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.

Background: The synthesis of the aliphatic polyesters obtained by the ring opening polymerization has been achieved using as initiators a large amount of organometallic compounds derivative from: Alkali metals, alkaline earth metals, transition metals and lanthanide metals. Of all these compounds, the lanthanide derivatives have acquired great importance in the synthesis of aliphatic polyesters, since these show a greater catalytic activity and also can provide polymer with characteristics that will be very useful in the design of biomaterials. Objective: It was proposed the synthesis of poly(L-lactida) (PL-LA) through a ring opening polymerization process of L-lactide initiated with samarium(III) acetate (Sm(OAc)3) under solvent-free melt conditions. The influence of different parameters of reaction, such as temperature, time, molar ratio monomer to initiator, on typical variables of polymers, e.g., conversion, dispersity, and molar mass, were analyzed. Methods: All polymerizations were carried out under solvent-free melt conditions in ampoules-like flasks, equipped with a magnetic stirrer. The obtained polyesters were characterized by size exclusion chromatography (SEC) and 1H-NMR. Results: The Sm(OAc)3 induces the polymerization of L-LA at high conversion, and produce polyesters with number-average molecular weights of 1.00 x 103 to 30.00 x 103 Dalton. The 1H-NMR analysis indicates a typical polymerization mechanism of coordination-insertion, with a breakdown of the acyl-oxygen bond of the L-LA. Conclusion: Sm(OAc)3 was an effective initiator for the ring-opening polymerization of L-LA. SEC chromatography showed that, at high temperatures and prolonged reaction times, the molar mass of the polyester decreases, which is associated with the transesterification collateral reactions that occur during the polymerization process.

Poly(ϵ-caprolactone)-b-poly(ethylene glycol)-b-poly(ϵ-caprolactone) (PCL-b-PEG-b-PCL) triblock copolymer were synthesized by mean anionic activation of the hydroxyl end groups of poly(ethylene glycol) in presence of diphenylmethylsodium. Copolymers were characterized by SEC, FT-IR and 1H-NMR spectroscopy, TGA and DSC. Size exclusion chromatographic analysis of obtained copolymers indicated incorporation of CL monomer into PEG without formation of PCL homopolymer. Characterization by FT-IR and 1H NMR spectroscopy of the resulting polymeric products, with respect to their structure, end-groups and composition, showed that they are best described as ester-ether-ester triblock copolymers, whose compositions can be adjusted changing the feeding molar ratio of PEG to CL. The thermal stability of triblock copolymers was less that PEG precursor, but higher that PCL homopolymer. Analysis by mean DSC showed that all copolymers were semi-crystalline and their thermal behavior depending on their composition.

Poly(e-caprolactone) (PCL) and poly(l-lactide) (PLA) were prepared by ring-opening polymerization (ROP) using samarium(III) acetate (SmAc3) and amino acid complexes of samarium(III) as initiators. The catalytic behavior of samarium(III) acetate and their respective amino acid complexes, Sm(2,2′-bipyridine)(Ln)3 (L1 = l-aspartic acid; L2 = l-glutamic acid and L3 = glycine), was studied. It could be observed that the amino acid structure and chemical bond type have great influence on the catalytic activity. Polymerization reaction temperature was 125 °C and the effect of time reaction on the conversion of monomers to polymers and the molecular weight were studied. The results indicate that the initiators induce the polymerization of e-caprolactone (e-CL) and l-lactide (l-LA). Polymers were characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (1H-NMR) and size exclusion chromatography (SEC). Polyesters with average-number molar mass of 1.50 × 103 − 104 Da were obtained. Based on the 1H-NMR end group analysis of low molecular weight of polymers, a coordination–insertion mechanism is proposed for the polymerization of lactones. Kinetics study indicated a polymerization rate of first order with respect to monomer concentration. Differences in the rates of polymerization in the four initiators appeared to be governed by the different degrees of steric hindrance in the initiator structure and monomer.

