Polymer Chemistry Blog

Ortyl et al. develop and implement new BODIPY-based derivatives as panchromatic, single-component polymerization photoinitiators.

Light is undeniably a fundamental tool for diverse chemical transformations in organic and polymer chemistry. Photopolymerization processes in particular, have gained widespread interest as they provide powerful, green methodologies for a variety of processes dealing with (bio)materials production, especially those involving 3D printing processes. In this aspect, due to their initiation efficiency, single-component photoinitiators are of particular interest. However, most known cationic photoinitiators have poor matching of their absorption characteristics with the emission characteristics of industrial UV light sources, the so-called medium pressure lamps and UV-LED and Vis-LED diodes.

To address this, Ortyl and collaborators developed and tested in 3D printing applications new BODIPY derivatives and more specifically iodonium salts based on a 4,4-difluoro-1,3,5,7,8-pentamethyl-4-bora-3a,4a-diaza-s-indecene (B-1) chromophore. The new tosylate, hexafluorophosphate, hexafluoroantimonate, and triflate iodonium B-1 salts were found initiate cationic photopolymerization  with the hexafluoroantimonate derivative showing the highest reactivity. All derivatives were found to efficiently initiate the polymerization of a variety of monomers (such as epoxides, ethers, glycidyls and oxetanes) as well as of hybrid monomers (such as divinyl/triacrylate and diepoxide/triacrylate). Importantly, diodes of wide spectrum could be used as light source in a wavelength range from 365 to 520 nm.  The applicability of the novel BODIPY derivative photoinitiators was demonstrated with 3D printing of epoxy and acrylic resins with good resolution. Moreover, the B-1 chromophore could also be used as a olorimetric sensor of the degree of photopolymerization.

In summary, BODIPY-based derivatives were developed as panchromatic, single-component polymerization photoinitiators.  

Tips/comments directly from the authors:

• Newly developed cationic photoinitiators based on the BODIPY chromophore effectively initiate various photopolymerization processes.
• The choice of the appropriate concentration of new photoinitiators, current intensity, type of diode, and monomers enable to obtain polymers with a high degree of polymerization (conversions up to even 95%).
• We described the application of the new BODIPy-based photoinitiators for photo-cured 3D printing. But the newly developed photoinitiators can be successfully used in other applications, such as temporary photo-cured coloured fillings for milk teeth for children.

Development of the first panchromatic BODIPY-based one-component iodonium salts for initiating the photopolymerization processes, Polym. Chem., 2021, 12, 6873-6893.

Link to the paper: https://pubs.rsc.org/en/content/articlehtml/2021/py/d1py01263k

You can follow Professor Ortyl on Twitter: @JoannaOrtyl

Dr. Kelly Velonia is an Advisory Board Member and a Web Writer for Polymer Chemistry. She joined the Department of Materials Science and Technology in 2007. Research in her group focuses on the synthesis and applications of bioconjugates and biopolymers.

Bonduelle et al. prove that ROPISA can be used to concomitantly yield well-defined amphiphilic copolypeptide chains and self-assembled nanostructures in a rapid, facile, and straightforward manner.

Synthetic polypeptides are among the most versatile building blocks to guide the formation of self-assembled nanomaterials through biomimetic structuring since they merge advantageous features of both synthetic polymers and proteins. Recent advances in ring-opening polymerization methodologies offer the unique ability to precisely design polypeptides fitting a particular function. In this context, application of polymerization-induced self-assembly (PISA) – i.e. in situ growth of a living amphiphilic polymer chain during its self-assembly into nanostructures- offers unprecedented possibilities for the synthesis of functional peptide-based nanomaterials. 

To address this possibility, Bonduelle, Lecommandoux and collaborators comprehensively studied the recently reported aqueous ROPISA process of N-carboxyanhydrides in the presence of α-amino-poly(ethylene oxide) initiators. A library of polypeptides was prepared from two NCA monomers derived from benzyl-L-glutamate (BLG-NCA) and L-Leucine (Leu-NCA) and a hydrophilic PEG_5kDa-NH₂ macroinitiator by varying the degree of polymerization. This combined one-pot synthesis and self-assembly was found to produce well-defined amphiphilic copolypeptide chains with narrow molar mass dispersity (Đ) values (between 1.12 and 1.17), controllable number-averaged molar mass M_n and good reaction yields. Importantly, in contrast to previous studies where nanomaterial morphology was essentially defined by the hydrophobic to hydrophilic ratio, the anisotropic rod-like nanostructure morphologies were dictated by the chemical nature of the NCA monomer and the secondary structure of the resulting polypeptides. In addition, ROPISA was found to provide control over the diameter of the produced rod-like self-assembled nanostructures. The β-sheet forming PLeu polypeptides were found to strongly favor the formation of long rods with high aspect ratio, as compared to α-helical PBLG polypeptides.

