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Paper of the month: Polymer synthesis by mimicking nature’s strategy: the combination of ultra-fast RAFT and the Biginelli reaction

24 Oct 2017

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.

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Paper of the month: Sequence-coded ATRP macroinitiators

27 Sep 2017

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:

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.
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.
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.

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Paper of the month: Block co-polyMOFs: assembly of polymer-polyMOF hybrids via iterative exponential growth and “click” chemistry

22 Aug 2017

Block copolymer (BCP) assemblies are derived from covalently linked polymer chains and can undergo phase separation and thus find use in a wide range of applications including micropatterning, battery and electronic technologies. Metal-organic frameworks (MOFs) are another class of self-assembled matter consisting of crystalline networks with angstrom-scale order and permanent porosity. Owing to these advantageous properties, they can enable functions such as gas, energy storage, catalysis and selective-separation.

In the Johnson group, impressive efforts have been made to merge amorphous polymer networks with multi-component supramolecular assembly to generate soft materials with novel properties. In their current contribution, the group expands these types of hybrid materials by reporting a novel BCP where one block is a uniform benzene dicarboxylate (BDC)-based oligomer synthesized by iterative exponential growth (IEG), and the other is polystyrene (PS) prepared by atom transfer radical polymerization (ATRP). In order to achieve this, the authors initially synthesized the BDC-based oligomer with a defined end-functionality bearing an alkyne group that would allow for further diversification. This alkyne group was subsequently used to couple to azide-terminated polystyrene. In the presence of Zn ions, this BCP forms a “block co-polyMOF” (BCPMOF) material comprised of polyMOF domains embedded in a PS matrix.

The presented work is the first demonstration that it is possible to generate a crystalline polyMOF-amorphous polymer hybrid material from a single diblock copolymer. As such, BCPMOFs represent a new composite material that possesses the processability of the polymers while exhibiting enhanced stability towards ambient conditions when compared to the isolated MOFs. The ultimate goal of the group is to obtain BCPMOFs with robust mechanical properties, high surface areas, and tunable, well-defined domain sizes.

Tips/comments directly from the authors:

In the synthesis of the mono benzylated diethyl 2,5-dihydroxyterephthalate, A, UV absorbance can readily distinguish the starting material from the product. The starting material elutes before the product and can be isolated for reuse.
As noted in the supporting information, side products can occur during the coupling reaction to form L₁. It is critical that ethanol is used to maintain the ethyl ester.
The coupling reactions tend to require longer reaction times as the molecular weight of the reactants grows. Upon scaling up of the reactions, do not be temped to increase the concentration too much as it can lead to side product formation.
The same synthetic protocols were used to form polyMOFs and block co-polyMOFs: 2.5 eq Zn(II) per BDC unit in L_n, was combined with L_n(PS) in DMF, heated at 100°C for 24h, followed by DMF washings to remove excess Zn(II) and unreacted ligand. Unlike the purification and isolation of L₂ and L₄, special care was taken for L₄PS-Zn to use minimal organic solvent due to the solubility imparted by polystyrene.

Block co-polyMOFs: assembly of polymer-polyMOF hybrids via iterative exponential growth and “click” chemistry
Polym. Chem., 2017, 8, 4488-4493, DOI: 10.1039/c7py00922d

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). Please visit this website for more information.

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Paper of the month: Acceleration and improved control of aqueous RAFT/MADIX polymerization of vinylphosphonic acid in the presence of alkali hydroxides

26 Jul 2017

Phosphonic acid functionalized polymers find use in a wide range of applications such as metal protection, polymer electrolyte membranes flame retardancy and dentistry. This is thanks to their unique characteristics including their acidic nature, stability, proton conductivity and metal binding ability. Vinyl phosphonic acid (VPA) in particular is a structurally simple example of such monomers which is not only affordable but also provides with a polymer (PVPA) with phosphonic acid groups that are directly attached to the backbone.

