Scalable and uniform 1D nanoparticles by synchronous polymerization, crystallization and self-assembly

Emerging Trends in Polymerization-Induced Self-Assembly

In this Perspective, we summarize recent progress in polymerization-induced self-assembly (PISA) for the rational synthesis of block copolymer nanoparticles with various morphologies. Much of the PISA literature has been based on thermally initiated reversible addition-fragmentation chain transfer (RAFT) polymerization. Herein, we pay particular attention to alternative PISA protocols, which allow the preparation of nanoparticles with improved control over copolymer morphology and functionality. For example, initiation based on visible light, redox chemistry, or enzymes enables the incorporation of sensitive monomers and fragile biomolecules into block copolymer nanoparticles. Furthermore, PISA syntheses and postfunctionalization of the resulting nanoparticles (e.g., cross-linking) can be conducted sequentially without intermediate purification by using various external stimuli. Finally, PISA formulations have been optimized via high-throughput polymerization and recently evaluated within flow reactors for facile scale-up syntheses.

Uniform shape monodisperse single chain nanocrystals by living aqueous catalytic polymerization

The preparation of polymer nanoparticles with a uniform size and shape, beyond spheres, is an unresolved problem. Here we report a living aqueous catalytic polymerization, resulting in particles grown by a single active site and composed of a single ultra high molecular weight polyethylene (UHMWPE) chain. The control on a molecular level (M_w/M_n = 1.1–1.2) and at the same time on a particle level (PDI < 0.05) together with the immediate deposition of the growing chain on the growing nanocrystal results in a distinct evolution of the particle morphology over time. These uniform nanocrystals are obtained as concentrated aqueous dispersions of > 10 wt-% (N ≈ 10¹⁹ particles L⁻¹) polymer content. Key to this robust procedure to single chain nanoparticles are long-lived water-stable Ni(II) catalysts that do not undergo any chain transfer. These findings are a relevant step towards polymer materials based on nanoparticle assembly.

The formation of polymer nanoparticles with a uniform size and shape, beyond spheres, is an unresolved problem. Here the authors show a living aqueous catalytic polymerization forming single crystal particles grown by a single active site and composed of a single ultra high molecular weight polyethylene chain.

Polymer-drug conjugate therapeutics: advances, insights and prospects

Polymer-drug conjugates have long been a mainstay of the drug delivery field, with several conjugates successfully translated into clinical practice. The conjugation of therapeutic agents to polymeric carriers, such as polyethylene glycol, offers several advantages, including improved drug solubilization, prolonged circulation, reduced immunogenicity, controlled release and enhanced safety. In this Review, we discuss the rational design, physicochemical characteristics and recent advances in the development of different classes of polymer-drug conjugates, including polymer-protein and polymer-small-molecule drug conjugates, dendrimers, polymer nanoparticles and multifunctional systems. Current obstacles hampering the clinical translation of polymer-drug conjugate therapeutics and future prospects are also presented.

Scalable preparation of alternating block copolymer particles with inverse bicontinuous mesophases

Block copolymer particles with controlled morphologies are of great significance in nanomaterials and nanotechnology. However, ordered inverse morphologies are difficult to achieve due to complex mechanism and formation conditions. Here we report scalable preparation of amphiphilic alternating block copolymer particles with inverse bicontinuous mesophases via polymerization-induced self-assembly (PISA). Concentrated dispersion copolymerizations (up to 40% solid content) of styrene (St) and pentafluorostyrene (PFS) employing a short poly(N,N-dimethylacrylamide) (PDMA29) stabilizer block lead to the formation of well-defined, highly asymmetric PDMA29-b-P(St-alt-PFS)x block copolymers with precise compositions and various morphologies, from simple spheres to ordered inverse cubosome mesophases. The particle morphology is affected by the molecular weight, solid content, and nature of the cosolvents. The cubosome structure is confirmed by electron microscopies and small angle X-ray scattering spectroscopy. This scalable PISA approach offers facile access to ordered inverse mesophases, significantly expanding the PISA morphology scope and enabling its applicability to the materials science fields.

