperature [72], which was attributed to the increased rate of initiation and particle formation. The overall observed activation energy (E0) for the dispersion copolymerization of PEO-MA or PEO-VB macromonomer and styrene was found to be 48 or 53 kj mol-1. These values are much lower than those (ca. 90-100 kj mol-1 [73]) obtained in the solution polymerization of low molecular weight monomers. 

Figure 2. Percentage conversion (%) and particle size (A) as a function of time for a dispersion polymerization and percentage conversion as a function of time for an equivalent solution polymerization...

Because of its heterophase nature, emulsion polymerization is generally more complicated than simple solution polymerization in which monomers and polymers are soluble in a suitably chosen solvent. In emulsion polymerization the different relative solubilities of monomers in water and in the polymer particles lead to different reaction locales and to different particle structures. Another complicating factor is the need to achieve and maintain colloidal stability throughout the polymerization and subsequent handling of the dispersions. Emulsion polymers can properly be called products by process since the process details exert such a powerful effect on the properties of the particles and resultant films. Consequently, an emulsion polymer is far more than a product defined by a simple polymer composition. 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

---