Hydrodynamic and electrooptical properties of star-shaped heteroarm fullerene (C60)-containing polymers in solutions

Star-shaped heteroarm polymers with a C₆₀ branching center and polystyrene and poly(2-vinylpyridine) arms of equal molecular masses have been studied by the methods of molecular hydrodynamics (translational diffusion and viscometry) and electrooptics (the Kerr effect). The experimental hydrodynamic data are interpreted in terms of the regular star model. The molecular masses and hydrodynamic sizes of star-shaped heteroarm polymers in solutions are estimated. A comparison of these values with the corresponding parameters of linear polymer-analogs (polystyrene and poly(2-vinylpyridine)) makes it possible to characterize the branching degree of macromolecules. The study of the electrooptical properties of the heteroarm polymer in benzene demonstrates the tendency of macromolecules toward aggregation.     

Internal organization and conformational characteristics of star-shaped polystyrene with fullerene C<Subscript>60</Subscript> as a branching center in deuterotoluene

The internal organization of star-shaped polystyrene macromolecules containing fullerene C₆₀ as a branching center is studied via small-angle neutron scattering in deuterotoluene. Analysis of the experimental data according to the Debye-Benoit approximation and the Fourier transformation of the momentum transfer dependences of scattering cross sections for the linear PS precursor and stars is used to determine their molecular masses (9 × 10³ and 5 × 10⁴) and gyration radii (∼2.7 and ∼5.5 nm), the gyration radius of the arm (∼3.4 nm), and the average functionality of the star (5.7). The behavior of scattering cross sections for the fullerene-containing polymer on the whole is described by the law of scattering for stars with Gaussian arms (the Benoit model). However, at the local level (within one chain segment), the fullerene center exerts a specific effect on the conformation of arms. As a result, their statistical flexibility decreases and eventually the size of the star increases by ∼30%. This finding conflicts with the Daoud-Cotton theory.     

Star-shaped polymers with a doubled fullerene (C60) branching center

Star-shaped regular homopolystyrenes with 22 arms and heteroarm polymers with 12 PS arms and 10 poly(2-vinypyridine) arms have been synthesized by consecutive coupling-functionalization-coupling reactions. The synthesis includes the following stages: the exhaustive grafting of fullerene C₆₀ by polystyryllithium chains (living hexaadducts); the coupling of hexaadducts with the use of dimethyldichlorosilane or 1,4-dibromobutane into twelve-arm macromolecules, where the branching center is composed of two covalently bonded fullerene C₆₀ molecules; functionalization of twelve-arm double-core PS stars during the action of excess dihalides (the replacement of lithium atoms with groups containing chlorine or bromine atoms); and the coupling of living chains of PS or poly(2-vinylpyridine) via reactions with halogen-containing groups at the branching center of double-core PS stars. Linear living polymers used as arms have been prepared by anionic polymerization. Exclusion chromatography has been used to control the individual stages of synthesis. The molecular characteristics of the PS precursor and of star-shaped polymers have been studied in terms of hydrodynamics and light scattering.     

Structural features of fullerene-containing star-shaped polystyrene molecules with Oligomer arms in solutions

The structure and conformational properties of star-shaped oligostyrenes containing fullerene C₆₀ as a branching center and short arms with lengths at the level of the persistent length or a segment of a polystyrene chain are studied by small-angle neutron scattering in deuterotoluene. The gyration radii of linear precursor oligomers (～0.4 and 0.6 nm) and corresponding star-shaped molecules (～1.1 and 1.4 nm) are calculated under the Guinier approximation. The linear oligomer (4–5 units) is found to be a rodlike molecule; arms of star-shaped molecules based on it assume the straightened conformations as well. Linear oligomer chains composed of 6–7 units deviate from the rodlike shape and acquire a certain flexibility in solution, but oligomer chains grafted onto the C₆₀ center preserve the extended conformations. There is no marked tendency toward screening of fullerene by radially extended arms. The number of branches in the star-shaped oligostyrenes corresponds to a functionality of f = 6 preset by the conditions of synthesis.     

