Laurent Chabanne

Monodisperse cylindrical micelles by crystallization-driven living self-assembly

Non-spherical nanostructures derived from soft matter and with uniform size-that is, monodisperse materials-are of particular utility and interest, but are very rare outside the biological domain. We report the controlled formation of highly monodisperse cylindrical block copolymer micelles (length dispersity < or = 1.03; length range, approximately 200 nm to 2 microm) by the use of very small (approximately 20 nm) uniform crystallite seeds that serve as initiators for the crystallization-driven living self-assembly of added block-copolymer unimers with a crystallizable, core-forming metalloblock. This process is analogous to the use of small initiator molecules in classical living polymerization reactions. The length of the nanocylinders could be precisely controlled by variation of the unimer-to-crystallite seed ratio. Samples of the highly monodisperse nanocylinders of different lengths that are accessible using this approach have been shown to exhibit distinct liquid-crystalline alignment behaviour.

Non-Centrosymmetric Cylindrical Micelles by Unidirectional Growth

Unidirectional Growth Block copolymers, in which two dissimilar polymers are covalently joined together, can be designed to form micelles in solution and can be used as self-assembling injectable gels for tissue engineering or wound healing. One challenge is to find ways to create asymmetrical structures, because normally, block addition would occur at both ends of the polymer chain. Rupar et al. (p. 559; see the Perspective by Pochan) devised a route to link together three diblock copolymers with a capping approach. Protecting one end during growth gave rise to asymmetrical structures. A capping approach is used to create asymmetrical block copolymer micelles through self-assembly. Although solution self-assembly of block copolymers (BCPs) represents one of the most promising approaches to the creation of nanoparticles from soft matter, the formation of non-centrosymmetric nanostructures with shape anisotropy remains a major challenge. Through a combination of crystallization-driven self-assembly of crystalline-coil BCPs in solution and selective micelle corona cross-linking, we have created short (about 130 nanometers), monodisperse cylindrical seed micelles that grow unidirectionally. These nanostructures grow to form long, non-centrosymmetric cylindrical A-B and A-B-C block co-micelles upon the addition of further BCPs. We also illustrate the formation of amphiphilic cylindrical A-B-C block co-micelles, which spontaneously self-assemble into hierarchical star-shaped supermicelle architectures with a diameter of about 3 micrometers. The method described enables the rational creation of non-centrosymmetric, high aspect ratio, colloidally stable core-shell nanoparticles in a manner that until now has been restricted to the biological domain.

Metallopolymers with emerging applications

A wide variety of metal-containing polymers, or ‘metallopolymers’, have become readily available over the past decade. This has led to a rapidly expanding interest in their properties and uses. These new materials combine the processing advantages of polymers with the functionality provided by the presence of metal centers. We illustrate a selection of applications of metallopolymers in areas such as sensors, memory and light-emitting devices, solar cells, nanolithography, photonic crystal displays, controlled release, and catalysis.

Probing the structure of the crystalline core of field-aligned, monodisperse, cylindrical polyisoprene-block-polyferrocenylsilane micelles in solution using synchrotron small- and wide-angle X-ray scattering.

The self-assembly of block copolymers in selective solvents represents a powerful approach to functional core-shell nanoparticles. Crystallization of the core can play a critical role in directing self-assembly toward desirable, nonspherical morphologies with low mean interfacial curvature. Moreover, epitaxial growth processes have been implicated in recent advances that permit access to monodisperse cylinders, cylindrical block comicelles with segmented cores and/or coronas, and complex hierarchical architectures. However, how the core-forming block crystallizes in an inherently curved nanoscopic environment has not been resolved. Herein we report the results of synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) studies of well-defined, monodisperse crystalline-coil polyisoprene-block-polyferrocenylsilane cylindrical micelles aligned in an electric field. WAXS studies of the aligned cylinders have provided key structural information on the nature of the PFS micelle core together with insight into the role of polymer crystallinity in the self-assembly of these and potentially related crystalline-coil block copolymers.

