Athina Anastasaki, Web Writer – Page 7

Author Archive

Paper of the month: Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation

28 Jun 2016

Williams et al. utilise addition-fragmentation chain transfer aqueous dispersion polymerisation for the synthesis of bespoke cationic nano-objects directly in water.

Polymerisation-induced self-assembly (PISA) via reversible addition-fragmentation chain transfer (RAFT) polymerisation enables the direct and efficient formation of various diblock copolymer morphologies (e.g. spherical micelles, work-like micelles, vesicles etc.) in aqueous solution. Here the first block is selected to be water-soluble, while the growing second block is water-insoluble and hence drives in situ self-assembly. This versatile approach can be conducted at much higher copolymer concentrations than traditional block copolymer self-assembly based on post-polymerisation processing.

Now Williams and co-workers report the synthesis of a range of cationic diblock copolymer nano-objects utilising a judicious binary mixture of chain transfer agents, namely non-ionic poly(glycerol monomethacrylate) (PGMA) and cationic poly[2-(methacryloyloxy)ethyl trimethylammonium chloride] (PQDMA) and using poly(2-hydroxypropyl methacrylate) (PHPMA) as the hydrophobic core-forming block. Systematic variation of the PQDMA mol fraction and the mean degree of polymerisation of the core-forming PHPMA block enabled the formation of well-defined spheres, worms or vesicles that remain cationic over a wide pH range.

Interestingly, higher cationic character led to the formation of kinetically-trapped spheres; this is because more effective electrosteric stabilisation prevents sphere-sphere fusion. In addition, using 5 mol% PQDMA stabiliser enabled preparation of a 12.5% w/w cationic worm gel that exhibited a zeta potential of +20 mV and a storage modulus of 137 Pa, as demonstrated by variable temperature rheology studies. This worm gel proved to be thermoresponsive: it underwent reversible degelation on cooling from 25 °C to 5 °C. Finally, such cationic gels exhibited weak antimicrobial activity towards the pathogen Staphylococcus aureus.

Tips/comments directly from the authors:

It is really important to map out a detailed phase diagram for the reliable and reproducible identification of pure copolymer phases. This is particularly true for the elusive worm phase, since this occupies relatively narrow phase space.
Using pairs of stabiliser blocks is a powerful and versatile means of tuning the copolymer morphology. If a wholly cationic stabiliser is used, only spheres can be obtained. However, using a binary mixture of a non-ionic and a cationic stabiliser allows access to cationic spheres, worms or vesicles. This is because the non-ionic stabiliser dilutes the charge density within the coronal layer. If maximum cationic character is desired, then the ionic block should have a higher degree of polymerisation than the non-ionic block. This will enable it to protrude from the layer of non-ionic stabiliser chains and influence the electrophoretic footprint of the diblock copolymer nano-objects.
When diluting thermoresponsive worm dispersions to the relatively low concentrations typically used for TEM or DLS analysis, it is important for dispersions to be stored at ambient temperature. This is because the thermoresponsive degelation behaviour becomes irreversible below a certain critical copolymer concentration. Thus storing highly dilute (< 1 %) dispersions in a refrigerator at 4-5 °C simply leads to kinetically-trapped spheres – worms are no longer reformed on returning to ambient temperature under these conditions.

Read this exciting research for free until 31/07/2016 through a registered RSC account:

Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation
M. Williams, N. J. W. Penfold, J. R. Lovett, N. J. Warren, C. W. I. Douglas, N. Doroshenko, P. Verstraete, J. Smets and S. P. Armes
Polym. Chem., 2016, 7, 3864-3873
DOI: 10.1039/C6PY00696E

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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).

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Paper of the month: Thermoresponsive hydrogels from triblock copolymers

06 Jun 2016

Despax et al. report the synthesis of triblock copolymers and their application as thermoresponsive hydrogels.

Temperature responsive gelators can benefit a wide range of biomedical applications and typically comprised of triblock copolymers with a central hydrophilic block and terminal blocks that undergo a hydrophilic to hydrophobic transition at a specific temperature. However, typical ABA triblock copolymers obtained from commercially available monomers require concentrations of at least 50-100 g L^-1.

Harrisson, Destarac and co-workers have managed to circumvent this by synthesizing high molecular weight triblock copolymers via low temperature reversible addition-fragmentation chain-transfer (RAFT) gel polymerization. The targeted triblock copolymers were based on polydimethylacrylamide (PDMA) as the long central hydrophilic block and poly(N-isopropylacrylamide) (PNIPAM) as the shorter terminal blocks and the gel formation was initially demonstrated via vial-inversion tests.

