Benzo-Fused Monomer Design toward Semiaromatic Polymers for a Circular Plastic Economy

ConspectusPlastic production has grown substantially over the past several decades, leading to massive consumption of nonrenewable fossil resources and the accumulation of plastic wastes, which have caused severe environmental problems. Consequently, the development of next-generation sustainable polymer materials is in high demand. The discovery of chemically recyclable polymers that can be efficiently transformed back into their pristine monomers by virtue of the reversibility of ring-opening polymerization (ROP) led to a major paradigm shift in redesigning sustainable polymers with an ideal circular polymer economy. Recent advancements in monomer design have demonstrated that various polymer systems including polyesters, polyacetals, polycarbonates, and others were feasible for closed-loop chemical recycling via ROP and ring-closing depolymerization (RCD). Our group has focused on the development of chemically recyclable semiaromatic polymers by exploiting the benzo-fusion strategy. The approach demonstrated here can be leveraged to promote the depolymerization of the corresponding polymers and to tune their material properties. Across several systems, we have observed a significant improvement in chemical recyclability because of the benzo-fusion and dramatic differences in material properties among polymers with different microstructures.To gain a better understanding of the structure-polymerization thermodynamics relationships, we developed a facile synthetic strategy to efficiently construct aromatic cyclic esters with stereodefined and diverse functionalities by using salicylic acid and its derivatives as aromatic building blocks. Continuingly, we targeted a biaryl-fused cyclic ester to access chemically recyclable polymers with axial chirality. Considering that the conjugation between aromatic rings and carbonyl groups could diminish the polymerization reactivity, we took inspiration from previous work and designed a new class of aliphatic-aromatic BPO monomers via a "nonadjacent ester" strategy. Gratifyingly, this system illustrated an impressive boost in polymerization reactivity, affording fully chemically recyclable polyesters. With these preliminarily remarkable findings, we developed a stereo- and sequence-controlled polymerization of BPO-based monomers with two stereogenic centers to furnish an isoenriched block polymer. The establishment of stereo- and sequence-controlled polymerization not only provides an effective and robust strategy to tailor the polymer property on the molecular level but also delivers various chemically recyclable materials capable of converting back to a single monomer. To further expand our strategy, we prepared a class of benzo-fused caprolactams bearing various substituents. This system having a "nonadjacent amide" group inherited the high reactivity of aliphatic lactams toward ROP, furnishing a series of semiaromatic polyamides with improved water resistance, transparency, and chemical recyclability. Semiaromatic polymers are a key class of engineering plastics renowned for their outstanding thermal and mechanical properties, making them ideal for high-performance applications. Collectively, this robust benzo-fusion strategy undoubtedly initiates the next wave of innovations toward the development of high-performance chemically recyclable semiaromatic polymers.