Hierarchical Macromolecular Structures: 60 Years after the Staudinger Nobel Prize II

Glycopolymers are synthetic macromolecules containing sugar moieties. They have shown promise in biorelated applications and the number of synthetic approaches for making these molecules is expanding rapidly. This field benefits from the rapid development of synthetic polymer chemistry, which has seen dramatic progress in the synthesis of functional glycopolymers. Strategies employed in glycopolymer synthesis have been generally carried out as either direct polymerization of glycomonomers or post-glycosylation of pre-formed polymers. This contribution is a short overview of some of the recent developments and will hopefully direct the reader to many papers of interest.

Graphene has remarkable physical properties, but existing production methods have severe deficiencies that limit its potential use in robust technologies. Opening a reliable and efficient synthetic route to graphene and its functionalized derivatives offers a path to overcome this obstacle for its practical application. Graphene can be regarded as a two-dimensional polymer (2D), and it is here argued that it, along with its derivatives, represents a realistic yet challenging target for polymer synthesis.

In order to demonstrate the possibility of such syntheses, an overview is presented on the evolution of phenylene-based macromolecules. It is shown how classical linear polyphenylenes can be expanded to increasingly more sophisticated structures involving two- and three-dimensional (3D) polyphenylene architectures. A crucial aspect of the meticulous synthetic design of these molecules has been the avoidance of defects within the structures, resulting in the precise control of their physical, especially optoelectronic, properties.

Linear conjugated polymers with defined optical properties have been made by controlling the degree of torsion between the benzene rings. This has included the development of efficient routes to ladder-type polymers and of step-ladder materials. Planar graphene molecules, or nanographenes, in a range of sizes and shapes have been fabricated by the controlled cyclodehydrogenation of 3D polyphenylene dendrimers. By combining knowledge gained from the synthesis of conjugated polymers, polyphenylene dendrimers, and nanographenes, it has proven feasible to make, either by solution or surface-bound methods, graphene nanoribbons with well-defined structures. These functional materials possess properties similar to graphene while displaying improved processability.

Finally, we review less-sophisticated paths towards graphene materials involving processing of graphene oxide, its reduction, and its hybridization with other components. These too have a role to play in acquiring functional graphenes where a lesser degree of control over the properties is required. This voyage of exploration towards the precise synthesis of conjugated phenylene-based polymers has thus had the dual objectives of fundamental research and practical materials science. En route we have had to meet the sometimes conflicting gauntlets thrown down by these two aims, which has at times involved trade-offs between the theoretically desirable and the reasonably accessible.

Amphiphilic polymers have the ability to self-assemble into supramolecular structures of great complexity and utility. Nowadays, molecular dynamics simulations can be employed to investigate the self-assembly of modestly sized natural and synthetic macromolecules into structures, such as micelles, worms (cylindrical micelles), or vesicles composed of membrane bilayers organized as single or multilamellar structures. This article presents a perspective on the use of large-scale computer simulation studies that have been used to understand the formation of such structures and their interaction with nanoscale solutes. Advances in this domain of research have been possible due to relentless progress in computer power plus the development of so-called coarse-grained intermolecular interaction models that encode the basic architecture of the amphiphilic macromolecules of interest.

The physical picture of the nematic conformation has been discussed on the basis of the rotational isomeric state analysis for segmented liquid-crystal (LC)-forming molecules, comprising mesogenic units on both sides of a flexible spacer. The disorientation angles (θ) of the two terminal mesogenic units are calculated for given spatial configurations of the spacer. The observed odd-even character of the thermodynamic quantities at the nematic (N)–isotropic (I) phase transition T _NI = ΔH _NI/ΔS _NI has been interpreted in terms of the profiles of the calculated distribution curves P(θ) − θ. Combined use of rotational isomeric state and ²H NMR techniques has led to an estimate of the conformer fraction in the nematic state. The transition entropies derived on this basis are favorably compared with the constant–volume transition entropies obtained from the pressure–volume–temperature measurement. The observed V–T relation indicates that the expansivity of the nematic LC phase is higher relative to that of the isotropic melt. It has been pointed out that the nematic conformation might possibly gain extra stability from the free volume effect in the LC state. These considerations offer an explanation why amazingly long flexible chain segments can be accommodated in the nematic fluid.

