Reversible switching between linear and ring polystyrenes bearing porphyrin end groups.

A route to macrocyclic polymers based on a new unimolecular ring-closure process has been investigated. It involves the direct end-to-end coupling of an α,ω-bis[chloroiron(III) meso-tetraphenylporphyrin] telechelic linear polystyrene synthesized by living polymerization followed by chain-end functionalization. The corresponding macrocyclic polystyrene was obtained readily and selectively by intramolecular condensation of the α,ω-bis[chloroiron(III) meso-tetraphenylporphyrin] polymer ends in the presence of a base to yield a diiron(III)-µ-oxobis(porphyrin) dimer as ring-closing unit. Addition of dilute HCl was shown to rapidly reconvert the diiron(III)-µ-oxobis(porphyrin) unit into the initial bis[chloroiron(III) porphyrin], demonstrating the selectivity and complete reversibility of the cyclization process. The synthesis and detailed structural characterization of the α,ω-homodifunctional precursor and the corresponding macrocyclic polystyrene along with an analysis of the porphyrin dimerization reaction using NMR spectroscopy and size-exclusion chromatography coupled with a diode array detector are presented.

Atomic force microscopy imaging and dilute solution properties of cyclic and linear polystyrene combs.

Large macrocyclic poly(chloroethyl vinyl ether)s (PCEVE)s of controlled ring size and narrow distribution were synthesized by a ring-closure process involving the intramolecular formation of acetal linkages between the two external blocks of linear ABC triblock precursors prepared by living cationic polymerization. The corresponding shape-persistent ring P(CEVE-g-PS) combs having macrocyclic poly(chloroethyl vinyl ether) backbones and polystyrene side chains were then synthesized by a "grafting onto" technique and characterized by size exclusion chromatography (SEC) analysis and atomic force microscopy (AFM) imaging of isolated molecules. Quantitative hydrolysis of the acetal linkages of the macrocyclic PCEVE backbone in acidic conditions yields the linear poly(chloroethyl vinyl ether)-g-polystyrene) homologue and allows a direct comparison of the characteristics and dimensions of cyclic and linear comb architecture. The influence of the chain architecture and PS graft dimensions on the dilute tetrahydrofuran (THF) solution properties, radius of gyration, and hydrodynamic radius of the comb copolymers is also studied and compared to data reported for linear and cyclic polystyrene chains.

Synthesis and AFM structural imaging of dendrimer-like star-branched polystyrenes.

We report on the synthesis of large and densely tough dendrimer-like star-branched polystyrenes of generation 3, specially designed for their visualization by AFM imaging. For this purpose, an iterative methodology involving a sequential three stages synthetic approach has been adopted. The precisely designed G-3 polymers appear on AFM images as completely flattened objects with a symmetrical square-like shape and an average diameter of 250 nm and a height of less than 1 nm. Imaging of their internal structure indicates that they are constituted of four interconnected lobes, in relation with the use of a tetrafunctional core precursor. In addition, AFM images show the presence inside each object of all the branching points which appear as white harder zones regularly distributed in the less dense and softer PS matrix.

Synthesis of macrocyclic copolymer brushes and their self-assembly into supramolecular tubes.

We report on an efficient route to design large macrocyclic polymers of controlled molar mass and narrow dispersity. The strategy is based on the synthesis of a triblock copolymer ABC, in which the long central block B is extended by two short A and C sequences bearing reactive antagonist functions. When reacted under highly dilute conditions, this precursor produces the corresponding macrocycle by intramolecular coupling of the A and C blocks. Chloroethyl vinyl ether was selected as the monomer for the central block B, because it can be readily derivatized into brushlike polymers by a grafting process. The corresponding macrocyclic brushes were decorated with polystyrene or randomly distributed polystyrene and polyisoprene branches. In a selective solvent for the polyisoprene branches, the macrocyclic brushes self-assemble into cylindrical tubes of up to 700 nanometers.

Water-soluble dendrigrafts bearing saccharidic moieties: elaboration and application to enzyme linked OligoSorbent Assay (ELOSA) diagnostic tests.

The synthesis of a series of water-soluble galactopyranose-functionalized polystyrene-polyvinyl ether dendrigrafts and their characterization (in solution and thin solid deposits) have been achieved. The presence of external galactopyranose groups on dendritic polymers has been exploited to prepare dendrigraft-oligonucleotide conjugates using a simple one-step coupling procedure with amino-ended oligonucleotides (ODNs). Several parameters such as the peripherical density of hydrophilic branches, the polymerization degree of polystyrene or poly(hydroxyethyl vinyl ether) blocks, and the number of galactopyranose groups were tuned. A capture test with short labeled complementary ODNs (25 bases) confirmed the presence of covalently bound ODNs on various kinds of dendrigrafts. The ability of the dendritic polymers to enhance the sensitivity of enzyme-linked oligosorbent assay (ELOSA) diagnostic tests (detection of hepatitis B virus, DNA target of 2400 bases) was then evaluated, especially the influence of the macromolecular architecture and the impact of the structural parameters. The dendrigraft-ODN conjugate with the lower saccharide external density was found to lead to a very significant amplification of the fluorescence signal, corresponding to a limit of sensitivity of 10(9) DNA copies per milliliter (instead of 10(11) DNA copies per milliliter without using dendrigrafts). Conversely, the dendrigrafts exhibiting a very high number of branches and galactopyranose groups at their periphery were not able to induce a better sensitivity due to steric hindrance generated by the peripheral congestion on these polymers.

