Synthesis of functional telechelic polydimethylsiloxanes by ion-exchangers catalysis

Slow Magnetic Relaxation in {[CoCxAPy)] 2.15 H2O}n MOF Built from Ladder-Structured 2D Layers with Dimeric SMM Rungs

We present the magnetic properties of the metal-organic framework {[CoCxAPy]·2.15 H2O}n (Cx = bis(carboxypropyl)tetramethyldisiloxane; APy = 4,4`-azopyridine) (1) that builds up from the stacking of 2D coordination polymers. The 2D-coordination polymer in the bc plane is formed by the adjacent bonding of [CoCxAPy] 1D two-leg ladders with Co dimer rungs, running parallel to the c-axis. The crystal packing of 2D layers shows the presence of infinite channels running along the c crystallographic axis, which accommodate the disordered solvate molecules. The Co(II) is six-coordinated in a distorted octahedral geometry, where the equatorial plane is occupied by four carboxylate oxygen atoms. Two nitrogen atoms from APy ligands are coordinated in apical positions. The single-ion magnetic anisotropy has been determined by low temperature EPR and magnetization measurements on an isostructural compound {[Zn0.8Co0.2CxAPy]·1.5 CH3OH}n (2). The results show that the Co(II) ion has orthorhombic anisotropy with the hard-axis direction in the C2V main axis, lying the easy axis in the distorted octahedron equatorial plane, as predicted by the ab initio calculations of the g-tensor. Magnetic and heat capacity properties at very low temperatures are rationalized within a S* = 1/2 magnetic dimer model with anisotropic antiferromagnetic interaction. The magnetic dimer exhibits slow relaxation of the magnetization (SMM) below 6 K in applied field, with a tlf ≈ 2 s direct process at low frequencies, and an Orbach process at higher frequencies with U/kB = 6.7 ± 0.5 K. This compound represents a singular SMM MOF built-up of Co-dimers with an anisotropic exchange interaction.

From Amorphous Silicones to Si-Containing Highly Ordered Polymers: Some Romanian Contributions in the Field

Polydimethylsiloxane (PDMS), in spite of its well-defined helical structure, is an amorphous fluid even at extremely high molecular weights. The cause of this behavior is the high flexibility of the siloxane backbone and the lack of intermolecular interactions attributed to the presence of methyl groups. These make PDMS incompatible with almost any organic or inorganic component leading to phase separation in siloxane-siloxane copolymers containing blocks with polar organic groups and in siloxane-organic copolymers, where dimethylsiloxane segments co-exist with organic ones. Self-assembly at the micro- or nanometric scale is common in certain mixed structures, including micelles, vesicles, et cetera, manifesting reversibly in response to an external stimulus. Polymers with a very high degree of ordering in the form of high-quality crystals were obtained when siloxane/silane segments co-exist with coordinated metal blocks in the polymer chain. While in the case of coordination of secondary building units (SBUs) with siloxane ligands 1D chains are formed; when coordination is achieved in the presence of a mixture of ligands, siloxane and organic, 2D structures are formed in most cases. The Romanian research group's results regarding these aspects are reviewed: from the synthesis of classic, amorphous silicone products, to their adaptation for use in emerging fields and to new self-assembled or highly ordered structures with properties that create perspectives for the use of silicones in hitherto unexpected areas.

Synthesis of Polysiloxaneimide with Piezoelectric Response

A new segmented polysiloxaneimide was synthesized by solution polycondensation of N, N bis(hydroxyethyl)benzophenontetracarboxylic diimide with siloxane diacid chloride. The copolymer was obtained with good yield and high molecular weight. The characterization by IR and 1H-NMR spectrometry and a study of the properties such as solubility and thermal behaviour of this copolymer are presented. A corona poling study was performed to obtain information about the piezoelectric behaviour of polysiloxaneimide. The remanent dielectric constants of the polarized samples were compared with those of well-known piezoelectric polymers.

Poly(Azomethine-Ester-Si1oxane)s: Synthesis and Thermal Behaviour

New siloxane-containing poly(azomethine-ester)s were synthesized by polycondensation reactions in solution, starting from three different azomethine mesogenic bisphenols: 4, 4'-oxo-bis(4-hydroxybenzylideneaniline), 1, 4-bis(4-hydroxybenzylideneamino)-phenylene, and 4, 4'-methylene-bis(4-hydroxybenzylideneaniline), and bis(carboxypropyl)siloxanes of various molecular weights. The reactions were performed at room temperature, activated by dicyclohexylcarbodiimide and catalysed by 4-(dimethylamino)pyridinium 4-toluenesulphonate, which is a novel synthesis method for siloxane polyesters. The polymers obtained were characterized by IR, 1 H-NMR, and UV spectroscopy, as well as by gel permeation chromatography and thermogravimetric analysis. Poly(azomethine-ester)s have good solubility in organic solvents and thermal oxidative stability. Six of the polymers obtained exhibited thermotropic liquid crystalline behaviour, evidenced by polarizing microscopy, differential scanning calorimetry, thermo-optical analysis, and wide-angle x-ray diffraction measurements. The melting and isotropization temperatures decrease with increase of the length of the siloxane spacer.

New Arylidene—Siloxane Polyesters

New siloxane-containing polyesters with arylidene mesogenic moieties were prepared by copolycondensation reactions between bis(hydroxybuthyl)siloxanes, 2,6-bis(4-hydroxybenzylidene)cyclohexanone, and diacid chlorides such as sebacoyl and terephthaloyl. The polymer structure was confirmed by IR and 1H-NMR spectroscopy. They were characterized by gel permeation chromatography and ultraviolet analyses. The thermotropic liquid crystalline behavior was investigated by differential scanning calorimetry and polarized optical microscopy.

Influence of Chemical Structure on Processing and Thermotropic Properties of Poly(siloxane-azomethine)s

A disiloxane dialdehyde was obtained from bis (chloromethyl)disiloxane and p-hydroxybenzal-dehyde and it was used in an equilibration (redistribution) reaction to synthesize an oligomeric dialdehyde. By solution polycondensation, starting from bis(formyl-p-phenoxymethyl)tetramethyldisiloxane and different organic diamines, poly(azomethine)s were obtained. In the case of aromatic diamines with high rigidity, which gave insoluble polymers, different approaches were tested in order to improve the solubility: the use of the dialdehyde having longer siloxane chain, or of a siloxane diamine in a copoly-condensation reaction. The polymers’ structures were confirmed by IR and 1H NMR spectroscopy and by elemental analysis. The structure-properties relationship was studied in terms of solubility, thermal and thermotropic behavior. Most of the obtained poly(azomethine)s has mesomorphic properties, which were studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction. The influence of aromatic diamines’ chemical structure on the processability of the siloxane-organic poly(azomethine)s was emphasized.