Sulfonamide-Supported Aluminum Catalysts for the Ring-Opening Polymerization of <i>rac</i>-Lactide

The synthesis, structures, and ring-opening polymerization (ROP) capability of a wide range of sulfonamide-supported aluminum alkyl and alkoxide complexes are reported. The synthesis of the new protio-ligands PhCH2N(CH2CH2NHSO2R)2 (R = Tol (15, H2N2TsNPh) or Me (16, H2N2MsNPh)) is described. These and the previously reported 1,2-C6H10(NHSO2R)2 (R = Tol (11, H2CyN2Ts) or Mes (12, H2CyN2SO2Mes)) and RCH2N(CH2CH2NHSO2Tol)2 (R = MeOCH2 (13, H2N2TsNOMe) or 2-NC5H4 (14, H2N2TsNpy)) reacted with AlEt3 to form Al(CyN2Ts)Et(THF) (17), Al(CyN2SO2Mes)Et(THF) (18), and Al(N2TsNR)Et (R = Ph (19), OMe (20), or py (21)), respectively. Subsequent reaction of these ethyl complexes with R′OH (R′ = iPr or Bn) resulted in protonolysis of the sulfonamide supporting ligands to yield a mixture of products including Al(OR′)3. In contrast, reaction of Al(OR′)Et2 (R′ = iPr, Bn, CH2CH2NH2, or CH2CH2NMe2) with various protio-ligands formed the sulfonamide-supported alkoxides Al(N2TsNpy)(OR′) (R′ = iPr (22) or Bn (23)), Al(N2MsNPh)(OR′) (R′ = iPr (26) or Bn (27)), Al(N2TsNR)(OCH2CH2NH2) (R = Ph (29), OMe (30), or py (31)), Al(CyN2Ts)(OCH2CH2NMe2) (32), and Al(N2TsNPh)(OCH2CH2NMe2) (33). Unexpectedly, reaction of Al(OiPr)Et2 with H2N2TsNOMe led to O-demethylation of the sulfonamide ligand. Reaction of AlMe2Cl with H2N2TsNPh gave [Al(NTs2NPh)Cl]2 (28). X-ray diffraction studies revealed four- or five-coordinate Cs-symmetric structures for 17−21, a five-coordinate C2-symmetric sulfonamide-bridged dimer for 28, and a five-coordinate Cs-symmetric monomer for 30 stabilized by intramolecular hydrogen bonding between the sulfonyl oxygens and the amine protons. Compounds 19, 21, 22−27, and 29−33 are all catalysts for the ROP of rac-lactide, with the alkoxide compounds 22−27 and 32 giving well-defined molecular weights and molecular weight distributions. These compounds were also active in the melt at 130 °C, giving atactic poly(rac-lactide) with moderate to narrow PDIs and extremely good control of Mn and high activity in the case of 23

Sulfonamide-Supported Aluminum Catalysts for the Ring-Opening Polymerization of <i>rac</i>-Lactide

The synthesis, structures, and ring-opening polymerization (ROP) capability of a wide range of sulfonamide-supported aluminum alkyl and alkoxide complexes are reported. The synthesis of the new protio-ligands PhCH2N(CH2CH2NHSO2R)2 (R = Tol (15, H2N2TsNPh) or Me (16, H2N2MsNPh)) is described. These and the previously reported 1,2-C6H10(NHSO2R)2 (R = Tol (11, H2CyN2Ts) or Mes (12, H2CyN2SO2Mes)) and RCH2N(CH2CH2NHSO2Tol)2 (R = MeOCH2 (13, H2N2TsNOMe) or 2-NC5H4 (14, H2N2TsNpy)) reacted with AlEt3 to form Al(CyN2Ts)Et(THF) (17), Al(CyN2SO2Mes)Et(THF) (18), and Al(N2TsNR)Et (R = Ph (19), OMe (20), or py (21)), respectively. Subsequent reaction of these ethyl complexes with R′OH (R′ = iPr or Bn) resulted in protonolysis of the sulfonamide supporting ligands to yield a mixture of products including Al(OR′)3. In contrast, reaction of Al(OR′)Et2 (R′ = iPr, Bn, CH2CH2NH2, or CH2CH2NMe2) with various protio-ligands formed the sulfonamide-supported alkoxides Al(N2TsNpy)(OR′) (R′ = iPr (22) or Bn (23)), Al(N2MsNPh)(OR′) (R′ = iPr (26) or Bn (27)), Al(N2TsNR)(OCH2CH2NH2) (R = Ph (29), OMe (30), or py (31)), Al(CyN2Ts)(OCH2CH2NMe2) (32), and Al(N2TsNPh)(OCH2CH2NMe2) (33). Unexpectedly, reaction of Al(OiPr)Et2 with H2N2TsNOMe led to O-demethylation of the sulfonamide ligand. Reaction of AlMe2Cl with H2N2TsNPh gave [Al(NTs2NPh)Cl]2 (28). X-ray diffraction studies revealed four- or five-coordinate Cs-symmetric structures for 17−21, a five-coordinate C2-symmetric sulfonamide-bridged dimer for 28, and a five-coordinate Cs-symmetric monomer for 30 stabilized by intramolecular hydrogen bonding between the sulfonyl oxygens and the amine protons. Compounds 19, 21, 22−27, and 29−33 are all catalysts for the ROP of rac-lactide, with the alkoxide compounds 22−27 and 32 giving well-defined molecular weights and molecular weight distributions. These compounds were also active in the melt at 130 °C, giving atactic poly(rac-lactide) with moderate to narrow PDIs and extremely good control of Mn and high activity in the case of 23