Despite the popularity and applicability of this polymer, the polymerization of VPA is typically slow yielding incomplete conversions which necessitates the need for additional costly and time-consuming purification steps. Destarac, Harrisson and co-workers were capable to circumvent this by investigating the effect of adding various alkali hydroxides to the conventional (free radical) and reversible addition-fragmentation transfer polymerization/macromolecular design by interchange of xanthates (RAFT/MADIX) radical polymerizations. Both types of polymerizations were strongly affected by the addition of NaOH. The authors found that by adding 1 equivalent of NaOH they could significantly increase the rate of the polymerization and the final conversion for both conventional and RAFT/MADIX polymerizations while larger quantities led to retardation of the reaction. A wide range of alkali hydroxides were also studied including H⁺, Li⁺, K⁺and NH₄⁺. It was shown that the dispersity of the final polymer decreases as the ionic radius of the counterion increases (H⁺ > Li⁺> Na⁺> K⁺> NH4⁺) while the acceleration of the polymerization follows the order Na⁺> K⁺> NH₄⁺> Li⁺ > H⁺). Overall, the fastest rates of polymerizations were obtained in the presence of 0.5 equivalent of NaOH, while the same concentration of KOH or NH₄OH allowed for a moderate acceleration on the polymerization rate combined with an improved control over the molar masses. Thus, this simple and cost-effective strategy can significantly improve the efficiency of the polymerization of VPA by simultaneously enhancing the reaction rate and the control over the molar masses.

Tips/comments directly from the authors:

Take care to control the temperature when neutralizing the VPA – the reaction is very exothermic!
Use NaOH for the most significant acceleration of polymerization and NH₄OH for the strongest reduction in dispersity of the polymer.
PVPA homopolymer can be precipitated in MeOH, but many PVPA-containing DHBCs must be purified by dialysis due to the small difference in solubility between PVPA and VPA and the low volatility of VPA.

Read this exciting research for free until 10/09/2017 through a registered RSC account.

Acceleration and improved control of aqueous RAFT/MADIX polymerization of vinylphosphonic acid in the presence of alkali hydroxides
Polym. Chem., 2017, 8, 3825-3832, DOI: 10.1039/c7py00747g

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Paper of the month: Structure/property relationships in copolymers comprising renewable isosorbide, glucarodilactone, and 2,5-bis(hydroxymethyl)furan subunits

29 Jun 2017

Polyesters obtained from bio-derived monomers are often used as building blocks with the ultimate aim of meeting consumer demands for high-performance and sustainable materials. To this end, Reineke, Tolman, and Lillie sought to establish how changing the ratio of the sustainable D-glucaro-1,4:6,3-dilactone containing α, ω-diene (GDLU) and isosorbide undecanoate (IU) may influence the thermal, chemical and mechanical properties of the acyclic diene metathesis (ADMET)-derived polymers. The authors synthesized a series of random copolymers consisting of a range of GDLU and IU compositions and fully characterized them by uniaxial tensile testing, small-amplitude oscillatory shear rheology, X-ray scattering, and hydrolytic degradation testing.

It was found that small compositional changes have a detrimental impact on their mechanical performance and degradability. In addition, the authors investigated which carbohydrate-based building block was most important in promoting the elasticity and shape-memory abilities of this class of materials. To address this issue, GDL or isosorbide were replaced with a different sustainable diol, 2,5-bis(hydroxymethyl) furan. Studies of the resulting copolymers indicated that GDLU is responsible for imparting both elasticity and shape memory properties. Further, more economical and environmentally-friendly routes for the synthesis of GDLU and IU feedstocks were also explored.

Tips/comments directly from the authors:

Acetonitrile appears to be the essential solvent for the scandium triflate-catalyzed esterification of GDL with 10-undecenoic acid anhydride.
10-Undecenoic acid anhydride can rapidly degrade on wet silica gel. To prevent this, oven dry (120 °C) the silica gel prior to its use and minimize the excess 10-undecenoic acid anhydride used in the reaction.
Due to the hydrolytic instability of GDL and GDL-containing polymers, they should be stored in a vacuum desiccator to protected from moisture to prevent degradation between uses.

Read this exciting research for free until 13/08/2017 through a registered RSC account.

Structure/property relationships in copolymers comprising renewable isosorbide, glucarodilactone, and 2,5-bis(hydroxymethyl)furan subunits
Polym. Chem., 2017, 8, 3746-3754, DOI: 10.1039/c7py00575j

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Paper of the month: Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight

05 Jun 2017

Block copolymers are of a great interest to the polymer chemistry community as they provide intermediate physicochemical properties when compared to the respective homopolymers. Sequential monomer addition, mechanistic transformation and coupling of different segments are three of the most popular approaches to obtain block copolymers. However, all these approaches suffer from several drawbacks such as being limited to monomers that can be polymerized only under the same polymerization mechanism or requiring extreme experimental precautions and elongated purification steps.

