Redox-controlled micellization of organometallic block copolymers

Polystyrene-block-polyferrocenylsilane (PS-b-PFS) diblock copolymers were stoichiometrically oxidized in solution using salts of the one-electron oxidant tris(4-bromophenyl)ammoniumyl. Due to a redox-induced polarity change for the PFS block, self-assembly into well-defined spherical micelles occurs. The micelles are composed of a core of partially oxidized PFS segments and a corona of PS. When the micellar solutions were treated with the reducing agent decamethylcobaltocene, the spherical micelles disassemble and regenerate unassociated and pristine PS-b-PFS free chains.     

Vesicular self-assembly of comb-dendritic block copolymers

New amphiphilic comb-dendritic block copolymers were developed as building blocks that self-assemble into stable vesicular structures with narrow size distribution.     

Polymerization-induced self-assembly of metallo-polyelectrolyte block copolymers

Cobaltocenium-containing polyelectrolyte block copolymer nanoparticles were prepared via polymerization-induced self-assembly (PISA) using aqueous dispersion RAFT polymerization. The cationic steric stabilizer was a macromolecular chain-transfer agent (macro-CTA) based on poly(2-cobaltocenium amidoethyl methacrylate chloride) (PCoAEMACl), and the core-forming block was poly(2-hydroxypropyl methacrylate) (PHPMA). Stable cationic spherical nanoparticles were formed in aqueous solution with low dispersity without adding any salts. The chain extension of macro-CTA with HPMA was efficient and fast. The effects of block copolymer compositions, solid content, charge density, and addition of salts were studied. It was found that the degree of polymerization of both the stabilizer PCoAEMACl and the core-forming PHPMA had a strong influence on the size of nanoparticles. © 2019 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 77–83     

Dynamic self-assembly of block copolymers regulated by time-varying building block composition via reversible chemical reaction

Science China Chemistry - Dynamic self-assembly processes occurring out of thermodynamic equilibrium underlie many forms of adaptive and intelligent behaviors in natural systems. Because of the...     

Electrochemically controlled self-assembly of an organometallic block copolymer

A new means of controlling the order-disorder transition of block copolymers is presented. By applying small electrical potentials (2 V/cm) to disordered solutions of an organometallic block copolymer, oriented ordered grains were obtained near the positive electrode. After reversing the electrical bias on the system, the ordered grains disappeared, and new, oriented, ordered regions were formed at the opposite electrode. Our work establishes the concept of electrochemical self-assembly for controlling order formation in block copolymers.     

Nanocrystal self-assembly with rod-rod block copolymers

Two distinct morphologies of hexylselenophene-hexylthiophene rod-rod block copolymer films can be prepared depending on the molecular weight of the sample (see picture: left M(n) =12.9, right M(n) =3.9?kg?mol(-1) ). These polymers can be used to organize spherical CdSe nanocrystals (yellow) into either dispersed or aligned hierarchical structures. Scale bars: 200?nm.     

Tandem interactions in the self-assembly of ionic block copolymers

The aim of the present review is to show how the phenomena of block copolymer self-assembly and interactions of ionic (or ionizable) groups in polymer systems can be combined to produce materials with versatile and unique behavior. In our discussion, we consider two classes of tandem interactions. First, block copolymers containing short ionic blocks and long nonionic blocks are investigated in organic media. In systems of this type, block copolymer self-assembly and short-range electrostatic interactions act in tandem, forming regular and highly-stable spherical structures. Next, we consider ionic (or ionizable) block copolymers dissolved in aqueous media. In this case, block copolymer self-assembly acts in tandem with the hydrophilic nature of the soluble blocks, resulting in a wide range of unique morphologies.     

