Polymer Chemistry Blog

Ishizuka et al. report the synthesis of polymeric microcapsules with a hydrophilic core using SPG membrane emulsification.

Polymeric nano/micro capsules can be used in many applications ranging from biomedicine to cosmetics, food industry and water treatment. However, the current approaches to prepare hollow particles possess a number of drawbacks including tedious and multistep synthesis and low encapsulation efficiency. In order to address these limitations, Zetterlund, Stenzel and co-workers have elegantly combined inverse emulsion periphery reversible addition-fragmentation chain transfer polymerization (RAFT) polymerization (IEPP) with shirasu porous glass membrane (SPG) emulsification to synthesize hollow particles with a hydrophilic core. An amphiphilic macroRAFT agent, namely poly(di (ethylene glycol)methyl ether methacrylate)17-block-poly (methyl methacrylate), was employed to prepare inverse emulsions with a relatively narrow size distribution. Adjusting the SPG membrane pore size from 0.2 to 3 μm allowed access to good control over the droplet size and subsequently the size of the final capsules. This is a significant improvement over conventional emulsions based techniques (e.g. (mini)emulsion or suspension systems) which typically generate either smaller or larger particles. Importantly, the synthesized emulsions were stable at room temperature for over a month maintaining their size and size distribution post polymerization. Finally, the authors demonstrated the successful encapsulation of a water soluble fluorescent dye without any significant effect on the droplet/microcapsule diameter. Therefore, this process showed great potential and can be applied to a wide range of droplets, enabling the encapsulation of various molecules in nano/micro capsules.


Tips/comments directly from the authors:

1. It is important to use a “hydrophobic membrane” to prepare inverse emulsions. Membranes are reusable after cleaning. Typically after each experiment, the membrane is first washed with THF to remove polymer, then with water to remove salt/water soluble guest molecules, subsequently with THF and finally with toluene at least three times to make sure the membrane is wetted with toluene.
2. In order to prepare monodisperse droplets, the continuous phase has to be stirred gently (in this study 160-170 rpm) during emulsification to create a flow which induces the detachment of droplets from the membrane surface and to prevent creaming (generated droplets would settle down without stirring). The stirring speed may differ depending on the specific emulsification condition.
3. During the crosslinking polymerization, a higher stirring rate (500-600 rpm) is typically required to prevent aggregation of particles. The optimum stirring rate may differ depending on the size of the reaction vessel and/or volume of the reaction mixture.

Read this exciting research for free until 25/02/2017 through a registered RSC account:

Synthesis of microcapsules using inverse emulsion periphery RAFT polymerization via SPG membrane emulsification
Polym. Chem., 2016, 7, 7047-7051, DOI: 10.1039/c6py01584k

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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please visit this website for more information.

Aksakal et al. report the Cu(0)-mediated living radical polymerization of acrylates using a British penny coin.

Cu(0)-mediated living radical polymerization, typically referred to as single electron transfer living radical polymerization (SET-LRP), is a versatile tool for the synthesis of advanced materials. The groups of Becer and Resmini have further highlighted this versatility by reporting the SET-LRP of acrylates catalyzed by a penny copper coin. Impressively, a wide range of hydrophobic and hydrophilic monomers were successfully polymerized yielding well-defined polymers with low dispersities, near-quantitative conversions and high end group functionality. The scope of the system was subsequently expanded to include the synthesis of star polymers through the core first approach. Interestingly, the authors used two type of coins, the first one (issued in 1971-1992) consisting of 97% of copper and the second one (issued after 1992) consisting of 6% of copper. Both coins exhibited near identical polymerization results. A series of polymerizations targeting different degrees of polymerization were also conducted, all proceeding with very good control over the molecular weight distributions. In comparison to traditional Cu(0)-wire systems, British penny coins have the additional advantage of prohibiting the induction period, which is typically observed for many SET-LRP reactions. Finally, the scalability of these polymerizations up to 50 gram scale was also successful and thus demonstrating an economic, efficient and easily accessible catalyst for SET-LRP of various acrylic monomers.


