Comparative Thermodynamic Studies of the Micellization of Amphiphilic Block Copolymers before and after Cyclization.

The enthalpy and entropy of micellization in water, ΔHmic and ΔSmic, respectively, of three linear amphiphilic BAB block copolymers consisting of either poly(methyl acrylate) (Mn ∼ 1200 and 700 Da) or poly(ethyl acrylate) (Mn ∼ 800 Da) as hydrophobic (B) segments and poly(ethylene oxide) (PEO) as the hydrophilic (A, Mn ∼ 3000 Da) segment were determined by isothermal titration calorimetry (ITC). The ΔHmic and ΔSmic of the cyclic AB block copolymers obtained by cyclization of the linear triblock copolymers were determined under the same conditions. The ΔHmic value of the cyclic copolymers was smaller than that of their linear precursors. The ΔSmic value showed the same trend, but the relative difference between the cyclized and linear copolymers was less pronounced. The hydrodynamic diameter (Dh), critical micelle concentration (CMC), molecular weight (Mw-mic), and second virial coefficient (A2) of the micelles were determined. The Dh value of the cyclic copolymer micelles was smaller than the linear counterpart. On the other hand, the CMC value became larger, whereas the A2 value was comparable or increased by cyclization. Overall, the results suggest that, in the unimer state, the hydrophobic segments of the cyclized copolymers form a tightly coiled structure to minimize contact with water, resulting in the smaller ΔHmic value. Contrary to the linear copolymer micelles, the cyclic copolymer micelles have no "dangling chains", which may explain the topology-driven slight difference in the ΔSmic value.

Self-Assembly of Linear and Cyclic Polylactide Stereoblock Copolymers with a Parallel and Antiparallel Chain Arrangement Distinguishing Their Directions on a Water Surface.

The self-assembly of molecules into a well-ordered structure is one of the most important processes in fabricating sophisticated materials. Here, we show that polymer chains can be self-assembled, distinguishing their direction (parallel or antiparallel), and could be a new useful scaffold for self-assembly in a controlled direction. The system that was used was a stereocomplex (SC) formation of linear and cyclic polylactide (PLA) stereoblock copolymers with a parallel and antiparallel chain arrangement in a Langmuir monolayer. The linear and cyclic stereoblock copolymers with a parallel arrangement formed a well-ordered lamellar SC in the first and second layers upon compression, but the linear and cyclic stereoblock copolymers with an antiparallel arrangement did not form a first-layer lamella and instead formed only the second-layer lamella. These results were only rationally explained by assuming that the enantiomeric PLA chains selectively assembled in a parallel direction, not in an antiparallel direction, in the SC. A simple polymer chain could be self-assembled, distinguishing the direction without a specific interaction group in it.

Programmed Polymer Folding with Periodically Positioned Tetrafunctional Telechelic Precursors by Cyclic Ammonium Salt Units as Nodal Points.

A programmed polymer folding process has been demonstrated by employing a pair of periodically positioned tetrafunctional, linear telechelic poly(THF)s having 5-membered cyclic ammonium salt groups, i.e., N-ethyl or N-phenylpyrrolidinium groups at both chain ends, and N, N-dialkylpyrrolidinium groups at the two interior positions, accompanying two units of a dicarboxylate counteranion to balance the charges, Ia and Ib, respectively. The electrostatic self-assembly and covalent fixation process has subsequently been applied, to cause the ring-opening reaction of the pyrrolidinium units by carboxylate counteranions under dilution. The obtained doubly cyclized polymer products, IIa from Ia and IIb from Ib, were characterized by 1H NMR and by a MALDI-TOF mass technique, to indicate the formation of polymeric constitutional isomers of either manacle-, 8-, or θ-form. The SEC peak deconvolution analysis of IIa showed the preferential formation of the manacle-form isomer over the 8- and the θ-form counterparts, to accord with the polymer folding of Ia, having the equivalent chemical reactivity of the linking groups, directed by the spatial distance between the folding points. On the other hand, the relevant SEC analysis of IIb showed the predominant formation of the 8-form isomer, consistent with the polymer folding of Ib promoted by the enhanced chemical reactivity of the N-phenylpyrrolidinium end groups over the interior N, N-dialkylpyrrolidinium groups.

Macrocyclic poly(p-phenylenevinylene)s by ring expansion metathesis polymerisation and their characterisation by single-molecule spectroscopy.