Synthesis and self-assembly of reactive H-shaped block copolymers

The H-shaped block copolymers (PTMSPMA)₂-PEG(PMPSTMSPMA)₂ with two compositions, (EG)₉₁-b-(TMSPMA)₉₂ and (EG)₄₅₅-b-(TMSPMA)₁₇₆ have been successfully synthesized by atom transfer radical polymerization (ATRP) of tri(methoxylsilyl)propyl methacrylate (TMSPMA) at room temperature in methanol. The initiation system applied was composed of 2,2-bis(methylene α-bromoisobutyrate)propionyl terminated poly(ethylene glycol) (Br₂PEGBr₂) with M _n = 4000 or 2000, CuBr and 2,2′-bipyridine. The macroinitiator, Br₂PEGBr₂, was prepared by the reaction of two hydroxyl groups terminated PEG with 2,2-bis(methylene α-bromoisobutyrate)propionyl chloride. The NMR spectroscopy and GPC measurements were used to characterize the structure and molecular weight and molecular weight distribution of the resultant copolymers. The H-shaped block copolymers Sam 1 and Sam 2 were self-assembled in DMF/water mixtures and then the trimethoxysilyl groups in PTMSPMA were cross-linked by condensation reaction in the presence of triethylamine. Stable large-compound vesicles with 10 nm diameter of cavities were formed for Sam 1 which contains a short PEG chain. However, the self-assembling of the Sam 2 in the selective solvents resulted in big vesicles aggregates. These two different morphologies of aggregates are attributed to their relative chain length of water soluble PEG. The vesicles formed from Sam1 with short PEG chains have big surface energy which will lead them to self-assemble further, forming large-compound vesicles. __________ Translated from Acta Polymerica Sinica, 2007, 10: 974–978 [译自: 高分子学报]     

The self-assembly mechanism of fibril-forming silk-based block copolymers

Triblock copolymers consisting of a silk-based ((Gly-Ala)(3)Gly-Glu) repeat flanked by hydrophilic outer blocks self-assemble into micrometer long fibrils in response to a trigger. Since the exact mechanism of the fibril formation remains unclear, we employ a multiscale modelling approach in combination with rare event simulations to elucidate key processes. Atomistic scale simulations on the silk-based block suggest a mechanism in which a polypeptide prefolded into a β-roll structure docks to the growing end of a fibril through the formation of Glu-Glu sidechain contacts. Subsequently it can slide to the optimal position before water is expelled to form a dry interface between the fibril end and the attaching block copolymer. In addition, we find that the folded state of the silk-based block is further stabilised through interactions with its neighboring block. Templated folding may also play a role in case a partially folded polypeptide attaches. The coarse-grained simulations indicate that the attachment and subsequent sliding is mediated by the hydrophilic flanks in a size dependent manner. The hydrophilic blocks prevent random aggregation and allow growth only at the end of the fibril. Our multiscale approach may be used for other fibril-forming peptides.     

Temperature-triggered reversible micellar self-assembly of linear-dendritic block copolymers

Polymeric micelles based on a thermoresponsive linear-dendritic block copolymer were completely disrupted into unimers upon cooling the solution to a temperature below its LCST and reversibly regenerated upon heating again.     

Unexpected consequences of block polydispersity on the self-assembly of ABA triblock copolymers

Controlled/"living" polymerizations and tandem polymerization methodologies offer enticing opportunities to enchain a wide variety of monomers into new, functional block copolymer materials with unusual physical properties. However, the use of these synthetic methods often introduces nontrivial molecular weight polydispersities, a type of chain length heterogeneity, into one or more of the copolymer blocks. While the self-assembly behavior of monodisperse AB diblock and ABA triblock copolymers is both experimentally and theoretically well understood, the effects of broadening the copolymer molecular weight distribution on block copolymer phase behavior are less well-explored. We report the melt-phase self-assembly behavior of SBS triblock copolymers (S = poly(styrene) and B = poly(1,4-butadiene)) comprised of a broad polydispersity B block (M(w)/M(n) = 1.73-2.00) flanked by relatively narrow dispersity S blocks (M(w)/M(n) = 1.09-1.36), in order to identify the effects of chain length heterogeneity on block copolymer self-assembly. Based on synchrotron small-angle X-ray scattering and transmission electron microscopy analyses of seventeen SBS triblock copolymers with poly(1,4-butadiene) volume fractions 0.27 ≤ f(B) ≤ 0.82, we demonstrate that polydisperse SBS triblock copolymers self-assemble into periodic structures with unexpectedly enhanced stabilities that greatly exceed those of equivalent monodisperse copolymers. The unprecedented stabilities of these polydisperse microphase separated melts are discussed in the context of a complete morphology diagram for this system, which demonstrates that narrow dispersity copolymers are not required for periodic nanoscale assembly.     