In order to address the latter challenge, Yagci, Yilmaz and co-workers developed the first metal free example of block copolymer formation in which they concurrently polymerize structurally different monomers from a junction point serving as two functional groups for each polymerization. Since atom transfer radical polymerization (ATRP) and ring opening polymerization (ROP) are not expected to interfere with each other and as such could be the ideal candidates for such a system. In order to test this hypothesis, a specifically designed initiator was synthesized possessing a tertiary bromine at one end (capable of initiating an ATRP reaction) and a hydroxyl functionality at the other end (capable for initiating a ROP reaction). Under carefully optimized conditions and in the presence of both the ATRP and ROP catalysts both methyl methacrylate (MMA) and ε- caprolactone could be simultaneously polymerized yielding a diblock copolymer with good agreement between theoretical and experimental value sand low dispersity values. Interestingly, the reaction took place on the roof of the chemistry department at Istanbul technical university utilizing natural sunlight as the light source. The applicability of this technique was further demonstrated by the simultaneous polymerization of different sets of monomers including n butyl acrylate- ε- caprolactone and methyl methacrylate-lactide combinations. As such, the successful combination of ATRP with ROP in the same reaction media allows for the facile one pot synthesis of block copolymers which can find use in further applications where excess of metals or inorganic residues would be undesirable.

Tips/comments directly from the authors:

1. All chemicals should be added into the reaction tube under nitrogen atmosphere in dark (perhaps by covering the outside of the tube with an aluminium foil) to avoid any premature light induced polymerization.
2. ROP polymerization can take place even in dark. Therefore, the ROP catalyst should be added last, and afterwards, the reaction tube should be exposed to sunlight as soon as possible. This way, one can provide the optimum conditions for the polymerizations to be realized simultaneously.
3. The method is best applicable in sunny days. Sunlight was deliberately selected as the most natural and simple way of light exposure. However, various other irradiation sources that emit in the wavelength regions matching with the absorption of appropriate sensitizers can also be used.

Read this exciting research for free until 17/07/2017 through a registered RSC account.

Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight
Polym. Chem., 2017, 8, 2899-2903, DOI: 10.1039/c7py00069c

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Paper of the month: Mild and efficient synthesis of ω,ω-heterodifunctionalized polymers and polymer bioconjugates

10 May 2017

The versatile and high yielding modification of polymer end groups is a critical tool for controlling materials properties. However, when multiple different functionalities are needed, pre-installation of two different functional groups at the polymer end groups is typically a tedious requirement. Sumerlin, Castellano and co-workers managed to circumvent this by developing a mild approach that enables the efficient synthesis of ω-ω-heterodifunctionalized polymers and polymer bioconjugates. Key to this strategy is the use of the recently introduced reagent benzotrifuranone (BTF) which allows the introduction of differentially “clickable” functional groups to monomethyl ether poly(ethylene glycol) amine (mPEG amine). In contrast to conventional polymer heterofunctionalization approaches that require high temperatures, significant excess of reagents and numerous synthetic steps, BTF serves as an ideal functionalization handle that operates at ambient temperature using near-stoichiometric amounts of reagents. Importantly, following functionalization of BTF with alkyne and alkene functional groups a fluorescent (coumarin) dye and biotin could be successfully conjugated to the end of mPEG-amine. These polymer bioconjugates were then able to bind avidin while showing an unexpected disruption of avidin tetramer formation. Overall, the compatibility of BTF with a broad scope of amine nucleophiles and thermally sensitive moieties (e.g. proteins) in combination with the highly efficient and mild nature of this reagent holds great promise for more elaborate heterofunctionalization strategies.

Tips/comments directly from the authors:

The reaction time of the first and second addition to BTF should be monitored (typically by thin layer chromatography) to ensure minimal over/under functionalization occurs.
The trisubstitution products are tolerant to many reaction conditions; however, when performing reactions that include radical intermediates, higher than usual reagent equivalents may be needed due to the radical scavenging nature of the phloroglucinol
When one-pot homodifunctionalizations are performed, be sure to add enough nucleophile to consume both electrophilic sites on the polymer end group and the three electrophilic sites on any unreacted BTF.
Regarding BTF synthesis: 1) Using fresh polyphosphoric acid and monitoring the reaction temperature is very important for the ring-closing in the last step of the synthesis, and 2) BTF and the mono- and difuranone derivatives are sensitive to silica gel, so avoid letting the compounds reside in a column too long during purification.

Read this exciting research for free until 21/06/2017 through a registered RSC account.

