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See detailTriblock copolymer based thermoreversible gels. 2. Analysis of the sol-gel transition
Yu, Jian Ming; Dubois, Philippe ULg; Teyssié, Philippe et al

in Macromolecules (1996), 29(16), 5384-5391

Syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers of various chemical compositions and different molecular weights have been studied in the presence of o-xylene ... [more ▼]

Syndiotactic poly(methyl methacrylate) (sPMMA)−polybutadiene (PBD)−sPMMA triblock copolymers of various chemical compositions and different molecular weights have been studied in the presence of o-xylene, which is a selective solvent for the central PBD block. Thermoreversible gels can be formed in a suitable curing temperature range (<35 °C) and at a high enough concentration (>1 wt %). The time dependence of the storage (G‘) and the loss (G‘‘) moduli has been measured in a frequency range of 0.08−1 Hz. The static and dynamic properties of the gels have been discussed on the basis of the scaling theory. At the gel point, where the loss angle (tan δc = G‘‘/G‘) is independent of frequency, typical power laws G‘(ω) G‘‘(ω) ωΔ have been observed. The scaling exponent Δ has been found equal to 0.70 ± 0.02 independently of the PBD and PMMA molecular weight, i.e., 36 000 < Mn (PBD) < 100 000 and 20 000 < Mn (PMMA) < 51 000. This exponent is also independent of the copolymer concentration and temperature in the investigated range, i.e., 2−7 wt % and 8−24 °C, respectively. This value of Δ agrees with theoretical predictions as well as with experimental values reported for some chemical gels; it is however different from the experimental values published for most physical gels. A PBD−PMMA diblock copolymer also forms a gel in o-xylene, although at higher concentration and lower temperature compared to the parent triblock copolymer. The scaling exponent Δ is then somewhat smaller, i.e., 0.61. [less ▲]

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See detailTheoretical investigation of the molecular structure of aluminium triisopropoxide and its complexes in ring-opening polymerization
Parenté, Vincent; Brédas, Jean-Luc; Dubois, Philippe ULg et al

in Macromolecular Theory and Simulations (1996), 5(3), 525-546

We present a theoretical study on aluminium triisopropoxide (Al(O1Pr)3) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AMI ... [more ▼]

We present a theoretical study on aluminium triisopropoxide (Al(O1Pr)3) using both empirical (Molecular Mechanics, MM, with Dreiding II force field) and quantum-chemical (Austin Model 1, AMI, semiempirical Hartree-Fock) techniques. We determine the most stable geometries for both the tetramer and tiimer of aluminium triisopropoxide as well as the thermodynamic characteristics of the equilibrium existing between these two aggregated structures. The theoretical results are compared to experimental data from X-ray diffraction and 27Al NMR measurements. For the tetramer, it appears that the optimal equilibrium geometries are in good agreement with the experimental X-ray diffraction geometry; another geometry is also obtained with both theoretical approaches, which is slightly less stable but of higher symmetry. On the basis of the most stable configurations for the tetramer and trimer aggregates, the variation of free enthalpy (AG) between the two aggregated structures has been estimated. The evolution of the theoretical AG values indicates a displacement of equilibrium towards the trimer species with temperature, in good agreement with experimental 1H and 27Al NMR data. Moreover, the AMI heats of formation show a gain of 33.9 kcal/mol due to the aggregation of four Al(O1Pr)3 instead of three, and thus a better stability of the tetramer. The molecular geometries being well described by the theoretical methods used in this study, we also present a model for the ring-opening polymerization complexes of ε-caprolactone and lactides. [less ▲]

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See detailDifunctional initiators based on 1,3-diisopropenylbenzene. 3. Synthesis of a pure dilithium adduct and its use as difunctional anionic polymerization initiator
Yu, Y. S.; Dubois, Philippe ULg; Jérôme, Robert ULg et al

in Macromolecules (1996), 29(8), 2738-2745

The addition reaction of tert-butyllithium (t-BuLi) to 1,3-diisopropenylbenzene (1,3-DIB) has been studied. Under optimized conditions, i.e., reaction in cyclohexane at −20 °C and dropwise addition of 1,3 ... [more ▼]

