The method is based on the stereo-controlled ring-opening metathesis polymerization (ROMP) of an olefinic monomer, cis-cyclooctene (COE), into either a trans-polyoctenamer rubber (TOR) or a cis-polyoctenamer rubber (COR) using ruthenium (Ru)-based catalysts. From screening assays, the researchers chose three representative catalysts: a Grubbs second-generation (G2) catalyst that readily leads to TOR formation (control); a photolatent bis-N-heterocyclic carbene (bis-NHC) catalyst (Ru-1) that only promotes TOR formation at high temperatures; and a stereoretentive catalyst (Ru-2) that promotes COR polymerization. To increase Ru-1 activity at room temperature, the pyrylium photoredox catalyst for ROMP, 2,4,6-tris(4-dodecylphenyl) pyrylium tetrafluoroborate (pyr.) was solubilized into the COE monomer solution. Blue-light irradiation of this solution resulted in a fast and substantial COE conversion into TOR, indicating that the stereochemistry of ROMP could be controlled by light.
Mechanical and structural analyses of the resulting polymers show that COR is soft and elastic (maximum stress (σm) of 12 MPa, Young’s modulus (E) ≈ 3 MPa, and strain at failure (εf) of approximately 800%), having similar mechanical properties to those of conventional thermoplastic elastomers. Nonetheless, its composition and structure are the same as those of a simple rubbery homopolymer. By contrast, TOR is a strong and stiff material (σm ≈ 23–27 MPa and E ≈ 800–1,000 MPa), optically opaque and has high crystallinity, resembling semicrystalline thermoplastics. Based on these findings, Rylski et al. realized that the mechanical properties of the resulting polyoctenamer could be modulated by controlling the stereochemistry of ROMP of COE using a mixture of Ru-1 and Ru-2 catalysts in combination with visible light. They use this concept to photopattern TOR domains in a COR matrix, creating composites with semicrystalline regions that are seamlessly integrated into the amorphous soft bulk and whose mechanical responses can be tuned simply by changing the pattern design. While the significance of this approach for the creation of functional multimaterials for technological applications is yet to be fully demonstrated, from a fundamental perspective, the work of Rylski et al. provides a simple but elegant approach to create multimaterials in which the interface between rigid and soft domains is cohesively integrated, similar to those found in biological materials.
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