Mild and efficient synthesis of ω,ω-heterodifunctionalized polymers and polymer bioconjugates
Polym. Chem., 2017,8, 2457-2461, DOI: 10.1039/C7PY00225D

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Paper of the month: Novel alkoxyamines for the successful controlled polymerization of styrene and methacrylates

31 Mar 2017

Reversible addition-fragmentation chain transfer polymerization (RAFT) and transition metal-mediated radical polymerization (TMM-RDRP) are two widely used techniques employed for the preparation of controlled polymeric architectures. However, both of them exhibit significant colouring and complete purification of the final materials is challenging. On the other hand, nitroxide mediated polymerization (NMP) requires no or minimal purification although designing alkoxyamines that can facilitate the controlled polymerization of both styrene and methacrylates is a challenge. In this contribution, Asua and co-workers employed three different alkoxyamines to study the homopolymerization of styrene and its chain extension with methacrylates. Upon careful evaluation of the reaction kinetics as well as variation of the polymerization temperature, high monomer conversions with uncompromised end group fidelity could be achieved. A wide range of molecular weights were targeted in order to identify the limitations of the system. It was found that for targeted degrees of polymerization beyond 333, there was an increased difference between theoretical and experimental molecular weights, due to the thermal initiation of styrene. In addition, the nature of the substituents in the nitroxide adduct was also found to be crucial for the controlled polymerization of styrene with bulkier adducts providing more extensive control. The retention of the reactive alkoxyamine chain end was further confirmed via nuclear magnetic resonance (NMR) and matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-ToF-MS). Well-defined PS-b-PBMA and PMMA-b-PS block copolymers were also successfully prepared highlighting these classes of alkoxyamines as a versatile mediator for the controlled polymerization of both methacrylates and styrene.

Tips/comments directly from the authors:

It is important to monitor the temperature throughout the polymerisation in order to ensure optimum kinetics.

The appropriate temperature should be employed for methacylates (< 100 °C) and styrene (> 110 °C). For instance, this can influence the final molar mass distributions of a block copolymer. The preparation of a poly(methyl methacrylate) macro-alkoxyamine first, followed by the chain extension with styrene provides a PMMA-b-PS block copolymer with moderate MMD. However, the preparation of the PS block first would yield a block copolymer with high dispersity value due, in part, to the high temperatures employed.

Several matrix/salt combinations should be tested when conducting MALDI-ToF MS analysis. The reactive chain ends might not be observed with some matrixes, such as trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB).

Read this exciting research for free until 16/05/2017 through a registered RSC account:

Novel alkoxyamines for the successful controlled polymerization of styrene and methacrylates
Polym. Chem., 2017,8, 1728-1736, DOI: 10.1039/c6py02190e

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Paper of the month: Vesicles of double hydrophilic pullulan and poly(acrylamide) block copolymers: a combination of synthetic- and bio-derived blocks

02 Mar 2017

Willersinn et al. report the synthesis of double hydrophilic pullulan and poly(acrylamide) block copolymers.

Double hydrophilic block copolymers (DHBC) self-assemble to various structures in aqueous solutions due to a strong difference in hydrophilicity. This is in contrast to amphiphilic block copolymers that self-assemble due to the insolubility of the hydrophobic block in water. In their recent contribution, Schmidt and co-workers were able to extend the principle of double hydrophilic self-assembly to novel polysaccharide-polyacrylamide block copolymers namely pullulan-b-poly(N,N-dimethylacrylamide) (Pull-b-PDMA) and pullulan-b-poly(N-ethylacrylamide) (Pull-b-PEA). The bio-derived pullulan block was obtained via acid catalyzed depolymerisation, while the polyacrylamide homopolymer blocks were synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization. Subsequently the blocks were conjugated via copper catalyzed azide alkyne cycloaddition (CuAAC). The presence of formed vesicular structures was investigated via cryogenic electron microscopy (cryo SEM), static light scattering (SLS) and dynamic light scattering (DLS) measurements showing diameters in the range of 200 to 500 nm. Finally, laser confocal scanning microscopy (LSCM) was employed using Rhodamine B staining or Rhodamine B labelled block copolymers to image the formed particle structures. These novel vesicular structures may find use in a wide range of applications including drug delivery.

Tips/comments directly from the authors:

1. The synthesis of block copolymers via CuAAc can be significantly simplified via the utilization of azide functionalized resins. As described in the paper the alkyne functionalized building block is utilized in excess. The resin is added after sufficient reaction time directly to the reaction mixture to react with residual alkyne end functionalized polymer and finally the resin is easily removed via filtration after the reaction. Therefore, no optimization of equivalents or absolute molecular weight data is needed to obtain a pure product without homopolymer contamination.
2. The choice of blocks is crucial for DHBC self-assembly in water. The blocks have to feature a significant difference in hydrophilicity. Moreover, high polymer concentrations are needed to obtain self-assembled structures.
3. For a pure DHBC self-assembly the utilized blocks should not feature LCST or other stimulus responsive behaviour that alters the solubility of the individual blocks. Otherwise the self-assembly might be driven via hydrophobic interactions as with an amphiphilic block copolymer.