The addition reaction of tert-butyllithium (t-BuLi) to 1,3-diisopropenylbenzene (1,3-DIB) has been studied. Under optimized conditions, i.e., reaction in cyclohexane at −20 °C and dropwise addition of 1,3-DIB to a 1:1 t-BuLi−triethylamine (Et3N) complex followed by a 1 h postreaction period, a pure diadduct is formed without concomitant oligomerization of 1,3-DIB. The behavior of this diadduct has been investigated, in apolar and polar solvents; triblock copolymers have been prepared and found to have interesting mechanical properties. [less ▲]

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See detailOligomerization and copolymerization of γ-butyrolactone - a monomer known as unable to homopolymerize, 1. Copolymerization with epsilon-caprolactone
Duda, Andrzej; Penczek, Stanislaw; Dubois, Philippe ULg et al

in Macromolecular Chemistry and Physics (1996), 197(4), 1273-1283

Copolymerization of γ-butyrolactone (γBL) with ε-caprolactone (εCL) initiated with aluminium isopropoxide trimer ([Al(OiPr)3 ]3, (A3)) is described. Copolymers with molecular weights ( ) up to 3 ; 104 and ... [more ▼]

Copolymerization of γ-butyrolactone (γBL) with ε-caprolactone (εCL) initiated with aluminium isopropoxide trimer ([Al(OiPr)3 ]3, (A3)) is described. Copolymers with molecular weights ( ) up to 3 ; 104 and containing up to 43 mol-% repeating units derived from γBL are prepared. Their molecular weight is controlled by the concentrations of the consumed comonomers and the starting concentration of initiator { = (86.09 ; [γBL]c + 114.14 ; [εCL]0)/3[Al(OiPr)3] + 60.10}. 13C NMR and DSC data are indicative of a pseudoperiodic or random copolymer structure. [less ▲]

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See detailMacromolecular engineering of polylactones and polylactides. 19. Kinetics of ring-opening polymerization of epsilon-caprolactone initiated with functional aluminum alkoxides
Dubois, Philippe ULg; Ropson, Nathalie; Jérôme, Robert ULg et al

in Macromolecules (1996), 29(6), 1965-1975

End-functional aliphatic polyesters have been synthesized by ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) and lactides (LA). Polymerization initiated with any of the functional ... [more ▼]

End-functional aliphatic polyesters have been synthesized by ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) and lactides (LA). Polymerization initiated with any of the functional aluminum alkoxides of the general structure Et(3-p)Al(OCH(2)X)(p), where p = 1 and 3, obeys a living ''coordination-insertion'' mechanism, under suitable conditions (temperature, concentration, and solvent), with the formation of alpha-functional chains of a predictable molecular weight. A kinetic study has emphasized the importance of the cyclic monomer (epsilon-CL or LA) and the key role of the initiator structure: the number of functional alkoxides per aluminum and the nature of the functional group (X = CH2Br, (CH2)(2)CH=CH2, (CH2)(2)NEt(2)). Activation enthalpy and entropy have been calculated for the epsilon-CL polymerization initiated by several aluminum alkoxides at various temperatures. The effects of solvent (toluene or tetrahydrofuran) and purification of the aluminum monoalkoxides by distillation under reduced pressure have also been considered. [less ▲]

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See detailIdentification and overexpression in Escherichia coli of a Mycobacterium leprae gene, pon1, encoding a high-molecular-mass class A penicillin-binding protein, PBP1
Basu, Joyoti; Mahapatra, Sebabrata; Kundu, Manikuntala et al

in Journal of Bacteriology (1996), 178(6), 1707-1711

Cosmid B577, a member of the collection of ordered clones corresponding to the genome of Mycobacterium leprae, contains a gene, provisionally called pon1, that encodes an 821-amino-acid-residue high ... [more ▼]

