Spontaneous Formation of Giant Unilamellar Vesicles from Microdroplets of a Polyion Complex by Thermally Induced Phase Separation

Water pump : Polyion complex (PIC) vesicles are spontaneously formed from PIC microdroplets, which are formed by mixing cationic and anionic polymers (see picture). The formation process can be reversibly controlled by local heating with a focused infrared laser that triggers microphase separation and subsequent water influx. The size of the resulting giant unilamellar vesicles is determined by the initial size of the PIC droplets.

     

Synthesis of Low‐Dimensional Polyion Complex Nanomaterials via Polymerization‐Induced Electrostatic Self‐Assembly

Nanostructured polyion complexes (PICs) are appealing in biomaterials applications. Yet, conventional assembly suffers from the weakness in scale‐up and reproducibility. Only a few low‐dimensional PICs are available to date. Herein we report an efficient and scalable strategy to prepare libraries of low‐dimensional PICs. It involves a visible‐light‐mediated RAFT polymerization of ionic monomer in the presence of a polyion of the opposite charge at 5–50 % w/w total solids concentration in water at 25 °C, namely, polymerization‐induced electrostatic self‐assembly (PIESA). A Vesicle, multi‐compartmental vesicle, and large‐area unilamellar nanofilm can be achieved in water. A long nanowire and porous nanofilm can be prepared in methanol/water. An unusual unimolecular polyion complex (uPIC)‐sphere‐branch/network‐film transition is reported. This green chemistry offers a general platform to prepare various low‐dimensional PICs with high reproducibility on a commercially viable scale under eco‐friendly conditions.     

Functional Polyion Complex Micelles for Potential Targeted Hydrophobic Drug Delivery

Polyion complex (PIC) micelles have gained an increasing interest, mainly as promising nano-vehicles for the delivery of various hydrophilic charged (macro)molecules such as DNA or drugs to the body. The aim of the present study is to construct novel functional PIC micelles bearing cell targeting ligands on the surface and to evaluate the possibility of a hydrophobic drug encapsulation. Initially, a pair of functional oppositely charged peptide-based hybrid diblock copolymers were synthesized and characterized. The copolymers spontaneously co-assembled in water into nanosized PIC micelles comprising a core of a polyelectrolyte complex between poly(L-aspartic acid) and poly(L-lysine) and a biocompatible mixed shell of disaccharide-modified poly(ethylene glycol) and poly(2-hydroxyethyl methacrylate). Depending on the molar ratio between the oppositely charged groups, PIC micelles varying in surface charge were obtained and loaded with the natural hydrophobic drug curcumin. PIC micelles’ drug loading efficiency, in vitro drug release profiles and antioxidant activity were evaluated. The preliminary results indicate that PIC micelles can be successfully used as carriers of hydrophobic drugs, thus expanding their potential application in nanomedicine.     

Reversible Metallo‐Supramolecular Block Copolymer Micelles Containing a Soft Core

An amphiphilic metallo‐supramolecular poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide) diblock copolymer containing a bis(2,2′:6′,2″‐terpyridine)ruthenium(II) complex as a supramolecular connection between the two constituting blocks was used to prepare stable aqueous micelles. The micelles were characterized by dynamic light scattering and atomic force microscopy. Individual micelles were observed together with aggregates of micelles. Only the addition of a large excess of competitive ligand caused the cleavage of the very stable ruthenium complex.     

Quasi‐Block Copolymers Based on a General Polymeric Chain Stopper

Quasi‐block copolymers (q‐BCPs) are block copolymers consisting of conventional and supramolecular blocks, in which the conventional block is end‐terminated by a functionality that interacts with the supramolecular monomer (a “chain stopper” functionality). A new design of q‐BCPs based on a general polymeric chain stopper, which consists of polystyrene end‐terminated with a sulfonate group (PS‐SO₃Li), is described. Through viscosity measurements and a detailed diffusion‐ordered NMR spectroscopy study, it is shown that PS‐SO₃Li can effectively cap two types of model supramolecular monomers to form q‐BCPs in solution. Furthermore, differential scanning calorimetry data and structural characterization of thin films by scanning force microscopy suggests the existence of the q‐BCP architecture in the melt. The new design considerably simplifies the synthesis of polymeric chain stoppers; thus promoting the utilization of q‐BCPs as smart, nanostructured materials.     

