Author Archive

Paper of the month: Reconsidering terms for mechanisms of polymer growth: the “step-growth” and “chain-growth” dilemma

An international group of polymer scientists from the International Union of Pure and Applied Chemistry (IUPAC) Subcommittee on Polymer Terminology (SPT) convey concerns with the basic terms typically used for classifying methods of polymer synthesis and initiate a dialogue with the broader polymer community to resolve terminology shortcomings.

In 1994 the IUPAC SPT highlighted long-standing problems with the widely used terms “step-growth polymerization” and “chain-growth polymerization,” which describe two discrete mechanisms of polymer growth, and depreciated their use since they do not describe the fundamental differences in the growth of polymers by these methods and are often confusing. To address this, the 1994 SPT members recommended the terms polycondensation and polyaddition for the two variants of “step-growth polymerization”, and similarly chain polymerization and condensative chain polymerization for the two variants of “chain-growth polymerization”. However, these terms have not been widely adopted by the community, and have also created confusion.

In this contribution, current IUPAC SPT members provide detailed descriptions of these two processes and outline concerns associated with the terms “step-growth,” “chain-growth,” and related terms. By discussing in detail the historical development of these terms and analyzing their use in current textbooks, the authors underline the lack of consensus in the terminology used within the polymer community. Interestingly, they demonstrate how the similarity of these terms leads to further confusion when translating into languages other than English. Finally, examples of polymerizations that cannot be classified under the umbrella of the existing definitions and have no designated terminology are discussed.

In 2019, IUPAC recognized the need to resolve these polymer terminology shortcomings and approved a project aimed to propose new terminologies. The authors, as members of the IUPAC SPT task group studying this issue, aim to clarify the naming of polymerisation processes and invite all members of the community to contribute by emailing to polymer.terminology@iupac.org.

Tips/comments directly from the authors:

  • We, the subcommittee of polymer nomenclature (SPT), want to raise attention to a long-standing dilemma in the terms that many of us use every day: “step-growth” and “chain-growth” polymerization.
  • A number of terms have been used over the past century to describe these two fundamental mechanisms of polymer growth, and many prominent polymer chemists have noted their shortcomings in textbooks.
  • We detail here the history of the terms, current usage in textbooks, and our specific concerns.
  • In particular, we invite feedback from the broader polymer community, including from students, lecturers, researchers, and anyone who uses polymer science regularly.

 

Reconsidering terms for mechanisms of polymer growth: the “step-growth” and “chain-growth” dilemma, Polym. Chem., 2022, 13, 2262-2270.

Link to the paper: https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py00086e

You can follow the authors on twitter: @IUPACPolymer

 

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.

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Paper of the month: The difference between photo-iniferter and conventional RAFT polymerization: high livingness enables the straightforward synthesis of multiblock copolymers

Lehnen et al. highlight the role of reversible deactivation as a key difference between photo-iniferter and conventional RAFT polymerization.

The use of light has become increasingly widespread in diverse polymerization approaches including reversible-addition fragmentation chain-transfer (RAFT) strategies. Among these, the photo-iniferter (PI)-RAFT polymerization in which light directly activates the chain transfer agent (CTA), has been shown to overcome several of the restrictions of conventional RAFT resulting in increased chain end fidelity. In this context, reversible deactivation is accepted to determine the fate of the growing radical via pathways that need to be understood to offer the means to further push the limits of PI-RAFT polymerization.  

To address this, Hartlieb and collaborators studied the PI-RAFT using an acrylamide (N-acryloyl morpholine) and a xanthate ((2-((ethoxycarbonothioyl)thio)propionic acid)). This monomer-CTA pair combination was selected on the basis of the low chain transfer capabilities (Ctr < 1) expected to result in high dispersities (>1.5). When targeting different degrees of polymerization (DP), the control over the molecular weight distribution was not found to significantly increase. However, control could be achieved through slow monomer addition that results in increasing the numbers of activation-deactivation events per monomer addition. Importantly, the high livingness associated with PI-RAFT proved to be invaluable in chain extension experiments since it was found to enable the straightforward, easy and rapid synthesis of very high molecular weight multiblock copolymers with up to 20 blocks and a high number of repeating units per block (DP = 25-100) with impressive precision.  