Heterogeneous Dehydrocoupling of Amine–Borane Adducts by Skeletal Nickel Catalysts

Skeletal Ni, produced by the selective leaching of Al from a Ni/Al alloy, has been successfully employed in the catalytic dehydrogenation of various amine-borane adducts. The combination of low cost and facile single-step synthesis make this system a potentially attractive alternative to the previously described precious metal and other first-row metal catalysts. The heterogeneous nature of the catalyst facilitates convenient product purification, and this is the first such system to be based on a first-row transition metal. Catalytic dehydrocoupling of Me(2)NH·BH(3) (1) and Et(2)NH·BH(3) (5) was demonstrated using 5 mol % skeletal Ni catalyst at 20 °C and produced [Me(2)N-BH(2)](2) (2) and [Et(2)N-BH(2)](2)/Et(2)N═BH(2) (6), respectively. The related adduct iPr(2)NH·BH(3) (7) was also dehydrogenated to afford iPr(2)N═BH(2) (8) but with significant catalyst deactivation. Catalytic dehydrocoupling of MeNH(2)·BH(3) (9) was found to yield the cyclic triborazane [MeNH-BH(2)](3) (10) as the major product, whereas high molecular weight poly(methylaminoborane) [MeNH-BH(2)](n) (11) (M(w) = 78 000 Da, PDI = 1.52) was formed when stoichiometric quantities of Ni were used. Similar reactivity was also observed with NH(3)·BH(3) (12), which produced cyclic oligomers and insoluble polymers, [NH(2)-BH(2)](x) (14), under catalytic and stoichiometric Ni loadings, respectively. Catalyst recycling was hindered by gradual poisoning. A study of possible catalyst poisons suggested that BH(3) was the most likely surface poison, in line with previous work on colloidal Rh catalysts. Catalytic dehydrogenation of amine-borane adducts using skeletal Cu and Fe was also explored. Skeletal Cu was found to be a less active dehydrogenation catalyst for amine-borane adducts but also yielded poly(methylaminoborane) under stoichiometric conditions on reaction with MeNH(2)·BH(3) (9). Skeletal Fe was found to be completely inactive toward amine-borane dehydrogenation.

Organic-metalloblock copolymers via photocontrolled living anionic ring-opening polymerization

A new method for the preparation of organic-organometallic diblock copolymers including a polyferrocenylsilane (PFS) metalloblock through photocontrolled ring-opening polymerization (ROP) is reported. Polystyrene (PS) homopolymers end-capped with a cyclopentadienyl group (1) were used as macroinitiators for the photocontrolled ROP of sila[1]ferrocenophanes [Fe(η-C5H4)2Si{C≡CtBu}2] 3a and [Fe(η-C5H4)2Si(Me)(C≡CSiMe3)] 3b to afford diblock copolymers with controlled molecular weights and block ratios, as well as low polydispersities (PDI < 1.2). Block copolymer PSm-b-[Fe(η-C5H4)2Si{C≡C(t-Bu)}2]n4 was clusterized with [Co2(CO)8], forming the highly metallized PSm-b-[Fe(η-C5H4)2Si{Co2(CO)6C2(t-Bu)}2]n (PS-bb-(Co-PFS), 7). The diblock PSm-b-[Fe(η-C5H4)2Si(Me)(C≡CH)]n6 was prepared by selective desilylation of PSm-b-[Fe(η-C5H4)2Si(Me)(C≡CSiMe3)]n5 was then reacted with ClAuP(n-Bu)3 in the presence of an amine as HCl acceptor to afford PSm-b-[Fe(η-C5H4)2Si(Me){C≡CAuP(n-Bu)3}]n (PS-bbb-(Au-PFS), 8). Preliminary studies on the self-assembly of these materials in thin films showed phase separation with metal-rich nanodomains within an organic matrix.

