Polymer Chemistry Blog

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.

Natural proteins are comprised of distinct secondary structure elements such as sheets, helices and coils. It is the combination of these diverse topologies that allow proteins to fulfil their functions. Owing to the properties of these structures, synthetic analogues of these materials are also of great interest to the polymer chemistry community. However, a covalent system where sheet, helix, and coil blocks are combined in a linear system has not been yet realized. Towards this direction, Weck, Elacqua and co-workers developed a new methodology to covalently link three distinct structures together with high fidelity without compromising the control over sequence. This was achieved by combining sequential ring-opening metathesis polymerization (ROMP) of sheet- and coil-forming monomers with palladium-mediated isocyanide polymerization of covalent coil-sheet-helix (ABC) and sheetcoil helix (BAC) domains. After polymerizing the initial sheet of coil-forming monomer through ROMP, the second monomer is introduced and subsequently polymerized yielding to a diblock comprised of sheet and coil structures. ROMP is then terminated by a special transfer agent that contains an isocyanide polymerization initiator. The telechelic diblock copolymers containing both coil-sheet and sheet-coil blocks, can serve as macroinitiators for the polymerization of a P-helix forming monomer. As such, this combination of sequential copolymerization and macroinitiation enables three different polymer chains to be linked covalently. Importantly, throughout the triblock copolymer synthesis, all individual blocks retained their secondary structures as evidenced by circular dicroism and fluorescence spectroscopies. The authors are confident that this work can be extended to the formation of a diverse array of tri- and multiblock copolymers enabling a range of new applications.

Tips/comments directly from the authors:

1. When synthesizing a topologically-diverse block copolymer, oftentimes it is necessary to use different polymerization techniques. If so, prudent selection and design of polymer backbone is key. First, select the class(es) of monomers you intend to employ. This will inform the type of polymerization method required, and subsequently, the initiator to be designed.

2. Ring-opening metathesis polymerization (ROMP) is a widely-used controllable polymerization method that allows for one to not only control molecular weight, but also is amenable to an iterative or tandem ROMP, which is desirable for sequence-controlled block copolymers

3. Performing ³¹P NMR spectroscopy after each step of block copolymer synthesis, especially before the final step to create the helical block, is crucial. It ensures that only one palladium species is present throughout.

4. Isocyanide polymerization mediated by palladium(II) is a robust technique; there is high functional group tolerance when synthesizing the initiator, which allows for the engineering of multipurpose catalysts like the one featured in this manuscript.

5. Topologically-diverse polymer backbones, such as sheets, helices, and coils, garner much interest from a biomimetic standpoint in the synthetic community. Judicious choice of polymer backbones, as well as block lengths, can inform characterization techniques, such as circular dichroism, fluorescence, and X-ray scattering to gain insights into topology.

6. We are available for any questions and to troubleshoot any issues you may have – please contact mw125@nyu.edu or eze31@psu.edu.

Synthesis of sheet-coil-helix and coil-sheet-helix triblock copolymers by combining ROMP with palladium-mediated isocyanide polymerization, Polym. Chem., 2018, 9, 5655-5659, DOI: 10.1039/C8PY01361F

About the webwriter

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Assistant Professor at ETH Materials Department.

A major requirement to synthesize polymeric materials via photopolymerizations is the efficiency of the photoinitiator. This is because the amount of initiated polymer chain is crucial for the outcome of the radical polymerization. Many factors have been reported to influence this efficiency including the UV-Vis absorption properties, the irradiation wavelengths, the dissociation quantum yields and the reactivity of primary radicals towards monomers. To enable “on demand” access to a wide range of photoinitiators performance, Gescheidt and co-workers developed a tool for predicting the initiator efficiency of various type 1 (α-cleavage) photoinitiators. To achieve this, a systematic analysis of the photoinitiator performance revealed the interplay of absorption properties, dissociation quantum yields, light intensities, irradiation wavelengths and kinetics.  It is important to note that important side reactions such as oxygen quenching have also been taken into account. The author’s simulations demonstrate that under ideal conditions any photoinitiator will function in an almost perfect way. However, the oxygen quenching mechanism combined with the subtleness of the photo-physical properties of the photoinitiator differentiates a well-suited photoinitiator from a less useful one. The authors conclude that a balanced combination of the intrinsic photoinitiator characteristics (i.e. absorption spectrum of initiator, quantum yields of dissociation, rate constants for primary radicals towards monomers), the major side reactions such as oxygen quenching, and the emission properties of the utilized light source (i.e. irradiation wavelengths, light intensity) is crucial to achieve the optimal initiation performance. As the authors nicely note in their conclusion, such a tool is not only useful for experienced researchers but it can also serve as an educational guide. The corresponding kinetic scheme is freely available on the author’s website and can be adjusted by any user.

Tips/comments directly from the authors:

1. A single property is not sufficient to reasonably classify a photoinitiator. A subtly interplay between absorbance, quantum yield, kinetics and the character of the light source is what matters.

2. The choice of a suitable photoinitiator strongly depends on the desired application (e.g. bulk polymerization or coatings). Here, the number of initiating radicals directs the efficiency of the polymerization and its robustness vs. oxygen inhibition.

3. The optical density of the formulation combined with the wavelength of irradiation control the thickness of the cured layer. It is crucial to be aware of the exact properties of the used light source (emission bands, intensity), since this drastically influences the amount of generated radicals and the initiation rate.

4. The Simulations rely on experimental parameters, which are straightforwardly determined (e.g. rate constants (Polym. Chem., 2018, 9, 38–47) and quantum yields (Photochem. Photobiol. Sci., 2018, 17, 660–669)) and provide an overall picture of a complex process such as the initiation of a photo-induced polymerization.

