Enantiomer-Selective Radical Polymerization of Bis(4-vinylbenzoate)s with Chiral Atom Transfer Radical Polymerization Initiating Systems

Abstract

We report a series of cyclopolymerizations of bis(4-vinylbenzoate) monomers through an atom transfer radical polymerization (ATRP) using chiral ligands. Cyclized polymers showing chiroptical properties were produced by polymerization of the racemic bifunctional monomer, rac-2,4-pentanediyl bis(4-vinylbenzoate) (rac-1), and enantiomerically unbalanced bifunctional monomers were recovered, providing substantial evidence for the enantiomer-selective polymerization of rac-1 mediated through the ATRP with chiral ligands. A comparison between the enantiomerically pure monomers, (2R,4R)-2,4-pentanediyl bis(4-vinylbenzoate) (RR-1) and (2S,4S)-2,4-pentanediyl bis(4-vinylbenzoate) (SS-1), revealed a drastic discrepancy in the rates of their homopolymerizations during the ATRP. The enantiomeric copolymerization of RR-1 and SS-1 indicated that the monomer reactivity ratio of RR-1 was higher than that of SS-1 for the ATRP with chiral ligands. The results of both homopolymerizations and copolymerizations clearly demonstrate the propensity for incorporation of the RR-1 enantiomer and thus the enantiomer-selective polymerization in this system. The monomer-structural effects on the enantiomer-selectivity were examined, and it was concluded that the chirality in the monomer is also essential for the enantiomer-selective polymerization as well as the chiral ligands for ATRP. Finally, the viability of the cyclopolymerization in the enantiomer-selective polymerization was proved using mono(4-vinylbenzoate) monomers incapable of cyclizing through the polymerization.

Controlled RAFT Cyclopolymerization of Oriented Styrenic Difunctional Monomers

“…For the synthesis of polystyrene‐like cyclopolymers, it is necessary to properly design a covalent bond network joining the two polymerizable aromatic moieties in such a way that both cyclization into large ring repeating units and propagation are still possible. The groups of Wulff17–18 and Kakuchi19–20 have developed a series of difunctional monomers incorporating styrene moieties and, by using them in a relatively low feed ratio, cyclocopolymerized them with a variety of other monomers (styrene, acrylates, etc.) for chiral induction in the macromolecular backbone.…”

Polymeric pseudo-crown ether for cation recognition via cation template-assisted cyclopolymerization

“…Cyclopolymerization [23][24][25][26][27][28][29][30][31][32][33][34][35][36] is a chain polymerization of bifunctional monomers via alternating propagation process of intramolecular cyclization and intermolecular addition, to give soluble linear polymers consisting of 'in-chain' cyclic structures. This strategy affords direct and quantitative incorporation of cyclic units into polymers as repeating units, in sharp contrast to the polymerization of monomers bearing a cyclic pendent group 37 or the postfunctionalization of linear polymers 38 .…”

“…This suppresses intermolecular propagation with only one of the two alkene units into pendent olefin-bearing polymers; otherwise the dangling olefin further causes intermolecular crosslinking of polymers to give insoluble gels and/or branched polymers [40][41][42][43] . So far, cyclopolymers with relatively large in-chain rings (at most 20-membered rings) generally required the elaborate design of bifunctional monomers that bring two olefins close, typically through a rigid spacer [29][30][31][32] , except for counterparts with small five-or six-membered rings from 1,6-dienes and 1,6-diynes [33][34][35][36] .…”

Polyacrylamide pseudo crown ethers via hydrogen bond-assisted cyclopolymerization

“…Selective cyclopolymerization involves the efficient intramolecular cyclization of divinyl monomers and the dangling olefin at the ω ‐end growing terminal; the inefficiency gave olefin‐bearing polymers that in turn induce intermolecular crosslinking to macroscopic gelation. Thus, the designer monomers carrying two olefins located adjacently through complex and/or rigid spacers are generally essential for efficient cyclopolymerization . However, this limitation has often made it difficult to directly synthesize cyclopolymers bearing large macrocyclic cavities (e.g., over 20 membered rings), while they are promising as intriguing functional materials typically for selective molecular recognition.…”