Asymmetric Polymerization Research Articles

Electric fields guide migration of epidermal stem cells and promote skin wound healing

Migration of epidermal stem cells (EpSCs) into wounds may play an important role in wound healing. Endogenous electric fields (EFs) arise naturally at wounds. Consistent with previous reports, we measured outward electric currents at rat skin wounds using vibrating probes. Topical use of prostaglandin E2 significantly promoted wound healing. However, it is not known whether EpSCs respond to EFs. We first isolated and characterized EpSCs from rat skin. We then demonstrated that EpSCs isolated from the epidermis migrated directionally toward the cathode in EFs of 50-400 mV/mm. The directedness values increased in a dose- and time-dependent fashion. The migration speed of EpSCs was significantly increased in EFs. EFs induced asymmetric polymerization of intracellular F-actin and activation of the extracellular signal-regulated kinase 1/2 and phosphatidylinositol-3-kinase (PI3K)/protein kinase B pathways. Inhibition of epidermal growth factor receptor, extracellular signal-regulated kinase 1/2, or PI3K significantly inhibited the cathodal distribution of F-actin and the electrotactic response of EpSCs. These data for the first time show that EpSCs possess obvious electrotaxis, in which the epidermal growth factor receptor-mitogen activated protein kinase-PI3K pathways are involved. These data thus suggest a novel aspect of electric signaling in wound healing-to stimulate and guide migration of EpSCs and to regulate wound healing.

Two modes of asymmetric polymerization of phenylacetylenes having anL-amino alcohol residue and two hydroxy groups

Four novel chiral phenylacetylenes having an L-amino alcohol residue and two hydroxymethyl groups were synthesized and polymerized by an achiral catalyst ((nbd)Rh+[η6-(C6H5)B−(C6H5)3]) or a chiral catalytic system ([Rh(nbd)Cl]2/(S)- or (R)-phenylethylamine ((S)- or (R)-PEA)). The two resulting polymers having an L-valinol or L-phenylalaninol residue showed Cotton effects at wavelengths around 430 nm. This observation indicated that they had an excess of one-handed helical backbones. Positive and negative Cotton effects were observed only for the polymers having an L-valinol residue produced by using (R)- and (S)-PEA as a cocatalyst, respectively, although the monomer had the same chirality. Even when the achiral catalyst was used, the two resulting polymers having an L-valinol or L-phenylalaninol residue showed Cotton effects despite the long distance between the chiral groups and the main chain. We have found the first example of a new type of chiral monomer, that is, a chiral phenylacetylene monomer having an L-amino alcohol residue and two hydroxy groups that was suitable for both modes of asymmetric polymerization, that is, the helix-sense-selective polymerization (HSSP) with the chiral catalytic system and the asymmetric-induced polymerization (AIP) with the achiral catalyst. The other two monomers having L-alaninol and L-tyrosinol were found to be unsuitable to neither HSSP nor AIP because of their polymers' low solubility. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Chiral Teleinduction in Asymmetric Polymerization of 3,5-Bis(hydroxymethyl)phenylacetylene Having a Chiral Group via a Very Long and Rigid Spacer at 4-Position

Abstract A new polymer prepared by polymerization of 3,5-bis(hydroxymethyl)phenylacetylene having a chiral menthyl group via a very long and rigid spacer by an achiral rhodium complex with triethylamine as an initiator showed strong CD absorptions assigned to the main chain in spite of the long distance (23 Å) between the chiral group and the polymerizing group in the monomer. This distance is the longest to the best of our knowledge. In addition, switching between the two chiralities of the resulting polymer was realized in the CD spectrum.

Starlike Polymer Brushes

We investigate the properties of polymer brushes made of symmetric and asymmetric starlike polymers in good solvent using Langevin dynamics simulations. Two types of populations coexist in such a brush, one of them being made of highly stretched polymers and a second one made of stars that are retracted inside the lower brush regions. Scaling properties of molecular tension, brush layer thickness, and brush density are in reasonable agreement with those of linear chain brushes. The dynamics of stars switching between both the retracted and the stretched states is analyzed, and an exponential scaling with the stretching energy difference is proposed. Most of our results are found in good agreement with recent SCF calculations of Polotsky et al., with one notable exception that their proposed transition from a longest path stretching mode to a uniform stretching mode, the latter being dominant at high densities, has not been supported by our data. Instead, the longest path stretching dominates at high densi...

