Polymerization, “cation pool initiator

Figure 3. Microsystem for the "cation pool initiated polymerization of...

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. 

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

Scheme 9.9 Cation-pool initiated polymerization of vinyl ethers...

Nagaki A, Kawamura K, Suga S, Ando T, Sawamoto M, Yoshida J (2004) Cation pool-initiated controlled/living polymerization using microsystems. J Am Chem Soc 126(45) 14702-14703... 

Therefore, as shown in the above example, the polymer end is really living within a residence time of 0.5 s at —78 °C in the cation pool-initiated polymerization using MCPT. 

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. 

Carbocationic Polymerization Using a Cation Pool" as an Initiator... 

The requirement of extremely fast initiation can be met by the use of a highly reactive cation pool as an initiator, which we have already discussed in the previous chapters. In fact, the use of the less sterically demanding, highly reactive N-acyliminium ion shown in Scheme 9.9 is quite effective for polymerization of vinyl ethers. 

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... 

Fig. 5 Flow microreactor system for polymerization of vinyl ether initiated by Af-acyliminium ion (cation pool). Ml, M2 micromixers Rl, R2 microtube reactors...

Scheme 14.1 The formation of the cation pool and the initiation of the polymerization [13].

Microsystem-controlled cationic polymerization technology requires extremely reactive initiators and cation pools serve as effective initiators for this technology. Usually carbocations are generated by a reversible process from their precursor. Yoshida et al. developed the cation pool method [35], in which carbocations are generated irreversibly by low-temperature electrolysis and are accumulated in relatively high concentration in the absence of nucleophiles. N-Acyliminium ions, alkoxycarbenium ions [36-40] and diarylcarbenium ions [41] have been generated by this method. Such cation pools are expected to serve as extremely reactive initiators for cationic polymerization. 

Table 14.1 Cationic polymerization of NBVE initiated by a pool of N-acyliminium ion 1 using a microflow system"...

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. 

Scheme 14.6 Cationic polymerization of vinyl ethers initiated by N-acyliminium ion pool.

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