Structure and supramolecular structures of star-shaped fullerene-containing heteroarm polymers in deuterotoluene

Self-organization of star-shaped polymers containing six PS arms and six polar polymer arms on a common C₆₀ branching center is studied by means of small-angle neutron scattering in deuterotoluene. The results are compared with the corresponding characteristics of six-arm star-shaped fullerene-containing PSs. It is shown that the incorporation of additional polar arms into a six-arm macromolecule leads to its compression due to an increase in the degree of coiling of polar chains in the nonpolar solvent. In solution, heteroarm stars give rise to supramolecular structures in the form of clusters whose dimensions and density depend on the nature of the polar arms. Stars containing PS and poly(2-vinylpyridine) arms are weakly associated, and the mean number of particles in an associate is ∼1.3. Hybrid polymers containing PS and poly(tert-butyl methacrylate) arms demonstrate capability for mutual penetration that favors the appearance of large structures that have a diameter of ∼50 nm and that include up to 12 macromolecules. Hybrid stars containing PS and diblock copolymer (poly(2-vinylpyridine)-poly(tert-butyl methacrylate)) arms exhibit moderate self-organization that manifests itself in the formation of chain associates built from four macromolecules.     

Effect of fullerene C60 branching center on the conformational properties of arms and the structure of star-shaped polystyrenes in solutions

The effect of the fullerene C₆₀ branching center on the structure and conformation of star-shaped polystyrenes with different arm lengths at equal concentrations in deuterotoluene (c = 1 g/dL) is studied by the method of small-angle neutron scattering. The analysis of neutron scattering for linear PS precursors and stars (the molecular masses of arms are ～7 × 10³ and ～4 × 10⁴) shows that the stars have ～6 arms that form a dense excluded-volume zone around a core inaccessible to other macromolecules. In low-molecular-mass stars (the molecular mass of the arm is ～7 × 10³), strengthening of the static rigidity of arms is observed; as a result, the size of arms increases relative to the size of free PS chains in a good solvent. At a greater length of arms (M ～ 4 × 10⁴), their individual properties are weakly pronounced in the correlation spectrum of the arm because of the interpenetration of arms, thereby demonstrating similarity in the structures of stars and their linear analogs. The mechanism controlling the effect of fullerene C₆₀ on the conformations of stars via solvent structuring by fullerene is discussed.     

Effect of the branching center structure on self-organization of fullerene-containing star-shaped polystyrenes in deuterotoluene

Small-angle neutron scattering method was used to study self-organization phenomena in regular fullerene-containing star-shaped polystyrenes differing in the number of arms (6 and 12) and branching center structure (one or two covalently bound molecules of C₆₀ fullerene) in deuterotoluene.     

Molecular and hydrodynamic characteristics of star-shaped polystyrenes with one or two fullerene (C<Subscript>60</Subscript>) molecules as a branching center

The molecular properties of regular star-shaped polystyrenes with different numbers of arms (6, 12, and 22) and different structures of the branching center (one or two covalently bonded fullerene C60 molecules) are studied by static light scattering; translational diffusion; and viscometry in toluene, THF, and chloroform. The lengths of the arms for the studied polymer stars are found to be the same. (The molecular mass of the arm is 8.1 × 10³.) The molecular mass and hydrodynamic sizes of macromolecules are estimated. It is shown that the conformational and hydrodynamic characteristics of polymer stars remain practically unchanged on passage from THF to chloroform. Compared to the linear analog, star-shaped fullerene-containing PSs are characterized by a higher density of the molecular coil. The shape of their macromolecules differs insignificantly from spherical, in consistence with theoretical predictions for polymer stars with relatively short arms.     

