Showing papers by "Virgil Percec published in 2008"

Induced helical backbone conformations of self-organizable dendronized polymers.

Abstract: Control of function through the primary structure of a molecule presents a significant challenge with valuable rewards for nanoscience. Dendritic building blocks encoded with information that defines their three-dimensional shape (e.g., flat-tapered or conical) and how they associate with each other are referred to as self-assembling dendrons. Self-organizable dendronized polymers possess a flat-tapered or conical self-assembling dendritic side chain on each repeat unit of a linear polymer backbone. When appended to a covalent polymer, the self-assembling dendrons direct a folding process (i.e., intramolecular self-assembly). Alternatively, intermolecular self-assembly of dendrons mediated by noncovalent interactions between apex groups can generate a supramolecular polymer backbone. Self-organization, as we refer to it, is the spontaneous formation of periodic and quasiperiodic arrays from supramolecular elements. Covalent and supramolecular polymers jacketed with self-assembling dendrons self-organize. The arrays are most often comprised of cylindrical or spherical objects. The shape of the object is determined by the primary structure of the dendronized polymer: the structure of the self-assembling dendron and the length of the polymer backbone. It is therefore possible to predictably generate building blocks for single-molecule nanotechnologies or arrays of supramolecules for bottom-up self-assembly. We exploit the self-organization of polymers jacketed with self-assembling dendrons to elucidate how primary structure determines the adopted conformation and fold (i.e., secondary and tertiary structure), how the supramolecules associate (i.e., quaternary structure), and their resulting functions. A combination of experimental techniques is employed to interrogate the primary, secondary, tertiary, and quaternary structure of the self-organizable dendronized polymers. We refer to the process by which we interpolate between the various levels of structural information to rationalize function as retrostructural analysis. Retrostructural analysis validates our hypothesis that the self-assembling dendrons induce a helical backbone conformation in cylindrical self-organizable dendronized polymers. This helical conformation mediates unprecedented functions. Self-organizable dendronized polymers have emerged as powerful building blocks for nanoscience by virtue of their dimensions and ability to self-organize. Discrete cylindrical and spherical structures with well-defined dimensions can be visualized and manipulated individually. More importantly, they provide a robust framework for elucidating functions available only at the nanoscale. This Account will highlight structures and functions generated from self-organizable dendronized polymers that enable integration of the nanoworld with its macroscopic universe. Emphasis is placed on those structures and functions derived from the induced helical backbone conformation of cylindrical self-organizable dendronized polymers.

Solvent Choice Differentiates SET-LRP and Cu-Mediated Radical Polymerization with Non-First-Order Kinetics

Abstract: In order to assess the role of the solvent mediating the disproportionation of Cu(I)X into Cu(0) and Cu(II)X2 during SET-LRP, a series of polymerizations of methyl acrylate initiated with methyl 2-bromopropionate and catalyzed by Cu(0)/Me6-TREN at 25 °C in DMSO, MeCN, and their mixtures were investigated. These polymerizations proceed in a dissimilar manner. SET-LRP of MA in DMSO, a solvent that mediates disproportionation, was exemplary of “ultrafast living radical polymerization” with complete conversion in 45 min following the expected first-order kinetics and providing perfect fidelity of the structure of the polymer chain ends. When the polymerization was performed in MeCN, a solvent in which Cu(I)X species do not disproportionate, it displayed nonlinear first-order kinetics with a poor retention of the bromine chain end functionality. The lack of first-order kinetics and decrease of chain end functionality with conversion indicate a nonliving polymerization in MeCN. Increasing the concentration of D...

Ultrafast SET-LRP of methyl acrylate at 25 °C in alcohols

Abstract: Alcohols are known to promote the disproportionation of Cu(I)X species into nascent Cu(0) and Cu(II)X. Therefore, alcohols are expected to be excellent solvents that facilitate the single-electron transfer mediated living radical polymerization (SET-LRP) mediated by nascent Cu(0) species. This publication demonstrates the ultrafast SET-LRP of methyl acrylate initiated with bis(2-bromopropionyloxy)ethane and catalyzed by Cu(0)/Me6-TREN in methanol, ethanol, 1-propanol, and tert-butanol and in their mixture with water at 25 °C. The structural analysis of the resulting polymers by a combination of 1H NMR and MALDI-TOF MS demonstrates the synthesis of perfectly bifunctional α,ω-dibromo poly(methyl acrylate)s by SET-LRP in alcohols. Moreover, this work provides an expansion of the list of solvents available for SET-LRP. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2745–2754, 2008

Nanomechanical function from self-organizable dendronized helical polyphenylacetylenes.

