Nucleobase Containing Synthetic Polymers: Advancing Biomimicry via Controlled Synthesis and Self-Assembly

The hydrogen-bonding recognition interactions of nucleobases are a fundamental property of nucleic acid chemistry and associated transcription, translation, and replication functions. Nucleobase interactions are central in protein biosynthesis, yielding sequence- and stereospecific macromolecules capable of assembly into precisely defined, complex shapes and morphologies that make up the machinery of life. As the understanding of nucleobases and their significance developed in the past century, chemists have inevitably sought to extend their function from a biological setting onto wholly synthetic platforms. Recent advances point to a burgeoning area of study which may soon bear fruit in some of the holy grails of polymer synthesis, namely sequence (and stereo) control, single chain manipulation, and controlled polymer folding. This Perspective seeks to summarize recent developments in the area of nucleobase containing polymers (including nucleobase mimics), with particular emphasis on controlled polymerization, self-assembly, and templating polymerization

A “Mix-and-Click” Approach to Double Core–Shell Micelle Functionalization

A micellar scaffold formed by self-assembly of a reversible addition–fragmentation chain transfer (RAFT)-synthesized amphiphilic diblock copolymer has been prepared to contain two orthogonal click-compatible functionalities in the core and shell. These functionalities (norbornenes in the core and terminal alkynes in the shell) have been used as handles to modify the micellar assembly in the core using tetrazine–norbornene chemistry or the shell using the copper-catalyzed azide–alkyne reaction. Additionally, both core and shell modifications were carried out in a tandem, one-pot process using the orthogonal chemistries mentioned above. In all cases the reactions were found to be highly efficient, requiring little excess of the modifying small molecule and very simple to perform under ambient conditions

Dibromomaleimide End Functional Polymers by RAFT Polymerization Without the Need of Protecting Groups

Polymers bearing the dibromomaleimide (DBM) group as a functional chain end have been synthesized by RAFT polymerization. A DBM functional chain transfer agent (CTA) was utilized to afford well-defined P^tBA, PMA, and PTEGA, without the requirement of protecting group chemistry. It was found that RAFT polymerization of NIPAM and styrene with this CTA was severely retarded/inhibited which is ascribed to their relatively low propagation rate constants compared to acrylates. This observation is accounted for by a reversible trapping of propagating radicals by the DBM group in RAFT polymerizations using a monomer with low <i>k</i>_p. However, further attempts to synthesize DBM-terminated P^tBA and PMA by ATRP using an analogous initiator were unsuccessful, and broad PDI were observed. Furthermore, highly efficient postpolymerization functionalization of DBM-terminated PMA produced by RAFT, with the model compound thiophenol was also demonstrated

Orthogonal Modification of Norbornene-Functional Degradable Polymers

Well-defined norbornene-functional poly­(carbonate)­s were prepared by ring-opening polymerization and utilized as multireactive polymeric scaffolds in a range of postpolymerization modifications. The norbornene-functional handles were shown to undergo facile reaction with azides via a 1,3-dipolar cycloaddition, tetrazines in the inverse electron demand Diels–Alder reaction and thiols via radical thiol-ene coupling. Furthermore, the above-mentioned chemistries were demonstrated in a sequential one-pot, three-step modification reaction illustrating the potential of these polymers as scaffolds to access multifunctionalized materials in an undemanding manner

Effect of Complementary Nucleobase Interactions on the Copolymer Composition of RAFT Copolymerizations

Methacryloyl-type monomers containing adenine and thymine have been successfully synthesized with good yields. The homopolymerization and copolymerization of these two new functional monomers were carried out using RAFT polymerization. The reactivity ratios of monomer pairs were measured and calculated using a nonlinear least-squares (NLLS) method, and the results confirmed that the monomer reactivities were dependent on the solvent used for polymerization. The presence and absence of hydrogen bonding affected the resultant copolymer composition where moderate alternating copolymers had a tendency to be formed in CHCl₃, while in DMF, statistical copolymers were formed. Furthermore, the glass transition temperatures of the copolymers were investigated, and the self-assembly of block copolymers made in solvents with different polarity were studied

Functionalized Organocatalytic Nanoreactors: Hydrophobic Pockets for Acylation Reactions in Water

The effect of covalently attaching 4-(dimethylamino)­pyridine (DMAP) functionality to the hydrophobic core of a polymeric micelle in water has been investigated in the context of acylation reactions employing non-water-soluble substrates. For this purpose a novel temperature-responsive polymeric micelle has been synthesized using reversible addition–fragmentation chain transfer (RAFT) polymerization techniques. The reactivity of the tethered organocatalyst within the nanostructure was found to be extremely high, improving in some cases the acylation rates up to 100 times compared to those for unsupported DMAP in organic solvents. Moreover, the catalytic nanoreactors have been demonstrated to be capable of reuse up to 6 times while maintaining high activity

Recyclable l‑Proline Functional Nanoreactors with Temperature-Tuned Activity Based on Core–Shell Nanogels