Programming chain-growth copolymerization of DNA hairpin tiles for in-vitro hierarchical supramolecular organization

Formation of biological filaments via intracellular supramolecular polymerization of proteins or protein/nucleic acid complexes is under programmable and spatiotemporal control to maintain cellular and genomic integrity. Here we devise a bioinspired, catassembly-like isothermal chain-growth approach to copolymerize DNA hairpin tiles (DHTs) into nanofilaments with desirable composition, chain length and function. By designing metastable DNA hairpins with shape-defining intramolecular hydrogen bonds, we generate two types of DHT monomers for copolymerization with high cooperativity and low dispersity indexes. Quantitative single-molecule dissection methods reveal that catalytic opening of a DHT motif harbouring a toehold triggers successive branch migration, which autonomously propagates to form copolymers with alternate tile units. We find that these shape-defined supramolecular nanostructures become substrates for efficient endocytosis by living mammalian cells in a stiffness-dependent manner. Hence, this catassembly-like in-vitro reconstruction approach provides clues for understanding structure-function relationship of biological filaments under physiological and pathological conditions.

Getting into Shape: Reflections on a New Generation of Cylindrical Nanostructures' Self-Assembly Using Polymer Building Blocks

Cylinders are fascinating structures with uniquely high surface area, internal volume, and rigidity. On the nanoscale, a broad range of applications have demonstrated advantageous behavior of cylindrical micelles or bottlebrush polymers over traditional spherical nano-objects. In the past, obtaining pure samples of cylindrical nanostructures using polymer building blocks via conventional self-assembly strategies was challenging. However, in recent years, the development of advanced methods including polymerization-induced self-assembly, crystallization-driven self-assembly, and bottlebrush polymer synthesis has facilitated the easy synthesis of cylindrical nano-objects at industrially relevant scales. In this Perspective, we discuss these techniques in detail, highlighting the advantages and disadvantages of each strategy and considering how the cylindrical nanostructures that are obtained differ in their chemical structure, physical properties, colloidal stability, and reactivity. In addition, we propose future challenges to address in this rapidly expanding field.

DNA-Inspired Strand-Exchange for Switchable PMMA-Based Supramolecular Morphologies

Inspired by nanotechnologies based on DNA strand displacement, herein we demonstrate that synthetic helical strand exchange can be achieved through tuning of poly(methyl methacrylate) (PMMA) triple-helix stereocomplexes. To evaluate the utility and robustness of helical strand exchange, stereoregular PMMA/polyethylene glycol (PEG) block copolymers capable of undergoing crystallization driven self-assembly via stereocomplex formation were prepared. Micelles with spherical or wormlike morphologies were formed by varying the molecular weight composition of the assembling components. Significantly, PMMA strand exchange was demonstrated and utilized to reversibly switch the micelles between different morphologies. This concept of strand exchange with PMMA-based triple-helix stereocomplexes offers new opportunities to program dynamic behaviors of polymeric materials, leading to scalable synthesis of "smart" nanosystems.

Straightforward access to biocompatible poly(2-oxazoline)-coated nanomaterials by polymerization-induced self-assembly

Poly(2-ethyl-2-oxazoline) chain transfer agents are employed in photoinitiated RAFT PISA, providing access to biocompatible core–shell polymeric nanostructures with various morphologies.

Scalable Fiber-like Micelles and Block Co-micelles by Polymerization-Induced Crystallization-Driven Self-Assembly

Self-assembled 1D block copolymer nanoparticles (micelles) are of interest for a range of applications. However, morphologically pure samples are often challenging to access, and precise dimensional control is not possible. Moreover, the development of synthetic protocols that operate on a commercially viable scale has been a major challenge. Herein, we describe the preparation 1D fiber-like micelles with crystalline cores at high concentrations by a one-pot process termed polymerization-induced crystallization-driven self-assembly (PI-CDSA). We also demonstrate the formation of uniform fibers by living PI-CDSA, a process in which block copolymer synthesis, self-assembly, and seeded growth are combined. We have demonstrated that the method is successful for block copolymers that possess the same composition as that of the seed (homoepitaxial growth) and also where the coronal chemistries differ to give segmented 1D fibers known as block co-micelles. We have also shown that heteroepitaxial growth allows the formation of scaled-up block co-micelles where the composition of both the core and corona was varied. These proof-of-concept experiments indicate that PI-CDSA is a promising, scalable route to a variety of polydisperse or uniform 1D nanoparticles based on block copolymers with different crystalline core chemistries and, therefore, functions.

