Bulk polymerizations

Polymerizations can be carried out in bulk monomer (either liquid or gaseous) or in an inert diluent, either as a slurry (suspension or emulsion) or in solution. 

This technique involves adding a small amount of catalyst or initiator and heating to a temperature where the initiator becomes active. In principle, it is the simplest and cheapest process and should give the purest product with no expense of solvent removal and recovery. However, the main problem is removal of heat from polymerization which for an average polymer is 500 kWh tonne of polymer. Hence, runaways are always a danger. Broad MWD often occurs. This can be somewhat overcome by using reactors with high 

Gaseous polymerization is now used in PE and PP manufacture, using either a fluid bed or helically stirred reactor (see section 1.15.1). In these processes, the catalyst is added to the reactor and is deposited on the polymer particles which then grow in size around the catalyst. These polymer particles are continuously removed and the break-up of the polymer by abrasion or expansion generates new (smaller) polymer/catalyst particles from the original powder. These continue to grow in size to become the seed polymer for the incoming catalyst. 

Bulk Polymerization. The monomers were also polymerized in bulk. To follow the process, a polarization microscope was used, which made it possible to observe phase separations or phase transitions during polymerization. 

A detailed physical Investigation of the polymers described here is discussed in Chapter 2  

Winsor, P.A., Liquid Crystals and Plastic Crystals, Ellis Horwood, Chichester, 197.  

In recent years, investigators working in the field of physics and chemistry of high-molecular compounds have been paying a lot of attention to the problem of C3 eating liquid crystalline systems (V14). The great interest displayed in the study of properties of such systems can most probably be accounted for by two main factors firstly, by the advances in studies into the structure, properties and practical use of low-molecular liquid crystals in physics, technology and medicine, and, secondly, by the studies of the nature and salient features of the liquid crystalline state in polymers as a specific state of macromolecu-lar substeuices. 

Advances in this field of research are associated with the development of methods for producing polymeric liquid-crystalline systems and controlling the processes of structure formation in polymers. 

Bulk polymerization consists of heating the monomer without solvent with initiator in a vessel. The monomer-initiator mixture polymerizes to a solid shape fixed by the shape of the polymerization vessel. The main practical disadvantages of this method are the difficulty in the removal of polymer from a reactor or flask and the dissipation of the heat evolved by the polymerization. 

In the use of polystyrene, the polymerization reaction is exothermic to the extent of 17 Kcal/mol or 200 BTU/lb (heat of polymerization). The polystyrene produced has a broad molecular weight distribution and poor mechanical properties. The residual monomer in the ground polymers can be removed using efficient devolatilization equipment. Several reviews are worthwhile consulting [42-44], 

The bulk polymerization of styrene to give a narrow molecular weight distribution has appeared in a U.S. patent [45]. The polydispersity reported was 

Several references to the bulk polymerization of styrene are worth consulting [46-50], Most consider a continuous bulk polymerization apparatus with some using spraying of the monomer through a nozzle. The controlled evaporation of unreacted monomer is one method of removing the heat of reaction. 

Before this experiment is carried out, the student must read the material safety data sheets (MSDS) for all the chemicals used as well as for the products. The instructor must approve that you have read and understood the MSDS for the safe handling of these materials. 

Bulk polymerization leads to very pure polymers, since only monomer, polymer, and initiator are present in the initial reaction mixture. At high conversion, however, transfer reactions that produce branched polymers can become noticeable. The gel effect (see Section 20.2.6.2) can cause overheating. Overheating leads to insufficient polymerization control, degradation, and possibly discoloration of the polymer. 

Some polymers, such as poly(vinyl chloride) or poly(acrylonitrile), are insoluble in their own monomers. In bulk polymerization, therefore, the polymer is precipitated at relatively low conversions. Since newly produced free radicals still have monomer available to them, the polymerization continues, but the kinetics of polymerization is complicated. Part of the growing chain resides in the precipitated phase, and thus the probability of chain termination is small. On the other hand, the rate of diffusion of the monomer to the growing chain end is considerably reduced, so that the propagation reaction is also affected. 

In bulk polymerization, also called mass or block polymerization, monomer and polymer (and initiator) are the only components. When only part of the monomer charge is converted to polymer, however, the problems encountered are more typical of the solution method, which is discussed next. We can differentiate between quiescent and stirred bulk polymerizations. Both methods are applied to systems where polymer is soluble in monomer and progressively increases in viscosity with conversion. In quiescent systems, gel formation, corresponding to infinite viscosity, can occur. Because of the heat of polymerization and autoacceleration, the reaction rate is difficult to control. Heat removal is impeded by high viscosity and low thermal conductivity. The removal of traces of unreacted monomer from the final product is difficult because of low diffusion rates. Conversion of all monomer is difficult for the same reason. 

