Alain Deffieux

Synthesis of Macrocyclic Copolymer Brushes and Their Self-Assembly into Supramolecular Tubes

We report on an efficient route to design large macrocyclic polymers of controlled molar mass and narrow dispersity. The strategy is based on the synthesis of a triblock copolymer ABC, in which the long central block B is extended by two short A and C sequences bearing reactive antagonist functions. When reacted under highly dilute conditions, this precursor produces the corresponding macrocycle by intramolecular coupling of the A and C blocks. Chloroethyl vinyl ether was selected as the monomer for the central block B, because it can be readily derivatized into brushlike polymers by a grafting process. The corresponding macrocyclic brushes were decorated with polystyrene or randomly distributed polystyrene and polyisoprene branches. In a selective solvent for the polyisoprene branches, the macrocyclic brushes self-assemble into cylindrical tubes of up to 700 nanometers.

Reactivity of Metallocene Catalysts for Olefin Polymerization: Influence of Activator Nature and Structure

In this review, various synthetic and structural aspects of methylaluminoxane and the elementary processes involved in olefin polymerization, as identified by various spectroscopic techniques, are detailed. Various boron-based counterions and their interactions with metallocenium cations are discussed. A comparison between MAO and boron-based activators in terms of catalytic activity and polymer characteristics is made. Finally, the use of the above system in the polymerization of cationically polymerizable monomers and acrylates is discussed.

Evaluation of the heterogeneity in linear low-density polyethylene comonomer unit distribution by differential scanning calorimetry characterization of thermally treated samples

Abstract A new procedure for analysing the structure of linear low-density polyethylene (LLDPE) based on differential scanning calorimetry (d.s.c.) has been developed. LLDPE with heterogeneous and homogeneous distributions of comonomer units have been annealed in stages at decreasing temperatures starting from the melt. With this new procedure, it has been possible to show on d.s.c. thermograms the appearance of various families of macromolecules in the case of heterogeneous copolymers. In the case of more homogeneous copolymers, various families are still present in the material but in a smaller number. Furthermore, using the various melting peaks that appeared on the d.s.c. thermogram during the heating of the annealed polymer and with the help of a calibration curve, it has been possible to determine qualitatively the composition of the LLDPE.

Synthesis, dimensions and solution properties of linear and macrocyclic poly(chloroethyl vinyl ether)-g-polystyrene comblike polymers

The grafting of polystyryllithium onto chloroethyl ether functions of poly(chloroethyl vinyl ether) (PCEVE) chains is a selective reaction which allows the complete substitution of chloride of CEVE units by polystyrene (PS) chains. Since the PCEVE polymer backbone and the PS grafts can be prepared by living cationic and anionic polymerization, respectively, it is possible to synthesize PCEVE-g-PS polymers possessing both a backbone with controlled dimensions and an adjustable number of branches of precise length. This procedure was applied to the synthesis of a series of linear and macrocyclic comblike PCEVE-g-PS based on the use of linear and cyclic PCEVE as reactive backbone, respectively. The synthesis, the characterization and the solution properties of these linear and cyclic comblike polymers are reported.

Isopentenyl diphosphate isomerase: A checkpoint to isoprenoid biosynthesis

Even if the isopentenyl diphosphate (IPP) isomerases have been discovered in the 50s, it is only in the last decade that the genetical, enzymatical, structural richness and cellular importance of this large family of crucial enzymes has been uncovered. Present in all living kingdoms, they can be classified in two subfamilies: type 1 and type 2 IPP isomerases, which show clearly distinct characteristics. They all perform the regulatory isomerization of isopentenyl diphosphate into dimethylallyl diphosphate, a key rate-limiting step of the terpenoid biosynthesis, via a protonation/deprotonation mechanism. Due to their importance in the isoprenoid metabolism and the increasing interest of industry devoted to terpenoid production, it is foreseen that the biotechnological development of such enzymes should be under intense scrutiny in the near future.

Imaging of catenated, figure-of-eight, and trefoil knot polymer rings.