Recyclable core–shell (CS) nanogels based on l-proline-containing hydrophobic cores with a thermoresponsive poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) shell have been synthesized via a seeded precipitation polymerization process. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to verify the successful addition of the shell and investigate the thermoresponsive properties of the nanostructures. The catalytic activity of the nanogels was assessed in a model asymmetric aldol reaction, where an enhancement was observed with increasing temperature, attributed to the hydrophobic nature of the PNIPAM shell. However, when a nanogel was synthesized with core–shell morphology based on a gradient of cross-linking density in the corona (GS), a dramatic drop in activity was observed at elevated temperatures: the collapse of the outer, lightly cross-linked, “corona” polymer chains appears to block access to the catalytic core. High activity and enantioselectivity were maintained in a number of recovery and reuse cycles, highlighting the recycling potential of these catalytic nanostructures

Entrapment and Rigidification of Adenine by a Photo-Cross-Linked Thymine Network Leads to Fluorescent Polymer Nanoparticles

Photo-cross-linking of nucleobase-containing polymer micelles was observed to result in fluorescent polymer nanoparticles. By varying the micelle assembly conditions, it was possible to probe the origins of this behavior. A number of factors were investigated, including the effect of omitting one of the nucleobases, blocking hydrogen-bonding interactions, detaching the nucleobase from the polymer backbone, and changing the degree of core cross-linking. Spectroscopic investigations were also carried out to further characterize the fluorescent nanoparticles. These data revealed that no new small molecule fluorophores were created during cross-linking and that a dense, hydrogen-bonded network of photodimerized thymine with entrapped adenine was required for fluorescence to arise. We conclude that rigidification and immobilization of adenine in this way leads to the enhancement of an already extant fluorescence pathway and suggests that synergistic covalent and supramolecular entrapment of profluorophores may provide a general strategy for the production of novel fluorescent polymer nanoparticles

Thermoresponsive Block Copolymer Core–Shell Nanoparticles with Tunable Flow Behavior in Porous Media

With the purpose of investigating new polymeric materials as potential flow modifiers for their future application in enhanced oil recovery (EOR), a series of amphiphilic poly(di(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) methyl ether methacrylate) [P(DEGMA-co-OEGMA)]-based core–shell nanoparticles were prepared by aqueous reversible addition–fragmentation chain transfer-mediated polymerization-induced self-assembly. The developed nano-objects were shown to be thermoresponsive, demonstrating a reversible lower-critical solution temperature (LCST)-type phase transition with increasing solution temperature. Characterization of their thermoresponsive nature by variable-temperature UV–vis and dynamic light scattering analyses revealed that these particles reversibly aggregate when heated above their LCST and that the critical transition temperature could be accurately tuned by simply altering the molar ratio of core-forming monomers. Sandpack experiments were conducted to evaluate their pore-blocking performance at low flow rates in a porous medium heated at temperatures above their LCST. This analysis revealed that particles aggregated in the sandpack column and caused pore blockage with a significant reduction in the porous medium permeability. The developed aggregates and the increased pressure generated by the blockage were found to remain stable under the injection of brine and were observed to rapidly dissipate upon reducing the temperature below the LCST of each formulation. Further investigation by double-column sandpack analysis showed that the blockage was able to reform when re-heated and tracked the thermal front. Moreover, the rate of blockage formation was observed to be slower when the LCST of the injected particles was higher. Our investigation is expected to pave the way for the design of “smart” and versatile polymer technologies for EOR applications in future studies

Untersuchung des Festigkeits- und Steifigkeitsverhaltens von Faserverbundwerkstoffen aus kaltaushaertenden Epoxidharzen fuer die Herstellung von Kleinflugzeugen. Teilbericht. Bd. 2 Ergebnisse der Harzgrundlagenuntersuchung

The present partial report of the research project documents the basic resin tests made. During a first test section, the softening temperatures of potentially suitable resin/hardener systems were determined by means of torsional oscillation tests. As a result of these tests, 4 systems were selected for more detailed investigations in the second test section. These additional tests comprised the determination of strength (tensile, compression, and shear) and determination of the associated elongations and elastic moduli in the tensile and compression tests. (orig./RHM)Der vorliegende Teilbericht des Forschungsvorhabens dokumentiert Harzgrundlagenuntersuchungen. In einem ersten Versuchsabschnitt wurden mit Hilfe von Torsionsschwingversuchen die Erweichungstemperaturen geeignet erscheinender Harz/Haertersysteme ermittelt. Als Ergebnis dieser Versuche wurden 4 Systeme fuer die weitergehende Untersuchung im zweiten Versuchsabschnitt ausgewaehlt. Diese weiteren Versuche umfassten die Ermittlung der Festigkeiten (Zug-, Druck-und Scherfestigkeit) sowie bei den Zug- und Druckversuchen die zugehoerigen Dehnungen und der Elastizitaetsmodule. (orig./RHM)Available from TIB Hannover: RO 5657(87-11)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman