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.     

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.     

Structural features of star-shaped fullerene (C60)-containing polystyrenes: Neutron scattering experiments

Structural features of star-shaped polyprotostyrene and polydeuterostyrene containing fullerene C₆₀ as a branching center have been studied by small-angle neutron scattering in benzene solutions. The results are compared with the corresponding characteristics of linear PSs, the molecular mass of which is equal to the molecular mass of one star arm in star-shaped macromolecules. The molecular masses of star-shaped polymers are estimated, and their branching center is shown to be hexafunctional. At relatively low concentrations of starshaped polymers in solutions, one can observe excluded volume effects, which are related by the presence of regions with higher densities at the center of a macromolecule. Using the Fourier transform of the scattering cross section, three-dimensional correlation functions are obtained, and the regular structure of stars is proved. Conclusions about the local correlations of units within one star arm and averaged correlations between units of neighboring arms within a given star are derived. An analysis of three-dimensional correlations shows that the centers of mass of all star arms are directed along orthogonal axes passing through the C₆₀ branching center of a star-shaped macromolecule.     

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.     

Anionic methods for synthesis of star polymers

Leading position among numerous methods for synthesis of star polymers is occupied, as regards their potential and diversity, by techniques based on the anionic polymerization. The review considers five basic approaches to application of the anionic polymerization mechanisms in relation to an agent used or procedure employed (methods with polyfunctional coupling agents, multifunctional initiators, polymerizing and nonpolymerizing divinyl agents; multistage methods, methods using C₆₀ fullerene). All groups of syntheses are illustrated by examples, and advantages of methods for synthesis of various homo- and heteroarm star structures are demonstrated. Particular attention is given to syntheses with C₆₀ fullerene. The potential of C₆₀ fullerene as a coupling agent for “living” polymer chains and methods for conversion of polymeric derivatives of C₆₀ (hexaadducts) to polyfunctional macroinitiators of anionic polymerization are described and techniques for functionalization of polymeric fullerene derivatives and their coupling into structures with a complex controllable architecture are presented.     

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.     

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.     

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.     

Synthesis of well‐defined miktoarm star polymers of poly(dimethylsiloxane) by the combination of chlorosilane and benzyl chloride linking chemistry

A series of novel four‐arm A₂B₂ and A₂BC and five‐arm A₂B₂C miktoarm star polymers, where A is poly(dimethylsiloxane) (PDMS), B is polystyrene (PS), and C is polyisoprene (PI), were successfully synthesized by the combination of chlorosilane and benzyl chloride linking chemistry. This new and general methodology is based on the linking reaction of in‐chain benzyl chloride functionalized poly(dimethylsiloxane) (icBnCl–PDMS) with the in‐chain diphenylalkyl (icD) living centers of PS‐DLi‐PS, PS‐DLi‐PI, or (PS)₂‐DLi‐PI. icBnCl–PDMS was synthesized by the selective reaction of lithium PDMS enolate (PDMSOLi) with the chlorosilane groups of dichloro[2‐(chloromethylphenyl)ethyl]methylsilane, leaving the benzyl chloride group intact. The icD living polymers, characterized by the low basicity of DLi to avoid side reactions with PDMS, were prepared by the reaction of the corresponding living chains with the appropriate chloro/bromo derivatives of diphenylethylene, followed by a reaction with BuLi or the living polymer. The combined molecular characterization results of size exclusion chromatography, ¹H NMR, and right‐angle laser light scattering revealed a high degree of structural and compositional homogeneity in all miktoarm stars prepared. The power of this general approach was demonstrated by the synthesis of a morphologically interesting complex miktoarm star polymer composed of two triblock terpolymer (PS‐b‐PI‐b‐PDMS) and two diblock copolymer (PS‐b‐PI) arms. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6587–6599, 2006     

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.     

Synthesis of low polydispersity polybutadiene and polyethylene stars by convergent living anionic polymerization

Star‐shaped polybutadiene stars were synthesized by a convergent coupling of polybutadienyllithium with 4‐(chlorodimethylsilyl)styrene (CDMSS). CDMSS was added slowly and continuously to the living anionic chains until a stoichiometric equivalent was reached. Gel permeation chromatography‐multi‐angle laser light scattering (GPC‐MALLS) was used to determine the molecular weights and molecular weight distribution of the polybutadiene polymers. The number of arms incorporated into the star depended on the molecular weight of the initial chains and the rate of addition of the CDMSS. Low molecular weight polybutadiene arms (M_n = 640 g/mol) resulted in polybutadiene star polymers with an average of 12.6 arms, while higher molecular weight polybutadiene arms (M_n = 16,000 g/mol) resulted in polybutadiene star polymers with an average of 5.3 arms. The polybutadiene star polymers exhibited high 1,4‐polybutadiene microstructure (88.3–93.1%), and narrow molecular weight distributions (M_w/M_n = 1.11–1.20). Polybutadiene stars were subsequently hydrogenated by two methods, heterogeneous catalysis (catalytic hydrogenation using Pd/CaCO₃) or reaction with p‐toluenesulfonhydrazide (TSH), to transform the polybutadiene stars into polyethylene stars. The hydrogenation of the polybutadiene stars was found to be close to quantitative by ¹H NMR and FTIR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 828–836, 2006     

Thermal degradation of a heteroarm starlike polymer with fullerene C60 core

The mechanism of thermal degradation of a 12-arm starlike polymer with fullerene C₆₀ core and equal number of polystyrene and poly(tert-butyl methacrylate) arms was studied by thermal desorption mass spectrometry. Thermal characteristics of the heteroarm stars were compared with those of six-arm starlike fullerene-containing polystyrenes and linear poly(tert-butyl methacrylate).     

