Discrete Miktoarm Star Block Copolymers with Tailored Molecular Architecture

Molecular architecture is a critical factor in regulating phase behaviors of the block copolymer and prompting the formation of unconventional nanostructures. This work meticulously designed a library of isomeric miktoarm star polymers with an architectural evolution from the linear-branched block copolymer to the miktoarm star block copolymer and to the star-like block copolymer (i.e., 3AB → 3(AB1)B2 → 3(AB)). All of the polymers have precise chemical composition and uniform chain length, eliminating inherent molecular uncertainties such as chain length distribution or architectural defects. The self-assembly behaviors were systematically studied and compared. Gradually increasing the relative length of the branched B1 block regulates the ratio between the bridge and loop configuration and effectively releases packing frustration in the formation of the spherical or cylindrical structures, leading to a substantial deflection of phase boundaries. Complex structures, such as Frank–Kasper phases, were captured at a surprisingly higher volume fraction. Rationally regulating the molecular architecture offers rich possibilities to tune the packing symmetry of block copolymers

Modular Preparation of Discrete Polyesters through Iterative Growth

Discrete macromolecules featuring precise chemical structures and uniform chain lengths are ideal model systems for resolving fundamental principles with an exceedingly high resolution. This work develops a robust approach to prepare orthogonally protected monomers for the convergent synthesis of discrete sequence-defined polymers. Malic acid derivatives bearing hydrophilic, hydrophobic, or omniphobic substituents were designed and synthesized. All of these monomers have the same chemical features and can be modularly connected following the same chemistry. Discrete polyesters with diverse composition and programmable monomer sequence, including homopolymers, diblock/triblock copolymers, and alternating polymers, were readily prepared. The resultant discrete species were fully characterized by nuclear magnetic resonance (NMR) spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-ToF MS). This study expands the diversity of monomers that can be applied in iterative growth, which is expected to serve as an efficient synthetic platform for precise macromolecular engineering

Discrete Giant Polymeric Chain with Precise Sequence and Regio-configuration: A Concise Multiblock Model System

The vastly expanding chemical and architectural parameter space of multiblock copolymers brings both opportunities and challenges for tailoring the structure and properties of a synthetic polymer. In this study, we reported a precise and concise multiblock model system to amplify the subtle structural variations and resolve the underlying phase principles. A series of giant polymeric chains based on polyhedral oligomeric silsesquioxane nanoparticles with an exact block number, regio-configuration, and sequence were designed and prepared. A close scrutiny of these sequence-/regio-isomers revealed that the interplay of geometric constraints, regio-configurations, and collective interactions dictates the self-assembled structures. Zigzag-packed lamellae with the layer normal perpendicular to the backbone were observed for the alternating chains, while a head-to-head arrangement with the lamellar normal along the chain direction was recognized in the case of block chains. Due to the geometric constraint, the alternating chains were exceedingly sensitive to the regio-regularity, leading to dramatically different phase stabilities. This work reveals the critical contribution of the block sequence and regio-configuration to phase behaviors, providing deeper insights toward rational structural engineering of multiblock copolymers

Discrete Diblock Copolymers with Precise Stereoconfiguration

This work develops an iterative growth approach to synthesize discrete oligo lactic acids with exactly defined stereoconfiguration by connecting enantiomeric monomers (i.e., L- and D-lactic acid) following a predesigned sequence. A library of diblock copolymers with uniform chain length was modularly prepared by conjugating the stereoisomeric blocks with a chemically incompatible chain. The precise chemical structure eliminates all molecular uncertainties associated with statistical distribution and decouples the intertwined variables. A rich collection of ordered structures, including unconventional Frank–Kasper A15 and σ phases, was captured. The stereoconfiguration exerts pronounced impacts on chain conformation, leading to appreciable variations of lattice dimension and phase stability. This study quantitatively assessed the critical contribution of stereoconfiguration on packing behaviors, calling for particular attention to this essential molecular parameter as an effective handle for rational structural engineering

Effect of Molecular Architecture and Symmetry on Self-Assembly: A Quantitative Revisit Using Discrete ABA Triblock Copolymers

The inherent statistical heterogeneities associated with chain length, composition, and architecture of synthetic block copolymers compromise the quantitative interpretation of their self-assembly process. This study scrutinizes the contribution of molecular architecture on phase behaviors using discrete ABA triblock copolymers with precise chemical structure and uniform chain length. A group of discrete triblock copolymers with varying composition and symmetry were modularly synthesized through a combination of iterative growth methods and efficient coupling reactions. The symmetric ABA triblock copolymers self-assemble into long-range ordered structures with expanded domain spacings and enhanced phase stability, compared with the diblock counterparts snipped at the middle point. By tuning the relative chain length of two end blocks, the molecular asymmetry reduces the packing frustration, and thus increases the order-to-disorder transition temperature and enlarges the domain sizes. This study would serve as a quantitative model system to correlate the experimental observations with the theoretical assessments and to provide quantitative understandings for the relationship between molecular architecture and self-assembly

Discrete Diblock Copolymers with Tailored Conformational Asymmetry: A Precise Model Platform to Explore Complex Spherical Phases