Two different molecular weight triblock copolymers were tested with the PDMA block varying from 58 kg mol^-1 to 421 kg mol^-1 showing self-supporting gels at 30 g L^-1and 6 g L^-1 concentration respectively, which is already a significant improvement over previously reported materials. As the vial-inversion test is subject to experimental variations, a more objective measure of the effect of the temperature was obtained from the evolution of the storage and loss moduli of aqueous polymer solutions.

For the lower molecular weight polymer, a two-step transition consisting of an initial thickening of the solution at the lower critical solution temperature (LCST) of PNIPAM occurred followed by gel formation at 38–39 °C requiring a minimum concentration of 20 g L^-1. For the longer polymer, only the second transition was observed; gel formation occurred at 40-45 °C with a minimum concentration of 4 g L^-1. With a storage modulus of only 0.1 Pa however, this gel is likely too soft for practical use.

In an attempt to improve the mechanical properties of the gels, 2-acrylamido-2-methylpropanesulfonic acid was also incorporated (20 mol% of DMA) resulting on the formation of self-supporting gels at 2 g L^-1, an order of magnitude improvement over previously-reported ABA copolymers. These results approach the performance obtained from exotic polymers such as polyisocyanopeptides.

Tips/comments directly from the authors:

High monomer concentrations are helpful to obtain high molecular weights. However, the polymerization of acrylamides is very exothermic so it is important not to exceed 30 wt%.
As very low initiator concentrations are used, it is important to thoroughly degas all solutions prior to polymerization.
Take care to exclude any air bubbles from the solution when carrying out rheology measurements.

Read this exciting research for free until 03/07/2016 through a registered RSC account:

Low concentration thermoresponsive hydrogels from readily accessible triblock copolymers
L. Despax, J. Fitremann, M. Destarac and S. Harrisson
Polym. Chem., 2016, 7, 3375-3377
DOI: 10.1039/C6PY00499G
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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).

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Paper of the month: Rapidly-cured isosorbide-based cross-linked polycarbonate elastomers

25 Apr 2016

Kristufek et al. report the synthesis of rapidly-cured isosorbide-based cross-linked polycarbonate elastomers.

Inexpensive starting materials from natural products (such as isosorbide, isomannide etc.) can allow for natural material to begin to compete with and (why not?) eventually replace petrochemicals as a source of monomers. Isosorbide-based materials in particular have attracted considerable attention due to both the rigidity of their fused ring systems and the easily-modifiable dual hydroxyl functionalities. However, the utility of isosorbide-based materials for elastomers is perhaps more limited. As such, in the current article, Wooley and co-workers aim to produce rapidly-photo-cross-linked isosorbide-based elastomers via thiol-ene chemistry that will have the additional potential to hydrolytically break down into their original building blocks.

This novel cross-linked network system was elegantly synthesized using a naturally-derived monomer, isosorbide dialloc (IDA) and cross linked with tri-methylpropane tris(3-mercaptopropionate) (TMPTMP) yielding IDA-co-TMPTMP, an optically transparent elastomer. All the IDA-co-TMPTMP networks were obtained by environmentally friendly methods including solvent-free conditions, low catalyst loading and UV irradiation. Importantly, a study of a constant UV cure time (1 minute) and variation of the thermal curing times led to the conclusion that this material is near its optimal thermal and mechanical properties without requiring post-cure heating.

The thermal decomposition temperature of the networks were consistent (320 °C) while the glass transition temperature remained below room temperature for all samples with a % elongation of 220-340%. The hydrolytic degradation of the material was also evaluated and found to afford 8.3±3.5% and 97.7±0.3% mass remaining after 60 days under accelerated basic and physiological neutral buffer conditions respectively. Finally, a cell viability assay and fluorescence imaging with adherent cells were also reported in order to show the potential of this material as a biomedical substrate. In conclusion, the rapid synthesis of this optically transparent flexible elastomer presented very interesting properties that could be very useful in biomedical applications or as environmentally-friendly materials.

Tips/comments directly from the authors:

Because DMPA dissolves slowly in the reaction mixture, it is important to keep it in the dark while mixing and allow it to fully dissolve, resulting in the most uniform materials.
Glass slides were used as the molds to maximize the light exposure to the reaction mixture of the two monomers, ensuring the rapid curing time.
During the degradation study, it is important to change the solution at short (ca. 2 days), constant intervals to provide consistent results.

