Controlled synthesis of amphiphilic biodegradable polylactide‐grafted dextran copolymers

The whole controlled synthesis of novel amphiphilic polylactide (PLA)‐grafted dextran copolymers was achieved. The control of the architecture of such biodegradable and potentially biocompatible copolymers has required a three‐step synthesis based on the “grafting from” concept. The first step consisted of the partial silylation of the dextran hydroxyl groups. This protection step was followed by the ring‐opening polymerization of D ,L ‐lactide initiated from the remaining OH functions of the partially silylated polysaccharide. The third step involved the silylether group deprotection under very mild conditions. Based on previous studies, in which the control of the first step was achieved, this study is focused on the last two steps. Experimental conditions were investigated to ensure a controlled polymerization of D ,L ‐lactide, in terms of grafting efficiency, graft length, and transesterification limitation. After polymerization, the final step was studied in order to avoid degradation of both polysaccharide backbone and polyester grafts. The chemical stability of dextran backbone was checked throughout each step of the synthesis. PLA‐grafted dextrans and PLA‐grafted (silylated dextrans) were proved to adopt a core‐shell conformation in various solvents. Furthermore, preliminary experiments on the potential use of these amphiphilic grafted copolymers as liquid/liquid interface stabilizers were performed. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2577–2588, 2004     

Comprehensive study on the formation of polyelectrolyte complexes from (quaternized) poly[2‐(dimethylamino)ethyl methacrylate] and poly(2‐acrylamido‐2‐methylpropane sodium sulfonate)

Polyelectrolyte complexes (PECs) have been prepared from well‐defined (quaternized) poly[2‐(dimethylamino)ethyl methacrylate] (PDMAEMA) and high molecular weight poly(2‐acrylamido‐2‐methylpropane sodium sulfonate) (PAMPSNa) after a thorough study of their viscometric properties. The effect of pH and quaternization degree of PDMAEMA on PECs stoichiometry has been examined. PEC‐based materials have been characterized in terms of thermal stability, equilibrium swelling degree, and free/bound water composition. The stoichiometry and swellability of the physically crosslinked hydrogels obtained from fully quaternized PDMAEMA/PAMPSNa complexes do not depend on pH. In contrast, PECs made of non quaternized PDMAEMA and PAMPSNa are highly affected by pH, and could reversibly disintegrate at pH ≥ 9. Partially quaternized PDMAEMA/PAMPSNa PECs exhibit intermediate properties and form stable loose structures in the whole investigated pH range. Finally, stable dispersions of PECs nanoparticles have been successfully produced from dilute solutions of the complementary polyelectrolytes. The nanoparticle average diameter as determined by dynamic light scattering proved to depend on the molar fraction of DMAEMA‐based subunits and on the initial polyelectrolyte concentration. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5468–5479, 2006     

Structural characterisation of a sol-gel copolymer synthesised from aliphatic and aromatic alkoxysilanes using 29Si-NMR spectroscopy

A hybrid organic-inorganic material was prepared from the hydrolytic co-condensation of 3-trimethoxysilylpropylmethacrylate (MAPTMS) and diphenyldiethoxysilane (DPHDES). The synthesis was performed in three steps: (1) MAPTMS pre-hydrolysis, (2) addition of DPHDES, and (3) further hydrolysis of the obtained mixture. ²⁹Si nuclear magnetic resonance spectroscopy was used to characterise the structural evolution of the mineral network during the synthesis. It revealed the catalytic effect of DPHDES on the condensation reaction of silanol groups, initially observed for pre-hydrolysed MAPTMS. In addition, it was clearly demonstrated that the formation of the highest condensed species of the aromatic oligomers (D₂ species) occurred only in the presence of aliphatic oligomers, which decrease the sterical hindrance about the silicon nuclei.     