Tips/comments directly from the authors:

Comments:

In this study, we provide direct evidence that the traditionally used Cu(0)-wire can be replaced with a copper coin, regardless to its year of issue. To avoid any induction period, this method can be employed for the synthesis of well-defined polymers of acrylates with both linear and star shaped initiators.


Tips:

1. Since the coins are usually contaminated due to prior circulation, we suggest for reproducibility purposes a quick rinse with a freshly prepared HCl before the polymerization.
2. As mentioned in the manuscript, both penny coins issued before and after 1992 exhibit near identical polymerization results. However, the coins issued after 1992 consist of 94% steel and are magnetic. Due to this, it should be taken into account that the magnetic stirrer can occasionally spin out of its axis. Therefore, we suggest the use of narrow and long Schlenk tube to avoid any splashing of the polymerization mixture.
3. Due to the reactivity of the polymerization mixture, samples for both NMR and GPC should be diluted immediately in order to avoid errors during kinetic sampling.
4. The inhibitor of monomers can be easily removed by passing over a plug of basic aluminium oxide. Due to the high viscosity of the OEGA₄₈₀ monomer, larger volumes can be diluted in a volatile solvent to decrease viscosity. Evaporation of the solvent will provide the inhibitor-free monomer.

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SET-LRP of acrylates catalyzed by a 1 penny copper coin
R. Aksakal, M. Resmini and C.R. Becer
Polym. Chem., 2016, 7, 6564-6569
DOI: 10.1039/C6PY01295G

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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please visit this website for more information.

Radlauer et al. report the regioregularity of ring-opening metathesis polymerization and cross metathesis reactions for the synthesis of block and heterotelechelic materials.

Among the two distinct olefin metathesis polymerization methods, namely acyclic diene metathesis (ADMET) polymerization and ring opening metathesis polymerization (ROMP), ROMP possesses the advantage of proceeding by a chain growth mechanism and has been demonstrated to deliver polymers with an R substituent every 8 carbons from 3-substituted cyclooctene (3RCOE) monomers. Although secondary metathesis such as cross metathesis (CM) can occur between chains during ROMP, when the selectivity of 3RCOE monomers was examined, no studies on the CM were conducted. Towards this direction, Hillmyer and co-workers determined the CM of 3RCOE derivatives to be both regio- and stereoselective, making only a small fraction of t-TT and t-HH errors. As only a marginal increase in errors over time was observed, it was concluded that the system reached an equilibrium between the formation and fixing of errors. This stereo- and regioselectivity can be extended by combining polymers with different degrees of polymerization, or by combining polymers with different R substituents leading to the formation of multiblock or statistical copolymers depending on how long the CM is allowed to proceed. In the latter case, differential scanning calorimetry (DSC) confirms the transition from two T_gs to one T_g corresponding to a multiblock and a statistical copolymer, respectively. Additionally, the location of end groups from an asymmetric chain transfer agent can be controlled, thus allowing access to predominantly heterotelechelic oligomers or polymers despite the CM reactions that can occur between the chains.

Tips/comments directly from the authors:

1. In our experience if a polymer turned brown, it was generally caused by the presence of deactivated Grubbs catalyst. To remove this contaminant at the end of the polymerization, the polymer was dissolved in chloroform and stirred with carbon black. Subsequent filtration and reprecipitation of the polymer generally yielded clear and colorless materials.
2. To obtain SEC data that was more easily interpreted, we used adjusted ratios of the polymers of different degrees of polymerization in the polymer-polymer cross metathesis experiments. Though the 1:1 scenario still showed immediate CM to an average N, the resolution between the two peaks was quite poor.

Any experiment involving the acetal-containing chain transfer agent required that each reaction component be filtered over basic alumina to avoid deprotection of the acetal moiety.

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Regioselective cross metathesis for block and heterotelechelic polymer synthesis
M.R. Radlauer, M.E. Matta and M.A. Hillmyer,
Polym. Chem., 2016, 7, 6269-6278,
DOI: 10.1039/C6PY01231K

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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please visit this website for more information.