Cosmid B577, a member of the collection of ordered clones corresponding to the genome of Mycobacterium leprae, contains a gene, provisionally called pon1, that encodes an 821-amino-acid-residue high-molecular-mass class A penicillin-binding protein, provisionally called PBP1. With similar amino acid sequences and modular designs, M. leprae PBP1 is related to Escherichia coli PBP1a and PBP1b, bienzymatic proteins with transglycosylase and transpeptidase activities. When produced in E. coli, His tag-labelled derivatives of M. leprae PBP1 adopt the correct membrane topology, with the bulk of the polypeptide chain on the surface of the plasma membrane. They defy attempts at solubilization with all the detergents tested except cetyltrimethylammonium bromide. The solubilized PBP1 derivatives can be purified by affinity chromatography on Ni2+-nitrilotriacetic acid agarose. They have low affinities for the usual penicillins and cephalosporins. [less ▲]

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See detailDifunctional initiator based on 1,3-diisopropenylbenzene. 2. Kinetics and mechanism of the sec-butyllithium/1,3-diisopropenylbenzne reaction
Yu, Y. S.; Dubois, Philippe ULg; Jérôme, Robert ULg et al

in Macromolecules (1996), 29(5), 1753-1761

Kinetics of the addition of s-butyllithium (s-BuLi) to 1,3-diisopropenylbenzene (1,3-DIB) has been studied, and the activation energy has been found to be 17.5 kcal/mol. Addition of s-BuLi to 1,3-DIB in a ... [more ▼]

Kinetics of the addition of s-butyllithium (s-BuLi) to 1,3-diisopropenylbenzene (1,3-DIB) has been studied, and the activation energy has been found to be 17.5 kcal/mol. Addition of s-BuLi to 1,3-DIB in a 2/1 molar ratio results in an oligomer rather than the desired diadduct. This observation has to be compared to the anionic polymerization of 1,3-DIB (hydrocarbon solvent at 50 °C) which leads to a polymer bearing ca. one double bond per monomeric unit. Li-NMR analysis of the addition of s-BuLi to 1,3-DIB (2/1 molar ratio) confirms the nonequivalence of the lithium cations and shows that ca. 30% s-BuLi remains unreacted when 1,3-DIB has been completely consumed. In the early stage of the addition, the propagation rate is faster than depropagation and the average molecular weight increases. When a depropagation−propagation quasi-equilibrium is reached, the oligomer molecular weight remains constant. It finally decreases when no DIB is left, as a result of depropagation. When the s-BuLi/1,3-DIB adduct is used as an initiator for styrene polymerization in the presence of a polar solvent, such as THF, initiation by monofuntional, difunctional, trifunctional, and tetrafunctional species is observed. In an apolar solvent, however, polystyrene of a narrow molecular weight distribution and a functionality of 2 is formed. This experimental observation more likely indicates that depolymerization of the 1,3-DIB oligomers occurs upon the addition of styrene. A mechanism which accounts for this propagation−depropagation process has been proposed and experimentally supported. [less ▲]

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See detailMacromolecular engineering of polylactones and polylactides. 21. Controlled synthesis of low molecular weight polylactide macromonomers
Barakat, Ibrahim; Dubois, Philippe ULg; Grandfils, Christian ULg et al

in Journal of Polymer Science. Part A, Polymer Chemistry (1996), 34(3), 497-502

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See detailPolymerization of β-butyrolactone initiated with Al(OiPr)3
Kurcok, Piotr; Dubois, Philippe ULg; Jérôme, Robert ULg

in Polymer International (1996), 41(4), 479-485

Polymerization of gamma-butyrolactone has been studied in toluene with Al(OiPr)3 as an initiator. The ring-opening polyaddition proceeds through a coordination-insertion mechanism at a very low rate. Well ... [more ▼]