This article briefly reviews research developments on “green polymer chemistry” and focuses on the studies recently performed by our group and related work by some other groups. The green character of polymer synthesis has been viewed from the standpoint of starting materials, polymerization catalyst, reaction solvent, and polymer recycling. Starting materials employ biobased renewable resources such as lactic acid (LA), itaconic anhydride (IAn), succinic anhydride, 1,4-butane diol, etc. Green catalysts include enzymes like lipase and protease. Green solvents are water, supercritical carbon dioxide, and ionic liquids; in particular, water is often used for emulsion systems. From LA and IAn, methacyloyl-polymerizable macromonomers were derived and their copolymerization with a (meth)acryroyl monomer in miniemulsion produced a graft copolymer having LA graft chains. The copolymers are classed as bioplastics from their biomass content (≥25 wt%) and are applicable for coatings. LA chain-containing comb polymers and a star-type polymer were prepared, the latter being currently employed as a coating material. The mechanism of catalysis of the enzymes in the oligomerization of LA alkyl esters was examined to reveal direct evidence that a deacylation step determines the enantioselection. Lipase catalysis was utilized for a polymer recycling system

Poly(ethylene glycol) (PEG) is the gold standard polymer for biomedical applications. PEG is known for its biocompatibility and antifouling properties and is widely used for bioconjugation. However, like other synthetic polymers in the field, PEG is not biodegradable, limiting its use for parenteral formulations and protein conjugation to a molecular weight range with a specific upper limit (commonly 40–60 kDa) to avoid polyether accumulation in human tissue. For these biomedical applications, but also for other purposes such as cleavable hydrogels and templates for porous membranes, several routes for the insertion of in-chain biocleavable moieties, such as acetals or disulfides, into PEG have been developed. Recently, the synthetic strategies have been extended from step-growth polymerizations of commercially available, telechelic PEGs to more sophisticated routes based on ethylene oxide (co)polymerizations, permitting the incorporation of predetermined breaking points at any position in the PEG chains.

Typical condensation polymerization is classified as a step-growth polymerization, in which the molecular weight of polymer obtained is difficult to control and the molecular weight distribution theoretically approaches 2 at high conversion. However, the mechanism of condensation polymerization of some monomers has been converted from step-growth to chain-growth by means of activation of the polymer end group by changing substituent effects between the monomer and the polymer, and activation of the polymer end group by intramolecular transfer to it of the catalyst. In this review, we describe the development of chain-growth condensation polymerization (CGCP) through the substituent effect and by catalyst transfer. Furthermore, construction of well-defined polymer architectures, such as block copolymers, star polymers, and graft copolymers by utilizing CGCP is also presented.

Metallopolymers are highly interesting materials with properties combining typical polymeric features with the properties of metal–ligand complexes. Thereby, the incorporation of different metal complexes into the polymeric material enables the tuning of the resulting material’s properties. In particular, ionic interactions between charged metal complexes and the corresponding counterions as well as reversible (switchable) metal–ligand interactions make these materials potentially interesting as self-healing materials. Compared to other self-healing polymers, the research on these materials is still in its infancy. This review summarizes the latest trends in the research regarding this class of materials.

Block copolymer (BCP) self-assembly (SA) is a useful tool for designing materials with tunable nanostructure as well as controllable multiscale, hierarchical structure. A combination of BCP SA with inorganic materials results in functional hybrid materials with ordered structures down to the nanoscale, thereby exploiting both the advantageous features of structure tunability from BCP SA and functionality from inorganic materials. Rather than a comprehensive review of the entire field of hybrid materials, this overview summarizes a variety of BCP-derived synthetic approaches developed over the last 10–15 years, with emphasis on work by the Wiesner group at Cornell University on hybrid materials with structural characteristics on multiple length scales. This encompasses hybrids with thermodynamic equilibrium-type BCP nanostructures, controlled nonequilibrium-type structure formation processes leading to structural asymmetries, as well as formation of hierarchical BCP materials with control over nanoscale and macroscale structures. Besides the development of wet-chemical methodologies for their synthesis, this overview also features some promising first applications of such materials. Results suggest that BCP SA directed synthetic approaches may provide routes to cost-effective and large-scale materials fabrication potentially useful for both, new materials discovery and study of fundamental structure – property correlations as well as exploration of the materials in a number of today’s most pressing applications including water filtration and energy conversion and storage.

Cyclic polymers have intrigued scientists for many years and their diffusion process may have important implications for polymer physics. In this contribution, we focus on the ring-closure method for synthesis of linear polymers made by “living” radical polymerization. In the first part, the probability of two chain ends being in a capture volume to undergo ring closure will be described using the well-known Gaussian chain end-to-end distance. The probability for knot or catenane structures will also be discussed. We then describe the thermodynamic Jacobson–Stockmayer theory for monocyclic ring closure, and an empirical equation based on kinetics. Finally, we give examples of ring closure for many different polymer systems, including the formation of many complex cyclic topologies.

The emulsion solvent evaporation technique is a method for preparing nanoparticles and nanocapsules that are particularly adapted for applications requiring materials with high purity and low toxicity, such as for biomedicine or electronics. We discuss here new important advances concerning the elucidation of the mechanism of nanoparticle formation, and the synthesis of nanoparticles with new structures or from new polymers.