Most of the investigations into cyanoacrylate polymerization have involved the study of dilute solutions using covalent bases such as amines or phosphines, so this information is not directly relevant to the polymerization of neat monomers between adherends. However, many of the prin- 

Another way to view this phenomenon is to consider the curing cyanoacrylate as a plasticizer-polymer mixture in which the unreacted monomer is the plasticizer. The Tg of a plasticized polymer is lower than that of unplasticized polymer, and the Tg drop is governed by the amount and efficiency of the plasticizer. In the present case, even if the polymer portion of the mixture has reached its ultimate Tg, the remaining monomer acts as a plasticizer that lowers the Tg to a value about 30 C above Tcure- 

Step growth polymers, such as polyesters, are often manufactured via bulk polymerization. The reactive species are mixed together in a stirred reactor designed to promote intimate contact bet veen the reactants. Variables such as temperature and pressure are used to control the molecular properties of the final polymer. 

We rarely use bulk polymerization methods to produce chain growth polymers. Bulk polymerization finds little use for these materials, because the process generates relatively few 

The simplest and most direct method of converting monomer to polymer is known as bulk or mass polymerization. A typical charge for a free-radical bulk polymerization might consist of a liquid monomer, a monomer-soluble initiator, and perhaps a chain-transfer agent. 

As simple as this seems, some serious difficulties can be encountered, particularly in free-radical bulk polymerizations. One of them is illustrated in Fig. 13.1, which indicates the course of polymerization for various concentrations of methyl methacrylate in benzene, an inert solvent The reactions were carefiiUy maintained at constant temperature. At low polymer concentrations, the conversion vs. time curves are described by (10.19). As polymer concentrations increase, however, a distinct acceleration of the rate of polymerization is observed which does not conform to the classical kinetic scheme. This phenomenon is known variously as autoacceleration, the gel effect, or the Tromsdorff ect. 

Example 1. The maximum possible temperature rise in a polymerizing batch may be calculated by assuming that no heat is transferred from the systeno. Estimate the adiabatic temperature rise for the bulk polymerization of styrene, Mlp— — 16.4 kcal/mol, molecular weight = 104. 

Solution. The polymerization of 1 mol of styrene liberates 16400 cdl (assuming complete conversion). In the absence of heat transfer, all this energy heats up the reaction mass. The heat capacities of organic compounds are often difficult to find, and since the reaction mass is going from monomer to polymer, which in general have different heat capacities, the heat capacity of the reaction mass changes with conversion and probably also with temperature. To a reasonable approximation, however, the heat capacity of most liquid organic systems may be taken as 0.5 cal/g°C. Thus, 

By keeping at least one dimension of the reaction mass small, permitting heat to be conducted out Polymethyl methacrylate sheets are cast between glass plates at a maximum thickness of S in. or so. 

If possible from the point of view of efficiency, polymerization of the reactants without any use of a dispersing medium is desired. This method is called Bulk Polymerization. However, in order to absorb the heat released in the reaction, other methods like suspension or emulsion polymerization are frequently used. In this case, the water absorbs heat and provides good control of temperature. Another (less popular) method is the solution (homogeneous or heterogeneous) polymerization, which is mostly utilized in ionic initiation. We compare advantages and disadvantages of each method. (Table 2-4 describes the most useful polymerization methods for some commercial polymers). 

This method is simple to operate, efficient in regard to reactor space utilization, and leads to a polymer that is pure without the retention of other species. On the other hand, the absorption of heat is quite difficult, leading to the danger of overheating that prevents controlled production. There is 

High impact polystyrene (HIPS) is a blend of PS that has been polymerized in the presence of polybutadiene. This leads to PS chains with grafted polybutadiene, as well as some free PS and PB, and the resultant polymer blend has improved impact strength. HIPS/MMT nanocomposites were formed though in-situ bulk polymerization in the presence of polybutadiene [86]. Intercalated nanocomposites with improved thermal stabihty were formed although the dispersion was different in the PS matrix phase compared to the rubber phase. 

Bulk polymerization initiated via more novel methods have also been used to form bulk PS-MMT nanocomposites. Zhang et al. [19] used gamma irradiation to initiate the polymerization of PS-MMT nanocomposites with different surface modifications (3, 34) and successfully prepared exfohated morphologies when reactive clay modifications were used. Uthirakumar et al. [51-54] modified MMT with a cationic radical initiator which was used to initiate the bulk polymerization of styrene. Because the polymerization was initiated from the clay surface and the monomer and the clay were suitably compatible, exfoliated morphologies were formed. 

As we have seen in the past few chapters, there are a number of reaction mechanisms used to make polymers. Some reactor designs (semibatch and CSTRs) have been discussed as ways to form uniform or well-characterized polymers and copolymers. This chapter covers some of the important considerations with scaling up polymerizations, starting with bulk polymerization, and takes into account how transport processes (heat and mass transfer) may influence the design of polymerization reactors. 

