Polymer Chemistry Blog

Haven et al. describe the efficiency of single monomer insertion via both kinetic simulations and experimental observations.

So-called sequence controlled materials have recently received considerable interest due to the precise and freely selectable order of monomers in a monodisperse chain. Such materials exhibit the precision of the peptides in all aspects and differentiate this approach from the synthesis of multiblock copolymers, where a significant dispersity (albeit <1.10 in many occasions) is displayed. Herein, Junkers and co-workers provide an in depth elucidation of the crucial factors that should be taken into account when performing single unit monomer insertion (SUMI) reactions. Both modelling and experimental data confirm that isolated yields of each insertion are comparatively low when going beyond the third monomer addition and as such, even lower yields must be expected for further monomer insertions. Kinetic simulations have shown that most reaction conditions play only a minor role for the success of the insertions and thus, a wide range of conditions can be applied for the synthesis of such materials. Moreover, the effect of the chain-length dependency on the SUMI reactions has also been critically evaluated. Importantly, the carefully optimized conditions obtained from microreactor experiments and kinetic modelling has been subsequently applied to upscale the SUMI reactions in a mesoflow reactor. Although the facile access to such materials demonstrates the pathway towards future developments in the synthesis of longer sequence controlled oligomers, the challenge remains whether oligomers with chain length above 5 will also be available

Tips/comments directly from the authors:

1. For Single Unit Monomer Insertion reactions (SUMIs), product yield optimization is by stopping the reaction after exactly one monomer equivalent consumption. The reaction rate, thus radical initiator concentration, temperature and overall monomer conversions play a minor role; SUMIs can thus be performed within few minutes.
2. To study the yield of a SUMI reaction, one needs to distinguish isolated yield from the yield in the crude product mixture. Practically, isolated yields are very dependent on the efficiency of the product isolation method. Yields from the crude can be obtained by careful calibration of mass spectra intensities.
3. As long as monomers with more or less equal reactivities are chosen, a yield of ~50% is the theoretical maximum.
4. Evaluation of experimental yields under optimized conditions show that the yield decreases with increasing length of the sequence-defined oligomers. This effect is attributed to a strong chain-length dependency of the monomer propagation rate coefficients.
5. For upscaling of SUMI reactions, micro- and mesoflow reactors offer the perfect solution.

Efficiency assessment of single unit monomer insertion reactions for monomer sequence control: kinetic simulations and experimental observations, by J.J. Haven, J. Vandenbergh, R. Kurita, J. Gruber and T. Junkers, Polym. Chem., 2015, 6, 5752-5765.

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Warwick (UK)/ Monash (Australia) research fellow working under the Monash Alliance. Visit http://haddleton.org/group-members for more information.

Zhao et al. describe the potential of the Biginelli polycondensation to improve metal bonding strength.

Recently, the introduction of efficient reactions (e.g. click reactions, Diels-Alder reactions) to polymer chemistry aiming to synthesize new condensation polymers with improved properties and characteristics has attracted considerable interest. However, on many occasions, the requirement of extensive synthetic steps (e.g. for monomer synthesis) in combination with the usage of unsafe reagents (e.g. toxic, explosive etc.) necessitates the need for the development of alternative strategies that will provide access to large scale functional materials. Towards this goal Tao and co-workers employed the Biginelli polycondensation reaction to polymerize a novel difunctional monomer consisting of benzaldehyde and beta-keto ester groups, to yield poly(dihydropyrimidin-2(1H)-one)s (poly(DHMPs) (M_n ~ 22000 g mol^-1) within 1 h. Interestingly and in contrast with the small molecular DHMPs, the Biginelli polycondensates presented metal bonding capability and adhesive properties (up to ~ 2.8 Mpa). In addition, when monomers containing more functional groups were employed, stronger tensile shear strength was demonstrated indicating that the cross-linked polymer network has a positive effect on the bonding strength (3.9-5.9 Mpa). Finally, the efficiency of the reaction was further demonstrated by performing the reaction using an electric heat gun. The preparation of the monomers on a large scale, the facile nature of the polymerisation and the excellent metal bonding performance paves the way for the synthesis of new functional polymers.

Tips/comments directly from the authors:

1.  When finishing the polycondensation of monomer AB, the final polymer should be precipitated into cold water immediately, because the viscosity of they system will increase after cooling down (solid can even be formed), which will make the purification challenging.

2.  When precipitating the polymer, adding some base (e.g. NaOH) in water is helpful for the removal of the acetic acid. In addition, strong stirring during the precipitation is necessary to achieve satisfactory purification.