Orthogonally Self‐Assembled Multifunctional Block Copolymers

We report the synthesis of telechelic poly(norbornene) and poly(cyclooctene) homopolymers by ring‐opening metathesis polymerization (ROMP) and their subsequent functionalization and block copolymer formation based on noncovalent interactions. Whereas all the poly(norbornene)s contain either a metal complex or a hydrogen‐bonding moiety along the polymer side‐chains, together with a single hydrogen‐bonding‐based molecular recognition moiety at one terminal end of the polymer chain. These homopolymers allow for the formation of side‐chain‐functionalized AB and ABA block copolymers through self‐assembly. The orthogonal natures of all side‐ and main‐chain self‐assembly events were demonstrated by ¹H NMR spectroscopy and isothermal titration calorimetry. The resulting fully functionalized block copolymers are the first copolymers combining both side‐ and main‐chain self‐assembly, thereby providing a high degree of control over copolymer functionalization and architecture and bringing synthetic materials one step closer to the dynamic self‐assembly structures found in nature.     

Induction of Secondary Structure through Micellization of an Oppositely Charged Pair of Homochiral Block‐ and Homopolypeptides in an Aqueous Medium

Polyion complex (PIC) formation is an attractive method for obtaining molecular assemblies owing to their facile fabrication process in aqueous media, but more insights are required in order to control the higher‐dimensional structures of polypeptide‐based PICs. Herein, the PIC formation behavior of oppositely charged homochiral polypeptides, poly‐l ‐lysine and poly(ethylene glycol)‐b‐poly(l ‐glutamate) (PEG‐PLG), and their secondary structures are carefully studied in water. PIC formation takes place in a polymer concentration‐dependent manner, and clear β‐sheet formation is observed at polymer concentrations ≥0.3 mg mL⁻¹. The results also confirm that multimolecular aggregation is a prerequisite for β‐sheet formation, which indicates that the inner hydrophobic environment of PICs is favorable for β‐sheet formation. Furthermore, the PEG weight fraction, stereoregularity of the polypeptide, and ionic strength of the solutions are found to be key factors for generating a secondary structure, presumably because these factors can contribute to the tuning of the inner environment of PICs. This method of producing water‐soluble nanoassemblies from oppositely charged polypeptides may expedite self‐assembly studies in biological systems and be incorporated into various molecular systems to exploit protein‐mimicking features.

     

Supramolecular Alternating Block Copolymers via Metal Coordination

Suprapolymers : The synthesis of symmetrically end‐functionalized polymers in a single step has been developed by means of ring‐opening metathesis polymerization by using a bimetallic ruthenium initiator and functional chain terminators. Self‐assembly of the resulting polymers allows for the formation of supramolecular alternating block copolymers (see figure).

     

Effect of Hydrophilic/Hydrophobic Block Ratio and Temperature on the Surface and Associative Properties of Oxyethylene and Oxybutylene Diblock Copolymers in Aqueous Media

Recent development in dispersion science and technology demands block copolymers with a variable block length and composition. To highlight that purpose, the surface active, associative, colloidal, and thermodynamic behavior of three diblock copolymers having different hydrophilic to hydrophobic ratio is reported here. Using surface tension and light scattering measurements, the micellization and adsorption behavior of polyoxyethylene and polyoxybutylene diblock copolymers of the type E_mB_n have been analyzed. Critical micelle concentration (CMC) and related thermodynamic parameters like free energy (ΔG_mic), enthalpy (ΔH_mic), and entropy (ΔS_mic) of micellization were calculated from CMC value using the closed association model. Likewise, the surface active parameters, like surface excess concentration (Γ₂), area per molecule (A₂), and thermodynamic parameters such as free energy (ΔG_ads), enthalpy (ΔH_ads), and entropy (ΔS_ads) of adsorption of polymer at the air/water interface, were also calculated at various temperatures. Static and dynamic light scattering techniques were employed for the determination of the weight-average molar (M_w), association number (N_w), polymer–water interaction (A₂), and micellar size in terms of hydrodynamic radii (R_h) of copolymer micelles. The effect of block length and solution temperature on the surface and micellar properties of these copolymers was also investigated.     

