Polymer Chemistry Blog

p-type organic semiconductor polymers can find a use in organic electronics, including organic light-emitting diodes (OLEDs), solar cells, and organic thin-film transistors. These materials offer unique characteristics over inorganic semiconductors such as flexibility and light weight. To maximize their potential, reversible deactivation radical polymerization (RDRP) methodologies are often used with traditional atom transfer radical polymerization and reversible addition/fragmentation chain transfer polymerization dominating in this area. To this end, Hudson and co-workers exploited Cu(0)-RDRP as an effective method for preparing functional acrylate-based polymers with p-type organic semiconductors as side chains. Impressively, all polymers were obtained in high yields (~ 90 %) with low dispersity and high end group functionality while the polymerizations displayed first order kinetics. Both low and high molecular weight polymers could be prepared in a facile manner and the choice of solvent seemed to be crucial to maintain good control over the molecular weight distributions. It should be highlighted that the described technique represents the most simple, low-cost and efficient way to synthesize these materials with improved end group functionality and yields over previous methods. The optical, electrochemical and thermal properties of each of these p-type materials were also carefully investigated with cyclic voltammetry and thermogravimetric analysis revealing the potential for further studies in optoelectronic applications. The Hudson group will now focus on the synthesis of more complex materials, including multiblock copolymers, and subsequently utilize them for optoelectronics.

Tips/comments directly from the authors:  

1. The Cu(0) wire should be prepared immediately before use for best activity, as substantial reductions in polymerization rate are observed when the wire is cleaned and stored.
2. Reducing the relative amount of Cu(0) wire when attempting the synthesis of high molecular weight polymers reduces the polymerization rate, but provides improved control over the polydispersity of the products.
3. For long-term storage all monomers should be stored in the freezer (–10 ºC), but are stable on the bench top under air for 1-2 days.
4. Yields of pure monomers 5a-c are substantially improved when purification is conducted quickly (<5 min) on a short silica column to minimize decomposition; the same urgency is not required for 5d.

Cu(0)-RDRP of acrylates based on p-type organic semiconductors, Polym. Chem., 2018, 9, 1397-1403, DOI: 10.1039/C8PY00295A

This article is free to read until 30 April

About the webwriter

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). Please, visit this site for more information.

Paclitaxel (Ptx) is one of the most widely used chemotherapeutic agents for the treatment of a broad range of human tumors. Polymer prodrugs are often employed to solve a number of issues associated with its limited water solubility, the absence of ionisable groups to enable Ptx salt formation and the short colloidal stability of its formulations. In a way analogous to polymer synthesis, the “grafting from” approach, also referred to here as “drug-initiated” consists of the controlled growth of a polymer from a drug. However, this approach is limited by the poor colloidal stability of hydrophobic drug-polymer nanocarriers and the lack of the direct synthesis of PEGylated prodrugs. Nicolas and co-workers managed to tackle these issues by developing a global method which enables the facile derivatization of Ptx followed by the subsequent reversible deactivation radical polymerization to design surfactant-free, Ptx-polymer prodrug nanocarriers with contrasting properties. In particular, nitroxide-mediated polymerization (NMP) and reversible addition-fragmentation chain transfer polymerization were elegantly selected to grow short polyisoprene or poly[(oligo(ethylene glycol) methyl ether methacrylate)] chains from Ptx in a controlled fashion. This allowed for the formation of either self-stabilized, all-hydrophobic Ptx-polymer prodrug nanoparticles or their PEGylated counterparts. Importantly, these prodrug nanocarriers exhibited high cytotoxicity on three different cancer cell lines, with chain length-cytotoxicity dependency and IC50 values comparable to those of the parent drug. This versatile approach demonstrates the robustness and the broad use of the drug-initiated method for the simple design of efficient polymer prodrug nanoparticles consisting of polymers of opposite nature, thus opening new perspectives in the nanomedicine field.

