Well-defined Molecular Weight Research Articles

Phase Behavior and Temperature-Responsive Molecular Filters Based on Self-Assembly of Polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene

This work describes the synthesis of temperature-responsive polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene triblock copolymers, i.e., PS-b-PNIPAM-b-PS, their self-assembly and phase behavior in bulk, and demonstration of aqueous thermoresponsive membranes. A series of PS-b-PNIPAM-b-PS triblock copolymers were synthesized using reversible addition−fragmentation chain transfer (RAFT) polymerization. The hydrophobic PS end blocks were selected to form the minority component, whereas the temperature-responsive PNIPAM midblock accounted for the majority component. The self-assembly and phase behavior in bulk of PS-b-PNIPAM-b-PS as well as selected blends with low molecular weight PNIPAM homopolymers were studied using transmission electron microscopy (TEM). Classical lamellar, cylindrical, spherical, and bicontinuous double gyroid morphologies were observed in the dried state. In aqueous solutions, the glassy PS domains act as physical cross-links, and hydrogels were therefore formed. The bulk block copolymer morphology had a strong effect on the degree of swelling in aqueous solutions upon cooling below the coil−globule transition temperature of the PNIPAM midblock. Bulk compositions with spherical PS domains and PNIPAM continuous phase swelled in water up to 58 times by weight, whereas composition having cylindrical PS domains or bicontinous gyroid structure in bulk swelled 20 or 10 times by weight, respectively. Finally, lamellar compositions did not show any swelling. Composite membranes for separation studies were prepared by spin-coating thin films of PS-b-PNIPAM-b-PS on top of meso/macroporous polyacrylonitrile (PAN) support membrane. The permeability was measured as a function of temperature using aqueous mixture of poly(ethylene glycol) (PEG) with several well-defined molecular weights. The permeability showed a temperature switchable on/off behavior, where higher permeability is obtained below transition temperature of PNIPAM, and the molecular cutoff limits for the PEG molecules are surprisingly lowbetween 108 and 660 g/mol. The results encourage to further develop and optimize these materials for responsive nanofiltration applications.

Novel Poly(phenylene vinylenes) with Well-Defined Poly(ε-caprolactone) or Polystyrene as Lateral Substituents: Synthesis and Characterization

New macromonomers containing dialdehyde functionalities placed at the middle or at the end of the chains were synthesized in two reaction steps. First, using ring-opening polymerization (ROP) of ε-caprolactone (CL) or atom transfer radical polymerization (ATRP) of styrene (St) in the presence of proper initiators provided well-defined low molecular weight polymers with dibromobenzene moieties. In the second step, using Suzuki couplings of these dibromobenzene functions with 4-formylphenyl boronic acid, macromonomers having 4,4‘-dicarbaldehyde terphenyl moieties were obtained. Poly(phenylenevinylene)s (PPVs), with lateral subtituents PSt or PCL chains respectively, were synthesized by following a Wittig polycondensation in combination with bis(triphenylphosphonium) salts in the presence of potassium tert-butoxide. The resulting PPVs were soluble in common organic solvents at room temperature as were the starting macromonomers. All of the starting and intermediate polymers and the final PPVs were characterized by using 1H and 13C NMR, IR, GPC, DSC, and TGA measurements. Optical properties of the polymers were followed by UV and fluorescence spectroscopy. The studied PPVs show a blue or green fluorescence in solution.

Organic Spirocyclic Initiators for the Ring-Expansion Polymerization of β-Lactones

We report spirocyclic imidazolidines derived from N-heterocyclic carbenes and β-lactones. These spirocycles are initiators for the zwitterionic ring-expansion polymerization of β-lactones to generate cyclic polyesters of well-defined molecular weight. The zwitterionic strategy based on the equilibrium between the zwitterionic intermediates and spiro macrocyclics enables the controlled synthesis of cyclic poly(β-lactone)s

Amphiphilic Model Conetworks of Polyisobutylene Methacrylate and 2-(Dimethylamino)ethyl Methacrylate Prepared by the Combination of Quasiliving Carbocationic and Group Transfer Polymerizations

