Graeme Cambridge

Complex and hierarchical micelle architectures from diblock copolymers using living, crystallization-driven polymerizations.

Block copolymers consist of two or more chemically distinct polymer segments, or blocks, connected by a covalent link. In a selective solvent for one of the blocks, core-corona micelle structures are formed. We demonstrate that living polymerizations driven by the epitaxial crystallization of a core-forming metalloblock represent a synthetic tool that can be used to generate complex and hierarchical micelle architectures from diblock copolymers. The use of platelet micelles as initiators enables the formation of scarf-like architectures in which cylindrical micelle tassels of controlled length are grown from specific crystal faces. A similar process enables the fabrication of brushes of cylindrical micelles on a crystalline homopolymer substrate. Living polymerizations driven by heteroepitaxial growth can also be accomplished and are illustrated by the formation of tri- and pentablock and scarf architectures with cylinder-cylinder and platelet-cylinder connections, respectively, that involve different core-forming metalloblocks.

Stimulus-Responsive Self-Assembly: Reversible, Redox-Controlled Micellization of Polyferrocenylsilane Diblock Copolymers

In depth studies of the use of electron transfer reactions as a means to control the self-assembly of diblock copolymers with an electroactive metalloblock are reported. Specifically, the redox-triggered self-assembly of a series of polystyrene-block-polyferrocenylsilane (PS-b-PFS) diblock copolymers in dichloromethane solution is described. In the case of the amorphous polystyrene(n)-b-poly(ferrocenylphenylmethylsilane)(m) diblock copolymers (PS(n)-b-PFMPS(m): n = 548, m = 73; n = 71, m = 165; where n and m are the number-averaged degrees of polymerization), spherical micelles with an oxidized PFS core and a PS corona were formed upon oxidation of more than 50% of the ferrocenyl units by [N(C(6)H(4)Br-4)(3)][SbX(6)] (X = Cl, F). Analogous block copolymers containing a poly(ferrocenylethylmethylsilane) (PFEMS) metalloblock, which has a lower glass transition temperature, behaved similarly. However, in contrast, on replacement of the amorphous metallopolymer blocks by semicrystalline poly(ferrocenyldimethylsilane) (PFDMS) segments, a change in the observed morphology was detected with the formation of ribbon-like micelles upon oxidation of PS(535)-b-PFDMS(103) above the same threshold value. Again the coronas consisted of fully solvated PS and the core consisted of partially to fully oxidized PFS associated with the counteranions. When oxidation was performed with [N(C(6)H(4)Br-4)(3)][SbF(6)], reduction of the cores of the spherical or ribbon-like micelles with [Co(η-C(5)Me(5))(2)] enabled full recovery of the neutral chains and no significant chain scission was detected.

Self-seeding in one dimension: an approach to control the length of fiberlike polyisoprene-polyferrocenylsilane block copolymer micelles.

Self-seeding is a phenomenon unique for polymer crystallization. Polymers have difficulty in crystallizing and, in most cases, only part of each polymer chain can be accommodated in the crystal lattice. As a result, polymers form crystals with lamellar structures terminated by surfaces containing chain folds. If long chains have to be integrated into the crystal in a short time, they will do so at the expense of lower crystallinity. As a consequence, polymer crystals inevitably consist of regions with different chain order and conformational entropy. Polymer crystals have a broad range of melting temperatures whose values depend upon the details of the crystallization process. In a typical self-seeding experiment, a crystalline polymer in the bulk state or suspended in a solvent is heated slightly above its normal melting point (as determined, for example, by differential scanning calorimetry; DSC) so that no residual crystals can be detected optically or spectroscopically. Cooling this melt or solution leads to the formation of polymer single crystals, normally in the form of thin plates uniform in size and thickness, which can be ideally suited for further applications. These single crystals are thought to be initiated by submicroscopic nuclei that survived the dissolution procedure. Since the discovery of self-seeding in the 1960s the process has attracted attention as a means of controlling the nucleation step of polymer crystallization without the need for external nucleating agents, to form uniform single crystals of homopolymers and block copolymers and also for materials applications. Polyferrocenyldimethylsilane (PFS) is a crystalline metalcontaining polymer with a range of interesting properties. PFS block copolymers and closely related materials selfassemble to form elongated micelles with a semicrystalline core. They are the only currently known synthetic polymers to form fiberlike micelles by a mechanism resembling that for the formation of amyloid fibers from soluble protein. Thus soluble polymeric “monomers” consisting of block copolymer unimers condense onto both ends of seed structures present in, or intentionally added to, the solution. PFS block copolymer fiber formation involves a conformation change driven by epitaxial crystallization of PFS moieties onto the open ends of the PFS core of existing micelles or seeds obtained by subjecting preformed fiberlike micelles to mild sonication. Thus, the number of micelles at the end of the growth process is determined by the number of seeds present at the beginning. Moreover the seeded growth experiments permit exquisite control over the types of structures obtained. For example, one type of PFS block copolymer such as PI-PFS (PI = polyisoprene) can be used to form the seed structure, and a different type of PFS block copolymer such as PFSPDMS (PDMS = polydimethylsiloxane) can be grown off the ends. In this way striking novel architectures referred to as “triblock co-micelles” can be prepared. 10] Our recent work targets a deeper understanding of the self-assembly process for PFS block copolymers in order to develop principles that can be extended to other coil-crystalline block copolymers. This may allow access to processable suspensions of semiflexible nanowires with useful optical or electronic properties. With proper control over their length and dimensions, such structures could be incorporated into optoelectronic devices or used in other applications. In 2009, Reiter and co-workers examined the mechanism of self-seeding in the melt for PFS homopolymer single crystals and for single crystals formed by P2VP-PEO block copolymers. (P2VP = poly(2-vinylpyridine), PEO = poly(ethylene oxide)). They showed for these two systems that the number density of the regenerated crystals decreased exponentially with the increase of the dissolution temperature but did not vary with the dissolution time. They also found a correlation in molecular orientation between a starting single crystal and the regenerated crystal clones formed through the self-seeding process. Their experiments established that single-crystal growth by self-seeding operates under thermodynamic control, consistent with the idea that upon heating, the less perfect crystals will melt and more perfect crystallites will survive. It is not a kinetic effect associated with polymer conformational memory effects. [*] J. S. Qian, Dr. G. Guerin, Y. J. Lu, G. Cambridge, Prof. M. A. Winnik Department of Chemistry, University of Toronto 80 St. George Street Toronto, Ontario, M5S 3H6 (Canada) Fax: (+ 1)416-978-0541 E-mail: mwinnik@chem.utoronto.ca