Abstract: Self-organizable dendronized helical polymers provide a suitable architecture for constructing molecular nanomachines capable of expressing their motions at macroscopic length scales. Nanomechanical function is demonstrated by a library of self-organized helical dendronized cis-transoidal polyphenylacetylenes (cis-PPAs) that possess a first-order phase transition from a hexagonal columnar lattice with internal order (ϕhio) to a hexagonal columnar liquid crystal phase (ϕh). These polymers can function as nanomechanical actuators. When extruded as fibers, the self-organizable dendronized helical cis-PPAs form oriented bundles. Such fibers have been shown capable of work by displacing objects up to 250-times their mass. The helical cis-PPA backbone undergoes reversible extension and contraction on a single molecule length scale resulting from cisoid-to-transoid conformational isomerization of the cis-PPA. Furthermore, we clarify supramolecular structural properties necessary for the observed nanomechanical f...

Effect of Cu(0) Particle Size on the Kinetics of SET-LRP in DMSO and Cu-Mediated Radical Polymerization in MeCN at 25 °C

Abstract: In order to estimate the effect of Cu(0) particle size in SET-LRP, a comparative analysis of Cu(0)/Me6-TREN-catalyzed polymerization of MA initiated with methyl 2-bromopropionate at 25 °C was performed in DMSO, a solvent that mediates the disproportionation of Cu(I)X, and in MeCN, a solvent in which Cu(I)X does not disproportionate Cu(I)X. Decreasing the Cu(0) particle size results in a marked increase in the apparent rate constant of propagation (kpapp). Decreasing the Cu(0) particle size from 425 to 0.05 μm (50 nm) increases the kpapp by almost an order of magnitude. Regardless of the Cu(0) particle size used, in DMSO a perfect SET-LRP occurs with a first-order polymerization in growing species up to 100% conversion. However, in MeCN the polymerization is not first order in growing species. The results presented here demonstrate that, in addition to the disproportionation of Cu(I)X/L into Cu(0) and Cu(II)X2/L, the particle size of Cu(0) plays a strong role in the kinetics during the entire polymerization.

A comparative analysis of SET‐LRP of MA in solvents mediating different degrees of disproportionation of Cu(I)Br

Abstract: Cu(I)Br/Μe 6 .TREN species are unstable and disproportionate into metallic Cu(0) and Cu(II)Br 2 /Μe 6 -TREN in DMSO, whereas in toluene are stable and do not undergo disproportionation, at least at 25 °C. To estimate the role of the disproportionating solvent in single electron-transfer living radical polymerization (SET-LRP) a comparative analysis of Cu(0)/Me 6 -TREN-catalyzed polymerization of MA initiated with methyl 2-bromopropionate at 25 °C was performed in DMSO and toluene. A combination of kinetic experiments and chain end analysis by 500-MHz 1 H NMR spectroscopy was used to demonstrate that disproportionation represents the crucial requirement for a successful SET-LRP of MA at 25 °C. In DMSO a perfect SET-LRP occurs and yields close to 100% conversion in 45 min. A first order polymerization in growing species up to 100% conversion and a PMA with perfectly functional chain ends are obtained. However, in toluene within 17 h only about 60% conversion is obtained, the polymerization does not show first order in growing species and therefore is not a living polymerization. Moreover, at 60% conversion the resulting PMA has only 80% active chain ends.

Molecular structure of helical supramolecular dendrimers.

Abstract: The molecular structure of helical supramolecular dendrimers generated from self-assembling dendrons and dendrimers and from self-organizable dendronized polymers was elucidated for the first time by the simulation of the X-ray diffraction patterns of their oriented fibers. These simulations were based on helical diffraction theory applied to simplified atomic helical models, followed by Cerius2 calculations based on their complete molecular helical structures. Hundreds of samples were screened until a library containing 14 supramolecular dendrimers and dendronized polymers provided a sufficient number of helical features in the X-ray diffraction pattern of their oriented fibers. This combination of techniques provided examples of single-92 and -113 helices, triple-61, -81, -91, and -121 helices, and an octa-321 helix that were assembled from crownlike dendrimers, hollow and nonhollow supramolecular crownlike dendrimers, hollow and nonhollow supramolecular disklike dendrimers, and hollow and nonhollow sup...