5. We are happy to answer any questions or remarks concerning our initiation model – please contact g.gescheidt-demner@tugraz.at.

FREE to read until 24th December

Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data, Polym. Chem., 2018, 9, 5107-5115, DOI: 10.1039/C8PY01195H

About the web writer

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.

Double-network hydrogels (DN gels) consist of two interpenetrating networks that are bound together by covalent and reversible interactions and can resist high deformation by reorganizing their structure. Magnetic hybrid hydrogels in particular have attracted considerable attention owing to the possibility of triggering the properties of the hydrogels with an external magnetic field. Fontaine, Montembault and co-workers further contributed to this field by developing a novel class of thermoresponsive hybrid double-crosslinked polymer networks materials that can rearrange and rebuild upon triggering a Diels-Alder (DA) reaction. Central to this approach is the use of iron oxide nanoparticles as the nano-crosslinkers and difuran-functionalized poly(ethylene oxide) as the diene partner for the thermoreversible DA reaction. The thermoreversibility of the network was confirmed by ¹H nuclear magnetic resonance (NMR) spectroscopy and rheological studies showing a fast gel/liquid state transition upon heating the sample. Importantly, the rheological properties of 3D networks with and without iron oxide nanoparticles were compared. The studies conclude that the presence of iron oxide nanoparticles in the network ensured that a gel-like structure was maintained after the retro DA reaction. These characteristics were attributed to the establishment of a secondary network through covalently integrated iron oxide nanoparticles. The unique combination of the Diels-Alder reaction with iron oxide nanoparticles to generate new reversible reticulated networks can pave the way for further applications mediated by magnetic hyperthermia stimuli.

Tips/comments directly from the authors:

1. The strategy used for the synthesis of difuran-functionalized diphosphonic acid terminated poly(ethylene oxide) by a combination of Kabachnik-Fields reaction and “click” copper-catalyzed 1,3-dipolar cycloaddition is a versatile method and poly(ethylene oxide) backbone can be replaced by a wide range of polymers that may bring new properties.

2. The formation of a 3D network via the Diels-Alder (DA) reaction with a trismaleimide is thermoreversible, with a faster retro-DA (rDA) reaction rate compared to the DA reaction, leading to an easier destruction of the 3D network than its formation.

3. The presence of phosphonic acid groups is needed to allow the crosslinking through interactions with the iron oxide nanoparticles and the formation of the 3D double-crosslinked network.

4. The 3D network is preserved even after the rDA reaction, demonstrating that the iron oxide nanoparticles serve as crossing points through strong bidendate Fe-O-P bonds. Furthermore, a gel-like structure is maintained at least in the limit of the percolation threshold.

5. The viscoelastic properties of the double-crosslinked gels demonstrates that the double-crosslinking leads to stiffer gels.

Read the full article for FREE until 26th November!

Thermoresponsive hybrid double-crosslinked networks using magnetic iron oxide nanoparticles as crossing points, Polym. Chem., 2018, 9, 4642-4650, DOI: 10.1039/C8PY01006D

About the web writer

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.

Direct arylation polymerization is a useful methodology that allows for the preparation of conjugated polymers. One of the great advantages of this technique is that it eliminates the use of toxic and pyrophoric reagents typically utilized to prepare monomers for other polymerization methods (e.g. Stille, Suzuki, etc.). Importantly, the technique allows the preparation of polymers with undetectable levels of homo-coupling or branching defects leading to the defect free synthesis of a wide range of conjugated polymer architectures such as homopolymers, donor-acceptors copolymers and porous polymers. However, the vast majority of direct arylation polymerization methodologies rely on the use of noble metals such as palladium (Pd), which is a costly, low abundant, relatively toxic, and unsustainable metal. To circumvent this, Thompson and co-workers were inspired by the small molecule copper catalyzed aryl-aryl cross-coupling for various iodinated arenes and electron-deficient heterocycles. To transition from small molecules to conjugated polymers, the group judiciously optimized the reaction conditions including the concentration, the nature of the solvent, the ligand, the temperature and the base employed. Conjugated polymers were then prepared in very high yields (up to 97%) with M_n of up to 10 kDa. NMR spectroscopy was used to characterize the recovered polymer products and confirmed the absence (or minimization) of undesired couplings. This report is the first example of perfectly alternating donor-acceptor conjugated polymers using copper catalyzed direct arylation polymerization thus offering an initial step towards the replacement of toxic metals like Pd. Future work will hope to address milder reaction conditions, lower catalyst loading, and a broader scope.

Tips/comments directly from the authors:  

1. Fresh CuI is recommended. It is recommended to acquire a fresh bottle or a freshly purified stock of CuI and store it under inert gas in a freezer.

2. The strict exclusion of air and moisture is advisable, and so care should be taken to thoroughly sparge the reaction vessel with inert gas before and after the addition of reagents.

3. The selection of base is critical for the polymerization, and the base should be finely ground and dried using a vacuum oven before use. The base can be stored in a desiccator after drying, but should be dispensed quickly to limit contact with moisture

4. After addition of the CuI, the copper-phenanthroline catalyst appears to form instantly. However, stirring at room temperature for a several minutes before heating may help ensure complete coordination of the phenanthroline to copper

This article is FREE to read and download until 21st September 2018

Copper catalyzed synthesis of conjugated copolymers using direct arylation polymerization  Polym. Chem., 2018, 9, 4120-4124, DOI: 10.1039/C8PY00913A

About the Web Writer

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.