Gavrilov et al. report the quantitative and in situ functionalization of PNIPAM in aqueous media.

One major issue in aqueous copper mediated approaches is the hydrolysis of the halide end group which significantly compromises the end group fidelity of the resulting materials and therefore limits post polymerization modifications. In this current contribution, Monteiro and co-workers have carefully investigated the kinetics of hydrolysis of poly(N-isoproplyacrylamide) (PNIPAM) obtained via a new single electron transfer (SET) polymerization method to reduce Cu(II) directly and quantitatively to Cu(0). It was shown that the rate of hydrolysis is independent of the molecular weight of the polymer and the copper content in solution and reaches completion in approximately 15 hours.

In order to circumvent the hydrolysis issue, the authors elegantly conducted an in situ azidation of the bromine end group resulting in the quantitative transformation of the bromine end groups in a matter of 30 seconds, as indicated by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-ToF-MS). This is quite a remarkable achievement given that organic solvents facilitate the same process in much longer reactions times, ranging from 10 to 24 hours. The fast reaction rate can be attributed to the enhanced solubility of sodium azide in water and the greater ion pair separation that facilitates nucleophilic attack.

In an attempt to further confirm the data obtained by MALDI-ToF-MS, the azide terminated PNIPAM was subsequently coupled to alkyne functional polymers (after purification) with coupling efficiencies greater than 97%. Thus, it was demonstrated that this approach can not only overcome the hydrolysis issues but also allow rapid synthesis of functional materials.

Tips/comments directly from the authors:

1. The method of addition of the reactants is important, and by varying the order of addition the polymerizations rates can vary.
2. It is important to note that the NaBH₄ is hygroscopic, and care should be taken to store it in a water-free environment as errors can arise from weighting wet reagents.
3. We believe that quantitative information can be obtained from the molecular weight distribution using the SEC/LND-model. This is especially important when determining quantitative coupling information of end-groups at high polymer molecular weights, as conventional techniques such as NMR and MALDI-ToF lose sensitivity. We use the weight distribution (i.e. w(M)) as the total weight of the reactants and products remains constant over the reaction; thus, the area under the w(M) vs M curve is the same before and after the reaction.

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Quantitative end-group functionalization of PNIPAM from aqueous SET-LRP via in situ reduction of Cu(II) with NaBH₄
M. Gavrilov, Z. Jia,, V. Percec and M.J. Monteiro
Polym. Chem., 2016, 7, 4802-4809
DOI: 10.1039/C6PY00968A

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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).

Despax et al. report the synthesis of triblock copolymers and their application as thermoresponsive hydrogels.

Temperature responsive gelators can benefit a wide range of biomedical applications and typically comprised of triblock copolymers with a central hydrophilic block and terminal blocks that undergo a hydrophilic to hydrophobic transition at a specific temperature. However, typical ABA triblock copolymers obtained from commercially available monomers require concentrations of at least 50-100 g L^-1.

Harrisson, Destarac and co-workers have managed to circumvent this by synthesizing high molecular weight triblock copolymers via low temperature reversible addition-fragmentation chain-transfer (RAFT) gel polymerization. The targeted triblock copolymers were based on polydimethylacrylamide (PDMA) as the long central hydrophilic block and poly(N-isopropylacrylamide) (PNIPAM) as the shorter terminal blocks and the gel formation was initially demonstrated via vial-inversion tests.

Two different molecular weight triblock copolymers were tested with the PDMA block varying from 58 kg mol^-1 to 421 kg mol^-1 showing self-supporting gels at 30 g L^-1 and 6 g L^-1 concentration respectively, which is already a significant improvement over previously reported materials. As the vial-inversion test is subject to experimental variations, a more objective measure of the effect of the temperature was obtained from the evolution of the storage and loss moduli of aqueous polymer solutions.

For the lower molecular weight polymer, a two-step transition consisting of an initial thickening of the solution at the lower critical solution temperature (LCST) of PNIPAM occurred followed by gel formation at 38–39 °C requiring a minimum concentration of 20 g L^-1. For the longer polymer, only the second transition was observed; gel formation occurred at 40-45 °C with a minimum concentration of 4 g L^-1. With a storage modulus of only 0.1 Pa however, this gel is likely too soft for practical use.