Rapidly-cured isosorbide-based cross-linked polycarbonate elastomers by T.S. Kristufek, S.L. Kristufek, L.A. Link, A.C. Weems, S. Khan, S.M. Lim, A.T. Lonnecker, J.E. Raymond, D.J. Maitland and K.L. Wooley, Polym. Chem., 2016, 7, 2045-2056, DOI: 10.1039/C5PY01659B

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please visit http://www.haddleton.org/users/athina-anastasaki for more information.

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Paper of the month: Thermoresponsive gels based on ABC triblock copolymers: effect of the length of the PEG side group.

23 Mar 2016

Constantinou & Georgiou report the synthesis of thermoresponsive triblock copolymers using group transfer polymerisation.

Thermoresponsive polymers can find use in a wide range of applications including tissue engineering and 3-D printing. For the successful synthesis of thermoresponsive gels several criteria need to be taken into account such as the composition, the molar mass, the concentration and the architecture. Georgiou’s group elegantly demonstrate the facile synthesis of such materials through group transfer polymerisation (GTP) thanks to its unique characteristics including scalability and faster reaction rates in comparison to conventional radical polymerisations. Different copolymers were targeted based on the ionic hydrophilic pH and thermoresponsive 2-(dimethylamino)ethyl methacrylate (DMAEMA), the non-ionic poly(ethylene glycol) (PEG)-based methacrylate (methoxy di-, penta-, and nona(ethylene glycol) methacrylate, DEGMA, PEGMA, and NEGMA), and the hydrophobic BuMA. The effect of the PEG side chain length and different compositions were systematically varied in order to investigate their effects on the thermoresponsive behaviour of the copolymers. Micelle formation was observed for all the terpolymers and the effective pK_as were affected by the hydrophobic BuMA content and the architecture. Interestingly, the cloud points were affected by both the composition (BuMA content) and the PEG side group length and increase as the hydrophilic content and the PEG length increased. The gel points were investigated over a wide range of temperatures and concentrations and found to be influenced by both the composition and the PEG side chain length. Stable gels were formed by the most hydrophobic and with the shortest PEG length macromonomers. In summary, it was demonstrated that the sol–gel transition can be tailored by varying both the PEG length as well as the composition of the polymers.

Tips/comments directly from the authors:

It is really important to monitor the temperature between additions during the one-pot synthesis. GTP is exothermic so when all monomer has converted to the polymer the temperature will drop back down so the next monomer can be added.
Since each addition/polymerisation step takes about 10-15 the reaction can be monitored in real time by gel permeation chromatography, if necessary.
Even though ideally all monomers have to be distilled this is not necessary when the GTP reaction is scaled up as long as the monomers are dry.
Gelation is also influenced by ionic strength so if salt is added to the polymer solutions the solution will gel at lower temperatures and concentrations.

Thermoresponsive gels based on ABC triblock copolymers: effect of the length of the PEG side group by A. P. Constantinou and T. K. Georgiou , Polym. Chem., 2016, 7, 2045-2056

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please visit her webpage for more information.

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Paper of the month: pH-Sensitive nanogates based on poly(L-histidine) for controlled drug release from mesoporous silica nanoparticles.

18 Feb 2016

Bilalis et al. report the design and synthesis of novel pH-sensitive nanogates based on poly(L-histidine) from mesoporous silica nanoparticles.

The development of novel drug delivery materials necessitates the combination of the knowledge from different scientific fields, including organic and inorganic chemistry. Among the wide range of hybrid organic/inorganic materials, mesoporous silica nanoparticles (MSNs) have attracted considerable attention thanks to their unique characteristics such as high surface area, large specific volume, controllable pore diameter, facile surface functionalization and nontoxicity. On the other hand, polypeptide-coated silica-based systems, including poly(L-histidine) (PHis), have shown great promise in preventing untimely release of drugs and as such the combination of PHis and MSNs can provide an excellent template for the design of advanced drug delivery systems for controlled release applications. To this end, Iatrou, Bilalis and co-workers have exploited surface-initiated ring-opening polymerization (ROP) to synthesize novel pH-sensitive poly(L-histidine)-grafted mesoporous silica nanoparticles through an amino-functionalized MSN intermediate. The successful grafting of the homopolypeptide chains from the surface of MSNs was demonstrated by Fourier Transform-infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) while size exclusion chromatography (SEC) confirmed the controlled character of the polymerization. Dynamic light scattering (DLS) and zeta potential analysis were also employed to ascertain the pH-responsive nature of the polypeptide-gated MSNs. In addition, drug loading and release studies were performed in order to verify the role of the grafted PHis chains as pH-sensitive nanogates for the MSN pores utilizing the model anticancer drug doxorubicin (DOX). DOX was efficiently loaded within the nanochannels of the hybrid MSN@PHis (~90%) and the drug entrapment and release pattern were proved to be pH-dependent with exert stability at physiological pH. The combination of the positive characteristics of MSNs and poly(L-histidine) enables the described materials as promising drug nanocarriers with potential in vitro and in vivo applications.