A series of polyacrylamide hydrogels (PAAm HGs) and polymers were synthesized by free radical polymerization in the same reaction conditions but using two different media as solvent: water/ethanol at 100/0, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70, and 20/80 (V/V) and water/DMSO at 100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70 and 20/80 (V/V). In most cases hydrogels were obtained in high yields and their swelling degree showed a significant dependence with the solvent used in the synthesis. When HGs were synthesized in water/ethanol mixture, the maximum equilibrium swelling (Seq) was reached when the water/ethanol ratio was 60/40(V/V), while for water/DMSO it was obtained for a water/DMSO ratio 80/20. In both cases, these swelling values were higher than when the HGs were synthesized in pure water. The swelling degree achieved by the HGs in water/ethanol was directly related to the yields and the pore size, but this behavior was not observed for the HGs obtained in water/DMSO. In all cases the HGs obtained in water/DMSO swell more than those obtained in water/ethanol. On the other hand, the molar mass of the PAAm polymers prepared in the same reaction conditions, decrease as the proportion of ethanol or DMSO increase. However, the molar mass reached by the polymers obtained in water/DMSO were higher than those obtained in water/ethanol.

Block copolymers of e-caprolactone (CL) and l-lactide (l-LA) were synthesized by sequential polymerization using diphenylzinc as initiator. The composition of the copolymers was adjusted changing the comonomers in ratio. Copolymers were characterized by 1H-NMR, 13C-NMR, DSC, and GPC. Results indicate that poly(e-caprolactone)-b-poly(l-lactide) (PCL-b-PLA) block copolymers had a narrow molecular weight distribution and well-controlled sequences without random placement.

Two series of copolymers of styrene with di-n-alkyl itaconates or methyl n-alkyl itaconates with n-alkyl side chains of 12, 14, 16, 18, and 22, using different compositions in the feed have been prepared in bulk via radical at 60 °C with AIBN as initiator. In most of the cases the copolymers were obtained in goods yields, their composition being close to the feed composition and their physical states depending on the composition and the itaconate structure. The copolymers were characterized by FTIR and NMR spectroscopy. The 1H NMR studies indicate that the copolymers have a mainly random distribution of the monomeric units. The thermal degradation process occurs in one step with decomposition temperatures intermediate between those of polyitaconate and polystyrene. Finally, the alkyl side chains of the itaconate moieties are able to crystallize in a paraffinic phase where the melting temperature and enthalpy increase as the itaconate content and the side chain length rise.

Starch complexes were obtained from activated and pregelatinized cassava starch (AS) or carboxymethyl starch (CMS) by using quaternary ammonium salts (QAS). The synthesis of the CMS was carried out by a single or double step carboxymethylation reaction with monochloroacetic acid. Different parameters were assayed to prepare the ionic complexes: CMS or AS as starting materials, anhydroglucose unit/QAS ratio, and length of the alkyl chain of the alkyltrimethylammonium bromide (C n TAB) salt. The products were characterized by FTIR, 1 H and 13 C NMR, and the Kjeldhal method. The results indicated a much more favorable complex formation when using CMS, obtaining the highest degree of substitution or complexation (0.72) and reaction efficiency (0.96) when C 18 TAB was employed. DSC and WAXS experiments evidenced the crystallization of the octadecyl chain introduced in the starch, exhibiting a dependence on sample water content. TGA and TVA analysis showed the stability of the complexes up to ∼150 °C and that degradation takes place in at least two stages with the release of the surfactant moiety in a first stage and the degradation of the starch skeleton in a second stage.

The physical and chemical modification of cassava starch was carried out to prepare compounds that were evaluated as corrosion inhibitors of carbon steel under alkaline conditions in 200 mg L −1 NaCl solutions. Two species were tested: an activated starch (AS) and a carboxymethylated starch (CMS) with two different degrees of substitution ( DS ). The success of the chemical procedure was confirmed by 13 C NMR. The DS was determined by back titration and products with 0.13 (CMS 0.13 ) and 0.24 (CMS 0.24 ) were obtained. The inhibitive properties were studied by means of electrochemical impedance spectroscopy. It was found that modified starches have corrosion inhibitive properties and that their protection level depends on the type and amount of active groups present in the molecules. AS showed better performance than CMS, whose inhibition ability increases with the degree of substitution. This result was explained by the strong ionic interaction between AS and ferrous cations, which was confirmed by the electrostatic potential mapping of the monomeric units. After the corrosion experiments, the surfaces also were investigated through atomic force microscopy. It was found that a densification of the inhibitive layer was responsible for the higher protection level afforded by AS after 24 h of immersion.

