Orthogonal Modification of Polymers via Thio-Bromo "Click" Reaction and Supramolecular Chemistry: An Easy Method Toward Head-to-Tail Self-Assembled Supramolecular Polymers

Assembling Complex Macromolecules and Self-Organizations of Biological Relevance with Cu(I)-Catalyzed Azide-Alkyne, Thio-Bromo, and TERMINI Double "Click" Reactions

In 2022, the Nobel Prize in Chemistry was awarded to Bertozzi, Meldal, and Sharpless "for the development of click chemistry and biorthogonal chemistry". Since 2001, when the concept of click chemistry was advanced by Sharpless laboratory, synthetic chemists started to envision click reactions as the preferred choice of synthetic methodology employed to create new functions. This brief perspective will summarize research performed in our laboratories with the classic Cu(I)-catalyzed azide-alkyne click (CuAAC) reaction elaborated by Meldal and Sharpless, with the thio-bromo click (TBC) and with the less-used, irreversible TERminator Multifunctional INItiator (TERMINI) dual click (TBC) reactions, the last two elaborated in our laboratory. These click reactions will be used to assemble, by accelerated modular-orthogonal methodologies, complex macromolecules and self-organizations of biological relevance. Self-assembling amphiphilic Janus dendrimers and Janus glycodendrimers together with their biological membrane mimics known as dendrimersomes and glycodendrimersomes as well as simple methodologies to assemble macromolecules with perfect and complex architecture such as dendrimers from commercial monomers and building blocks will be discussed. This perspective is dedicated to the 75th anniversary of Professor Bogdan C. Simionescu, the son of my (VP) Ph.D. mentor, Professor Cristofor I. Simionescu, who as his father, took both science and science administration in his hands, and dedicated his life to handling them in a tandem way, to their best.

Dual Function of β-hydroxy Dithiocinnamic Esters: RAFT Agent and Ligand for Metal Complexation

The reversible addition-fragmentation chain-transfer (RAFT) process has become a versatile tool for the preparation of defined polymers tolerating a large variety of functional groups. Several dithioesters, trithiocarbonates, xanthates, or dithiocarbamates have been developed as effective chain transfer agents (CTA), but only few examples have been reported, where the resulting end groups are directly considered for a secondary use besides controlling the polymerization. We here demonstrate that β-hydroxy dithiocinnamic esters represent a hitherto overlooked class of materials, which were originally designed for the complexation of transition metals but might as well act as reversible CTA. Modified with a suitable leaving group (R-group), these vinyl conjugated dithioesters indeed provide reasonable control over the polymerization of acrylates, acrylamides, or styrene via the RAFT process. Kinetic studies revealed linear evolutions of molar mass with conversion, while different substituents on the aromatic unit had only a minor influence. Block extensions prove the livingness of the polymer chains, although extended polymerization times may lead to side reactions. The resulting dithiocinnamic ester end groups are still able to form complexes with platinum, which verifies that the structural integrity of the end group is maintained. These findings open a versatile new route to tailor-made polymer bound metal complexes. This article is protected by copyright. All rights reserved.

Oligomer Sensor Nanoarchitectonics for “Turn-On” Fluorescence Detection of Cholesterol at the Nanomolar Level

Sensitive and rapid monitoring of cholesterol levels in the human body are highly desirable as they are directly related to the diagnosis of cardiovascular diseases. By using the nanoarchitectonic approach, a novel fluorescent conjugated oligofluorene (OFP-CD) functionalized with β-cyclodextrin (β-CD) was assembled for “Turn-On” fluorescence sensing of cholesterol. The appended β-CD units in OFP-CD enabled the forming of host-guest complexes with dabsyl chloride moieties in water, resulting in fluorescence quenching of the oligofluorene through intermolecular energy transfer. In the presence of cholesterol molecules, a more favorable host-guest complex with stoichiometry 1 cholesterol: 2 β-CD units was formed, replacing dabsyl chloride in β-CD’s cavities. This process resulted in fluorescence recovery of OFP-CD, owing to disruption of energy transfer. The potential of this nanoarchitectonic system for “Turn-On” sensing of cholesterol was extensively studied by fluorescence spectroscopy. The high selectivity of the sensor for cholesterol was demonstrated using biologically relevant interfering compounds, such as carbohydrates, amino acids, metal ions, and anions. The detection limit (LOD value) was as low as 68 nM, affirming the high sensitivity of the current system.