Ring expansion metathesis polymerisation (REMP) has proven to be a viable approach to prepare high purity cyclic polymers. Macrocyclic polymers with a fully conjugated defect free backbone are of particular interest as these polymers have no end groups that can act as charge traps. In this work soluble macrocyclic poly(p-phenylenevinylene)s ( cPPVs) have been prepared directly via the REMP of substituted paracyclophanedienes. Single-molecule spectroscopy of the two topological forms of PPV i.e., linear ( lPPV) and cyclic ( cPPV) revealed that lPPV exists in an extended conformation whereas the cPPV adopts a restricted ring-like conformation. Despite such large differences in the chain conformation, the spectral properties of the two compounds are unexpectedly very similar, and are dominated by torsional deformations in relatively short conjugated segments.

Topological Polymer Chemistry Designing Complex Macromolecular Graph Constructions.

The precision design of topologically intriguing macromolecular architectures has continuously been an attractive challenge in polymer science and polymer materials engineering. A class of multicyclic polymer topologies, including three subclasses of spiro, bridged, and fused forms, are particularly unique not only from a topological geometry viewpoint but also from their biochemical relevance to programmed folding structures. In this Account, we describe recent progress in constructing this class of macromolecules, in particular by means of an electrostatic self-assembly and covalent fixation (ESA-CF) protocol, in which ion-paired polymer self-assemblies are employed as key intermediates. All three dicyclic constructions having either 8 (spiro), manacle (bridged), or θ (fused) forms, as well as a tricyclic trefoil (spiro) graph, have been constructed by the ESA-CF process. Moreover, a triply fused-tetracyclic macromolecular K3,3 graph has been constructed using a uniform-size dendritic polymer precursor having six cyclic ammonium salt end groups carrying two units of a trifunctional carboxylate counteranion. Remarkably, the K3,3 graph is known in topological geometry as a prototypical nonplanar graph and has been identified as topologically equivalent to some multicyclic polypeptides (cyclotides) produced through the intramolecular S-S bridging with cysteine residues. A series of single cyclic (ring) polymers having one, two, and even three designated functional groups at the prescribed positions along their cyclic backbone segment (kyklo-telechelics) have also been obtained by the ESA-CF protocol. And in conjunction with a tandem alkyne-azide addition (i.e., click) and an olefin metathesis (i.e., clip) reaction, the precision design of complex multicyclic macromolecular architectures has been achieved. Thus, a series of tri-, tetra-, and even hexacyclic polymer topologies of spiro- and bridged-forms and three doubly fused-tricycle (ß-, γ-, and δ-graph) forms, as well as a triply fused-tetracyclic and a quadruply fused-pentacyclic form (unfolded tetrahedron-graph, and "shippo"-form, respectively) were effectively constructed. Furthermore, the hybrid multicyclic polymer constructions comprised of three subclasses of spiro, bridged, and fused units have been produced using complementary pairs of single cyclic and dicyclic kyklo-telechelic precursors obtainable by the ESA-CF process. Upon these synthetic developments, we are now entering into an exciting new era of polymer science and polymer materials engineering based on the precision design of polymer topologies, which appears comparable to the "Cambrian explosion period" experienced in the evolution of life systems.

Load-Induced Frictional Transition at a Well-Defined Alkane Loop Surface.

Self-assembled monolayers (SAMs) have attracted considerable attention as a tool to confer desirable properties on material surfaces. So far, molecules used for the SAM formation are generally limited to linear ones and thus chain ends dominate the surface properties. In this study, we have successfully demonstrated unique frictional properties of a SAM composed of alkane loops from cyclic alkanedisulfide on a gold substrate, where both sulfurs are bound to gold. The frictional response was proportional to the load. However, once the load went beyond a threshold value, the frictional response became more dominant. Such a frictional transition was reversible and repeatable and was not discerned for a corresponding SAM composed of n-alkyl chains. The load-induced change in the frictional response from the alkane loops could be associated with the conformational change of the alkane loops. The present results differ from most studies, in which the surface properties are designed on the basis of functional chain end groups.

Synthesis of Cyclic Polymers and Characterization of Their Diffusive Motion in the Melt State at the Single Molecule Level.

We demonstrate a method for the synthesis of cyclic polymers and a protocol for characterizing their diffusive motion in a melt state at the single molecule level. An electrostatic self-assembly and covalent fixation (ESA-CF) process is used for the synthesis of the cyclic poly(tetrahydrofuran) (poly(THF)). The diffusive motion of individual cyclic polymer chains in a melt state is visualized using single molecule fluorescence imaging by incorporating a fluorophore unit in the cyclic chains. The diffusive motion of the chains is quantitatively characterized by means of a combination of mean-squared displacement (MSD) analysis and a cumulative distribution function (CDF) analysis. The cyclic polymer exhibits multiple-mode diffusion which is distinct from its linear counterpart. The results demonstrate that the diffusional heterogeneity of polymers that is often hidden behind ensemble averaging can be revealed by the efficient synthesis of the cyclic polymers using the ESA-CF process and the quantitative analysis of the diffusive motion at the single molecule level using the MSD and CDF analyses.