In summary, ROPISA provides a rapid, facile, and straightforward methodology for the synthesis of rigid polypeptide-based nanomaterials at high solid content with tunable anisotropy.  These new results strengthen the potential of using ROPISA to obtain polypeptides with N-carboxyanhydrides and open new avenues towards the design of functional nanomaterials.

Tips/comments directly from the authors:

• The polymerization of NCAs via ROPISA can easily be contaminated by homopolymers that originate from the hydrolysis of a monomer to an amino acid that then initiates the polymerization process. Salt concentration and temperature are important parameters to control this secondary reaction.
• The agitation of the reaction medium is a key factor in the ROPISA process: insufficient stirring makes the synthesis too inhomogeneous and does not provide good results. At high solid contents (above 10%) and in some experiments, hydrogels are obtained, a phenomenon we are currently studying.
• Other molar mass of poly(ethylene glycol) can be used: we have performed ROPISA with PEG2k or PEG10k with the two monomers described. We obtain in most cases anisotropic nano-objects.

Citation to the paper: Aqueous ROPISA of α-amino acid N-carboxyanhydrides: polypeptide block secondary structure controls nanoparticle shape anisotropy, Polym. Chem., 2021,12, 6242-6251

Link to the paper: https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00995h

Dr. Kelly Velonia is an Advisory Board Member and a Web Writer for Polymer Chemistry. She joined the Department of Materials Science and Technology in 2007. Research in her group focuses on the synthesis and applications of bioconjugates and biopolymers.

Wehr et al. introduced chirality into aqueous block copolymer (BCP) self-assemblies in a study discriminating the effect of tacticity from that of crystallinity.

Amphiphilic block copolymers (BCPs) bearing mostly atactic hydrophobic polymers on the main chain have long served as building blocks to produce nanocompartments with diverse morphologies and a variety of (bio)technological applications. More recently, the use of isotactic hydrophobic blocks has attracted significant interest since the effect of stereoregularity of the main chain of BCPs on the formation and morphology of aqueous self-assemblies has not yet been elucidated. Recent studies on isotactic hydrophobic blocks are based on crystallisation-driven self-assembly (CDSA) not allowing to unravel the differences between atactic and isotactic BCPs and associate them with morphological changes in BCP assemblies. CDSA additionally prevents higher complexity and applicability since the formed crystalline membranes generally lack flexibility and fluidity or require handling in temperatures above their melting or glass transition temperatures (T_g) to form well-controlled self-assemblies.

To discriminate the effect of tacticity from that of crystallinity in aqueous self-assemblies of amphiphilic BCPs, Gaitzsch, Meier and collaborators synthesized poly(butylene oxide)-block-poly(glycidol) (PBO-b-PG) BCPs, differing solely in their tacticities (R/S, R and S). Despite the differences in their stereochemistry, all polymers displayed similar thermal and structural behaviour, proving that stereoregularity did not induce crystallinity or formation of secondary structures in bulk or in solution. However, the nanoscopic polymersomes (i.e. small unilamellar vesicles, SUVs) composed of the different BCPs expressed stability differences when studying self-assembly into homogenous phases of SUVs. Interestingly, only the atactic BCPs formed microscopic giant unilamellar vesicles (GUVs) which were stable over several hours while GUVs composed of isotactic BCPs ruptured within minutes after formation. The ability of atactic PBO-b-PG to form microreactors was highlighted by reconstituting the membrane protein OmpF in the GUV membrane via microfluidics and performing an enzyme reaction inside its lumen.

This study differentiates for the first time the effect of tacticity from that of crystallinity in aqueous self-assemblies of amphiphilic BCPs and is expected pave the way in designing versatile vesicles with fluid membranes composed of atactic or isotactic BCPs. Studies of the interplay of membrane chirality with transmembrane proteins or guests in nano- and micro- compartments are now within reach. 

