Polymerization, “cation pool

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

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

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. 

Scheme 9.9 Cation-pool initiated 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... 

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... 

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. 

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. 

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. 

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