Formation and Properties of Vesicles from Cyclic Amphiphilic PS-PEO Block Copolymers.

Linear polystyrene-poly(ethylene oxide)-polystyrene (PS-PEO-PS) block copolymers and corresponding cyclized PS-PEO counterparts with three different PS molecular weights were synthesized and self-assembled to investigate the effects arising from the topology. Linear PS5-PEO45-PS5 (L1) and cyclic PS10-PEO45 (C1) formed micelles. As previously reported for poly(n-butyl acrylate) and PEO block copolymers, the micelles from C1 showed more than 30 °C higher phase transition temperature (cloud point, Tc) than those from L1. Linear PS10-PEO45-PS10 (L2) and cyclic PS20-PEO45 (C2) resulted in the formation of a structure called large compound micelles. Self-assembly of linear PS40-PEO48-PS40 (L3) and cyclic PS86-PEO48 (C3) lead to the formation of vesicles. The vesicles were characterized by TEM, DLS, and SLS. Remarkably, the vesicles from L3 (Tc = 69, 59, and 48 °C in the presence of 1, 5, and 10 wt % of NaCl, respectively) were found to be somewhat more thermally stable than those from C3 (Tc = 62, 52, and 43 °C in the presence of 1, 5, and 10 wt % of NaCl, respectively). This trend of the thermal stability was counterintuitively opposed to the case of the micelles. Moreover, Tc of the vesicles was controlled by the ratio of L3 and C3.

Light- and Heat-Triggered Reversible Linear-Cyclic Topological Conversion of Telechelic Polymers with Anthryl End Groups.

This study demonstrates the comprehensive investigation on the reversible linear-cyclic topological conversion of hydrophilic and hydrophobic polymers with various molecular weights. The reactions were triggered by light or heat, which reversibly dimerize and cleave the anthryl or coumarinyl end groups of the telechelics. Poly(ethylene oxide) telechelics with anthryl end groups attached through electron-donating (Ant-O-PEO and Ant-CH2-PEO) and electron-withdrawing (Ant-CO2-PEO) linking groups were synthesized. While Ant-O-PEO and Ant-CH2-PEO decomposed upon photoirradiation at 365 nm, Ant-CO2-PEO efficiently cyclized through the photodimerization of the anthryl end groups both in water and in organic solvents shown by NMR, SEC, and MALDI-TOF MS. The lower the molecular weight, the faster the cyclization proceeded. When cyclized Ant-CO2-PEO was heated at 150 °C in bulk, the polymers quantitatively converted back into the original linear topology. Furthermore, repeatable linear-cyclic topological conversion was confirmed. The reversible topological conversion of hydrophobic poly(tetrahydrofuran) telechelics with anthryl end groups (Ant-PTHF) was also successful. In addition, poly(ethylene oxide) telechelics with coumarinyl end groups (Cou-PEO) were also cyclized by irradiation at 365 nm in water. However, the cyclization hardly occurred when performed in methanol likely due to the lack of sufficient hydrophobic interaction of the coumarinyl end groups. Cyclized Cou-PEO was irradiated at 254 nm to test for linearization, finding the linear product and cyclic precursor were likely photoequilibrated.

Cyclic polymers revealing topology effects upon self-assemblies, dynamics and responses.

A variety of single- and multicyclic polymers having programmed chemical structures with guaranteed purity have now become obtainable owing to a number of synthetic breakthroughs achieved in recent years. Accordingly, a broadening range of studies has been undertaken to gain updated insights on fundamental polymer properties of cyclic polymers in either solution or bulk, in either static or dynamic states, and in self-assemblies, leading to unusual properties and functions of polymer materials based on their cyclic topologies. In this article, we review recent studies aiming to achieve distinctive properties and functions by cyclic polymers unattainable by their linear or branched counterparts. We focus, in particular, on selected examples of unprecedented topology effects of cyclic polymers upon self-assemblies, dynamics and responses, to highlight current progress in Topological Polymer Chemistry.

NMR Relaxometry for the Thermal Stability and Phase Transition Mechanism of Flower-like Micelles from Linear and Cyclic Amphiphilic Block Copolymers.