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.

A series of comb like poly(p-n-alkylstyrene)s with linear alkyl groups, containing even numbers of carbon atoms from 12 up to 22 were synthesized by radical polymerization and characterized by FTIR, 1H NMR, TGA, TVA DSC, and WAXS. All polymers were obtained in good yields and were soluble in organic solvents as chloroform, dichloromethane or tetrahydrofuran and insoluble in methanol; they are stable up to temperatures near 300 °C. On the other hand, the alkylic side chains of all series are able to crystallize in a paraffinic phase in which the melting temperature and enthalpy increase with the methylene number of the n-alkyl chain.

Poly(α- n -alkyl β- l -aspartate)s, P n AALAs, are comb-like polymers with rod-like helical conformations which present crystallinity for long alkyl side chains ( n > 10). Dielectric spectroscopy experiments for P n AALAs (10 ⩽ n ⩽ 18) show the existence at low temperatures of two secondary modes corresponding to localized motions of the carbonyl group. At higher temperatures two cooperative mobilities coexist. The first corresponds to the segmental mobility of the amorphous side chains and has the characteristic of a polyethylene-like dynamic glass transition common to various series of nanophase segregated side chain polymers with different morphologies. A second segmental mode is attributed to the restricted rotational motion of the rigid rods which is hindered when the spacing between rods decreases. An extension to the results found for other polymer series with shorter lateral chains is presented, showing the tendency to approximate amorphous polyethylene dynamics in these confined nanodomains of increasing sizes.

A series of acrylamide/itaconic acid (AAm/IA), acrylamide/monomethoxyethyl itaconate (AAm/MEI), and acrylamide/dimethoxyethyl itaconate (AAm/DEI) hydrogels were synthesized in AAm/comonomer molar ratios of 100/0, 90/10, 80/20, and 70/30. In all cases, almost quantitative yields were obtained, and the swelling ability increased when comonomer proportion increased. AAm/MEI and AAm/IA reach swelling values more than 40,000, and both systems swelled more than AAm/DEI. This fact pointed out that the carboxylic acid groups present in IA and MEI are mainly responsible for the swelling. All of these hydrogels are stable up to 150 °C as indicated by the TGA measurements. For comparison, the synthesis of a series of N-2-hydroxyethyl acrylamide/itaconic acid (HEAAm/IA), N-2-hydroxyethyl acrylamide /monomethoxyethyl itaconate (HEAAm/MEI), and N-2-hydroxyethyl acrylamide /dimethoxyethyl itaconate (HEAAm/DEI) hydrogels were also carried out. The swelling behavior of HEAAm/IA and HEAAm/MEI was opposite to the former; in these cases the presence of the itaconate comonomer diminish the swelling ability. A diffusion mechanism study was carried out for all the series.

A series of complexes from poly(aspartic acid) and alkyltrimethylammonium bromide surfactants with linear alkyl groups, containing even number of carbon atoms from 12 up to 22 were synthesized. The complexes were obtained in good yields, are soluble in organic solvents and stable up to temperatures near 170 °C. On the other hand, the paraffinic side chains are able to crystallize depending on the length of the surfactant n-alkyl chain being the main chain of the complexes is arranged in an extended conformation.

A series of complexes from poly(itaconic acid) and poly(mono methyl itaconate) and alkyltrimethylammonium bromide surfactants with linear alkyl groups, containing even number of carbon atoms from 12 up to 22 were synthesized. The complexes were obtained in good yields and were soluble in organic solvents and stable up to temperatures near 150 °C. On the other hand, the paraffinic side chains are able to crystallize depending on the methylene number of the surfactant n-alkyl chain.