Tips/comments directly from the authors:  

1. The drug-initiated NMP of isoprene from Ptx is a very simple yet efficient method to prepare surfactant-free, stable polymer prodrug nanoparticles with high drug payload, without any protection/deprotection chemistry.
2. When using the AMA-SG1 alkoxyamine for Ptx derivatization, the resulting Ptx-AMA-SG1 alkoxyamine is obtained as a mixture of diastereomers (this is related to the two chiral centers of the alkoxyamine). The signals from the NMR spectrum should be carefully assigned. Alternatively, the diastereomers can also be separated by column chromatography with a less polar eluent.
3. Mn of PEGMA-based prodrugs are higher than those of PI-based prodrugs because shorter POEGMA chains hardly precipitate compared to PI with similar Mn. Dialysis was not attempted because of potential hydrolytic cleavage between the drug and the polymer (especially with the diglycolate linker)

Self-stabilized, hydrophobic or PEGylated paclitaxel polymer prodrug nanoparticles for cancer therapy, Polym. Chem., 2018, 9, 687-698, DOI: 10.1039/C7PY01918A

This article is free to read until 16 April 2018

About the webwriter

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). Please, visit this site for more information.

The undesired adsorption of bacteria, proteins and other biomolecules on surfaces of biomedical devices often triggers the formation of biofilms causing severe systemic infections. In order to circumvent this, functional polymeric coatings with antifouling and/or antimicrobial properties are typically used. Towards this direction, Wurm, Lienkamp and co-workers developed photo-reactive poly(phosphoester)s (PPEs) which form surface-attached polymer networks and hydrogels. To achieve this, a benzophenone-functionalized cyclic phosphate monomer was synthesized and subsequently copolymerized with ethylene ethyl phosphate (EEP) yielding hydrophilic functional polymers. Upon terpolymerization with additional comonomers polymeric materials with pentyl (PEP), furfuryl (FEP) or butenyl (BuEP) pendants groups were obtained. Importantly, all polymerizations were well-controlled with good agreement between theoretical and experimental molecular weights and low dispersity values. The copolymerization kinetics were carefully monitored via real-time ₃₁P nuclear magnetic resonance spectroscopy indicating a gradient-like structure. The cross-linked surface attached PPE networks were then formed by spin-coating these polymers onto pre-functionalized substrates followed by UV irradiation. Importantly, the layer thickness could be varied between 56 and 263 nm and was dependant on the applied polymer and the hydrophilicity of the substrates. Atomic force microscopy was also employed to further characterize these materials showing a homogeneous and smooth morphology with static contact angles of 20-26° (for specific networks) and revealing hydrophilic surfaces. Given the biocompatible nature of PPEs, these networks can potentially be promising anti-fouling coatings candidates for biomedical devices such as implants or catheters. In addition, initial functionalization of the substrates using furane-containing PPE-coatings demonstrated that additional modifications can be performed therefore paving the way for more complex surface architectures.

Tips/comments directly from the authors:  

1. Synthesis of PPEs must be conducted under strict exclusion of moisture.
2. The resulting copolymers are extremely hydrophilic. Thus, care must be taken to immediately cross-link them after spin-coating, or else they will de-wet from the surface.

Surface-attached poly(phosphoester)-hydrogels with benzophenone groups, Polym. Chem., 2018, 9, 315-326, DOI: 10.1039/c7py01777d

About the webwriter

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). Please, visit this site for more information.

Polymeric amphiphiles can find use in a wide range of applications including detergents, catalysts and drug delivery vehicles. However, new polymeric biocompatible micelles with increased stability, loading efficiency and degradability are still required to address related challenges in the drug delivery field. To this end, Kim and co-workers designed and synthesized a novel epoxide monomer namely tetrahydropyranyl glycidyl ether (TGE). A series of amphiphilic diblock co-polymers were subsequently synthesized with PTGE consisting of a hydrophobic pH-responsive block with poly ethylene glycol (PEG) being the hydrophilic part. These PEG-b-PTGE diblock copolymers showed superior stability in biological media, higher loading capacity, tunable release and controllable degradation when compared to the acrylic analogue 1-ethoxyethyl glycidyl ether (EEGE). The enhanced stability and tunability of the PTGE block were attributed to the increased hydrophophicity and the tight association between the chair conformations of the cyclic TGE side chains. All diblock copolymers exhibited low dispersity values and controlled molecular weights. The high stability of these micelles in combination with their high biocompatibility highlight their potential to be used in drug delivery. In summary, the developed new class of monomers and polymers will contribute to the advanced of polyethers as promising candidates for biomedical applications and beyond.

