Polymer Chemistry Blog

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.