Hybrids of Reduced Graphene Oxide and Hexagonal Boron Nitride: Lightweight Absorbers with Tunable and Highly Efficient Microwave Attenuation Properties

Sandwichlike hybrids of reduced graphene oxide (rGO) and hexagonal boron nitride (<i>h</i>-BN) were prepared via heat treatment of the self-assemblies of graphene oxide (GO) and ammonia borane (AB). TG-DSC-QMS analysis indicate a mutually promoted redox reaction between GO and AB; 900 °C is a proper temperature to transfer the hybrids into inorganic sandwiches. XRD, XPS, and Raman spectra reveal the existence of <i>h</i>-BN embedded into the rGO frameworks. High-resolution SEM and TEM indicate the layer-by-layer structure of the hybrids. The content of <i>h</i>-BN can be increased with increase of the mass ratio of AB and the highest heat treatment temperature. The complex permittivity and the microwave absorption are tunable with the variation of the content of <i>h</i>-BN. When the mass ratio of GO/AB is 1:1, the microwave absorption of the hybrid treated at 900 °C is preferable in the range of 6–18 GHz. A minimum reflection loss, −40.5 dB, was observed at 15.3 GHz for the wax composite filled with 25 wt % hybrids at the thickness of 1.6 mm. The qualified frequency bandwidth reaches 5 GHz at this thickness with a low surface density close to 1.68 kg/m². The layer-by-layer structure of the hybrid makes great contributions to the increased approaches and possibilities of electron migrating and hopping, which has both highly efficient dielectric loss and excellent impedance matching for microwave consumption

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors

Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300–530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion

Hierarchical Reduced Graphene Oxide Ridges for Stretchable, Wearable, and Washable Strain Sensors

Recently, flexible and wearable devices are increasingly in demand and graphene has been widely used due to its exceptional chemical, mechanical and electrical properties. Building complex buckling patterns of graphene is an essential strategy to increase its flexible and stretchable properties. Herein, a facile dimensionally controlled four-dimensional (4D) shrinking method was proposed to generate hierarchical reduced graphene oxide (rGO) buckling patterns on curved substrates mimicking different parts of the uniforms. The reduced graphene oxide ridges (rGORs) generated on the spherical substrate seem isotropic, while those generated on the cylindrical substrate are obviously more hierarchical or oriented, especially when the cylindrical substrate are shrinking via two steps. The oriented rGORs are superhydrophobic and strain sensitive but obviously anisotropic along the axial and circumferential directions. The sensitivity of rGORs along the axial direction is much higher than those along the circumferential direction. In addition, the intrinsic solvent barrier property of graphene enables the crack-free rGORs an excellent chemical protective performance, withstanding DCM immersion for more than 2.5 h. The flexible rGORs-based strain sensors can be used to detect both large and subtle human motions and activities by achieving high sensitivity (maximum gauge factor up to 48), high unidirectional stretchability (300–530%), and ultrahigh areal stretchability (up to 2690%). Excellent durability was also demonstrated for human motion monitoring with resistance to hand rubbing, ultrasonic cleaning, machine washing, and chemical immersion