Tips/comments directly from the authors:  

1. The synthesis of the TGE monomer is a very simple, one-step procedure, but the moisture should be strictly controlled during the synthesis. The residual water can result in byproduct, thus lowering the yield after purification.

2. The polymerization using organic superbase t-BuP4 is a very simple and reliable method; however, the t-BuP4 must be handled and stored carefully by removing the moisture. Otherwise, it may cause a lower degree of polymerization than targeted one and self-initiation process. Thus, any source for moisture should be carefully removed in solvent, initiator and monomer.

The power of the ring: a pH-responsive hydrophobic epoxide monomer for superior micelle stability, Polym. Chem., 2017, 8, 7119-7132, DOI: 10.1039/c7py01613a

About the webwriter

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). Please visit this site for more information.

Dynamically crosslinked polymeric materials have received significant attention owing to their unique characteristics including the introduction of mechanical properties and the possibility to extend a material’s lifetime. These materials can typically find use in a wide range of applications such as coatings and elastomers. Konkolewicz and co-workers significantly contributed towards this direction by developing a facile synthesis of dynamic materials with thiol-maleimide based adducts. Maleimides are of particular importance as they consist of a highly reactive vinyl group for thiol-Michael addition reactions and typically demonstrate very high yields under mild conditions. To synthesize such materials, a thiol-maleimide cross linker (2-((1-(2-(acryloyloxy)ethyl)-2,5-dioxopyrrolidin-3-yl)thio)ethylacrylate) was initially synthesized and subsequently incorporated into a polymer matrix of hydroxyethyl acrylate. The properties of the elastomeric materials were then carefully evaluated by tensile testing, creep recovery, swelling studies, differential scanning calorimetry and rheological experiments. It was found that these polymeric materials showed dynamic behaviours like self-healing and malleability at elevated pH values and temperatures. In addition, these materials possess significant healing properties and are mechanically stable towards creep deformation at room temperature and pressure. Their stimuli responsive self-healing, elastic, malleable and mechanically stable nature in combination with the facile nature of the synthesis paves the way for potential utilization in different applications that require enhanced properties and functions.

Tips/comments directly from the authors:

1. The synthetic techniques used to make the thiol-Michael based crosslinker (TMMDA) are very simple, but extra care should be given to store the crosslinker in the refrigerator or freezer. Storing the crosslinker at room temperature may result in background polymerization and eventually lead to loss of the crosslinker.

2. Although conventional free radical polymerization was used as a tool for polymerization, other polymerization techniques can be used as well. Although, reactivity of the thiol moiety has to be considered in that case.

3. Self-healing polymers are commonly responsive to single stimulus (e.g. temperature responsive Diels-Alder based polymer or light responsive disulfide polymer). TMMDA crosslinked materials developed in this paper have self-healing properties with both temperature and pH stimulus, giving them enhanced functionality and responsive character.

4. Dynamic materials synthesized in this article, based on the thiol-Michael reaction, showed malleability or reshape ability in response to both elevated temperature and pH. As a result, materials can be re-shaped into new configurations upon application of stimuli.

5. The thiol-Michael adducts are essentially static in the absence of thermal and pH stimulus, making the materials mechanically stable and creep resistant under ambient conditions.

Dual stimuli responsive self-healing and malleable materials based on dynamic thiol-Michael chemistry, Polym. Chem., 2017, 8, 6534-6543, DOI: 10.1039/C7PY01356F

About the webwriter

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). Please, visit this link for more information.

Nature is capable of synthesizing an unlimited number of biomacromolecules (e.g. proteins) with remarkable structures and functions by simply starting from only 20 amino acids. This process lies in the precise sequence-controlled polymerization of amino acids to control the primary structures of polypeptide precursors, followed by a highly efficient post-translation modification in order to define these structures.

Inspired by nature’s strategy to synthesize proteins, Tao and co-workers developed a two-stage method to synthesize a large number of polymers with precisely controlled structures, different functionalities and various molecular diversities. Key to their strategy is the combination of controlled radical polymerization and post-polymerization modification. Specifically, reversible addition-fragmentation chain transfer (RAFT) polymerization was utilized to synthesize the polymer precursors starting from only 3 acrylamide monomers. By repeating the polymerization with different monomer sequences, 6 triblock copolymers with controlled chain ends, molecular weights and molar mass distributions were obtained. The different polarity of all synthesized precursors was then confirmed by reverse-phase high performance liquid chromatography (HPLC). The triblock copolymers were subsequently modified via the Biginelli reaction to rapidly generate 60 derivatives in a high-throughput (HPT) manner. HTP analyses was also conducted as an efficient and quick way to verify specific functionalities (e.g. radical scavengers, metal chelating agents, etc.).

