Archive for the ‘Paper of the Month’ Category

Paper of the month: Locally controlling dynamic exchange reactions in 3D printed thiol-acrylate vitrimers using dual-wavelength digital light processing

Rossegger et al. employ a photolatent catalyst for the local activation of topological rearrangements in thermo-activated vitrimers.

Vitrimers are covalent adaptable polymers networks which have recently attracted tremendous interest thanks to their unique feature of switching from a classic thermoset behaviour to a malleable plastic upon heating. In particular, at low temperature, vitrimers exhibit properties similar to a thermoset (e.g. rigid, brittle, opaque, high strength, good chemical resistance, etc.). Instead, heating vitrimers to temperatures above their topological freezing temperature, leads to activation and exchange of the covalent bonds within the networks thereby allowing the polymer chains to flow like viscoelastic liquids. However, one of the main limitations of this thermoresponsive feature is the lack of spatial control. In their current contribution, Schlögl and coworkers report a novel photocatalyst that can introduce spatial control to vitrimers. In particular, triphenylsulfonium phosphate was used as a photocatalyst to release strong Brønsted acids in a vitrimer region exposed to UV light (365 nm). The acids subsequently catalyse the bond exchange of vitrimer networks only in this local UV-exposed region, thus fully controlling the vitrimeric property. Furthermore, this new chemistry was not only confirmed by stress relaxation studies but was also applied to develop shape-changing vitrimer materials. Importantly, the triphenylsulfonium phosphate catalyst is stable at high temperatures and transparent in the visible light region. As such, visible light (405 nm) could be used to prepare the vitrimer in 3D structures without introducing any Brønsted acid. Subsequently, UV light was successfully used to change the shape of the vitrimer by locally activating the photocatalyst. The authors anticipate that this new spatial control technology enables the fabrication of sophisticated soft active devices that can change shape in a programmable manner. We look forward to reading more about such fantastic development from the Schlögl group.

 

Tips/comments directly from the authors:

 

  • Owing to their strong Brønsted acidity and high thermal stability, photoacid generators are able to catalyze thermo-activated transesterifications in hydroxyl ester networks.
  • Stress relaxation kinetics increase with rising catalyst content and rising irradiation dose.
  • Since activation of the photoacid generator and the curing of the network can be achieved simultaneously by irradiating the desired layers with UV-A light (365 nm), a compromise between sufficient activation and resolution has to be made.
  • Prior to the shape memory experiments it is important to thermally anneal the networks to form additional crosslink sites by hydrogen bonding, which leads to a change in thermal and mechanical properties. After 4 h at 140 °C, the network properties remain constant and the printed test specimen are able to repeatedly undergo shape changes after the programming step.
  • Photoacid generators are highly versatile transesterification catalysts and can be applied for imparting dynamic network properties in numerous photopolymer systems. Network architecture can be conveniently adjusted by the structure and functionality of the monomers and/or crosslinkers.

 

Citation to the paper: Locally controlling dynamic exchange reactions in 3D printed thiol-acrylate vitrimers using dual-wavelength digital light processing, Polym. Chem., 2021,12, 3077-3083, DOI: 10.1039/d1py00427a

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d1py00427a 

 

Athina AnastasakiDr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

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Paper of the month: Synthesis, characterization and self-assembly of linear and miktoarm star copolymers of exclusively immiscible polydienes

Ntetsikas et al. synthesised a series of linear and miktoarm star copolymers to study their self-assembly behaviour in bulk.

Block copolymers consisting of high 1,4-microstructure-content polybutadiene (PB1,4) blocks  and high 3,4-content polyisoprene (PB1,4b-PI3,4) blocks self-assemble, due to their incompatibility, to form different nanostructures useful for various applications, such as electronic devices, nanotechnology and optoelectronics. In this work, Avgeropoulos and co-workers report a new synthetic procedure for the preparation of four linear PB1,4b-PI3,4 diblock copolymers and eight asymmetric miktoarm star copolymers and investigate the effect of the architecture (linear versus non-linear) on microphase separation and final nanostructure of these copolymers. Furthermore, the results of this study have been compared with the PS(PI1,4)n (PS: polystyrene) well studied and established systems.

