Microflow systems control

Livingness of the Microflow System-controlled Cationic Polymerization... 

In Section 9.4.1 we discussed ideal living polymerization. Let us examine the livingness of the microflow system-controlled cationic polymerization here. 

Therefore, all requirements for living polymerization seem to be satisfied, at least in a practical sense, in the microflow system controlled cationic polymerization. The livingness strongly depends on the reaction time. In a very short period of time highly reactive intermediates, in this case reactive propagating polymer ends, can survive and they can be utilized for subsequent reactions when different monomers or a terminating reagent are added. This concept is quite similar to that discussed in Section 6.3. 

Figure 9.5 Two different types of living polymerization (a) conventional living polymerization and (b) microflow system-controlled living polymerization...

The concept of flash chemistry can be applied to polymer synthesis. Cationic polymerization can be conducted in a highly controlled manner by virtue of the inherent advantage of extremely fast micromixing and fast heat transfer. An excellent level of molecular weight control and molecular-weight distribution control can be attained without deceleration caused by equilibrium between active species and dormant species. The polymerization is complete within a second or so. The microflow system-controlled cationic polymerization seems to be close to ideal living polymerization within a short residence time. 

Nagaki A, Tomida Y, Yoshida J (2008) Microflow-system-controlled anionic polymerization of styrenes. Macromolecules 41(17) 6322-6330... 

Nagaki A, IwasaM T, Kawamura K et al (2008) Microflow system controlled carbocationic polymerization of vinyl ethers. Chem Asian J 3 1558-1567... 

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

Concept of Microflow System-controlled Polymerization Technology (MCPT)... 

One of the advantages of controlled/living polymerization is that the method allotvs flexible synthesis of structurally defined block copolymers composed of different monomers, which would offer greater opportunities for the synthesis of organic materials with interesting properties. This is also true for microflow system controlled polymerization technology (MCPT). 

Flow-Based Systems Needle-type sensors with a fluid flowing over the sensor tip seem to resist biofouling and extend sensor lifetime.31 There are numerous methods that have been investigated for flow-based sensors, such as microperfusion systems,75 microdialysis,76 77 and ultrafiltration.78 Reduced fouling was found with an open microflow system where slow flow of protein-free fluid over the sensor surface at the implant site is effected.73 Different from the other flow-based sensors, the open microflow is controlled by the subcutaneous tissue hydrostatic pressure and does not require a pump. 

Nagaki et al. (2008) also demonstrated the use of sec-BuLi 84 in a microflow system for the anionic polymerization of styrene 88, as a means of attaining a high degree of control over the molecular weight distribution of the resulting polymer. Employing a solution of styrene 88 (2.0 M) in THF and sec-BuLi 84 (0.2 M) in hexane and a tubular reactor... 

Yoshida and coworkers also developed a microreaction system for cation pool-initiated polymerization [62]. Significant control of the molecular weight distribution (Mw/Mn) was achieved when N-acyliminium ion-initiated polymerization of butyl vinyl ether was carried out in a microflow system (an IMM micromixer and a microtube reactor). Initiator and monomer were mixed using a micromixer, which was connected to a microtube reactor for the propagation step. The polymerization reaction was quenched by an amine in a second micromixer. The tighter molecular weight distribution (Mw/M = 1.14) in the microflow system compared with that of the batch system (Mw/M > 2) was attributed to the very rapid mixing and precise control of the polymerization temperature in the microflow system. 

This book provides an outline of the concept of flash chemistry for conducting extremely fast reactions in a highly controlled manner using microflow systems. In the following chapters, we will discuss the background, the principles, and applications of flash chemistry. 

Why is fast chemical synthesis needed The most appropriate answer to this question is because we can just do it with our present knowledge and technologies. Extremely fast reactions that are complete within a second used to be difficult to control on a preparative scale because we were using conventional macrobatch reactors. However, we are now able to conduct such reactions in a controlled manner with the aid of microflow systems constructed with micro-structured reactors and microreactor technology. 

Short residence time The length of time that the solution remains inside the reactor is called the residence time. In a flow system, the residence time can be adjusted by changing the length of the channels and flow speed. In microflow systems, the residence time can be greatly reduced because of the small size of the channel. This feature of microflow systems is extremely useful in controlling reactive species. In short, unstable reactive species can be transferred to another location to be used in the next reaction before they decompose. Therefore, chemical conversions that are impossible in... 

As demonstrated in this chapter, a number of microfluidic devices of various structures and sizes for extremely fast mixing, heat exchanging and residence time control have been developed based on conventional and modern fabrication technologies. Microflow systems composed of such microfluidic devices are expected to serve as powerful tools for conducting extremely fast, highly exothermic reactions in a highly controlled manner to effect flash chemistry, where desired products are formed within milliseconds to seconds. 

Chlorination reactions are also highly exothermic and the use of microflow systems is quite effective for conducting the reaction in a controlled manner. Various chlorination reactions including chlorination of toluene derivative to obtain benzyl chlorides, chlorination of acetic acid to obtain chloroacetic acid, and radical chlorination of alkanes using microflow systems have been reported. 

The reactions shown above, and others, indicate that electrophilic nitration of aromatic compounds serves as a good example of flash chemistry, and microflow systems will be widely utilized for conducting these types of violent and hazardous reactions under highly controlled conditions in industry in the future. 

It is important to note that the first step and the second step are relatively fast. In a macrobatch system, however, it takes time (a minute or so) to complete the addition of a reagent or a substrate. During that time the reactive intermediate might decompose. In a microflow system, however, the reaction time can be greatly reduced to avoid decomposition of the unstable reactive intermediates. The concept of reactive intermediate control based on a short residence time, which we have already discussed in Chapter 6, can be applied in this case. 

---