A series of amphiphilic polymer conetworks (APCN) of the hydrophobic polyisobutylene methacrylate macromonomer (PIBMA) and the ionizable hydrophilic 2-(dimethylamino)ethyl methacrylate (DMAEMA) were synthesized by combining, for the first time, two controlled polymerization methods, quasiliving carbocationic polymerization (QLCCP) and group transfer polymerization (GTP). First, isobutylene (IB) was polymerized by QLCCP, and the resulting IB 7-mer (PIB) was modified to yield PIBMA with exact chain end-functionality of 1.0. Subsequently, PIBMA was successfully polymerized sequentially by GTP with DMAEMA (comonomer) and ethylene glycol dimethacrylate (EGDMA, cross-linker) using a bifunctional GTP initiator, 1,4-bis(methoxytrimethylsiloxymethylene)cyclohexane. Amphiphilic conetworks with linear segments of well-defined molecular weights (model conetworks) and a randomly cross-linked conetwork with linear segments characterized by a broad size distribution between the cross-links were prepared. Four of the mod...

Effects to Improve the Quality of Life: Color, Performance and Protection from Ciba Specialty Chemicals

Ciba Specialty Chemicals creates effects to improve the quality of life. Building on its technical competencies, the company has developed major innovative product groups that provide color, performance and protection. Antioxidants and light stabilizers protect plastics, coatings, photographic pictures and wood from degradation induced by sunlight, air, ozone, heat, fire and mechanical stress. UV absorbers protect human skin from sunburn and photo-induced ageing. Pigments and dyes add beautiful colors to objects of everyday life, serve as recording media for optical information storage on digital versatile discs (DVDs) and are used as color filters in liquid crystal displays (LCDs). Effect pigments allow creation of color and visual effects by interference. Polymeric flocculants are essential components for solid–liquid separation processes in wastewater treatment and paper production. Photolatent catalysts are key components for light-induced polymerization reactions. New sterically hindered N-alkoxyamines are initiators for nitroxide mediated radical polymerization (NMP) reactions leading to new pigment dispersants with well-defined molecular weights and narrow molecular weight distribution. New N-acyloxyamines are proposed as peroxide substitutes for controlled degradation of high molecular weight polypropylene. A new clarifier can be used to produce polypropylene with enhanced transparency and excellent mechanical properties. Progress in enabling sciences such as nanotechnology and biotechnology together with new technologies entering the markets will offer outstanding opportunities for new developments in specialty chemicals.

Grafting polymers onto carbon black surface by trapping polymer radicals

Polystyrene, poly(styrene- co-maleic anhydride), poly[styrene- co-(4-vinylpyridine)] and poly(4-vinylpyridine) with well-defined molecular weights and polydispersities were synthesized using 4-hydroxyl-2,2,6,6-tetramethylpiperidin-1-oxyl (HTEMPO)-mediated radical polymerization initiated by azobisisobutyronitrile or benzoyl peroxide. The resultant polymers were grafted onto carbon black surface through a radical trapping reaction at 130 °C in DMF. 1H NMR, TGA, TEM, AFM, DSC and dynamic light scattering were used to characterize the carbon black grafted with polymers. It was found that the carbon black grafted with polystyrene and poly(styrene- co-maleic anhydride) could be dispersed in THF, chloroform, dichloromethane, DMF, etc., and the carbon black grafted with poly(4-vinylpyridine) and poly[styrene- co-(4-vinylpyridine)] could be well dispersed in ethanol.

