Poly(ferrocenylsilane)‐block‐Polylactide Block Copolymers

A PFS/PLA block copolymer was studied to probe the effect of strong surface interactions on pattern formation in PFS block copolymer thin films. Successful synthesis of PFS‐b‐PLA was demonstrated. Thin films of these polymers show phase separation to form PFS microdomains in a PLA matrix, and ultrathin films (<5 nm) formed SINPATs on silicon and mica. The SINPATs consisted of strongly surface‐adsorbed PLA blocks on top of which the PFS blocks dewetted into sphere‐like features. The lateral spacing between these features was regular, and was typically much larger than the length scale associated with regular block copolymer phase separation.

     

Toroidal Micelles of Uniform Size from Diblock Copolymers

Uniform nanodonuts: Stable toroidal micelles that have a highly uniform size and shape spontaneously self‐assemble from a selective THF/ethanol solvent mixture (see 3D AFM image). The donut‐shaped micelles can be used as a template to grow gold nanoparticles, which form along the ring surface.

     

DNA Terminal Mismatch‐Induced Stabilization of Polymer Micelles from RAFT‐Generated Poly(N‐isopropylacrylamide)‐DNA Block Copolymers

Temperature‐responsive diblock copolymers made of poly(N‐isopropylacrylamide) (PNIPAAm) generated by reversible addition‐fragmentation chain transfer (RAFT) polymerization and a single‐stranded DNA (ssDNA) self‐assembled into polymer micelles. The micelles consisted of the PNIPAAm core surrounded by the ssDNA corona with a hydrodynamic diameter up to 300 nm in an aqueous medium above the lower critical solution temperature. In a medium of high ionic strength, the formation of the fully matched duplex with the complementary ssDNA on the surface of the polymer micelles induced rapid and spontaneous aggregation. By contrast, the micelles remained dispersed under the identical conditions when single‐base‐substituted ssDNA was added to form the corresponding terminal‐mismatched duplex on the micellar surface. This highly sequence‐selective process took place irrespective of the size of the PNIPAAm core.     

Formation of Core‐Shell‐Corona Micellar Complexes through Adsorption of Double Hydrophilic Diblock Copolymers into Core‐Shell Micelles

Summary: The complexation between polystyrene‐block‐poly(acrylic acid) (PS‐b‐PAA) micelles and poly(ethylene glycol)‐block‐poly(4‐vinyl pyridine) (PEG‐b‐P4VP) is studied, and a facile strategy is proposed to prepare core‐shell‐corona micellar complexes. Micellization of PS‐b‐PAA in ethanol forms spherical core‐shell micelles with PS block as core and PAA block as shell. When PEG‐b‐P4VP is added into the core‐shell micellar solution, the P4VP block is absorbed into the core‐shell micelles to form spherical core‐shell‐corona micellar complexes with the PS block as core, the combined PAA/P4VP blocks as shell and the PEG block as corona. A model is suggested to characterize the core‐shell‐corona micellar complexes.

Schematic formation of core‐shell‐corona (CSC) micellar complexes by adsorption of PEG‐b‐P4VP into core‐shell PS‐b‐PAA micelles.     

Synthesis and characterization of biodegradable A–B–A triblock copolymers containing poly(ϵ‐caprolactone) A blocks and poly(trans‐4‐hydroxy‐L‐proline) B blocks

Novel, biodegradable poly(?‐caprolactone)‐block‐poly(trans‐4‐hydroxy‐N‐benzyloxycarbonyl‐L ‐proline)‐block‐poly(?‐caprolactone) triblock copolymers were synthesized by ring‐opening polymerization from dihydroxyl‐terminated macroinitiator poly(trans‐4‐hydroxy‐N‐benzyloxycarbonyl‐L ‐proline) (PHpr) and ?‐caprolactone (?‐CL) with stannous octoate as the catalyst. The molecular weights were characterized with gel permeation chromatography and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. With an increase in the contents of ?‐CL incorporated into the copolymers, a decrease in the glass‐transition temperature (T_g) was observed. The T_g values of copoly(4‐phenyl‐?‐caprolactone) and copoly(4‐methyl‐?‐caprolactone) were higher than T_g of copoly(?‐caprolactone). Their micellar characteristics in an aqueous phase were investigated with fluorescence spectroscopy, dynamic light scattering, and transmission electron microscopy. The block copolymers formed micelles in the aqueous phase with critical micelle concentrations in the range of 1.00–1.36 mg L^?1. With higher molecular weights and hydrophobic components in the copolymers, a higher critical micelle concentration was observed. As the feed weight ratio of antitriptyline hydrochloride (AM) to the polymer increased, the drug loading increased. The micelles exhibited a spherical shape, and the average size was less than 250 nm. The in vitro hydrolytic degradation and controlled drug release properties of the triblock copolymers were also investigated. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4268–4280, 2006     