Phosphorus-Containing Block Copolymers from the Sequential Living Anionic Copolymerization of a Phosphaalkene with Methyl Methacrylate

Although living polymerization methods are widely applicable to organic monomers, their application to inorganic monomers is rare. For the first time, we show that the living poly(methylenephosphine) (PMP_n ^- ) anion can function as a macroinitiator for olefins. Specifically, the phosphaalkene, MesP=CPh₂ (PA), and methyl methacrylate (MMA) can be sequentially copolymerized using the BnLi-TMEDA initiator system in toluene. A series of PMP_n -b-PMMA_m copolymers with narrow dispersities are accessible (Đ=1.05-1.10). Analysis of the block copolymers provided evidence for -P-CPh₂ -CH₂ -CMe(CO₂ Me)- switching groups. Importantly, this indicates that the -P-CPh₂ ^- anion directly initiates the anionic polymerization of MMA and stands in stark contrast to the isomerization mechanism followed for the homopolymerization of PA. For the first time, the glass transition of a PMP_n homopolymer has been measured (T_g =45.1 °C, n=20). The PMP_n -b-PMMA_m copolymers do not phase separate and show a single T_g which increases with higher PMMA content.

Probing the Growth Kinetics for the Formation of Uniform 1D Block Copolymer Nanoparticles by Living Crystallization-Driven Self-Assembly

Living crystallization-driven self-assembly (CDSA) is a seeded growth method for crystallizable block copolymers (BCPs) and related amphiphiles in solution and has recently emerged as a highly promising and versatile route to uniform core-shell nanoparticles (micelles) with control of dimensions and architecture. However, the factors that influence the rate of nanoparticle growth have not been systematically studied. Using transmission electron microscopy, small- and wide-angle X-ray scattering, and super-resolution fluorescence microscopy techniques, we have investigated the kinetics of the seeded growth of poly(ferrocenyldimethylsilane)- b-(polydimethylsiloxane) (PFS- b-PDMS), as a model living CDSA system for those employing, for example, crystallizable emissive and biocompatible polymers. By altering various self-assembly parameters including concentration, temperature, solvent, and BCP composition our results have established that the time taken to prepare fiber-like micelles via the living CDSA method can be reduced by decreasing temperature, by employing solvents that are poorer for the crystallizable PFS core-forming block, and by increasing the length of the PFS core-forming block. These results are of general importance for the future optimization of a wide variety of living CDSA systems. Our studies also demonstrate that the growth kinetics for living CDSA do not exhibit the first-order dependence of growth rate on unimer concentration anticipated by analogy with living covalent polymerizations of molecular monomers. This difference may be caused by the combined influence of chain conformational effects of the BCP on addition to the seed termini and chain length dispersity.

Recent Trends in Metallopolymer Design: Redox-Controlled Surfaces, Porous Membranes, and Switchable Optical Materials Using Ferrocene-Containing Polymers

Metallopolymers with metal functionalities are a unique class of functional materials. Their redox-mediated optoelectronic and catalytic switching capabilities, their outstanding structure formation and separation capabilities have been reported recently. Within this Minireview, the scope and limitations of intriguing ferrocene-containing systems will be discussed. In the first section recent advances in metallopolymer design will be given leading to a plethora of novel metallopolymer architectures. Discussed synthetic pathways comprise controlled and living polymerization protocols as well as surface immobilization strategies. In the following sections, we focus on recent advances and new applications for side-chain and main-chain ferrocene-containing polymers as (i) remote-switchable materials, (ii) smart surfaces, (iii) redox-responsive membranes, and some recent trends in (iv) photonic structures and (v) other optical applications.

