Search

Polímeros de origen natural como el almidón han sido vistos por muchos autores como una alternativa atractiva y viable como sustitutos de algunos polímeros sintéticos, ya que ellos y sus derivados resultan ser poco nocivos para el medio ambiente y su uso podría reducir la contaminación producida por polímeros derivados del petróleo. Sin embargo, debido a su carácter hidrófilo sus propiedades se ven comprometidas con el grado de humedad del ambiente, lo cual constituye una limitación muy importante a la hora de desarrollar productos derivados del mismo. Es por esta razón que se han desarrollado diferentes métodos de modificación química con la finalidad de convertirlo en un material hidrófobo, lo que facilita no solo obtener productos resistentes a la humedad, sino también a la degradación hidrolítica. Esto, además, permite la obtención de materiales más fáciles de moldear y amigables con el medio ambiente y, por lo tanto, útiles para la fabricación de utensilios. En esta revisión se presentan muchos de los avances obtenidos en las diferentes vías de modificación química dirigidas hacia la síntesis de almidones hidrófobos, en especial la obtención de almidones eterificados y esterificados, haciendo énfasis especialmente en la influencia de los medios de síntesis y las condiciones de reacción.

Copolymers of racemic β-butyrolactone ((R,S)-BL) and e-caprolactone (CL), were synthesized by ring-opening polymerization initiated by sodium hydride (NaH). The initiator exhibited a satisfactory catalytic activity, producing copolymers whose yields are greatly influenced by the feed monomer ratio, CL/BL. All polymers obtained were characterized by nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and wide angle X-rays scattering, WAXS. The molar composition of copolyesters determined by 1H-NMR spectra, showed that the incorporation of CL is favoured over the incorporation of (R,S)-BL. Gel permeation chromatography and 13C-NMR spectra indicated that CL/BL copolymers had block sequence distribution. The TGA analysis of copolymers showed that these copolymers are stable up to temperatures near 200°C, followed by a decomposition process in two steps; the first one is attributed to the (R,S)-BL block degradation and the second to the remaining PCL block. The crystallization process of these copolymers was studied by DSC and WAXS showing that the amorphous (R,S)-BL segments chains did not affect the crystallinity of the PCL blocks.

In this work, the behavior of the crystallizable blocks within polystyrene- b -poly(ethylene oxide)- b -poly(e-caprolactone) linear triblock copolymers is studied. The apparent crystallization and melting temperatures of PEO and PCL can be coincident as a result of their close values, their dependence on the molecular weight of the blocks and on the relative amount of each component within the copolymers. When the content of both PEO and PCL is low, typically 20% or less, both blocks crystallized coincidentally upon cooling from the melt at very large supercoolings ( T c =−40 °C), suggesting that homogeneous nucleation has taken place. A S 63 EO 16 C 20 copolymer (where subscripts denote the weight fraction of the components) exhibited a small angle X-ray scattering (SAXS) pattern at room temperature that suggests a cylindrical morphology for the minor components, where the crystallization of PEO and PCL blocks can take place subsequently near −40 °C in a hard confinement fashion within the vitreous PS matrix. We have employed differential scanning calorimetry (DSC) to study the self-nucleation process and the results indicate that the difficulty in generating enough self-seeds to nucleate every isolated microdomain causes the disappearance of Domain II (or exclusive self-nucleation Domain ) and therefore a direct transition from complete melting ( Domain I ) to self-nucleation and annealing ( Domain III ) is observed. When the content of both PEO and PCL blocks is larger than 35%, a coincident crystallization can be observed as well as coincident melting. For a S 15 EO 37 C 48 triblock copolymer, PS is a minor phase and mixed spherulites composed of both PCL and PEO blocks were observed by Polarized Optical Microscope (POM) at appropriate supercoolings. In this case, the self-nucleation behavior is standard, and through its use the crystallization and melting process of both blocks can be separated. Wide angle X-ray scattering (WAXS) at different temperatures was used to corroborate the sequential melting of both semicrystalline blocks deduced by self-nucleation. Isothermal crystallizations were also followed by WAXS as a function of time and the results showed that depending on the crystallization temperature ( T c ), only the PCL block crystallizes (at high T c ), or the PCL block crystallizes first followed by the PEO at later times (at lower T c ).

Two poly(β-L-aspartate)s of general formula –[–HN–CH(COOR)–CH2–CO–]– containing ethylene oxide units in the side chain [R=–CH2CH2OCH3 (IVa), and –CH2–CH2OCH2CH2OCH3 (IVb)] have been prepared by anionic ring-opening polymerization of the corresponding optically pure β-lactams. In both cases, stereoregular polymers with molecular weights above 2×105 were obtained in satisfactory yields. These polyamides were fully characterized by combustion analysis, FTIR and 1H–13C NMR spectroscopy, and DSC. Whereas polymer IVa attained was completely free of imide units, side reactions giving rise to imidation could not be entirely repressed in the polymerization of the β-lactam leading to IVb. X-Rays from oriented samples revealed that polymer IVa crystalizes in a hexagonal structure made up of 4* 13/4 helices which bear great conformational resemblance with the well known α-helix in polypeptides. A similar conformation seems to be adopted by polymer IVb, although in this case, side chains tend to remain in a disordered state hindering the crystallization of the helices.

The (S)-4-alkoxo-2-azetidinecarboxylic acids are optically active β-lactam derivatives of aspartic acid, which are used as precursors of carbapenem-type antibiotics and poly-β-aspartates. The crystal structures of three (S)-4-alkoxo-2-azetidinecarboxylic acids with alkyl chains with 10, 12 and 16 C atoms were solved using parallel tempering and refined against the X-ray powder diffraction data using the Rietveld method. The azetidinone rings in the three compounds display a pattern of asymmetrical bond distances and an almost planar conformation; these characteristics are compared with periodic solid-state, gas-phase density-functional theory (DFT) calculations and MOGUL average bond distances and angles from the CSD. The compounds pack along [001] as corrugated sheets separated by approximately 4.40 Å and connected by hydrogen bonds of the type N—H...O.

Random and block copolymers containing styrene and mono n-alkyl itaconates with n = 12, 14, 16, 18 and 22 have been prepared by free radical polymerization and nitroxide-mediated radical polymerization. The molar fraction of the mono n-alkyl itaconate in block copolymers increased with increasing size of the alkyl group in the ester substituent. A calorimetric study of all these copolymers was performed. No glass transition (Tg) was observed in the temperature range - 30ºC-120ºC for random copolymers. Melting process attributed to crystalline order in the long side chains was observed for some copolymers depending on their composition and of the monoitaconate chain side length.

The crystal structures of the four-membered heterocycles (S)-(−)-4-oxo-2-azetidinecarboxylic acid (I) and 3-azetidinecarboxylic acid (II) were solved by direct methods using powder synchrotron X-ray diffraction data. The asymmetry of the oxoazetidine and azetidine rings is discussed, along with the hydrogen bonding.

The diphenylzinc-water system was used as catalyst for ethylene oxide polymerization in benzene solution at 60°. The system is greatly influenced by the molar ratio of water to diphenylzinc. H2O/Ph2Zn, the maximum catalyst activity being found for a ratio of unity. Ph2Zn alone and molar ratios of 0.25, 0.5, 1.5, 1.75 and 2.0 gave very low conversion to polymer. For a molar ratio of unity, the yield of polymer and the molecular weight increase with time. The reaction is first order with respect to monomer with kP = 5.7 × 10−5 sec−1 mol−1 l.