Monomers are usually unsaturated, cyclic or low-molecular compounds containing two or more functional groups, mainly including acrylics, epoxies, styrenes, alcohols, anhydrides, etc.
These monomers are the most important raw materials in polymer polymerization. Different types of monomers endow polymer materials with diversified functions, so that polymer materials not only have the function of transmitting, converting or storing substances, energy and information, and have functions such as chemical reactivity, photosensitivity, conductivity, catalysis, biocompatibility, pharmacology, and magnetism.
The manufacture of plastics requires the use of a variety of chemical raw materials, including monomers, catalysts, stabilizers, and more. Among them, monomers are the main raw materials for synthetic plastics. They can be derivatives of fossil fuels such as petroleum and natural gas, or renewable resources such as vegetable oil and starch.
Plastic is an indispensable key material in modern society. However, most of the plastics will be used as non-degradable garbage after they are used, which will have a huge negative impact on the environment. The material properties of plastics processed through mechanical recycling are severely degraded, limiting further applications. In recent years, the concept of closed-loop recycling of plastics has become the focus of the world's polymer science community. This method synthesizes polymer materials by designing specific monomers, and then directly converts them into original monomers, so as to realize resource recycling and use at the same level. . Although some examples of closed-loop recycled plastics have been reported, their properties (crystallinity, glass transition temperature, stress/strain, etc.) are often not characterized for commercial applications.
Recently, some researchers have proposed that monothiolactide monomers can be used as a new class of monomers to prepare closed-loop recyclable plastics with excellent performance. The ring-opening polymerization of monothiolactide monomer has high regioselectivity, which can effectively suppress the side reaction of thioesterification during the polymerization process, and realize controllable ring-opening polymerization. The resulting polymer can be efficiently and selectively recovered to the original monothiolactide monomer through a simple sublimation device. At the same time, the obtained recycled polymers exhibit mechanical properties comparable to those of petroleum-based low-density polyethylene (LDPE), especially toughness (>400% elongation at break).
Experimental and computational studies reveal key role of monothioxation strategy in highly selective polymerization, recycling, and performance. Reducing the ring strain endows the monothiolactide monomer with chemical recyclability; the regioselective ring opening of the monomer at the acyl-S bond site realize the polymerization of the main chain ester-thioester completely alternating structure.
Single-crystal X-ray diffraction and computational studies further reveal that the structure of alternating thioester and ester bonds enhances the n-π* interactions within the polymer molecule, the rigid ring structure of the monomer limits the n-π* interaction in the molecule, thereby suppressing the side reaction of thioesterification during the polymerization process without sacrificing the activity of the monomer and realize the controlled ring-opening polymerization of monothiolactide.
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Reference
- Tough while Recyclable Plastics Enabled by Monothiodilactone Monomers
J. Am. Chem. Soc., 2023