In particular, the authors combined anionic polymerization and selective chlorosilane chemistry to prepare four different sets of linear and star copolymers. Each set included one linear diblock copolymer with similar molecular characteristics to the corresponding PB1,4(PI3,4)2 and PB1,4(PI3,4)3 miktoarm stars. All copolymers were carefully characterized by size-exclusion chromatography (SEC), membrane osmometry (MO) and nuclear magnetic resonance (NMR) indicating a high degree of molecular and compositional homogeneity. Differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) were used to verify microphase separation and reveal the effect of the architecture on the adopted topologies. Through such a comprehensive characterization the authors have discovered that the high chain flexibility provided by the two polydiene segments affords promising properties previously unattainable from the corresponding triblock copolymers of these polydienes with polystyrene.

This work paves the way for further studies of material properties such as rheology and binary blends of the pure linear and non-linear copolymers with corresponding homopolymers (either hPB1,4 or hPI3,4).

We look forward to further exciting findings from the Avgeropoulos’ group.

Tips/comments directly from the authors:

  • Morphological characterization studies reveal the coherence of theoretical studies on the PS(PI1,4)n system and the experimental results of the PB1,4(PI3,4)n system (PS is substituted by PI3,4).
  • The only discrepancies from the relevant PS/PI system were found for two linear copolymers, where in both samples, hcp cylinders of the minority phase in the matrix of the majority were observed, instead of the expected DG cubic structure morphology.
  • The almost identical electron densities between the two polydienes led to impossible morphological characterization through small angle X-ray scattering (SAXS) and only transmission electron microscopy results verify the adopted morphology for each copolymer.
  • The adopted well-ordered nanostructures lead to the assumption that the segment–segment interaction parameter between the two polydienes of high 1,4-microstructure (∼92%) for the PB and ∼55–60% 3,4-microstructure for the PI is well above zero.
  • It was really exciting to verify that if the 3,4-microstructure for the PI blocks was not within the regime of ∼55–60% then a homogeneous structure was adopted (no microphase separation).
  • This regime of ∼55–60% 3,4-microstructure for the PI segments can be achieved by just adding a very small amount of a polar additive (∼1ml of THF) in the polymerization solvent ( 200 ml of benzene).

 

Citation to the paper: Synthesis, characterization and self-assembly of linear and miktoarm star copolymers of exclusively immiscible polydienes, Polym. Chem., 2021,12, 2712-2721, DOI: 10.1039/D1PY00258A

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00258a#!divAbstract

 

Professor Athina AnastasakiDr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

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Paper of the month: Mechanically tough yet self-healing transparent conductive elastomers obtained using a synergic dual cross-linking strategy

Zhang et al. demonstrate a simple methodology for the synthesis of mechanically tough and self-healing materials.

Polymers that simultaneously exhibit high mechanical toughness and self-healing properties are essential to develop the next generation of materials for various applications including optoelectronics, sensors and healthcare devices. In this work, He, Li and co-workers developed a new route for the synthesis of a tough and self-healing supramolecular network by introducing Al(III)-carboxyl complexes into photo-polymerizable deep eutectic solvent. The resulting elastomers were mechanically tough and self-healing and demonstrated high transparency and stretchability as well as ionic conductivity. In particular, thanks to the synergic interaction of H-bonds and coordination bonds within the polymer matrix, over 80% self-healing efficiency for the damaged film could be obtained after 72 hours at ambient temperature. At the same time, the interaction between the fractures interfaces allows for the quick recovery of ion channels, therefore inducing electrical self-healing properties. The materials have been thoroughly characterized by a number of techniques including nuclear magnetic resonance, electrochemical impedance spectroscopy, differential scanning calorimetry, dynamic mechanical analysis, and tensile experiments. The developed methodology is simple, environmentally friendly and fast. The authors envision that their fundamental design concept will significantly contribute to the development of the next generation of tough and self-healing materials for potential use in flexible electronics and other applications.