Synthesis of Conjugated Polymers in Chiral Nematic Liquid Crystal Fields

AbstractThis review presents the progress made on current research in the synthesis of π‐conjugated polymers in chiral nematic liquid crystal (N*‐LC) fields through either chemical or electrochemical polymerization. The following are noteworthy advances in the field of π‐conjugated polymers. (i) Helical aliphatic conjugated polymers of helical polyacetylene (H‐PA) were synthesized using asymmetric interfacial polymerization in N*‐LC fields. (ii) Helical aromatic conjugated polymers with electroactive properties, such as poly(3,4‐ethylenedioxythiophene) (PEDOT) and polybithiophene (PBT), were synthesized using asymmetric electrochemical polymerization in N*‐LC fields. (iii) Chiral aromatic conjugated copolymers, such as the poly(bithienylene‐phenylene) (PBTP) derivative, were synthesized using the asymmetric Stille polycondensation reaction in N*‐LC fields. Throughout the document, it has been emphasized that by using N*‐LC systems as asymmetric polymerization fields, helix formation is possible for aliphatic and aromatic π‐conjugated polymers without chiroptical substituents on the side chains.

Primitive chain network simulations for asymmetric star polymers

For branched polymers, the curvilinear motion of the branch point along the backbone is a significant relaxation source but details of this motion have not been well understood. This study conducts multi-chain sliplink simulations to examine effects of the spatial fluctuation and curvilinear hopping of the branch point on the viscoelastic relaxation. The simulation is based on the primitive chain network model that allows the spatial fluctuations of sliplink and branch point and the chain sliding along the backbone according to the subchain tension, chemical potential gradients, drag force against medium, and random force. The sliplinks are created and∕or disrupted through the motion of chain ends. The curvilinear hopping of the branch point along the backbone is allowed to occur when all sliplinks on a branched arm are lost. The simulations considering the fluctuation and the hopping of the branch point described well the viscoelastic data for symmetric and asymmetric star polymers with a parameter set common to the linear polymer. On the other hand, the simulations without the branch point motion predicted unreasonably slow relaxation for asymmetric star polymers. For asymmetric star polymers, further tests with and without the branch point hopping revealed that the hopping is much less important compared to the branch point fluctuation when the lengths of the short and long backbone arms are not very different and the waiting time for the branch point hopping (time for removal of all sliplinks on the short arm) is larger than the backbone relaxation time. Although this waiting time changes with the hopping condition, the above results suggest a significance of the branch point fluctuation in the actual relaxation of branch polymers.

Synthesis and asymmetric polymerization of chiral maleimides bearing an aza crown ether

Chiral N-substituted maleimide derivatives bearing L-phenylalanine- or L-leucine-introduced aza crown ether ((S)-A15C5PAMI, (S)-A15C5LMI) were synthesized from maleic anhydride, corresponding amino acids and aza crown ether, and polymerized with an organometal/chiral ligand. The number-average-molecular weights and the specific optical rotations of the polymers were 700–5600 and −105.8° to −38.3°, respectively. The specific optical rotations of all polymers tended to a positive direction, compared with model compounds ((S)-A15C5PASI ([α]435=−218.1°), (S)-A15C5LSI ([α]435=−215.1°)). Asymmetric inductions in the main chains of the polymers were investigated by the measurements of specific optical rotation, circular dichroism. Asymmetric anionic homopolymerizations of (S)-(–)-N-maleoyl-L-leucine-aza-15-crown-5 ((S)-A15C5LMI) and (S)-(–)-N-maleoyl-L-phenylalanine-aza-15-crown-5 ((S)-A15C5PAMI) were carried out with chiral anionic initiator consisting of organometal and chiral ligand to obtain optically active polymers. Chiroptical properties and structures of the polymers obtained were analyzed by circular dichroism, ultraviolet, nuclear magnetic resonance and gel permeation chromatography measurements. The additional effects of (R)-cyclohexylethylamine (to the obtained poly((S)-A15C5LMI) were investigated.

