Sébastien Maria

Hiromu Kaneyoshi

Krzysztof Matyjaszewski

Rinaldo Poli
email: poli@lcc-toulouse.fr

Effect of Electron Donors on the Radical Polymerization of Vinyl Acetate Mediated by [Co(acac)]2: degenerative transfer versus reversible homolytic cleavage of an organocobalt(III) complex

Keywords: Controlled radical polymerization, organometallic, homolytic bond cleavage

The molecular structure of bis(acetylacetonate)cobalt(II) (Co(acac)2) in solution and in the presence of the electron donors (ED) pyridine (py), NEt3 and vinyl acetate (VOAc) was investigated using 1 H NMR spectroscopy in C6D6. The extent of formation of ligand adducts, Co(acac)2(ED)x, varies in the order py > NEt3 > VOAc (no interaction). Density functional theory (DFT) calculations on model system agree with Co-ED bond strengths decreasing in the same order. The effect of electron donors on the Co(acac)2mediated radical polymerization of VOAc was examined at 30 o C by the addition of excess py or NEt3 to the complex in the following molar ratio ([VOAc]0/[Co]0/[V-70]0/[py or NEt3]0=500/1/1/30). Whereas, as previously reported (A. Debuigne, J.-R. Caille, R. Jerome, Angew. Chem. Int. Ed. 2005, 44, 1101.), the polymerization showed long induction periods in the absence of ED, a controlled polymerization process, though the level of control was poorer, took place without induction period in the presence of ED. The effective polymerization rate decreased in the order py > NEt3. A similar behavior was found when these electron donors were added to an ongoing Co(acac)2-mediated radical polymerization of VOAc. On the basis of the NMR and DFT studies, it is proposed that the polymerization is controlled by the reversible homolytic cleavage of an organocobalt(III) dormant species in the presence of ED. Conversely, the faster polymerization after the induction period in the absence of ED is due to a degenerative transfer process with the radicals produced by the continuous decomposition of the excess initiator. Complementary experiments provide additional results in agreement with this interpretation.

degenerative transfer, cobalt coordination chemistry, DFT calculations

Introduction

Recent progress in controlled radical polymerization (CRP) [START_REF] Ak | Controlled/Living Radical Polymerization: From Synthesis to Materials[END_REF] processes allows the synthesis of various well-defined polymer structures in a controlled fashion. Three different CRP mechanisms have been extensively investigated (Scheme 1). [START_REF] Goto | [END_REF] The first mechanism (eq. 1) is based on a spontaneous reversible homolytic cleavage of a dormant chain end and is exemplified by nitroxide-mediated polymerization (NMP) [3] or organometallic radical polymerization (OMRP). [4] The second process (eq. 2) involves a catalytic reversible homolytic cleavage of a carbon-halogen bond via a redox process, which occurs in atom transfer radical polymerization (ATRP). [5] The third one (eq. 3) is based on a thermodynamically neutral bimolecular exchange between propagating radicals and a dormant species. Degenerative transfer (DT) polymerization with alkyl iodides, [6] organotellurium or organostibine species [7] belongs in this category as well as reversible addition-fragmentation chain transfer (RAFT) [START_REF] Chiefari | Handbook of Radical Polymerization[END_REF] polymerization and macromolecular architecture design by interchange of xanthates (MADIX). [START_REF] Corpart | PCT Int. Appl.; WO[END_REF] Recently, organocobalt compounds have been shown to provide the controlled polymerization of acrylates, acrylic acid and vinyl acetate by this degenerative transfer mechanism. [START_REF] Ab | ACS National Meeting[END_REF] Insert Scheme 1 One of the most promising CRP processes for conducting a successful controlled polymerization of VOAc is the recently reported Co(acac)2-mediated polymerization. [4e, 4f] This was initiated with 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70) in the presence of bis(acetylacetonate)cobalt(II) (Co(acac)2), resulting in the formation of poly(vinyl acetate) (PVOAc) with predetermined Mn and low polydispersity. This system has been proposed to follow the OMRP mechanism (Scheme 1, equation 1 with X = Co(acac)2). There is, however, an important feature of this efficient polymerization system that needs clarification. It concerns the puzzling long induction times, when the polymerization is carried out in bulk. The offered explanation [4e] was based on the effect of the unreacted Co(acac)2 on the radical dissociation equilibrium (a manifestation of the persistent radical effect). In other words, the radical dissociation equilibrium would lie far on the side of the organocobalt(III) species until nearly all the Co(acac)2 complex is consumed, followed by the established of an equilibrium that is suitably placed to insure a rapid and controlled polymerization process. However, it is hard to accept that a persistent radical effect could result in such a sharp transition for the free radicals concentration. We will herein propose an alternative explanation to account for all the observed data.