Tips/comments directly from the authors:

  • Although in this paper we only report on the preparation of transparent conductive polymers from Al(III)-based polymerizable deep eutectic solvents (PDESs), this method can also be extended to other types of metal cations such as copper, zinc, cobalt and nickel ions. However, considering that some metal ions are coloured, the prepared conductive elastomers may be suitable for other areas of application.
  • During the experiments, we also found that PDES with trivalent iron ions could not be polymerized under UV light. The reasons for this phenomenon are not yet clear and we will continue to investigate.
  • The prepared transparent conductive elastomers are somewhat hygroscopic and therefore need to be kept dry during storage and use to maintain their performance stability.
  • If a high transparency film is desired, a release film should be necessary to insulate the polymer from oxygen during polymerization to maintain the homogeneity of the polymer structure.

Citation to the paper: Mechanically tough yet self-healing transparent conductive elastomers obtained using a synergic dual cross-linking strategy, Polym. Chem., 2021,12, 2016-2023, DOI:10.1039/D0PY01760D

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01760d 

 

Professor Athina AnastasakiDr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

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Paper of the month: Synthesis of disulfide-bridging trehalose polymers for antibody and Fab conjugation using a bis-sulfone ATRP initiator

Forsythe and Maynard present a new strategy for the direct synthesis of disulphide-bridging trehalose polymers using a bis-sulfone ATRP initiator.

Conjugated polymers are often used to enhance the properties of therapeutic molecules such as antibodies and antigen binding fragments (Fabs). In this work, Forsythe and Maynard present a new strategy to prepare polymer-antibody/Fab conjugates by employing bis-sulfone end-groups installed via a functionalized atom transfer radical polymerization initiator. In particular, a bis-sulfone initiator was first synthesized and subsequently subjected to activators generated electron transfer (AGET) polymerization using ascorbic acid as the reducing agent. Upon optimizing the ligand concentration (special care was taken here as an excess of ligand causes a detrimental side reaction which results in broadening of the molecular weight distributions), disulphide-reactive trehalose polymers could be efficiently prepared with controlled molecular weight and fairly low dispersity, thus confirming a controlled polymerization. The polymers were then conjugated to a full Immunoglobulin G and its Fab fragment and the reaction proceeded quantitatively as confirmed by western blot and mass spectrometry. The stability of the resulting conjugates was then assessed by accelerated heat stress studies where the trehalose polymer was found to considerably increase the thermal stability of both Herceptin and Herceptin Fab. Importantly, this new strategy allows for a facile way to synthesize polymeric bioconjugates without the need for time consuming post-polymerization modification methods while also exhibiting very good monomer compatibility. As the authors conclude, they anticipate a continued exploration in the field of antibody and protein conjugation and we look forward to reading the next exciting findings from the Maynard group.

 

Tips/comments directly from the authors:

  • The bis-sulfone functionality is a robust system for the production of protein-polymer conjugates. However, due to its base-sensitivity, care needs to be taken when incorporating it into polymers. Since common ligands for AGET ATRP display reactivity towards the bis-sulfone, both reaction temperature and concentration should be kept low.
  • For performing conjugations using the bis-sulfone, we found it important to do a two-step reduction and alkylation where the reducing agent (DTT) was removed prior to the conjugation. To help avoid re-oxidation of the disulfides during this process, we used a buffer containing EDTA to prevent trace metal-mediated oxidation and additionally used desalting columns that allow for rapid removal of excess DTT.
  • While we demonstrate the applicability of the bis-sulfone initiator for AGET ATRP, the chemistry should be amenable to other controlled polymerizations such as RAFT. Incorporation into a chain transfer agent could further expand the diversity of chemistry available for antibody conjugation.
  • Trehalose polymers stabilized the antibody and Fab to temperature increases. However, the same polymer could also increase the stability of the conjugates in vivo since these polymers have improved the pharmacokinetics of other proteins.

