Afang Zhang

Tuning Polymer Thickness: Synthesis and Scaling Theory of Homologous Series of Dendronized Polymers

The thickness of dendronized polymers can be tuned by varying their generation g and the dendron functionality X. Systematic studies of this effect require (i) synthetic ability to produce large samples of high quality polymers with systematic variation of g, X and of the backbone polymerization degree N, (ii) a theoretical model relating the solvent swollen polymer diameter, r, and persistence length, lambda, to g and X. This article presents an optimized synthetic method and a simple theoretical model. Our theory approach, based on the Boris-Rubinstein model of dendrimers predicts r approximately n(1/4)g(1/2) and lambda approximately n(2) where n = [(X - 1)(g) - 1]/(X - 2) is the number of monomers in a dendron. The average monomer concentration in the branched side chains of a dendronized polymer increases with g in qualitative contrast to bottle brushes whose side chains are linear. The stepwise, attach-to, synthesis of X = 3 dendronized polymers yielded gram amounts of g = 1-4 polymers with N approximately = 1000 and N approximately = 7000 as compared to earlier maxima of 0.1 g amounts and of N approximately = 1000. The method can be modified to dendrons of different X. The conversion fraction at each attach-to step, as quantified by converting unreacted groups with UV labels, was 99.3% to 99.8%. Atomic force microscopy on mixed polymer samples allows to distinguish between chains of different g and suggests an apparent height difference of 0.85 nm per generation as well as an increase of persistence length with g. We suggest synthetic directions to allow confrontation with theory.

Thermoresponsive dendronized polymers with tunable lower critical solution temperatures

A series of first (PG 1) and second generation (PG 2) dendronized polymers were synthesized which exhibit fast and sharp phase transitions with negligible hystereses in aqueous solutions and apparent lower critical solution temperatures (LCSTs) in the range of 33-49 degrees C.

EPR Spectroscopic Characterization of Local Nanoscopic Heterogeneities during the Thermal Collapse of Thermoresponsive Dendronized Polymers

Despite these efforts,a molecular-scale picture of what happens when thermores-ponsive polymers start to dehydrate at a certain temperature,subsequently collapse, and then assemble to mesoglobules,does not exist. This absence severely hampers rationalmaterials design.In an exploratory research effort aimed at detectingunusual properties of dendronized polymers,

Stimuli-Responsive Zwitterionic Block Copolypeptides: Poly(N-isopropylacrylamide)-block-poly(lysine-co-glutamic acid)

Synthesis of novel zwitterionic block copolypeptides, poly(N-isopropylacrylamide)-block-poly(L-glutamic acid-co-L-lysine) [PNiPAM(n)(PLG(x)-co-PLLys(y))m , where n is the number-average degree of polymerization (DP(n)) of PNiPAM block, x and y are the mole fraction of glutamic acid and lysine residues, respectively, and m is the total DP(n) of the peptide block], and their stimuli-responsiveness to temperature and pH variation in aqueous solutions are described. Initiated with the amino-terminated poly(N-isopropylacrylamide) (PNiPAM(n)-NH2), ring-opening polymerization (ROP) of a mixture of gamma-benzyl-L-glutamate N-carboxyanhydride (BLG-NCA), and Boc-L-lysine N-carboxyanhydride (BLLys-NCA) afforded the block copolypeptides PNiPAM(n)(PBLG(x)-co-PBLLys(y))m, with a poly(N-isopropylacrylamide) block together with a random copolypeptide block, which was then deprotected with HBr/trifluoroacetic acid into the double hydrophilic block copolypeptides, PNiPAM(n)(PLG(x)-co-PLLys(y))m. Their block ratios and lengths, as well as the amino acid residue ratios in the random copolypeptide block are varied (n = 360, x = 0.4-0.5, y = 0.4-0.6, and m = 220-252). The secondary structures of the copolypeptides in aqueous solution at different pH conditions were examined. Phase transitions in aqueous solutions induced by both pH and temperature variation were investigated by (1)H NMR spectroscopy. The transitions induced by temperature were also explored by turbidity measurements using UV/vis spectroscopy for their lower critical aggregation temperature (LCAT) determination. Furthermore, these aggregation processes were followed by dynamic light scattering measurements.

Low toxic, thermoresponsive dendrimers based on oligoethylene glycols with sharp and fully reversible phase transitions.

Novel first (G1) and second (G2) generation dendrimers based on three-fold branched oligoethylene glycol dendrons are efficiently synthesized which show characteristic thermoresponsive behavior and negligible cytotoxicity (for G2).

Formation of a Mesoscopic Skin Barrier in Mesoglobules of Thermoresponsive Polymers

With the combination of molecular scale information from electron paramagnetic resonance (EPR) spectroscopy and meso-/macroscopic information from various other characterization techniques, the formation of mesoglobules of thermoresponsive dendronized polymers is explained. Apparent differences in the EPR spectra in dependence of the heating rate, the chemical nature of the dendritic substructure of the polymer, and the concentration are interpreted to be caused by the formation of a dense polymeric layer at the periphery of the mesoglobule. This skin barrier is formed in a narrow temperature range of ~4 K above T(C) and prohibits the release of molecules that are incorporated in the polymer aggregate. In large mesoglobules, formed at low heating rates and at high polymer concentrations, a considerable amount of water is entrapped that microphase-separates from the collapsed polymer chains at high temperatures. This results in the aggregates possessing an aqueous core and a corona consisting of collapsed polymer chains. A fast heating rate, a low polymer concentration, and hydrophobic subunits in the dendritic polymer side chains make the entrapment of water less favorable and lead to a higher degree of vitrification. This may bear consequences for the design and use of thermoresponsive polymeric systems in the fast growing field of drug delivery.