In an attempt to improve the mechanical properties of the gels, 2-acrylamido-2-methylpropanesulfonic acid was also incorporated (20 mol% of DMA) resulting on the formation of self-supporting gels at 2 g L^-1, an order of magnitude improvement over previously-reported ABA copolymers. These results approach the performance obtained from exotic polymers such as polyisocyanopeptides.

Tips/comments directly from the authors:

1. High monomer concentrations are helpful to obtain high molecular weights. However, the polymerization of acrylamides is very exothermic so it is important not to exceed 30 wt%.
2. As very low initiator concentrations are used, it is important to thoroughly degas all solutions prior to polymerization.
3. Take care to exclude any air bubbles from the solution when carrying out rheology measurements.

Read this exciting research for free until 03/07/2016 through a registered RSC account:

Low concentration thermoresponsive hydrogels from readily accessible triblock copolymers
L. Despax, J. Fitremann, M. Destarac and S. Harrisson
Polym. Chem., 2016, 7, 3375-3377
DOI: 10.1039/C6PY00499G
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About the webwriter

Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).

Kristufek et al. report the synthesis of rapidly-cured isosorbide-based cross-linked polycarbonate elastomers.

Inexpensive starting materials from natural products (such as isosorbide, isomannide etc.) can allow for natural material to begin to compete with and (why not?) eventually replace petrochemicals as a source of monomers. Isosorbide-based materials in particular have attracted considerable attention due to both the rigidity of their fused ring systems and the easily-modifiable dual hydroxyl functionalities. However, the utility of isosorbide-based materials for elastomers is perhaps more limited. As such, in the current article, Wooley and co-workers aim to produce rapidly-photo-cross-linked isosorbide-based elastomers via thiol-ene chemistry that will have the additional potential to hydrolytically break down into their original building blocks.

This novel cross-linked network system was elegantly synthesized using a naturally-derived monomer, isosorbide dialloc (IDA) and cross linked with tri-methylpropane tris(3-mercaptopropionate) (TMPTMP) yielding IDA-co-TMPTMP, an optically transparent elastomer. All the IDA-co-TMPTMP networks were obtained by environmentally friendly methods including solvent-free conditions, low catalyst loading and UV irradiation. Importantly, a study of a constant UV cure time (1 minute) and variation of the thermal curing times led to the conclusion that this material is near its optimal thermal and mechanical properties without requiring post-cure heating.

The thermal decomposition temperature of the networks were consistent (320 °C) while the glass transition temperature remained below room temperature for all samples with a % elongation of 220-340%. The hydrolytic degradation of the material was also evaluated and found to afford 8.3±3.5% and 97.7±0.3% mass remaining after 60 days under accelerated basic and physiological neutral buffer conditions respectively. Finally, a cell viability assay and fluorescence imaging with adherent cells were also reported in order to show the potential of this material as a biomedical substrate. In conclusion, the rapid synthesis of this optically transparent flexible elastomer presented very interesting properties that could be very useful in biomedical applications or as environmentally-friendly materials.

Tips/comments directly from the authors:

1. Because DMPA dissolves slowly in the reaction mixture, it is important to keep it in the dark while mixing and allow it to fully dissolve, resulting in the most uniform materials.
2. Glass slides were used as the molds to maximize the light exposure to the reaction mixture of the two monomers, ensuring the rapid curing time.
3. During the degradation study, it is important to change the solution at short (ca. 2 days), constant intervals to provide consistent results.

Rapidly-cured isosorbide-based cross-linked polycarbonate elastomers by T.S. Kristufek, S.L. Kristufek, L.A. Link, A.C. Weems, S. Khan, S.M. Lim, A.T. Lonnecker, J.E. Raymond, D.J. Maitland and K.L. Wooley, Polym. Chem., 2016, 7, 2045-2056, DOI: 10.1039/C5PY01659B