Tips/comments directly from the authors:

• Kinetic measurements of the polymerisations of racemic and enantiopure monomers revealed that both enantiomers reacted in the same speed. Hence in case of synthesising the atactic polymer, an ideal statistical distribution of m and r diads is achieved.
• The microfluidic technique applied in here was essential to form the GUVs. Other approaches to form GUVs were not successful due to the instability of the self-assemblies especially of the stereoregular polymers.
• We essentially took commercially available but expensive enantiopure monomers to generate highly stereoregular polymers, which formed less stable (i.e. inferior) self-assemblies than the cheaply accessible atactic polymers. However, it allowed us to falsify the common believe that a higher order in polymers always leads to better defined structures or a higher stability.

Citation to the paper: Fully amorphous atactic and isotactic block copolymers and their self-assembly into nano- and microscopic vesicles, Polym. Chem., 2021,12, 5377-5389.

Link to the paper: https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00952d

Dr. Kelly Velonia is an Advisory Board Member and a Web Writer for Polymer Chemistry. She joined the Department of Materials Science and Technology in 2007. Research in her group focuses on the synthesis and applications of bioconjugates and biopolymers.

Wang et al. synthesized amphiphilic block copolymers bearing photoswitches and evaluated the effect of the photoswitch number and position on solution self-assembly.

Diverse applications of amphiphilic block copolymers (BCPs) stem from their ability to self-assemble into nanostructures with well-defined architectures the shape of which has been shown to bear significant effect on BCP nanostructure properties and applications. In this aspect, photo-triggered polymer vesicles (polymersomes) have been extensively investigated for on-demand cargo delivery as light-triggered conformational changes of the BCPs offer macroscopic actuation of the nanocarriers and enable reversible mass transport through the vesicular membrane without permanent disruption. However, the effect of the number and location of the photoswitches in the BCP on their conformational change has been challenging to study.

To address this, Kim and collaborators synthesized amphiphilic block copolymers composed of hydrophilic polyethylene glycol (PEG) blocks and discrete oligo(phenyllactic acid) (OPLA) blocks containing hydrazone-based photoswitches at specific positions. The photoswitches were selected on the basis of their ability to undergo E–Z isomerization upon light irradiation causing a conformational change on the hydrophobic block. As a result, vesicles formed via cosolvent self-assembly were shown to undergo a reversible shape transformation upon irradiation with UV or visible light. Importantly, the location and number of photoswitches per polymer was shown to have a significant effect. When the hydrazone-based photoswitch was embedded in the middle of the hydrophobic OPLA chains a dramatic membrane deformation was observed causing reversible shape transformation from polymeric vesicles to urchin-like structures. In contrast, when the hydrazone-based photoswitches were embedded at the junction of the hydrophilic and the hydrophobic block, the self-assembled nanocarriers did not undergo shape transformation when irradiating with different light sources. This indicates that the position of the switch in the hydrophobic moiety of the BCP is a decisive factor determining the shape transformation of the nanoparticles driven by the light-induced configurational change of the hydrazone-based photoswitches. It was further shown that when the number of photoswitches embedded in the OPLA chains was increased, the extent of shape transformation was significantly enhanced.

This study offers new insights on the design and development of BCPs for the fabrication of polymersomes tailored for a wide range of potential applications involving on-demand release of cargo molecules.

Citation to the paper: On-demand shape transformation of polymer vesicles via site-specific isomerization of hydrazone photoswitches in monodisperse hydrophobic oligomers, Polym. Chem., 2021,12, 5027-5036.

Link to the paper: https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00981h

Dr. Kelly Velonia is an Advisory Board Member and a Web Writer for Polymer Chemistry. She joined the Department of Materials Science and Technology in 2007. Research in her group focuses on the synthesis and applications of bioconjugates and biopolymers.

Mertens et al. synthesized a series of sequence-defined oligoampholytes and evaluated their properties.

Polyampholytes are polymers compromising of positively and negatively charged groups and present a number of distinct properties which allows them to be employed in many applications including antifouling coatings and drug delivery systems. Polyampholytes are usually produced by conventional polymerizations strategies yielding materials with varied chain lengths and monomer composition, thus compromising the purity and the properties of these molecules. Du Prez’s group has contributed to this field by developing a chemical platform based on thiolactone chemistry which allowed the introduction of a variety of side-chain functionalities employing acrylic, amine or halide building blocks. In their current contribution, novel sequence-defined oligoampholytes with carboxylic acid and tertiary amine side-chains were elegantly synthesized through a thiolactone-based solid-phase synthesis protocol, in which the side-chain functionalities were incorporated through a thiol-Michael reaction. Although initially acrylate-based building blocks were employed to introduce the tertiary amine, severe side reactions were observed including hydrolysis or methanolysis. Instead, a tertiary amine bearing vinyl sulfonamide proved to be an efficient and stable alternative for the used acrylates and thus could be directly integrated into the synthetic protocol. A wide range of oligomers (up to a hexamer) were synthesized with an alternating or block-like arrangement of the tertiary amine and acid side-chains. These perfectly uniform oligomers were shown to be soluble in water and exhibited UCST-type thermoresponsive behaviour in alcoholic/aqueous mixtures. Such properties were influenced by both the length and the sequence of the oligoampholytes. As the authors elude in their conclusion, the ability to fully control the arrangement of the amphiphilic groups is a powerful tool to design the properties and functions of these uniform macromolecules.

