Micromixing controlled polymerization

Figure 14.5 H NMR spectrum (600 MHz, in CDCI3) of the polymer obtained by the micromixing-controlled polymerization of NBVE, which was initiated by 1 and terminated by allyltrimethylsilane.

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. 

Polymerization of vinyl ethers initiated by an A -acyUminium ion pool has demonstrated that the molecular weight and the molecular weight distribution can be controlled by using the flow microreactor system (Fig. 10.5) [4, 5]. An iV-acyh-minium ion generated and accumulated by the cation pool method was used as an initiator, which was mixed with a vinyl ether at high speed using a micromixer. The polymerization proceeded in a flow microreactor and was complete within a short residence time. An amine was then introduced using a micromixer to terminate the polymerization. Thus, this polymerization can be called flash polymerization. 

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. 

Along with micromixing, the control of macromixing or RTD is also important for efficient operation of the polymerization reactor. For continuous polymerization, the reactor should accomplish the following requirements ... 

Superfocus interdigital multilamination micromixer can achieve better control than a macrobatch reactor, and the PDI obtained is very close to the theoretical limiting value of 1.5. As the characteristic dimension of the microdevice increases the reactive medium cannot be fully homogenized by diffusion transport before leaving the system, resulting in a high PDI and a loss in control of the 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. 

Interdigital-type feed micromixers were recently applied as a reactor for a nitroxide-mediated radical polymerization, demonstrating a control over the molecular weight distributions as a result of an improved control of the co-polymerization reaction [78, 79]. 

Cationic polymerization was performed in a microreaction system consisting of two micromixers coimected by microcapDlaries [4]. A method that used irreversible generation and accumulation of highly reactive cations in the absence of nucleophile was apphed for polymerization. The molecular weight distribution was controlled by extremely fast micromixing, and the resulting polymer could be used for followup reactions. 

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... 

The effect of macromixing on the performance of chemical reactors is dealt with in standard text books [2] [4] [6] and will not be discussed here. Practical examples of micromixing effects are less known. From the discussion presented in Sec. 4 and influence of micromixing may be expected when a controlling step of the chemical process (time constant t ) competes with the micromixing process (time constant. This may happen especially in three domains fast and complex reaction systems, polymerization reactions, and precipitation (crystallization) reactions. Three examples of such micromixing effects are presented below. 

Polymerization and polycondensation reactions. Micromixing may control the efficiency of initiator utilization, and molecular weight dis-tributions ... 

Generally, a laminar MF reactor contains several inlets, namely, a micromixer, a reaction microchannel, a characterization compartment (optional), and an outlet (Figure 8.3). The molecular weight of the resulting polymer is controlled by varying the polymerization time (by changing the time of residence of the reactants in the reactor) and the concentrations of the monomer and initiator. 

In Chap. 8, we have learned that a flow microreactor system incorporating a micromixer is useful in controlling competitive consecutive reactions. The ultimate reaction system in which reactions occur in chains, or consecutively, is polymerization. This chapter describes how we can exploit the advantages of the flow microreactor system in controlling the molecular weight or molecular weight distribution in polymerization reactions, including cationic polymerizalion and anionic polymerization. 

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