Citation to the paper: Synthesis of disulfide-bridging trehalose polymers for antibody and Fab conjugation using a bis-sulfone ATRP initiator, Polym. Chem., 2021,12, 1217-1223, DOI: 10.1039/D0PY01579B

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01579b 

 

Athina AnastasakiDr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

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Paper of the month: Digital light processing 3D printing with thiol–acrylate vitrimers

Rossegger et al. present a new transesterification catalyst which can be applied in thiol-acrylate vitrimer systems enabling the fabrication of precise 3D objects.

image describing the work

Vitrimers are a unique class of materials that possess the remarkable property to be thermally processed in a liquid state while maintaining their network integrity. This property is induced owing to various thermo-activated exchange reactions including the catalyzed transesterification of hydroxyl ester moieties. However, the possibility to introduce dynamic covalent bonds into 3D printable photopolymers is challenging and the printed objects often suffer from a range of limitations such as low resolution, poor surface quality and lack of versatility. In addition, conventional transesterification catalysts exhibit poor solubility and present additional compromises on cure rate and pot life of photocurable resins. To this end, Schlögl and co-workers introduced a mono-functional oligomeric methacrylate phosphate as a new and efficient transesterification catalyst. The catalyst has many advantageous characteristics: it is liquid, easily dissolved in a range of acrylic monomers and can be covalently incorporated into the network across its methacrylate group. Once photo-cured, the dynamic thiol-click networks are able to rapidly undergo thermo-activated rearrangements of their network topology as shown by stress relaxation experiments. Importantly, when applied in thiol-acrylate vitrimer systems, precise 3D objects with 500 µm features using bottom-up digital light processing can be obtained. When compared to other commonly employed catalysts, the mono-functional methacrylate phosphate is superior both in terms of solubility and stress relaxation, thus unlocking a new toolbox of photocurable vitrimers.

 

Tips/comments directly from the authors:

  • Owing to their strong Brønsted acidity, organic phosphates are able to catalyze transesterifications in hydroxyl ester networks. They exhibit a better performance in catalyzing exchange reactions in dynamic photopolymers compared to Lewis acids such as Zn(OAc)2.
  • Stress relaxation kinetics increase with rising catalyst content. However, the catalyst content should not exceed 50 mol% as the resin formulation is getting destabilized. Below 50 mol%, the thiol-click resin is stable over several weeks. This is a clear advantage compared to conventional transesterification catalysts, which initiate thiol-Michael reactions and lead to a premature gelation of thiol-click resins.
  • Prior to the shape memory experiments it is important to thermally anneal the networks to form additional crosslink sites by hydrogen bonding, which lead to a change in thermal and mechanical properties. After 4 h at 180 °C, the network properties remain constant and the printed test specimen are able to repeatedly undergo shape changes after the programming step.
  • Organic phosphates are highly versatile transesterification catalysts and can be applied for imparting dynamic network properties in numerous photopolymer systems. Network architecture can be conveniently adjusted by the structure and functionality of the monomers and/or crosslinkers.

 

Citation to the paper: Digital light processing 3D printing with thiol–acrylate vitrimers, Polym. Chem., 2021,12, 639-644, DOI: 10.1039/D0PY01520B

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01520b

 

Professor Athina AnastasakiDr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

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Paper of the month: Ring opening polymerization of ε-caprolactone through water

Atta et al. demonstrates a simplified ROP protocol which operates in the absence of any inert gas and without the need of drying any of the reaction’s reagents.

image describing the work

Ring Opening Polymerization (ROP) is arguably one of the most popular methodologies to synthesize biodegradable materials such as polycaprolactone (PCL) and poly (lactic acid) (PLA). However, a major drawback of this approach which severely limits its applicability is that it typically operates under completely moisture-free conditions, as water is well-known to deactivate the catalyst and terminate the propagating chains. To avoid water contamination, highly specialized equipment (e.g., Schlenk lines or glove boxes) as well as anhydrous reagents have to be employed which makes the process particularly tedious for both experts and non-experts. To overcome this, Gormley and co-workers have developed two elegant and simple methods that allow for the facile synthesis of PCL through ROP in a laboratory oven and without using any inert gas or dry reagents. In the first technique, a vacuum oven was employed to evaporate water from a traditional ROP reaction with stannous octoate as the catalyst while in the second approach titanium isopropoxide was utilized to simultaneously quench residual water and catalyze ROP. Impressively, and despite the simplicity of those methodologies, a range of chain lengths could be synthesized (degree of polymerization 25-500) with relatively good control over the molecular weight distributions of PCL (Đ < 1.5 for all cases). It is highlighted that a large excess of water impurities (750 ppm) could be tolerated by both methods yielding well-defined polymers at quantitative conversions. This work represents a great example of a simplified ROP which operates in the absence of complicated reactions set ups and can be performed in any laboratory. As the authors also remark, targeting even higher molecular weights or achieving even lower dispersity values will be the next challenge to address and we very much look forward to the next developments by the Gormley group.