High Molecular Weight Poly(butyl methacrylate) by Reverse Atom Transfer Radical Polymerization in Miniemulsion Initiated by a Redox System

The reverse atom transfer radical polymerization of butyl methacrylate in miniemulsion, initiated with the redox pair hydrogen peroxide/ascorbic acid and mediated with copper(II) bromide tris[2-di(2-ethylhexyl acrylate)aminoethyl]amine, produced high molecular weight poly(butyl methacrylate) (Mn = 989 900, PDI = 1.25). The miniemulsion was carried out with 15% solids (based on 100% conversion), 10 wt % of the nonionic surfactant Brij 98 (based on monomer), and 3.8 wt % hexadecane (based on monomer), with the final weight-average particle diameter less than 110 nm. The use of the redox pair to initiate the polymerization also facilitated a relatively fast rate of polymerization. The polymerizations were carried out at 60 °C and typically reached their maximum conversion (with degrees of polymerization up to 6900) of ∼80% in only 8 h. Additionally, the polymerization of methyl methacrylate produced well-defined high molecular weight polymers with a controlled degree of polymerization and narrow molecular we...

Synthesis, characterization and application of donor-acceptor block copolymers in nanostructured bulk heterojunction solar cells

The synthesis and properties of two semiconducting donor-acceptor (D-A) block copolymers and their application in nanostructured bulk heterojunction solar cells are reported. The donor segments were obtained via nitroxide mediated polymerization of either bis(4-methoxyphenyl)-4'-vinylphenylamine or N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-[1,1'biphenyl]-4,4'-diamine. Narrow-distributed macroinitiators, poly{bis(4-methoxyphenyl)-4'-vinylphenylamine} (PvDMTPA) and poly{N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-[1,1'biphenyl]-4,4'-diamine} (PvDMTPD) were used to polymerize the acceptor monomer perylene diimide acrylate (PerAcr) to yield block copolymers with well-defined molecular weights. Different diblock copolymers, PvDMTPA-block-PPerAcr and PvDMTPD-block-PPerAcr, with high perylene diimide weight fractions were prepared. The block copolymers exhibited efficient fluorescence quenching. Transmission electron microscopy revealed wire-like and worm-like nanostructures throughout bulk samples. Thin film photovoltaic devices showed short circuit currents of up to 1.21 mA/cm2 and power conversion efficiencies η of 0.32% under AM 1.5 illumination conditions.

Soluble Poly(p‐phenylenevinylene)s through Ring‐Opening Metathesis Polymerization

Ring opening of substituted cyclophanedienes by ruthenium-based metathesis catalysts gives soluble phenylenevinylene homopolymers of well-defined molecular weight with an alternating cis-trans microstructure (see picture, R=OC8H17). The living character of this polymerization is demonstrated by controlled chain extension.

PBI-1402: A Low Molecular Weight Synthetic Hematopoietic Growth Stimulant.

PBI-1402 is a non-toxic, well-defined low molecular weight synthetic hematopoietic growth stimulant. PBI-1402 promotes the proliferation and maturation of hematopoietic progenitors (myeloid and erythroid populations) yielding a biological efficacy comparable to G-CSF, GM-CSF and EPO in in vitro human bone marrow cell proliferation and colony formation assays. An additive effect is observed when PBI-1402 is combined with G-CSF, GM-CSF and EPO. In human bone marrow colony assay, PBI-1402 enhances the differentiation of pluripotent stem cells: CFU-GEMM, CFU-GM with a predominant effect on BFU-E. Furthermore, PBI-1402 exerts its activity via a different mechanism of action than EPO and stem cell factor (SCF) and at an earlier stage on more immature hematopoietic progenitors. PBI-1402 is targeted as an adjunct to cancer chemo/radiotherapy, bone marrow transplantation and diseases involving neutropenia and anemia.

Molecular Cloning of Protein-Based Polymers

Protein-based biopolymers have become a promising class of materials for both biomedical and pharmaceutical applications, as they have well-defined molecular weights, monomer compositions, as well as tunable chemical, biological, and mechanical properties. Using standard molecular biology tools, it is possible to design and construct genes encoding artificial proteins or protein-based polymers containing multiple repeats of amino acid sequences. This article reviews some of the traditional methods used for constructing DNA duplexes encoding these repeat-containing genes, including monomer generation, concatemerization, iterative oligomerization, and seamless cloning. A facile and versatile method, called modules of degenerate codons (MDC), which uses PCR and codon degeneracy to overcome some of the disadvantages of traditional methods, is introduced. Re-engineering of the random coil spacer domain of a bioactive protein, WPT2-3R, is used to demonstrate the utility of the MDC method. MDC re-constructed coding sequences facilitate further manipulations, such as insertion, deletion, and swapping of various sequence modules. A summary of some promising emerging techniques for synthesizing repetitive sequence-containing artificial proteins is also provided.

