Showing papers on "Copolymer published in 1999"

Development of a Universal Alkoxyamine for “Living” Free Radical Polymerizations

Abstract: Examination of novel alkoxyamines has demonstrated the pivotal role that the nitroxide plays in mediating the “living” or controlled polymerization of a wide range of vinyl monomers. Surveying a variety of different alkoxyamine structures led to α-hydrido derivatives based on a 2,2,5-trimethyl-4-phenyl-3-azahexane-3-oxy, 1, skeleton which were able to control the polymerization of styrene, acrylate, acrylamide, and acrylonitrile based monomers. For each monomer set, the molecular weight could be controlled from 1000 to 200 000 amu with polydispersities typically 1.05−1.15. Block and random copolymers based on combinations of the above monomers could also be prepared with similar control. In comparison with 2,2,6,6-tetramethylpiperidinoxy (TEMPO), these new systems represent a dramatic increase in the range of monomers that can be polymerized under controlled conditions and overcome many of the limitations associated with nitroxide-mediated “living” free radical procedures. Monomer selection and functional...

Self-Assembly of Ordered Microporous Materials from Rod-Coil Block Copolymers

Abstract: Rod-coil diblock copolymers in a selective solvent for the coil-like polymer self-organize into hollow spherical micelles having diameters of a few micrometers. Long-range, close-packed self-ordering of the micelles produced highly iridescent periodic microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes whose diameter, periodicity, and wall thickness depended on copolymer molecular weight and composition. Addition of fullerenes into the copolymer solutions also regulated the microstructure and optical properties of the microporous films. These results demonstrate the potential of hierarchical self-assembly of macromolecular components for engineering complex two- and three-dimensional periodic and functional mesostructures.

Giant Wormlike Rubber Micelles

Abstract: A low molecular weight poly(ethyleneoxide)-poly(butadiene) (PEO-PB) diblock copolymer containing 50 weight percent PEO forms gigantic wormlike micelles at low concentrations (<5 percent by weight) in water. Subsequent generation of free radicals with a conventional water-based redox reaction leads to chemical cross-linking of the PB cores without disruption of the cylindrical morphology, as evidenced by cryotransmission electron microscopy and small-angle neutron scattering experiments. These wormlike rubber micelles exhibit unusual viscoelastic properties in water.

Thermoreversible Gelation of PEG−PLGA−PEG Triblock Copolymer Aqueous Solutions

Abstract: Aqueous solutions of new biodegradable triblock copolymers, poly(ethylene glycol-b-(dl-lactic acid-co-glycolic acid)-b-ethylene glycol) (PEG−PLGA−PEG), have shown to have sol-to-gel (lower transiti...

Combinatorial screening of complex block copolymer assembly with self-consistent field theory

Abstract: We describe a numerical implementation of self-consistent mean field theory for block copolymers that requires no assumption of the mesophase symmetry. The method is amenable to massively parallel screening of self-assembly in complex block copolymer systems, characterized by high-dimensional parameter spaces. We illustrate the method by screening for novel mesophases in linear $\mathrm{ABCA}$ tetrablock copolymers; several previously unreported mesophases have been identified. Extension to multicomponent systems, such as blends of block copolymers and homopolymers, is straightforward.

Morphological phase diagram for a ternary system of block copolymer ps310-b-paa52/dioxane/h2o

Abstract: The partial ternary phase diagram was investigated for the polystyrene310-block-poly(acrylic acid)52 copolymer in dioxane/water mixtures in regions in which self-assembled nanoaggregates of various morphologies are seen. Both fractionated and unfractionated copolymers were used; the unfractionated copolymer contains homopolystyrene. The study was carried out over the range of water contents from 0 to 45 wt % and copolymer concentrations from 0.1 to 10 wt %. Freeze-drying transmission electron microscopy (TEM), turbidity measurements, as well as static and dynamic light scattering were employed. Because of the proximity of the melting points and boiling points of water and dioxane, quenching and subsequent freeze-drying of solution samples can be employed to preserve aggregate morphologies. The morphologies can then be observed using TEM. The reversibility of various morphological transitions was examined by means of TEM and turbidity measurements. With increasing water content, the sequence of copolymer s...

Atom transfer radical polymerization of 2-hydroxyethyl methacrylate

Abstract: Atom transfer radical polymerization (ATRP) has been used to directly prepare linear pHEMA of controlled molecular weight and low polydispersity. Reaction conditions were adjusted to successfully polymerize the functional, polar methacrylate monomer. These adjustments included the use of a mixed solvent system consisting of methyl ethyl ketone and 1-propanol, lowering the temperature to 50 °C or less and using an alkyl bromide initiator with a copper chloride catalyst. The Cu/2,2‘-bipyridyl (bpy) complex, normally a heterogeneous mixture in nonpolar, organic solvents, was completely soluble in the above solvent system. A block copolymer was prepared with a poly(methyl methacrylate) macroinitiator. The monomer can also be protected and polymerized under conditions very similar to those used for the ATRP of methyl methacrylate.

Molecular and Mesoscopic Structures of Transparent Block Copolymer-Silica Monoliths

Abstract: Mesoscopically ordered, transparent silica-surfactant monoliths have been prepared using amphiphilic triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymer species to organize polymerizing silica networks. The block copolymer acts as a structure- directing agent, as the aqueous silica cations partition within the hydrophilic regions of the self-assembled system and associate preferentially with the PEO blocks. Subsequent polymerization of the silica precursor species under strongly acidic conditions (pH 1) produces a densely cross-linked silica network that may be mesoscopically organized by the block copolymer species into composites with characteristic ordering length scales of >10 nm. When this is accompanied by slow evaporation of the aqueous solvent, such composite mesostructures can be formed into transparent and crack-free monoliths (e.g., 2.5 cm diameter 3 mm thick). Distributions and dynamics of the PEO and PPO copolymer blocks within the silica matrix were investigated in situ using 29 Si{ 1 H} and 13 C{ 1 H} two-dimensional solid-state heteronuclear correlation NMR techniques and 1 H NMR relaxation measurements. Mesostructural ordering was determined by X-ray diffraction and transmission electron microscopy. The degree of microphase separation and the resulting mesostructure of bulk samples were found to depend strongly upon the concentration of block copolymer, with higher concentrations producing higher degrees of order.

Elastomeric Nanofibers of Styrene-Butadiene-Styrene Triblock Copolymer

Abstract: Nanofibers of a commercial styrene-butadiene-styrene triblock copolymer were electrospun from solution, and collected either as a nonwoven elastomeric fabric, or on a layer of graphite that was evaporated onto a glass microscope slide. The resulting nanofibers were elastic, birefringent, and most had diameters around 100 nm. A few thin, beaded fibers were found among the smooth nanofibers. The diameter of the fibers between the beads was as small as 3 nm. After staining with osmium tetroxide, the nanofibers were examined using transmission electron microscopy. Separated phases of styrene and butadiene blocks were observed. The single-phase domains were irregular in shape, but elongated along the axis of the fiber. Wide-angle X-ray diffraction patterns showed a weak indication of molecular orientation along the fiber axis, and the birefringence confirmed that such orientation was present. The single-phase domains grew larger in nanofibers that were held at room temperature (∼ 25 °C) for several days. Annealing at a temperature 70 °C greatly accelerated the growth of the single-phase domains. The nanofibers softened and flattened on the evaporated graphite during annealing.

Novel macromolecular architectures based on aliphatic polyesters : Relevance of the coordination-insertion ring-opening polymerization

Abstract: Recent developments in the macromolecular engineering of aliphatic polyesters have been overviewed. First, aluminum alkoxides mediated living ring opening polymerization (ROP) of cyclic (di)esters, i.e., lactones, lactides, glycolide, is introduced. An insight into this so-called “coordination-insertion” mechanism and the ability of this living polymerization process to prepare well-defined homopolymers, telechelic polymers, random and block copolymers is then discussed. In the second part, the combination of the living ROP of (di)lactones with other well-controlled polymerization mechanisms such as anionic, cationic, free radical, and metathesis polyadditions of unsaturated comonomers, as well as polycondensations, is reported with special emphasis on the design of new and well-tailored macromolecular architectures. As a result of the above synthetic breakthrough, a variety of novel materials have been developed with versatile applications in very different fields such as biomedical and microelectronics.