Tips/comments directly from the authors:

  • The rational goal of this work is to enable the ROP reaction in an oven without inert gas environment and without drying or purifying the reagents.
  • The most exciting aspect of this work is to enable non-experts to synthesize custom polymers.
  • TTIP plays multiple roles in this ROP reaction. It not only initiates and catalyzes the polymerization reaction but also eliminates water from the reaction medium.
  • It is important for the audience that we should perform this experiment with minimal mixing time (within 1-5 sec) as water present in the air can contaminate CL.
  • The purity of CL can be easily checked by TTIP. A precipitate of TiO2 was formed when the water content of CL was above 750 ppm, and a cloudy solution was observed.

Citation to the paper: Ring opening polymerization of ε-caprolactone through water, Polym. Chem., 2021,12, 159-164, DOI: 10.1039/D0PY01481H

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01481h 

Professor Athina AnastasakiDr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

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Paper of the month: Enzyme-responsive polymeric micelles with fluorescence fabricated through aggregation-induced copolymer self-assembly for anticancer drug delivery

Yan et al. develop new enzyme-responsive polymeric micelles with potential applications in cancer therapy.

image describing the work

One of the most exciting and fast-growing topics in polymer chemistry is the synthesis of amphiphilic copolymers that can self-assemble into nanoparticles. Hydrophobic compounds such as cancer drugs can be encapsulated in the core of these self-assembled nanoparticles, thus protecting them from degradation or unwanted interactions with healthy cells. In addition, advances in polymer end-group functionalization allow the conjugation of special ligands on the nanoparticle surface which are responsible for directing the nanoparticles to cancer cells. Upon reaching the tumours (or being taken up by cancer cells), the nanoparticles must release the encapsulated drugs in order to kill the cancer cells. This drug release step requires the use of stimuli-responsive smart polymers that can switch from hydrophobic to hydrophilic upon exposure to stimuli. Temperature, pH, and enzyme-responsive polymers are therefore developed to release drugs on-demand. In this work, Zhao and co-workers further advance the field by synthesizing new fluorescent nanoparticles which can release a cancer drug (doxorubicin) while simultaneously turning off the fluorescent signal when the drug is released. This was achieved by efficiently coupling a tetraphenylethene moiety onto poly(acrylic acid). The hydrophobic property of the tetraphenylethene moiety induces the self-assembly of the resulting diblock copolymers into fluorescent nanoparticles via an aggregation-induced self-assembly mechanism. Upon exposure of the fluorescent nanoparticles to esterase, this enzyme can hydrolyze the ester bond between the tetraphenylethene side chain and the polymer backbone. The enzyme-catalyzed hydrolysis reaction turns the hydrophobic block back to the water-soluble poly(acrylic acid) block and therefore, disassembles the nanoparticles and also turns the fluorescent signal off. The diblock copolymer has poly(ethylene glycol) as the corona-forming block which possesses negligible toxicity to healthy cells. Therefore, this new copolymer is very promising for drug delivery applications, especially when monitoring the drug release is essential.

Citation to the paper: Visible light enabled para-fluoro-thiol ligation, Polym. Chem., 2020, 11, 7704-7713, DOI: 10.1039/D0PY01328E

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py01328e

Professor Athina Anastasaki Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

 

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Paper of the month: Visible light enabled para-fluoro-thiol ligation

Engelke and Truong demonstrate a light-induced para-fluoro-thiol reaction using the photogeneration of a superbase.

