The polymerization method refers to the means used to complete a polymerization process. According to the mechanism behind it, different polymerization methods are suitable for different types of polymerization. For example, the polymerization methods used in chain-growth polymerization usually include bulk polymerization, suspension polymerization, solution polymerization and emulsion polymerization. Furthermore, since free radicals are relatively stable, the above four methods can be used for free radical polymerization; for anionic and cationic polymerization, however, because of sensitivity of the active centers to impurities, solution polymerization or bulk polymerization is often preferred. What's more, regards to step-growth polymerization, the commonly used methods are melt polycondensation, solution polycondensation, interfacial polycondensation and solid state polycondensation.
Bulk polymerization is proceeded in the absence of any solvent or dispersant and is thus the simplest method frequently used for addition polymerization. The reaction is initiated under the action of the initiator or light, heat, radiation, etc. It takes advantages of simple operation and pure product; however, its disadvantage is that the heat transfer of reaction system is difficult. Industrial applications of free radical bulk polymerization production of polymers mainly include polymethyl methacrylate (PMMA), high-pressure polyethylene (PE) and polystyrene (PS).
This method uses mechanical stirring to mix the monomer in an aqueous medium, such as water, while the monomers polymerize, forming spheres of polymer. Since the polymerization is carried out in a liquid phase, the reaction system is easy to transfer heat, and the resulting polymer is easy to separate and wash, so its purity is high. On the contrary, the major disadvantage is that the polymer droplets are easy to stick and thereby creaming occurs during the polymerization process. Suspension polymerization is mainly used in the production of polyvinyl chloride (PVC), polystyrene (PS), poly(styrene-acrylonitrile) as well as polymethyl methacrylate (PMMA).
This method is to dissolve the monomer and initiator (or catalyst) in a non-reactive solvent for polymerization. Advantages of solution polymerization are low viscosity, fast heat transfer, and easy control of the polymerization temperature. Conversely, its apparent disadvantages are that the polymer obtained is with a relatively low polymerization degree and poor purity, as well as the small amount of solvent mixed in the system is not easy to remove. In industry, some important polymers obtained through this pathway include polyacrylonitrile (PAN), polyacrylic acid (PAA), polyethylene (HDPE, LLDPE), aromatic polyamides and etc.
It is a kind of polymerization in which the monomer is dispersed in a liquid phase with the initiator dissolved under the action of an emulsifier to form an emulsion. Rather than occurring in emulsion droplets, polymerization takes place in the latex/colloid particles that form spontaneously in the first few minutes of the process. Moreover, a large amount of water is also easy to dissipate the heat. Nevertheless, the emulsifier encapsulated in the polymer particles is not easy to remove, which would affect the performance of the final product. Several commercially important polymers are produced through emulsion polymerization, such as styrene butadiene rubber, neoprene rubber, nitrile rubber and polyvinyl chloride latex.
This type of polymerization only contains monomers and a small amount of catalyst, and is conducted above the melting point (generally 10-25°C higher than the melting point). The reaction temperature of melt polycondensation, generally above 200°C, is much higher than that of chain-growth polymerization. For polycondensation reactions with a low reaction rate at room temperature, increasing the reaction temperature is usually beneficial to speed up the reaction and shorten the reaction time. However, also because of the high reaction temperature, various side reactions are easy to occur in the melt polycondensation procedure, such as cyclization, cracking, oxidative degradation, and decarboxylation. Some industrial polymers synthesized by this way include polyester, polycarbonate (PC), polyamide (PA), etc.
The solution polycondensation of monomers and catalysts proceeds in a solvent. Similar to solution polymerization, its advantages are low system viscosity, fast heat transfer, and easy control of system temperature. Because of the addition of extra solvent, the need to recover the solvent not only increase the cost but also make the operation complicated. The scale of industrial application of solution polycondensation is second only to the melt polycondensation. The products produced by solution polycondensation include polyaramid, polysulfone, polyphenylene ether, paint, coating, etc.
Interfacial polycondensation occurs at the interface between two immiscible phases (generally two liquids), resulting in a polymer that is constrained to the interface. It can be characterized by the following four aspects: a. multiphase reaction; b. low and irreversible reaction temperature; c. reaction rate is a diffusion control process; d. average molecular weight is less relative to the ratio of ingredients. Its disadvantages are the high synthesis cost and the complicated recovery process of solvent. Industrially, interfacial polymerization has been extensively researched and used to produce polyamides, polyanilines, polyimides, polyurethanes, polyureas, polypyrroles, polycarbonates and etc.
By contrast to the melt polycondensation, solid state polycondensation is carried out below the melting point or softening point of the raw materials (monomers and polymers). It takes unique advantages in the preparation of polymers with high molecular weight and high purity, polymers that are susceptible to decompose above the melting point, and inorganic condensation polymers. The disadvantages, on the contrary, are low reaction rate, long reaction time, diffusion control process and obvious autocatalysis. This method is still in the research stage and there are few industrialized products.
In most cases, one polymer can be synthesized via several different polymerization methods. The choice of polymerization method mainly depends on the properties and morphology of the polymer to be synthesized, average molecular weight and molecular weight distribution. When producing polymers of good quality, choosing the right polymerization method can lower the equipment investment, production cost, and waste pollution, which should always be the primary consideration.