Fundamental Principles of Polymeric Materials, Third Edition. Christopher S. Brazel and Stephen L. Rosen. 2012 John Wiley Sons, Inc. Published 2012 by John Wiley Sons, Inc. 

FIGURE 12.1 Polymerization of methyl methacrylate at 50 °C in the presence of henzoyl peroxide (a thermal initiator) in bulk (100%) or solution polymerization using various concentrations (10-80%) of monomer in benzene. 

The difficulties of bulk polymerization are compounded by the inherent nature of the reaction mass. Vinyl monomers have rather large exothermic heats of polymerization, typically between -10 and -21 kcal/mol. Organic systems also have low heat capacities and thermal conductivities, about half those of aqueous solutions. Thus, the temperature can rise very quickly. To top it all off, the tremendous viscosities prevent effective convective (mixing) heat transfer. As a result, the overall heat-transfer coefficients are very low, making it difficult to remove the heat generated by the reaction. This raises the temperature, further increasing the rate of reaction (see Example 9.2) which in turn increases the rate of heat evolution, and can ultimately lead to disaster To quote Schildknecht [2] on laboratory bulk polymerizations, If a complete rapid polymerization of a reactive monomer in large bulk is attempted, it may lead to loss of the apparatus, the polymer or even the experimenter.  

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Elbert R, Laschewsky A and Ringsdorf H 1985 Hydrophilic spacer groups in polymerizable lipids— formation of biomembrane models from bulk polymerized lipids J. Am. Ohem. Soc. 107 4134-41... 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

The assumption that k values are constant over the entire duration of the reaction breaks down for termination reactions in bulk polymerizations. Here, as in Sec. 5.2, we can consider the termination process—whether by combination or disproportionation to depend on the rates at which polymer molecules can diffuse into (characterized by kj) or out of (characterized by k ) the same solvent cage and the rate at which chemical reaction between them (characterized by kj.) occurs in that cage. In Chap. 5 we saw that two limiting cases of Eq. (5.8) could be readily identified ... 

Bulk and solution polymerizations are more or less self-explanatory, since they operate under the conditions we have assumed throughout most of this chapter. A bulk polymerization may be conducted with as few as two components monomer and initiator. Production polymerization reactions are carried out to high conversions which produces several consequences we have mentioned previously ... 

The enthalpy of the copolymerization of trioxane is such that bulk polymerization is feasible. For production, molten trioxane, initiator, and comonomer are fed to the reactor a chain-transfer agent is in eluded if desired. Polymerization proceeds in bulk with precipitation of polymer and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. The mixing requirements for the bulk polymerization of trioxane have been reviewed (22). Raw copolymer is obtained as fine emmb or flake containing imbibed formaldehyde and trioxane which are substantially removed in subsequent treatments which may be combined with removal of unstable end groups. 

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

Because the polymerization occurs totally within the monomer droplets without any substantial transfer of materials between individual droplets or between the droplets and the aqueous phase, the course of the polymerization is expected to be similar to bulk polymerization. Accounts of the quantitative aspects of the suspension polymerization of methyl methacrylate generally support this model (95,111,112). Developments in suspension polymerization, including acryUc suspension polymers, have been reviewed (113,114). 

The first quantitative model, which appeared in 1971, also accounted for possible charge-transfer complex formation (45). Deviation from the terminal model for bulk polymerization was shown to be due to antepenultimate effects (46). Mote recent work with numerical computation and C-nmr spectroscopy data on SAN sequence distributions indicates that the penultimate model is the most appropriate for bulk SAN copolymerization (47,48). A kinetic model for azeotropic SAN copolymerization in toluene has been developed that successfully predicts conversion, rate, and average molecular weight for conversions up to 50% (49). 

Acrylonitrile and its comonomers can be polymerized by any of the weU-known free-radical methods. Bulk polymerization is the most fundamental of these, but its commercial use is limited by its autocatalytic nature. Aqueous dispersion polymerization is the most common commercial method, whereas solution polymerization is used ia cases where the spinning dope can be prepared directly from the polymerization reaction product. Emulsion polymerization is used primarily for modacryhc compositions where a high level of a water-iasoluble monomer is used or where the monomer mixture is relatively slow reacting. 

Bulk Polymerization. The bulk polymerization of acrylonitrile is complex. Even after many investigations into the kinetics of the polymerization, it is stiU not completely understood. The complexity arises because the polymer precipitates from the reaction mixture barely swollen by its monomer. The heterogeneity has led to kinetics that deviate from the normal and which can be interpreted in several ways. 