 

Graphical abstract for the paper

Post polymerisation modification of macromolecules enables the rapid synthesis of a wide range of polymers with different properties from the same starting material. One promising post polymerisation modification strategy is para-fluoro-thiol reaction (PFTR) which can be further expanded through the use of light as an external stimulus. In this work, Engelke and Truong describe a facile method towards light-enabled PFTR by employing the thioxanthone- 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) salt for light-induced activation of PFTR. The authors exploit this strategy in polymer chemistry by grafting various thiol-containing molecules to a post polymerisation modified backbone and by inducing polymer crosslinking.

The very fast release of DBU under visible light irradiation (blue light, 420 nm) allows for efficient para-fluorothiol ligation which can be used in the synthesis of small thioether molecules. Importantly, this photochemical process could be realized in very high yields (typically >85%) and the product can be easily isolated from the salt by products. The unique aspect of this approach is the temporal control over the photo-induced ligation compared to all other reactions employing photogeneration of a base catalyst, where the reaction continues even when the light is turned off. The PFT ligand could also be initiated by sunlight thus offering for a non-invasive and low-cost technique for the fabrication and modification of complex macromolecular structures. The authors are confident that such strategy mediated by external stimuli will be highly advantageous for soft lithography applications generating micro-and nanosized architectures.

 

 

Tips/comments directly from the authors:

 

1)  Don’t be turned off by the synthesis of the caged DBU.  It’s very straightforward and most of the steps are high yielding. In the step where poly(phosphoric acid) is used, a mechanical stirrer is highly recommended as the mixture can become quite viscous.

2)  The exciting aspect of this work is the efficient uncaging by sunlight which, as an energy source, is non-destructive, low-cost and pollution-free.

3)  Since DBU is a popular organocatalyst for some very important polymerization techniques, such as the ring opening polymerization of lactides and cyclic carbonates, or the polycondensation of isocyanate and polyols. This strategy could be employed for light-mediated polymerization of such monomers, enabling the synthesis of materials that would not be possible to access otherwise.

4)  It was quite fun to see the CO2 bubble (through a cannula) as soon as the light was switched on!

 

Citation to the paper: Visible light enabled para-fluoro-thiol ligation, Polym. Chem., 2020, 11, 7015-7019, DOI: 10.1039/D0PY01373K

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py01373k

 

Athina Anastasaki

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

 

 

 

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Paper of the month: Thermoresponsive properties of poly(acrylamide-co-acrylonitrile)-based diblock copolymers synthesized (by PISA) in water

Audureau et al. report the synthesis of UCST-thermoresponsive diblock copolymers using reversible addition-fragmentation chain transfer (RAFT) polymerization in aqueous media.

Image describing the paper

Thermoresponsive polymers have attracted widespread interest in both fundamental research and industrial applications due to their special ability to change properties upon cooling or heating. Polymers exhibiting upper critical solution temperature (UCST) are soluble in a solvent above the UCST but precipitate from the same solvent when cooling below its critical temperature. In the large UCST polymer family, the statistical copolymer poly(acrylamide-co-acrylonitrile) (P(AAm-co-AN)) has gained increasing interest and has been used to prepare nanoparticles for drug delivery, cancer theranostics/chemotherapy and photoacoustic imaging. However, a method for scalable synthesis of thermoresponsive P(AAm-co-AN) block copolymer in water and in-situ self-assembly of the resulting copolymers into nanoparticles remains elusive. In this work, Rieger, Stoffelbach and co-workers employed polymerization-induced self-assembly technique (PISA) to synthesize, for the first time in water, well-defined P(AAm-co-AN) block copolymers which self-assembled into nanoparticles. Importantly, the rare worm-like morphology was successfully obtained, which paves the way for developing better cancer drug delivery systems since nanoworms have distinct and advantageous properties when compared to their spherical counterparts such as long circulation time, high accumulation in tumour and deep tumour penetration. Furthermore, an interesting worm-to-sphere morphological transition was observed upon heating the nanoworms solution. This is in contrast to previous reports where a worm-to-sphere transition was only demonstrated upon cooling and therefore, offers a new promising strategy to design novel smart nanoparticles for various applications.

