Molecular weight control, semibatch

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

FIGURE 6.14 Proposed scheme for control of copolymer composition and average molecular weights in semibatch methyl methacrylate/butyl acrylate emulsion copolymerizations. Reprinted (adapted) with permission from Vieira RAM, Sayer C, Lima EL, Pinto JC. Closed-loop composition and molecular weight control of a copolymer latex using near-infrared spectroscopy. Ind Eng Chem Res 2002 41 2915-2930. 2002 American Chemical Society. 

Kreft, T Reed WF. Predictive control and verification of conversion kinetics and polymer molecular weight in semibatch free radical homopolymer reactions. Eur Polym J 2009 45 2288-2303. 

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. 

When operating under semibatch Policy II, common practice is to maintain the reactor contents at low or starved monomer concentrations. This provides for relatively straightforward temperature control and overall reactor operation. However, when such low monomer concentrations are used over the duration of the polymerization, the potential for significant long-chain branching and crosslinking exists. The molecular weight profile would, therefore, be radically different from a batch process. 

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. 

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 13.12 Results of predictive control of for free radical polymerization of Am in semibatch operation. In batch mode, decreases monotonically in time. By computing conditions for isoreactivity (Equation 13.63c) constant during the reaction was achieved. By operating Am flow into the reactor in the flooded regime (Equation 13.63b), a predictable increase in during the semibatch reaction was achieved. Adapted with permission from Kreft T, Reed WF. Predictive control and verification of conversion kinetics and polymer molecular weight in semi-batch free radical homopolymer reactions. Eur Polym J 2009 45 2288-2303. 

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. 

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

Advanced control of a semibatch reactor involves driving the reaction from an initial state to a specified final state in some manner which is judged to be the best in terms of productivity or product quality. Nonlinear model predictive control may be onployed to drive the reaction along a trajectory, which maximizes some predetermined objective functional [25]. Such a procedure presumes high-quality online sensors for polymer properties (monomer conversion and number-average molecular weight), an accurate mathematical model, and an optimization-based controller which allows control objectives rather than just setpoints to be included. 

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