Micellar aggregation in blends of linear and cyclic poly(styrene-b-isoprene) diblock copolymers.

The morphology of micelles formed from blends of linear and cyclic poly(styrene-b-isoprene) (PS-b-PI) block copolymers has been investigated in solution using dynamic light scattering (DLS) and in thin solid deposits by atomic force microscopy (AFM) and transmission electron microscopy under cryogenic conditions (cryo-TEM). Micelles of the pure cyclic PS(290)-b-PI(110) copolymers are wormlike cylindrical objects built by unidirectional aggregation of 33 nm wide sunflower micelles, while the linear block copolymer having the same volume fraction and molar mass forms spherical micelles 40 nm in diameter. The DLS, AFM, and cryo-TEM results consistently show that the addition of the linear copolymer (even for amounts as low as 5% w/w) to the cyclic copolymer rather favors the formation of spherical micelles at the expense of the cylindrical aggregates. Those results clearly show that the linear block copolymer chains can be used to stabilize the thermodynamically unstable elementary sunflower micelle. The thermal stability of the micelles (from the pure copolymers and from the blends) has been examined in solid deposits with in situ AFM measurements. Coalescence starts at about 70 degrees C, and the surface roughness shows a two-step decrease toward a fully homogeneous and flat structure.

Molecular containers based on amphiphilic PS-b-PMVE dendrigraft copolymers: topology, organization, and aqueous solution properties.

The synthesis, characteristics, and properties of amphipatic, water-soluble dendrigrafts, with a polystyrene core and polystyrene-b-poly(methyl vinyl ether) (PS-b-PMVE) diblock as external branches, are described. The dendrigrafts are observed by AFM and TEM as egglike or long cylindrical objects which can self-organize intramolecularly in segregated subdomains forming flowerlike or strings of flowerlike objects. In organic solvents the dendrigrafts behave as fully soluble isolated macromolecules and show in water a low critical solubility temperature (LCST) at t > 30 degrees C. The ability of the amphiphilic PS-b-PMVE dendrigrafts to complex and transport in water organic (pyrene) and metallo-organic (manganese tetraphenyl porphyrin) molecules is investigated. The possibility to stabilize the high oxidation state of metallo-porphyrin complexes through their encapsulation into the dendrigraft is shown.

Control of the morphology of linear and cyclic PS-b-PI block copolymer micelles via PS addition.

We have studied the effect of polystyrene (PS) homopolymer addition on the morphology of self-assembled block copolymer micelles made from linear or cyclic poly(styrene-b-isoprene), PS-b-PI, in a selective solvent for the PI block (heptane). Both copolymers have the same composition: the degree of polymerization is 290 for the PS block, and 110 for the PI block, and we focused on the influence of the addition of small amounts of PS homopolymer on the micellar morphology. For the copolymer concentrations considered, the linear copolymer self-organizes into spherical micelles while the cyclic copolymer forms cylindrical micelles. PS and PI chains constitute the core and the corona of these micelles, respectively, due to the different affinity of the blocks for heptane. Consequently, the PS homopolymer added is "solubilized" into the micellar core. Dynamic light scattering (DLS) data combined with atomic force microscopy (AFM) results show that the addition of PS homopolymer induces a drastic change in the micellar organization. Indeed, a morphological transition, from spheres to cylinders for the linear copolymer, and from cylinders to vesicles for the cyclic copolymer, is observed. These results highlight the fact that a small incorporation of PS homopolymer is clearly sufficient to modify the morphology (size and shape) of the micelles. This approach could be a key parameter for the design/control of micelles for specific applications in nanotechnology.

Vesicles made of PS-PI cyclic diblock copolymers: in situ freeze-drying cryo-TEM and dynamic light scattering experiments.

We have studied the morphology of self-assembled micelles made of linear and cyclic poly(styrene-b-isoprene) PS-b-PI block copolymers dispersed in selective solvents of the PI block (n-heptane, n-decane). Up to a copolymer concentration of 5 mg mL(-1), the micelles made from linear block copolymer chains adopt a spherical shape. Those arising from cyclic copolymer chains having exactly the same molar mass and volume fraction self-assemble into (i) planar sunflower-shaped particles at low concentration (c < 0.1 mg mL(-1)), (ii) giant wormlike micelles at intermediate concentration (0.1 mg mL(-1) < c < 2 mg mL(-1)) and (iii) vesicles at higher concentration (2 mg mL(-1) < c < 5 mg mL(-1)). Those results were obtained using dynamic light scattering and in situ freeze-drying cryo-transmission electron microscopy. In this contribution, we discuss the effects of concentration and temperature on the morphology of the self-assembled particles made from both linear and cyclic PS-PI copolymers, and highlight the surprising vesicle formation in cyclic block copolymer solutions.