A first attempt was made to produce novel ABC triblock terpolymers with three potentially crystallisable blocks: polyethylene (PE), poly(ethylene oxide) (PEO), and poly(e-caprolactone) (PCL). Polybutadiene-b-poly(ethylene oxide) diblock copolymers were synthesized by living anionic polymerization. Then, a non-catalyzed thermal polymerization of e-caprolactone from the hydroxyl end group of the PB-b-PEO diblock precursors was performed. Finally, hydrogenation by Wilkinson catalyst produced PE-b-PEO-b-PCL triblock terpolymers. Side reactions were detected that lead to the formation of undesired PCL-b-PEO diblock copolymers, however, these impurities were successfully removed by purification. A range of triblock terpolymers with PCL and PEO minor components were prepared. Topological restrictions on the PEO middle block prevented this block from crystallizing while the complex crystallization behavior of the PE and PCL blocks was documented by DSC and WAXS measurements.

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 ).

A series of poly(mono n-alkyl itaconate)s, poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s with n ¼ 12; 14, 16, 18 and 22 have been prepared by radical polymerization. NMR studies point out that poly(methyl n-alkyl itaconate)s and poly(di n-alkyl itaconate)s are mainly syndiotactic polymers whereas poly(mono n-alkyl itaconate)s are obtained as almost atactic polymers. The characterization performed by DSC, solid state 13 C CP/MAS NMR and X-ray diffraction, indicates that the side chains of poly(mono n-alkyl itaconate)s and poly(methyl n-alkyl itaconate)s derivatives with more than 12 carbon atoms are able to crystallize in hexagonal lattices. In the case of poly(di n-alkyl itaconate)s, when the side chains contain 12 or more carbon atoms, they are able to crystallize also in hexagonal lattices. q 2003 Elsevier Science Ltd. All rights reserved.

Hairy random copoly(β,l-aspartate)s were prepared by anionic ring opening polymerization of mixtures of (S)-4-octadecoxycarbonyl and (S)-4-benzyloxycarbonyl 2-azetidinones at 3:1, 1:1 and 1:3 molar ratios. The three resulting copolymers had molar ratios of octadecyl to benzyl units of 68:32, 44:56 and 12:88, respectively. They all were found to adopt the rigid α-helix-like conformation characteristic of poly(β,l-aspartate)s, but only the first one displayed crystallization of the alkyl side chains. This copolymer showed thermochromic behavior with color changes taking place when heated above the side chain melting temperature. These results evidenced the ability of the benzyl group to be accommodated within the layered structures of comb-like poly(β,l-aspartate)s and to modify the temperature range in which chromatic changes are observed in these systems.

We have recently prepared a series of Polystyrene-b-Poly(ethylene oxide)-b-Polycaprolactone (PS-b-PEO-b-PCL or SEOCL) triblock copolymers of varying compositions and molecular weights. These ABC triblock copolymers present the peculiarity that two of the three blocks are able to crystallize upon cooling from an already phase segregated melt. When either of the crystallizable blocks or both are a minor phase, a fractionated crystallization process develops. The confinement of crystallizable blocks in the nanoscopic scale enables the clear observation in some cases of exclusive crystallization from homogeneous nuclei of two components within the triblock copolymer. The homogeneous nature of the nucleation was deduced since the supercooling attained is the maximum possible before vitrification of the material takes place. The self-nucleation domains were also found to depend on the composition and molecular weight of the copolymers. The block copolymers exhibited a marked decrease in crystalline memory and when the crystallizable blocks constitute minor phases, the self-nucleation domain disappears. The reason behind this behavior is that only at lower self-nucleation temperatures the density of self-nuclei becomes high enough to include at least one crystal fragment per confined microdomain in view of their vast numbers (e.g., 10 16 /cm 3 ).

