A highly isotactic stereoblock PLA has been obtained using the achiral heteroscorpionate zwitterionic zinc complexes as catalysts, for the first time, via a chain-end control mechanism.
Despite many merits, biobased and biodegradable poly(lactic acid) (PLA) suffers from inherently high flammability, significantly impeding its practical applications in the areas of packaging, fibers, electric and electronics, etc. Phosphoramidesbased flame retardants have demonstrated exceptional flame retardancy in PLA only at very low loading levels. Unfortunately, exiting synthetic processes of phosphoramides often involve the use of a huge amount of organic solvents, presenting a potential threat to the ecosystem and human beings, in addition to high production costs. Herein, we report a one-pot, solvent-and catalyst-free, and green approach for the synthesis of a polyphosphoramide (DM-H) via two-step polycondensation between dimethyl methylphosphonate (DMMP) and 1,6-hexanediamine (HDA). As-synthesized DM-H exhibits exceptional flame-retardant efficiency in the PLA while preserving the mechanical properties of the PLA. The addition of only 2 wt % DM-H enables the PLA to achieve a desirable V-0 rating during vertical burning tests and a high limited oxygen index (LOI) value of 29.7%, because the DM-H can act both in the gas and condensed phases. Moreover, the tensile strength is of 59.7 MPa comparable to 62.1 MPa of the bulk PLA. This work offers an innovative and green approach for the synthesis of polyphosphoramides to create high-performance sustainable PLA materials with balanced flame retardancy and mechanical performances.
Heterocyclic-fused cyclopentadienyl
scandium bis(alkyl) complexes L
1–4
Sc(CH2SiMe3)2THF ((5-Me-1-Ph-cyclopenta[b]pyrrol-4-yl)Sc(CH2SiMe3)2THF (1), (2,5-Me2-3-Ph-6H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF (2), (2,4,5,6-Me4-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF (3), (2,3,4,5,6-Me5-4H-cyclopenta[b]thiophenyl)Sc(CH2SiMe3)2THF) (4)) were facilely
synthesized by alkane elimination reaction of Sc(CH2SiMe3)3(THF)2 with the heterocyclic-fused
cyclopentadienyl ligands HL
1–4
in high yields. Complexes 1–4 were characterized by 1H and 13C NMR
spectroscopies and X-ray diffraction analyses as THF-solvated monomers,
adopting a half-sandwich geometry. Upon activation of [Ph3C][B(C6F5)4]/Al
i
Bu3, these half-sandwich scandium complexes displayed
various activities toward the copolymerization of ethylene (E) and
dicyclopentadiene (DCPD). Complex 1, supported by the
phenyl-substituted pyrrole-fused cyclopentadienyl ligand, showed a
slightly higher activity than the phenyl-substituted thiophene-fused
cyclopentadienyl complex 2. Among the thiophene-fused
cyclopentadienyl complexes 2–4, 4, bearing pentamethyl substituents, showed the highest activity
of 2.9 × 106 g/molSc·h·bar. The
resultant copolymers had adjustable DCPD incorporation varying from
14.0 up to 46.1 mol %, of which the alternating poly(E-alt-DCPD) had a high T
g of 166 °C.
In addition, no cross-linking was observed in the copolymers, suggesting
that these catalytic systems were highly regioselective for the two
active double bonds within DCPD.
The scandium bis(alkyl) complex bearing the phosphazene ligand L 1 Sc(CH 2 SiMe 3 ) 2 (1) (L 1 = N(PPh 2 NPh) 2 ) reacted with an equimolar amount of 2,6-diisopropylaniline to afford the corresponding mixed alkyl/anilido complex L 1 Sc[NHC 6 H 3 ( i Pr) 2 ](CH 2 SiMe 3 ) (2). Under mild conditions (20 °C, 4 h or 0 °C, 12 h), complex 2 could be swiftly transformed to the terminal imido complex L 1 Sc N[C 6 H 3 ( i Pr) 2 ](DMAP) 2 (4) in the presence of DMAP (DMAP = 4-N,N-dimethylaminopyridine). Correspondingly, treatment of the yttrium and lutetium bis(alkyl) complexes L 2 Ln(CH 2 SiMe 3 ) 2 (L 2 = N[Ph 2 PNC 6 H 3 ( i Pr) 2 ] 2 ; Ln = Y (7), Lu ( 8)) with equimolar amounts of 2,6-diisopropylaniline gave the mixed alkyl/anilido complexes L 2 Ln[NHC 6 H 3 ( i Pr) 2 ](CH 2 SiMe 3 ) (Ln = Y (9), Lu ( 10)), which, however, underwent dealkylation of the Ln−CH 2 SiMe 3 species at temperatures of 60 °C for 9 and 100 °C for 10 to afford bis(anilido) complexes L 2 Ln[NHC 6 H 3 ( i Pr) 2 ] 2 (Ln = Y (11), Lu ( 12)) as redistribution products. All these complexes have been characterized by 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy and X-ray diffraction analyses, and clear structural insight into the behavior of an imido functionality on a lanthanide metal center was provided.
A CpPN-based scandium terminal imido complex was isolated, which could induce the intramolecular C-H bond activation of a phenyl group even at room temperature.
We disclosed a novel water-soluble photocatalyst that could promote aerobic oxidative hydroxylation of arylboronic acids to furnish phenols in excellent yields. This transformation uses visible-light irradiation under environmentally friendly conditions, that is, water-soluble catalyst, metal-free, green oxidant, room temperature.
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