Implications of monomer and initiator structure on the dissociative electron-transfer step of SET-LRP

Abstract: The heterolytic dissociation process associated with the activation of Single Electron-Transfer Living Radical Polymerization is examined through the use of energy profile modeling. Monomer and initiator structure is correlated with the approximate activation barriers, energies of electrostatic ion-radical pair formation, and stability of ion-radical pair generated from the counteranion halide leaving group and the radical atom with partial positive charge density induced by its electron-withdrawing substituent. Energy profiles permit access not just to one, but to all local minima, in the dissociation pathway and the identification of a global minimum. The location and energy of this global minimum allows for the placement of various initiators and dormant propagating macroradicals on the spectrum between stepwise and concerted dissociative electron-transfer. The barrier for the activation step for alkyl-halides derived from acrylates, vinyl halides, and styrenes, as well as from initiators bearing electron-withdrawing groups is decreased in comparison to relatively more electron-rich alkyl halides. This rate enhancement is explained through the sticky dissociative model wherein electron-transfer is accelerated by the formation of strong ion-radical pairs between radicals with partial positive charge density and their counteranion leaving group. Greater electron-withdrawing capacity of the alkyl halide substituent increases the stability of the ion-radical pair, reduces its equilibrium bond length, and accelerates electron-transfer. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5663–5697, 2008

Hollow spherical supramolecular dendrimers.

Abstract: The synthesis of a library containing 12 conical dendrons that self-assemble into hollow spherical supramolecular dendrimers is reported. The design principles for this library were accessed by development of a method that allows the identification of hollow spheres, followed by structural and retrostructural analysis of their Pm3n cubic lattice. The first hollow spherical supramolecular dendrimer was made by replacing the tapered dendron, from the previously reported tapered dendritic dipeptide that self-assembled into helical pores, with its constitutional isomeric conical dendron. This strategy generated a conical dendritic dipeptide that self-assembled into a hollow spherical supramolecular dendrimer that self-organizes in a Pm3n cubic lattice. Other examples of hollow spheres were assembled from conical dendrons without a dipeptide at their apex. These are conical dendrons originated from tapered dendrons containing additional benzyl ether groups at their apex. The inner part of the hollow sphere a...

Functionally terminated poly(methyl acrylate) by SET‐LRP initiated with CHBr3 and CHI3

Abstract: The single-electron transfer living radical polymerization (SET-LRP) of methyl acrylate initiated with bromoform (CHBr 3 ) and iodoform (CHI 3 ) and catalyzed by Cu(0)/Me 6 -TREN in DMSO at 25 °C provides a reliable method to prepare poly (methyl acrylate) (PMA) with active chain ends and controlled structure that can undergo subsequent functionalization to provide strategies for the synthesis of different block copolymers and other complex architectures. A detailed kinetic and structural analysis was used to assess the scope and the limitations of CHBr 3 and CHI 3 as initiators under SET-LRP conditions.

SET‐LRP of acrylates in the presence of radical inhibitors

Abstract: The Cu(0)/Me6-TREN-catalyzed single-electron transfer mediated living radical polymerization (SET-LRP) of methyl acrylate in the presence of the classic 4-methoxyphenol free radical inhibitor was investigated. Kinetic experiments, combined with 1H NMR, and MALDI-TOF MS analysis of the resulting polyacrylates demonstrated that SET-LRP is a robust synthetic method that does not require the purification of the monomers to remove the radical inhibitor. It is anticipated that these results will contribute to the expansion of technological and fundamental applications of SET-LRP since it allows the synthesis of polymers with a structural perfection that previously was not accessible by any other method, starting from unpurified monomers, solvents, initiators, and ligands. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3174–3181, 2008

