Suspension polymerization

Suspension polymerization is a heterogeneous radical polymerization process that uses mechanical agitation to mix a monomer or mixture of monomers in a liquid phase, such as water, while the monomers polymerize, forming spheres of polymer.

This process is used in the production of many commercial resins, including polyvinyl chloride (PVC), a widely used plastic, styrene resins including polystyrene, expanded polystyrene, and high-impact polystyrene, as well as poly(styrene-acrylonitrile) and poly(methyl methacrylate).^[1]

IUPAC definition

Polymerization in which polymer is formed in monomer, or monomer-solvent droplets
in a continuous phase that is a nonsolvent for both the monomer and the formed polymer.

Note 1: In suspension polymerization, the initiator is located mainly in the monomer phase.

Note 2: Monomer or monomer-solvent droplets in suspension polymerization have
diameters usually exceeding 10 μm.^[2]

Particle properties

Suspension polymerization is divided into two main types, depending on the morphology of the particles that result. In bead polymerization, the polymer is soluble in its monomer and the result is a smooth, translucent bead. In powder polymerization, the polymer is not soluble in its monomer and the resultant bead will be porous and irregular.^[3] The morphology of the polymer can be changed by adding a monomer diluent, an inert liquid that is insoluble with the liquid matrix. The diluent changes the solubility of the polymer in the monomer and gives a measure of control over the porosity of the resulting polymer.^[4]

The polymer beads that result can range in size from 100 nm to 5 mm. The size is controlled by the stirring speed, the volume fraction of monomer, the concentration and identity of the stabilizers used, and the viscosities of the different components. The following equation derived empirically summarizes some of these interactions:

${\bar {d}}=k{D_{v}\cdot R\cdot \nu _{m}\cdot \epsilon \over D_{s}\cdot N\cdot \nu _{l}\cdot C_{s}}$

d is the average particle size, k includes parameters related to the reaction vessel design, D_v is the reaction vessel diameter, D_s is the diameter of the stirrer, R is the volume ratio of the monomer to the liquid matrix, N is the stirring speed, ν_m and ν_l are the viscosity of the monomer phase and liquid matrix respectively, ε is the interfacial tension of the two phases, and C_s is the concentration of stabilizer. The most common way to control the particle size is to change the stirring speed.^[4]

See also

• Radical Polymerization
• Polymer
• Polymerization
• Step-Growth Polymerization
• Superabsorbent Polymer

References

1. Vivaldo-Lima, E., Wood, P., and Hamielec, A. (1997). "An Updated Review on Suspension Polymerization". Ind. Eng. Chem. Res. 36: 939–965. doi:10.1021/ie960361g.
2. "Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)" (PDF). Pure and Applied Chemistry. 83 (12): 2229–2259. 2011. doi:10.1351/PAC-REC-10-06-03.
3. Kotoulas, Costas & Kiparissides, Costas (2006). "A Generalized Population Balance Model for the Prediction of Particle Size Distribution in Suspension Polymerization Reactors". Chemical Engineering Science. 61: 332–346. doi:10.1016/j.ces.2005.07.013.
4. Arshady, R. (1992). "Suspension, Emulsion, and Dispersion Polymerization: A Methodological Survey". Colloid. Polym. Sci. 270: 717–732. doi:10.1007/BF00776142.