The radical polymerization of methyl n-octadecyl itaconate initiated with AIBN, produces poly(methyl n-octadecyl itaconate) with a yield of 47% and a molecular weight of about 105 g/mol. 13C-NMR studies point out that this polymer is mainly syndiotactic. The combined studies of DSC and solid state 13C CP/MAS NMR indicate that the polymer side chains are able to crystallize in a paraffinic phase, presumably in an hexagonal form which melts at 44°C with a ΔH of 4.8Kcal/mol.

The preparation of polystyrene-b-poly(ethylene oxide)-b-poly(-caprolactone) triblock co- polymers using three different strategies is described. Experimentally, a method we have called "thermal" is the simplest one of the three employed. However, it presents serious limitations for the preparation of high molecular weight triblock copolymers with low polydispersity. The titration method is a more elegant alternative to prepare the PS-b-PEO-b-PCL triblock copolymers. Nevertheless, the stoichiometric titration of the precursor terminal hydroxyl groups requires much expertise, and it becomes more difficult as the concentration of such terminal groups decreases when the precursor molecular weight increases. Finally, the sequential method may be regarded as the most convenient one. However, the experimental steps involved a solvent exchange during polymerization that must be performed under conditions of strict purity and high vacuum. Several triblock and pentablock copolymers of varying compositions and molecular weights were successfully prepared. The fact that these copolymers contain two crystallizable blocks, i.e., PEO and PCL, makes them very interesting from a structural point of view. The copolymers were found to be phase segregated according to evidence provided by DSC and WAXS. The crystallization and melting behavior of both PEO and PCL blocks is greatly affected by composition. Fractionated crystallization phenomena were detected when either one or both blocks constituted the minor components within the block copolymers.

The structure and thermal properties of the phase B of poly(α-alkyl-β-L-aspartate)s abbreviated PAALA-n (n being the number of carbon atoms in the linear alkyl side chain) with n = 14 and 16 were determined using X-ray diffraction and DSC methods, respectively. These data together with those previously reported by us for the dodecyl deriviative were used to perform Monte Carlo simulations of these comb-like polymers. The results allowed to predict the solubility of different gases in the phase B of PAALA-n. The variation of both the unoccupid space and the excess chemical potential with the size of the penetrant were computed. An attractive interaction between the polymer matrix and the penetrants was founs when small gases were considered. The results have been compared with those recently obtained for crystalline poly(α-alkyl-β-L-aspartate)s bearing short linear alkyl side chains.

Copoly(α-alkyl-β,l-aspartate)s containing n-octadecyl and n-butyl side groups at different ratios were prepared by anionic ring-opening polymerization of the corresponding optically pure (S)-4-alkoxycarbonyl-2-azetidinones. Random copolymers were obtained by polymerization of comonomer mixtures. Diblock copolymers were achieved by sequential copolymerization using the living poly(α-n-octadecyl-β,l-aspartate) block to initiate the polymerization of (s)-4-n-butoxycarbonyl-2-azetidinone as a second block. Composition and sequence distribution were characterized by NMR with the help of a model copoly(β,l-aspartate) made of α-n-dodecyl and α-benzyl β,l-aspartate units. All the copolymers were found to adopt the layered structure made of 13/4 helices described previously for comb-like poly(α-n-alkyl-β,l-aspartate) homopolymers. Copolymers containing at least 50% of n-octadecyl side groups have these groups crystallized in an interlayer microphase and aligned normal to the main helical chain. Melting of the paraffinic crystallites happened within the 40–60°C temperature range with formation of a liquid-crystal phase in which side chains are molten but retain the alignment of the low temperature phase. Different from what is known to happen to poly(α-n-octadecyl-β,l-aspartate), no indications on the occurrence of a second structural transition were observed at higher temperatures.

The helix−coil conformational transition taking place in a family of poly(β-peptide)s, namely, poly(α-alkyl-β-aspartate)s (Mw > 100 000), upon addition of acid or by changes in temperature has been investigated. Results obtained for a set of optically pure polymers differing in the size and shape of the alkyl side chain evidenced that the transition is a phenomenon common to all poly(β-l-aspartate)s. Experimental conditions required for transition were found to be independent of the constitution of the poly(β-peptide). Investigation of poly(β-d,l-aspartate)s with different enantiomeric composition and configurational sequence revealed the occurrence of the helix−coil transition in block stereocopolymers for any d/l ratio but not in stereocopolymers with a statistical distribution of the antipode units.

