Semibatch composition control

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

Tip 13 (related to Tip 12) Copolymerization, copolymer composition, composition drift, azeotropy, semibatch reactor, and copolymer composition control. Most batch copolymerizations exhibit considerable drift in monomer composition because of different reactivities (reactivity ratios) of the two monomers (same ideas apply to ter-polymerizations and multicomponent cases). This leads to copolymers with broad chemical composition distribution. The magnirnde of the composition drift can be appreciated by the vertical distance between two items on the plot of the instantaneous copolymer composition (ICC) or Mayo-Lewis (model) equation item 1, the ICC curve (ICC or mole fraction of Mj incorporated in the copolymer chains, F, vs mole fraction of unreacted Mi,/j) and item 2, the 45° line in the plot of versus/j. 

Flint 3. Semibatch or continuous reactors involve the continuous (over a certain period or periods of time) or intermittent flow of monomer (or other ingredients, like components of an initiator system, solvent or CTA) into the polymerizing mixture. This flow may in general have several beneflcial effects, ranging from extra cooling to more flexibility for molecular weight and branching control, all the way to polymerization rate and composition control. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

In emulsion polymerizations semibatch operation provides better control of the particle size of the product. The properties of the product polymers can be modified, also, by continuous or intermittent changes in the composition of the monomer feed in emulsion copolymerizations, where a given monomer can be preferentially concentrated in the interior or on the surface of the final particles, as described in Chapter 8. 

Figure 11. Average copolymer composition with time in response to step change in monomer ratio in a controlled semibatch reactor. Key -------, MA ------, ST.

A second motivation for using a monomer-feed semibatch procedure is to control copolymer composition and/or particle morphology. Delayed feed of part of the more reactive monomer can be used to eliminate or reduce the composition drift of the copolymer. The delayed feed of a comonomer mixture when the reactor is operated in the monomer-starved regime can also be used to prevent copolymer composition drift. Such operations will produce polymer particles with more uniform morphology. Different monomer addition schemes can be employed to control nonuniform particle morphology (see papers by Bassett et al. in General References 7 and 9). 

A second reason for intermediate monomer feed locations is to control copolymer composition and particle morphology. Unlike the batch reactor in which the more reactive monomer reacts first, the copolymer produced in a CSTR should have uniform composition. If all monomers are fed to the first reactor, however, the polymer formed in the downstream reactors will contain less of the most reactive monomer. Thus, the use of intermediate feed locations in a CSTR system is analogous to the programmed addition of reactive monomers in semibatch reactors. The location and rates of the various monomer streeims will influence copolymer composition and particle morphology. 

In ordinary batch copolymerization there is usually a considerable drift in monomer composition because of different reactivities of the two monomers (based on the values of the reactivity ratios). This leads to a copolymer with a broad chemical composition distribution (CCD). In many cases (depending on the specific final product application) a composition drift as low as 3-5% cannot be tolerated, for example, copolymers for optical applications on the other hand, during production of GRIN (gradient index) lenses, a controlled traj ectory of copolymer composition is required. This is partly circumvented in semibatch operation where the composition drift can be minimized (i.e., copolymer composition can be kept constant ) by feeding a mixture of the monomers to the reactor with the same rate by which each of them is consumed in the reactor. 

Semibatch copolymerization is most often done in an attempt to maintain a reasonably constant copolymer composition when the comonomers are of widely varying reactivities [1]. Semibatching of initiator is often done to maintain temperature control in a heat transfer-limited kettle, and semibatch addition of initiator or chain transfer agent may be used to maintain a desired MWD. Quantitative strategies for semibatching may be developed through empirical experimentation at the bench or pilot scale, or, if accurate mathematical models are available, classical trajectory optimization techniques may be used [2,3]. 

The batch emulsion polymerization is commonly used in the laboratory to study the reaction mechanisms, to develop new latex products and to obtain kinetic data for the process development and the reactor scale-up. Most of the commercial latex products are manufactured by semibatch or continuous reaction systems due to the very exothermic nature of the free radical polymerization and the rather limited heat transfer capacity in large-scale reactors. One major difference among the above reported polymerization processes is the residence time distribution of the growing particles within the reactor. The broadness of the residence time distribution in decreasing order is continuous>semibatch>batch. As a consequence, the broadness of the resultant particle size distribution in decreasing order is continuous>semibatch>batch, and the rate of polymerization generally follows the trend batch>semibatch>continuous. Furthermore, the versatile semibatch and continuous emulsion polymerization processes offer the operational flexibility to produce latex products with controlled polymer composition and particle morphology. This may have an important influence on the application properties of latex products [270]. 

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. 

The semibatch approach, where policies are developed for selective reagent feeds to the reactor, has been extensively elaborated, especially for emulsion polymerization, and in the context of controlling composition during copolymerization reactions [68-72,109]. Discussions are provided in Chapters 4, 7, 12, 17-19, and 21. Sun et al. developed model-based semibatch monomer feeding policies for controlled radical polymerization (CRP) [73, 74]. Vicente et al. [75,76] controlled composition and molecular weight distribution in emulsion copolymerization in an open-loop method by maintaining the ratio of comonomers. Yanjarappa et al. [77] synthesized, via a sanibatch method, copolymers with constant composition for biofunctionalization. General semibatch policies are reviewed by Asua [78]. 

FIGURE 13.15 Low composition drift of Q9 in a batch reaction and semibatch experiments in which F. was predictively arranged to either decrease or increase. In the latter case, a blend of Am/Q9 copolyelectrolyte and hompolymeric Q9 was produced. Reprinted (adapted) with permission from Kreft T, Reed WF. Predictive control of average composition and molecular weight distributions in semibatch free radical copolymerization reactions. Macromolecules 20()9 42 5558-5565. 2009 American Chemical Society. 

Sun X, Luo Y, Wang R, Li, B-G, Liu B, Zhu S. Programmed synthesis of copolymer with controlled chain composition distribution via semibatch RAFT copolymerization. Macromolecules 2007 40 849-859. 

Kreft T, Reed WF. Predictive control of average composition and molecular weight distributions in semibatch free radical copolymerization reactions. Macromolecules 2009 42 5558-5565. 

---