Supramolecular Helical Miktoarm Star Polymers

We report a hydrogen-bonded supramolecular miktoarm star polymer containing three distinct helical arms. Our design involves two helical poly(methacrylamide) arms connected by a barbituric acid (Ba) at the center, prepared through the reversible addition-fragmentation chain-transfer polymerization with a bifunctional agent. Together with a telechelic helical poly(isocyanide) end-functionalized with a Hamilton Wedge (HW) that is complementary to Ba, the two components assemble into an AB₂-type star copolymer. The assembly is driven by the hydrogen bonding between HW and Ba, which is quantified by ¹H NMR titration and isothermal titration calorimetry. Gel-permeation chromatography provides evidence for the formation of the desired miktoarm star architecture. This strategy of site-specific functionalization on helical polymers provides a modular approach to preparing nonlinear supramolecular ensembles with topologically diverse building blocks.

Star polymer networks: a toolbox for cross-linked polymers with controlled structure

The synthesis of precisely controlled polymer networks has been a long-cherished dream of polymer scientists. Traditional random cross-linking strategies often lead to uncontrolled networks with various kinds of defects. Recent...

Comparing C2=O and C2=S Barbiturates: Different Hydrogen-Bonding Patterns of Thiobarbiturates in Solution and the Solid State

Carbonyl-centered hydrogen bonds with various strength and geometries are often exploited in materials to embed dynamic and adaptive properties, with the use of thiocarbonyl groups as hydrogen-bonding acceptors remaining only scarcely investigated. We herein report a comparative study of C2=O and C2=S barbiturates in view of their differing hydrogen bonds, using the 5,5-disubstituted barbiturate B and the thiobarbiturate TB as model compounds. Owing to the different hydrogen-bonding strength and geometries of C2=O vs. C2=S, we postulate the formation of different hydrogen-bonding patterns in C2=S in comparison to the C2=O in conventional barbiturates. To study differences in their association in solution, we conducted concentration- and temperature-dependent NMR experiments to compare their association constants, Gibbs free energy of association ∆Gassn., and the coalescence behavior of the N-H‧‧‧S=C bonded assemblies. In Langmuir films, the introduction of C2=S suppressed 2D crystallization when comparing B and TB using Brewster angle microscopy, also revealing a significant deviation in morphology. When embedded into a hydrophobic polymer such as polyisobutylene, a largely different rheological behavior was observed for the barbiturate-bearing PB compared to the thiobarbiturate-bearing PTB polymers, indicative of a stronger hydrogen bonding in the thioanalogue PTB. We therefore prove that H-bonds, when affixed to a polymer, here the thiobarbiturate moieties in PTB, can reinforce the nonpolar PIB matrix even better, thus indicating the formation of stronger H-bonds among the thiobarbiturates in polymers in contrast to the effects observed in solution.

Synthesis of fully degradable cationic polymers with various topological structures via postpolymerization modification by using thio-bromo “click” reaction

Poly(ε-caprolactone) (PCL) based and fully degradable cationic polymers with various topological structures and tunable charge densities were prepared via postpolymerization modification using thio-bromo “click” chemistry.

Thio-Bromo "Click" Reaction Derived Polymer-Peptide Conjugates for Their Self-Assembled Fibrillar Nanostructures

The synthesis and self-assembly of peptide-polymer conjugates into fibrillar nanostructures are reported, based on the amyloidogenic peptide KLVFF. A strategy for rational synthesis of polymer-peptide conjugates is documented via tethering of the amyloidogenic peptide segment LVFF (Aβ_17-20 ) and its modified derivative FFFF to the hydrophilic poly(ethylene glycol) monomethyl ether (mPEG) polymer via thio-bromo based "click" chemistry. The resultant conjugates mPEG-LVFF-OMe and mPEG-FFFF-OMe are purified via preparative gel permeation chromatography technique (with a yield of 61% and 64%, respectively), and are successfully characterized via combination of spectroscopic and chromatographic methods, including electrospray ionization time-of-flight mass spectrometry. The peptide-guided self-assembling behavior of the as-constructed amphiphilic supramolecular materials is further investigated via transmission electron microscopic and circular dichroism spectroscopic analysis, exhibiting fibrillar nanostructure formation in binary aqueous solution mixture.

The CuAAC: Principles, Homogeneous and Heterogeneous Catalysts, and Novel Developments and Applications

The copper-catalyzed azide/alkyne cycloaddition reaction (CuAAC) has emerged as the most useful "click" chemistry. Polymer science has profited enormously from CuAAC by its simplicity, ease, scope, applicability and efficiency. Basic principles of the CuAAC are reviewed with a focus on homogeneous and heterogeneous catalysts, ligands, anchimeric assistance, and basic chemical principles. Recent developments of ligand design and acceleration are discussed.