Star-shaped polymers with the fullerene C60 branching center

The anionic methods for the synthesis of homo- and heteroarm (hybrid) star-shaped polymers using fullerene C₆₀ aPre considered. The possibilities of fullerene C₆₀ as an agent of combination of living polymer chains and the procedures of transformation of polymer derivatives of C₆₀ (hexaadducts) into polyfunctional macroinitiators of anionic polymerization of vinyl monomers are shown. The methods for functionalization of polymer fullerene derivatives and their combinations into structures of complex controlled architecture are presented. The structural features and initiating properties of the living polymer fullerene derivatives and their role in the formation of heteroarm star-shaped macromolecules with the controlled number of branches and predetermined molecular weight characteristics of the arms are discussed. The hydrodynamic properties of the star-shaped fullerene-containing polymers are considered. The data on the small-angle neutron scattering study of self-organization of the stars in solutions are presented.     

Star‐shaped siloxane polymers with various cyclic cores: Synthesis and properties

Six new star‐shaped polydimethylsiloxane (PDMS) with various cyclic siloxane cores were synthesized by the “grafting onto” method. The polymers obtained feature low dispersity and well‐defined structures. Two of them, Q ₈ ‐PDMS and D ₁₂ ‐PDMS , have a three‐dimensional spatial structure traditional for star‐shaped polymers where the arms are arranged in all directions from the branching center. The other four polymers, D ₄ ‐PDMS – D ₈ ‐PDMS , have quite a different spatial geometry (cis‐structure) where all the PDMS‐arms are arranged on one side of the branching center plane. Such star‐shaped structures were not reported before. The structures and purity of the polymers obtained were confirmed using a set of physicochemical methods of analysis. The effect of the macromolecule structure on the properties of the target polymers was identified. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019     

Method of determining the degree of branching from the gel permeation chromatogram for star-shaped SBS thermoplastic block copolymers

A novel GPC calculation method has been developed for characterizing star-shaped styrene–butadiene block copolymers (SBS). This method enables us to determine the degree of branching (number of arms per molecule) of the synthesized polymer without the need of a priori measurement of the true molecular weights of the SBS star polymer and its linear polymeric arm. To illustrate the simplicity of this method, nearly monodispersed three-arm and four-arm model star polymers have been purposely synthesized by linking living diblock polymeric arms of the polystyrene-block-polybutadiene type with silicon tetrachloride as the multifunctional linking agent. The good agreement between the degree of branching calculated from the GPC chromatogram and that actually measured by MALL (multiple angle laser light scattering) has corroborated the validity of the calculation method. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3393–3401, 1997     

Star-shaped polymers by living cationic polymerization. VIII. Size and shape of star poly(vinyl ether)s determined by dynamic light scattering and computer simulation

The sizes and shapes of star-shaped poly(vinyl ether)s, prepared by living cationic polymerization, were studied by dynamic light scattering and molecular mechanics-based computer simulation. The hydrodynamic radii (R_h) of star poly(isobutyl vinyl ether)s (4a; M?_w = 2.2 × 10⁴ ? 1.7 × 10⁵) determined by dynamic light scattering were in the range from 30 to 90 Å in tetrahydrofuran or ethyl acetate. Consistent with the expected multiarmed architecture of 4a, the radius for a given number (f) of arms per molecule increased with the degree of polymerization [DP(arm)] of the arms, and for a fixed DP(arm), the radius increased with f. The relationship between arm number f and the “shrinking” factor h [R_h(star)/R_h(linear)] was consistent with multibranched structures for the star polymers. These results are supported by those for the molecular weight itself; the apparent weight-average molecular weights by size-exclusion chromatography are less than the corresponding absolute values by static light scattering. The dependence of h on f suggests some degree of asymmetry in the star shape. Similar results were also obtained by the computer simulation of potential energy-minimized conformations of the arms, which implied almost spherical but slightly asymmetric shapes. The computer simulation also demonstrated that the star polymer (4b) with pendant hydroxyl groups in the arms is smaller in size than the corresponding alkyl (isobutyl) (4a) with the identical arm number and arm degree of polymerization. © 1995 John Wiley & Sons, Inc.     