Most of the available characterization techniques for macromolecules are based on the determination of the average properties (chemical composition, tacticity, dimensional parameters, melting temperature, etc) of an entire set of molecules in a given system. In this context, imaging techniques could offer a quite different and unique approach that allows the analysis of macromolecules as single objects. However, isolated polymer chains are often too diffuse to be distinctly and directly visualized. To circumvent this problem, a magnification process of the size and density of macromolecules prior to imaging can be used to homogeneously enlarge isolated polymer chains before observation by atomic force microscopy (AFM). Numerous studies have been devoted over the past decades to the investigation of the properties of isolated biomolecules by molecular imaging. The visualization of single DNA molecules has shown that DNA can exist in the form of single and interlocked rings of various complexities. 4] Since then, several examples of catenated and knotted singleand double-stranded DNA rings have been reported. Continuous progress in this area is clearly associated with the development of imaging techniques that allow the investigation of the molecular structure 11] and mechanical properties of isolated macromolecules. Besides biomacromolecules, a limited number of reports have addressed the imaging of isolated polyelectrolyte chains, and even fewer studies of the AFM investigation of neutral polymer chains at the molecular level have been reported. Indeed, neutral linear polymer chains, which have sizes that range from a few to several hundred nanometers, possess a cross section of only a few atoms and are softer than biomolecules and polyelectrolytes. Moreover, their local concentration can widely fluctuate through chain motion because of weaker interactions with substrates. This fluctuation makes the linear polymer chains highly diffuse objects that cannot be easily observed by imaging microscopy. To increase the contrast of the images, selective adsorption of salts has been used by Minko and co-workers in the case of polyelectrolyte chains. To date, AFM visualization and characterization of single synthetic non-ionic macromolecules has been mostly limited to macromolecules with invariant shape such as carbon nanotubes, comb polymers,and dendrigrafts, which possess higher mass densities along their backbone than linear polymers. Therefore, one method to permit molecular imaging of single linear, cyclic, and starlike main-chain polymers could involve magnification of the macromolecules by homogeneously increasing their mass density along the whole of the chain, thus making their cross-section thicker and more readily detectable by the AFM tip. This magnification can be achieved, for example, by uniform grafting of well-defined oligomers onto the backbone of the target macromolecules, a strategy that requires long polymer chains with anchoring sites homogeneously distributed along the backbone chain for “grafting-onto” or “grafting-from” processes. These reaction procedures are common in the synthesis of comb polymers, which, because of their highly branched structure, have been thoroughly characterized at the molecular level by AFM imaging. 29, 31] To obtain a representative image of the starting macromolecule, the grafting reaction should preserve the chain integrity and its architecture in order to give a homogeneous translation of its initial dimensions. These criteria mean that the grafting reaction should be highly selective and efficient, and currently hamper this approach and restrict the number of macromolecules with an appropriate number of homogenously distributed grafting sites of suitable reactivity. When these conditions are fulfilled, magnified macromolecules can be prepared and individually visualized and characterized as representative models of the initial polymer chains. This approach is illustrated in Figure 1 for a ring polymer synthesis. The architecture, shape, and dimensions of each macromolecule that constitutes a given polymer system can indeed be determined by using this approach. The visualization of the targeted macromolecules as well as the presence and characteristics of other macromolecular structures that form through secondary reaction processes is also permitted. Very recently, we applied this approach to the characterization of macrocyclic polymers formed by end-to-end cyclization of a heterofunctional precursor. The linear precursor used for the cyclization was an ABC triblock copolymer comprising a long central B block surrounded by two short A and C sequences that bear mutually reactive functions. The main B block consisted of chloroethyl vinyl ether (CEVE) units, each with a reactive site for anchoring side chains (Figure S1 in the Supporting Information). After the cyclization step, the reactive chloride group of the CEVE units was used to graft polystyryl lithium chains of [*] M. Schappacher, A. Deffieux CNRS; Universit Bordeaux Laboratoire de Chimie des Polym res Organiques, ENSCPB 16, Avenue Pey Berland, 33607 Pessac Cedex (France) Fax: (+ 33)5-5640-8487 E-mail: deffieux@enscpb.fr

Ring-Opening Polymerization of L-Lactide Efficiently Triggered by an Amido-Indole. X-ray Structure of a Complex between L-Lactide and the Hydrogen-Bonding Organocatalyst

