Molecular weight 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. 

Similarly, small amounts of N2O are detected at approximately 400 C, and this product was observed during most of the ammonia oxidation trials. Srinivasan et al. (1997) and Srinivasan (1998) did not report the presence of N2O during studies on the oxidation of NHj in a prototype MIT microreactor design. This may have been due to an overlap with CO2 since these species have the same molecular weights. 

Microreactors have also been used for ionic polymerization or polycondensation processes. Nagaki et al. [136] have synthesized polystyrene-poly(alkyl methacrylate) block copolymers by butyllithium initiated anionic polymerization in an integrated flow microreactor system. A high level of control of molecular weight was achieved at temperatures between -28 and +24 °C due to fast mixing, fast heat transfer, and residence time control. Santos and Metzger... 

Therefore, coils of functionalized macromolecules with surrounding solvent molecules fixed to chains at catalytic sites behave like isolated microreactors. The activity of sites inside the microreactor depends on the properties of loose low-molecular weight analogues [130, 132, 133]. 

The stable encapsulation of biological cargo is important for applications in drug delivery. However, for some applications, enhanced permeability can be advantageous, for example in the design of microreactors. In this case, diffusion of low molecular weight species can initiate and maintain reactions occurring inside the capsule (see Sect. 4). 

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... 

Controlled anionic polymerization of alkyl methacrylates initiated by 1,1-diphenyUiexyllithium using a flow microreactor gives the corresponding poly (aUcyl methacrylate)s with high level of control of molecular weight under easily accessible temperatures compared with conventional batch macropolymerization, e.g., —28°C for methyl methacrylate (MMA) (MJMn = 1.16), 0°C for butyl methacrylate (BuMA) MJM = 1.24), and 24°C for tert-butyl methacrylate (f-BuMA) (Mw/Mn = 1.12). Precise control of the reaction temperature and fast mixing of a monomer and an initiator seem to be responsible (Fig. 16) [161]. 

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). 

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