Living random and block copolymerization of ethene and propene on a tailor-made phenoxyimine catalyst and characterization of the resulting high molecular weight PE-block-P(E-co-P) block copolymers

The phenoxyimine catalyst (bis-(N-(3′,5′-diiodo-salicylidene)-2,6-difluoroaniline)-titanium(IV)-dichloride) was tailored to enable random and block copolymerisation of propylene and ethylene with compositions covering the entire feasible composition range. Well-defined high molecular weight diblock copolymers of the type PE-block-P(E-co-P), consisting of a semi-crystalline polyethylene and a soft P(E-co-P) block, were prepared and evaluated with respect to propylene content and the block lengths. The characterisation by means of AFM, CRYSTAF data and high temperature chromatographic elution provided the experimental evidence that no homo-polyethylene and less than 5% of the random copolymer are formed as by-products.

Ultra high molar mass poly[2-(dimethylamino)ethyl methacrylate] via atom transfer radical polymerization

Well-defined high molecular weight poly[2-(dimethylamino)ethyl methacrylates] [poly(DMAEMA)s] with molar masses up to ∼1×10 6 g/mol were successfully synthesized via atom transfer radical polymerization (ATRP). This was achieved by using p-toluenesulfonyl chloride( p-TsCl)/CuCl/1,1,4,7,10,10-hexamethyl-triethylenetetramine(HMTETA) initiator/catalyst complex in methanol/water mixture. Well-controlled/‘living’ behavior was demonstrated throughout the reaction, up to high monomer conversion. The PDI value remained low at 1.26 even for a polymer with very high molecular weight at 1.1×10 6 g/mol. We believe this is the first successful case where controlled ATRP produces a polymer with molar mass exceeding a million!

Novel Monodisperse PEG−Dendrons as New Tools for Targeted Drug Delivery: Synthesis, Characterization and Cellular Uptake

Dendrimers, dendrons, and hyperbranched polymers are gaining popularity as novel drugs, imaging agents, and drug delivery systems. They present advantages of well-defined molecular weight, multivalent surfaces, and high drug carrying capacity. Moreover, it is emerging that such architectures can display unique endocytic properties. As poly(ethylene glycol) (PEG) is widely used for protein and drug conjugation, the aim of this study was for the first time to synthesize novel, branched PEG-based architectures, to define their cytotoxicity and, via preparation of Oregon green (OG) conjugates define the effect of structure on their cellular uptake. Five PEG-based dendrons were synthesized using monodisperse Fmoc-amino PEG propionic acid (M(w) = 840) as a monomer, and cadaverine, tris(2-aminoethyl)amine or lysine as the branching moieties. These were diamino,bisPEG (M(w) = 1300); triamino,trisPEG (Mw = 1946); tetraamino,tetraPEG (M(w) = 3956); monocarboxy,diamino,bisPEG (M(w) = 1346); and monocarboxy,tetraamino,tetraPEG (M(w) = 3999). These products had NH(2) or both NH(2) and COOH terminal groups and the identity was verified by amino group analysis and ESI-TOF mass spectroscopy. Purity was determined by HPLC. Representative structures were not toxic towards an endothelial-like cell line (ECV304) at concentrations up to 4 mg/mL (over 72 h). At 37 degrees C, all of the OG-labeled PEG dendrons showed progressive uptake by ECV304 cells, but tetraamino,tetraPEG showed the greatest rate of internalization over the first 20 min. Cellular uptake was inhibited at 4 degrees C, and PEG dendron localization to perinuclear vesicles was confirmed by fluorescence microscopy. These well-defined novel architectures have potential for further development as targetable drug delivery systems or tools for construction of structurally defined modified surfaces.