Synthesis and morphology of new asymmetric star polymers of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) and their non-volatile memory device applications

We report the synthesis, morphology, and properties of new 3-arm AB22 and 5-arm AB44 asymmetric amphiphilic star polymers of poly[4-(9,9-dihexylfloren-2-yl)styrene]-block-poly(2-vinylpyridine) (P(St-Fl)-b-P2VP), with narrow molecular weight distributions (Mw/Mn < 1.11). The asymmetric star polymers were synthesized by living anionic polymerization using 1,1-diphenylethylene (DPE)-functionalized P(St-Fl) with two or four benzyl bromide moieties, followed by linking reaction with a living anionic P2VP. Depending on the arm numbers of P2VP, lamellar and hexagonally perforated lamellae morphologies were observed for the AB22 and AB44P(St-Fl)-b-P2VP from the results of SAXS, WAXS, and TEM. The [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) incorporated in the polymer matrix increased the order degree of the liquid-crystal-like morphology in the P(St-Fl) block. The prepared AB22 and AB44P(St-Fl)-b-P2VP could be blended with PCBM for electrical write-once-read-many times (WORM) memory devices application. The threshold voltage (−2.8 to −3.9 V) and ON/OFF ratio (103 to 106) of the WORM characteristic were obtained by tuning the PCBM content. The ON or OFF state of the WORM memory devices could remain over 104 s without any degradation. The optical absorption and photoluminescence results indicated the formation of charge transfer complexation between PCBM and the 2-vinylpyridine or pendent fluorene chromophores. The switching behavior was further explained by the charge injection dominated thermionic emission in the OFF state and field-induced charge transfer in the ON state. These experimental results provide a new strategy to prepare a conjugated moiety containing asymmetric star polymers and their potential electrical memory device applications.

Light driven asymmetric polymerization: an approach for tele-control reaction

Chiroptical activity tuning polymerization in liquid crystal (LC) electrolyte solution under irradiation of UV or Vis light was performed.

Asymmetric polymerizations of chiral 4‐benzyl‐2‐ethynyloxazoline with rhodium catalyst and chiroptical properties of the polymers

Optically active 4-alkyl-2-ethynyloxazoline derivatives (BnEOx) were polymerized with rhodium catalysts. The polymerization in toluene produced polymer with the highest absolute values of specific rotation ([α](D) = -77.3°). The yields, molecular weights, and specific rotations of poly(BnEOx)s were influenced by polymerization conditions. The copolymerization with phenylacetylene (PA) was effective to increase the molecular weight of the copolymer. It is interesting to note that the copolymers exhibited positive specific rotations ([α](D) = +4.7° to +62.5°) despite the fact that [α](D) s of BnEOx and the homopolymer are negative sign. The chiroptical properties were investigated by the chiral/achiral copolymerization of BnEOx with PA. The copolymerizations of BnEOx with PA gave copolymers containing higher order structure such as one-handed helical conformation. Furthermore, induced Cotton effects were observed in the π-π* transition region of conjugated main chain depending a complex of these polymers with zinc triflate salt in tetrahydrofuran solution, indicating the formation of chiral supramolecular aggregates.

Advances in synthetic optically active condensation polymers - A review

The study of optically active polymers is a very active research field, and these materials have exhibited a num- ber of interesting properties. Much of the attention in chiral polymers results from the potential of these materials for sev- eral specialized utilizations that are chiral matrices for asymmetric synthesis, chiral stationary phases for the separation of racemic mixtures, synthetic molecular receptors and chiral liquid crystals for ferroelectric and nonlinear optical applica- tions. Recently, highly efficient methodologies and catalysts have been developed to synthesize various kinds of optically active compounds. Some of them can be applied to chiral polymer synthesis. In a few synthetic approaches for optically active polymers, chiral monomer polymerization has essential advantages in applicability of monomer, apart from both asymmetric polymerization of achiral or prochiral monomers and enantioselective polymerization of a racemic monomer mixture. The following are the up to date successful approaches to the chiral synthetic polymers by condensation polymer- ization reaction of chiral monomers.

