Continuous bulk free radical polymerization

In bulk- and solution-phase free-radical polymerization, there is a tradeoff between molecular weight and polymerization rate. This is especially true for controlled/living radical polymerization. In emulsion polymerization, however, high molecular weight polymers can be made at fast polymerization rates. Emulsion polymerization is a type of radical polymerization that is frequently used for making polymers of high molecular weight. The most common type of emulsion polymerization is an oil-in-water emulsion, in which droplets of monomer (the oil) are emulsified with surfactants in a continuous phase of water. 

Jaisinghani and Ray (40) also predicted the existence of three steady states for the free-radical polymerization of methyl methacrylate under autothermal operation. As their analysis could only locate unstable limit cycles, they concluded that stable oscillations for this system were unlikely. However, they speculated that other monomer-initiator combinations could exhibit more interesting dynamic phenomena. Since at that time there had been no evidence of experimental work for this class of problems, their theoretical analysis provided the foundation for future experimental work aimed at validating the predicted phenomena. Later studies include the investigations of Balaraman et al. (43) for the continuous bulk copolymerization of styrene and acrylonitrile, and Kuchanov et al. (44) who demonstrated the existence of sustained oscillations for bulk copolymerization under non-isothermal conditions. Hamer, Akramov and Ray (45) were first to predict stable limit cycles for non-isothermal solution homopolymerization and copolymerization in a CSTR. Parameter space plots and dynamic simulations were presented for methyl methacrylate and vinyl acetate homopolymerization, as well as for their copolymerization. The copolymerization system exhibited a new bifurcation diagram observed for the first time where three Hopf bifurcations were located, leading to stable and unstable periodic branches over a small parameter range. Schmidt, Clinch and Ray (46) provided the first experimental evidence of multiple steady states for non-isothermal solution polymerization. Their... 

Continuous stirred-tank reactors (CSTRs) are used for large productions of a reduced number of polymer grades. Coordination catalysts are used in the production of LLDPE by solution polymerization (Dowlex, DSM Compact process [29]), of HDPE in slurry (Mitsui CX-process [30]) and of polypropylene in stirred bed gas phase reactors (BP process [22], Novolen process [31]). LDPE and ethylene-vinyl acetate copolymers (EVA) are produced by free-radical polymerization in bulk in a continuous autoclave reactor [30]. A substantial fraction of the SBR used for tires is produced by coagulating the SBR latex produced by emulsion polymerization in a battery of about 10 CSTRs in series [32]. The CSTRs are characterized by a broad residence time distribution, which affects to product properties. For example, latexes with narrow particle size distribution cannot be produced in CSTRs. 

Many thermoplastics are heterogeneous (or heterophase) because they contain liquid or rubber dispersions that improve their physical properties with respect to those of the continuous brittle phase. Examples of this are the softening of PVC by the presence of phthalate droplets and the improved toughness of HIPS or the polymer of acrylonitrile-butadiene-styrene (ABS) by addition of PBD-based rubber particles. This chapter will focus on the (heterogeneous, bulk and free-radical) polymerizations leading to the production of HIPS and PVC. 

Lewis and Volpert continue the discussion of the isothermal form of frontal polymerization in Chapter 5. Isothermal frontal polymerization is also a localized reaction zone that propagates but because of the autoacceleration of the rate of free-radical polymerization with conversion. A seed of poly(methyl methacrylate) is placed in contact with a solution of a peroxide or nitrile initiator, and a front propagates from the seed. The monomer diffuses into the seed, creating a viscous zone in which the rate of polymerization is faster than in the bulk solution. The result is a front that propagates but not with a constant velocity because the reaction is proceeding in the bulk solution at a slower rate. This process is used to create gradient refractive index materials by adding the appropriate dopant. 

Water as diluent has obvious advantages and has been developed for many free-radical polymerizations. Thus suspension polymerization involves the dispersion of the non-miscible monomer in water as droplets (0.1-5 mm diameter) by means of agitation and protective colloids or dispersing agents (e.g. polyvinyl alcohol, PVAL), and adding a monomer-soluble initiator. The polymer ends up approximately the same size as the original droplets and the system can be viewed as many small bulk polymerizations. As water is the continuous phase the viscosity remains constant and good heat transfer occurs. This process is used for PVC. 

Many advantages are shown by emulsion and suspension polymerization. First, the continuous water phase can act as an excellent heat conductor and allow the heat to be removed from the system. This is an effective way to increase the polymerization rate of many reactions. Second, the viscosity remains close to that of water and is not dependent on molecular weight since the polymer molecules are contained within the particles. Third, emulsion polymerization is unique in the sense that an increase in molar mass can be achieved without reducing the rate of polymerization. Therefore, high molecular weight polymers can be obtained at fast polymerization rates. By contrast, there is a tradeoff between molecular weight and polymerization rate in bulk and solution free-radical polymerization. 

Styrene can be polymerized to a high conversion if a small amount, say 20%, of an unreactive solvent is present. It is often referred to as a bulk polymerization even though some solvent is present. Most atactic polystyrene is produced by free-radical polymerization in this manner. Several continuous processes have been described in some detail [9], In one process, linear-flow reactors are used in series (Figure 5.3). Each reactor contains a series of sections that are agitated and provided with heat transfer tubes. A free-radical initiator may be used. In order to lower viscosity, especially in the latter stages, a solvent, typically ethyl benzene, is present to the extent of about 20%. Polymerization to high conversion is obtained in the last reactor as the temperature is increased... 

If an oil-soluble monomer is dispersed in a continuous aqueous phase without the use of surfactants, suspension polymerization results. The viscosity of the resulting suspension will remain essentially constant over the course of the polymerization. Oil-soluble free radical initiators are used to effect polymerization. The monomer is dispersed into beads by the action of an agitator. Since little or no surfactant is used, no emulsification takes place, and, if the agitation is stopped, the monomer will form a separate bulk phase, usually above the aqueous phase. The monomer is polymerized by the initiator within the droplets, forming polymer beads of approximately the same size as the monomer droplets (0.1-10 mm diameter). The product can be readily separated from the aqueous phase (via filtration or decantation) in the form of macroscopic particles or beads, which can be easily packaged and/or transported. Heat transfer is facihtated by the presence of the continuous aqueous phase. Blocking agents such as clays or talcs are used to prevent particle ag-... 

---