Continuous polymerization control

Continuous Polymerization Control Strategies hi continuous polymerizations, temperature and pressure are controlled in much the same way as in batch systems. 

For continuing polymerization to occur, the ion pair must display reasonable stabiUty. Strongly nucleophilic anions, such as C/ , are not suitable, because the ion pair is unstable with respect to THE and the alkyl haUde. A counterion of relatively low nucleophilicity is required to achieve a controlled and continuing polymerization. Examples of anions of suitably low nucleophilicity are complex ions such as SbE , AsF , PF , SbCf, BE 4, or other anions that can reversibly coUapse to a covalent ester species CF SO, FSO, and CIO . In order to achieve reproducible and predictable results in the cationic polymerization of THE, it is necessary to use pure, dry reagents and dry conditions. High vacuum techniques are required for theoretical studies. Careful work in an inert atmosphere, such as dry nitrogen, is satisfactory for many purposes, including commercial synthesis. 

Continuous polymerization processes for PA-6,6 have been reported for over 30 years.5,6,28 Prepolymerization in tubular (Fig. 3.21) or baffled reactors is particularly well suited to continuous polymerization. The polymerization of prepolymers to high-molecular-weight materials in a continuous process is more difficult to control as small differences is molecular weights result in large differences in viscosities. Viscosity differences result in different hold-up times in die reactor and thus nonhomogeneous products. 

G.R. Meira. Forced oscillations in continuous polymerization reactors and molecular weight distribution control. A survey. J. Macromol. Sci.- Rev. Macro-mol. Chem., 20(2) 207-241, 1981. 

The two prominent variables to be controlled in a continuous polymerization system are the reaction temperature and the monomer conversion achieved in the reaction system. Final polymer properties are directly influenced by changes in these process... 

A recent paper by Kiparissides, et al. (8) details a mathematical model for the continuous polymerization of vinyl acetate in a single CSTR. Operating conditions were shown to exist in which either steady-state operation or sustained conversion oscillations would occur for vinyl acetate. Experimental results for both cases were successfully simulated by their model. In addition, regulatory conversion control policies were considered in which both initiator feed rate and emulsifier feed rate were used as manipulated variables (Kiparissides (9)). The problem of conversion control in the operating region in which sustained conversion oscillations occur is one of significant commercial importance. Most commonly, however, a uniform concentration of emulsifier is required in the emulsion recipe and, hence, emulsifier flow rate cannot be used as a manipulated variable. 

Figure 20. Simulated conversion response of continuous polymerization system to a load disturbance under closed-loop control with IAE optimum controller tuning constants and manipulation of initiator flow rate at 0.06 mol/L H20 surfactant and 50°C catalyst feed concentration—STD feedback (-) vs.

Thus, the use of alkyllithium initiation offers the synthetic chemist a tool of enormous flexibility for "tailor-making" polymers of precise structure. Control of molecular weight, molecular-weight distribution, diene structure, branching, monomer-sequence distribution, and functionality can conveniently be achieved by such techniques as incremental or sequential addition of monomer, initiators, or modifier, programming of temperature, continuous polymerization, or the use of multifunctional reagents. 

Another problem associated with the batch technique is poor reaction control (unsatisfactory stirring, temperature control, etc). To overcome the problems outlined above a semi-continuous polymerization technique has been introduced [27]. In this technique a mixed monomer/inifer feed is added at a sufficiently low constant rate to a well stirred, dilute BC13 charge. Due to stationary conditions maintained during the whole polymerization, well-defined telechelic products with symmetrical end groups and theoretical polydispersities could be obtained. The kinetics of the polymerization has been discussed and the DPn equation has been derived. In contrast to the batch technique, the DPn for the semi-continuous technique is simply given by the [monomer]/[inifer] ratio. Thus, very reactive or unreactive inifers, unsuitable for batch polymerization, can also be used in the semi-continuous process. 

Along with micromixing, the control of macromixing or RTD is also important for efficient operation of the polymerization reactor. For continuous polymerization, the reactor should accomplish the following requirements ... 

Fig. 2 Continuous polymerization apparatus A, CO2 cylinder B, monomer C, initiator solution D, continuous syringe pumps E, syringe pumps FI, steady-state filter G, thermo-stated autoclave H, static mixer I, chiller/heater unit J, effluent cooler K, gas chromatograph VI, V2, four-way valves V4, three-way valves V5, V6, two-way valves V7, heated control valve [51]... 

Figure 1.5 Schematic of BASF s early tower process for the continuous polymerization of styrene. This configuration was designed by C. Wulff and E. Dorrer in the early 1930s. Polymerization was thermally initiated and the exotherm controlled by heat transfer tubes (courtesy of BASF, Ludwigshafen)...

In certain forms of the material, the microporous polymer creates exactly two distinct, interwoven but disconnected porespace labyrinths, separated by a continuous polymeric dividing wall. This opens up the possibility of performing enzymatic, catalytic or photosyndietic reactions in controlled, ultrafinely microporous polymeric materials with the prevention of recombination of the reaction products by their division into the two labyrinths. These features combine with specific surface areas for reaction on the order of lO -lO square meters per gram, and with the possibility of readily controllable chirality and porewall surface characteristics of the two labyrinths. 

Winyl polymerization as a rule is sensitive to a number of reaction variables, notably temperature, initiator concentration, monomer concentration, and concentration of additives or impurities of high activity in chain transfer or inhibition. In detailed studies of a vinyl polymerization reaction, especially in the case of development of a practical process suitable for production, it is often desirable to isolate the several variables involved and ascertain the effect of each. This is difficult with the conventional batch polymerization technique, because the temperature variations due to the highly exothermic nature of vinyl polymerization frequently overshadow the effect of other variables. In a continuous polymerization process, on the other hand, the reaction can be carried out under very closely controlled conditions. The effect of an individual variable can be established accurately. In addition, compared to a batch process, a continuous process normally gives a much greater throughput per unit volume of reactor capacity and usually requires less labor. 

During the authors investigation of acrylamide polymerization in aqueous solutions, a laboratory scale continuous process, with reactors of 2- or 3-liter capacity, was developed. It offered simple and flexible operation, and close control of conditions. This article describes the technique adopted and some experimental results showing the effect of individual variables on the molecular weight of the polymer formed. A theoretical treatment of the continuous polymerization process has been made recently by Jenkins (4). The empirical data obtained in the present work are examined with the aid of theoretical relationships. 

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