An iron-cyclopentadienyl bond cleavage mechanism for the thermal ring-opening polymerization of dicarba[2]ferrocenophanes

In order to gain insight into the mechanism for the thermal ring-opening polymerization of strained dicarba[2]ferrocenophanes, the thermal reactivity of selected examples of these species with different substitution patterns has been explored. When heated at 300 °C dicarba[2]ferrocenophanes meso/rac-[Fe(η5-C5H4)2(CHPh)2] (mesomeso/racrac-7) and meso-[Fe(η5-C5H4)2(CHCy)2] (mesomeso-13) were found to isomerize or to undergo disproportionation, respectively. These processes are apparently general for dicarba[2]ferrocenophanes with one or more non-hydrogen substituents at each carbon atom in the dicarba bridge and both appear to involve homolytic cleavage of the C–C bond in the bridge as a key step. In striking contrast, derivatives containing either one or no non-hydrogen substituents on the bridge such as {Fe[η5-C5H4]2[CH(Ph)CH2]} (15) and [Fe(η5-C5H4)2(CH2)2] (17) undergo thermal ring-opening polymerization (ROP) under similar conditions (300 °C, 1 h). Thus, thermolysis of 15 yielded polyferrocenylethylene {Fe[η5-C5H4]2[CH(Ph)CH2]}n (16a) with a broad molecular weight distribution (Mw = 13,760, PDI = 3.27). Analysis of 16a by MALDI-TOF mass spectrometry suggested that the material was macrocyclic. Thermal treatment of linear polyferrocenylethylenes {Fe[η5-C5H4]2[CH(Ph)CH2]}n with narrow molecular weight distributions (prepared by photocontrolled ROP) at 300 °C confirmed that the macrocycles detected form directly, and not as a result of depolymerization. Copolymerizations of 15 with 17 and of 15 with the deuterated species [Fe(η5-C5H4)2(CD2)2] (dd44-17) were conducted in order to probe the bond cleavage mechanism. Comparative NMR spectroscopic analysis of the resulting copolymers 18 and dd44-18, respectively, and of homopolymer 16a, indicated that thermal ROP does not occur via a homolytic C–C bridge cleavage mechanism. A series of thermolysis experiments were conducted with MgCp2 (Cp = η5-C5H5) at 300 °C, which resulted in the isolation of ring-opened species formed from 15 and 17, and indicated that the Fe–Cp bonds can be cleaved under the thermal ROP conditions employed. The studies indicated that a chain growth process that involves heterolytic Fe–Cp bond cleavage in the monomers is the most probable mechanism for the thermal ROP of dicarba[2]ferrocenophanes.

Controlled thiol–ene post-polymerization reactions on polyferrocenylsilane homopolymers and block copolymers

Various thiols were reacted with poly(ferrocenylmethylvinylsilane) (PFMVS) homopolymers using the radical-mediated thiol–ene reaction with a view to preparing metallopolymers with diverse functional groups. Post-polymerization thiol–ene reactions on poly(ferrocenyldimethylsilane)-b-poly(ferrocenylmethylvinylsilane) (PFDMS-b-PFMVS) diblock copolymers with dodecanethiol and octadecanethiol afforded diblocks with a long hexane-soluble block and a short crystalline, hexane-insoluble PFDMS block. The thiol–ene reactions provided sufficient control to allow access to diblocks that were partially substituted, thus leaving vinyl groups that might subsequently be used for further post-polymerization reactions.

Functionalization of Poly(ferrocenyldimethylsilane) via Lithiation of the Cyclopentadienyl Rings

The metallation of the cyclopentadienyl (Cp) ligands of poly(ferrocenyldimethylsilane) (PFDMS) can be performed by reaction with the Schlossers base pair t-BuLi/KOt-Bu in THF. Subsequent treatment with a series of electrophiles affords a range of Cp-substituted polymers with up to an average of 1.8 new substituents per repeating unit. NMR studies on polymers containing trimethylsilyl groups and deuterium on the Cp rings are indicative of high regioselectivity with selective metallation at the β-carbon.