A rigid, chiral, dendronized polymer with a thermally stable, right-handed helical conformation

First- and second-generation dendronized polymethacrylates PG1 and PG2 carrying chiral 4-aminoproline-based dendrons were obtained on the half-gram scale in high molar masses (PG1: M(n)=5 x 10(6) g mol(-1), PG2: M(n)=1x10(6) g mol(-1)) by spontaneous (radical) polymerization of the corresponding vinyl macromonomers. NMR spectroscopic studies on PG2 together with its unprecedented high glass transition temperature (T(g)>200 degrees C, decomp) and structural parameters provided by atomistic MD simulations show this polymer to be rather rigid. Optical rotation and CD measurements revealed that PG2 adopts a helical conformation that remains unchanged over wide ranges of temperature and solvent polarity. It is also retained when the polymer is deprotected (and thus positively charged, de-PG2) at its terminal amino groups, by which the mass and steric demand of the dendrons is reduced by roughly 50 %. Molecular dynamics simulations on models of PG2 reveal its helical conformation to be right-handed, irrespective of backbone tacticity, and initial results also indicate that de-PG2 retains the right-handedness.

ILQINS Hexapeptide, Identified in Lysozyme Left-Handed Helical Ribbons and Nanotubes, Forms Right-Handed Helical Ribbons and Crystals

Amyloid fibrils are implicated in over 20 neurodegenerative diseases. The mechanisms of fibril structuring and formation are not only of medical and biological importance but are also relevant for material science and nanotechnologies due to the unique structural and physical properties of amyloids. We previously found that hen egg white lysozyme, homologous to the disease-related human lysozyme, can form left-handed giant ribbons, closing into nanotubes. By using matrix-assisted laser desorption ionization mass spectrometry analysis, we here identify a key component of such structures: the ILQINS hexapeptide. By combining atomic force microscopy and circular dichorism, we find that this fragment, synthesized by solid-phase peptide synthesis, also forms fibrillar structures in water at pH 2. However, all fibrillar structures formed possess an unexpected right-handed twist, a rare chirality within the corpus of amyloid experimental observations. We confirm by small- and wide-angle X-ray scattering and molecular dynamics simulations that these fibrils are composed of conventional left-handed β-sheets, but that packing stresses between adjacent sheets create this twist of unusual handedness. We also show that the right-handed fibrils represent a metastable state toward β-sheet-based microcrystals formation.

Thermoresponsive Supramolecular Dendronized Polymers

Combining the concepts of supramolecular polymers and dendronized polymers provides the opportunity to create bulky polymers with easy structural modification and tunable properties. In the present work, a novel class of side-chain supramolecular dendronized polymethacrylates is prepared through the host-guest interaction. The host is a linear polymethacrylate (as the backbone) attached in each repeat unit with a β-cyclodextrin (β-CD) moiety, and the guest is constituted with three-fold branched oligoethylene glycol (OEG)-based first-(G1) and second-generation (G2) dendrons with an adamantyl group core. The host and guest interaction in aqueous solution leads to the formation of the supramolecular polymers, which is supported with (1)H NMR spectroscopy and dynamic light scattering measurements. The supramolecular formation was also examined at different host/guest ratios. The water solubility of hosts and guests increases upon supramolecular formation. The supramolecular polymers show good solubility in water at room temperature, but exhibit thermoresponsive behavior at elevated temperatures. Their thermoresponsiveness is thus investigated with UV/Vis and (1)H NMR spectroscopy, and compared with their counterparts formed from individual β-CD and the OEG dendritic guest. The effect of polymer concentration and molar ratio of host/guest was examined. It is found that the polar interior of the supramolecules contribute significantly to the thermally-induced phase transitions for the G1 polymer, but this effect is negligible for the G2 polymer. Based on the temperature-varied proton NMR spectra, it is found that the host-guest complex starts to decompose during the aggregation process upon heating to its dehydration temperature, and this decomposition is enhanced with an increase of solution temperature.

Thermoresponsive supramolecular dendronized copolymers with tunable phase transition temperatures

Supramolecular copolymerization from a mixture of comonomers with different hydrophilicities may offer an easy and convenient access to form thermoresponsive supramolecular copolymers with varied phase transition temperatures by simply varying ratios of the comonomers. Herein, supramolecular dendronized copolymers (SDCPs) were constructed via host–guest interaction from a linear polymer carrying β-cyclodextrin units (the host) and a mixture of two adamantyl-cored second generation oligo(ethylene glycol) dendrons (the guests). The phase transition temperatures of these SDCPs can be tuned continuously in the range of 34–56 °C by varying the ratios of the dendritic guests. To check the possible effects of steric hindrance and hydrophilicity of the guests on the supramolecular complexation, isothermal titration calorimetry was applied to follow the complexation process of the first (G1) and second generation (G2) dendritic guests. Proton NMR spectroscopy was utilized to follow dehydration and collapse processes of SDCPs. It was found that the dehydration and collapse of the OEG units initiated decomposition of the dendritic guests from the supramolecular copolymers, and the more hydrophobic guest dissociated at a much lower temperature than that for the more hydrophilic one.