Three Mechanisms of Asymmetric Polymerization of Phenylacetylenes Having an l-Amino Ether Residue and Two Hydroxy Groups

Three novel chiral phenylacetylenes having an octyloxyethanolamine residue derived from a l-aminoalcohol and two hydroxymethyl groups were synthesized and polymerized by two achiral catalysts ((nbd)Rh+[η6-(C6H5)B−(C6H5)3] and [Rh(nbd)Cl]2/triethylamine (TEA)) and a chiral catalytic system ([Rh(nbd)Cl]2/(S)- or (R)-phenylethylamine ((S)- or (R)-PEA)). All of the resulting polymers showed Cotton effects at wavelengths around 430 nm. This observation indicated that they had an excess of one-handed helical backbones. Positive and negative Cotton effects were observed for the polymers having an l-valinol residue produced by using (S)- and (R)-PEA as a cocatalyst, respectively, although the monomers had the same chirality. The two polymers having an l-alaninol or l-phenylalaninol residues obtained by using (S)- and (R)-PEA as a cocatalyst showed CD absorptions with identical signs. Therefore, we found that the chiral monomer having an l-valinol residue was suitable for both modes of asymmetric polymerization, t...

Precise Synthesis of Novel Ferrocene-Based Star-Branched Polymers by Using Specially Designed 1,1-Diphenylethylene Derivatives in Conjunction with Living Anionic Polymerization

The precise synthesis of novel ferrocene-based regular and asymmetric star-branched polymers by a methodology using specially designed 1,1-diphenylethylene derivatives in conjunction with living anionic polymerization of ferrocenylmethyl methacrylate (FMMA) is described. The methodology involves three reaction steps, i.e., (1) introduction of 3-(tert-butyldimethylsilyloxymethyl)phenyl (SMOP) group(s) at the polymer chain end or in-chain, (2) conversion of the SMOP group(s) to α-phenyl acrylate function(s), and (3) a linking reaction of the α-phenyl acrylate function(s) with the living anionic polymer of FMMA or methyl methacrylate. By developing this methodology, a variety of 3-arm $$ {\text{AA}^{\prime}_{2}} $$ , A2B, AB2, ABC and 4-arm A4, A3B, A2B2, A2BC, and ABC2 star-branched polymers with well-defined structures have been successfully synthesized. The A, B, and C segments are poly(FMMA), polystyrene, and poly(methyl methacrylate), respectively.

Helical Crystal Assemblies in Nonracemic Chiral Liquid Crystalline Polymers: Where Chemistry and Physics Meet

Chirality transfer (or amplification) in soft materials is an important topic for understanding how chiral assemblies develop from the atomic level to macroscopic objects. This is an interdisciplinary area which is critically associated with the evolution from asymmetric chemistry to asymmetric physics, and, more specifically, in biomaterials, liquid crystals, and polymers. In this review, we discuss our recent studies on the conditions for chirality transfer across different length scales. We observed that the formation of helical structures in different length scales is a typical process in chirality transfer. This is illustrated by a series of nonracemic chiral main-chain liquid crystalline (LC) polyesters, PETs(R*-n). The polymers contained a different number of methylene units in the chain backbone (from 7 to 11). All of the PETs(R*-n) macroscopically exhibited an LC chiral smectic C (SC*) phase, a chiral smectic A (SA*) phase, and a twist grain boundary smectic A (TGBA*) phase with increasing temperature. The atomic chiral centers caused the helical conformation, and then, helical lamellar crystals were found in these polymers. The crystal structures were determined by X-ray diffraction and electron diffraction techniques. Dark field transmission electron microscopic images revealed that the molecular orientation in these helical crystals is double twisted. Our systematic studies showed that chirality transfer from one length scale to another is neither automatic, nor necessary, and it critically depends upon the packing scheme of the chiral building blocks on each length scale. It was particularly surprising that in this series of polyesters, the odd−even effect exists across many length scales. This includes not only thermodynamic properties such as the SC*, the SA*, and the TGBA* LC phase transition temperatures, but also the helical crystal handedness.

