Asymmetric Cationic Polymerization of Benzofuran through a Reversible Chain-Transfer Mechanism: Optically Active Polybenzofuran with Controlled Molecular Weights

Abstract

Benzofuran (BzF) is a prochiral, 1,2-disubstituted, unsymmetric cyclic olefin that can afford optically active polymers by asymmetric polymerization, unlike common acyclic vinyl monomers. Although asymmetric cationic polymerization of BzF was reported by Natta et al. in the 1960s, the polymer structure has not been clarified, and there are no reports on molecular weight control. Herein, we report dual control of the optical activity and molecular weight of poly­(BzF) using thioether-based reversible chain-transfer agents for asymmetric cationic polymerization with β-amino acid derivatives as chiral additives and aluminum chloride as a catalyst. This asymmetric moderately living cationic polymerization leads to an increase in molecular weight and specific optical rotation with monomer conversion. In addition, asymmetric block polymers consisting of opposite absolute configurational segments were synthesized using both enantiomers sequentially as chiral additives. Finally, a comprehensive analysis of the polymerization products and the model reaction revealed that the optical activity of poly­(BzF) originates from the threo-diisotactic structure, which occurs by regio-, trans-, and enantioselective propagation.

Asymmetric Ion-Pairing in Stereoselective Vinyl Polymerization

“…More recently, Kamigaito and co-workers revisited the initial system disclosed by Natta 12 for the polymerization of benzofuran. 73 Through a more rigorous mechanistic investigation and ligand derivatization, they were able to achieve the highest optical activities ([α]) to date for poly(benzofuran) (93.4°), which enabled a polymerization that demonstrated semiliving character, the characterization of the relevant optically active stereosequences, and the realization of molecular weights greater than 500 kDa. As an example of structure−selectivity relationships uncovered, (S)-β-phenylalanine as a ligand for AlCl 3 provided high activity but low selectivity.…”

Living Cationic Ring‐Opening Polymerization of Hetero Diels–Alder Adducts to Give Multifactor‐Controlled and Fast‐Photodegradable Vinyl Polymers

“…In particular, living polymerization techniques have played an important role in simultaneously controlling the structural parameters, including molecular weight, dispersity, end‐group, and polymer architectures [1–8] . Furthermore, building on the advantages of living polymerizations, fine‐tuning of sequence, regioregularity, and stereoregularity of vinyl polymers has long been investigated to synthesize vinyl polymers with enhanced properties and functions [9–33] . However, although tremendous efforts have been devoted to developing a powerful synthetic method, simultaneous control of the multiple structural parameters for obtaining highly well‐defined polymers has remained a challenging task.…”

Synthesis and Degradation of Vinyl Polymers with Evenly Distributed Thioacetal Bonds in Main Chains: Cationic DT Copolymerization of Vinyl Ethers and Cyclic Thioacetals

“…[ 59 , 60 , 61 , 62 , 63 , 64 , 65 ] In particular, thioacetals or thioethers serve as efficient and reversible chain‐transfer agents for the cationic polymerization of vinyl ethers and styrene derivatives and are relatively stable and easy to handle even in air. [ 60 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 ] In this cationic polymerization, the vinyl monomers are formally inserted into the terminal C−S bonds derived from the sulfur compounds via reversible generation of the propagating cationic species based on the DT mechanism. The fast interconversion between the dormant C−S species and the propagating cationic species enables the precise control of polymer chain lengths.…”