Xiaohui Kang

Sequence-controlled polymers have attracted interest due to their structural precision, which enables for functions reminiscent of natural macromolecules. Switchable polymerization offers a powerful platform for one-pot block copolymer synthesis, but maintaining orthogonal chemoselectivity becomes challenging when monomer mixtures involve congeners with similar reactivities. Even when such selectivity is established, additional comonomers can generate active species that perturb the original chemoselectivity. Here, we report a switchable polymerization strategy that enables the precise one-pot synthesis of polyacetal-block-polyester copolymers from a four-component mixture of epoxides, o-phthalaldehyde (OPA), and nadic anhydride (NA). Using an organoboron catalyst, the system exhibits strict chemoselectivity: at room temperature, only the terminal epoxide (PO) and OPA copolymerize to form the polyacetal block, while the internal epoxide (CHO) and NA remain inert; upon heating, the system switches to CHO/NA copolymerization to construct the polyester block. Comprehensive characterization confirmed that the bulky anhydrides NA, OA, and MeNA yielded tapering-free block architectures, while the less sterically hindered anhydrides gave only minor tapering, and mechanistic studies reveal that the orthogonal selectivity arises from distinct kinetic preferences of the resting state at each stage. The method accommodates various epoxides and anhydrides, providing a versatile platform for precision synthesis of sequence-controlled block copolymers.

Iron complexes are renowned for promoting vinyl-selective polymerization of diene monomers, whereas systems capable of delivering high cis-1,4-selectivity remain relatively scarce. Herein, a series of α-diimine FeCl2 complexes featuring axial Fe...H–C anagostic interactions were synthesized and evaluated for 1,3-butadiene polymerization. Upon activation with AlR3/[Ph3C][B(C6F5)4], these complexes produce polybutadienes with much-enhanced cis-1,4-selectivity of up to 86%. Notably, the cis-1,4-selectivity remains largely insensitive to steric variations at distal ligand positions across the examined series (H vs tBu, cis-1,4-content of 81–84%), showing clear contrast to conventional α-diimine iron systems (cis-1,4-content <50%).

Efficient organic Meerwein-type ion pairs enable controlled CROP of cyclic esters for metal-free polyesters.

Addressing the challenge of efficient lignin depolymerization, this study focuses on environmentally friendly choline amino acid ionic liquids ([Ch][AA] ILs) and systematically investigates the microscopic mechanism of β-O-4 bond cleavage in a lignin model compound (GG) using density functional theory (DFT). This study systematically compares three potential pathways: ionic liquid acid catalysis, base catalysis, and stepwise acid-base synergistic catalysis, proposing for the first time a highly advantageous "stepwise acid-base synergistic catalytic mechanism". The results demonstrate that compared to single base and acid catalysis, which exhibit excessively high energy barriers, the synergistic pathway effectively lowers the energy barrier of the rate-determining step through the tautomerism of key intermediates. The results indicate that the synergistic pathway exhibits a relative energetic advantage. Whereas, accounting for computational accuracy, the acid-catalyzed pathway may be considered as a competing reaction pathway under experimental conditions. Furthermore, the study reveals the impact of anion structure on catalytic activity: on the one hand, polar groups on the anion side chain can construct a strong hydrogen bond network and ion-dipole interactions, significantly reducing the rate-determining step energy barrier; on the other hand, long alkyl side chains effectively stabilize key transition states by enhancing noncovalent interaction. This study not only elucidates the key roles of multisite synergistic effects and van der Waals interactions in lignin depolymerization at the molecular level but also provides theoretical support for the design of efficient, low-toxicity catalytic systems for lignin valorization.

