Thermotropic aromatic copolyesters of catechol

Three new series of thermotropic aromatic main-chain polymers were synthesized and studied by differential scanning calorimetry and hot-stage polarized microscopy. The polymers were random copolyesters of p-hydroxybenzoic acid (from 60 to 10 mol %), catechol (from 20 to 45 mol %), and one of the following dicarboxylic diacids: terephthalic acid (series 1 ), 2,6-naphtalenedicarboxylic acid (series 2 ), and 1,2-bis-p-carboxyphenoxy ethane (series 3 ). Copolyesters with more than 25 mol % catechol in their structures were soluble in common organic solvents such as chloroform. All the synthesized copolyesters showed hysteresis of the heat capacity at the mesophase glass transition region and nematic mesophases above their T_g's which were stable over very broad ranges of temperature. Copolyesters containing the 2,6-naphtylenedioyl group showed the most stable nematic phases due to the increased anisometry of the 2,6-naphtylenedioyl unit compared to that of the therephthaloyl or the 4-oxybenzoyl units. © 1992 John Wiley & Sons, Inc.     

Thermotropic copolyesters of 4-hydroxybenzoic acid

Copolyesters of 4-hydroxybenzoic acid were prepared by thermal polycondensation of 4-acetoxybenzoic acid with various acetylated comonomers, such as 4-mercaptobenzoic acid, 3-hydroxybenzoic acid, N-(4-carboxyphenyl)4-hydroxyphthalimide or hydroquinone in combination with 1,12-dodecane bistrimellitimide. The copolyesters were characterized by elemental analyses, inherent viscosities, ¹H- or ¹³C NMR spectroscopy, DSC-measurements, WAXS-measurements at temperatures between 25 and 425°C, microscopic observation, thermomechanical and thermogravimetrical analyses. Copolyesters containing 4-mercaptobenzoic acid or N-(4-carboxyphenyl)4-hydroxyphthalimide possess a reversible first order phase transition which represent a change of modifications and not a melting process. Both classes of copolyesters adopt the same kind of high temperature modification as poly(4-hydroxybenzoate), namely a pseudo-hexagonal chain packing. In contrast, copolyesters of 3-hydroxybenzoic acid or copoly(ester imide)s with aliphatic spacer containing more than 50% 4-hydroxybenzoic acid form a nematic phase over a broad temperature range. However, in the case of copolyesters derived from 3- and 4-hydroxybenzoic acid the homogeneous nematic melt is thermodynamically unstable and gradually turns into a heterogeneous more or less crystalline material with blocks of 4-hydroxybenzoate units.     

Biomimetic selectivity

Synthetic organic chemistry normally achieves selectivity by manipulation of the intrinsic reactivity of the substrate, but enzyme use is quite a different principle. The geometry of the enzyme-substrate complex determines enzymatic selectivity, completely overwhelming any normal selective reactivities. Biomimetic chemistry aims to imitate the enzymatic style. Some early approaches used attached reagents or templates to direct photochemical and free radical processes, with a combination of geometric and reactivity control. Recent work uses a mimic of the enzyme class cytochrome P-450 to achieve the selective hydroxylations of steroids with complete domination by the geometry of the catalyst-substrate complex.     

Sequentially ordered,thermotropic aromatic copolyesters: Synthesis and comparison of their properties with random copolyesters

Synthetic methods of aromatic copolyesters having ordered comonomer sequence are described. In order to obtain a regular sequence copolyester, one first has to prepare a dimeric or trimeric compound with a predesigned sequence via a multistep route and then polycondense it with another difunctional compound. Low reaction temperature is required to avoid the occurrence of undesired interchange reactions that would lead to randomization of the comonomer sequence. Sequentially ordered copolyesters exhibit significant differences in thermal transitions, crystalline properties, and occasionally even in liquid crystallinity when compared with the properties of the corresponding random copolyesters. According to the preliminary results randomization in comonomer sequence of sequentially regular aromatic copolyesters above their melting points is rather rapid.     

Axial correlation lengths for aromatic copolyesters

X‐ray methods were used to investigate the structure of Vectra® and Xydar® polyesters. Xydar® is based on the aromatic polyester prepared from p‐hydroxybenzoic acid (HBA), terephthalic acid, and biphenol. Vectra® consists of HBA and 2‐hydroxy‐6‐naphthoic acid. X‐ray fiber diagrams of these materials consist of a series of nonperiodic layer lines, which are consistent with a structure consisting of parallel chains of a random comonomer sequence. The meridional peak positions are predicted accurately for fully extended, infinite chains, and the observed and calculated intensities are also in reasonably good agreement. However, there is a less adequate match between the observed and calculated peak profiles, which are predicted to be much sharper than those observed. The latter agreement is improved with a model consisting of finite chain segments with a slightly nonlinear (sinuous) conformation. The best agreement for the observed peak profiles is obtained with an ordered segment length of 90 ± 6 Å and a sinuosity of g = 3.67 ± 0.08% for Vectra® fibers and with an ordered segment length of 104 ± 6 Å and a sinuosity of g = 3.33 ±0.08% for Xydar® fibers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1839–1847, 2001     