The dramatic effect of architecture on the self-assembly of block copolymers at interfaces

Dramatic morphological changes are observed in the Langmuir-Blodgett (LB) film assemblies of poly(ethylene glycol)-b-(styrene-r-benzocyclobutene) block copolymer (PEG-b-(S-r-BCB)) after intramolecular cross-linking of the S-r-BCB block to form a linear-nanoparticle structure. To isolate architectural effects and allow direct comparison, the linear block copolymer precursor and the linear-nanoparticle block copolymer resulting from selective intramolecular cross-linking of the BCB units were designed to have exactly the same molecular weight and chemical composition but different architecture. It was found that the effect of architecture is pronounced with these macromolecular isomers, which self-assemble into dramatically different surface aggregates. The linear block copolymer forms disklike surface assemblies over the range of compression states, while the linear-nanoparticle block copolymer exhibits long (>10 microm) wormlike aggregates whose length increases as a function of increasing cross-linking density. It is shown that the driving force behind the morphological change is a combination of the altered molecular geometry and the restricted degree of stretching of the nanoparticle block because of the intramolecular cross-linking. A modified approach to interpret the pi-A isotherm, which includes presence of the block copolymer aggregates, is also presented, while the surface rheological properties of the block copolymers at the air-water interface provide in-situ evidence of the aggregates' presence at the air-water interface.     

Morphological studies on Sn-O coordination driving self-assembly of well-defined organotin-containing block copolymers

A tin-oxygen coordination driving self-assembly was developed in the block copolymers containing organotin, which were prepared by the radical addition-fraction transfer（RAFT） method and characterized by the gel-permeation chromatography（GPC） and 1H-NMR. And the self-assemblies of these block copolymers with various chain length ratios in the different concentrations in CHCl3 were stable according to the results of DLS and TEM. Additionally, it was also given an insight investigation on the regulation of self-assembly of the block copolymers by adding dibutyltin dichloride and a possible mechanism was proposed.     

Shell-crosslinked nanoparticles through self-assembly of thermoresponsive block copolymers by RAFT polymerization

A reversible addition-fragmentation chain transfer (RAFT) agent, the methyl-2-(n-butyltrithiocarbonyl)propanoate (MBTTCP) has shown to be efficient in controlling the polymerization of N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPAM) and N-acryloyloxysuccinimide (NAS). Two different strategies have been studied to synthesize block copolymers based on one PNIPAN block and the other a random copolymer of DMA and NAS. When a PNIPAM trithiocarbonate-terminated is used as macromolecular chain transfer agent for the polymerization of a mixture of NAS and DMA, well-defined P(NIPAM-b-(NAS-co-DMA)) block copolymers were obtained with a low polydispersity index. These thermoresponsive block copolymers dissolved in aqueous solution at 25 °C and self-assembled into micelles when the temperature was raised above the LCST of the PNIPAM block. The micelle shell containing NAS units was further crosslinked using a primary diamine in order to get shell-crosslinked nanoparticles. Upon cooling below the LCST of PNIPAM this structure may easily reorganize to form nanoparticles with a water filled hydrophilic core.     

Synthesis and self-assembly in bulk of star-shaped block copolymers based on helical polypeptides

A series of star-shaped copolymers containing poly (styrene) (PS) and poly(γ-benzyl-l-glutamate) (PBLG) were synthesized by click reaction from alkyne- and azide-functionalized homopolymers. The α-azide PS star-shaped homopolymer was synthesized by copper-mediated atom transfer radical polymerization from a bromine-containing star-shaped initiator, and α-alkyne PBLG homopolymers were synthesized by ring-opening polymerization of γ-benzyl-l-glutamate N-carboxyanhydride with an amino-containing α-alkyne initiator. The molecular structures of the homopolymers and star-shaped block copolymers were confirmed by ¹H nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography analysis. The self-assembling behavior of the star-shaped block copolymers in bulk was investigated using wide-angle X-ray diffraction, and small-angle X-ray scattering. For star-shaped block copolymer at lower f _PS of ~0.27, PBLG segment was assigned to a hexagonally packed-cylinder structure (Φ _H) based on the rod-like α-helix conformation as shown in FTIR spectra. By increasing f _PS to ~0.42, a proposed microphase-separated double-hexagonal morphology was observed, in which PBLG rods formed the core of the columns by interdigitated packing in Φ _H phase with PS domain as the matrix in a larger hexagonal columnar structure. The proposed structure was based on calculation from the simulated molecular length of each block of the copolymer and experimental analyses.     