A series of poly(α- n -alkyl β- l -aspartate)s (where n is the number of carbon atoms in the linear alkyl side chain, with n = 12, 18 and 22), was studied by differential scanning calorimetry. The effect of the length of alkyl side chain on the non-isothermal crystallization kinetics on cooling was investigated. It was found that the ΔH c and crystallization rate of PAALA samples on cooling increase with the alkyl side chain length. A similar trend is observed for the activation energy of the crystallization process. It was found that the Johnson-Mehl-Avrami model cannot be applied for a quantitative description of non-isothermal d.s.c. data corresponding to this crystallization process.

The structure in the solid state of two helical nylon 3 derivatives, poly(α- n -hexyl-β- l -aspartate) and poly(α- n -octyl-β- l -aspartate), was investigated by molecular mechanics simulation methods. It was found that both polymers adopt the 13 4 helix previously observed for poly(β- l -aspartate)s bearing alkyl side chains of small size. The hexyl derivative could not form stable tridimensional structures. This is in agreement with reported experimental results which showed such polymers to be uncrystallizable. In the case of the octyl derivative, a crystal structure arranged in layers with a rhombic unit cell in the P 2 1 2 1 2 space group appeared to be energetically favoured. Side chain trajectories within the interhelical space were found to vary depending on the position of the residue along the repeating unit of the helix. Side chains protruding normal to the sheets are almost extended whereas those located along the sheets are folded. A quantitative estimation of the penetration of the side chains into the intermolecular space has been made on the basis of interatomic distances measurements. © 1997 Elsevier Science Ltd.

Poly(e-caprolactone) (PCL) was prepared by ring- opening polymerization initiated with samarium acetate. The effects of reaction temperature, molar ratio of monomer/ initiator and reaction time on the conversion of e-CL to PCL and the molecular weight were investigated. The temperature was varied between 80 and 150 °C and the molar ratio of monomer to initiator between 107 and 1000 mol/mol. The results indicatethatthe samariumacetate (SmAc3) induces the polymerization of e-CL to high conversion and polyesters with number-average molecular weights of 3.43×10 3 to 12.60×10 3 were obtained. The polymers were characterized by Nuclear Magnetic Resonance ( 1 H-NMR), size exclusion chromatography (SEC). Based on the 1 H-NMR end-group analysis of low-molecular-weight PCL, it was proposed a coordination-insertion mechanism for the polymerization. Kinetics study indicated that the polymerization rate is first- order with respect to monomer concentration.

The present work describes the characterization and thermal properties of hydrophobic starch obtained by the esterification of cassava starch with acyl imidazoles, acid chlorides and methyl ester derivatives of fatty acids with n-alkyl chains with 12–22 carbon atoms, in order to compare the dependence of their properties as a function of the length of the side chain and the methodology used for their synthesis. The n-acyl starches presented degrees of substitution (DS) between 0.06 and 1.2. Most of the derivatives obtained with acyl imidazoles were found to be stable at temperatures up to 300 °C, whereas those synthesized with acid chlorides or methyl ester decomposed below. Finally, when the n-acyl starches were substituted with n-alkyl side chains of 16 or more carbon atoms, they were capable to crystallize in separate paraffinic phases independent of the starch backbone.

Different polymerization methods were used for the preparation of poly(α-isobutyl-β-D,L-aspartate)s containing variable ratios of D- to L-aspartic units and the microstructure of the resulting stereocopolymers was examined by NMR spectroscopy. Anionic ring-opening polymerization in solution of enantiomeric mixtures of α-isobutyl-β-D- and L-aspartalactams was found to proceed stereoselectively rendering block copolymers composed of right- and left-handed helical sequences. Configurationally statistical copolymers were obtained instead when the enantiomeric lactam mixtures were polymerized in the bulk. Random stereocopolymers could be prepared also by polycondensation in solution of mixtures of pentachlorophenyl α-isobutyl-β-D and -L-aspartates. The conformation in solution and the crystal structure of the resulting copolymers were investigated in connection with their stereochemical configuration and these features compared with those displayed by optically pure poly(α-isobutyl-β-L-aspartate). © 1996 John Wiley & Sons, Inc.