Core-Polymerized Reactive Micelles from Heterotelechelic Amphiphilic Block Copolymers

Abstract: Amphiphilic poly(ethylene glycol)-b-polylactide (PEG/PLA) copolymers with an aldehyde group at one end and a methacryloyl group at the other chain end were synthesized by anionic polymerization. The efficiencies of the functionalization at both ends were almost quantitative. The amphiphilic block copolymers formed micelles in aqueous media. Acetal groups on the micelle surface were quantitatively converted to aldehyde groups by an acid treatment. The end methacryloyl group located in the core of the micelle was polymerized effectively to form core−shell-type nanoparticles having reactive aldehyde groups on the surface. The size of the reactive nanoparticle was 20−30 nm which was constant with temperatures up to 60 °C. The stability of the micelle was also confirmed by a sodium dodecyl sulfate (SDS) treatment. When SDS was added to the nanosphere solution to 20 mg/mL, the particle was not collapsed. The particle was stable enough even in organic solvents. This functionalized micelle having high stability i...

Fluorene copolymers and devices made therefrom

Abstract: This invention relates to: (a) fluorene copolymers comprising at least 10 percent (by residual monomeric units) of 9-substituted and/or 9,9-disubstituted fluorene moieties and at least two other monomeric units containing delocalized p-electrons, (b) polymer blends comprising at least 10 weight percent of a copolymer specified in (a), and (c) electronic devices (such as polymer light emitting diodes) containing one or more films derived from these copolymers.

Viscoelastic Behavior of Semidilute Solutions of Multisticker Polymer Chains

Abstract: Multisticker polymer chains consisting of water-soluble polyacrylamides hydrophobically modified with low amounts of N,N-dihexylacrylamide have been prepared by a free radical micellar polymerization technique. This process gives multiblock copolymers in which the number and length of the hydrophobic blocks can be tuned by varying the surfactant over hydrophobe molar ratio. The viscoelastic behavior of semidilute solutions of various series of copolymers with variable molecular weights (Mw ≈ 4.2 × 104−2.7 × 106), hydrophobe contents ([H] = 0.5−2 mol %), and hydrophobic block lengths (NH = 1−7 units per block) has been investigated as a function of polymer concentration, C, using steady-flow and oscillatory experiments. In the semidilute range, two different regimes can be clearly distinguished in the zero-shear viscosity η0 = f(C) curves: a first unentangled regime where the viscosity increase rate strongly depends on NH and [H]; a second entangled regime where the viscosity follows a scaling behavior of...

Propylene olefin copolymers

Abstract: Improved thermoplastic polymer blend compositions comprising an isotactic polypropylene component and an alpha-olefin and propylene copolymer component, said copolymer comprising crystallizable alpha-olefin sequences. In a preferred embodiment, improved thermoplastic polymer blends are provided comprising from about 35% to about 85% isotactic polypropylene and from about 30% to about 70% of an ethylene and propylene copolymer, wherein said copolymer comprises isotactically crystallizable propylene sequences and is predominately propylene. The resultant blends manifest unexpected compatibility characteristics, increased tensile strength, and improved process characteristics, e.g., a single melting point.

Nitroxide-controlled free-radical copolymerization of vinyl and divinyl monomers. 2. Gelation

Abstract: Some important features of the gel prepared by the nitroxide-mediated free-radical copolymerization of styrene with a small amount of 4,4‘-divinylbiphenyl were studied. Owing to the “living” character of this system, the cross-linking reaction in this system proceeds highly homogeneously (randomly) without forming any microgels. The gel prepared by this method showed remarkable differences from the one prepared in the conventional system, in regard to the relative fraction and swelling ratio of the gel studied as a function of monomer conversion. It was also found that the critical number density of cross-links at the gel point agreed with the mean-field theoretical value of Flory and Stockmayer (FS) within a factor 2. This value was exceptionally small compared with those previously reported for the conventional free-radical systems, which are typically 1 or 2 orders of magnitude larger than the FS value. These results, along with the previous study on the reactivity of the pendant vinyl of this system (...