Synthesis of a Heterotelechelic Helical Poly(methacrylamide) and its Incorporation into a Supramolecular Triblock Copolymer

We report the first heterotelechelic helical poly(methacrylamide) (PMAc) bearing orthogonal supramolecular binding sites on its chain-ends synthesized through a combination of reversible addition-fragmentation chain-transfer (RAFT) polymerization and thiol-bromo "click" chemistry. The heterotelechelic PMAc was assembled with two monotelechelic polymers featuring different secondary structures, namely a coil-like poly(styrene) and a helical poly(isocyanide), resulting in the formation of a coil-helix-helix supramolecular triblock copolymer through orthogonal metal coordination and hydrogen bonding interactions. Triblock assembly was confirmed through 1H NMR spectroscopy, isothermal titration calorimetry (ITC) and viscometry. The individual polymer blocks retained their secondary structures in the final triblock copolymer, as evidenced by circular dichroism (CD) spectroscopy. Our synthetic strategy expands the toolbox of triblock copolymers featuring structural motifs similar to the ones found in proteins and provides the potential for the development of other complex multifunctional polymeric ensembles.

Construction and morphology of non-covalently double-crosslinked supramolecular polymer networks

A straightforward synthesis of α-Ba, ω-TAP functionalized polymers, Ba-PnBuA-TAP, is reported, leading to the formation of double-crosslinked supramolecular networks driven via the sequential hydrogen-bonding association and metal-coordination.

Synthesis of polymer-linked copper(i) bis(N-heterocyclic carbene) complexes of linear and chain extended architecture

Novel PS-based mechanophores of linear and chain-extended architecture are synthesized obtaining bis(NHC) complexes with more than one Cu(i) center per chain and molecular weights of up to 50 000 g mol⁻¹.

One-Pot Synthesis of Thermoresponsive Amyloidogenic Peptide-Polymer Conjugates via Thio-Bromo "Click" Reaction of RAFT Polymers

A synthetic strategy to efficiently prepare main-chain peptide-polymer conjugates probing their aggregation in solution is described. An in situ tandem reaction based on aminolysis/thio-bromo "click" reaction is performed to tether an amyloidogenic peptide fragment amyloid-β_17-20 (Leu-Val-Phe-Phe (LVFF)) to the ω-chain end of poly(diethylene glycol methyl ether acrylate) (PDEGA), prepared via reversible addition fragmentation chain transfer polymerization. Structural confirmation of the constructed conjugates PDEGA-LVFF (M_n,SEC = 5600, Ð = 1.21), (M_n,SEC = 7600, Ð = 1.16), and (M_n,SEC = 8900, Ð = 1.15) is successfully made by combined studies of ¹ H NMR, size-exclusion chromatography, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, and electrospray ionization time-of-flight (ESI-TOF) mass spectrometry. The effect of the peptidic constituent on the thermoresponsive behavior of the polymer is examined by UV-vis spectroscopy, and the self-assembly behavior of the amphiphilic conjugate is further exploited, exhibiting micellar morphology in aqueous solution.

Visible-light-mediated cleavage of polymer chains under physiological conditions via quinone photoreduction and trimethyl lock

We introduce a click and visible-light triggered unclick approach via thio-bromo reaction and hydroquinone photoreduction/trimethyl lock cleavage for polymer modifications.

Supramolecular semifluorinated dendrons glued by weak hydrogen-bonds

Uniform nanoparticles with a diameter of ∼50 nm were generated from supramolecular semifluorinated dendritic networks, driven by weak heterocomplementary hydrogen-bonding moieties.

Controllable Supramolecular Polymerization Promoted by Host-Enhanced Photodimerization

In this letter, we report a new method of controllable supramolecular polymerization, taking advantage of host-enhanced photodimerization. The low-molecular-weight supramolecular oligomers were formed by noncovalent complexation between cucurbit[8]urils (CB[8]) and the bifunctional monomers (DBN) with Brooker's merocyanine moiety (MOED) on either end. Interestingly, when irradiated with UV light, the supramolecular oligomers could transform into supramolecular polymers with high molecular weight. The molecular weight of supramolecular polymers could be controlled by varying the irradiation time. It is highly anticipated that this work can enrich the methods on the modulation of supramolecular polymerization.

