In mass or bulk polymerization, the reaction mixture consists mainly of monomers, and in the case of free-radical or ionic polymerization, of vinyl monomers and a soluble initiator. Thus, the polymerization is carried out in undiluted monomers. This type of polymerization is frequently used for step-growth polymers.
Mass polymerization has several advantages over other polymerization techniques; since there is no solvent or diluent present, it provides polymers of high clarity and high(er) molecular weight. It is also a very environmental friendly polymerization method since no purification is required and the final product is a 100% solid resin. However, bulk polymerization has many limitations and drawbacks; for example, this polymerization technique is not practical for very exothermic reactions because the temperature is hard to control, particularly at a later stage when the viscoity is high. Exceptions are vinyl monomers with a low reactivity and enthalpy of polymerization. Two examples are poly(methyl methacrylate) and polystyrene. But even then, the polymerization has to be conducted in thin and long reactors with large heat exchangers. Another major drawback of mass polymerization is the significant viscosity increase with increasing molecular weight which can cause problems with removal of volatile byproducts such as water which is a common byproduct of condensation reactions. It is also likely that the kinetics of the reaction changes when the molecular weight is high and the monomer concentration low (see Trommsdorff effect).
Bulk polymerization is much more suited for step-growth polymers because these polymers have a (much) lower heat of polymerization and reactivity.
Bulk polymerizations can proceed either homogenous or inhomogenous, depending on the solubility of the growing polymer chains in the monomer. An example of a heterogenous bulk polymerization is vinyl chloride. The process takes place in three stages.1 During the first and third stage, the reaction mixture consists only of a single phase in which polymerization proceeds according to the kinetics of a homogeneous reaction. During the second stage, two phases are present simultaneously. In the beginning of this stage, globular polymer particles are formed by precipitation from the solution. In the range of 1 - 10% conversion, the globular particles are stable against aggregation and the number of particles is nearly constant. However, between 10 and 20% conversion, secondary structures form by coalescence, which is completed at about 20% conversion. Above about 20%, a constant monomer concentration in the growing particles is maintained by diffusion of monomers from the liquid phase into the polymer droplets. This stage lasts to about 75% conversion. In the third stage above 75% conversion, the monomer phase completely disappeares and a conventional homogeneous polymerization takes place in the monomer swollen polymer phase. This type of polymerization is sometimes called precipitation polymerization because the polymer is insoluble in the monomer in the later stage of polymerization and thus precipitates. It should not be confused with dispersion polymerizations such as emulsion polymerization and suspension polymerization. Precipitation polymerization gives usually less regular particles as a result of little or no stabilizers present and the particles are usually larger.
Examples for a homogenous bulk polymerization are methyl methacrylate and styrene. However, these polymers are often produced via other polymerization techniques such as emulsion or suspension polymerization. Bulk polymerization is more often used for step-growth polymers such as polyurethanes, polycaprolactam (Nylon 6), polycarbonates and polyester (PET).