3.  During the metal bonding test, evenly heating could improve the efficiency of bonding. An open system for the volatilization of water will also enhance the glue effect.

4.  As the monomer A2B2 is viscous, gentle heating prior to use is helpful for measuring.

From drug to adhesive: a new application of poly(dihydropyrimidin-2(1H)-one)s via the Biginelli polycondensation, by Y. Zhao, Y. Yu, Y. Zhang, X. Wang, B. Yang, Y. Zhang, Q. Zhang, C. Fu, Y. Wei and L. Tao, Polym. Chem., 2015, 6, 4940-4945.

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Warwick (UK)/ Monash (Australia) research fellow working under the Monash Alliance. Visit http://haddleton.org/group-members for more information.

Martin et al. describe the synthesis of multiblock copolymers via RAFT polymerization at room temperature.

The preparation of high-order multiblock copolymers in a one pot process using reversible addition-fragmentation chain transfer (RAFT) is highly attractive due to the rapid polymerization rates, the achievement of quantitative conversions for each block, the lack of purification steps between the intermediate monomer additions (time effective and resource effective) and the narrow molecular weight distributions that can be attained. The “secret” of this success is the choice of high k_p acrylamide monomers and water as the reaction solvent allowing for full monomer conversion to be obtained whilst employing very low amounts of free radical initiator. However, applying this polymerization methodology to lower k_p monomers, such as methacrylates and acrylates, can be problematic as a higher concentration of initiator will be required to “push” the reaction towards completion and side reactions are also likely to occur at elevated temperatures, typically employed for this polymerization protocol. High temperatures are also disadvantageous for the polymerization of monomers that possess an LSCT upon polymerization (e.g. N-isopropyl acrylamide (NIPAM)).

In this work, a new approach to prepare multiblock copolymers via room temperature aqueous RAFT is presented. The authors implement the suitable redox couple tert-butyl hydroperoxide/ascorbic acid (TBHP/AsAc) to polymerize both acrylate and acrylamide multiblock copolymers with low dispersity values and high end-group fidelity exemplified by several in situ chain extensions. The challenge of working with slightly lower k_p monomers is also highlighted as both low and high molecular weight tailing is evident for the acrylate multiblocks whilst only gradual broadening and no shoulders are observed for the acrylamide analogues. A multiblock that consists of both acrylamide and acrylate monomers has also been targeted, demonstrating the versatility of the approach to obtain more complex multiblock structures. The main advantage of this work is the possibility of incorporating thermoresponsive blocks (e.g. NIPAM and diethyl acrylamide (DEA)) in the multiblock composition and limiting side reactions, often occurring at higher temperatures. Another interesting feature of this paper is the ability to control the polymerization of more hydrophobic (and not water soluble) monomers (e.g. methyl acrylate and ethylene glycol methyl ether acrylate) which were also successfully included in the multiblock sequence with a high degree of control. In contrast with multiblock copolymers obtained via single electron transfer living radical polymerization (SET-LRP) or atom transfer living radical polymerization (ATRP) methods, RAFT offers the additional advantage of allowing the incorporation of acidic monomers in the multiblock composition. The next challenge to tackle is to polymerize even lower k_p monomers (such as methacrylates) with a similar level of control.

Tips/comments directly from the authors:

1) When working at room temperature the viscosity is high. To avoid a loss of MW control after few block extension, a strong stirring for a good homogenization of the polymerization mixture is necessary.

2) The mixing of acrylate and acrylamide blocks is rather difficult because of the difference in reactivity of each family of monomers. Normally poly(acrylates) are better reinitiating group than poly(acrylamides) and therefore should be polymerized first.

3) In the redox initiator couple tert-butyl hydroperoxide/ascorbic acid (TBHP/AsAc), we found that a lot less AsAc could be used than that reported, and yet still give efficient initiation. In fact we observed that AsAc could act as an inhibitor of the radical polymerization. We are currently investigating the optimal ratio of the reducing and oxidizing agents.

4) The water soluble hydroxyethylacrylate monomer needs to be carefully purified because of diacrylate contamination, which is responsible for the shoulder observed at high MW on SEC analyses.

Preparation of complex multiblock copolymers via aqueous RAFT polymerization at room temperature, by L. Martin, G. Gody and S. Perrier, Polym. Chem., 2015, 6, 4875-4886

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Warwick (UK)/ Monash (Australia) research fellow working under the Monash Alliance. Visit http://haddleton.org/group-members for more information.