Chiral arylnickel complexes as highly active initiators for screw-sense selective living polymerization of 1,2-diisocyanobenzenes

Optically active chiral organonickel complexes served as efficient chiral initiators for living aromatizing polymerization of 1,2-diisocyanobenzene derivatives, which afford optically active helical poly(quinoxaline-2,3-diyl)s up to 84% s.e. (screw-sense excess). In comparison with asymmetric polymerization with the corresponding organopalladium initiators, the nickel initiators show a much greater polymerization rate, while the selectivity remains high. The organonickel initiators can be generated in situ from nickel(0) precursors with the corresponding enantiopure (S,S)-2-(4,5-diphenylimidazolin-2-yl)phenyl chloride, leading to the convenient synthesis of highly stereo-controlled poly (quinoxaline-2,3-diyl)s.

Asymmetric catalytic emulsion polymerization in chiral micelles

Achiral substituted acetylene monomers undergo aqueous catalytic emulsion asymmetric polymerizations in chiral micelles consisting of SDS and amino acid, providing optically active helical polymer emulsions. The asymmetric polymerizations lead to one-handed helical polymers, which are the origin of the optical activity of polymer emulsions.

Toward De Novo Recapitulation of Cytoskeleton Dynamics with DNA Nanotubes

The polymerization and depolymerization of cytoskeleton can transduce chemical energy into mechanical energy that drives cellular motility, such as chromosome segregation to cell protrusion. De novo recapitulation of the cytoskeletal phenomena with synthetic material would test our understanding of the design principles of polymerization motors. In the DNA nanotechnology tool box, DNA nanotubes are arguably the closest counterparts of cytoskeleton and we aim to utilize such structures to synthesize artificial cytoskeletons with various levels of complexity. DNA nanotube polymerization was monitored using total internal reflection microscopy at the single molecule level. We observed that DNA nanotubes exhibit asymmetric polymerization, similar to actin and microtubule polymerizations. We measured the association and dissociation rates of DNA nanotube polymerization at different monomer concentrations and temperatures and compared our measurements with the polymerization theory developed for the cytoskeleton and the kinetic DNA tile assembly model. Finally, the coupling between DNA nanotube polymerization and a DNA nanotechnology analog of nucleotide hydrolysis could potentially recapitulate cytoskeleton-based dynamics, such as treadmilling and dynamic instability, where the polymerization and depolymerization co-exist at steady state without ever reaching equilibrium.

From helical polyacetylene to helical graphite: synthesis in the chiral nematic liquid crystal field and morphology-retaining carbonisation

This tutorial review presents current progress in the synthesis of helical polyacetylene (H-PA) using a chiral nematic liquid crystal reaction field as an asymmetric polymerisation solvent. In addition, we review the morphology-retaining carbonisation for helical graphite using H-PA as a carbonisation precursor. Carbonisation of H-PA films, via iodine doping, is demonstrated to produce carbon and graphitic films bearing completely preserved morphologies and even helical nanofibril structures.

Stress Relaxation of Comb Polymers with Short Branches

We measured the linear and nonlinear rheology of model polyisoprene comb polymers with a moderate number (5−18) of short (marginally entangled to unentangled) branches and highly entangled backbones. The hierarchical modes of relaxation were found to govern both the linear and nonlinear response. Appropriate modification of tube-model theory for entangled branches, inspired by recent work on asymmetric star polymers (where the short branch behaves as effectively larger on small time scales), provided a framework for quantitative predictions of the linear viscoelastic spectra. The extended nonlinear stress relaxation data over a wide time range (via time−temperature superposition) obeys time−strain separability and allows extraction of two damping functions: one for the branches at short times and one for the diluted backbone at long times. Both exhibit signatures of the comb architecture. The comb damping function at short times, shifted relative to the branch relaxation, is dominated by the retraction of...

Photoinduced Racemization of an Optically Active Helical Polymer Formed by the Asymmetric Polymerization of 2,7‐Bis(4‐tert‐butylphenyl)fluoren‐9‐yl Acrylate

A trick of light: An optically active preferred-handed helical polymer synthesized by asymmetric anionic polymerization underwent stereomutation upon photoirradiation, whereas the helix remained stable upon heating (see picture). This photoinduced racemization of a synthetic helical polymer occurred without any changes in chemical bonding, such as bond formation, bond cleavage, or the isomerization of a double bond.