N-(3,5-Bis(trifluoromethyl)phenyl)-1H-indole-2-carboxamide 1e is an efficient hydrogen-bonding organocatalyst for the ring-opening polymerization of l-lactide. This new catalytic species does control the dispersity (1.08) and molecular weight (3460 g/mol vs 3064 in theory) of the poly(l-lactides) prepared in 2 h. (1)H NMR analysis showed that compound 1e complexes l-lactide in CDCl(3) through the two available NH groups (amide and indole). In particular, the catalytic species appeared to be mainly the H-bonding donor amide (1e in extended conformation, alone or dimer (1e)(2)) and, to a lesser extend, the dual H-bonding amido-indole (1e in its the pinched conformation). The first X-ray structure of the complex between a H-bonding organocatalyst and l-lactide also revealed a tight H-bonded network between the dimer (1e)(2) and l-lactide.

Rubber Elongation Factor (REF), a Major Allergen Component in Hevea brasiliensis Latex Has Amyloid Properties

REF (Hevb1) and SRPP (Hevb3) are two major components of Hevea brasiliensis latex, well known for their allergenic properties. They are obviously taking part in the biosynthesis of natural rubber, but their exact function is still unclear. They could be involved in defense/stress mechanisms after tapping or directly acting on the isoprenoid biosynthetic pathway. The structure of these two proteins is still not described. In this work, it was discovered that REF has amyloid properties, contrary to SRPP. We investigated their structure by CD, TEM, ATR-FTIR and WAXS and neatly showed the presence of β-sheet organized aggregates for REF, whereas SRPP mainly fold as a helical protein. Both proteins are highly hydrophobic but differ in their interaction with lipid monolayers used to mimic the monomembrane surrounding the rubber particles. Ellipsometry experiments showed that REF seems to penetrate deeply into the monolayer and SRPP only binds to the lipid surface. These results could therefore clarify the role of these two paralogous proteins in latex production, either in the coagulation of natural rubber or in stress-related responses. To our knowledge, this is the first report of an amyloid formed from a plant protein. This suggests also the presence of functional amyloid in the plant kingdom.

Use of “TMA-depleted” MAO for the activation of zirconocenes in olefin polymerization

The behavior of two differently prepared methylaluminoxanes (MAOs) on the activation process of racEt(Ind)2ZrCl2 for olefin polymerization was studied. Contrarily to commercial product, the two MAOs used are characterized by a low content of free trimethylaluminium (TMA). MAO-A was prepared by pumping off the TMA contained in commercial MAO, whereas MAO-B was obtained by thermal treatment of TMA with benzophenone. The different elementary steps in the zirconocene activation process depending on the amount and nature of added MAO (commercial and modified ones) were investigated by UV/visible spectroscopy. In all cases, monomethylation of racEt(Ind)2ZrCl2 rapidly takes place after addition of small amounts of MAO. Then, abstraction of the second chloride of the metallocene occurs upon addition of larger amounts of MAO (Al/Zr<200) leading to the formation of highly active catalytic species for hexene polymerization. This is characterized by a bathochromic shift of the zirconocene absorption band. The role of TMA is clearly pointed out by comparing the modified MAO/zirconocene systems with the one based on commercial MAO. The influence of structure and composition of MAO on the activation process is also clearly underlined by the very different amounts of activator needed to get metallocene active species in the presence of commercial and modified cocatalysts.

Activation of iPr(CpFluo)ZrCl2 by methylaluminoxane, 4. UV/visible spectroscopic study in hydrocarbon and chlorinated media

The olefin polymerization catalytic system iPr(CpFluo)ZrCl 2 /MAO was investigated by UV/ visible absorption spectroscopy in solvents of various polarity at 20°C. It was shown that the UV/visible main absorption band of the zirconocene, which can be related to the Ligand to Metal Charge Transfer bands (LMCT), varies greatly upon incremental addition of MAO or AlMe 3 . For low [AlMe 3 ]/[Zr] or [MAO]/[Zr] ratios (Al:Zr 1000) with this system is attributed to a reduction of active species concentration, as indicated by the intensity decrease of their UV/visible absorption band. This reveals the low stability of active species formed from iPr(CpFluo)ZrCl 2 compared to those from rac-Et(Ind) 2 ZrCl 2 .