New Insights into RAFT Dispersion Polymerization-Induced Self-Assembly: From Monomer Library, Morphological Control, and Stability to Driving Forces

Polymerization-induced self-assembly (PISA) has been established as an efficient, robust, and versatile approach to synthesize various block copolymer nano-objects with controlled morphologies, tunable dimensions, and diverse functions. The relatively high concentration and potential scalability makes it a promising technique for industrial production and practical applications of functional polymeric nanoparticles. This feature article outlines recent advances in PISA via reversible addition-fragmentation chain transfer dispersion polymerization. Considerable efforts to understand morphological control, broaden the monomer library, enhance morphological stability, and incorporate multiple driving forces in PISA syntheses are summarized herein. Finally, perspectives on the future of PISA research are discussed.

Scalable and cleavable polysaccharide nanocarriers for the delivery of chemotherapy drugs

While polysaccharide-based nanocarriers have been recognized for their crucial roles in tumor theranostics, the industrial-scale production of nanotherapeutics still remains a significant challenge. Most current approaches adopt a postpolymerization self-assembly strategy that follows a separate synthetic step and thus suffers from subgram scale yields and a limited range of application. In this study, we demonstrate the kilogram-scale formation of polysaccharide-polyacrylate nanocarriers at concentrations of up to 5 wt% through a one-pot approach - starting from various acrylate monomers and polysaccharides - that combines aspects of hydrophobicity-induced self-assembly with the free radical graft copolymerization of acrylate monomers from polysaccharide backbones into a single process that is thus denoted as a graft copolymerization induced self-assembly. We also demonstrate that this novel approach is applicable to a broad range of polysaccharides and acrylates. Notably, by choosing a crosslinker that bears a disulfide group and two vinyl capping groups to structurally lock the nanocarriers, the products are rendered cleavable in the reducing environments encountered at tumor sites and thus provide ideal candidates for the construction of anticancer nanotherapeutic systems. In vitro and in vivo studies demonstrated that the use of this nanocarrier for the delivery of doxorubicin hydrochloride (DOX) significantly decreased the side effects of DOX and improved the bio-safety of the chemotherapy accordingly.

STATEMENT OF SIGNIFICANCE

While polysaccharide-based nanocarriers have been recognized for their crucial roles in tumor theranostics, the industrial-scale production of these nanotherapeutics still remains a significant challenge. Most current approaches adopt a post-polymerization self-assembly strategy which that follows a separate synthetic step, and thus suffers from sub-gram scale yields and a limited range of application. In this study, the hydrophobic effect was combined with free radical polymerization to facilitate the graft copolymerization-induced self-assembly (GISA) of acrylate monomers with various hydrophobicities to construct cleavable polysaccharide-polyacrylate nanocarriers at a high efficiency with excellent potential for industrial-scale production. We envision that these nanocarriers will contribute to the development of tumor nanotheranostics that combine the biological functionalities of polysaccharides with the unmatched application-specific flexibility of nanocarriers.

RAFT Polymerization-Induced Self-Assembly as a Strategy for Versatile Synthesis of Semifluorinated Liquid-Crystalline Block Copolymer Nanoobjects

Polymerization-induced self-assembly is demonstrated as a powerful platform for the synthesis of block copolymers comprising a semifluorinated liquid-crystalline block. This strategy transforms the deficiency of polymer insolubility encountered in traditional homogeneous solution protocols to the strength for dispersion polymerization, thus, enabling direct access to polymorphic block copolymer nanoobjects at high concentrations and with quantitative conversions. The versatility of this strategy is highlighted by polymerizations in a wide selection of inexpensive solvents, from nonpolar to highly polar, to afford various block copolymers with distinct combinations of amorphous/crystalline or hydrophilic/hydrophobic/fluorinated segments. The utility of the nanoparticles is demonstrated as robust Pickering emulsifiers for commonly considered good solvents.

