Controlled active radical polymerization with stereoscopic regularity

On May 4, 2023, Professor Zhong Mingjiang’s research group at Yale University published a new study entitled “Rare earth–cobalt bimetallic catalysis mediates stereocontrolled living radical polymerization of acrylamides” in the journal Nature Synthesis.

The study was developed by designing a series of “Lewis acid covalently linked porphyrin cobalt catalysts” (LACoP), under the bimetallic catalytic system, the active radical polymerization with adjustable stereoscopic regularity is realized. This method regulates the material properties of the bulk phase, solution and interface morphology of the polymer by changing the stereotactic regularity of the polymer.

The corresponding author of the paper is Professor Zhong Mingjiang, the first author is Zhang Xiaowei, a doctoral student in the laboratory, and the other authors include postdoctoral researcher Lin Fei (currently a distinguished researcher of Nankai University) and doctoral student Cao Mengxue.

The tacticity of a polymer is one of the main microstructural parameters that determine its material properties. The control of stereoscopic regularity can be achieved by coordination polymerization and ion polymerization, but these polymerization methods are suitable for a narrow range of monomers, and the compatibility of functional groups is also very limited. In contrast, radical polymerization has a wider monomer range and higher reaction compatibility. However, because the general carbon radical itself is planar symmetric structure and is not easy to interact with chiral additives or catalysts, achieving stereocontrolled radical polymerization has always been a problem in polymer chemistry.

Based on previous explorations, the authors designed a series of “Lewis acid covalently linked porphyrin cobalt catalysts”. In this design, the polydentate cyclic ligand azeca ether (ACE), covalently linked to porphyrin cobalt, binds to Lewis acids (e.g., rare earth metal cations) to produce a bimetallic complex of rare earth metals and porphyrin cobalt. The porphyrin cobalt center regulates photocatalytic reactive radical polymerization, while covalently linked rare earth metal ions regulate the stereochemistry of radical polymerization (Figure 1d).

Figure 1: Schematic diagram of radical polymerization under stereoscopic regularity control.

The authors successfully achieved free radical polymerization with high stereoregularity control by improving the binding affinity and geometry of the ACE ligand and selecting appropriate rare earth metal ions (Figures 2a, 2b). This series of catalysts can be applied to different kinds of acrylamide monomers (Figure 2c). The authors characterized the crystallinity and associated thermal properties of polydimethylbenzenamide (polyDMAA) with different stereotactic regularities by differential scanning calorimetry (DSC) and X-ray wide-angle diffraction (WAXS). The glass transition temperature (Tg) of the whole polyDMAA (95% m) is 109°C, which is 20°C lower than that of random polyDMAA (51% m). The DSC temperature curve of polyDMAA observed the missing first-order phase transitions: exothermic crystallization peak (Tc 177°C) and endothermic melting peak (Tm 258°C), indicating the formation of quasicrystalline structure. In addition, clear diffraction from the crystal phase was observed in the WAXS plots of the two homogeneous polyDMAAs (Figures 2d, 2e, 2f).

Figure 2: LACoP-mediated stereoscopic regularity controllable active radical polymerization and related polymer characterization.

By comparing the kinetic properties under different reaction conditions, the authors provide a mechanistic explanation for the reactive radical polymerization catalyzed by LACoP. In LACoP-1dUnder the conditions, by increasing the concentration of rare earth metal ions, the polymerization was significantly accelerated and accompanied by a more prominent stereoscopic regularity control, indicating that covalently linked rare earth metal ions achieved selective chelation of the end of the polymer chain.

The authors suggest that the quality of stereoscopic control can be improved by inhibiting other chelated species that compete with end-of-strand chelation. Thus, in conventional radical polymerization and common porphyrin cobalt1aIn the induced LRP, the content of meso diads changes with the gradual consumption of monomers due to the difference in the binding strength of monomers and polymers to Lewis acid. And LACoP-1dThe amount of meso diads in the induced aggregation is consistent (Figure 3F).

Figure 3: Kinetic and mechanistic studies of polymerization.

The authors elucidated the effect of polymer stereotactic regularity on the properties of polymer materials in bulk phase, interface, solution and other forms by measuring the lower critical dissolution temperature (LCST), surface adhesion capacity and conductivity of polymers at different regularities (Figure 4). The polymerization method developed in this paper can realize the diversification of the properties of polymer materials without changing the chemical composition, and provides new ideas for the design of future industrial or functional polymers.

Figure 4: Polymer properties under stereoscopic regularity regulation.

(Source: Science Network)

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