In summary this study highlights the role of reversible deactivation and employs the high livingness of PI-RAFT to demonstrate its enormous potential for the synthesis of polymeric materials and more specifically segmented macromolecules.

Tips/comments directly from the authors:

  • We want to emphasize how fast and easy polymerization reactions can be performed using this technique as the shown xanthate is an extremely powerful iniferter
  • The shown multiblocks were produced in a very straight forward way; no rigorously clean or inert conditions or specialized equipment.
  • The photo-iniferter process is older than RAFT polymerization but its full potential isn’t used yet.  

 

The difference between photo-iniferter and conventional RAFT polymerization: high livingness enables the straightforward synthesis of multiblock copolymers, Polym. Chem., 2022, 13, 1537-1546

Link to the paper: https://pubs.rsc.org/en/content/articlelanding/2022/py/d1py01530c

Link to Dr Matthias Hartlieb’s group website: https://www.uni-potsdam.de/polybio

You can follow Dr Matthias Hartlieb on Twitter: @PolyBioPotsdam

 

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.

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Paper of the month: Iron-containing poly(ionic liquid) membranes: a heterogeneous Fenton reaction and enhanced anti-fouling ability

Guan et al. develop iron-containing poly(ionic liquid) (Fe-PIL) membranes with anti-fouling properties enhanced by a heterogeneous Fenton reaction.

The life span of membranes used in various separation technologies is often limited by fouling causing decreased performance and large economic costs. Recently, poly(ionic liquid)s (PILs) have been employed to prepare membranes with a large range of applications due to their unique material properties including excellent stability, processability and flexibility. Although PIL membranes are less prone to fouling and easier to clean due to their charged nature, the problem of irreversible pollutant deposition can limit their efficiency.  

To address this, Zhang and collaborators developed iron-containing PIL (Fe-PIL) membranes and used them as catalysts for heterogenous Fenton reaction. Poly(4-vinylpyridine)-b-polysulfone-b-poly(4-vinylpyridine) (PSF-b-P4VP) blend membranes were synthesized via Cu(0)-RDRP. The pore size and hydrophilicity of the membranes fabricated via NIPS, were found to depend on the block ratio of the polymer. A quaternization reaction followed by coordination with Fe(II) bromide was employed to generate the Fe-PILs on the surface of the polysulfone blends.  The membranes were shown to possess low surface roughness, increased hydrophilicity, anti-fouling properties and scalability. The dispersibility of the catalyst and the catalytic efficiency in heterogeneous Fenton reactions were shown to be excellent in a broad pH range from acidic to neutral and basic conditions. More importantly, the Fe-PIL membranes exhibited superior synergistic performance with filtration in the dynamic heterogeneous Fenton reaction and excellent reusability as they could be maintained well after five cycles.

In summary this study combines PIL membrane technology with dynamic heterogeneous catalysis (Fenton reaction) to create reusable PILs that address the issue of membrane fouling.

 

Iron-containing poly(ionic liquid) membranes: a heterogeneous Fenton reaction and enhanced anti-fouling ability, Polym. Chem., 2022,13, 130-138

Link to the paper: https://pubs.rsc.org/en/content/articlelanding/2022/py/d1py01345a

 

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.

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Paper of the month: Development of the first panchromatic BODIPY-based one-component iodonium salts for initiating the photopolymerization processes

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.

 

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Paper of the month: Aqueous ROPISA of α-amino acid N-carboxyanhydrides: polypeptide block secondary structure controls nanoparticle shape anisotropy

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 PEG5kDa-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 Mn 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.

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Paper of the month: Fully amorphous atactic and isotactic block copolymers and their self-assembly into nano- and microscopic vesicles

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 (Tg) 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.

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Paper of the month: On-demand shape transformation of polymer vesicles via site-specific isomerization of hydrazone photoswitches in monodisperse hydrophobic oligomers

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 EZ 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.

 

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