Polymerization of gamma-butyrolactone has been studied in toluene with Al(OiPr)3 as an initiator. The ring-opening polyaddition proceeds through a coordination-insertion mechanism at a very low rate. Well defined -isopropylester, -hydroxy poly(-butyrolactone)s (PBL) are formed with a narrow molecular weight distribution at low monomer-to-initiator molar ratios, When this ratio is higher (ca. 170), a competition occurs between propagation and side reactions, i.e. elimination, inter- and intra-molecular transesterifications and thermal degradation, which is responsible for a loss of control of the PBL molecular charcteristics. The addition of a Lewis base (1 equivalent of nicotine/Al) to the Al-alkoxide initiator has no significant effect on the polymerization rate, although the chain microstructure is deeply affected since predominantly syndiotactic PBL chains are formed (63% syndio-diads) in contrast to a completely atactic polymer in the absence of nicotine. [less ▲]

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See detailAn improved preparation method of benzyl and thenyl triphenylphosphonium salt
Zhang, Jian-Xin; Dubois, Philippe ULg; Jérôme, Robert ULg

in Synthetic Communications (1996), 26(16), 3091-3095

New benzyl and thenyl phosphonium salts have been prepared by an improved method in solution from the parent alcohols and triphenylphosphine hydrobromide. Benzyl-type alcohols containing a tertiary amine ... [more ▼]

New benzyl and thenyl phosphonium salts have been prepared by an improved method in solution from the parent alcohols and triphenylphosphine hydrobromide. Benzyl-type alcohols containing a tertiary amine can also be converted within high yields to phosphonium salts by removing the formed water by azeotropic distillation. Solvent (methylene chloride, chloroform and acetonitrile) and temperature have been chosen in close relation to the alcohol reactivity. Purification is straightforward and yields are relatively high. [less ▲]

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See detailMacromolecular engineering of polylactones and polylactides. 20. Effect of monomer, solvent, and initiator on the ring-opening polymerization as initiated with aluminum alkoxides
Ropson, Nathalie; Dubois, Philippe ULg; Jérôme, Robert ULg et al

in Macromolecules (1995), 28(23), 7589-7598

Structure and coordinative association of the propagating species have been studied by H-1, Al-27, and C-13 NMR and viscosimetry in case of epsilon-caprolactone (E-CL) and lactides (LA) polymerization ... [more ▼]

Structure and coordinative association of the propagating species have been studied by H-1, Al-27, and C-13 NMR and viscosimetry in case of epsilon-caprolactone (E-CL) and lactides (LA) polymerization initiated by aluminum alkoxides. In toluene, aluminum triisopropoxide, a commonly used initiator, forms tetrameric (A(4)) and/or trimeric (A(3)) aggregates, which are dissociated upon the addition of the cyclic monomer. Actually, LA is responsible for the complete deaggregation of both A(3) and A(4) into single species (A(1)) trisolvated by the monomer (Al coordination number, CN = 6), whereas A(3) is selectively deaggregated by epsilon-CL. This behavior is consistent with the higher stability of A(4) compared to A(3). Whatever the monomer, three chains are initiated by the active A(1) species. When conversion of epsilon-CL is complete, the active aluminum trialkoxide sites are solvated by one (CN = 4) and more probably three (CN = 6) carbonyl groups of the attached poly-epsilon-caprolactone chains. At the complete LA conversion, only one carbonyl group of the polylactide chains is coordinated to Al (CN = 4). In THF, there is a competition between monomer and solvent for coordination to Al, which explains why polymerization is slower in THF than in toluene. In both toluene and THF, diethylaluminum monoalkoxide forms cyclic trimers that are solvated by the monomer (epsilon-CL) at least on the NMR time scale. Nevertheless, a fractional kinetic order in initiator is observed which gives credit to a fast reversible dissociation of the epsilon-CL-solvated trimers. [less ▲]

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See detailAnionic polymerization of pivalolactone initiated by alkali metal alkoxides
Jedlinski, Zbigniew; Kurcok, Piotr; Kowalczuk, Marek et al

in Macromolecules (1995), 28(21), 7276-7280

Polymerization of pivalolactone (alpha,alpha-dimethyl-beta-propiolactone) initiated with alkali metal alkoxides has been studied. It has been revealed that after addition of an alkoxide anion, from the ... [more ▼]