Tips/comments directly from the authors:

It is really important to strictly maintain the reported time of reaction during the synthesis of MSNs using TEOS. Longer or less time of reaction will lead to larger or smaller nanoparticles, respectively.
It should be noted that the functionalization of the surface of MSNs with APTES was conducted before the removal of CTAB so as to avoid the grafting of PHis chains from the MSN nanopores.
When following the reported functionalization procedure, a LiOH solution must be used in order to remove HCl traces from the amino groups of MSNs after the removal of CTAB.
The loading procedure of DOX into the MSN nanopores should take place at acidic pH (3.0). In that way the maximum drug encapsulation is ensured, because the PHis nanogates are in an opened state (fully protonated and thus hydrophilic).

pH-Sensitive nanogates based on poly(L-histidine) for controlled drug release from mesoporous silica nanoparticles by P. Bilalis, L.-A. Tziveleka, S. Varlas and H. Iatrou, Polym. Chem., 2016, 7, 1475-1485, DOI: 10.1039/C5PY01841B

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Paper of the month: Lights on! A significant photoenhancement effect on ATRP by ambient laboratory light

25 Jan 2016

Zhang et. al. report a significant photoenhancement effect on classical ATRP and ATRP derivatives by ambient laboratory light.

Since the introduction of atom transfer radical polymerisation (ATRP) by Matyjaszewski and Sawamoto, a large diversity of other copper adjuncts have attracted considerable interest including activator regenerated by electron transfer (ARGET)-ATRP, activator generated by electron transfer (AGET)-ATRP, initiators for continuous activator regeneration (ICAR)-ATRP and single electron transfer living radical polymerisation (SET-LRP). Recently photoinduced copper radical polymerization has also drawn significant attention as CuBr₂, typically referred to as deactivating species, can be reduced in situ generating the active CuBr species in the presence of UV or visible light. In this contribution, Jordan and co-workers investigated on whether the typical laboratory light would have any considerable impact on standard ATRP reactions. Interestingly, the vast majority of the ATRP techniques (with the exception of ARGET-ATRP) demonstrated a remarkable photoenhancement effect by ambient light, originated from common fluorescent lamps. It was observed that when less copper complex is utilized for the polymerizations, a stronger influence of the ambient light in the monomer conversion is evident and this effect was significant even in the presence of additional reducing agents. As a general rule, it was concluded that the slower the polymerization is, the more pronounced the effect of the ambient laboratory light can be. As it was proved that it makes a difference if one is performing an ATRP reaction with the hoods on and off, the effect of the laboratory light on the polymerization can no longer be neglected and the authors encourage the report of the light conditions in typical ATRP experiments in order to ensure the reproducibility.

Tips/comments directly from the authors:

Comments:
In this study, we provide conclusive evidence that common laboratory light especially originating from fluorescence lamps has a significant impact on ATRP. This is most probably one main reason why reproducibility of ATRP reactions are not as it should be which impairs further development of controlled radical polymerization techniques. As shown, the impact of light is different for the different ATRP recipes and will also strongly vary with the:

1. Type of complex formed regarding the metal and the ligands;
2. Quantity/concentration of metal complex formed and surely;
3. Type of illumination (natural light, type of fluorescent lamps installed in the laboratory and light intensity in the reaction vial). Strong influence were found for the “hood light illumination” (see Fig. 1 for description and Fig. 2 for experimental data) which was also very surprising for us. We therefore suggest to check the type of light bulbs/source of light, consider their emission spectra and resulting intensity at the location of the reaction and possibly provide these details in the experimental section to ensure reproducibility.