Conformational asymmetry of block copolymers is a critical molecular parameter dictating the self-assembly behaviors. This work develops an efficient approach to construct block copolymers with uniform chain length and tunable conformational mismatch. Three model discrete diblock copolymers based on γ-alkyl-α-hydroxy glutaric acid and lactide monomers were prepared through the iterative growth approach. The conformational asymmetry can be adjusted via simple substitution of the hydrocarbon side chains. The precise chemical structure rules out all molecular uncertainties associated with statistical distribution, providing a delicate platform for quantitatively resolving the intricate details and underlying principles. Diverse ordered structures, including the Frank–Kasper σ and A15 phases and quasicrystalline phase, were captured. A phase portrait with an exceptionally high compositional resolution was mapped, demonstrating clearly that the spherical packing region expands and the complex phases emerge as the conformational asymmetry increases. This study explicitly correlates the origin of the intriguing structures with the intrinsic molecular parameters, providing deep insights into the formation and evolution of the complex phases in block copolymers

Modulation of the Complex Spherical Packings through Rationally Doping a Discrete Homopolymer into a Discrete Block Copolymer: A Quantitative Study

The Frank–Kasper phase and quasicrystalline phase are an intriguing class of complex crystalline structures, which so far are sporadically observed only in a limited number of block copolymers. Incorporation of a homopolymer into a block copolymer has recently been demonstrated as an effective and robust approach to regulate the formation and evolution of these complex spherical phases. The experimental explorations, however, suffer from inherent chain length distribution of the blending stocks. In this study, we quantitatively assessed the phase behaviors of the block copolymer/homopolymer binary blends using discrete species with a precise chemical structure and uniform chain length, ruling out all interferences associated with chemical heterogeneities. Diverse spherical packings, including σ, A15, C15, and C14 phases, were captured by rationally tuning the chain length and loading content of the homopolymer. The short chains swell the spherical core and drive a transition toward the lattices with a lower interfacial curvature (i.e., σ → A15 → HEX), whereas the long chains localize in the center of the core and prompt the formation of the Frank–Kasper phases with the increasing particle volume asymmetry (C15 and C14). The experimental observation validates the recent theoretical advances, demonstrating that the blending strategy is a robust approach for structural engineering

Confined Self-Assemblies of Chiral Block Copolymers in Thin Films

Self-assembly of chiral block copolymers (BCPs*) can give rise to ordered chiral nanostructures, that is, a helical phase (H* phase), via chirality transfer from the molecular level to mesoscale. In the present work, we reported the self-assembly of BCPs* under one-dimensional spatial confinement. The morphological dependence of self-assembled BCPs* on the molecular weights and the film thickness was investigated. As chiral nanostructures, the H* phase can be formed in bulk, nonchiral nanostructures that were observed in the thin films. Also, the topology effect of self-assembly of BCPs* was examined. The self-assembly of BCPs* with a star-shaped topology exhibited a distinct morphology compared with that of linear BCPs*. The present work provides new insight into the chirality transfer of macromolecules under spatial confinement

Precisely Encoding Geometric Features into Discrete Linear Polymer Chains for Robust Structural Engineering

Molecular shape is an essential parameter that regulates the self-organization and recognition process, which has not yet been well appreciated and exploited in block polymers due to the lack of precise and efficient modulation methods. This work (i) develops a robust approach to break the intrinsic symmetry of linear polymers by introducing geometric features into otherwise homo­geneous chains and (ii) quantitatively highlights the critical contribution of molecular geometry/architecture to the self-assembly behaviors. Iteratively connecting homologous monomers of different side chains according to pre-designed sequences generates discrete polymers with exact chemical structure, uniform chain length, and programmable side-chain gradient along the backbone, which transcribes into diverse shapes. The precise chemistry eliminates all the defects and heterogeneities, providing a delicate platform for fundamental inquiries into the role of molecular geometry. A rich collection of unconventional complex phases, including Frank–Kasper A15 and σ phases, as well as a dodecagonal quasicrystal phase, were captured in these rigorous single-component systems. The self-assembly behaviors are strikingly sensitive to subtle variations of geometry, such that simply migrating a few methylene units among the side chains would generate substantial differences in lattice size or phase stability, or even trigger a phase transition toward distinct structures. The phenomena can be rationalized with a geometric argument that nonuniform side chain distribution leads to conformational mismatch between two immiscible blocks, resulting in varied interfacial curvatures and distinct lattice symmetries. The profound contribution demonstrates that molecular geometry is an effective and robust parameter for structural engineering

Local Chain Feature Mandated Self-Assembly of Block Copolymers

This work demonstrates an effective and robust approach to regulate phase behaviors of a block copolymer by programming local features into otherwise homogeneous linear chains. A library of sequence-defined, isomeric block copolymers with globally the same composition but locally different side chain patterns were elaborately designed and prepared through an iterative convergent growth method. The precise chemical structure and uniform chain length rule out all inherent molecular defects associated with statistical distribution. The local features are found to exert surprisingly pronounced impacts on the self-assembly process, which have yet to be well recognized. While other molecular parameters remain essentially the same, simply rearranging a few methylene units among the alkyl side chains leads to strikingly different phase behaviors, bringing about (i) a rich diversity of nanostructures across hexagonally packed cylinders, Frank–Kasper A15 phase, Frank–Kasper σ phase, dodecagonal quasicrystals, and disordered state; (ii) a significant change of lattice dimension; and (iii) a substantial shift of order-to-disorder transition temperature (up to 40 °C). Different from the commonly observed enthalpy-dominated cases, the frustration due to the divergence between the native molecular geometry originating from side chain distribution and the local packing environment mandated by lattice symmetry is believed to play a pivotal role. Engineering the local chain feature introduces another level of structural complexity, opening up a new and effective pathway for modulating phase transition without changing the chemistry or composition