Dynamic Ordering and Phase Segregation in Hydrogen-Bonded Polymers

Hydrogen bonds (H-bonds) constitute highly relevant structural units of molecular self-assembly. They bridge biological and synthetic sciences, implementing dynamic properties into materials and molecules, not achieved via purely covalent bonds. Phase segregation on the other hand represents another important assembly principle, responsible for, e.g., cell compartimentation, membrane-formation, and microphase segregation in polymers. Yet, despite the expanding elegant synthetic strategies of supramolecular polymers, the investigation of phase behavior of macromolecules driven by H-bonding forces still remains in its infancy. Compared to phase segregation arising from covalently linked block copolymers, the generation of phase segregated nanostructures via supramolecular polymers facilitates the design of novel functional materials, such as those with stimuli-responsive, self-healing, and erasable-material properties. We here discuss the phase segregation of H-bonding polymers in both the solution and solid state, wherein the molecular recognition elements are based on multiple H-bonding moieties, such as thymine/2,6-diamino-pyridine (THY/DAP), thymine/diamino triazine (THY/DAT), and barbiturate/Hamilton wedge (Ba/HW) elements. The specific aggregation of a series of different H-bonding polymers in solution, both linear and dendritic polymers, bearing heterocomplementary H-bonding moieties are described, in particular focusing on the issue of phase segregation. The exploitation of H-bonded supramolecular dendrons with segregating polymer chains leads to the formation of three-phase segregated hierarchical micelles in solution, purely linking the components via H-bonds, in turn displaying a versatile spectrum of segregated morphologies. We also focus on segregation effects of H-bonded amorphous and crystalline polymers: thus the formation of nanostructures, such as disordered micelles and well-ordered body centered cubic (BCC) packed spheres from telechelic polymers bearing H-bonding moieties at the chain ends is observed. Finally, we discuss the discovery of novel functional microphase separated self-healing supramolecular architectures, illustrating dynamic and self-healing properties with an almost complete recovery of the initial mechanical performances healing within 24h at 30 °C. Collectively, our studies prove that phase segregation in H-bonding polymers is an important principle, capable to generate nanostructures and dynamic properties not achieved in covalently linked polymers. The results discussed illustrate that a rational architectural design within H-bonding polymer systems in interplay with phase segregation in both the amorphous and crystalline state opens perspectives to develop artificial supramolecular systems approaching the level of complexities and properties present in nature's biomaterials.

Hierarchical Micelles via Polyphilic Interactions: Hydrogen-Bonded Supramolecular Dendrons and Double Immiscible Polymers

We report a simple strategy to form three-phase segregated hierarchical micelles via a counterbalanced phase segregation/self-assembly process. Our methodology relies on a cooperative polyphilic phase segregation, paralleled by a self-assembly process induced by hydrogen-bonds to afford the generation of supramolecular multicompartment dendrons. The versatile preparation of such hierarchical morphologies is evidenced on the basis of a series of supramolecular dendrons, composed of semifluorinated copolymers, homopolymers, or nonfluorinated polymers. We do have designed and prepared mid- and α,ω-barbiturate (Ba) functionalized poly(n-butyl acrylates), Ba-(PnBuA-Ba)2, together with a series of heterocomplementary α,ω-Hamilton wedge (HW) functionalized polymers via reversible addition-fragmentation chain transfer (co)polymerization. To enable subtle phase segregation processes, the semifluorinated homo- and copolymers HW-P(nBuA-co-PFPA)-HW (prepared via copolymerization of nBuA with 2,2,3,3,3-pentafluoropropyl acrylate (PFPA)) and HW-PPFPA-HW, as well as the nonfluorinated polymer HW-PnBuA-HW and HW-PI-HW (PI, polyisoprene), have been generated. Selective intermolecular complexation between Ba-(PnBuA-Ba)2 and the complementary polymers (such as HW-P(nBuA-co-PFPA)-HW, HW-PPFPA-HW or HW-PI-HW) leads to the successful generation of supramolecular dendrons as evidenced by (1)H NMR and diffusion-ordered NMR spectroscopy, together with the formation of well-defined disc-like nano-objects as demonstrated by microscopy investigations. Transmission electron microscopy demonstrates a unique, uncommon phase behavior showing remarkable three-phase segregated hierarchical micelles, indicative of the desired micellar multicompartments.

Accessing libraries of bifunctional block copolymers using two distinct pentafluorophenyl moieties

The suitability of pentafluorophenyl (PFP) esters and PFP alkyl moieties for orthogonal dual functionalization of block copolymer segments is evaluated. While amidation followed by para-fluoro–thiol substitution is a viable reaction sequence, thiol-based substitution can occur on PFP esters at a high rate, thereby hampering the viability of the reverse sequence.

Catechol/boronic acid chemistry for the creation of block copolymers with a multi-stimuli responsive junction

A series of well-defined nitrocatechol and boronic acid end-functionalized homopolymers were synthesized via the RAFT process, followed by their coupling reactions leading to block copolymers integrating a multi-stimuli responsive link in between the blocks.

One-pot synthesis and self-assembly of supramolecular dendritic polymers

A X–Y₂ type heterotropic single-chain polymer, Ba-(PnBuA-HW)₂, is prepared in a one-pot two-step reaction, subsequently self-assembling into supramolecular dendrimers, which are displaying solvent-dependent disc-like hierarchical nanoscopic structures as evidenced by AFM.