Nano Grape Formation by isotactic‐Poly(methacrylic acid)‐block‐Poly(butyl acrylate)

A novel approach is employed to produce core–corona nanospheres, which introduces a stereoregular hydrophilic part to an amphiphilic block copolymer. The resultant morphology is reported using isotactic‐poly(methacrylic acid)‐block‐poly(butyl acrylate). Infrared spectroscopy revealed a supramolecular interaction, and X ray diffraction revealed the crystallization of the outer isotactic‐poly(methacrylic acid) part. The nanostructure, which looks like a nanosized ‘grape’, was formed when nanospheres and nanofibers coexisted simultaneously and partially fused.

     

Multi‐Morphological Complex Aggregates Formed from Amphiphilic Poly(ethylene oxide)‐block‐Polydimethylsiloxane‐block‐Poly(ethylene oxide) Triblock Copolymers

Summary: Amphiphilic triblock copolymers (PEO_x‐b‐PDMS_y‐b‐PEO_x) with different block lengths were synthesized and multi‐morphological complex crew‐cut, star‐like, and short‐chain aggregates were prepared by self‐assembly of the given copolymers. The morphologies and dimensions of the aggregates can be well controlled by variation of the preparation conditions. TEM, SEM, FFR‐TEM, and LLS studies show the resulting morphologies range from LCMs, unilamellar or multilayer vesicles, LCVs, porous spheres to nanorods.

TEM images of the vesicles formed from PEO‐b‐PDMS‐b‐PEO.     

Sphere‐to‐Rod Transition of Micelles formed by the Semicrystalline Polybutadiene‐block‐Poly(ethylene oxide) Block Copolymer in a Selective Solvent

We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)‐block‐poly(ethylene oxide), in the selective solvent n‐heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5 kg · mol⁻¹, respectively. In this solvent, micellization into a liquid PEO‐core and a corona of PB‐chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at −30 °C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (−196 °C; liquid nitrogen) leads to the transition from spherical to rod‐like micelles. With time these rod‐like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO‐cores of the rod‐like micelles increases. The transition from a spherical to a rod‐like morphology can be explained by a decrease of solvent power of the solvent n‐heptane for the PB‐corona chains: n‐Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology.

     

Defined Poly[styrene‐block‐(ferrocenylmethyl methacrylate)] Diblock Copolymers via Living Anionic Polymerization

Ferrocenylmethyl methacrylate (FMMA) is one of the very few metallocene‐based monomers that are promising candidates for truly living anionic polymerization. Nevertheless, FMMA homopolymers with a narrow polydispersity, or block copolymerization studies that result in satisfying blocking efficiencies, are unknown so far. Here we describe a procedure that leads to highly regular FMMA‐based polymers for the first time, characterized by polydispersity indices (PDI) of less that 1.05 and very high blocking efficiencies (>95%) in sequential copolymerization with styrene. Some of the obtained poly[styrene‐block‐(ferrocenylmethyl methacrylate)]s show unusual microphase morphologies, presumably the consequence of high T_gs causing ‘frustrated’ non‐equilibrium states.

     

The Influence of Pendant Hydroxyl Groups on Enzymatic Degradation and Drug Delivery of Amphiphilic Poly[glycidol‐block‐(ε‐caprolactone)] Copolymers

An amphiphilic diblock copolymer PG‐b‐PCL with well‐controlled structure and pendant hydroxyl groups along hydrophilic block was synthesized by sequential anionic ring‐opening polymerization. The micellization and drug release of PG‐b‐PCL copolymers using pyrene as a fluorescence probe were investigated for determining the influences of copolymer composition and lipase concentration on drug loading capacity and controlled release behavior. The biodegradation of PG‐b‐PCL copolymers was studied with microspheres as research samples. It has been concluded that the polar hydroxyl groups along each repeat unit of hydrophilic PG block in PG‐b‐PCL copolymer have great influences on drug encapsulation, drug release, and enzymatic degradation of micelles and microspheres.

     

Self‐Assembly in Water of Poly(acrylic acid)‐Based Diblock Copolymers Synthesized by Nitroxide‐Mediated Polymerization

Summary: Amphiphilic diblock copolymers consisting of a hydrophilic block, poly(acrylic acid), and a hydrophobic block, polystyrene, were synthesized by direct nitroxide‐mediated polymerization using the PS block as a macro‐initiator for the first time. Several techniques were used to characterize the amphiphilic block copolymers (size exclusion chromatography, NMR spectroscopy). The proposed method can lead to samples with a broad range of composition and molar mass. Preliminary studies of their self‐assembly in aqueous medium using fluorescence spectroscopy and small‐angle neutron scattering are presented.

