Conversion styrene polymerization

Peaking and Non-isothermal Polymerizations. Biesenberger a (3) have studied the theory of "thermal ignition" applied to chain addition polymerization and worked out computational and experimental cases for batch styrene polymerization with various catalysts. They define thermal ignition as the condition where the reaction temperature increases rapidly with time and the rate of increase in temperature also increases with time (concave upward curve). Their theory, computations, and experiments were for well stirred batch reactors with constant heat transfer coefficients. Their work is of interest for understanding the boundaries of stability for abnormal situations like catalyst mischarge or control malfunctions. In practice, however, the criterion for stability in low conversion... 

Figure 4. Temperature vs. conversion for batch-mass styrene polymerization (4)...

Figure 5. Number average degree of polymerization vs, conversion for hatch-mass styrene polymerization (4j...

Hamielec and coworkers (, 42, 43) have conducted extensive experimental and theoretical studies with styrene polymerization in CSTR s. Theirs represent probably the first published work in this area at commercially interesting temperatures and conversions relating theory to experiment, and determining the effects of reactor configuration and conditions on conversion, molecular weight and MWD. 

Experimental work with styrene in tubular reactors has been reported (39) where viscosities were relatively low due to conversions below 32%. However, Lynn ( ) has concluded that a laminar flow tubular reactor for styrene polymerization is probably technically infeasible due to the distortion in velocity... 

TREYBiG AND ANTHONY Atiionic Styrene Polymerization TABLE IV. EXPERIMENTAL MONOMER CONVERSIONS DEGREES OF POLYMERIZATION FOR CFSTR RUNS 1 313 AND -15... 

SAMPLE NO. POLYMERIZATION TIME (min) CONVERSION STYRENE IN MONOMER INITIAL STYRENE IN MONOMER GPC 2 CHROMATOGRAM VARIANCE GPC INJECTED INTO TOTAL AMOUNT OF POLYMER INJECTED (mg) VOLUME INJECTED GAUSSIAN FIT TO GPC 2 CHROMATOGRAM... 

The effect of the nitrone stmcture on the kinetics of the styrene polymerization has been reported. Of all the nitrones tested, those of the C-PBN type (Fig. 2.29, family 4) are the most efficient regarding polymerization rate, control of molecular weight, and polydispersity. Electrophilic substitution of the phenyl group of PBN by either an electrodonor or an electroacceptor group has only a minor effect on the polymerization kinetics. The polymerization rate is not governed by the thermal polymerization of styrene but by the alkoxyamine formed in situ during the pre-reaction step. The initiation efficiency is, however, very low, consistent with a limited conversion of the nitrone into nitroxide or alkoxyamine. 

The process begins in a prepolymerizer, which is a water-jacketed reactor with a mixer in it. See Figure 23—12.) The styrene is partially polymerized by adding the peroxide initiator and heating to 240—250°F for about four hours. About 30% of the styrene polymerizes and the reactor contents become syrupy goo. Thats about as far as the prepolymer step can go—30% conversion— because the mixing and heat transfer gets very inefficient as the goo gets thicker, and the polymerization becomes hard to control. 

Case 3 behavior occurs when the particle size is sufficiently large (about 0.1-1 pm) relative to kt such that two or more radicals can coexist in a polymer particle without instantaneous termination. This effect is more pronounced as the particle size and percent conversion increase. At high conversion the particle size increases and k, decreases, leading to an increase in h. The increase in h occurs at lower conversions for the larger-sized particles. Thus for styrene polymerization it increases from 0.5 to only 0.6 at 90% conversion for 0.7-pm particles. On the other hand, for 1.4-pm particles, n increases to about 1 at 80% conversion and more than 2 at 90% conversion [Chatterjee et al., 1979 Gerrens, 1959]. Much higher values of h have been reported in other emulsion polymerizations [Ballard et al., 1986 Mallya and Plamthottam, 1989]. Methyl methacrylate has a more pronounced Trommsdorff effect than styrene and vinyl acetate, and this results in a more exaggerated tendency toward case 3 behavior for methyl methacrylate. 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

Fig. 14. Plot of conversion against polymerization time (SDS 0.3 g, water 3 ml, styrene 40 ml, 40 °C). ( ) Gel (O) bulk...