Atom Transfer Radical Polymerization in Supercritical Carbon Dioxide

Abstract: Atom transfer radical polymerization (ATRP) has been successfully carried out in supercritical carbon dioxide (scCO2) for the polymerization of fluorinated (meth)acrylates. In particular, well-controlled polymerizations have been obtained with the use of a fluoroalkyl-substituted 2,2‘-bipyridine ligand. Block copolymers comprised of fluorinated (meth)acrylates and poly(MMA) (MMA = methyl methacrylate) or poly(DMAEMA) (DMAEMA = 2-(dimethylamino)ethyl methacrylate) were produced in scCO2 by ATRP. In addition, the dispersion polymerization of MMA using ATRP in scCO2 in the presence of a fluorinated polymeric surfactant stabilizer was successfully carried out to yield poly(MMA) latex particles with controlled molecular weight and a narrow molecular weight distribution.

Preparation and mechanical properties of polystyrene–clay hybrids

Abstract: Polystyrene–clay hybrids (PSCHs) were prepared by melt blending a styrene vinyloxazoline copolymer with organophilic clay. In the PSCHs, the silicate layers of the clay were delaminated and dispersed homogeneously to the nanometer level. The moduli of the PSCHs were higher than that of the PS copolymer. For example, the tensile modulus of the PSCH with 5 wt % clay was 1.4 times higher compared to that of the PS copolymer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3359–3364, 1999

Preparation of Platinum Colloids on Polystyrene Nanospheres and Their Catalytic Properties in Hydrogenation

Abstract: Well-dispersed platinum colloids have been prepared on polystyrene nanospheres with surface-grafted poly(N-isopropylacrylamide) (PNIPAAm) via the reduction of PtCl62- by ethanol. A range of monodisperse nanospheres was obtained by emulsifier-free dispersion copolymerization of styrene with a PNIPAAm macromonomer. The surface compositions of the polystyrene nanospheres before and after immobilization of Pt colloids have been investigated by X-ray photoelectron spectroscopy. Peak fitting of the C 1s core-line spectra provides evidence for the presence of PNIPAAm chains at the surface of polystyrene nanospheres. These immobilized Pt colloids were found to be active and stable heterogeneous catalysts for the hydrogenation of allyl alcohol in water. Both the particle size and activity of platinum colloids showed a marked dependence on the level of surface PNIPAAm. The average diameter of an immobilized Pt colloid is 15.0 A and largely unchanged after seven cycles in hydrogenation, as confirmed by TEM. The immo...

A new copolymerization process leading to poly(propylene carbonate) with a highly enhanced yield from carbon dioxide and propylene oxide

Abstract: Using excessively loaded propylene oxide (PO) as a solvent, the copolymerization of carbon dioxide (CO2) and PO was carried out with zinc glutarate catalyst, consequently producing poly(propylene carbonate) of high molecular weight in a high yield (64–70 g polymer per gram of catalyst) never achieved before. Both the PO used as solvent and the excessively loaded CO2 were fully recoverable, respectively, and reusable for their copolymerization, indicating that this is a clean, green polymerization process to convert CO2 to its polycarbonate. The polymer yield was further improved by scaling up the copolymerization process. Among zinc glutarate catalysts prepared through several synthetic routes, one from zinc oxide delivered the highest yield in the copolymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1863–1876, 1999

Core-shell gyroid morphology in a poly(isoprene-block-styrene-block- dimethylsiloxane) triblock copolymer

Abstract: We report a new morphology in a poly(isoprene-block-styrene-block-dimethylsiloxane) ABC triblock copolymer. The most probable symmetry of the morphology (Ia3d) was determined from two-dimensional SAXS measurements performed on a sample with long-range order induced by an oscillatory shear field. By comparing the predictions of self-consistent field theory with TEM and SAXS and SANS powder diffraction patterns, we deduced that the morphology is a core−shell gyroid comprised of two independent and triply periodic poly(dimethylsiloxane) networks encased in poly(styrene) shells and separated by a continuous poly(isoprene) domain. Thus, this unique nanostructure divides space into five independent, three-dimensionally continuous domains.

