Control of copolymer composition

Most monomers have different reactivity ratios, which lead to production of copolymers that do not have the same composition of the monomer mixture. In batch copolymerization, the copolymer produced at the beginning of the process is richer in the most reactive monomer, while the copolymer becomes richer in the less reactive monomer at the end of the batch. This composition drift causes the production of heterogeneous polymer mixtures, which may be deleterious for the performance of the polymer material. With the exception of the azeotropic reactions, most copolymerization systems experience composition drifts during batch copolymerizations, which must be corrected if homogeneous copolymer materials are to be produced. For this reason, copolymerizations are usually performed in semibatch (with manipulation of monomer feed flow rates) or continuous mode. 

The previous discussion leads to the definition of the fourth classical control problem, which is the control of the copolymer composition along the reaction batch (or at the end of the batch). This objective is normally attained through manipulation of monomer feed flow rates [44, 45]. The feed stream usually contains the most reactive monomer species, so that composition control is obtained by keeping the concentration of the most reactive monomer concentration at the desired low levels throughout the batch time. It is important to emphasize that implementation of monomer feed strategies may lead to runaway conditions in the presence of heat transfer limitation [ 46 ], which partially explains why control of copolymer composition in emulsion reactors is normally attained by working under starved conditions. 

It is very important to notice that the strategy used to control the copolymer composition may exert a significant impact on the MWD of the final polymer material. Therefore, it is advisable to design control strategies for the simultaneous control of copolymer composition and the MWD in copolymerization reactions. 

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J2 Control of copolymer composition using monomer-starved conditions... 

A further development of the concept of copolymer composition control has been the invention of power feeds by Bassett and Hoy [64,65]. This takes the starve-fed control of copolymer composition one stage further by providing an easy means of continuously varying the composition of the comonomer mixture being fed into the reaction vessel, and may be considered as a means of achieving controlled composition drift. The simplest power feed Systran employs two monomer feed tanks and is illustrated in Figure 7.4a, whitli shows that tank 2 feeds into tank 1 from which monomer is supplied to the reaction vessel. Consider the mass balance in tank 1 with respect to monomer A ... 

Other strategies for controlling copolymer composition Although the use of monomer-starved conditions for control of copolymer composition is widespread, the low monomer feed rates which need to be used give rise to low rates of copolymerization and have significant effects upon the molar mass and molar mass distribution of the copolymers formed (see Section 7.4.4.4). Hence, alternative procedures have been developed which facilitate higher feed rates, but nevertheless allow for control of copolymer composition. These procedures are briefly described in this section. 

A numerical technique that has become very popular in the control field for optimization of dynamic problems is the IDP (iterative dynamic programming) technique. For application of the IDP procedure, the dynamic trajectory is divided first into NS piecewise constant discrete trajectories. Then, the Bellman s theory of dynamic programming [175] is used to divide the optimization problem into NS smaller optimization problems, which are solved iteratively backwards from the desired target values to the initial conditions. Both SQP and RSA can be used for optimization of the NS smaller optimization problems. IDP has been used for computation of optimum solutions in different problems for different purposes. For example, it was used to minimize energy consumption and byproduct formation in poly(ethylene terephthalate) processes [ 176]. It was also used to develop optimum feed rate policies for the simultaneous control of copolymer composition and MWDs in emulsion reactions [36, 37]. 

Colloid Polymer Science 278, No.4, April 2000, p.375-9 COPOLYMERIZATION OF STYRENE AND REACTIVE SURFACTANTS IN A MICROEMULSION CONTROL OF COPOLYMER COMPOSITION BY ADDITION OF NONREACTIVE SURFACTANT Pyrasch M Tieke B Koln,Universitat... 

FIGURE 6.13 Proposed scheme for control of copolymer composition in semibatch suspension AAJVA copolymerizations. Reprinted (adapted) with permission from Silva FM, Lima EL, Pinto JC. Control of copolymer composition in suspension copolymerization reactions. Ind Eng Chem Res 2004 43 7312-7323. 2004 American Chemical Society. 

Silva FM, Lima EL, Pinto JC. Control of copolymer composition in suspension copolymerization reactions. Ind Eng Chem Res 2004 43 7312-7323. 

Vicente M, Leiza JR, Asua JM. Simultaneous control of copolymer composition and MWD in emulsion copolymerization. AlChE J 2001 47 1594-1606. 

Wang R, Luo Y, Li B, Sun X, Zhu S. Design and control of copolymer composition distribution in living radical polymerization using semi-batch feeding policies a model simulation. Macromol Theor Simul 2006 15 356-368. 

In general, the optimization of polymerization processes [2] focuses on the determination of trade-offs between polydispersity, particle size, polymer composition, number average molar mass, and reaction time with reactor temperature and reactant flow rates as manipulated variables. Certain approaches [3] apply nonhnear model predictive control and online, nonlinear, inferential feedback control [4] to both continuous and semibatch emulsion polymerization. The objectives include the control of copolymer composition. 

We have seen in the previous chapters how the composition of a random copolymer can influence many of its important properties, including solubility, degree of crystallinity, Tg, and Tm- The control of copolymer composition is therefore of great practical importance. 

Figure 3.7 Butyl acrylate-Veova 9 control of copolymer composition distribution by online Raman...

Figure 4.12 Two possible set-ups for power-feed control of copolymer composition.

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