Tips:
1. The various metal complexes used in ATRP are most probably photosensitive but to a different extent. Thus, the influence of laboratory light will vary.
2. Perform the ATRP reactions always under the same light settings with the same light source and provide details (type of lamp, light intensity at each location of the reaction) in the experimental section.
3. Especially modern fluorescent lamps have a higher emission in the blue range and thus may have a stronger/other influence upon conversion/kinetics of the ATRP. Check the emission spectra of the lamps installed in the laboratory and especially in the chemical fume hoods.

Lights on! A significant photoenhancement effect on ATRP by ambient laboratory light by Tao Zhang Dan Gieseler and Rainer Jordan, Polym. Chem., 2016, 7, 775-779

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Elings Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).Visit her webpage for more information.

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Paper of the month: Solvent-free mechanochemical synthesis of a bicyclononyne tosylate: a fast route towards bioorthogonal clickable poly(2-oxazoline)s

23 Dec 2015

Glassner et. al. report a new solvent-free mechanochemical synthesis of a bicyclononyne tosylate.

The strain promoted azide-alkyne cycloaddition of cyclooctynes (SPAAC) is a popular bioorthogonal reaction that enables the tracking of biomacromolecules in living systems. Among the various cylooctynes that have been developed for SPAAC reactions, derivatives of bicyclo[6.1.0]non-4-yne (BCN) have attracted considerable interest for a number of reasons including their low hydrophobicity and ability to undergo fast cycloadditions not only with azides but also with nitrones. At the same time, poly(2-oxazoline)s (PAOx) represent a promising class of polymers for biomedical applications that can easily incorporate clickable end-groups by employing functional initiators or terminators. In this paper, Hoogenboom and co-workers present the first mechanochemical synthesis of a BCN tosylate (BCN-OTs) initiator via solvent-free reaction conditions utilizing high speed vibration milling instead of manual grinding. The BCN-OTs was subsequently used for the cationic ring opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx) yielding a well-defined polymer and demonstrating controlled/living polymerization features such as narrow molecular weight distributions and high chain end fidelity. This represents a rapid route to prepare defined BCN functionalized PEtOx that can be used for bioorthogonal strain-promoted conjugation reactions. This was successfully demonstrated for the synthesis of a PS-b-PEtOx copolymer and a PEtOx-protein conjugate. As such, BCN-OTs may find potential applications as intermediate in the synthesis of functional BCN derivatives and BCN-PAOx. In addition, the ball-milling methodology utilized for this study is a promising tool for the synthesis of other unstable/highly reactive tosylates.

Tips/comments directly from the authors:

1) KOH and K₂CO₃ should be finely grounded and dried in a vacuum oven prior to use.

2) During the synthesis of BCN-OTs, removal of solvents has to be performed at ambient temperature (turn of the heating bath of the rotary evaporator).

3) BCN-OTs has to be used immediately for the next step because of its limited stability at ambient conditions.

Solvent-free mechanochemical synthesis of a bicyclononyne tosylate: a fast route towards bioorthogonal clickable poly(2-oxazoline)s by M. Glassner, S. Maji, V. de la Rosa, N. Vanparijs, K. Ryskulova, B. De Geest and R. Hoogenboom, Polym. Chem., 2015, 6, 8354-8359

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Warwick (UK)/ Monash (Australia) research fellow working under the Monash Alliance. Visit http://haddleton.org/group-members for more information.

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Paper of the month: Synthesis and characterization of TEMPO- and viologen-polymers for water-based redox-flow batteries

25 Nov 2015

Janoschka et al. report TEMPO and viologen containing polymers for water-based redox-flow batteries.

