Radical chemistry in polymer science: an overview and recent advances

Radical chemistry is one of the most important methods used in modern polymer science and industry. Over the past century, new knowledge on radical chemistry has both promoted and been generated from the emergence of polymer synthesis and modification techniques. In this review, we discuss radical chemistry in polymer science from four interconnected aspects. We begin with radical polymerization, the most employed technique for industrial production of polymeric materials, and other polymer synthesis involving a radical process. Post-polymerization modification, including polymer crosslinking and polymer surface modification, is the key process that introduces functionality and practicality to polymeric materials. Radical depolymerization, an efficient approach to destroy polymers, finds applications in two distinct fields, semiconductor industry and environmental protection. Polymer chemistry has largely diverged from organic chemistry with the fine division of modern science but polymer chemists constantly acquire new inspirations from organic chemists. Dialogues on radical chemistry between the two communities will deepen the understanding of the two fields and benefit the humanity.

Conformational Switch of Arylopeptide: Helix-Helix Transition Based on Side Chain Solvation

Controlling the structural transition between well-defined architectures found in living system is essential in polymer chemistry as well as material science. Herein, the reversible conformational switch of a non-natural polypeptide with an aromatic ring (2,6-naphthalene spacer) on its peptide backbone, referred to as an arylopeptide, between two distinct well-defined helical structures (extended 3₁ -helix and contracted 4₁ -helix) using side chain solvation is demonstrated. The folding selectivity of the arylopeptide and found that the affinity between the solvent and side chains is an essential factor for determining the global structure is investigated. A thermoresponsive arylopeptide bearing oligoether groups (─(CH₂ CH₂ O)₉ CH₃ )) on the side chain is designed, which exhibited unique lower critical solution temperature behavior and converted from the 3₁ to the 4₁ -helix depending on the temperature. Furthermore, the solvent affinity of the entire polymer by combining substituents (─(CH₂ CH₂ O)₃ CH₃ and ─C₁₂ H₂₅ ) with different properties on the side chains to achieve a spring-like expansion-contraction system that allows interconversion between 3₁ - and 4₁ -helices is adjusted.

Folding control of a non-natural glycopeptide using saccharide-coded structural information for polypeptides

We synthesized “glyco-arylopeptides”, whose folding structure significantly changes depending on the kind of saccharide in their side chain. The saccharide moiety interacts with the main chain via hydrogen bonding, and the non-natural polypeptides form two well-defined architectures—(P)-3₁- and (M)-4₁-helices—depending on the length of the saccharide chains and even the configuration of a single stereo-genic center in the epimers.

Synthesis of an optically active polymer containing a planar phthalimide backbone by asymmetric polymerization

Herein we present the precise design and synthesis of a novel polymer backbone that induces a helical structure through asymmetric polymerization reactions of a phthalimide-based monomer catalyzed by a planar-chiral cyclopentadienyl–ruthenium complex.

Side-Chain-Driven Dual Structural System of Poly-Arylopeptide: Selective Helical Formation Derived from Aromatic Ring Flips on the Backbone

A methodology for producing dual structural systems of macromolecules, which involves flipping the unsymmetrical aromatic rings on the main chain is presented. Previously, we reported a non-natural polypeptide containing an aromatic ring on the peptide backbone, called a poly "arylopeptide". Herein, we used 2,6-naphthalene rings as axially unsymmetrical spacers, which has two geometrical isomers, anti and syn, to create dual structural properties. The miniscule energy difference between the two geometrical isomers can be amplified by incorporating the 2,6-naphthylene units into the polypeptide backbone, which creates a thermodynamic driving force for the formation of two specific global structures (i.e., 3₁-helix or 4₁-helix) biased toward one side geometrical isomer depending on the side chain. Additionally, the 3₁-helix can be switched to the 4₁-helix upon addition of a small amount of additives, indicating a conformational conversion from an identical sequence. The developmental dual helical systems exploit basic molecular geometry and can serve as a design platform for synthetic polymers.

Functionalizable Stereocontrolled Cyclopolyethers by Ring-Closing Metathesis as Natural Polymer Mimics

Whereas complex stereoregular cyclic architectures are commonplace in biomacromolecules, they remain rare in synthetic polymer chemistry, thus limiting the potential to develop synthetic mimics or advanced materials for biomedical applications. Herein we disclose the formation of a stereocontrolled 1,4‐linked six‐membered cyclopolyether prepared by ring‐closing metathesis (RCM). Ru‐mediated RCM, with careful control of the catalyst, concentration, and temperature, selectively affords the six‐membered‐ring cyclopolymer. Under optimized reaction conditions, no metathetical degradation, macrocycle formation, or cross‐linking was observed. Post‐polymerization modification by dihydroxylation afforded a novel polymer family encompassing a poly(ethylene glycol) backbone and sugar‐like functionalities (“PEGose”). This strategy also paves the way for using RCM as an efficient method to synthesize other stereocontrolled cyclopolymers.

Post-synthetic functionalization of a polysulfone scaffold with hydrazone-linked functionality

The synthesis, characterization, and post-synthetic functionalization of a readily functionalized step-growth linear polymer derived from divinyl sulfone (DVS) and tert-butylcarbazate (TBC) is presented.