Polymer Chemistry Blog

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 

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.

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 

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.

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

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

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.

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

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

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.

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

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 (F_AN) in the P(AAm-co-AN) block, tunable by the initial AN fraction (f_AN) 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 (F_AN) ranging from 0.3 to 0.5.

3) In addition to F_AN, the presence of a hydrophilic PDMAc block and the DP_n 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:

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.

Xu and co-workers utilize model trimers as a guide for discrete oligomer synthesis.

Biomacromolecules such as DNA and proteins exhibit perfect sequence precision which allows them to fulfill a number of biological functions. A long-lasting challenge in polymer chemistry is to mimic these biopolymers through synthetic analogues. In particular, single unit monomer insertion technique has emerged as a powerful tool to synthesize sequence-defined polymers with perfect uniformity. Key to this approach is the alternating addition of electron-donor and acceptor monomers which can be utilized to prepare long polymer chains through sequential monomer radical additions occurring one unit at a time. Despite notable progress in the last decades, such alternative and sequential monomer additions often produce complex radical reaction kinetics which makes the formation of diverse polymer sequences challenging. To this end, simplifying reaction processes and establishing simple reaction kinetics is essential to bring a rapid and reliable synthesis. In this work, Xu and co-workers describe a methodology to address this challenge by employing model trimers as a guide for the synthesis of sequence-defined polymers. Central to the design is the sequential and alternating PET-RAFT SUMI technology which enables the acquisition of full kinetic data, thus providing a very useful insight over both reaction rates and yields. Four different families of α,β-disubstituted vinyl monomers (N-phenylmaleimide (PMI), fumaronitrile (FCN) and dimethyl fumarate (DMF) and indene (Ind)) were employed to prepare nine model trimers. These model compounds were subsequently utilized to guide the synthesis of longer discrete polymers (pentamers with diverse monomer sequences) through multiple insertions yielding materials with high isolated yields. The authors’ findings were supported by nuclear magnetic resonance and mass-spectrometry which were used to establish reaction rate and product purity respectively. The authors anticipate that their method can also be applied to other vinyl polymers and different RAFT initiation systems. Such monodispersed materials with perfect sequence control are expected to find use in a range of applications.

Tips/comments directly from the authors:

1)  The use of automated flash chromatography can effectively simplify the SUMI product purification and allows for a more efficient and reproducible synthesis.

2) The online-NMR spectroscopy is an effective technique to monitor RAFT agent and monomer conversion in RAFT SUMI.

3)  There are several diastereoisomers for each of SUMI products that would show different polarities in column chromatography and complicated NMR spectra. Careful implementation and thoughtful data analysis are required.

4) The characterization of long chain oligomers (more than four monomer units) is quite challenging. Only mass spectrometry is available for the structure confirmation. Therefore, the model trimers are very important to guide the synthesis of long chain oligomers. All triad sequences in long chain oligomers can be found in the model trimers.

5). The established model trimers and kinetics data could also provide experimental and theoretical guidance for the synthesis of alternating polymers and investigation of mechanism and kinetics of radical copolymerization.

Citation to the paper: Sequential and alternating RAFT single unit monomer insertion: model trimers as the guide for discrete oligomer synthesis, Polym. Chem., 2020, 11, 4557-4567, DOI: 10.1039/d0py00390e

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00390e

Read more papers from our Pioneering Investigators 2021 collection here!

About the web writer

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.

D’Agosto, Lansalot and co-workers report the synthesis of nanoparticles with various shapes through the RAFT/MADIX PISA polymerization of vinyl acetate.

Polymerization-induced self-assembly (PISA) is a widely used technique that allows access to the formation of a range of polymeric nanoparticles including spheres, worms and vesicles. Although this methodology has been very successful with dispersion polymerizations, emulsion polymerization systems are mostly limited to the preparation of spherical particles. Poly(vinyl acetate) latexes are obtained by emulsion polymerization and find use in many industrial applications but yet, the preparation of higher ordered morphologies remains challenging. D’Agosto, Lansalot and co-workers were able to circumvent this by conducting the emulsion polymerization of vinyl acetate at higher temperatures anticipating that this would not only lead to much faster reaction kinetics but also to the softening of the polymeric nanoparticles allowing for increased flexibility and rearrangements. Indeed, the aqueous macromolecular design via interchange of xanthate (MADIX)-mediated emulsion polymerization of vinyl acetate from a poly(ethylene glycol) with a xanthate chain-end macro-CTA led to well-controlled polymerizations with high blocking efficiency accompanied with the formation of stable latexes. By judiciously adjusting the targeted degree of polymerization, the authors triggered for the first time the morphological transformation from spherical to higher ordered morphologies and observed the formation of vesicles (with different sizes) as well as worm-like nanoparticles. In particular, the worm-like morphology could alternatively be observed by increasing the solid content from 10 to 15 wt%. The data was supported by very nice cryo-TEM images which depicted all the discussed morphologies. The range of obtained shapes were attributed to the high water solubility of vinyl acetate combined with the low Tg of PVAc. The presented elegant findings enhance our fundamental understanding on emulsion PISA systems where polymerization temperature and solid content significantly affect the resulting morphology.

Tips/comments directly from the authors:

1. The polymerization takes place above the T_g of the forming PVAc block, which seems to be key for accessing non spherical morphologies in VAc PISA.
2. PEG-b-PVAc block copolymers are obtained in very short times.
3. This system provides an interesting medium for investigating the impact of several parameters on the morphologies obtained through PISA processes.
4. Extension of this strategy to other non-activated monomers, for instance in the copolymerization of vinyl acetate and ethylene, seems accessible.

Citation to the paper: Poly(ethylene glycol)-b-poly(vinyl acetate) block copolymer particles with various morphologies via RAFT/MADIX aqueous emulsion PISA, Polym. Chem., 2020, 11, 3922-3930, DOI: 10.1039/d0py00467g

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00467g

Read more papers on PISA in our Polymerisation-Induced Self Assembly themed collection here!

About the web writer:

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.