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Vesicles of double hydrophilic pullulan and poly(acrylamide) block copolymers: a combination of synthetic- and bio-derived blocks
Polym. Chem., 2017,8, 1244-1254, DOI: 10.1039/c6py02212j

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Paper of the month: Probing the causes of thermal hysteresis using tunable Nagg micelles with linear and brush-like thermoresponsive coronas

31 Jan 2017

Blackman et al. report the synthesis of thermoresponsive polymers with tunable aggregation numbers in order to study the causes of thermal hysteresis.

Thermoresponsive polymers are materials that exhibit a change in their solubility over a temperature range. Thanks to this unique characteristic, these polymers can be used as smart and switchable materials for a wide range of biomedical applications. However, the cloud points upon cooling and heating a thermoresponsive polymer do not coincide because the process of equilibration takes time. The temperature interval between the cloud points upon cooling and heating is called hysteresis and the reversibility of these polymer’s thermal transitions can be influenced by many factors. In order to shine a light to these factors, O’Reilly, Gibson and Blackman synthesized well-defined, responsive amphiphilic block copolymers containing four different thermoresponsive corona blocks and assembled them in micellar structures in aqueous media. All micelles were designed to have tunable aggregation number enabling the study of the effects of altering the corona chemistry, chain confinement and core hydrophobicity on the thermoresponsive behavior, specifically the degree of hysteresis. It was found that higher core hydrophobicities were associated with a higher degree of hysteresis due to differences in core hydration. Linear corona chains capable of forming polymer-polymer hydrogen bonding interactions (e.g. pNIPAM) showed a greater hysteresis than those that could not (pDEAm). Importantly, the authors demonstrates that polymers with a brush-like architecture (pDEGMA and pOEGMA) exhibit irreversible phase transitions at a critical chain density, owing to irreversible nanoscale rearrangement in the precipitated bulk. These findings offer a deeper and more comprehensive understanding of stimuli-responsive self-assemblies and further highlight the complexity of hysteresis in thermoresponsive polymer systems.

Tips/comments directly from the authors:

1. It is important to use a Peltier system fitted with a reference cell with an internal temperature probe in order to accurately measure the hysteresis in the cooling curve. We have found that those without this feature typically over-estimate the hysteresis by a few degrees.
2. When synthesizing the pOEGMA-containing diblock copolymers, careful consideration of the RAFT CTA was necessary; the use of 2-Cyano-2-propyl dodecyl trithiocarbonate, as was employed for pDEGMA-containing diblock copolymers, resulted in a significant amount of high molecular weight polymers in the molecular weight distribution. Additionally, lower conversions had to be employed in order to reduce the presence of such species.
3. It was also important to employ multiple angle light scattering coupled with an algorithm such as REPES, which could enable us to investigate the molecular weights of the major fast mode and filter out scattering from spurious slow modes typically found in thermoresponsive polymer systems attributed to non-Brownian interactive behavior.

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Probing the causes of thermal hysteresis using tunable Nagg micelles with linear and brush-like thermoresponsive coronas
Polym. Chem., 2017, 8, 233-244.
DOI: 10.1039/C6PY01191H

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Polymer Chemistry Blog

Archive for the ‘Paper of the Month’ Category

Paper of the month: Polymer synthesis by mimicking nature’s strategy: the combination of ultra-fast RAFT and the Biginelli reaction

Paper of the month: Sequence-coded ATRP macroinitiators

Paper of the month: Block co-polyMOFs: assembly of polymer-polyMOF hybrids via iterative exponential growth and “click” chemistry

Paper of the month: Acceleration and improved control of aqueous RAFT/MADIX polymerization of vinylphosphonic acid in the presence of alkali hydroxides

Paper of the month: Structure/property relationships in copolymers comprising renewable isosorbide, glucarodilactone, and 2,5-bis(hydroxymethyl)furan subunits

Paper of the month: Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight

Paper of the month: Mild and efficient synthesis of ω,ω-heterodifunctionalized polymers and polymer bioconjugates

Paper of the month: Novel alkoxyamines for the successful controlled polymerization of styrene and methacrylates

Paper of the month: Vesicles of double hydrophilic pullulan and poly(acrylamide) block copolymers: a combination of synthetic- and bio-derived blocks

Paper of the month: Probing the causes of thermal hysteresis using tunable Nagg micelles with linear and brush-like thermoresponsive coronas

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