Multi-armed micelles and block co-micelles via crystallization-driven self-assembly with homopolymer nanocrystals as initiators.

We report the preparation of multi-armed micelles and block co-micelles using the crystallization-driven self-assembly of crystalline-coil polyferrocenylsilane block copolymers from nanocrystals of the homopolymer. The resulting multi-armed micelles possessed hierarchical multipod structures with monodisperse and tunable arm lengths. The termini of the arms remained active to the addition of further block copolymer unimers, and multi-armed block co-micelles with segmented arm chemistries and variable segment sequences were prepared. Coronal cross-linking followed by nanocrystal dissolution led to the release of non-centrosymmetric AB cylindrical diblock co-micelles.

Reversible cross-linking of polyisoprene coronas in micelles, block comicelles, and hierarchical micelle architectures using Pt(0)-olefin coordination.

Previous work has established that polyisoprene (PI) coronas in cylindrical block copolymer micelles with a poly(ferrocenyldimethylsilane) (PFS) core can be irreversibly cross-linked by hydrosilylation using (HSiMe(2))(2)O in the presence of Karstedts catalyst. We now show that treatment of cylindrical PI-b-PFS micelles with Karstedts catalyst alone, in the absence of any silanes, leads to PI coronal cross-linking through Pt(0)-olefin coordination. The cross-linking can be reversed through the addition of 2-bis(diphenylphosphino)ethane (dppe), a strong bidentate ligand, which removes the platinum from the PI to form Pt(dppe)(2). The Pt(0) cross-linking of PI was studied with self-assembled cylindrical PI-b-PFS block copolymer micelles, where the cross-linking was found to dramatically increase the stability of the micellar structures. The Pt(0)-alkene coordination-induced cross-linking can be used to provide transmission electron microscopy contrast between PI and poly(dimethylsiloxane) (PDMS) corona domains in block comicelles as the process selectively increases the electron density of the PI regions. Moreover, following the assembly of a hierarchical scarf-shaped comicelle consisting of a PFS-b-PDMS platelet template with PI-b-PFS tassels, Pt(0)-induced cross-linking of the PI coronal regions allowed for the selective removal of the PFS-b-PDMS center, leaving behind an unprecedented hollowed-out scarf structure. The addition of Karstedts catalyst to PI or polybutadiene homopolymer toluene/xylene solutions resulted in the formation of polymer gels which underwent de-gelation upon the addition of dppe.

Self-Seeding in One Dimension: A Route to Uniform Fiber-like Nanostructures from Block Copolymers with a Crystallizable Core-Forming Block

One-dimensional micelles formed by the self-assembly of crystalline-coil poly(ferrocenyldimethylsilane) (PFS) block copolymers exhibit self-seeding behavior when solutions of short micelle fragments are heated above a certain temperature and then cooled back to room temperature. In this process, a fraction of the fragments (the least crystalline fragments) dissolves at elevated temperature, but the dissolved polymer crystallizes onto the ends of the remaining seed fragments upon cooling. This process yields longer nanostructures (up to 1 μm) with uniform width (ca. 15 nm) and a narrow length distribution. In this paper, we describe a systematic investigation of factors that affect the self-seeding behavior of PFS block copolymer micelle fragments. For PI(1000)-PFS(50) (the subscripts refer to the number average degree of polymerization) in decane, these factors include the presence of a good solvent (THF) for PFS and the effect of annealing the fragments prior to the self-seeding experiments. THF promoted the dissolution of the micelle fragments, while preannealing improved their stability. We also extended our experiments to other PFS block copolymers with different corona-forming blocks. These included PI(637)-PFS(53) in decane, PFS(60)-PDMS(660) in decane (PDMS = polydimethylsiloxane), and PFS(30)-P2VP(300) in 2-propanol (P2VP = poly(2-vinylpyridine)). The most remarkable result of these experiments is our finding that the corona-forming chain plays an important role in affecting how the PFS chains crystallize in the core of the micelles and, subsequently, the range of temperatures over which the micelle fragments dissolve. Our results also show that self-seeding is a versatile approach to generate uniform PFS fiber-like nanostructures, and in principle, the method should be extendable to a wide variety of crystalline-coil block copolymers.