Starlike Block Copolymers with Amphiphilic Arms as Models for Unimolecular Micelles

Abstract: In this paper, we describe the synthesis of novel, controlled starlike molecular architectures with six and twelve amphiphilic arms, respectively. These molecules form unimolecular micelles in solution, which respond in a unique fashion to the polarity of the solvent by changing their molecular geometry. Certain amphiphilic block copolymers form micelles in solvents which are selective for either one block or the other. 1-5 These represent supermolecular self-assembled structures composed of individual amphiphiles in a dynamic equilibrium, i.e., exchange of individual components between the micelles and/or the solution occurs readily. The organized structure of polymeric micelles may be stabilized by intramolecular cross-linking. 6 On the other hand, unimolecular micelles cannot participate in such dynamic equilibria since the individual amphiphiles are covalently fixed. Unimolecular micelles have been reported from certain graft copolymers. 7 However, to mimic the fundamental parameters associated with traditional micelles, a very controlled polymeric molecular architecture is required. This is provided, for example, with dendrimers, which display controlled, three-dimensional branched structures. A variety of amphiphilic dendritic unimolecular micelles have been described in the literature. 8-11 Multiarm, starlike block copolymers, on the other hand, present the opportunity for a new class of unimolecular micelles. Fre ́ch t et al. have integrated both approaches by endcapping a hydrophilic 4-arm poly(ethylene glycol) star polymer with hydrophobic dendritic blocks. 12 Our approach to preparing unimolecular micelles involves the design of novel starlike block copolymers with six and twelve arms, respectively. To produce these architectures, we have utilized atom transfer radical polymerization (ATRP)13,14 in the consecutive polymerization of tert-butyl acrylate (tBA) and methyl methacrylate (MMA) from the branched initiators shown in Figure 1. Subsequent selective deprotection of the acrylate block with trifluoroacetic acid leads to a structure with a poly(acrylic acid) (PAA) hydrophilic block, and PMMA as the hydrophobic component. We recently reported the synthesis and versatility of these starlike initiators in the controlled polymerization of MMA homo and random copolymers. 15,16 Although other research groups have recently reported the synthesis of star-polymers by ATRP techniques, 17,18 to our knowledge this work represents the first preparation of unimolecular micelles using ATRP techniques. The characteristics of the macroinitiators and the block copolymers are summarized in Table 1. 19,20When run to less than 60-70% conversion, both 6and 12-arm PtBAs have a polydispersity<1.1. The molecular weight determined by 1H NMR from the ratio of integrated peak areas of the aromatic signals of the initiator core and thetert-butyl ester groups is about 7000 g/mol for the 6-arm polymer (DP∼ 12 per arm) and 16500 for the 12-arm polymer (DP∼ 14 per arm), respectively. By using these functionalized PtBA macroinitiators for the subsequent polymerization of MMA, starlike block copolymers could be obtained. The molecular weight of the PMMA block was determined to be 43000 g/mol (6-arm) and 58000 g/mol (12-arm) by 1H NMR analysis of the ratio of the integrated peak areas from the methyl ester groups relative to the initiator cores. Figure 2 shows the GPC traces of both the 6-arm macroinitiators and the resulting block copolymers. The polydispersities of the starlike block copolymers are<1.3 for both examples. Quantitative deprotection of the PtBA block with trifluoroacetic acid in methylene chloride results in the desired amphiphilic block copolymers. By changing the order of monomer addition in the polymerization, the reversed block copolymers, i.e., hydrophobic core with a hydrophilic shell, were obtained with similarly low polydispersities (Table 1, Figure 