Linear and cyclic amphiphilic block copolymers consisting of poly(ethylene oxide) (PEO) as the hydrophilic segment and poly(methyl acrylate) or poly(ethyl acrylate) as the hydrophobic segments were synthesized and self-assembled to form flower-like micelles. The micelles from linear (methyl acrylate)12(ethylene oxide)73(methyl acrylate)12 (Mn = 1000-3200-1000, l-MOM) showed a cloud point (Tc) at 46 °C by the transmittance of the micellar solution, whereas that of cyclic (methyl acrylate)30(ethylene oxide)79 (Mn = 2600-3500, c-MO) increased to 72 °C, as previously reported. DLS showed comparable diameters (l-MOM, 14 nm; c-MO, 12 nm) and Tc values (l-MOM, 48 °C; c-MO, 75 °C). For the investigation of the difference in Tc and the phase transition mechanism based on the polymer topology, NMR relaxometry was performed to determine the spin-lattice (T1) and spin-spin (T2) relaxation times. A decrease in T2 of the PEO segment in both l-MOM and c-MO was observed above Tc, suggesting that slow large-scale motions, such as the detachment of a chain end from the core, bridging, and interpenetration of the micelles, were inhibited. T1 of the PEO segment in l-MOM continuously increased in the experimental temperature range, indicating that the segment is hydrated even above its Tc. On the other hand, that of c-MO reached a ceiling above its Tc, likely due to the prevention of the rotation of the PEO main chain bonds caused by dehydration. Similar results were obtained for linear (ethyl acrylate)8(ethylene oxide)79(ethyl acrylate)8 (Mn = 800-3500-800, l-EOE) and its cyclic (ethyl acrylate)15(ethylene oxide)78 (Mn = 1500-3400, c-EO).

Folding Construction of a Pentacyclic Quadruply fused Polymer Topology with Tailored kyklo-Telechelic Precursors.

A pentacyclic quadruply fused polymer topology has been constructed for the first time through alkyne-azide addition (click) and olefin metathesis (clip) reactions in conjunction with an electrostatic self-assembly and covalent fixation (ESA-CF) process. Thus, a spiro-type, tandem tetracyclic poly(tetrahydrofuran), poly(THF), precursor having two allyloxy groups at the opposite positions of the four ring units was prepared by the click-linking of one unit of an eight-shaped precursor having alkyne groups at the opposite positions with two units of a single-cyclic counterpart having an azide and an alkene group at the opposite positions. Both are obtainable through ESA-CF. The subsequent metathesis clip-folding of the tetracyclic precursor could afford a pentacyclic quadruply fused polymer product, of "shippo" form, in 19% yield.

SN2 regioselectivity in the esterification of 5- and 7-membered azacycloalkane quaternary salts: a DFT study to reveal the transition state ring conformation prevailing over the ground state ring strain.

The nucleophilic esterification of 5- and 7-membered N-phenylcyclic ammonium salts resulted in distinctive regioselectivity, despite their comparable ring strain in the ground states relative to the corresponding cyclopentane and cycloheptane (both 25.9 kJ mol(-1)). The former underwent a selective ring-opening reaction, while the latter predominantly underwent ring-emitting with concurrent ring-opening reactions. A DFT study of the model compounds revealed that the regioselection in the 5- and 7-membered azacycloalkane quaternary salts is plausibly directed by the transition state ring conformation, and not by the ground state ring strain. Remarkably, at the ring-opening transition state, the 5-membered cyclic skeletal structure expands toward the unstrained and thus less frustrated 6-membered cyclohexane conformation. On the other hand, the 7-membered counterpart expands at the ring-opening transition state toward the more frustrated 8-membered cyclooctane conformation to promote the alternative ring-emitting process.

Constructing a macromolecular K(3,3) graph through electrostatic self-assembly and covalent fixation with a dendritic polymer precursor.

A triply fused tetracyclic macromolecular K(3,3) graph has been constructed through electrostatic self-assembly of a uniformly sized dendritic polymer precursor having six cyclic ammonium salt end groups carrying two units of a trifunctional carboxylate counteranions, and subsequent covalent conversion by the ring-opening reaction of cyclic ammonium salt groups at an elevated temperature under dilution. The K(3,3) graph product was isolated from the two constitutional isomers by means of a recycling SEC technique, as the hydrodynamic volume of the triply fused tetracyclic K(3,3) product is remarkably contracted in comparison with another isomer having a ladder form in solution.

Click construction of spiro- and bridged-quatrefoil polymer topologies with kyklo-telechelics having an azide group.