Although indan formation is significant in styrene polymerizations, j3-proton elimination is much faster than intramolecular alkylation [292]. Unsaturated styrene and a-methylstyrene dimers are prepared quantitatively under high dilution at elevated temperatures without cyclization to indan derivatives [293]. In this case, the carbocations must be quenched before intramolecular cyclization becomes significant at high conversion. However, indan formation competes with depropagation at temperatures above 50° C, which is much too high for unsaturated styrene dimers (D =) to homopolymerize. As outlined in Eq. (95), the unsaturated dimers form indans (Din) in the presence of acid. 

In 1988 Faust and Kennedy reported that controlled/living styrene polymerization would be possible with a combination of l-(p-methylphe-nyl)ethyl acetate [CH3ChH4CH(CH3)-OCOCH3 the adduct of acetic acid with p-methylstyrene] and BCI3 in CH3CI solvent below -30° C [214]. Similar systems were also reported by Matyjaszewski and Lin [27,117,208], Although the M of the polymers increases linearly with conversion, the controlled/living nature of these polymerizations is rather obscured by very broad MWDs where the MJM ratios sometimes exceed 6. 

There is further evidence that radical termination reactions are diffusion-controlled. For many polymers, the rate of polymerization shows a sudden increase when the fraction of polymer produced reaches values near 15 to 30 per cent. In the case of methyl methacrylate, Matheson et al. found that, at 30 C and 15 per cent conversion, kt has decreased 160 fold, while kp has not changed appreciably. Vaughan " has proposed a simple diffusion model which is in reasonable accord with the data on styrene polymerization at high conversions. 

Systems Where Radical Desorption is Negligible. Styrene and methyl methacrylate emulsion polymerization are examples of systems where radical desorption can be neglected. In Figures 4 and 5 are shown comparisons between experimental and theoretical conversion histories in methyl methacrylate and styrene polymerization. The solid curves represent the model, and it appears that there is excellent agreement between theory and experiment. The values of the rate constants used for the theoretical simulations are reported in previous publications (, 3). The dashed curves represent the corresponding theoretical curves in the calculation of which gel-effect has been neglected, that is, ktp is kept constant at a value for low viscosity solutions. It appears that neglecting gel-effect in the simulation of styrene... 

Kinetics of initiated radical polymerization of styrene is well studied at the initial stages. At the high-conversion of polymerization the growth of viscosity of media- influences the mechanism and kinetics of process greatly (12),In our research we have attempted to obtain tne empirical dependence of some rate constants of initiated polymerization of styrene on conversion with the aim of using them at mathematical simulation of the process. 

To calculate the rates of styrene polymerization with the help of experimental data on conversion-time the method of the digital differentiation using five points (16) was applied. In table V the values of Kp/Kt for different conversions are represented, the values have been obtained during the styrene polymerization in the presence of different initial concentrations of BzjOa at temperature of 70° C, Up to conversion of - 40% the ratio of Kp/Kt somewhat increases smd does not depend on the initial concentration of initiator. But at more high stages of polymerization the difference of MW of polymer being formed reveals itself and the ratio of Kp/Kt increases as quickly as the initial concentration of initiator decreases. Dependences of Kp/Kt on the con-... 

Another concern is what to do with the acid catalyst at the end of the process. Typically, styrene polymerization is carried out to 50-90% conversion. The unreacted styrene is flashed from the polymer product, condensed, and recycled into the reactor feed. If the materials of construction allow, a volatile acid catalyst could also be continuously recycled through the system, theoretically reaching a point where no fresh catalyst feed would be required. If corrosion in the devolatilization and recycle system is a concern, then the neutralization option mentioned above might be considered. 

F7-21g Sketch the polypiet concentration, Pj, mole fraction of polymer with j monomer units, yj, and the corresponding weight fraction, Wj, for j = 2, 10, 30 as a function of monomer conversion in Styrene polymerization for (a) Termination by means other than combination. 

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