Development of Enzyme Loaded Polyion Complex Vesicle (PICsome): Thermal Stability of Enzyme in PICsome Compartment and Effect of Coencapsulation of Dextran on Enzyme Activity

Applications of enzymes are intensively studied, particularly for biomedical applications. However, encapsulation or immobilization of enzymes without deactivation and long‐term use of enzymes are still at issue. This study focuses on the polymeric vesicles “PICsomes” for encapsulation of enzymes to develop a hecto‐nanometer‐scaled enzyme‐loaded reactor. The catalytic activity of a PICsome‐based enzyme nanoreactor is carefully examined to clarify the effect of compartmentalization by PICsome. Encapsulation by PICsome provides a stability enhancement of enzymes after 24 h incubation at 37 °C, which is particularly helpful for maintaining the high effective concentration of β‐galactosidase. Moreover, to control the microenvironment inside the nanoreactor, a large amount of dextran, a neutral macromolecule, is encapsulated together with β‐galactosidase in the PICsome. The resulting dextran‐coloaded nanoreactor contributes to the enhancement of enzyme stability, even after exposure to 24 h incubation at −20 °C, mainly due to the antifreezing effect.

     

Micellization Phenomena in Semicrystalline Block Copolymers: Reflexive and Critical Views on the Formation of Cylindrical Micelles

Investigations on the self‐assembly of block copolymers in solution have in some way a less well‐studied history than the study of their phase separation in the solid state, and many aspects are yet not completely understood. Here we focus on the behavior of a specific class of copolymers, namely semicrystalline block copolymers, capable of forming cylindrical aggregates in a solvent selective for the non‐crystalline, complementary block. A common model of micellization is proposed, in principle applicable to most of these copolymeric systems.

     

Fabrication of Dendrimer‐Based Polyion Complex Submicrometer‐Scaled Structures with Enhanced Stability under Physiological Conditions

Submicrometer‐scaled (subμ‐) self‐assembled materials have been developed based on polyion complex (PIC) formation, in particular for biomedical‐applications. However, sufficient stability under physiological conditions is required for their practical use. In this study, PIC formation behavior is examined using a block aniomer, poly(ethylene glycol)‐b‐poly(aspartic acid), and homocatiomers, poly(l ‐lysine) (LPK) and dendritic poly(l ‐lysine) (DPK) with different generations, to elucidate the contribution of the dendritic architecture to stability enhancement. LPK‐based PIC shows a subμ‐vesicular structure only at 25 °C in the absence of NaCl; in contrast, DPK‐based PIC forms a subμ‐structure under physiological salt concentration and temperature conditions, even when the number of charges of a single molecule is much smaller than that of LPK. Moreover, the formation of subμ‐vesicular and ‐spherical micellar structures is dependent on DPK generation. Thus, the molecular backbone architecture of the PIC component plays an important role not only in expanding the preparation conditions and enhancing stability, but also in controlling the self‐assembled structures, mainly due to the spatially restricted structures of dendrimers.

     

Novel One‐Step Route for Preparing Amphiphilic Block Copolymers of Styrene and N‐Isopropylacrylamide in a Microemulsion

Summary: Copolymerizations of St and NIPAM have been carried out through interfacial‐initiated microemulsion polymerization in a frozen state. FT‐IR and NMR spectroscopies confirm the occurrence of copolymerization between the two monomers. DSC analysis shows the existence of two glass transition temperatures of the resultant copolymers. The micellization of the copolymers is investigated by DLS and the temperature‐responsive behavior of the resultant micelles is observed. DSC and DLS results reveal the block feature of the obtained copolymers. Thus amphiphilic poly(styrene‐block‐N‐isopropylacrylamide) is prepared by a one‐step interfacial‐initiated microemulsion polymerization.

Hydrodynamic radius of the micellar particles formed by (left), and a typical DSC trace of (right), the poly(styrene‐block‐N‐isopropylacrylamide) prepared here.     

Modular Synthesis of Functional Block Copolymers

Summary: The free‐radical addition of ω‐functional mercaptans to the vinyl double bonds of 1,2‐polybutadiene‐block‐poly(ethylene oxide) copolymers was used for modular synthesis of well‐defined functional block copolymers. The modification reaction proceeds smoothly and yields quantitatively functionalized block copolymers (¹H NMR and FT‐IR spectroscopy) without disturbing the molecular‐weight distribution of the parent copolymer (PDI < 1.09, size exclusion chromatography).

The modular synthetic pathway towards the functional block copolymers reported here.