Unprecedented tetracyclic polymer topologies with spiro- and a bridged-type quatrefoil forms are effectively constructed through an alkyne-azide, click-linking reaction by employing a kyklo-telechelic poly(tetrahydrofuran), poly(THF), precursor having an azide group, obtained through an electrostatic self-assembly and covalent fixation (ESA-CF) process, and complementary tetrafunctional alkyne reagents of either a pentaerythritol derivative or a four-armed star telechelic polymer precursor.

Single-molecule study on polymer diffusion in a melt state: effect of chain topology.

We report a new methodology for studying diffusion of individual polymer chains in a melt state, with special emphasis on the effect of chain topology. A perylene diimide fluorophore was incorporated into the linear and cyclic poly(THF)s, and real-time diffusion behavior of individual chains in a melt of linear poly(THF) was measured by means of a single-molecule fluorescence imaging technique. The combination of mean squared displacement (MSD) and cumulative distribution function (CDF) analysis demonstrated the broad distribution of diffusion coefficient of both the linear and cyclic polymer chains in the melt state. This indicates the presence of spatiotemporal heterogeneity of the polymer diffusion which occurs at much larger time and length scales than those expected from the current polymer physics theory. We further demonstrated that the cyclic chains showed marginally slower diffusion in comparison with the linear counterparts, to suggest the effective suppression of the translocation through the threading-entanglement with the linear matrix chains. This coincides with the higher activation energy for the diffusion of the cyclic chains than of the linear chains. These results suggest that the single-molecule imaging technique provides a powerful tool to analyze complicated polymer dynamics and contributes to the molecular level understanding of the chain interaction.

Regioselective ring-emitting esterification on azacyclohexane quaternary salts: a DFT and synthetic study for covalent fixation of electrostatic polymer self-assemblies.

A regioselective nucleophilic esterification upon six-membered, thus considered unstrained, azacyclohexane quaternary salts has been disclosed by DFT calculations using a model compound and subsequent experimental studies of nucleophilic substitution on N-phenyl-3,3-dimethylpiperidinium salt groups at the polymer chain ends by carboxylate anions. An exclusive ring-emitting esterification was proposed theoretically and confirmed experimentally to produce a simple ester group, in contrast to less robust amino-ester linkages through an alternative ring-opening process with strained five-membered ammonium salts. This reaction was subsequently applied to a prototypical process of an electrostatic self-assembly and covalent fixation (ESA-CF) technique to produce a ring polymer having simple ester linking units.

Tuneable enhancement of the salt and thermal stability of polymeric micelles by cyclized amphiphiles.

Cyclic molecules provide better stability for their aggregates. Typically in nature, the unique cyclic cell membrane lipids allow thermophilic archaea to inhabit extreme conditions. By mimicking the biological design, the robustness of self-assembled synthetic nanostructures is expected to be improved. Here we report topology effects by cyclized polymeric amphiphiles against their linear counterparts, demonstrating a drastic enhancement in the thermal, as well as salt stability of self-assembled micelles. Furthermore, through coassembly of the linear and cyclic amphiphiles, the stability was successfully tuned for a wide range of temperatures and salt concentrations. The enhanced thermal/salt stability was exploited in a halogen exchange reaction to stimulate the catalytic activity. The mechanism for the enhancement was also investigated. These topology effects by the cyclic amphiphiles offer unprecedented opportunities in polymer materials design unattainable by traditional means.

Systematic Synthesis of Block Copolymers Consisting of Topological Amphiphilic Segment Pairs from kyklo- and kentro-Telechelic PEO and Poly(THF).

A set of four types of block copolymers consisting of topological amphiphilic segment pairs was effectively synthesized via kyklo- (functionalized cyclic) and kentro- (center-functionalized linear) telechelic poly(ethylene oxide) (PEO) and poly(tetrahydrofuran) (poly(THF)). Accordingly, kyklo- and kentro-telechelic PEO with an ethynyl group was newly prepared from relevant linear PEO precursors with quinuclidinium end groups and an ethynyl-functionalized dicarboxylate counteranion by the electrostatic self-assembly and covalent fixation (ESA-CF) process. Similarly, kyklo- and kentro-telechelic poly(THF) with an azido group was obtained. The PEO and poly(THF) telechelics were subjected to click chemistry to systematically produce amphiphilic block copolymers with two symmetric topological forms, that is, an "8" shape (IC·IIC) and a four-armed star shape (IL·IIL), and two asymmetric topological forms, that is, twin-tailed tadpole shapes (IL·IIC and IC·IIL) with respect to the hydrophilic-hydrophobic plane.