The conformation of three poly(α-alkyl β-l-aspartate)s, abbreviated PAALA-n, with the alkyl group being methyl, ethyl, and propyl (n = 1, 2, and 3), was examined by X-ray diffraction, polarized infrared spectroscopy, and molecular mechanics calculations. The crystal structure was modeled and refined against diffraction data with the linked-atom least-squares methodology (LALS). Two crystal forms reminiscent of the well-known α-helix and pleated β-sheet structures typical of polypeptides were characterized for the methyl derivative. The helical form is a pseudohexagonal lattice of right-handed 17/4 helices stabilized by intramolecular hydrogen bonds. The β form is made of chains in nearly extended conformation and hydrogen bonded to each other to form sheets which stack together in a metrically orthorhombic crystal lattice. In addition to these two forms, a second folded form made of right-handed 4/1 helices arranged in a tetragonal structure was characterized for the ethyl derivative. Poly(α-propyl β-l-as...

Poly(beta-l-aspartate)s are known to take up helical conformations reminiscent of the alpha-helix of polypeptides. The isobutyl, n-butyl, and 2-methoxyethyl esters have been examined by polarized ir spectroscopy in order to discriminate between the left (1L) and right (2R)-handed conformations, which are known to be compatible with the 13/4-helix adopted by these polyamides when crystallized in the hexagonal form. Dichroic ratios obtained from samples stretched in poly(ethylene oxide) together with orientation measurements made by x-ray diffraction were used to estimate the transition moment directions of amide A, I, and II bands with respect to the fiber axis. These were compared to those calculated by modeling simulations to conclude that the right-handed conformation consisting of 14-membered hydrogen-bonded rings is the correct model for the 13/4-helix. These results give definite support to earlier molecular mechanics calculations, which had shown that the 2R model is energetically favored over the 1L by about 2.5 kcal/(mol residue).

The structure and thermal properties of a series of novel comblike polyamides derived from nylon 3, poly(α-n-alkyl β-L-aspartate)s (n being the number of carbons in the linear alkyl side group) with n = 6, 8, 12, 18, and 22, were investigated. Polarizing infrared and solid-state 13 C CP-MAS NMR measurements revealed that these polyamides adopt α-helix-like conformations and that these structures are retained at high temperatures. Two first-order transitions at temperatures T 1 and T 2 separating three structurally distinct phases, namely, A, B, and C, were characterized for polymers with n ≥ 12 by DSC and X-ray and electron diffraction methods. T 1 and T 2 ranged from -15 to +75 °C and from 50 to 129 °C, respectively, for n increasing from 12 to 22. Phase A (T T 2 ) was interpreted as a uniaxial arrangement of independent helices embedded in the amorphous side chain matrix. Microscope optical observations suggested that a cholesteric-nematic rearrangement is probably implied in the B-C transition. Members with n = 6 and 8 displayed a peculiar behavior. The octyl derivative crystallized by annealing in a three-dimensional structure composed of 13/4 helices of the type reported for poly(β-L-aspartate)s bearing short side chains while the uncrystallized polymer was arranged as in phase B. On the contrary, the hexyl derivative could neither crystallize nor organize in a layered structure. It was concluded from this study that the title compounds follow closely the general pattern of behavior described for poly(γ-n-alkyl α-L-glutamate)s, a family of thermotropic polypeptides that has received great attention in recent years.

