Stability of star-shaped RAFT polystyrenes under mechanical and thermal stress

Well-defined three-arm and four-arm star polymers designed via a Z-group approach carrying trithiocarbonate functionalities at the core are prepared via reversible addition-fragmentation chain transfer (RAFT) polymerization featuring molecular weights of Mn{,}SEC = 156 kDa{,} D = 1.16 (3-arm) and Mn{,}SEC = 162 kDa{,} D = 1.15 (4-arm) based on multi-angle laser light scattering (MALLS) detection{,} respectively. The star-shaped polystyrenes are subjected (in bulk) to thermal stress in the temperature range between 140 and 200 [degree]C from 10 minutes up to 96 h. The thermally treated 3-arm and 4-arm star polymers are analyzed via size exclusion chromatography (SEC) to quantify the degradation process at variable temperatures as a function of time under an argon atmosphere. Cleavage rate coefficients of the star polymers are deduced as a function of temperature{,} resulting in activation parameters for the cleavage process{,} i.e. Ea = 131 kJ mol-1; A = 3.93 [times] 1011 s-1 (Mn{,}SEC = 156 kDa{,} D = 1.16{,} 3-arm star) and Ea{,} = 134 kJ mol-1; A = 9.13 [times] 1011 s-1 (Mn{,}SEC = 162 kDa{,} D = 1.15{,} 4-arm star){,} respectively. Processing of the star-shaped polymers is mimicked via a small scale counter rotating twin screw extrusion to achieve nonlinear shear and elongation flow under pressure. Furthermore{,} a rheological assessment via the linear shear deformation region (small amplitude oscillatory shear{,} SAOS) allows for a correlation of the processing conditions with the thermal degradation properties of the star polymers in the melt. Zero shear viscosity ([small eta]0) as a criterion of the degradation process is measured in the rheometer and correlated to the weight-average molecular weight{,} Mw

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

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Single chain folding of synthetic polymers by covalent and non-covalent interactions: Current status and future perspectives

The present feature article highlights the preparation of polymeric nanoparticles and initial attempts towards mimicking the structure of natural biomacromolecules by single chain folding of well-defined linear polymers through covalent and non-covalent interactions. Initially, the discussion focuses on the synthesis and characterization of single chain self-folded structures by non-covalent interactions. The second part of the article summarizes the folding of single chain polymers by means of covalent interactions into nanoparticle systems. The current state of the art in the field of single chain folding indicates that covalent-bond-driven nanoparticle preparation is well advanced, while the first encouraging steps towards building reversible single chain folding systems by the use of mutually orthogonal hydrogen-bonding motifs have been made. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Star and miktoarm star block (co)polymers viaself-assembly of ATRP generated polymer segments featuring Hamilton wedge and cyanuric acid binding motifs

Hamilton wedge (HW) end-functionalized poly(styrene) (PS-HW, Mn = 5400 g mol-1, PDI = 1.06), HW mid-chain functionalized poly(styrene) (PS-HW-PS, Mn = 4600 g mol-1, PDI = 1.04), cyanuric acid (CA) end-functionalized poly(styrene) (PS-CA, Mn = 3700 g mol-1, PDI = 1.04) and CA end- functionalized poly(methyl methacrylate) (PMMA-CA, Mn = 8500 g mol-1, PDI = 1.13) precursors were successfully synthesized via a combination of atom transfer radical polymerization (ATRP) and copper catalyzed azide-alkyne cycloaddition (CuAAC). The precursor polymers were characterized via size exclusion chromatography (SEC) and 1H NMR with respect to both molecular weight and structure. Supramolecular homopolymer (PS-HW-PS-CA), block copolymer (PS-HWPMMA-CA), star polymer (PS-HW-PS - PS-CA) as well as miktoarm star polymer (PS-HW-PS - PMMA- CA) were formed in solution in high yields at ambient temperature (association close to 89% for PS-HW-PS-CA, 90% for PS-HW-PS - PS-CA and 98% for PS-HW-PS - PMMA-CA) via H-bonding between the orthogonal recognition units, HW and CA. The formation of supramolecular polymers was confirmed via 1H NMR at ambient temperature in deuterated methylene chloride (CD 2Cl2) solution. © The Royal Society of Chemistry 2011

Single chain self-assembly of well-defined heterotelechelic polymers generated by ATRP and click chemistry revisited

Well-defined heterotelechelic poly(styrene) carrying thymine/ diaminopyridine (DAP) (Mn,SEC = 9300, PDI = 1.04) and Hamilton wedge (HW)/cyanuric acid (CA) (Mn,SEC = 8200, PDI = 1.04) bonding motifs are prepared via a combination of controlled/living radical polymerization and copper catalyzed azide/alkyne "click" chemistry and are subsequently self-assembled as single chains to emulate-on a simple level-the self-folding behavior of natural biomacromolecules. Hydrogen nuclear magnetic resonance (1H NMR) in deuterated dichloromethane and dynamic light scattering analyses provides evidence for the hydrogen bonding interactions between the α-thymine and ω-DAP as well as α-CA and ω-HW chain ends of the heterotelechelic polymers leading to circular entropy driven single chain self-assembly. This study demonstrates that the choice of NMR solvent is important for obtaining well-resolved NMR spectra of the self-assembled structures. In addition, steric effects on the HW can affect the efficiency of the self-assembly process. © 2011 Wiley Periodicals, Inc