In summary, the authors presented an efficient strategy to prepare and characterize large libraries of polymers with diverse structures and functions.

Tips/comments directly from the authors:

1. For the Biginelli reaction, acetic acid/MgCl₂ is an efficient solvent/catalyst system to smoothly get the targeted compounds. However, this system is not as efficient for aliphatic aldehydes. Fortunately, the Biginelli reaction has been studied for more than 100 years and as such, many other solvent/catalyst systems have been established in this time. Thus, people can choose different conditions to perform the Biginelli reaction for the post-polymerization modification depending on the specific requirements and applications.

2. For the high throughput analysis of radical scavengers, the oxygen in the air might also quench the radical, and the radical colour was found to fade faster in summer than in winter. Thus, the use of fresh reagents and careful recording of the temperature is recommended.

3. The ultra-fast RAFT was used in the present work as a model polymerization to prepare copolymers. The authors believe other advanced controlled radical polymerization techniques (SET-ATRP, photo-induced CRPs, sulfur-free RAFT, etc.) can also be used to prepare multiblock copolymers, especially when thermo-sensitive monomers are used.

Polymer synthesis by mimicking nature’s strategy: the combination of ultra-fast RAFT and the Biginelli reaction, Polym. Chem., 2017, 8, 5679-5687, DOI: 10.1039/c7py01313b

About the webwriter

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). Please visit this link for more information.

The development of strategies that allow the translation of the precise monomer sequence control achieved in nature over macromolecular structure (e.g. DNA) to whole synthetic systems is an exciting field in current polymer science. In particular, the fabrication of sequence-defined polymers paves the way for a diverse range of applications. For example, these macromolecules can be used to store monomer-coded information. Lutz and his team has pioneered this field and recently described the synthesis of digitally-encoded polyurethanes using an orthogonal solid-phase iterative approach. This class of materials is particularly interesting thanks to their unique physicochemical properties and straightforward sequencing by tandem mass spectrometry.

In the current contribution, Lutz and co-workers expand the application pool of these materials by covalently linking sequence-coded oligomers to other synthetic polymers. Sequence-coded oligourethanes were initially synthesized by orthogonal solid phase iterative chemistry on a modified Wang resin. While still attached to the solid support, the ω-OH-termini of the oligourethanes were transformed into atom transfer radical polymerization (ATRP) initiators by esterification with α-bromoisobutyryl bromide. Then, the oligomers were detached from the solid support and their cleaved α -COOH-termini were esterified with ethanol yielding monodisperse ATRP macro initiators. Upon polymerization of styrene from these precise oligomers, well-defined blocky architectures were obtained containing sequence-coded oligourethane segments. The polymerization was well-controlled, yielding materials with narrow molecular mass distributions and good agreement between theoretical and experimental molecular weights. Importantly, for a given macroinitiator length, the coded sequence of oligourethane had no influence on the ATRP process. Overall, these exciting results open up interesting perspectives for the development of plastic materials containing sequence-coded traceability barcodes.

Tips/comments directly from the authors:

1. Sequence-coded oligourethanes have an interesting tendency to crystallize, which is currently under investigation. Consequently, these oligomers are usually relatively easy to characterize in solution directly after their synthesis but may become less soluble in standard solvents with time and storage.
2. As mentioned in the communication, the tendency toward self-organization of the oligourethanes might influence their macroinitiator behavior. The preliminary results shown in this communication indicate that a controlled radical polymerization behavior is attainable with these macroinitiators. However, a deeper understanding of the initiation step is probably mandatory.
3. The atom transfer radical polymerization of styrene was chosen as a simple polymerization model in the present work. Nevertheless, other controlled radical polymerization techniques might, of course, be considered for preparing such materials.

Sequence-coded ATRP macroinitiators Polym. Chem., 2017, 8, 4988-4991, DOI: 10.1039/C7PY00496F

About the webwriter

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). Please, visit this link for more information.