Molecular weight distribution microreactors

Iwasaki T, Yoshida J (2005) Free radical polymerization in microreactors. Significant improvement in molecular weight distribution control. Macromolecules 38 1159-1163... 

The reaction was faster in the microreactor than in the batch system, which is assigned to be a mixing effect [57]. The molecular weight distribution obtained in the microreactor is slightly narrower than that of the batch reactor, which is attributed to the good thermal management of the microreactor. 

In Chapter 9, we mentioned that the use of microreactors leads to a significant improvement in the control of the molecular-weight distribution in free radical polymerization by virtue of superior heat-transfer efficiency.Free-radical polymerization reactions are usually highly exothermic, so precise temperature control is essential to carry out these reactions in a highly controlled manner. Thus, from an industrial viewpoint, a major concern with free-radical polymerization is the controllability of the reaction temperature. Temperature control often arises as a serious problem during the scale-up of a bench process to industrial production. In this section, we will discuss the numbering-up of microreactors to increase production volumes in radical polymerization in industry. 

For values of this Peclet number well below 1, as encountered in microreactors, a narrow molecular weight distribution can be achieved, while higher values, like those encountered in macroscale reactors, induce a drastic increase in the polydispersity index (Fig. 6.32) [37,48]. Therefore, microreactors can lead to better control over bulk or semi-dilute polymerization processes. 

Recently, it has been demonstrated that good control of molecular weight and molecular weight distribution can be attained by using microreactor systems without stabilizing the carbocationic intermediates. The concept of this new technology (flow-microreactor-system-controlled polymerization) is described in the following... 

An example of microreactor systems for block copolymerization is shown in Fig. 7. The first monomer IBVE is mixed with TfOH in the first micromixer (Ml). Introduction of the second monomer (NB VE or EVE) at the second micromixer M2 results in the formation of the polymer of higher molecular weight with narrow molecular weight distribution [128]. Block copolymerization can be carried out with any combination and with either order of monomer addition, as shown in Table 3, demonstrating that the present method serves as a flexible method for the synthesis of block copolymers. Therefore, flow-microreactor-system-controlled polymerization can serve as a powerful method for synthesis of structurally well-defined polymers and copolymers in industry. 

In a conventional anionic polymerization of styrenes in polar solvents in a batch macroreactor, major drawbacks include the requirement of low temperature such as —78°C. In contrast, Nagaki et al. reported that controlled anionic polymerization of styrene can be conducted under easily accessible conditions such as 0°C in a polar solvent using a flow microreactor to obtain the polystyrene with narrower molecular weight distribution (M = 1,200-20,000, MJM = 1.09-1.13) (Fig. 9) [146]. Moreover, the molecular weight can be easily controlled by changing the flow rates of monomer and initiator solutions. Furthermore, these methods can be... 

RAFT polymerizations of W-isopropylacrylamide (NIPAM) as monomer and a trithiocarbonate as chain transfer agent have been carried out using a flow microreactor under homogeneous conditions (Fig. 26) [210]. In a flow process, an increase in the inner diameter of the tube results in slightly lower conversions and wider molecular weight distributions. Polymerization rates in a flow microreactor are considerably higher than those of batch polymerization because of uniform heating (Table 6). 

A silicon-glass-based flow microreactor system that is suitable for long periods of use has been developed and applied to the polymerization of amino acid NCAs [229]. The flow microreactor exhibits excellent controllability of the molecular weight distribution. Moreover, a single flow microreactor can produce 100 mg/min of copoly(Lys-Leu). This means that more than 200 g of copoly(Lys-Leu) acids can be produced in 2 months. 

Table 9 Molecular weight and molecular weight distribution obtained in microreactor synthesis...

In recent years, the main focus of the patents has shifted from the development of novel microstructured device to the well-defined application of the device to proper reaction process. Homogeneous liquid reactions are occupying a large part of patents. The main improvement of these patents is to achieve the faster reaction in microreactor than batch reactor or to conduct the reaction with hazardous chemicals without human exposure. Microreactors also can be applied to multiphase and gas-phase reactions. Recently, the carbonylation reaction using carbon monoxide in microreactor has been reported [15]. The polymerization process can be also controlled with microreactors. The microreactor leads to the concise synthesis of polymers with a narrow molecular weight distribution. 

The benefit of microreaction systems is not limited to cationic or radical polymerization. Amino acid polymerization using a-amino acid V-carboxyanhydride (NCA) yielded better molecular weight distribution when performed in a microreactor [5]. Also, the average molecular weight could be controlled by changing the flow rates. 

For butyl acrylate (BA), the molecular weight distribution was found to be narrower than that for the batch reactor, as can be seen in Figure 12.5. The PDI for this polymer is then lower in the microreactor system (Table 12.1). The difference was smaller but still noticeable for benzyl methacrylate (BMA) and methyl methacrylate (MMA) and almost zero for vinyl benzoate (VBz) and styrene (St) (Table 12.2). The authors claimed that the observed results are directly related to the superior heat transfer ability of the microtube reactor. The more exothermic the polymerization reaction. 

Figure 12.5 Molecular weight distribution of poly(butyl acrylate) produced in the microreactor system (solid line) and in the macroscale batch reactor (dashed line). The residence time was 4 min. From Ref [10].

Micromrxers in conjunction with serial microreactors can also be used effectively for LRP reactions, particularly for mixing viscous living polymer melts with non-viscous monomer for block copolymer production. For example, poly(n-butyl acrylate) can be synthesized in a microtube reactor via an N M P reaction, then the viscous homopolymer melt can be efficiently mixed with low-viscosity styrene monomer via a micromixer [90]. This can then be followed by N M P of the styrene on to the poly (w-butyl acrylate) chains in a second microtube reactor, thus creating a block copolymer. This technique gives a narrower molecular weight distribution product than comparable batch reactions. 

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