Brownian dynamics simulation study on the self-assembly of incompatible star-like block copolymers in dilute solution

We study the self-assembly of symmetric star-like block copolymers (A(x))(y)(B(x))(y)C in dilute solution by using Brownian dynamics simulations. In the star-like block copolymer, incompatible A and B components are both solvophobic, and connected to the center bead C of the polymer. Therefore, this star-like block copolymer can be taken as a representative of soft and deformable Janus particles. In our Brownian dynamics simulations, these "soft Janus particles" are found to self-assemble into worm-like lamellar structures, loose aggregates and so on. By systematically varying solvent conditions and temperature, we build up the phase diagram to illustrate the effects of polymer structure and temperature on the aggregate structures. At lower temperatures, we can observe large worm-like lamellar aggregates. Upon increasing the temperature, some block copolymers detach from the aggregate; this phenomenon is especially sensitive for the polymers with less arms. The aggregate structure will be quite disordered when the temperature is high. The incompatibility between the two parts in the star-like block copolymer also affects the self-assembled structures. We find that the worm-like structure is longer and narrower as the incompatibility between the two parts is stronger.     

Nanotubes from the self-assembly of asymmetric crystalline-coil poly(ferrocenylsilane-siloxane) block copolymers

Block copolymers with a high asymmetry normally give spherical starlike micelles in a solvent selective for the longer block. We have discovered that samples of poly(ferrocenyldimethylsilane-b-dimethylsiloxane) (PFS-b-PDMS) with block ratios of 1:12 form nanotubes in n-hexane and n-decane, which are poor solvents for PFS. Two block copolymer samples PFS(40)-b-PDMS(480) (M(n) = 45 300, PDI = 1.01) and PFS(80)-b-PDMS(960) (M(n) = 90 500, PDI = 1.01) were synthesized by sequential anionic polymerization. When self-assembly occurs, the PFS blocks aggregate and crystallize to form a shell with a cavity in the middle of the tube, while the PDMS blocks form the corona. The nature of these structures was elucidated by conventional transmission electron microscopy and dark-field scanning transmission electron microscopy. Time- and temperature-dependence studies revealed that a variety of morphologies are formed initially depending on the conditions of sample preparation, but most of them eventually rearrange to form nanotubules. The lengths of the tubes can be varied with time and with the choice of solvents. We have been able to grow nanotubes with lengths reaching 0.1 mm. The presence of the hollow core was confirmed by trapping tetrabutyllead in the cavity and performing energy-dispersive X-ray measurements on the resulting structure.     

Electrostatic self-assembly of neutral and polyelectrolyte block copolymers and oppositely charged surfactant

We investigated the phase behavior and the microscopic structure of the colloidal complexes constituted from neutral/polyelectrolyte diblock copolymers and oppositely charged surfactant by dynamic light scattering (DLS) and small-angle neutron scattering (SANS). The neutral block is poly(N-isopropylacrylamide) (PNIPAM), and the polyelectrolyte block is negatively charged poly(acrylic acid) (PAA). In aqueous solution with neutral pH, PAA behaves as a weak polyelectrolyte, whereas PNIPAM is neutral and in good-solvent condition at ambient temperature, but in poor-solvent condition above approximately 32 degrees C. This block copolymer, PNIPAM-b-PAA with a narrow polydispersity, is studied in aqueous solution with an anionic surfactant, dodecyltrimethylammonium bromide (DTAB). For a low surfactant-to-polymer charge ratio Z lower than the critical value ZC, the colloidal complexes are single DTAB micelles dressed by a few PNIPAM-b-PAA. Above ZC, the colloidal complexes form a core-shell microstructure. The core of the complex consists of densely packed DTA+ micelles, most likely connected between them by PAA blocks. The intermicellar distance of the DTA+ micelles is approximately 39 A, which is independent of the charge ratio Z as well as the temperature. The corona of the complex is constituted from the thermosensitive PNIPAM. At lower temperature the macroscopic phase separation is hindered by the swollen PNIPAM chains. Above the critical temperature TC, the PNIPAM corona collapses leading to hydrophobic aggregates of the colloidal complexes.     

Dendron-mediated self-assembly of highly PEGylated block copolymers: a modular nanocarrier platform

PEGylated dendron coils (PDCs) were investigated as a novel potential nanocarrier platform. PDCs self-assembled into micelles at lower CMCs than linear copolymer counterparts by 1-2 orders of magnitude, due to the unique architecture of dendrons. MD simulations also supported thermodynamically favourable self-assembly mediated by dendrons.