Thermal and pyrolysis studies of copolyesters

Random copolyesters of dimethyl terephthalate (DMT), ethylene glycol (EG), and butane-1,4-diol (BD) and the homopolyesters poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) have been subjected to degradation and pyrolysis studies. Differential thermal analysis (DTA) showed that the decomposition temperature is dependent on the percentage of EG and BD present in the copolyesters. Thermal volatilization analysis (TVA) also showed that the decomposition temperature is dependent on the percentage of EG and BD present in the copolyesters. The trend for the decomposition temperatures obtained from TVA studies for these copolyesters is similar to such other thermal properties as melting temperature T_m, ΔH_f, ΔH_c, etc. The subambient thermal volatilization analysis (SATVA) curves obtained for these polymers are also presented. The SATVA curve is the fingerprint of the total volatile products formed during the degradation in high vacuum. The isothermal pyrolysis of these materials was carried out in high vacuum at 450°C. The products formed were separated in a gas chromatograph and were subsequently identified in a mass spectrometer. The major pyrolysis products from PBT were butadiene and tetrahydrofuran, whereas those from PET were ethylene and acetaldehyde. The ratio of acetaldehyde to ethylene increases with the EG content in the copolyester, suggesting a different decomposition mechanism compared to the decomposition mechanism of PBT and PET.     

Segmented block copolyesters using click chemistry

Copper(I) catalyzed azide‐alkyne 1,3‐Huisgen cycloaddition reaction afforded the synthesis of triazole‐containing polyesters and segmented block copolyesters at moderate temperatures. Triazole‐containing homopolyesters exhibited significantly increased (～40 °C) glass transition temperatures (T_g) relative to high temperature, melt synthesis of polyesters with analogous structures. Quantitative synthesis of azido‐terminated poly(propylene glycol) (PPG) allowed for the preparation of segmented polyesters, which exhibited increased solubility and mechanical ductility relative to triazole‐containing homopolyesters. Differential scanning calorimetry demonstrated a soft segment (SS) T_g near ?60 °C for the segmented polyesters, consistent with microphase separation. Tensile testing revealed Young's moduli ranging from 7 to 133 MPa as a function of hard segment (HS) content, and stress at break values approached 10 MPa for 50 wt % HS segmented click polyesters. Dynamic mechanical analysis demonstrated an increased rubbery plateau modulus with increased HS content, and the T_g's of both the SS and HS did not vary with composition, confirming microphase separation. Atomic force microscopy also indicated microphase separated and semicrystalline morphologies for the segmented click polyesters. This is the first report detailing the preparation of segmented copolyesters using click chemistry for the formation of ductile membranes with excellent thermomechanical response. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Biomimetic asymmetric catalysis

Enzymes are the core for biological transformations in nature. Their structures and functions have drawn enormous attention from biologists as well as chemists since last century. The large demand of bioactive molecules and the pursuit of efficiency and greenness of synthesis have spurred the rapid development of biomimetic chemistry in the past several decades. Biomimetic asymmetric catalysis, mimicking the structures and functions of enzymes, has been recognized as one of the most promising sy...     

Biomimetic Polyene Cyclizations

Certain polyenic substances having trans olefinic bonds in a 1,5 relationship can be induced to undergo stereospecific, non-enzymic, cationic cyclization to give polycyclic products with all-trans (“natural”) configuration. These transformations appear to mimic in principle the biogenetic conversion of squalene into polycyclic triterpenoids. Acetal and allylic alcohol functions are effective initiators for these cyclizations, and methylacetylenic end groups are particularly useful terminators since they lead to five-membered rings. Thus a tetraenic acetal having no chiral centers has been converted in a single step into a tetracyclic product having seven such centers. The process is highly stereoselective, giving only two of 64 possible racemates. Systems have also been developed for effecting the total synthesis of the steroid nucleus in a single step starting from a substrate containing only one ring. The mechanism of these biomimetic cyclizations and also that of their enzymic counterparts is still unknown, but the balance of the evidence favors a synchronous process.     

Thermotropic copolyesters containing the 1,4-cyclohexylenedimethylene spacer

Poly[oxy(2-methyl-1,4-phenylene)oxyterephthaloyl-co-oxy(2-methyl-1,4-phenylene)oxy-1,4-cyclohexanediacetoyl] (I), poly[oxy(2-chloro-1,4-phenylene)oxyterephthaloyl-co-oxymethylene-1,4-cyclohexylenemethyleneoxyterephthaloyl] ( II ), and poly[oxy(2-methyl-1,4-phenylene)oxyterephthaloyl-co-oxymethylene-1,4-cyclohexylenemethyleneoxyterephthaloyl] ( III ) were synthesized and shown to form birefringent fluid states in the melt.     