Alkyl chloride initiators for SET‐LRP of methyl acrylate

Abstract: Single-electron transfer living radical polymerization (SET-LRP) proceeds by an outer-sphere single-electron transfer mechanism that induces a heterolytic bond cleavage of the initiating and propagating R-X (where X = Cl, Br, and I) species. Therefore, unlike the homolytic bond cleavage mechanism claimed for ATRP, SET-LRP is expected to show a small dependence of the nature of the halide group on the apparent rate constant of activation. This means the R-X with X = Cl, Br, and I must all be efficient initiators for SET-LRP and no chain transfer must be observed in the case of initiators with X = Br and I. Here, we report the SET-LRP of methyl acrylate initiated with the alkyl chlorides methyl-2-chloropropionate (MCP) and chloroform (CHCl3) and catalyzed by Cu(0)/Me6-TREN/CuCl2 in DMSO at 25 °C. A combination of kinetic and structural analysis was used to elucidate the MCP and CHCl3 initiating behavior under SET-LRP conditions, and to demonstrate the very small dependence of the SET-LRP apparent rate constant of propagation on X while providing polymers with well defined architecture. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4917–4926, 2008

Thixotropic twin-dendritic organogelators.

Abstract: Twin-dendritic organogelators have been prepared through selective functionalization of N- (3-amino-propyl)-1,3-propanediamine (APPDA) with self-assembling dendrons by using 1,1'-carbonyldiimidazole (CDI). Subsequent modification of the APPDA linker provided an additional degree of structural diversity by which to tailor the gelator self-assembly in bulk or in the gel state. These compounds are able to gel cyclohexane, toluene, n-butyl acetate, ethyl acetate, dichloromethane, and tetrahydrofuran. 3,4-Disubstituted apical branching units provided the most efficient organogelators and show a propensity to form thixotropic gels, wherein the gel recovers its elasticity after being subjected to shear. Structural and retrostructural analysis of the twin-dendritic organogelators reveals the bulk structural characteristics to be indicative of the subsequent gel properties. Diverse self-organized arrays were identified in bulk and all are able to form gels, thus indicating the role of quasiequivalence in mediating self-assembly in the gel state. Furthermore, we have found that porous columnar mesophases provide a strategy by which to prepare thixotropic gels. We demonstrate the importance of weak lateral hydrogen bonding within a column stratum versus hydrogen bonding along the length of the column for forming porous columnar mesophases and, by extension, thixotropic gels.

Nanomechanical Function Made Possible by Suppressing Structural Transformations of Polyarylacetylenes

Abstract: Self-assembling dendrons sterically disfavor the cisoid conformation through which 6π-electrocyclization proceeds in cis-polyarylacetylenes. Through the same mechanism, selected self-organizable dendronized helical cis-polyphenylacetylenes (cis-PPAs) undergo an unprecedented thermally induced cisoid-to-transoid conformational isomerism in bulk. The motion of helix (un)winding can be converted to unidirectional motion in self-organizable dendronized cis-PPAs. By eliminating 6π-electrocyclization, nanomechanical actuators capable of lifting a US dime have been prepared from self-organizable dendronized cis-PPAs.

Supramolecular structural diversity among first-generation hybrid dendrimers and twin dendrons.

Abstract: Hybrid dendrimers, obtained by complete monofunctionalization of the peripheral amines of a "zero-generation" polyethyleneimine dendrimer, provide structurally diverse lamellar, columnar, and cubic self-organized lattices that are less readily available from other modified dendritic structures. The reaction of tris(2-aminoethyl)amine (TREN) with 4-dodecyloxybenzimidazolide provides only the corresponding zero-generation TREN dendrimer. From the mixture of tri- and disubstituted TREN derivatives obtained from first-generation self-assembling dendritic imidazolides, the hybrid dendrimer and a twin dendron could be separated, purified, and characterized. The hybrid dendrimers display smectic, columnar hexagonal (Phi(h)), and cubic (Pm_3n) lattices. The TREN twin dendrons, on which only two peripheral amines have been acylated, exhibit centered-rectangular columnar (Phi(r-c)), Phi(h), and Pm_3n lattices. The existence of a thermoreversible Phi(h)-to-Pm_3n phase transition in the first-generation hybrid dendrimers and twin dendrons is exploited to elucidate an epitaxial relationship between the two mesophases. We postulate a mechanism by which the transition proceeds. The thermoreversible Phi(h)-to-Pm_3n phase change is accompanied by optical property changes that are suitable for rudimentary signaling or logic functions. This structural diversity reflects the quasiequivalence of flat-taper and conical self-assembling dendrons and the ability of flexible dendrimers to accommodate concomitant conformational and shape changes.