Schematic of the formation of the PS‐b‐PAA block copolymers and their micellization in aqueous media.     

Synthesis and Aqueous Self‐Assembly of a Polyferrocenylsilane‐block‐poly(aminoalkyl methacrylate) Diblock Copolymer

Water‐soluble cylindrical micelles with an organometallic core are formed by self‐assembly of the first polyferrocenylsilane‐block‐polyacrylate block copolymer, synthesized by anionic polymerization, in water at pH 8. A transmission electron microscopy image of the micelles is shown in the Figure.     

Thermo‐ and pH‐Responsive Micelles of Poly(acrylic acid)‐block‐Poly(N,N‐diethylacrylamide)

Summary: The bis‐hydrophilic block copolymer, poly(acrylic acid)₄₅‐block‐poly(N,N‐diethylacrylamide)₃₆₀, was obtained after hydrolysis of poly(tert‐butyl acrylate)₄₅‐block‐poly(N,N‐diethylacrylamide)₃₆₀, synthesized by sequential anionic polymerization of tert‐butyl acrylate (tBA) and N,N‐diethylacrylamide (DEAAm) in the presence of Et₃Al. The polymer is stimuli‐sensitive with respect to both pH and temperature in aqueous solution, reversibly forming spherical crew‐cut micelles with PDEAAm‐core (〈R_h〉_z = 21.5 nm) under alkaline conditions for T > 35 °C as well as inverse star‐like micelles with an expanded PAA‐core (〈R_h〉_z = 43.8 nm) under acidic conditions for T < 35 °C, as indicated by dynamic light scattering.

Modes of micelle formation for poly(acrylic acid)₄₅‐block‐poly(N,N‐diethylacrylamide)₃₆₀ in aqueous solution depending on the pH and temperature.     

Molar Mass and Structural Characteristics of Poly[(lactide-co-(aspartic acid)] Block Copolymers

Summary: We report on various synthetic procedures for the preparation of biodegradable and biocompatible poly(lactide-co-aspartic acid) block copolymers based on natural monomeric units – lactic acid and aspartic acid. Multiblock poly(lactide-co-aspartic acid) copolymers of different comonomer composition were synthesized by heating a mixture of L-aspartic acid and L,L-lactide in melt without the addition of any catalyst or solvent and with further alkaline hydrolysis of the cyclic succinimide rings to aspartic acid units. Diblock poly(lactide-co-aspartic acid) copolymers with different block lengths were prepared by copolymerization of amino terminated poly(β-benzyl-L-aspartate) homopolymer and L,L-lactide with subsequent deprotection of the benzyl protected carboxyl group by hydrogenolysis. The differences in the structure, composition, molar mass characteristics, and water-solubility of the synthesized multiblock and diblock poly(lactide-co-aspartic acid) copolymers are discussed.     

Giant Biodegradable Poly(ethylene glycol)‐block‐Poly(ε‐caprolactone) Polymersomes by Electroformation

Here, the formation of giant enzyme‐degradable polymersomes using the electroformation method is reported. Poly(ethylene glycol)‐block‐poly(ε‐caprolactone) polymersomes have been shown previously to be attractive candidates for the detection of bacterial proteases and protease mediated release of encapsulated reporter dyes and antimicrobials. To maximize the efficiency, the maximization of block copolymer (BCP) vesicle size without compromising their properties is of prime importance. Thus, the physical‐chemical properties of the BCP necessary to self‐assemble into polymeric vesicles by electroformation are first identified. Subsequently, the morphology of the self‐assembled structures is extensively characterized by different microscopy techniques. The vesicular structures are visualized for giant polymersomes by confocal laser scanning microscopy upon incorporation of reporter dyes during the self‐assembly process. Using time correlated single photon counting and by analyzing the fluorescence decay curves, the nanoenvironment of the encapsulated fluorophores is unveiled. Using this approach, the hollow core structure of the polymersomes is confirmed. Finally, the encapsulation of different dyes added during the electroformation process is studied. The results underline the potential of this approach for obtaining microcapsules for subsequent triggered release of signaling fluorophores or antimicrobially active cargo molecules that can be used for bacterial infection diagnostics and/or treatment.     