Tips/comments directly from the authors:

• The tertiary amine side-chains are attached to the backbone through a β-thioester bond resulting from the thiol-Michael addition, which was found to be susceptible towards hydrolysis and methanolysis. Prolonging the carbon spacer between the ester and the tertiary amine reduced the rate of this undesired degradation, yet it could not be completely inhibited.
• Compared to acrylates, vinyl sulfonamides are a class of Michael acceptors that received considerably less attention within the area of polymer chemistry, although they can provide favourable properties. The thiol-vinyl sulfonamide adduct is stable towards hydrolysis, unlike an acrylate, while the reaction kinetics are comparable. These aspects are highlighted within the present work, as well as in the work of others.
• While we demonstrate the synthesis of these oligomers together with the fundamental characterisation of their properties, further research is required to establish in-depth structure-property relationships.

Citation to the paper: Sequence-defined oligoampholytes using hydrolytically stable vinyl sulfonamides: design and UCST behaviour, Polym. Chem., 2021,12, 4193-4204, DOI: 10.1039/D1PY00662B

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00662b 

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.

Sardon’s group investigate the ring opening polymerization of cyclic esters using novel photo acid generators.

Light has emerged as a powerful stimulus allowing for spatial and temporal control over polymerization kinetics, macromolecular sequence, and composition and has enabled a number of high-end applications including coatings, microelectronics, additive manufacturing and 3D printing. However, the potential of light in polymer chemistry is far from being fully exploited. Photopolymerization is currently dominated by (controlled) radical polymerizations of vinyl monomers. Little attention has been paid to photo-induced cationic ring-opening polymerization (CROP) of cyclic esters. In a recent contribution to Polymer Chemistry, Sardon and co-workers developed six new photocatalysts for light-mediated CROP. Upon exposure to light, the new photocatalysts release strong sulfonic acids that can trigger the CROP of ε-caprolactone. The authors particularly focused on imino-sulfonates and aryl-sulfonates based photocatalysts, and this strategy was hugely successful. Complete monomer conversion was obtained after only 5 minutes of irradiation. This is an impressively high polymerization rate despite the catalyst efficiency being typically strongly related to the chromophore and the sulfonate substituent. In addition, several of these photocatalysts are stable even at 100 °C and were successfully used to produce not only linear biodegradable polymer polymers but also crosslinking poly(ε-caprolactone) exhibiting excellent mechanical properties. Furthermore, the authors employed density functional theory calculation to propose a photodissociation mechanism. The studied photopolymerization has also been successfully applied to surface coating. The potential applications of these new photocatalysts are certainly not limited to photocoating, and therefore, we look forward to seeing further exciting applications of these photocatalysts.

Tips/comments directly from the authors:

• The efficiency of the PAGs was observed to be highly dependent on the chromophore and the photolabile bond; imino-sulfonates were more capable of producing sulfonic acids than aryl-sulfonates. However, their preparation is tedious and requires a multistep synthesis process. Aryl-sulfonates are not as efficient but their synthesis is performed in a single step in excellent yields up to 90%.

• Imino-sulfonate based photocatalyst were able to promote the ring opening polymerization of ε-caprolactone at room temperature but 3 h were required for getting full conversion. Nevertheless, as several of these catalyst were stable up to 100 °C we were able to get full conversion in just 5 minutes at 100 °C

• To further expand the applications of the studied PAGs, we demonstrated the ability of the photoacid to promote the crosslinking at room temperature in the presence of a dilactone. While the crosslinking reaction was successful, long reaction times were required for reaction completion, making impractical the use of these photocatalyst in 3 D printing applications.

Citation to the paper: Novel imino- and aryl-sulfonate based photoacid generators for the cationic ring-opening polymerization of ε-caprolactone, Polym. Chem., 2021,12, 4035-4042, DOI: 10.1039/D1PY00734C

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2021/py/d1py00734c#!divAbstract

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.