Redox-flow batteries (RFBs) provide an inexpensive, safe and long lasting approach towards the development of stationary large-scale energy storage. Typically, RFBs employ redox- active materials that dissolve in either aqueous or organic solutions, although many of the existing systems challenge the chemical stability of installed battery components and impose a safety risk. Vanadium salts in sulphuric acid consist the most thoroughly studied system and when aqueous solutions are additionally employed further advantages can be achieved including low corrosivity, high safety and affordable size-exclusion membranes. In this current contribution, Schubert and co-workers present the screening of two series of redox-active polymers that can potentially find application in water-based polymer redox-flow batteries (pRFBs). The rheological and electrochemical properties of both viologen and TEMPO containing polymers have been investigated in details. The viologen-homopolymer poly(1-methyl-1’-(4-vinylbenzyl)-[4,4’-bipyridine]-1,1’-diium dichloride) was found to exhibit good solubility in water (even in the reduced state) and a redox potential of -0.4 V (vs Ag/AgCl), demonstrating its suitability as an anode material. On the other hand, the TEMPO-polymer showed reduced solubility in aqueous solution and thus requiring a solubilizing comonomer. Although poly(ethyleneglycol)-based comonomers and methacrylamide were subsequently tested, the unfavourable LCST and the requirement of high mole fractions respectively led to the investigating of a third alternative. Pleasingly, poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl-co-2-(methacryloyloxy)-N,N,N-trimethylethane ammonium chloride) proved to be a suitable cathode material for pRFBs when a choline moiety was used as the solubilizing unit. In summary, the presented work paves the way for the production of economical energy storage devices that are safe, metal free and utilise all-organic raw materials.

Tips/comments directly from the authors:

1. It should be noted that the redox-active polymer solutions can be used straight from the reaction vessel and without further purification for the designated application in pRFBs.

2. The TEMPO-polymers are excellent surfactants and prone to foam formation. The authors highly encourage the reader to come up with ideas for potential applications of this redox-active, radical-bearing surfactant.

3. When preparing polymers for pRFBs the molar mass of the polymer should be tailored in order to fit the size-exclusion membrane (dialysis membrane) used in the battery. The dispersity Đ should be low to ensure good rheological properties.

Synthesis and characterization of TEMPO- and viologen-polymers for water-based redox-flow batteries, by T. Janoschka, S. Morgenstern, H. Hiller, C. Friebe, K. Wolkersdörfer, B. Häupler, M.D. Hager and U.S. Schubert, Polym.Chem., 2015, 6, 7801-7811

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Paper of the month: Micro-patterned polymer brushes by a combination of photolithography and interface-mediated RAFT polymerization for DNA hybridization

27 Oct 2015

Cimen et al. report the fabrication of micro-patterned polymer brushes for DNA attachment and subsequent hybridization.

The covalent immobilization of DNA probes has been extensively studied over the past ten years. Among others approaches, the binding of protected organic molecules on the silicon surfaces followed by the deprotection of the attached groups is perhaps one of the most popular routes. However, the harsh conditions typically utilized for the deprotection have a detrimental effect on the quality of the surface and as such alternative approaches have also been exploited including growth of brushes from a range of substrates.

In this paper, Caykara and Cimen investigated the micro-patterning of poly(6-azidohexylmethacrylate) [poly(AHMA)] brushes via a combination of photolithography and interface-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization for DNA hybridization. Upon immobilization of the RAFT agent on the Si surface followed by the attachment of 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)propionic acid (TTC5) (photoresist-coated substrate), poly(AHMA) brushes were grown via interface-mediated RAFT polymerization. AFM was used to determine the thickness of the polymer brush which was found to approach the average thickness of a uniform (AHMA) brush film. Propargylamine was subsequently used to functionalize the azide groups of poly(AHMA) via a click reaction as confirmed by X-ray photoelectron spectroscopy (XPS). The micro-patterned poly(AHMA) brushes with amine pendant groups were then chemically modified in order to function as selective adsorption sites for DNA hybridization. In order to form a non-active background, the probe DNA molecules were successfully coupled onto the micro-patterned poly(AHMA) brushes, as shown by AFM and ellipsometry. The probe DNA modified surfaces were subsequently incubated in the presence of complementary DNA and the successful hybridization of complementary DNA molecules on the micro-patterned poly(AHMA) was confirmed by confocal microscopy and AFM. This simplified and straightforward approach provides a platform for the immobilization of complicated structures on silicon surfaces with potential applications in biosensing and the production of novel materials.

Tips/comments directly from the authors:

1. 11-amino-1-undecene (AUD) cannot be attached directly to the hydrogen terminated silicon surface because of the free amino functional groups. So, in this study, the amino group was protected by the t-butyloxycarbonyl group (t-BOC). The protection reaction is easily performed but during the vacuum purification of the product, control of the temperature and pressure is of vital importance. Gentle heating and constant vacuum pressure must be carried out to prevent the decomposition of t-BOC-AUD.

2. Si-H surface can be easily oxidized at atmosphere. So, rinsing of Si-H surface and coating of t-BOC-AUD must be performed immadiately in a glove box.

