Polymer Chemistry Blog

Nanotransfer printing is a technique often used to construct complex patterns by employing elastomeric stamps and relying on surface chemistries. This has enabled not only the assembly of complex constructs but also the effective integration of heterogeneous materials. In the current manuscript, Campos and co-workers significantly contributed to this direction by introducing a nanotransfer technique, termed soft pattern-transfer printing, which does not rely on adhesive layers or external stimuli. As a result, a cost and time efficient high throughput processing platform is being developed. To achieve this, representative organic thin films of P3HT homopolymers, self-assembled diblock copolymers and functionalized perylene diimide small molecules were employed as inks for micron-sized array of patterns ranging from squares, lines, polygons and rings. Importantly, hierarchical patterns were obtained through microns-sized arrays of self-assembled block copolymers. In addition, to build layers of complex structures onto the same film, the technique can be repeated through sequential printing. As the authors elude in their conclusion, such high-fidelity pattern transfer work is very promising for potential uses in a number of areas such as the construction of van der Waals heterostructures interfaced with self-assembled block copolymer thin films and the development of platforms to investigate the influence of hierarchical patterning on cell differentiation.

Tips/comments directly from the authors:

1. Solvent-vapor induced self-assembly of diblock copolymer thin films is an attractive approach to achieve long-range microphase segregation.
2. Achieving solvent-vapor induced self-assembly of diblock copolymer thin films directly on exfoliated materials is particularly challenging because of the macroscopic topographical heterogeneities which disrupt the film integrity.
3. Moreover, the generation of hierarchical patterns, particularly with one length scale in the nanometer regime, often involves lithographic processes which are difficult to scale.
4. A simple contact-based approach is presented for transfer of polymeric materials (e.g. self-assembled block copolymers, homopolymers, small molecules), with well-defined edge resolution (<20 nm) and high fidelity of nanoscale pattern transfers.
5. To avoid warped or cracked transfers, it is critical to handle PDMS stamps with care, avoiding excessive mechanical deformation, and to apply minimal pressure.
6. Importantly, we show successful transfer of solvent-vapor induced self-assembled diblock copolymer films onto 2D materials (e.g. boron nitride).
7. The transfer of micron-scale patterns of self-assembled diblock copolymers with nanoscale features yield hierarchical ordering.
8. Patterns resulting from sequential soft nanotransfer printing resemble Moiré patterns, large-scale interference patterns. Such complex patterns may be used to impart local physical and electronic perturbations.

Read this article for free until the 31st July!

Hierarchical patterns with sub-20 nm pattern fidelity via block copolymer self-assembly and soft nanotransfer printing, Polym. Chem., 2019, 10, 3194-3200, DOI: 10.1039/C9PY00335E

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.

Controlled radical polymerization strategies are often exploited to tailor the properties of functional polymer networks. With the recent developments in external stimuli to regulate polymerization, the use of light has received significant attention as it enables the synthesis of materials with precise spatial, temporal, and sequence control.  In order to design structurally tailored and engineered macromolecular (STEM) networks, Matyjaszewski and co-workers proposed a new, metal-free approach to prepare well-defined networks. To achieve this, the authors selectively activated the fragmentation of trithiocarbonate reversible addition-fragmentation chain-transfer (RAFT) agents by visible light RAFT iniferter photolysis coupled with RAFT addition-fragmentation process. Through this two-step synthesis, different materials could be polymerized yielding compositionally and mechanically differentiated networks. Upon carefully selecting the crosslinker as well as the RAFT inimer, three different types of primary polymethacrylate networks could be generated under green light. The obtained networks were further enriched by the addition of methyl acrylate and dimethylacrylamide under blue light, resulting in soft and stiff gels respectively. Importantly, dynamic mechanical analysis was utilized to characterize the mechanical properties of both the starting and the final materials and to determine their glass transition temperatures. Such STEM networks significantly expand the toolbox of polymer and material science.

Tips/comments directly from the authors:

1. Structurally tailored and engineered macromolecular (STEM) networks are versatile materials containing latent functional groups accessible for post-synthesis modifications to afford new chemical and material properties.
2. The network synthesis and modifications were controlled using dual wavelengths (green and blue). The primary network was synthesized under green light irradiation, and the subsequent modifications were performed under blue light.
3. Initial network synthesis involves incorporation of two RAFT photoiniferters with similar Z groups (thioalkyl) but different R groups (either a tertiary or secondary carbon radical) to enable activation of one RAFT agent over the other under green light. This is followed by activation of both RAFT agents for secondary modification under blue light.
4. The n to π^* electronic transition at 520 nm affords photolysis of trithiocarbonate with 4-cyanopentanoic acid R-leaving group under green light leading to generation of tertiary carbon radicals promoting polymerization of methacrylates. The second trithiocarbonate RAFT agent with propionic acid R-leaving group is also incorporated into this network during this process as a RAFT methacrylate monomer or dimethacrylate crosslinker.
5. Selective activation under green light is made possible as the addition of 4-cyanopentanoicacid radical to trithiocarbonate RAFT agent with propionic acid R-leaving group does not lead to fragmentation as radical stabilization energies of tertiary radicals are higher than secondary radicals.
6. Therefore, the methacrylate/dimethacrylate RAFT agent with propionic acid R-leaving group remains inert under green light and can only be activated under blue light (465 nm) where the n to π^* electronic transition lies.
7. Both RAFT agents (secondary and tertiary leaving groups) are then activated in a second step which involves soaking in a second monomer (acrylate or acrylamides) into the network followed by polymerization under blue light.
8. Depending on the functionality of the second monomer, the post-modified network can be either softer or stiffer with different responses to polarity (hydrophilicity/hydrophobicity).