Simple and rapid enzymatic assay of ornithine decarboxylase activity

Since the induction of putrescine synthesis by ornithine decarboxylase (ODC) is observed in many pathological and physiological processes, a useful and simple method to assay this enzyme activity should be an interesting tool to quantify the biological importance of its induction. An enzymatic method to assay ODC is reported here. This method is based on the reaction between putrescine and soya diamine oxidase. The reaction releases H(2)O(2), which is measured by a colorimetric method. The validation of this method showed good accuracy (98+/-5% of recovery). High precision and reproducibility were obtained. A linearity with a correlation coefficient of 0.999 in the range of 2.5-25 nmol was obtained. This method is also rugged and specific. The application of the assay of ODC activity showed that it is useful as a rapid and simple tool for assaying ODC activity in vitro. Comparison with the HPLC determination of ODC activity shows strong correlation along with the high accuracy of the two methods.     

Reaction de metathese des olefines induite photochimiquement en presence d'un complexe de metal de transition : V. Intervention d'un complexe dinucleaire du tungstene

The photoinduced metathesis [W(CO)₆CCl₄hν] of cis- and trans-RCH=CHMe gives 2-butene in a very high trans but low cis stereoselectivity. Propene behaves like a cis olefin. These results are consistent with the proposal of a dinuclear tungsten intermediate, which requires high steric constraints in the transalkylidenation step.     

Microgels and ionic associations in solutions of cellulose diacetate

Solutions of cellulose diacetate (CDA) from two sources (cotton linters and wood pulp Floranier) were analysed in various solvents by size exclusion chromatography (SEC). Without special precautions, the SEC chromatograms presented three peaks — or prehumps — before the main polymer peak. The first prehump which could be eliminated by ultracentrifugation corresponded to microgels whose sugar composition was determined. These microgels were also investigated by electron microscopy, X-ray and electron diffraction analysis. They corresponded mainly to cellulose triacetate (CTA-II) in the case of CDA from cotton linters and a mixture of CTA-II and xylan diacetate (XDA) in the case of CDA from the wood pulp Floranier. The second and third prehumps could be attributed to ionic effects corresponding to the association of remaining sulfate groups on the CDA molecules with residual calcium. It was found that these ionic effects could be eliminated by the addition of LiBr or LiCl to the elution solvents. This led to chromatograms devoid of prehumps.Presented in part at the Cellulose '91 meeting in New Orleans.     

Steric effects—III: Composition of the E's parameter. Variation of alkyl steric effects with substitution. Role of conformation in determining sterically active and inactive sites

The steric effect of alkyl groups as parameterized by E'_s covers a range of over seven powers of ten. In this article an attempt is made to portray in mechanism terms the manifold nature of the steric effect as a function of substitution, i.e. the composition of the steric effect. One most consider the contribution of the topological carbon sites as a function of the true coordinates of these atoms. To this end, we evaluate the geometry of these alkyl groups in carboxylic acid indirectly by a search for the probable conformations of these acids. The starting point in this study is the determination of the minimum energy conformations in carboxylic acids using the empirical force field method based on analogous alkylmethyl ketone models.This approach to the ground state behaviour of these acids leads directly to a division of the complete set of alkyl groups into two subsets within which a Conformational sequence or filitation is associated with the substitution process which generates the successive alkyl groups of that subset. It is shown that eclipsed conformations are preferred in the case of secondary and tertiary groups bearing Et and Me sub-groups, giving way to preferred bisected conformations when t-Bu and i-Pr sub-groups are present. For further elucidation of steric perturbation terms these two subsets must be considered independently. These important observations lead to an overall coherent interpretation of steric effects.Three regions of distinct behaviour as a function of substitution have been identified. In Region I (the so-called “mormal” region, where the contribution of the introduction of successive methyl groups to the overall E'_s increases monotonically), besides excellent topological site correlations, an excellent hybrid correlation is proposed. For a conformationally homogeneous (i.e. eclipsed) set of alkyl groups the overall E'_s of groups is well expressed by a linear combination of the weighted contribution of sub-groups and the Newman six-number. This correlation leads to an interpretation of the composition of the overall steric effect of an alkyl group, as measured by the E'_s parameter, in terms of conformationally defined sites and stresses the steric importance of certain privileged sites. This result is consistent with the molecular mechanics viewpoint of preferred conformations but at variance with previous assumptions.The levelling effect (Region II), in the light of conformational information, is understood in terms of sterically active and inactive sites, while the inversion effect (Region III) is considered to be the result of molecular distortion brought about by relief of local internal steric strain within the alkyl groups. The X-ray structure of i-Pr₃CCO₂H illustrates this point.