 

Tips/comments directly from the authors:

 

1)  The thermoresponsive properties of the copolymers crucially depend on the molar fraction of acrylonitrile (FAN) in the P(AAm-co-AN) block, tunable by the initial AN fraction (fAN) in the monomer feed. As AN is volatile, a closed Schlenk system should be used to avoid monomer evaporation during polymerization and produce polymers with predictable properties.

2) P(AAm-co-AN) statistical copolymers exhibited a typical UCST-type thermal transition for acrylonitrile molar fractions (FAN) ranging from 0.3 to 0.5.

3) In addition to FAN, the presence of a hydrophilic PDMAc block and the DPn of the polymer blocks also impact the thermoresponsiveness.

 

Citation to the paper: Thermoresponsive properties of poly(acrylamide-co-acrylonitrile)-based diblock copolymers synthesized (by PISA) in water), Polym. Chem., 2020, 11, 5998-6008, DOI: 10.1039/D0PY00895H. Link to the paper here.

More papers on PISA can be found at our themed collection here!

 

About the web writer:

Professor Athina Anastasaki

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she has joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

 

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Paper of the month: Single-chain crosslinked polymers via the transesterification of folded polymers: from efficient synthesis to crystallinity control

Terashima and co-workers report efficient synthetic systems of single-chain crosslinked polymers.

 

Crosslinked polymers have emerged as a class of unique materials which find use in a diverse range of applications such as drug delivery, dispersants and coating industries. Typically, those materials are made through a combination of controlled polymerization and crosslinked methods. In this work, Terashima and co-workers prepared a range of single-chain crosslinked polymers with controlled crystallization. This was achieved by the intramolecular transesterification of random copolymers compromising of octadecyl methacrylate, 2-hydroxyethyl methacrylate, and methyl acrylate. Those copolymers were self-folded in organic media (octane was used as the solvent) through the association of the hydroxyl groups to form reverse micelles. Upon synthesis, the micelles were intramolecularly crosslinked by an efficient transesterification of the methyl acrylate units with the hydroxyl groups to produce polymer nanoparticles with pending octadecyl groups. The materials synthesized were thoroughly characterized by a number of techniques including nuclear magnetic resonance, gel permeation chromatography, small angle X-ray scattering and dynamic light scattering. The developed system allowed for the efficient control of the molecular weight of the crosslinked polymers owing to the precise synthesis of the precursors prepared by living radical polymerization. Importantly, the degree of crosslinking was found to control the crystallinity of the products. Last but not least, a relatively high concentration could be used (up to 50 mg ml-1).  As the authors allude to in their conclusion, their work has paved the way to the production of well-defined polymeric nanoparticles that can be employed for surface coating, painting, optical plastics and cosmetics.

 

Tips/comments directly from the authors:

 

1) Intramolecular crosslinking of folded polymers in organic media via transesterification affords the precision and high-throughput synthesis of single-chain crosslinked polymer nanoparticles.

2) The molecular weight of the crosslinked polymers can be controlled as desired at the stage of the synthesis of the precursor polymers by controlled radical polymerization.

3) Transesterification between hydroxyl groups and methyl acrylate units efficiently proceeds within the cores of folded micelles to fix the folded structures in a specific solvent.

4) SEC-MALLS analysis is essential to characterize single-chain crosslinked polymers. Because of the compact structures, the apparent molecular weight of the crosslinked polymers by the general RI detector with PMMA standard calibration turns smaller than that of the non-crosslinked precursor polymers. If the absolute weight-average molecular weight of the crosslinked polymers by the MALLS detector is also close to that of the precursor polymers, you can conclude that the products consist of single chain-crosslinked polymers.

5) Crystallinity of the bulk polymers is controlled by tuning the degree of intramolecular crosslinking. This is an interesting approach to control the thermal and physical properties of solid polymer materials.

Citation to the paper: Single-chain crosslinked polymers via the transesterification of folded polymers: from efficient synthesis to crystallinity control, Polym. Chem., 2020, 11, 5181-5190, doi.org/10.1039/D0PY00758G

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00758g

About the web writer:

Professor Athina Anastasaki

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

 

 

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