Interfacial Behavior of Anionically Synthesized Amphiphilic Star Block Copolymers Based on Polybutadiene and Poly(ethylene oxide) at the Air/Water Interface

We describe the preparation and the surface properties of monolayers of a new set of (PB-b-PEO)4 amphiphilic four-arm star block copolymers. A divergent anionic polymerization method yielded copolymers with well-defined molecular weights, block volume fractions, and architecture. Initially, tri- and tetracarbanionic initiators were used to initiate the polymerization of butadiene followed by end-capping of living chains with ethylene oxide. The tri- and tetrafunctionalities of the resulting hydroxyl-terminated star polydienes were confirmed by 1H NMR spectroscopy. The four-arm star polymeric alcohols were then titrated with diphenylmethylpotassium to yield the analogous potassium alkoxides. The resulting macroinitiators were used to polymerize corona blocks of ethylene oxide. Different samples of well-defined (PB-b-PEO)4 amphiphilic star block copolymers exhibiting narrow molar mass distribution were prepared with poly(ethylene oxide) over a range of volume fractions. Isotherm experiments at the air/water...

Polymer-layered silicate nanocomposites prepared through in situ reversible addition–fragmentation chain transfer (RAFT) polymerization

Reversible addition–fragmentation chain transfer (RAFT) polymerizations were performed in the presence of organically modified clays and successfully prepared polystyrene-, poly(methyl methacrylate)-, and poly(n-butyl acrylate)-layered silicate nanocomposites. The polymers had well-defined molecular weights and low polydispersities, as expected from RAFT polymerizations. The morphology of polystyrene-, and poly(n-butyl acrylate)-nanocomposites were found to be exfoliated using montmorillonite modified with N,N-dimethyl-n-hexadecyl-(4-vinylbenzyl) ammonium (MMT-VB16). In the case of PMMA nanocomposite, the structure was a mixture of intercalated and exfoliated when MMT-VB16 was used, while the use of montmorillonite modified with 2-methacryloyloxyethyl-hexadecyldimethyl ammonium (MMT-MA16) resulted in exfoliation.

Polysaccharide-Induced Order-to-Order Transitions in Lyotropic Liquid Crystals

We report on the order-to-order transitions of lyotropic liquid crystals formed by self-assembled monogylcerides and water in the presence of polysaccharides of various molecular weights. The phase diagram of monoglyceride-water-polysaccharide systems, their morphology, and the topology of liquid crystalline structures were determined by combining optical cross-polarization, oscillatory shear rheometry, and small-angle X-ray scattering. The presence of hydrophilic mono-, oligo-, and polysaccharides in the water domains of liquid crystalline phases resulted in a general decrease of the cubic-to-hexagonal transition temperature. Provided that the sugar could fit within the water channels, the decrease was observed to be dependent on the polysaccharide concentration but independent of its molecular weight. For isotropic bicontinuous cubic phases, monomeric sugars such as glucose were reported to shrink the lattice parameter of the structure without inducing phase transitions. However, when a polymeric form of glucose was used, such as dextran, transitions from the gyroidal Ia3d cubic phase to double diamond Pn3m cubic phases were observed at well-defined molecular weights of polysaccharide. These results were interpreted in terms of size exclusions of polymer sugars by the water domains of the liquid crystal phases as well as the different topologies of water channels. Molecular dynamics simulations of polysaccharides in the water environment were performed to support these findings.