2). Preliminary1H NMR investigations show that either block can be selectively collapsed by changing solvent as indicated schematically in Figure 3. Unfortunately for the protiated monomers, the signals of the main chain protons of the PMMA block ( δ 0.5-2 ppm) overlap those of the PAA block. Consequently, changes in the micellar molecular geometry cannot be determined directly from these overlapping proton signals. However, the aromatic signals of the initiator core are easily monitored by 1H NMR. This provides an indirect reporter for the solvation of the block directly attached to the core. In a nonselective solvent like acetone which solvates both blocks, the signals of the initiator † IBM Research Center. ‡ Stanford University. (1) Munk, P.; Prochazka, K.; Tuzar, Z.; Webber, S. E. Chemtech1998, October, 20-28. (2) Halperin, A.Macromolecules1991, 24, 1418-1419. (3) Ma, Y.; Cao, T.; Webber, S. E. Macromolecules1998, 31, 1773-1778. (4) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168-3181. (5) Pispas, S.; Poulos, Y.; Hadjichristidis, N. Macromolecules1998, 31, 4177-4181. (6) Thurmond, K. B., II; Kowalewski, T.; Wooley. K. L. J. Am. Chem. Soc.1997, 119, 6656-6565. (7) Kikuchi, A.; Nose, T.Macromolecules1996, 29, 6770-6777. (8) Hawker, C. J.; Wooley, K. L.; Fre ́chet, J. M. J.J. Chem. Soc., Perkin Trans. 11993, 1287. (9) Newkome, G. R.; Morrefield, C. N.; Baker, G. R.; Saunders, M. J.; Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1178-1180. (10) Tomalia, D. A.; Berry, V.; Hall, M.; Hedstrand, D. M. Macromolecules 1987, 20, 1164-1167. (11) Stevelmans, S.; van Hest, J. C. M.; Jansen, J. F. G. A.; Van Boxtel, D. A. F. J.; de Brabander-van den Berg, E. M. M.; Meijer, E. W. J. Am. Chem. Soc. 1996, 118, 7398-7399. (12) Gitsov, I.; Fre ́chet, J. M. J.J. Am. Chem. Soc. 1996, 118, 37853786. (13) Patten, T. E.; Matyjaszewski, K. AdV. Mater. 1998, 10, 901-915. (14) Matyjaszewski, K. Controlled Radical Polymerization ; Matyjaszewski, K., Ed.; ACS Symp. Ser. No. 685; American Chemical Society: Washington, D.C., 1998. (15) Heise, A.; Hedrick, J. L.; Trollsås, M.; Miller, R. D.; Frank, C. W. Macromolecules1999, 32, 231-234. (16) Heise, A.; Hedrick, J. L.; Trollsås, M.; Miller, R. D.; Frank, C. W. Polym. Prepr.1998, 39, 627. (17) Angot, S.; Murthy, K. S.; Tanton, D.; Gnanou, Y. Macromolecules 1998, 31, 7218-7225. (18) Ueda, J.; Kamigaito, M.; Sawamoto, M.acromolecules1998, 31, 6762-6768. (19) Hedrick, J. L.; Trollsa ̊s, M.; Hawker, C. J.; Atthoff, B.; Claesson, H.; Heise, A.; Miller, R. D.; Mecerreyes, D.; Je ́rôme, R.; Dubois, Ph. Macromolecules1998, 31, 8691. (20) Coca, S.; Davis, K.; Miller, P.; Matyjaszewski, K. Polym. Prepr.1997, 38, 689-690. Figure 1. 6(left) and 12-arm (right) atom transfer radical polymerization (ATRP) initiators. 8647 J. Am. Chem. Soc. 1999,121,8647-8648

Sugar-Installed Polymer Micelles: Synthesis and Micellization of Poly(ethylene glycol)−Poly(d,l-lactide) Block Copolymers Having Sugar Groups at the PEG Chain End

Abstract: A poly(ethylene glycol)−poly(d,l-lactide) block copolymer (PEG−PLA) having a site specifically protected-sugar group at the PEG chain end was synthesized through a successive ring-opening polymerization of ethylene oxide and d,l-lactide using a metalated protected sugar as an initiator. Removal of protective groups from the sugar residue in the block copolymer was quantitatively carried out using 80% trifluoroacetic acid at room temperature, yielding a block copolymer having a glucose or galactose residue at the chain end in a regioselective manner. Polymer micelles having sugar residues on the surface were then prepared by dialyzing an N,N-dimethylacetamide solution of the sugar-bearing PEG−PLA block copolymer against water. Dynamic light-scattering measurement of the polymer micelle solution revealed that the scaled characteristics line width had essentially no angular dependence, consistent with the spherical geometry of the polymer micelle. The diameter and polydispersity index of the polymer micelle,...