Read the full paper now for FREE until 12th July!

Transformation of gels via catalyst-free selective RAFT photoactivation, Polym. Chem., 2019, 10, 2477-2483, DOI: 10.1039/C9PY00213H

About the webwriter

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.

Being able to achieve perfect sequence of the repeating units/monomers has recently attracted significant attention within the polymer chemistry community and the ultimate aim is to achieve similar monomer precision with natural biomacromolecules, such as DNA or proteins. Ouchi and co-workers contributed to this direction by exploring in detail the iterative single unit monomer radical addition of a bulky tertiary, adamantyl and isopropyl methacrylic monomer (IPAMA) in order to yield sequence-defined oligo- or poly(methacrylates) in high yields. The authors focused on improving the efficiency of the single unit addition and eliminating all unfavourable products, which is a significant challenge of this technique. To achieve this, the introduction of an activated ester for the alkyl halide or the adduct was essential in improving the accuracy of the single unit addition of IPAMA. In particular, a 4 step cycle consisting of “radical addition”, “transformation”, “selective cleavage” and “active esterification” was elegantly developed. Although one more step was required to change the electron density of the halogen terminal, the efficiency of single unit addition was enhanced and high yields were obtained. Importantly, and despite the yields being close to 100%, the authors suggest that the additional introduction of some supporter such as solid resin would make the presented approach much more scalable and practical.

Tips/comments directly from the authors:

1. The adamantyl and isopropyl methacrylic monomer (IPAMA) shows no homopolymerization ability due to the bulkiness.  The double bond of IPAMA is active enough for radical species like general methacrylic monomers and thus single unit addition is anticipated under the condition of reversible deactivation radical polymerization or controlled radical polymerization.
2. The tertiary and bulky ester pendant can be transformed into less bulky alkyl substituent.  Such transformation allows iterative  single unit addition to give sequence-defined methacrylic oligomers and polymers.
3. The introduction of an activated ester in an alkyl halide dormant species for ATRP allows quantitative single unit radical addition of IPAMA, in contrast to general alkyl halide resulting in bimolecular coupling and/or less quantitative reaction.
4. The activated ester pendant on the penultimate unit for the adduct is transformed into alkyl ester (transformation), followed by selective cleavage of the terminal IPAMA unit under acidic condition.  The relative high molecular weight of N-hydroxy-5-norbornene-2,3-dicarboxyimide ester as activated ester was useful for study in single unit addition by SEC.
5. The excess amount of IPAMA (10 eq for the halide) is required to complete the radical addition at a suitable rate.  The unreacted monomer can be removed by preparative SEC or silica column chromatography.
6. The Cp*-based ruthenium complex with bisphosphine monoxide was useful as the catalyst for single unit radical addition.  The copper catalyst with dNbpy was also available.  Other copper catalysts were not studied.  The high efficiency in the radical addition is desirable for synthesis of sequence-defined methacrylic oligomers and polymers in high yields.
7. Temperature is important to balance the quantitative reaction with the speed.
8. COMU was best among condensing agents for the esterification studied in this work.

Read the full article for FREE until 3rd June!

Precise control of single unit monomer radical addition with a bulky tertiary methacrylate monomer toward sequence-defined oligo- or poly(methacrylate)s via the iterative process, Polym. Chem., 2019, 10, 1998-2003, DOI: 10.1039/C9PY00096H

About the webwriter

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.

Oxygen is considered detrimental for radical polymerizations and as such traditional deoxygenation strategies (e.g. freeze pump thaw, nitrogen sparging, etc.) are typically required for the complete removal of oxygen. However, such methods may also possess drawbacks (e.g. lack of reproducibility) and as such alternative polymerization strategies that do not require external deoxygenation have been developed. To this end, Wilson, Perrier, Tanaka and co-workers reported the ultrafast polymerization of a range of acrylamide monomers in water exploiting reversible addition-fragmentation chain-transfer (RAFT) polymerization in the presence of air. The authors used microvolume insert vials as the reaction vessels and found that good control over the molecular weight and the dispersity could be maintained at very low volumes (down to 2 μl scale). Importantly, the resulting materials were successfully chain extended multiple times by sequential monomer additions allowing the facile synthesis of pentablock copolymers with a final volume of the reaction mixture not exceeding 10 μl. Nuclear magnetic resonance and gel permeation chromatography have been used to characterize the materials which were found to reach very high monomer conversions accompanied with low molecular weight distributions. These results demonstrate that RAFT polymerization can be used as a high-throughput screening method for the preparation of complex sequence-controlled multiblock copolymers. The authors are currently looking at expanding the scope of their investigation to include the synthesis of more complex structures and investigate their applicability to biological sciences.