Arm‐first synthesis of star polymers with polywedge arms using ring‐opening metathesis polymerization and bifunctional crosslinkers

This work presents a two‐step, one‐pot process to make star polymers with polywedge arms. In a one‐pot reaction, after the polywedge arms are synthesized, crosslinker species are added to the reaction, rapidly forming star polymers. Crosslinker species with different degrees of conformational freedom were designed and synthesized and their capacity to generate star polymers was evaluated. Mass conversions up to 92% and stars with up to 17 arms were synthesized with the most rigid crosslinker. The effects of arm molecular weight and molar ratio of crosslinker to arm on mass conversion and arms per star were explored further. Finally, the size‐molecular weight scaling relationship for polywedges with linear and star architectures was compared, corroborating theoretical results regarding star polymers with arms much larger than their core. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 732–740     

Effect of fullerene C60 as a branching center on molecular and polarization properties of star-shaped polystyrenes

The behavior of molecules of a star-shaped six-arm polystyrene with a covalently bound fullerene C₆₀ as a branching center was studied by viscometry and by measuring the electrooptical Kerr effect and the dielectric polarization in solutions. It was shown that polarization and electrooptical characteristics of a fullerene-containing polystyrene (C₆₀ ～ 3 wt %) differ by an order of magnitude or even greater from the corresponding characteristics of the parent polymer. A comparison of the above properties with the analogous characteristics of the model hexaadduct (the products of reaction between octyllithium and fullerene C₆₀) demonstrated that a difference in the behavior of the star-shaped polystyrene and its parent analog is associated with the structural features of the branching center, among which is the occurrence of six proton addends that are bonded rather weakly to the fullerene cage in the hexaadducts under study.     

Molecular dynamics of star‐shaped poly(L‐lactide)s in tetrahydrofuran as solvent monitored by fluorescence spectroscopy

Linear telechelic, α,ω‐ditelechelic, and star‐shaped tri‐, tetra‐, penta‐, and hexa‐arm poly(L ‐lactide)s (PLAs) fitted at every arm with pyrene end group have been prepared. Internal dynamics and mobility of the PLA chains in tetrahydrofuran solution at 25 °C, with regard to the number of PLA arms in one macromolecule and the individual arm average degree of polymerization, was followed by fluorescence spectroscopy. Analysis of both static and time‐resolved spectra of the star‐shaped polymers revealed dynamic segmental motion resulting in end‐to‐end cyclization, accompanied by an excimer formation. Probability and rate of the latter reaction increased with increasing number of arms and with decreasing their polymerization degree. Moreover, time‐resolved measurements revealed that for macromolecules containing few arms (2 or 3) the pyrene moieties are located in the interior of the star‐shaped PLAs, whereas in the instance of the higher number of arms (4–6) they are located at the periphery of the star‐shaped PLAs. Thus, increasing the number of arms leads to their stretching away from the center of the star‐shaped PLA macromolecule. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4586–4599, 2005     

Synthesis of fluorine‐containing star‐shaped poly(vinyl ether)s via arm‐linking reactions in living cationic polymerization

Various types of fluorine‐containing star‐shaped poly(vinyl ether)s were successfully synthesized by crosslinking reactions of living polymers based on living cationic polymerization. Star polymers with fluorinated arm chains were prepared by the reaction between a divinyl ether and living poly(vinyl ether)s with fluorine groups (C₄F₉, C₆F₁₃, and C₈F₁₇) at the side chain using cationogen/Et_1.5AlCl_1.5 in a fluorinated solvent (dichloropentafluoropropanes), giving star‐shaped fluorinated polymers in high yields with a relatively narrow molecular weight distribution. The concentration of living polymers for the crosslinking reaction and the molar feed ratio of a bifunctional vinyl ether to living polymers affected the yield and molecular weight of the star polymers. Star polymers with block arms were prepared by a linking reaction of living block copolymers of a fluorinated segment and a nonfluorinated segment. Heteroarm star‐shaped polymers containing two‐ or three‐arm species were synthesized using a mixture of different living polymer species for the reaction with a bifunctional vinyl ether. The obtained polymers underwent temperature‐induced solubility transitions in various organic solvents, and their concentrated solutions underwent sol–gel transitions, based on the solubility transition of a thermoresponsive fluorinated segment. Furthermore, a slight amount of fluorine groups were shown to be effective for physical gelation when those were located at the arm ends of a star polymer. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Star‐branched polystyrenes by nitroxide living free‐radical polymerization