Dendronized polymers that have a cylindrical shape and a helical polymer backbone at the core of the cylinder are able to undergo reversible stretching and contraction of the helix. As the helix expands, the cylindrical macromolecule elongates like a molecular mechanical actuator. When the polymers are self-organized in a columnar lattice, the cylinders can be aligned and the extension of the individual molecules is amplified to macroscopic dimensions and can be employed to perform work. Relationships between the complex architecture of these polymers, their organization in bulk, and emergent function are discussed as an example of the remarkable opportunities that remain to be explored as we commemorate the 60th anniversary of Hermann Staudinger receiving the Nobel Prize for Chemistry.

Self-assembled polymeric nanoscale systems that are robust yet adaptive are of primary importance for fabricating multifunctional stimuli-responsive nanomaterials. Noncovalent interactions in water can be strong, and biological systems exhibit excellent robustness and adaptivity. Synthetic amphiphiles can also result in robust assemblies in water. Can we rationally design water-based noncovalent polymers? Can we program them to perform useful functions that rival covalent materials? We review here advancements related to these questions, focusing on aromatic self-assembly in aqueous media. Regarding functional materials, we present examples from our work on water-based recyclable noncovalent membranes, which can be used for size-selective separations of nanoparticles and biomolecules. These systems introduce the paradigm of noncovalent nanomaterials as a versatile and environmentally friendly alternative to covalent materials. We also address emerging rational design principles for creating 1D, 2D, and 3D functional nanoarrays hierarchically assembled from well-defined molecular units in aqueous media, enabling new synthetic strategies for fabricating complex water-based materials.

Life as we know it is impossible without formation of hierarchical structures. First and foremost, proteins, that is, sequence-specific polypeptides, are nature′s vanguard in this respect. Peptoids and polypeptoids are structural isomers and analogs to peptides and polypeptides. Here, we review the advancements over the last few years on biomimetic hierarchical structures obtained using polypeptoids. Although the inherently more flexible amide bond in peptoids make the stabilization of secondary structure challenging, it also gives us a tool to direct the conformation of the amide bond by design. As will be seen, this is a particularly important feature of peptoids.

Thermo- and pH-responsive microgels were prepared from solubilized native elastin by crosslinking of the elastin lysine residues with poly(ethylene glycol) diglycidyl ether (PEG-DGE) and with bis(sulfosuccinimidyl) suberate (BS3). In the first case, a peptide-PEG conetwork was obtained whereas, in the second case, the elastin peptides were interlinked with hydrophobic bridges. The structure of the microgels was controlled by the ratio of crosslinker to elastin and by performing the crosslinking reaction in an inverse minielemulsion, yielding particles with a diameter in the submicron range. Depending on the degree of crosslinking, the hybrid microgels exhibited a volume change transition at 37 and 35.5°C and pH responsivity in the range of 5–7 for microgels crosslinked with PEG-DE and BS3, respectively. This temperature- and pH-responsive behavior can be assigned to the well-investigated coacervation of elastin peptides, demonstrating that the elastin functionality is abolished only by rather dense crosslinking. In spite of the broad distribution in the molecular weight of the elastin molecules, the microgels remained soluble. Light scattering and sedimentation experiments demonstrated that the coacervation occurred predominantly intramolecularly, i.e., by collapse in the core while the corona stabilized the colloidal dispersion against precipitation. Preliminary experiments were conducted to evaluate the suitability of these microgels for use as a drug-release system and demonstrated cytocompatibility, enzymatic degradability by elastase, and entrapping and slow release of a water-soluble biopolymer (Texas Red-labeled dextran with M _w = 70,000). In summary, we present an easy entry to functional biohybrid microgels, where the responsiveness to temperature and pH can be exploited further for application of the microgel as a drug carrier.

Nanophase separation, self-assembly, and molecular nanostructure design of liquid polyelectrolytes afford new families of ionic liquids containing nanometer-scaled compartments. Key intermediates of nanostructured polymeric ionic liquids (nanoPILs) are PILs with micelle-like topologies, block copolymers and polymer electrolytes dissolved in ionic liquids (ILs), and nanoparticle dispersions. In contrast to micellar ILs, micelle-like nanoPILs consist of a nonionic hyperbranched polyether core with low glass transition temperature and covalently attached alkyl-substituted IL moieties in its periphery. Such hyperbranched nanoPILs are thermally stable dispersants, nanoreactors, and transporters that are useful in nanoparticle synthesis and polymer melt compounding. As new molecular carbon/polyelectrolyte composite materials, tree-like nanoPILs are grafted onto functionalized graphene. Here, we highlight recent progress made in nanoPIL science and engineering, illustrated by selected examples.