3. In the photolithography process, coating of the photoresist was carried out under yellow light. Until removing of the unexposed areas, the surfaces must be kept under safe light.

4. To avoid loss of resolution during the UV exposure, obtaining a thin and homogenous coating on the silicon surface is extremely important. Therefore, the silicon wafers should be cleaved into uniform square or rectangle pieces before the lithography process and the surfaces must be modified homogeneously.

Micro-patterned polymer brushes by a combination of photolithography and interface-mediated RAFT polymerization for DNA hybridization, by D. Cimen and T. Caykara, Polym. Chem., 2015, 6, 6812-6818

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Paper of the month: Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst

28 Sep 2015

Zhang et al. report a highly efficient separation and recycling of an iron catalyst in a p-xylene/PEG-200 biphasic system exploiting Fe(III)-mediated ICAR ATRP.

Atom transfer radical polymerization (ATRP) is a well-established polymerization protocol which allows access to the facile preparation of well-defined materials. As copper is considered an unwanted contamination in some applications, a significant attention has been directly towards the investigation of ATRP catalyst separation and recycling. However, most of the recycling studies are conducted with copper catalysts neglecting other catalytic species such as iron which are less toxic, abundant and biocompatible. Inspired by the successful application of biphasic systems in organic synthesis, Cheng, Zhang and co-workers utilized a PEG-200/p-xylene biphasic system to afford a thermo-regulated phase-separable catalysis (TPSC) via Fe(III)-mediated initiators for continued activator regeneration ATRP (ICAR ATRP). Although PEG-200 and p-xylene are immiscible at ambient temperature, they become homogeneous when heated to 70 °C. Upon commencement of the polymerization, followed by a subsequent cooling period, the reaction mixture separates in two phases. The PEG-200 phase includes the catalyst complex and could be re-used 10 times while still maintaining high catalyst activity while the p-xylene layer contains well-defined polymers with less than 4 ppm of catalyst. Importantly, the versatility and robustness of this protocol was demonstrated by the polymerization of a large diversity of monomers, including methacrylates, acrylates and styrene. In all cases, narrow molecular weight distributions (Ð <1.27) were obtained while high end-group fidelity was verified through successful chain extension experiments that confirmed the “living”/controlled nature of the system. This novel strategy complements previous studies in the field and clearly shows a trend of using alternative metals for controlled polymerizations while at the same time recycling the catalyst to minimize cost and purification steps.

Tips/comments directly from the authors:

1. Iron catalysts have unique advantages over copper catalysts from the view point of catalyst abundancy, biocompatibility and toxicity. Therefore, iron catalysts are better candidates than others for the synthesis of polymeric materials, especially those used for biomedical applications, by the ATRP method.

2. For this Fe(III)-mediated ICAR ATRP, it should be noted that choosing a facile and highly efficient separation biphasic TPSC system for the features of homogeneous catalysis at high temperatures (polymerization temperature) and phase separation at room temperature is important.

3. In this system iron catalyst complexes can be separated and recycled in situ more than 10 times. However, a small amount of PEG-200 may dissolve in p-xylene, as a consequence, we can add some fresh PEG-200 to keep a more efficient TPSC strategy.

4. The organic phase (p-xylene layer with the resultant polymers) can be transferred at room temperature by simple decantation and washed with p-xylene in recycling procedure.

Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst, by B. Zhang, X. Jiang, L. Zhang, Z. Cheng and X. Zhu, Polym. Chem., 2015, 6, 6616-6622

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Author Archive

Paper of the month: Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation

Paper of the month: Thermoresponsive hydrogels from triblock copolymers

Paper of the month: Rapidly-cured isosorbide-based cross-linked polycarbonate elastomers

Paper of the month: Thermoresponsive gels based on ABC triblock copolymers: effect of the length of the PEG side group.

Paper of the month: pH-Sensitive nanogates based on poly(L-histidine) for controlled drug release from mesoporous silica nanoparticles.

Paper of the month: Lights on! A significant photoenhancement effect on ATRP by ambient laboratory light

Paper of the month: Solvent-free mechanochemical synthesis of a bicyclononyne tosylate: a fast route towards bioorthogonal clickable poly(2-oxazoline)s

Paper of the month: Synthesis and characterization of TEMPO- and viologen-polymers for water-based redox-flow batteries

Paper of the month: Micro-patterned polymer brushes by a combination of photolithography and interface-mediated RAFT polymerization for DNA hybridization

Paper of the month: Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst

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