Synthesis of One-Component Nanostructured Polyion Complexes via Polymerization-Induced Electrostatic Self-Assembly

Nanostructured polyion complexes (PICs) are expected to serve as novel platforms to stabilize and deliver drugs, proteins, and nucleic acids. Yet, traditional self-assembly suffers from lack of scale-up and reproducibility. Particularly for one-component PICs, only spheres are available to date. Here, we report an efficient and scalable strategy to prepare one-component low-dimensional PICs. It involves visible-light-mediated RAFT iterative polymerization of opposite-charge monomers at 25% w/w solids in water at 25 °C. Sphere-film-vesicle transition and charge-/medium-tunable shape selectivity are reported. One-component PIC nanowire, ultrathin film, vesicle, tube, and surface-charged vesicle are easily prepared, and vesicle-polymerization is fulfilled, using this new strategy. This strategy provides a general platform to prepare one-component low-dimensional PICs with tailorable morphologies and high reproducibility on commercially viable scale under eco-friendly conditions.

Synthesis of transition-metal-free and sulfur-free nanoparticles and nanocapsules via reversible complexation mediated polymerization (RCMP) and polymerization induced self-assembly (PISA)

Transition-metal-free and sulfur-free synthesis of spheres, worms, and vesicles via the combination of organocatalyzed living radical polymerization and PISA.

Solvent dependent pathway complexity and seeded supramolecular polymerization

cNDI-1 exhibits an off-pathway aggregate in cyclic hydrocarbon (MCH) but produces a helical supramolecular polymer in linear alkane (decane) by well-defined J-aggregation.

Polymerization-induced self-assembly of liquid crystalline ABC triblock copolymers with long solvophilic chains

Polymerization-induced self-assembly was exploited to investigate the self-assembly behavior of liquid crystalline triblock copolymers with long solvophilic chains.

Topological engineering of amphiphilic copolymers via RAFT dispersion copolymerization of benzyl methacrylate and 2-(perfluorooctyl)ethyl methacrylate for polymeric assemblies with tunable nanostructures

RAFT dispersion copolymerization of benzyl methacrylate and 2-(perfluorooctyl)ethyl methacrylate enables the regulation of the nanostructure of polymer assemblies.

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.

Dimensional Control and Morphological Transformations of Supramolecular Polymeric Nanofibers Based on Cofacially-Stacked Planar Amphiphilic Platinum(II) Complexes

Square-planar platinum(II) complexes often stack cofacially to yield supramolecular fiber-like structures with interesting photophysical properties. However, control over fiber dimensions and the resulting colloidal stability is limited. We report the self-assembly of amphiphilic Pt(II) complexes with solubilizing ancillary ligands based on polyethylene glycol [PEG_n, where n = 16, 12, 7]. The complex with the longest solubilizing PEG ligand, Pt-PEG₁₆, self-assembled to form polydisperse one-dimensional (1D) nanofibers (diameters <5 nm). Sonication led to short seeds which, on addition of further molecularly dissolved Pt-PEG₁₆ complex, underwent elongation in a "living supramolecular polymerization" process to yield relatively uniform fibers of length up to ca. 400 nm. The fiber lengths were dependent on the Pt-PEG₁₆ complex to seed mass ratio in a manner analogous to a living covalent polymerization of molecular monomers. Moreover, the fiber lengths were unchanged in solution after 1 week and were therefore "static" with respect to interfiber exchange processes on this time scale. In contrast, similarly formed near-uniform fibers of Pt-PEG₁₂ exhibited dynamic behavior that led to broadening of the length distribution within 48 h. After aging for 4 weeks in solution, Pt-PEG₁₂ fibers partially evolved into 2D platelets. Furthermore, self-assembly of Pt-PEG₇ yielded only transient fibers which rapidly evolved into 2D platelets. On addition of further fiber-forming Pt complex (Pt-PEG₁₆), the platelets formed assemblies via the growth of fibers selectively from their short edges. Our studies demonstrate that when interfiber dynamic exchange is suppressed, dimensional control and hierarchical structure formation are possible for supramolecular polymers through the use of kinetically controlled seeded growth methods.