Although bulk polymerization of acrylonitrile seems adaptable, it is rarely used commercially because the autocatalytic nature of the reaction makes it difficult to control. This, combined with the fact that the rate of heat generated per unit volume is very high, makes large-scale commercial operations difficult to engineer. Lastiy, the viscosity of the medium becomes very high at conversion levels above 40 to 50%. Therefore commercial operation at low conversion requires an extensive monomer recovery operation. 

Copolymerization is effected by suspension or emulsion techniques under such conditions that tetrafluoroethylene, but not ethylene, may homopolymerize. Bulk polymerization is not commercially feasible, because of heat-transfer limitations and explosion hazard of the comonomer mixture. Polymerizations typically take place below 100°C and 5 MPa (50 atm). Initiators include peroxides, redox systems (10), free-radical sources (11), and ionizing radiation (12). 

The procedure of forming copolymers dates back to the early 1940s when only phenoHc resins were avaHable. Copolymers were produced by bulk polymerization of phenol [108-95-2] and formaldehyde [50-00-0]. Because the resulting soHd product had the shape of the vessel in which polymerization... 

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

The vast majority of commercial apphcations of methacryhc acid and its esters stem from their facile free-radical polymerizabiUty (see Initiators, FREE-RADICAl). Solution, suspension, emulsion, and bulk polymerizations have been used to advantage. Although of much less commercial importance, anionic polymerizations of methacrylates have also been extensively studied. Strictiy anhydrous reaction conditions at low temperatures are required to yield high molecular weight polymers in anionic polymerization. Side reactions of the propagating anion at the ester carbonyl are difficult to avoid and lead to polymer branching and inactivation (38—44). 

Bulk Polymerization. This is the method of choice for the manufacture of poly(methyl methacrylate) sheets, rods, and tubes, and molding and extmsion compounds. In methyl methacrylate bulk polymerization, an auto acceleration is observed beginning at 20—50% conversion. At this point, there is also a corresponding increase in the molecular weight of the polymer formed. This acceleration, which continues up to high conversion, is known as the Trommsdorff effect, and is attributed to the increase in viscosity of the mixture to such an extent that the diffusion rate, and therefore the termination reaction of the growing radicals, is reduced. This reduced termination rate ultimately results in a polymerization rate that is limited only by the diffusion rate of the monomer. Detailed kinetic data on the bulk polymerization of methyl methacrylate can be found in Reference 42. 

Three bulk polymerization processes are commercially important for the production of methacrylate polymers batch cell casting, continuous casting, and continuous bulk polymerization. Approximately half the worldwide production of bulk polymerized methacrylates is in the form of molding and extmsion compounds, a quarter is in the form of cell cast sheets, and a quarter is in the form of continuous cast sheets. 

DADC HomopolymeriZation. Bulk polymerization of CR-39 monomer gives clear, colorless, abrasion-resistant polymer castings that offer advantages over glass and acryHc plastics in optical appHcations. Free-radical initiators are required for thermal or photochemical polymerization. 

Bulk polymerization has been studied at relatively low temperatures and in toluene and carbon tetrachloride solutions carried to low conversions (12). The effects of temperature and different organic peroxide initiators have been observed. The molecular weight of soluble polymer after 3% conversion is ca — 19,000 and is somewhat dependent on initiator concentration or temperature between 35 and 65 °C. With di-2-methylpentanoyl... 

Fig. 1. Bulk polymerization of diethylene glycol bis(aHylcarbonate) at 45°C with initial addition of 3.0% diisopropyl percarbonate. Rates of polymerization as measured by density and catalyst consumption decrease with time at a given temperature (14).

The Hquid monomers are suitable for bulk polymerization processes. The reaction can be conducted in a mold (casting, reaction injection mol ding), continuously on a conveyor (block and panel foam production), or in an extmder (thermoplastic polyurethane elastomers and engineering thermoplastics). Also, spraying of the monomers onto the surface of suitable substrates provides insulation barriers or cross-linked coatings. 

Bulk Polymerization. The spontaneous polymerisation of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen. Very pure monomer does not polymerize under these conditions. Irradiation by either uv or y-rays (26,28) also induces polymerization of VDC. 

Fig. 1. Bulk polymerization of vinyHdene chloride at 45°C, with 0.5 wt % benzoyl peroxide as initiator (29).

Bulk Polymerizations. In the bulk polymerization of vinyl acetate the viscosity increases significantly as the polymer forms making it difficult to remove heat from the process. Low molecular weight polymers have been made in this fashion. Continuous processes are known to be used for bulk polymerizations (68). 

Glass-Transition Temperature. The T of PVP is sensitive to residual moisture (75) and unreacted monomer. It is even sensitive to how the polymer was prepared, suggesting that MWD, branching, and cross-linking may play a part (76). Polymers presumably with the same molecular weight prepared by bulk polymerization exhibit lower T s compared to samples prepared by aqueous solution polymerization, lending credence to an example, in this case, of branching caused by chain-transfer to monomer. 

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