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     

Fullerene-based processable polymers as plausible acceptors in photovoltaic applications

To develop n-type processable polymeric materials for photovoltaic applications, a series of polymers was synthesized by grafting fullerene C₆₀ onto polystyrene (PS). Grafted polymers were studied and compared with PS:fullerene blends. Electronical and electrical properties were first measured to define the minimal amount of C₆₀ required for solar cells application. Then, thermal properties and rheological behavior of grafted polymers were analyzed to determine whether they could be processed from the melt. A throughout experimental study revealed that C₆₀-grafted polymers exhibit two thresholds. The first threshold at 3–4 vol % (detected by electrical conductivity, electron mobility, and melt viscosity measurements) is associated to the percolation of C₆₀ molecules. The second threshold (evidenced by glass transition and melt viscosity measurements and confirmed by optical and atomic force micrograph observations) at about 12–13 vol % is assumed to be related to the formation of C₆₀ aggregates. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Anionic Synthesis of Heteroarm,Star-Branched Polymers. Scope and Limitations

Abstract

The scope and limitations of the following methods for the synthesis of heteroarm star-branched polymers are presented: (a) divinylbenzene linking reactions; (b) coupling reactions with macromolecular silyl halides; and (c) 1,1-diphenylethylene-based living linking reactions. The methodology of using l, 3-bis(1-phenylethenyl)benzene to prepare A₂B₂ hetero 4-armed stars is reviewed, and the synthesis of well-defined (polystyrene)₂(1,4-polybutadiene)₂ heteroarm stars is described. The synthesis of well-defined ABC heteroarm star polymers using macromonomers functionalized with 1, 1-diphenylethylene groups is described.     

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.     

Synthesis and pyrolysis of ABC type miktoarm star copolymers with polystyrene,poly(lactic acid) and poly(ethylene glycol) arms

An ABC type miktoarm star copolymer possessing polystyrene (PS), poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) arms was synthesized by combining Atom Transfer Radical Polymerization (ATRP) and Ring Opening Polymerization (ROP) with two click chemistries, namely thiol–ene and copper catalyzed azide–alkyne cycloaddition (CuAAC). For this purpose, a core 1-(allyloxy)-3-azidopropan-2-ol with allyl and azide functionalities was synthesized in two steps. Then, clickable polymers, polystyrene with thiol functionality (PS–SH) and poly(ethylene glycol) with alkyne functionality (PEG–acetylene) were independently prepared. As the first step of the grafting onto process, PS–SH was thiol–ene clicked onto the core to yield PS–N₃–OH. The second arm was then incorporated onto the core by the Ring Opening Polymerization (ROP) of l-(?)-Lactide (LA) using as PS–N₃–OH initiator and tin(II) 2-ethylhexanoate as catalyst. Finally, alkyne–PEG–acetylene was bonded to the resulting PLA–PS–N₃ using CuAAC click reaction. All intermediates, related polymers at different stages and final PS–PLA–PEG miktoarm star copolymer were characterized by ¹H NMR, FT-IR, SEC and DP-MS analyses. Direct pyrolysis mass spectrometry, (DP-MS) analyses of PS–PLA–PEG and all intermediate polymers indicated that the decomposition of PS and PEG chains occurred almost independently, following the degradation mechanisms of the corresponding homopolymers. On the other hand, during the pyrolysis of PS–PLA–PEG, elimination of H₂O during the decomposition of PEG chains at the early stages of pyrolysis caused hydrolysis of PLA chains and increased the yields of CO₂, CO and units involving unsaturation and/or crosslinked structure.     

Synthesis and characterization of well-defined poly(tert-butyl acrylate) star polymers

Star polymers with different numbers and lengths of poly(tert-butyl acrylate) (PTBA) arms were obtained via atom transfer radical polymerization. Aliphatic alcohols with different number of hydroxyl groups varying from 3 to 6 and calix[4]arenes based on pyrogallol with 12 and 16 phenol groups were transformed to bromoester derivatives to prepare multifunctional ATRP initiators used as the cores of the stars. The star polymers were characterized by GPC with refractive index, multiangle laser light scattering and viscosimetric detectors. The molar masses of the stars reached 70,000 g/mol and the molar mass dispersities did not exceed 1.2. To elucidate the compact structure of the stars, their true molar masses were determined by GPC with triple detection (RI-MALLS-Visco) and compared with the apparent molar masses obtained from the calibration with linear poly(tert-butyl acrylate) standards. The intrinsic viscosities of the PTBA stars of the same molar mass decreased with the number of star arms but were always lower than the intrinsic viscosities of the analogue linear PTBA polymers. The values of the branching ratio g′ decreased with increasing number of arms indicating more compact structure of stars. The branching ratio g′ was correlated to the empirical predictions.     

Neutron Scattering Studies of Structure and Self-Assembly of Star-Shaped Polymers with Fullerene Centres in Solutions

Protonated star-shaped polystyrenes with single and double fullerene C₆₀ core and the hybrid stars with pairs of polar and non-polar arms (tertbuthylmetacrylate, polystyrene) have been studied in deuterated toluene (20 °C) by small-angle neutron scattering at low and moderate polymer concentrations (c₁ ∼ 1 g/dl, c₂ ∼ 3–6 g/dl) to evaluate the peculiarities of fullerene centre action on polymers self-assembly in solutions. As we found, the cores composed of two fullerenes, linked via Si(CH₃)₂-bridge, induce stars' anisotropic interactions and association into chain-like structures (correlation radius ∼400–600 nm). Meanwhile, the single-core stars of polystyrene and hybrids organize globular clusters (size ∼ 10³ nm) those geometry do not change significantly by polymer content variation.