Synthesis and characterization of functional aliphatic copolyesters

Functional aliphatic copolyesters of succinic acid (SA) and citric acid (CA) were synthesized via direct copolycondensation in the presence of 1,4‐butanediol, with titanium(IV) butoxide as a catalyst. The effects of the comonomer and comonomer ratio on the polycondensation and the melting and glass‐transition temperatures were investigated. The melting temperature was very sensitive to the molar ratio of the SA–CA comonomer units. The chain extension of this poly(butylene succinate citrate) was carried out with hexamethylene diisocyanate. The intrinsic viscosity, crystallinity percentage, and rheological properties of these copolyesters were also studied. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3232–3239, 2002     

Copolyamides and copolyesters containing the phenoxasilin ring

Copolyamides and copolyesters containing the phenoxasilin ring were prepared from 2,8-dichloroformyl-10,10-diphenylphenoxasilin, isophthaloyl chloride and m-phenylenediamine or bisphenol A by interfacial polycondensation in chloroform-aqueous alkali mixture. They were obtained in yields of 80% or above and at relatively high viscosities up to 1.30 dl/g. The copolymers with high phenoxasilin content were freely soluble in dimethylacetamide, dimethylformamide and N-methyl-2-pyrrolidone, but decreasing phenoxasilin content led to copolymers with slight solubilities in these solvents; the copolyesters also dissolved in chloroform, m-cresol and phenol-sym tetrachloroethane (60:40 in wt%). Flexible transparent films were obtained from chloroform solutions of the copolyesters, but the films cast from DMF solutions of the copolyamides became brittle as the phenoxasilin content decreased. The phenoxasilin-containing copolymers hardly degraded below 400° and had good thermal stability. Introduction of the phenoxasilin ring into the polymer backbones by copolycondensation did not reduce thermal stability.     

Aromatic-aliphatic copolyesters—II. Various polycondensation processes

Aromatic aliphatic copolyesters, using hydroquinone, resorcinol, 4,4′-dihydroxybiphenyl (DHBP) 2,2 bis(4-hydroxyphenyl)propane and 4,4′-dihydroxydiphenyl sulphone (DHDPS) as bisphenols and ethylene glycol as diol, have been synthesized by interfacial, low temperature and high temperature solution condensation. Relative reactivities of these bisphenols and ethylene glycol have been evaluated by various polycondensation methods at a fixed ratio of bisphenol/glycol. Decrease in the extent of polymerization and viscosity was observed by incorporation of aliphatic diol. Viscosity was also influenced by the chemical structure of the bisphenol.     

Synthesis and eutectic behavior of liquid crystalline copolyesters

A series of liquid crystalline copolyesters, derived from 1,4‐hydroxy‐benzoic acid (HBA), 6‐hydroxy‐2‐naphthoic acid (HNA), terephthalic acid (TA), and hydroquinone (HQ), were prepared; crystallization, melting and solid‐state structure of the copolyesters were studied by using differential scanning calorimetry (DSC) and wide‐angle x‐ray diffraction (WAXD). It was found that the variation of melting point of the copolyesters with increasing HBA mol % exhibits eutectic melting behavior at a constant mole ratio of HNA, and the extrapolated eutectic temperature decreases linearly with increasing HNA mol %. WAXD analysis of the copolyesters indicates that the d‐spacing related to three‐dimensional order increases first and then decreases with increasing HBA mol %. The increase of the d‐spacing, consistent with looser packing of chains, leads to the reduction of melting point and most likely accounts for the eutectic behavior observed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2171–2177, 2009     

Synthesis of copolyesters via interchange reactions with polypivalolactone

The synthesis of copolyesters via interchange reactions of polypivalolactone (PPVL) with bisphenol-A polycarbonate (PC) and dimethyl terephthalate (DMT) was studied. Random copolyesters with high thermal stability were obtained by heating PPVL with PC and DMT in the melt at 280 °C in the presence of tetrabutylorthotitanate (TnBT) as a catalyst. Results from SEC measurements showed that the values for M_n of these copolyesters ranged from 7,000 to 16,000. The process for preparing these copolyesters was studied in detail and based on the results of these studies, the reactions occurring during copolyester synthesis were discussed. It was found that in the early stages of the process mainly interchange reactions occurred between PPVL and PC chains, whereas the amount of terephthalate units increased only after longer reaction times.     

水凝胶仿生柔性电子学

水凝胶力学性质与生物组织相似,生物相容性好,在生物电子学领域具有独特的优势.受生物组织——如皮肤、神经、肌肉等启发,发展了具有仿生结构和功能的水凝胶材料.以这种水凝胶材料制作而成的柔性电子器件具有感知温度、压力、应变、电场等外界刺激的功能,可模拟生物组织的传感能力,在仿生电子皮肤,人工肌肉,人工神经等领域具有重要的应用...