Higher-order assembly of crystalline cylindrical micelles into membrane-extendable colloidosomes

Crystallization-driven self-assembly of diblock copolymers into cylindrical micelles of controlled length has emerged as a promising approach to the fabrication of functional nanoscale objects with high shape anisotropy. Here we show the preparation of a series of crystallizable diblock copolymers with appropriate wettability and chemical reactivity, and demonstrate their self-assembly into size-specific cylindrical micelle building blocks for the hierarchical construction of mechanically robust colloidosomes with a range of membrane textures, surface chemistries and optical properties. The colloidosomes can be structurally elaborated post assembly by in situ epitaxial elongation of the membrane building blocks to produce microcapsules covered in a chemically distinct, dense network of hair-like outgrowths. Our approach provides a route to hierarchically ordered colloidosomes that retain the intrinsic growth activity of their constituent building blocks to permit biofunctionalization, and have potential applications in areas such as biomimetic encapsulation, drug delivery, catalysis and biosensing.

Functional nanoscale objects can be prepared via crystallization-driven self-assembly of diblock copolymers. Here the authors show the self-assembly of crystalline block copolymers into size-specific cylindrical micelles for the hierarchical construction of mechanically robust colloidosomes with a range of membrane textures.

Self-Assembly of Double Hydrophilic Poly(2-ethyl-2-oxazoline)-b-poly(N-vinylpyrrolidone) Block Copolymers in Aqueous Solution

The self-assembly of a novel combination of hydrophilic blocks in water is presented, namely poly(2-ethyl-2-oxazoline)-b-poly(N-vinylpyrrolidone) (PEtOx-b-PVP). The completely water-soluble double hydrophilic block copolymer (DHBC) is formed via copper-catalyzed polymer conjugation, whereas the molecular weight of the PVP is varied in order to study the effect of block ratio on the self-assembly process. Studies via dynamic light scattering, static light scattering as well as microscopy techniques, e.g., cryo scanning electron microscopy or laser scanning confocal microscopy, show the formation of spherical particles in an aqueous solution with sizes between 300 and 400 nm. Particles of the DHBCs are formed without the influence of external stimuli. Moreover, the efficiency of self-assembly formation relies significantly on the molar ratio of the utilized blocks. The nature of the formed structures relies further on the concentration, and indications of particular and vesicular structures are found.

Complex and Hierarchical 2D Assemblies via Crystallization-Driven Self-Assembly of Poly(l-lactide) Homopolymers with Charged Termini

Poly(l-lactide) (PLLA)-based nanoparticles have attracted much attention with respect to applications in drug delivery and nanomedicine as a result of their biocompatibility and biodegradability. Nevertheless, the ability to prepare PLLA assemblies with well-defined shape and dimensions is limited and represents a key challenge. Herein we report access to a series of monodisperse complex and hierarchical colloidally stable 2D structures based on PLLA cores using the seeded growth, "living-crystallization-driven self-assembly" method. Specifically, we describe the formation of diamond-shaped platelet micelles and concentric "patchy" block co-micelles by using seeds of the charge-terminated homopolymer PLLA₂₄[PPh₂Me]I to initiate the sequential growth of either additional PLLA₂₄[PPh₂Me]I or a crystallizable blend of the latter with the block copolymer PLLA₄₂-b-P2VP₂₄₀, respectively. The epitaxial nature of the growth processes used for the creation of the 2D block co-micelles was confirmed by selected area electron diffraction analysis. Cross-linking of the P2VP corona of the peripheral block in the 2D block co-micelles using Pt nanoparticles followed by dissolution of the interior region in good solvent for PLLA led to the formation of novel, hollow diamond-shaped assemblies. We also demonstrate that, in contrast to the aforementioned results, seeded growth of the unsymmetrical PLLA BCPs PLLA₄₂-b-P2VP₂₄₀ or PLLA₂₀-b-PAGE₈₀ alone from 2D platelets leads to the formation of diamond-fiber hybrid structures.

Dimensionally controlled water-dispersible amplifying fluorescent polymer nanoparticles for selective detection of charge-neutral analytes

Fluorescent nanoparticles composed of poly(p-phenylenevinylene) block copolymers were prepared by the facile one-step process and exhibited discriminative detection of neutral explosives against charged molecules.