Polymerization of pivalolactone (alpha,alpha-dimethyl-beta-propiolactone) initiated with alkali metal alkoxides has been studied. It has been revealed that after addition of an alkoxide anion, from the initiator, onto the carbonyl carbon atom of the monomer the selective cleavage of the acyl-oxygen bond of the monomer leads to the formation of alkoxide propagating species. Thus, it has been demonstrated that, in contrast to alpha-unsubstituted beta-lactones and higher lactones, the anionic polymerization of pivalolactone proceeds through either alkoxide or carboxylate propagation centers, depending on the nature of initiator used, i.e. an alkoxide or a carboxylate, respectively. In the former case, however, cyclic oligomers are also formed which are unusual in the anionic polymerization of other beta-lactones. This indicates that, whatever the lactone ring size (four-, six-, or seven-membered lactones), intramolecular transesterification reactions can take place if alcoholate ions are propagating species. [less ▲]

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See detailAnalyse morphologique par microscopie à force atomique de films minces de copolymères et de mélanges de polymères
Leclère, P; Viville, P; Lazzaroni, R et al

Conference (1995, September 02)

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See detailSubstituent effect in anionic polymerization of β-lactones initiated by alkali metal alkoxides
Kurcok, Piotr; Matuszowicz, Andrzej; Jedlinski, Zbigniew et al

in Macromolecular Rapid Communications (1995), 16(7), 513-519

The influence of methyl substituent on the mechanism of the ring-opening polymerization of -lactones initiated by alkali metal alkoxides is discussed. Attention has been paid to the effect of the ... [more ▼]

The influence of methyl substituent on the mechanism of the ring-opening polymerization of -lactones initiated by alkali metal alkoxides is discussed. Attention has been paid to the effect of the substituent position in the monomer molecule on the ring-opening mechanism, the 3,3-dimethyl-2-oxetanone (pivalolactone), 4-methyl-2-oxetanone (beta-butyrolactone) and 2-oxetanone (beta-propiolactone) being chosen as model monomers. Moreover, it was found unexpectedly that in the case of pivalolactone polymerization, besides open-chain polymers, cyclic oligomers are produced. [less ▲]

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See detailRecent advances in ring-opening polymerization of lactones and related compounds
Lögfren, Anders; Albertsson, Ann-Christine; Dubois, Philippe ULg et al

in Journal of Macromolecular Science - Reviews in Macromolecular Chemistry and Physics (1995), C35(3), 379-418

Polyesters are currently synthesized by a step-growth mechanism from a mixture of a diol and a diacid (or a diacid derivative), or from a hydroxy-acid when available. Ring-opening polymerization (ROP) of ... [more ▼]

Polyesters are currently synthesized by a step-growth mechanism from a mixture of a diol and a diacid (or a diacid derivative), or from a hydroxy-acid when available. Ring-opening polymerization (ROP) of lactones and related compounds is an alternative method for the synthesis of aliphatic polyesters. Comparison of these two mechanisms is clearly in favor of the polyaddition process. The drawbacks of polycondensation are well known:high temperatures and long reaction times are required to produce high-molecular-weight chains. Even though conversion of the hydroxyl and acid group is close to completion, any departure from the reaction stoichiometry has a very detrimental effect on the chain length. [less ▲]

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See detailPolylactone based Biomedical Materials : Chemistry and Breakthrough Applications
Dubois, Philippe ULg; Barakat, I; Degée, P et al

Poster (1992, November 22)

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See detailSynthetic biomedical polymers
Grandfils, Christian ULg; Dubois, Philippe ULg; Barakat, I et al

Poster (1991, November 28)

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See detailSynthetic Biomedical Polymers (Part B)
Nihant, N; Schugens, C; Barakat, I et al

Poster (1991, May 31)

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