Microflow systems distribution

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... 

Table 31 Comparison of the product distribution obtained in batch and microflow systems for the arylation of octafluorocyclopentene 213...

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. 

Let us briefly touch on polymerization of typical vinyl monomers using the microflow system. It is well known that the polymerization of butyl acrylate (BA) is very fast, highly exothermic, and very difficult to control in a macrobatch reactor. As shown in Figure 9.10, the molecular-weight distribution is not narrow. Polymerization in a microflow system is also very fast and is almost complete within the residence time of 5 min (Figure 9.11). However, the superior molecular-weight distribution... 

Table 9.2 Effect of the microflow system on molecular-weight distribution control...

Polymerization of benzyl methacrylate (BMA) is much slower than that of BA. Although the yield of the polymer increased with an increase in the residence time, the polymerization did not complete within 12 min. The value of M /Mn was much smaller than that for BA, both in the microflow system and the macrobatch system. The effect of the microflow system on molecular-weight distribution control is, however, smaller than for the BA case. Probably, temperature control for BMA polymerization is better than that for BA polymerization, even in the macrobatch system, because heat generation per unit time for BMA polymerization seems to be much less than that for BA polymerization. 

Vinyl benzoate (VBz) polymerization is slower than MMA polymerization. It is noteworthy that M /Mn for the polymer obtained in the microflow system is very similar to that for the macrobatch system, suggesting that the superior heat removal ability of the microflow system is not important for molecular-weight distribution control in VBz polymerization. This is presumably because the heat generation in VBz polymerization is smaller and controllable even in the macrobatch system on the laboratory scale. 

Studies on the relative rates of the polymerization are helpful in obtaining a deeper insight into the effect of the microflow system on molecular-weight distribution control. It can be seen from Figure 9.11, where the polymer yield obtained in the microflow system is plotted against the reaction time (residence time) for each monomer, that the rate of polymerization increases in the order St < VBz < MMA < BMA < BA. This trend is consistent with the propagation rate constants reported in the literature (Table 9.2). It is reasonable to consider that a similar order is... 

In summary, microflow systems are quite effective for molecular-weight distribution control of very fast, highly exothermic free-radical polymerizations. The superior heat transfer ability of the microflow system in comparison with conventional macrobatch systems seems to be responsible for the high molecular-weight distribution controllability. It should be noted that the controllability is much lower than is achieved by conventional living free-radical polymerization, because residence time control does not work for controlling radical intermediates. The lifetime of a radical intermediate is usually much shorter than the residence time in a microflow system. It is also noteworthy that the more rapid and exothermic the polymerization is, the more effective the microflow system is. These facts speak well for the potentiality of microflow systems in the control of highly exothermic free-radical polymerization without deceleration by reversible termination. 

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. 

Free-radical polymerization can also be conducted in microflow systems. A fairly good level of molecular-weight control and molecular-weight distribution control can be attained, although the level is not as high as those of conventional living-radical polymerizations. 

The example described above indicates that a numbering-up microflow system consisting of several microtube reactors is quite effective for conducting radical polymerization. Precise temperature control by effective heat transfer, which is one of the inherent advantages of microflow systems, seems to be responsible for the effective control of the molecular-weight distribution. The data obtained with the continuous operation of the pilot plant demonstrate that the microflow system can be applied to relatively large-scale production, and speaks well for the potential of microchemical plants in the polymer industry. 

Living cationic polymerization of vinyl ethers initiated by an SnCU/RCl catalytic system can be carried out in a continuous microflow system, which consists of a mutilamination micromixer M (channel width = 40 pm, IMM) and a microtube reactor R (Figure 14.1). A solution of a monomer and RCI is mixed with a solution of SnCU using the micromixer at —78 °C and the resulting mixture was allowed to react in the microtube reactor at the same temperature. For example, isobutyl vinyl ether (IBVE) was polymerized using functionalized initiators to obtain end-functionalized polymers of narrow molecular weight distribution (Mw/M < 1.2) (Scheme 14.4). 

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. 

The present cation pool-initiated polymerization using a microflow system can be applied to other vinyl ethers such as isobutyl vinyl ether (IBVE) and tert-butyl vinyl ether (TBVE) (Table 14.2). The corresponding macroscale batch polymerization results in much poorer molecular weight distribution control. 

A microflow system consisting of a T-shaped micromixer and a microtube reader is effedive for the polymerization (Figure 14.6). The polymerization is complete within a residence time of 0.37-1.5 s at — 25 °C (almost quantitative yield). The degree of molecular weight distribution control depends strongly on the inner diameter of the mixer and the flow rate, as depided in Table 14.3. M / M decreased with decrease in the mixer inner diameter, presumably because faster mixing is achieved by a mixer... 

An example of microflow systems for block copolymerization is shown in Figure 14.11. The first monomer IBVE is mixed with TfOH in the first micromixer (Ml). Introduction of the second monomer (NBVE or EVE) at the second micromixer M2 results in the formation of a polymer of higher molecular weight with a narrow molecular weight distribution. Block copolymerization can be carried out with any combination and with either order of monomer addition, as shown in Table 14.4, demonstrating that the present method serves as an effective technique for the synthesis of block copolymers. The observations illustrate the possibility of using MCPT in the synthesis of structurally well-defined polymers and copolymers both in the laboratory and industry. 

Cationic polymerization without stabilization of a carbocationic intermediate can be carried out in a microflow system. Good molecular weight control and molecular weight distribution control are attained by virtue of characteristic features of microflow systems (microflow-systempolymerization technology, MCPT). Conventional controlled/living cationic polymerization based on cation stabilization can be also carried out in a microflow system. 

Fig. 6. Topological analysis of 2-AG distribution in different layers of rat hippocampus. Selected microstructure from hippocampus was laser-cut under the microscope (A) and analyzed using a state-of-the-art microflow LC/MS system. A heat map was generated to show topological differences in 2-AG levels (B).

In flowing systems, the complex interplay between interfacial, gravitational, viscous and inertial forces is responsible for a variety of phase distributions and flow patterns. The dominant interfadal forces combined with the laminar nature of the flow result in very regularly shaped gas-liquid and liquid-liquid interfaces characteristic of multiphase microflows. Courbin et al. described dynamic wetting morphologies of a flat surface that is microstructured with a forest of posts upon droplet impact [44], Eijkel and co-workers [42, 48] provided a more general review of surface tension effects in the context of nanofluidic systems. The importance of interfadal forces with respect to gravity is described by the dimensionless Bond number. 

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