Switchable polarity solvents

The second procedure we employed involved the use of a switchable polarity solvent derived from a low-boiling secondary amine and carbon dioxide (6) [40]. 

Another approach to polymer/catalyst separation is to replace the use of costly wasteful solvents with a procedure that produces little to no waste, and requires no catalyst modification. These switchable polarity solvents (SPS), as developed by Jessop and coworkers, are able to utilize the reaction of secondary amines with C02 to reversibly form carbamate salt ionic liquids (Scheme 8.10) [66]. Upon the completion of the polymerization reaction, EtBuNH (Et = ethyl, Bu = n-butyl) is added to dissolve the polymer/catalyst mixture. The subsequent bubbling of C02 through this solution results in a precipitation of the polymer product, such that the purified polymer can be isolated by simple filtration. The catalyst can then be recovered by distilling the amine/carhamate mixture, and the entire system can (in theory) be recycled (Scheme 8.11). 

Phan L, Andreatta JR, Horvey LK et al (2008) Switchable-polarity solvents prepared with a single liquid component. J Org Chem 73(1) 127-132... 

Figure 9.7 Polymerization of styrene in switchable polarity solvent consisting of DBU and 1-propanol. [Reprinted with permission from Ind. Eng. Chem. Res. 2008, 47, 539-545. Copyright 2008 American Chemical Society.]...

Switchable polarity solvents have recently been used in two chemical syntheses the polymerization of styrene (Figure 9.7) and carbon dioxide-epoxide copolymerization (Figure 9.8). In the first study,a non-stoichiometric mixture of DBU and 1-propanol was used as this gave a less viscous reaction mixture, which facilitated the filtration step and the isolation of the polystyrene. The recovered ionic liquid was turned back into the less polar mixture using nitrogen and could be reused four times with the addition of some makeup solvent to replace that lost in the filtration step. 

Figure 9.8 Copolymerization of carbon dioxide in a switchable polarity solvent.

Scheme 8.10 Formation of switchable polarity carbamate solvent.

Like in other chiroptical switches (Section 5.3.1), solvent polarity was found to play an important role. Diastereoselective cyclization was observed in THF and toluene, but not in nonpolar solvents such as n-hexane. Upon photoexcitation, diarylethenes 24 (Scheme 11) can adopt a planar and a twisted conformation, and photocyclization only proceeds through the planar conformation. In the case of chiral diarylethene 27a, there are two diastereomeric planar conformations leading to the diastereomers of the cyclic product 27b. The stereoselectivity in the photocyclization process is enhanced because of a decrease in the excited state energy of the unreactive twisted form, providing a relaxation pathway for the less favorable planar diastereoisomer in more polar solvents. Chiral photochromic diarylethenes are among the most prominent photoswitches known today, featuring nondestructive read-out, excellent reversibility, and the potential for construction of switchable molecular wires and modulation of liquid crystalline phases (see Section 5.5.3).[40,411... 

We found that the CD sign of the chiral aggregates around the CD reversal point (THF/methanol 55%/45%) is switchable, depending on the solvent addition order either Method I (methanol added into THF solution of the polysilane) or Method II (THF solution of the polysilane added into methanol), as shown in Fig. 21. This solvent polarity dependent CD switching may be related to the polymeric seed chirality in the initial stage of the polymer chirality amplification process, which can be detected as Cotton CD signal active polymer aggregates. 

As with most discoveries, these ideas then passed through a lull before takeoff. Then the Chicago-New York axis of Yang and Turro in the USA investigated the first example (6) in the host-guest mould [10]. Their focus was on the demonstration of exciplex emission of (6) in aqueous solution due to protection by an encapsulating /J-cyclodextrin. Ever since Weller s experiments, exciplex emissions had been observed to fall off rapidly in intensity as solvent polarity was increased [4], A formally related example due to Tazuke from 1982 must, however, be pointed out, where a hydrophobic polymer microdomain in mixed and neat aqueous solution allows exciplex emission from pyrene and dimethyl-aniline pendants [11], The importance of (6) in the present context stems from the correlation of pH-dependent emissions from the naphthalene moiety and from the exciplex. Co-occurrence of externally switchable photophysics and supramolecular phenomena would later become common in the research literature [12]. 

The switching of a switchable solvent (a) Reversible protonation of 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) in the presence of an alcohol and carbon dioxide, (b) Polarity switching in reaction (a), (c) Miscibility of decane with the alcohol-DBU mixture (non-polar) under nitrogen and separation of decane from the ionic liquid (polar) under carbon dioxide. [Reprinted with permission from Nature 2005, 436, 1102. Copyright 2005 Nature Publishing Group.]... 

Possibly the least explored and newest options available to the green chemist are liquid polymer solvents (Chapter 8) and switchable and tunable solvents (Chapter 9). Unreactive low molecular weight polymers or those with low glass transition temperatures can be used as non-volatile solvents. In particular, poly(ethyleneglycols) and poly(propyleneglycols) have been used recently in a range of applications. Probably the most important recent additions to our toolbox are switchable solvents. New molecular solvents have been discovered that can be switched from non-volatile to volatile or between polar and nonpolar environments by the application of an external stimulus. Gas-expanded liquids will also be discussed in Chapter 9, as carbon dioxide can be used as a solubility switch and to reduce the environmental burden of conventional solvents. 

A switchable solvent is one that can be made to change important physical properties, such as polarity or hydrophobicity, under the influence of an external factor, which may be a change of temperature or pressure or the addition or removal of a... 

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