Aliphatic polyesters, widely used in biomedicine due to their biocompatibility and biodegradability, are typically synthesized via the ring-opening polymerization (ROP) of cyclic esters. Although traditional metal catalysts are highly active, their biological toxicity limits their applications. Organocatalysts, particularly natural organic molecules, offer safer alternatives. We explored the ROP mechanisms of cyclic esters (L-Lactide (L-LA), ε-caprolactone (ε-CL), and δ-valerolactone (δ-VL)) catalyzed by phosphonium carboxybetaines (PCBs, (PhR)₃P⁺(CH₂)₂COO⁻, R = H(PCB), F(PCB-F) and OMe(PCB-OMe)) through density functional theory (DFT) computations. The DFT results revealed that the ROP of cyclic esters follows a bifunctional–cooperative activation mechanism, wherein the phosphonium moiety (Ph₃P⁺(CH₂)₂) activates the monomer via an extensive hydrogen-bonding interaction network, and the carboxylate (COO⁻) serves as a proton acceptor to enhance the nucleophilicity of the initiator phenylpropanol (PPA). In contrast, unsubstituted PCB exhibited the lowest energy barrier, being consistent with the highest catalytic activity among PCB derivatives observed experimentally. Moreover, a series of novel PCB derivatives (Ph₃P⁺(CH₂)_nCOO⁻, n = 3–6 (PCB1-PCB4)) were designed by regulating the carbon spacer length, and their catalytic performances were computationally tested. The designed catalyst PCB2 (Ph₃P⁺(CH₂)₄COO⁻) exhibited higher activity for the ROP of L-LA, attributed to providing sufficient flexibility to minimize deformation while improving proton-accepting capability. Similarly, PCB2 also demonstrated superior catalytic activity for δ-VL and the more challenging ε-CL monomer. This work not only clarifies the intrinsic catalytic nature of these zwitterionic organocatalysts, but also provides an effective strategy for the rational design of high-performance, metal-free catalysts for the synthesis of sustainable polyesters.

DFT reveals a favorable Sn-coordinated ring-opening pathway for EOx, while bulk ROP affords high-molar-mass PEOx with a regular chain microstructure, rapid crystallization, and strong melt viscoelasticity.

Neutral inverse-sandwich lanthanum arene complexes with the parent benzene tetraanion are still limited. Here, we report the neutral inverse-sandwich lanthanum benzene complexes [(Cp^Ar5)La(THF)_n]₂(μ-η⁶:η⁶-C₆H₆) (Cp^Ar5 = η⁵-C₅Ar₅, Ar = ⁱPr₂-C₆H₃-3,5; n = 0, 2; n = 1, 2-THF) supported by a superbulky penta-arylcyclopentadienyl ligand. Complex 2 was isolated from the reduction of the half-sandwich lanthanum diiodide precursor [(Cp^Ar5)LaI₂(THF)₂] (1) in benzene by the K/KI reductant, yielding 69%. The reaction of complex 2 with 0.5 equiv of (HBBN)₂ enabled C-H bond functionalization of the benzene tetraanion, generating a novel borylated product [(Cp^Ar5)La]₂(μ-η⁶:η⁶-C₆H₅BBN) (3). NMR analyses, single-crystal X-ray diffraction, and UV-vis spectroscopic studies demonstrated that complexes 2, 2-THF, and 3 share a [La³⁺-(arene)^4--La³⁺] electronic structure, which was further confirmed by density functional theory (DFT) calculations. Moreover, treatment of complex 2 with Me₃SiN₃ afforded a product [(Cp^Ar5)La{N(SiMe₃)₂}]₂(μ-N₃)₂ (4) via four-electron reduction and subsequent Si-N bond activation. Additionally, the dimeric peroxo complex [(Cp^Ar5)La(THF)]₂(μ-η²:η²-O₂)₂ (5) was detected in the reaction of 2 with O₂. The redox reactivity of 2 shows its great potential in the multielectron reduction of unsaturated substrates, functioning as a La(I) synthon.

Synthesis of dipyrrolyl acetonitriles and benzhydryl pyrroles by multi-functionalization of alkenes with sulfonyl pyrroles was developed under visible-light irradiation.