The conformation of the two title poly(β-L-aspartate)s was analyzed by both molecular mechanics calculations and the linked-atom least-squares (LALS) methodology using X-ray diffraction data previously published by us. A right-handed 13/4 helix composed of 14-member hydrogen-bonded rings was found to be the most favored conformation for the so-called hexagonal form in which both compounds are known to crystallize. The preferred conformation for the tetragonal form, which is the second crystal modification adopted by the n-butyl derivative, consisted of a right-handed 4/1 helix. Different modes of packing compatible with experimental data were examined for each crystal form, the most favored being those corresponding to space groups P2 1 and P4 1 for hexagonal and tetragonal, respectively. These results appear to be substantially identical to those earlier described for poly(α-isobutyl β-L-aspartate) which is the only poly(β-L-aspartate) investigated so far by these methods. Comparison between the relative stabilities calculated for the two polymorphs of each polymer allowed one to account for the different tendency to crystallize in the tetragonal form that is experimentally observed among poly(β-L-aspartate)s possessing side chains of intermediate size.

A number of poly(α-n-alkyl- l -aspartate)s, which are nylon-3 derivatives with an alkoxycarbonyl side group attached to every repeat unit, have been prepared by non-assisted anionic polymerization of the corresponding (S)-4-alkoxycarbonyl-2-azetidinones. Side reactions leading to imidation were found to take place to an extent which is highly dependent on both the polarity of the polymerization solvent and the nature of the alkyl side chain. Highly stereoregular polymers without detectable imidation and molecular weights in the range 105 to 5 × 105 were obtained in dimethyl sulfoxide for n = 4, 6, 8 and 12 and in CH2Cl2 for n = 1, 2 and 18 (n=number of carbon atoms in the alkyl side group). A study of the thermal stability of these new polyamides showed that they melt with decomposition to largely imidated polymers when heated at temperatures above 250°C. X-Ray analysis of oriented samples revealed that members with n = 1, 2, 4 and 8 are able to form crystal structures with chains arranged in a helical conformation similar to the α-helix typical of polypeptides. This type of conformation seems also to be adopted by all other members of the series. For n = 6 and 12, however, the polymer chains did not crystallize but they became aligned side by side with alkyl side groups remaining in a disordered state.

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.

Cyclization of L-aspartic acid to (S)-4-benzyloxycarbonyl-2-azetidinone followed by transesterification of this compound in the presence of titanium (IV) tetrabutoxide proved to be an exceedingly efficient method of general application for the preparation of (S)-4-alkoxycarbonyl-β-lactams.

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.

Poly (ethylene oxide) (PEO) samples, obtained using diphenylzinc-water as initiator, have been used to determine some characteristics of this polymer. The unperturbed dimensions of PEO have been obtained by the application of various theoretical treatments to viscometry measurements carried out in a good solvent. The following relationship between intrinsic viscosity and weight average molecular weight has been obtained [ŋ]/dL.g•1 = 3.99x10•4M0.69 W.

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.

A series of poly(alpha-n-alkyl beta-L-aspartates) which are nanophase self-assembled comb-like polymers has been studied by dielectric spectroscopy in a broad frequency range (10(-2) < or = nu < or = 3 x 10(6) Hz), with n-alkyls side chains of various lengths, 10 < or = n < or =18. In every member of the series the same relaxations were identified after the decomposition of the experimental isothermal trace in up to three peaks with relaxation times distributions. The strength, width and average relaxation time for all the relaxation modes were determined for each material. Besides the local low temperature, Arrhenius modes, two relaxation modes, alpha and alpha(PE), present a cooperative character whose dynamics are not affected by the side chains melting. The alpha(PE) relaxation is a polyethylene-like glass transition of the amorphous side chains and its dynamics is strongly dependent on the n value due to the increasing restrictions imposed by the self-assembled confinement. The strength of the alpha(PE) relaxation mode increases as the lateral chains loose their 2D order. The restricted chopstick motion of the rigid rods is thought to be the origin of the alpha mode; this motion is hindered at temperatures where the cage size decreases as a result of the increasing disorder with temperature.

Polymerisation par coordination du monomere a un atome de zinc terminal des sites actifs, suivie d'insertions d'unites monomeriques a cet atome de zinc

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.