Star‐branched polystyrenes, with polydispersity indices of 1.15–1.56 and 4–644 equal arms, were synthesized by the reaction of 2,2,6,6‐tetramethylpiperidin‐1‐yloxy (TEMPO)‐capped polystyrene (PS‐T) with divinylbenzene (DVB). The characterization of PS‐T and the final star polymers was carried out by size exclusion chromatography, low‐angle laser light scattering, and viscometry. The degree of branching of the star polymers depended on the DVB/PS‐T ratio and the PS‐T molecular weight. An asymmetric (or miktoarm) star homopolymer of the PS_nPS′_n type was made by the reaction of the PS_n symmetric star, which had n TEMPO molecules on its nucleus and consisted of a multifunctional initiator, with extra styrene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 320–325, 2001     

Influence of arm length and number on star-shaped poly(2-isopropyl-2-oxazoline) aggregation in aqueous solutions near cloud point

Four-arm star-shaped poly(2-isopropyl-2-oxazolines) (PiPrOx₄) are synthesized by cationic polymerization on t-butylcalix[4]arene macroinitiator. The obtained samples differ by polymerization degree of arms N_PiPrOx = 9 and 25 and are characterized in chloroform. The behavior in aqueous solutions is studied by light scattering methods and compared with the results of investigation of eight-arm star with similar structure. Three types of particles are observed in solution of short-arm PiPrOx₄ at room temperature, whereas only two particle types are present in long-arm star solution. Arm shortening leads to widening of the phase transition interval. The arm number decreasing reduces the phase transition temperature by 1°C.     

Thermal stability of the fullerene‐chain link in 6‐arm PS stars with a C60 core

The thermal stability of well‐defined hexa‐adducts (PS)₆C₆₀ in solution at temperatures around 100 °C has been studied by multi‐detector Size Exclusion Chromatography. The degradation reaction corresponds to a quantitative release of the polystyrene arms from the fullerene core through thermal cleavage of the PS‐C₆₀ link. From the kinetics of formation of cut arms and the progressive decrease of the stars' functionality, we could establish that the reaction follows a stepwise “breaking” mechanism where a 6‐arm star is first converted to a 5‐arm star, then to a 4‐arm star, and so on down to the ungrafted arm. Furthermore, not only does the thermal stability of the PS? C₆₀ bond increase if the functionality of the star decreases, but the difference is large enough to allow determination of the kinetics constants for the first three steps. The activation energy for the breaking of an arm‐C₆₀ link is about 65 kJ/mol. The stability of (PS)₆C₆₀ slightly decreases with an increase of the arm length. MALDI‐TOF mass spectroscopy has shown that both C? C bonds in α and β positions to C₆₀ can be cut, but the breaking of the direct fullerene‐arm bond is favored. We have also found that a polyisoprene? C₆₀ bond is about seven times less stable than a PS‐fullerene link upon heating. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4820–4829, 2004     

Structural features and initiating properties of the hexaadduct of polystyryllithium and fullerene C60 in tetrahydrofuran and their role in synthesis of heteroarm star-like polymers

Structural transformations of the hexaadduct of polystyryllithium and fullerene C₆₀ in the course of interaction with a proton donor or 1,1-diphenylethylene and at initiation of styrene polymerization were studied, and their role in the formation of heteroarm star-like polymers with C₆₀ center of branching and arms of polystyrene and poly-2-vinylpyridine was elucidated.