Entropic effects on the supramolecular self-assembly of macromolecules

We report the transfer of entropic chain length effects into the realm of supramolecular chemistry and thereby demonstrate a macromolecular method to tune the reaction equilibria of hydrogen bonding motifs via the application of substituents of differing lengths and masses while not altering the actual recognition units to achieve a difference in the degree of association. The supramolecular adducts are characterized via temperature-dependent nuclear magnetic resonance (NMR) spectroscopy. (Figure Presented). © 2015 American Chemical Society

Single-chain self-folding of synthetic polymers induced by metal-ligand complexation

The controlled folding of a single polymer chain is for the first time realized by metal- complexation. α,ω-Bromine functional linear polymers are prepared via activators regenerated by electron transfer (ARGET) ATRP (M̄n,SEC = 5900 g mol-1, D strok sign = 1.07 and 12 000 g mol-1, D strok sign = 1.06) and the end groups of the polymers are subsequently converted to azide functionalities. A copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is carried out in the presence of a novel triphenylphosphine ligand and the polymers to afford homotelechelic bis-triphenylphosphine polymeric-macroligands (MLs) (M̄n,SEC = 6600 g mol-1, D strok sign = 1.07, and 12 800 g mol-1, D strok sign = 1.06). Single-chain metal complexes (SCMCs) are formed in the presence of Pd(II) ions in highly diluted solution at ambient temperature. The results derived via 1H and 31P{1H} NMR experiments, SEC, and DLS unambiguously evidence the efficient formation of SCMCs via metal ligand complexation. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Bioinspired dual self-folding of single polymer chains via reversible hydrogen bonding

With the long term aim of preparing synthetic macromolecules that mimic the folding actions of natural biomacromolecules, a single synthetic polymer chain containing two distinct and orthogonal hydrogen bonding recognition motifs has been synthesized using an atom transfer radical polymerization (ATRP) and orthogonal ligation strategy. The hydrogen bonding recognition units, based on both three-point thymine (Thy)-diaminopyridine (DAP) and six-point cyanuric acid (CA)-Hamilton wedge (HW) interactions, induced - at low concentrations - a single chain self-folding process. The self-assembly process was monitored - initially between small molecule models - by proton nuclear magnetic resonance ( 1H NMR) spectroscopy, revealing full orthogonality of the two recognition pairs, HW-CA and Thy-DAP. Dynamic as well as static light scattering (DLS and SLS) analyses of the macromolecular self-assembly systems provide unambiguous evidence for the hydrogen-bonding interactions between both the Thy-DAP and CA-HW units leading to well-defined dual point single chain self-folding, indicating that more complex single chain self-assemblies based on synthetic polymers should be able to mimic - on a simplified level - the folding actions of natural biomacromolecules. The reversibility of the self-folding action depends on temperature as confirmed via 1H-NMR spectroscopy in [D 2]tetrachloroethane. © 2012 The Royal Society of Chemistry

Constructing star polymers via modular ligation strategies

Branched polymers result in a more compact structure in comparison to linear polymers of identical molecular weight, due to their high segment density which affects the crystalline, mechanical, and viscoelastic properties of the polymer. Star polymers constitute the simplest form of branched macromolecules where all of the chains - or arm segments - of one macromolecule are linked to a centre defined as the core. Over recent years, modular ligation reactions - some of which adhere to click criteria - have enabled the synthesis of a variety of star polymers via efficient polymer-polymer conjugations. While the modified Huisgen [3 + 2] dipolar copper catalyzed azide and alkyne cycloaddition (CuAAC) has been widely employed for macromolecular star synthesis, Diels-Alder and hetero Diels-Alder reactions offer alternative pathways which allow for similarly efficient macromolecular conjugations. Moreover, combinations of these protocols afford the synthesis of more complex star polymer structures which previously had not been achievable

ABC-type miktoarm star terpolymers accessed by H-bonding driven supramolecular self-assembly

A supramolecular ABC-type miktoarm star polymer was prepared using a Hamilton wedge (HW) mid-chain functionalized polyethylene glycol-b-polystyrene (PEG-HW-PS) block copolymer and an α-cyanuric acid (CA) chain-end functional linear homopolymer poly-(n-butylacrylate) (CA-PnBA). The PEG-HW-PS element prepared via a combination of atom transfer radical polymerization (ATRP) and copper-catalyzed azide alkyne cycloaddition (CuAAC) (Mn = 13,700 g mol−1, PDI = 1.04). The CA-PnBA strand was synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization (Mn = 11,300 g mol−1, PDI = 1.06). The quantitative formation of a well-defined supramolecular ABC-type miktoarm star polymer was unambiguously proven via proton nuclear magnetic resonance (1H NMR) spectroscopy, diffusion ordered NMR spectroscopy (DOSY) and dynamic light scattering (DLS) analyses