High-Conversion Bulk Polymerizations

In many polymerizations, a marked increase in rate is observed toward the end of the reaction instead of the expected gradual decrease caused by the depletion of the monomer and initiator. This auto-acceleration is a direct result of the increased viscosity of the medium, and the effect is most dramatic when polymerizations are carried out in the bulk phase or in concentrated solutions. The phenomenon, sometimes known as the Trommsdorff-Norrish or gel effect, is caused by the loss of the steady state in the polymerization kinetics. 

When the viscosity of the reaction medium increases, the different steps in the polymerization reaction become diffusion controlled at stages that depend on the 

FIGURE 3.2 (a) Schematic representation of the polymerization rate vs. conversion, (b) Schematic representation of the vitrification points for a bulk polymerization of poly(methyl methacrylate) at various polymerization temperatures. The solid line represents the volume fraction of polymer at which polymerization stops, as a function of polymerization temperature. 

O Driscoll has proposed that the auto-acceleration can be modeled by recognizing that the termination reaction is diffusion controlled but will also depend on the size of the chain involved. The critical chain length for entanglement then becomes an important parameter, and two termination rate constants can also be defined, one for chains smaller than and one for large entangled chains. These are, respectively, and k. If v is the kinetic chain length and Vp is the conventional steady-state polymerization, then the observed rate v is given by 

Good agreement between the predicted rate from Equation 3.17 and the experimental values obtained for the bulk polymerization of methyl methacrylate has been reported. 

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Russell GT, Napper DH, Gilbert RG. Initiator efficiencies in high conversion bulk polymerizations. Macromolecules 1988 21 2141-2148. 

Poly (ethylene terephihalate) and nylon-6,6 manufacture are homogeneous bulk step-growth reactions. The molecular weight of the polymer produced is limited by the high viscosity of the reaction mixture at very high conversions. Post polymerization techniques such as that described in connection with reaction (5-39) can be used to increase the polymer molecular weight for some applications. 

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 ... 

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. 

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. 

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 [Lm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reaciors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCh solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

The importance of the cage reaction increases according to the viscosity of the reaction medium. This contributes to a decrease in initiator efficiency with conversion. 15 1 155 Stickler and Dumont156 determined the initiator efficiency during bulk MMA polymerization at high conversions ca 80%) to be in the range 0.1-0.2 depending on the polymerization temperature. The main initiator-derived byproduct was phenyl benzoate. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

In this manuscript we review the principles of bulk and solution polymerization with particular emphasis on high conversion (high polymer concentrations) rate of polymerization and molecular weight development. 

In the literature there is only one serious attempt to develop a detailed mechanistic model of free radical polymerization at high conversions (l. > ) This model after Cardenas and 0 Driscoll is discussed in some detail pointing out its important limitations. The present authors then describe the development of a semi-empirical model based on the free volume theory and show that this model adequately accounts for chain entanglements and glassy-state transition in bulk and solution polymerization of methyl methacrylate over wide ranges of temperature and solvent concentration. 

Attainment of a maximum double bond conversion is typical in multifunctional monomer polymerizations and results from the severe restriction on bulk mobility of reacting species in highly crosslinked networks [26]. In particular, radicals become trapped or shielded within densely crosslinked regions known as microgels, and the rate of polymerization becomes diffusion limited. Further double bond conversion is almost impossible at this point, and the polymerization stops prior to 100% functional group conversion. In polymeric dental composites, which use multifunctional methacrylate monomers, final double bond conversions have been reported ranging anywhere from 55-75% [22,27-29]. 

Poly(acrylic acid) is not soluble in its monomer and in the course of the bulk polymerization of acrylic acid the polymer separates as a fine powder. The conversion curves exhibit an initial auto-acceleration followed by a long pseudo-stationary process ( 3). This behaviour is very similar to that observed earlier in the bulk polymerization of acrylonitrile. The non-ideal kinetic relationships determined experimentally in the polymerization of these two monomers are summarized in Table I. It clearly appears that the kinetic features observed in both systems are strikingly similar. In addition, the poly(acrylic acid) formed in bulk over a fairly broad range of temperatures (20 to 76°C) exhibits a high degree of syndiotacticity and can be crystallized readily (3). 

The second large-scale process was the batch mass suspension process. Monsanto did the pioneer work on this (41). In this process, prepolymerization is carried out in bulk and main polymerization in suspension the latter is taken to conversions of over 99%. In contrast to the continuous mass process, peroxide starters are used in order to achieve a high conversion at tolerable reaction times. Figure 3 shows a basic flow diagram of such a plant. A detailed discussion of advantages and disadvantages of the two processes can be found in R. Bishop s monograph published in 1971 (42), and it is continued in a paper by Simon and Chappelear in 1979 (43). It was a decisive factor for the economic success of impact polystyrene that these processes had been completely developed and mastered in theory and practice. 

Use of Pseudomonas cepacia lipase (lipase PS) or Porcine pancreatic lipase does allow for the enzymatic ROP of lactide. Matsumura and coworkers reported polymers with extraordinarily high molecular weights (Mw up to 270 kDa) and very narrow PDI (<1.3) [135-137]. However, high temperatures (130°C) were needed to achieve good conversions, and polymerizations proceeded only when conducted in bulk. It is conceivable that another non-enzymatic mechanism contributed in these polymerizations. In fact, Koning and coworkers synthesized copolymers... 

The bulk polymerization effects the special needs to remove the heat of reactions, and moreover, high conversions cannot be reached because the viscosity of the polymer increases drastically with conversion. In order to avoid a high viscosity of the end product before discharging, the mass polymerization is carried out in solution. Ethylbenzene is a common solvent. 

Because of the highly exothermic nature of acrylonitrile polymerization, bulk processes arc not normally used commercially. Howevei. a commercially feasible process lor bulk polymerization in a continuous stirred lank reactor has been developed. The heat nl reaction is controlled hy operating at relatively low conversion levels and supplementing the normal jacket cooling with reflux condensation of umcaclcd monomer... 

The locus of reaction during an emulsion polymerization is nearly exclusively within particles in which the ratio of polymer to monomer is high enough so that the reacting fluid is quite viscous. Relative to bulk polymerization, this is beyond the start of the Mgel effect and one should expect the influence of restricted diffusion of the polymer to be felt during the entire emulsion polymerization reaction. The most common approach for treating this behavior in bulk polymerization is to treat the termination rate constant, k, as a function of conversion level... 

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