End-to-End Coupling and Network Formation Behavior of Cylindrical Block Copolymer Micelles with a Crystalline Polyferrocenylsilane Core

Cylindrical block copolymer micelles with a crystalline poly(ferrocenyldimethylsilane) (PFDMS) core and a long corona-forming block are known to elongate through an epitaxial growth mechanism on addition of further PFDMS block copolymer unimers. We now report that addition of the semicrystalline homopolymer PFDMS(28) to monodisperse short (ca. 200 nm), cylindrical seed micelles of PFDMS block copolymers results in the formation of aggregated structures by end-to-end coupling to form micelle networks. The resulting aggregates were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). In some cases, a core-thickening effect was also observed where the added homopolymer appeared to deposit and crystallize at the core-corona interface, which resulted in an increase of the width of the micelles within the networks. No evidence for aggregation was detected when the amorphous homopolymer poly(ferrocenylethylmethylsilane) (PFEMS(25)) was added to the cylindrical seed micelles whereas similar behavior to PFDMS(28) was noted for semicrystalline polyferrocenyldimethylgermane (PFDMG(30)). This suggested that the crystallinity of the added homopolymer is critical for subsequent end-to-end coupling and network formation to occur. We also explored the tendency of the cylindrical seed micelles to form aggregates by the addition of PI-b-PFDMS (PI = polyisoprene) block copolymers (block ratios 6:1, 3.8:1, 2:1, or 1:1), and striking differences were noted. The results ranged from typical micelle elongation, as reported in previous work, at high corona to core-forming block ratios (PI-b-PFDMS; 6:1) to predominantly end-to-end coupling at lower ratios (PI-b-PFDMS; 2:1, 1:1) to form long, essentially linear structures. The latter process, especially for the 2:1 block copolymer, led to much more controlled aggregate formation compared with that observed on addition of homopolymers.

Fiberlike Micelles Formed by Living Epitaxial Growth from Blends of Polyferrocenylsilane Block Copolymers

Poly(ferrocenyldimethylsilane) (PFS) block copolymers form fiberlike micelles by a seeded growth process. This paper describes the effect of adding similar amounts of PFS block copolymers, PFS-PDMS and PFS-PI, to a common micelle seed. The lengths of the micelles obtained were strongly influenced by the degree of polymerization of the corona-forming blocks. The change in length was due to a change in the number of polymer molecules per unit length of the micelle.

Seeded Growth and Solvent-Induced Fragmentation of Fiberlike Polyferrocenylsilane–Polyisoprene Block Copolymer Micelles

Addition of a concentrated solution of PI(1000) -PFS(50) dissolved in THF to a solution of PI(1000) -PFS(50) seed micelles in decane led to the formation of uniform elongated fiberlike micelles with a narrow length distribution. When additional THF (>10 vol.-%) was added to the micelles, the micelle length decreased and the contour-length distribution broadened. This effect was shown to be inconsistent with a transition to an equilibrium, in which individual polymer molecules dissociated from and added to existing micelles. Rather, it appears that the polar solvent induced fragmentation of the fiberlike micelles.

Evaluation of the Cross Section of Elongated Micelles by Static and Dynamic Light Scattering

We describe simultaneous static (SLS) and dynamic light scattering (DLS) measurements on dilute solutions of a series of poly(ferrocenyldimethylsilane-b-isoprene) (PFS(50)-PI(1000)) block copolymer micelles of uniform length in tert-butyl acetate (tBA) and in decane. The subscripts in the term PFS(50)-PI(1000) refer to the mean degree of polymerization of each block. The SLS experiments show that in both solvents the micelles formed are elongated and rigid. We also observed that the large length of the PI block (1000 units) contributes to the SLS signal. From the SLS data, we calculated the mass per unit length (linear aggregation number), as well as the cross section of the micelles in both solvents. Interestingly, the linear aggregation number and the micelle cross sections, as deduced by SLS, were the same in decane and in tBA. However, the fitting of DLS data indicates that the hydrodynamic cross section of the micelles in tBA is much larger than that in decane, and both values are larger than the values determined by SLS. We hypothesize that the difference between cross sections deduced from SLS and DLS data fitting is related to the shape of the segment density profile of the corona. In tBA, the PI chains are more stretched than in decane, increasing the hydrodynamic radius of the micelle cross section.