Influence of the isomeric structures of butyl acrylate on its single‐electron transfer‐degenerative chain transfer living radical polymerization in water Catalyzed by Na2S2O4

Abstract: The aim of this work is to the study the influence of the isomer struc- tures of butyl acrylate monomer on the single-electron transfer/degenerative chain transfer mediated living radical polymerization (SET-DTLRP). The kinetic of isobutyl acrylate is determined for the first time by SET-DTLRP in water catalyzed by sodium dithionite. The plots of number-average molecular weight versus conversion and ln((M)0/(M)) versus time are linear, demonstrating a controlled polymerization. The influence of the isomer t-butyl, i-butyl, and n-butyl on the kinetics, properties, and stereochemistry of the reactions was assessed. To the best of our knowledge, there is no previous report dealing with the synthesis of PiBA by any LRP approach in aque- ous medium. The results presented in this work suggest that the stability provided by the acrylate side group has an important influence in the polymerization process.

Synthesis of poly(ethyl acrylate) by single electron transfer-degenerative chain transfer living radical polymerization in water catalyzed by Na2S2O4

Abstract: Living radical polymerization of ethyl acrylate was achieved by single-electron-transfer/degenerative-chain transfer mediated living radical polymerization in water catalyzed by sodium dithionite. The plots of number-average molecular weight versus conversion and In[M]/[M] versus time are linear, indicating a controlled polymerization. This method leads to the preparation of α,α-di(iodo)poly(ethyl acrylate) (α,ω-di(iodo)PEtA) macroinitiator that can be further functionalized. The molecular weight distributions were determined using a combination of three detectors (TriSEC): right-angle light scattering, a differential viscometer and refractive index. The method studied in this work represents a possible route to prepare well-tailored macromolecules made of ethyl acrylate in environmental friendly reaction medium. To the best of our knowledge there is no previous report dealing with the synthesis of PEtA by any LRP approach in aqueous medium. Furthermore, the method described in this article was successfully applied in pilot scale reactions under industrial production conditions.

Long-range electron transport in a self-organizing n-type organic material

Abstract: Nondispersive time-of-flight electron transport has been observed in a self-organizing perylene diimide derivative, (3,4,5Pr)12G1-3-perylenetetracarboxyldiimide, yielding long-range transport mechanisms that correlate with molecular packing in several structural phases. The consistency of temperature and electric field dependences of electron mobility in the isotropic and columnar Φh1 phases with static rather than dynamic disorder formalisms provides a link between molecular packing, energetic and positional disorders, and magnitudes of mobility.

A comparative analysis of self-assembly in poly(methacrylates) with bulky side substituents of different molecular masses in the solid state and in solutions

Abstract: A comparative investigation of self-assembly in poly(methacrylates) with bulky substituents based on gallic acid both in the condensed state and in hexane solutions has been performed by using the methods of SAXS and DSC. The size and shape of the formed supramolecular aggregates have been analyzed for the polymers of two different degrees of polymerization (150 and 21). In fresh solutions of a low-molecular-mass polymer, extended piles composed of several molecules are formed. Each molecule from a disk with a diameter of ∼4.4 nm, which is similar to the diameter of columnar phase cylinders in the solid state. In solutions of a highmolecular-mass polymer, several molecules form a wormlike particle. The observed supramolecular aggregates are unstable: within several months, their order breaks down and particles are dissolved.

Sequential Ni‐Catalyzed Borylation and Cross‐Coupling of Aryl Halides via in situ Prepared Neopentylglycolborane.

Abstract: A procedure for NiCl2(dppp)-catalyzed pinacolborylation and neopentylglycolborylation that utilizes in situ prepared inexpensive pinacolborane and neopentylglycolborane is reported. The scope of this reaction was demonstrated with a variety of aryl bromides and iodides. The resulting aryl neopentylglycolboronic esters undergo a NiCl2(dppe)-catalyzed cross-coupling with aryl halides, resulting in an extremely efficient and cost-effective method for the synthesis of functional biaryls, dendritic building blocks, and other complex architectures.