Polymerization microflow systems

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. 

Many organic reactions suffer from the formation of significant amounts of polymeric by-products. Faster reactions than mixing might be responsible in many cases. To avoid such undesirable side reactions, slow addition and high dilution techniques are often used. The examples shown here, however, indicate that such reactions can be conducted much more easily and selectively using a microflow system without deceleration by slow addition or high dilution conditions. 

Cation-pool Initiated Polymerization of Vinyl Ethers Using a Microflow System ... 

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. 

The polymerization can be restarted when the same or a different monomer is added. The use of a different monomer leads to block copolymerization. Within the short residence time in a microflow system at low temperature, the polymer chain was really living. In fact, block copolymerization by adding the second monomer in the microflow system has already been achieved by using a strong proton acid, such as trifluoromethanesulfonic acid (TfOH), as an initiator in a microflow system (see Section 9.4.8). 

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. 

The example discussed in the previous sections illustrates the potential of microflow systems, in conjunction with a strong acid initiator, such as a cation pool, to effect cationic polymerization in a highly controlled manner without the deceleration inherent in the dynamic equilibrium... 

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

Table 9.1 POLYMER SYNTHESIS BASED ON FLASH CHEMISTRY Block polymerization using the microflow system ... 

Living-radical polymerization can be performed using microflow systems and several studies have been reported in the literature. Such studies mainly focus on making numbers of polymers for screening using minimum quantities of raw materials. 

Radical polymerization can be conducted in microflow systems. A typical system for laboratory-scale radical polymerization that consists of a mixer and microtube reactors is shown in Figure 9.9. ... 

Figure 9.9 Microflow system for polymerization. Ml, T-shape micromixer Rl, R2, R3, microtube reactors...

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... 

Figure 9.11 Relative rates of polymerization in the microflow system. Copyright 2005 American Chemical Society...

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. 

Simulation of Free-radical Polymerization in Microflow Systems ... 

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. 

Further applications of flash polymerization in microflow systems will hopefully appear in the field of polymer science and technology in the future. 

Polymerization is an important process for the synthesis of polymers in the chemical industry, and the use of microflow systems has attracted significant research interest. 

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. 

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... 

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