Helical Structures ofN-Alkylated Poly(p-benzamide)s

Poly(p-benzamide)s 1 bearing a chiral side chain on the nitrogen atom were synthesized by chain-growth polycondensation methodology. The polyamides exhibited well-defined molecular weights with narrow polydispersities. Solutions of the polyamides in several organic solvents (CH(3)CN, CHCl(3), and CH(3)OH) showed dispersion type CD signals characteristic of coupled-oscillator and much larger as compared with the corresponding monomer. The CD signals were dependent on the temperature and molecular weight of the polyamides but independent of the solvent, as far as examined. An exciton model analysis of the absorption and CD spectra provided a clear-cut picture for the secondary structure of these polyamides in solution that the N-alkylated poly(p-benzamide)s possess a right-handed helical conformation ((P)-helix). In the solid states, the results of X-ray crystallographic analysis of 4-(methylamino)benzoic acid oligomers substantiated that they have a helical conformation with three monomer units per turn.

Unfolding, Aggregation, and Seeded Amyloid Formation of Lysine-58-Cleaved β2-Microglobulin

Beta(2)-microglobulin (beta(2)m) is the amyloidogenic protein in dialysis-related amyloidosis, but the mechanisms underlying beta(2)m fibrillogenesis in vivo are largely unknown. We study a structural variant of beta(2)m that has been linked to cancer and inflammation and may be present in the circulation of dialysis patients. This beta(2)m variant, DeltaK58-beta(2)m, is a disulfide-linked two-chain molecule consisting of amino acid residues 1-57 and 59-99 of intact beta(2)m, and we here demonstrate and characterize its decreased conformational stability as compared to wild-type (wt) beta(2)m. Using amide hydrogen/deuterium exchange monitored by mass spectrometry, we show that DeltaK58-beta(2)m has increased unfolding rates compared to wt-beta(2)m and that unfolding is highly temperature dependent. The unfolding rate is 1 order of magnitude faster in DeltaK58-beta(2)m than in wt-beta(2)m, and at 37 degrees C the half-time for unfolding is more than 170-fold faster than at 15 degrees C. Conformational changes are also reflected by a very prominent Congo red binding of DeltaK58-beta(2)m at 37 degrees C, by the evolution of thioflavin T fluorescence, and by changes in intrinsic fluorescence. After a few days at 37 degrees C, in contrast to wt-beta(2)m, DeltaK58-beta(2)m forms well-defined high molecular weight aggregates that are detected by size-exclusion chromatography. Atomic force microscopy after seeding with amyloid-beta(2)m fibrils under conditions that induce minimal fibrillation in wt-beta(2)m shows extensive amyloid fibrillation in DeltaK58-beta(2)m samples. The results highlight the instability and amyloidogenicity under near physiological conditions of a slightly modified beta(2)m variant generated by limited proteolysis and illustrate stages of amyloid formation from early conformational variants to overt fibrillation.

Biological Evaluation of Polyester Dendrimer: Poly(ethylene oxide) “Bow-Tie” Hybrids with Tunable Molecular Weight and Architecture

High molecular weight (MW) polymers have shown promise in terms of improving the properties and the efficacy of low MW therapeutics. However, new systems that are highly biocompatible, are biodegradable, have well-defined MW, and have multiple functional groups for drug attachment are still needed. The biological evaluation of a library of eight polyester dendrimer-poly(ethylene oxide) (PEO) bow-tie hybrids is described here. The group of evaluated polymers was designed to include a range of MWs (from 20000 to 160000) and architectures with the number of PEO arms ranging from two to eight. In vitro experiments revealed that the polymers were nontoxic to cells and were degraded to lower MW species at pH 7.4 and pH 5.0. Biodistribution studies with (125)I-radiolabeled polymers showed that the high MW carriers (>40000) exhibited long circulation half-lives. Comparison of the renal clearances for the four-arm versus eight-arm polymers indicated that the more branched polymers were excreted more slowly into the urine, a result attributed to their decreased flexibility. Due to their essentially linear architecture that does not provide for good isolation of the iodinated phenolic moieties, the polymers with "two arms" were rapidly taken up by the liver. The biodistributions of two long-circulating high MW polymers in mice bearing subcutaneous B16F10 tumors were evaluated, and high levels of tumor accumulation were observed. These new carriers are therefore promising for applications in drug delivery and are also useful for improving our understanding of the effect of polymer architecture on pharmacokinetic properties.