Tips/comments directly from the authors:

1. In general, the aqueous ultrafast RAFT conditions (Polym. Chem., 2015, 6, 1502-1511) used in our work can also be scaled up (> 50 ml), however, depending on the set up, it may take longer time to permit sufficient heat transfer.
2. The protocol is limited to acrylamidic monomer family in solvent mixture that constitutes mostly water to permit ultrafast polymerisation open to air without prior deoxygenation with quantitative monomer conversion. In addition, changing RAFT agent with a more stabilizing R group requires some modification to the protocol due to a longer induction period.
3. Scaling down works very well in microvolume inserts, using centrifuge to spin down the reaction mixture to the bottom. Caution has to be taken when spinning inserts/vials inside a centrifuge, as leaving it spinning for too long may break the vials.
4. For sequential chain extensions, the reactions vessels were cooled with liquid nitrogen, which was admittedly an overkill. Instead, it can also be cooled with ice-water bath. Cooling between blocks is essential at microscale for premixing the sequential monomer solution and subsequent centrifuge is advised to spin down the mixture again before reheating.
5. For multiple reactions, a piece of cardboard was punctured and used as a platform for multiple inserts to be conveniently placed in an oil bath at the same time.
6. The master mix containing PATBC (the RAFT agent) to target DP25, may appear somewhat cloudy with only 20% dioxane (of the total solvent volume added) especially when cooled or stored in refrigerator, however it will turn clear upon heating.
7. When targeting high DP (>100), although some dioxane was used in our paper, the monomer (DMA, NAM) can sufficiently solubilise the PABTC without any co-organic solvents.

Read the full Open Access article: Microscale synthesis of multiblock copolymers using ultrafast RAFT polymerisation, Polym. Chem., 2019, 10, 1186-1191, DOI: 10.1039/C8PY01437J

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.

Hyperbranched polyphenylenes and cyanate esters are two unique classes of materials that possess complementary properties. On one side, polyphenylenes are good insulators with remarkable solubility owing to their dense packing and the strongly twisted structure hinder π-conjugation respectively. Cyanate esters are also well renowned for their thermal stability as thermosetting materials. To combine these properties, Voit and co-workers investigated the copolymerisation of the two monomers 3,3′-(1,4-phenylene)bis(2,4,5-triphenylcyclo-pentadienone) and 2,2-bis(4-cyanatophenyl) propane through a Diels-Alder cycloaddition where carbon monoxide is released as a side product. The polymerisation was followed by UV/Vis spectroscopy and the structure of the oligomers could be further investigated by in-depth NMR studies. Importantly, the catenation proved to be completely statistical and independent of the temperature of the polymerization while the obtained oligomers can be cured via a trimerisation reaction of the terminal OCN-groups. Finally, the polymerisation and crosslinking reaction kinetics were also studied and upon crosslinking the resins exhibit high thermal resistance and transparency as well as a high refractive index. Thus, the resulting materials simultaneously possess the strengths of polyphenylene polymers while retaining the curing potential of the cyanate esters but at only the tenth of the activation energy of pure cyanate monomers, lowering the risk factors during handling. As the authors elegantly conclude, materials with such unique characteristics may find application in integrated optics.

Tips/comments directly from the authors:

1. The Diels-Alder cycloaddition with these substrates requires high temperatures. However, under these conditions the trimerisation reaction of cyanate esters also takes place. To avoid the premature crosslinking of the system while maintaining the cyanate ester termination a special protocol was developed. During the Diels-Alder reaction the ratios where adjusted to obtain a oligomer terminated with cyclopentadienone groups and only 15 minutes prior to the end of the reaction one additional equiv. of cyanate ester was added.
2. The cyclopentadienone possesses a deep purple color while the polymer is colorless. Therefore, UV/Vis spectroscopy can be a powerful tool to track the reaction, but a simple look inside the reaction vial already gives indications on the state of the reaction.
3. While cyclopentadienone monomers are sometimes challenging to synthesize there is a wide variety of commercial cyanate ester monomers and prepolymers allowing for a high degree of tunability of the resulting resin without changing the cyclopentadienone unit.
4. Different to fully phenylene-based systems which are difficult to analyze by ¹³C NMR spectroscopy, the reaction with cyanate results in pyridine and cyanurate structures that can be well identified thus improving the structural characterization of such oligomers.

Read the full paper for FREE until 1st April 2019!

A Diels–Alder reaction between cyanates and cyclopentadienone-derivatives – a new class of crosslinkable oligomers, Polym. Chem., 2019, 10, 698-704

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.