Carbon Anionic Chiral Initiators for Asymmetric Anionic Polymerization of Achiral Isocyanates

Abstract

Novel optically active carbon anionic initiators bearing a chiral oxazole substituent on fluorene ring, (S)-1-(9H-fluoren-2-yl)-4-isopropyl-4, 5-dihydrooxazole lithium ((S)-1-FIDD-Li) and (S)-2-(9H-fluoren-2-yl)-4-isopropyl-4, 5-dihydrooxazole lithium ((S)-2-FIDD-Li), were synthesized. Anionic polymerizations of achiral polyisocyanates with the chiral initiators were investigated and optical rotation of the obtained polymers were attributed to asymmetric induction of the chiral initiators. The crowded substituent of initiator ((S)-2-FIDD-Li) seems to reduce the polymerizability of isocyanates and yet enhances the chiral induced ability in polymerization.

Synthetic Control of Helical Polyisocyanates by Living Anionic Polymerization toward Peptide Mimicry

“…Optically active polyisocyanates from chiral monomers were first proven by Goodman and Chen , and more deeply explored by Green, − Okamoto, ,− Zentel, − and Lee − (Figure a). Chiral initiators were utilized by Okamoto ,,, and Novak followed by many other researchers − (Figure b). The end-capping reaction using functional terminators was less efficient before living anionic polymerization. ,, Later, Lee utilized chiral terminators in the living anionic polymerization of HIC and demonstrated successful terminal chiral induction in PHIC (Figure c). ,, Indeed, living anionic polymerization has been demonstrated to be compatible with chiral monomers, − chiral initiators, , and chiral terminators. ,− Meanwhile, noncovalent chiral induction for achiral polyisocyanates was first found by Green, − and various systems of host–guest interaction were developed by Okamoto and Kakuchi. , …”

“…Green and co-workers discovered important fundamental principles of macromolecular chiral amplification through circular dichroism (CD) analysis of chiral polyisocyanate homo- and copolymers. − ,− The chiral amplification in polyisocyanate homopolymers is attributed to the cooperation of a number of unit chiral biases having a small force (“cooperative effect”) . For copolymers from chiral and achiral isocyanate comonomers, chiral components (sergeants) force the stereospecific configuration of achiral components (soldiers) so that the copolymer can adopt a single-handed helix sense (“sergeants-and-soldiers effect”). , For chiral polyisocyanate copolymers from two enantiomers, the minority of enantiomers follow the configuration mode of the majority so that the copolymer can adopt a single-handed helix sense preferred by the chirality of the majority (“majority-rules effect”). ,, Okamoto and co-workers demonstrated the helix-sense-selective anionic polymerization of achiral isocyanates using some chiral initiators. ,,, They found the amplification of terminal chirality on the short chain length scale of polyisocyanates, which was later termed the “covalent chiral domino (CCD) effect” …”

Chiroptical Cross‐Linked Polymers Grown via Radical Polymerization around Chiral Nanosilica

“…There are some effective ways to synthesize chiral polymers that began from achiral monomers in conjunction with chiral reagents or catalysts. For instance, helix-sense-selective polymerization can afford chiral polymers with stable helical conformations via polymerization of achiral monomers having a large steric hindrance, in which the direction of the helicity could be entirely controlled by chiral catalysts, [19][20][21] initiators, [22,23] solvents, [24][25][26] and additives. [27][28][29] Besides that, chiral molecular imprinting is the simple technique of creating chiral recognition holes inside polymeric matrixes, which are complementary to the template of chiral molecules.…”