Poly(methyl silsesquioxane)/amphiphilic block copolymer hybrids and their porous derivatives: Poly(styrene‐block‐acrylic acid) and poly(styrene‐block‐3‐trimethoxysilylpropyl methacrylate)

Porous poly(methyl silsesquioxane) (PMSSQ) films were prepared from PMSSQ/amphiphilic block copolymer (ABC) hybrids, and this was followed by spin coating and multistep baking. The ABCs were poly(styrene‐block‐acrylic acid) (PS‐b‐PAA) and poly(styrene‐block‐3‐trimethoxysilylpropyl methacrylate) (PS‐b‐PMSMA), which were synthesized by living polymerization. The chemical bonding between the ABCs and PMSSQ resulted in significant differences in the morphologies and properties of the hybrids and their porous derivatives. Both intramolecular and intermolecular hydrogen bonding existed in the PMSSQ/PS‐b‐PAA hybrid and led to macrophase separation. Through the modification of the chemical structure from the poly(acrylic acid) segment to PMSMA, covalent bonding between PMSSQ and PMSMA occurred and prevented the macrophase separation and initial pyrolysis of the ABC. Modulated differential scanning calorimetry results also suggested a significant difference in the miscibility of the two hybrid systems. The chemical bonding resulted in higher retardation of the symmetry‐to‐nonsymmetry Si? O? Si structural transformation for PMSSQ/PS‐b‐PMSMA than for PMSSQ/PS‐b‐PAA according to Fourier transform infrared studies. The pore size of the nanoporous thin film from the PMSSQ/PS‐b‐PMSMA hybrid was estimated by transmission electron microscopy to be less than 15 nm. The refractive index and dielectric constant of the prepared porous films decreased from 1.354 to 1.226 and from 2.603 to 1.843 as the PS‐b‐PMSMA loading increased from 0 to 50 wt %, respectively. This study suggests that chemical bonding in hybrid materials plays a significant role in the preparation of low‐dielectric‐constant nanoporous films. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4466–4477, 2004     

Synthesis of Continuous Mesoporous Alumina Films with Large‐Sized Cage‐Type Mesopores by Using Diblock Copolymers

Mesoporous alumina films with large‐sized cage‐type mesopores were prepared by using commercially available diblock copolymer (PS‐b‐PEO) and economic inorganic salt (AlCl₃) as aluminum source. The obtained mesopore sizes drastically expand from 35 nm to 80 nm when the amount of ethanol in the precursor solutions were controlled. More interestingly, under an optimized amount of ethanol as co‐solvent, there was no significant change of micelle morphology on the substrate, even though the relative amount of PS‐b‐PEO to alumina source was dramatically varied. When the amount of alumina precursor was decreased, the pore walls gradually became thinner, thereby improving pore connectivity. The ordered mesoporous alumina films obtained in this study exhibit high thermal stability up to 1000 °C, and their frameworks are successfully crystallized to γ‐alumina phase. This technique could also be applicable for creating other metal oxide thin films with large mesopores.     

Cover Picture: Toroidal Micelles of Uniform Size from Diblock Copolymers (Angew. Chem. Int. Ed. 25/2009)

Pure toroidal micelles of highly uniform shape and size are presented by T. Chang et al. in their Communication on page 4594 ff. The donut‐shaped micelles are prepared by spontaneous self‐assembly of a polyisoprene‐block‐poly(2‐vinylpyridine) copolymer, and are stable enough to act as a template for the growth of gold nanoparticles along the ring surface.

     

Emulsion‐Induced Ordered Microporous Films Based on Micelles of Amphiphilic Poly(ethylene oxide)‐block‐Poly(methyl methacrylate) Diblock Copolymers

Well‐defined PEO‐b‐PMMA was prepared, initiated by macroinitiator PEO‐Br, by means of ATRP, where esterification of the terminal hydroxyl group of PEO with 2‐bromoisobutyryl bromide yielded a macroinitiator PEO‐Br. Highly ordered microporous films (hexagonal pattern) were constructed by emulsion micelles of such amphiphilic diblock copolymer formed from a solution with CHCl₃/H₂O/THF = 100:5:10 (v/v). We also constructed the microporous films using diblock copolymer by the current water‐assisted method.

     

Surface Stabilization of Diblock PEG‐PLGA Micelles by Polymerization of N‐Vinyl‐2‐pyrrolidone

This paper presents a new approach to improving the physical stability of biodegradable poly‐(ethylene glycol)‐block‐poly[(DL ‐lactic acid)‐co‐(glycolic acid)] (PEG‐PLGA) micelles. A hydroxyl‐terminated PEG monomethacrylate (PEGmer) macroinitiator was used to prepare a methacrylate‐end‐capped PEG‐PLGA diblock copolymer by the ring‐opening polymerization of D ,L ‐lactide and glycolide. The surface‐exposed methacrylate groups in the shell layer of the micelles can be polymerized with N‐vinyl‐2‐pyrrolidone. The resulting micelles show substantially enhanced stability.