The second feature is related to the exact nature of the Co(acac)2 species in solution under the polymerization conditions. The nature of Co(acac)2 in noncoordinating solvents has been addressed by Cotton and Soderberg on the basis of visible and near-IR spectroscopy. The compound is mononuclear tetrahedral in dilute solutions, whereas it affords octahedral coordination through the formation of oligonuclear aggregates at higher concentrations. [11] An early X-ray diffraction study of anhydrous Co(acac)2, obtained by sublimation, showed a tetranuclear Co4(acac)8 arrangement where octahedral coordination for the Co II ions is achieved by sharing of oxygen donors. [12] A different polymorph, described as containing square-planar mononuclear units, has been characterized more recently. [13] The authors underline the absence of significant intermolecular interactions, but a closer look at the molecular packing (structure retrieved from the Cambridge Crystallographic Data Centre) [13] reveals that the molecules are in fact loosely stacked on top of each other as shown in ScScheme 2(a). Thus, each Co atom is effectively 6coordinated, with the 5 th and 6 th axial coordination positions being occupied by thedelocalized electron density of the acac ligand of neighboring molecules. A sterically more encumbered analogue, Co[tBuC(O)CHC(O)tBu]2, on the other hand, adopts a tetrahedral structure rather than a square planar arrangement, as shown in ScScheme 2(b). [14] It is also known that the addition of small amounts of neutral donors yields complex equilibria. For instance, the addition of pyridine yields complexes having py/Co ratios of 1:2, 1:1 and 2:1, the first two species being described as dinuclear, [Co(acac)2]2(py) and [Co(acac)2(py)]2. [15] Indeed, X-ray crystallography shows that the analogous 1:1 adduct with water is dinuclear, [Co(acac)2(H2O)]2, [16] but a mononuclear 5-coordinate Co(acac)2(ED) adduct (ED = electron donor) has also been crystallographically characterized for ED = 2aminopyridine. [17] Many EDs have been shown to add to Co(acac)2 and yield 1:2 adducts, Co(acac)2(ED)2. X-ray crystallographic analyses of numerous derivatives (e.g. ED = H2O, [16,18] MeOH, [19] pyridine, [20] imidazole, [21] …), revealed that these complexes have an trans-octahedral geometry, as shown in ScScheme 2(c). All these observations indicate the propensity of Co(acac)2 to establish weak coordinative interactions with electron donors leading to octahedral coordination. A reasonable question to ask, therefore, is whether the monomer vinyl acetate might be able to establish a coordinative interaction with Co(acac)2 through the oxygen lone pairs of the ester function, since the bulk CRP of VOAc mediated by Co(acac)2 with V-70 is conducted in the presence of excess VOAc compared to Co(acac)2 ([VOAc]0/[Co]0>100) at 30 o C. [4e-h] Insert ScScheme 2 Moreover, the cobalt complex proposed as the dormant species in Scheme 1 (Equation 1, X = Co(acac)2) is only pentacoordinated, whereas Co III is known to strongly prefer a 6-coordinated, octahedral coordination geometry. This suggests that one molecule of reactant or other available ED may occupy the 6 th available coordination site. In this report, we explore the molecular structure of the Co(acac)2 complex in solution using 1 H NMR spectroscopy and DFT calculations. The effect of EDs such as py and triethylamine on the Co(acac)2-mediated polymerization of VOAc is also discussed. This work has allowed to correlate the catalytic performance of Co(acac)2 with the nature of EDs and to reveal a previously unsuspected interplay, which is tuned by the ED coordination, between OMRP and DT pathways.

Experimental Section

Characterization. [START_REF] Ak | Controlled/Living Radical Polymerization: From Synthesis to Materials[END_REF] H NMR spectra were collected at 30 o C using a Bruker 250 MHz spectrometer. Deuterated benzene and acetone-d6 were degassed and stocked in a Schlenk tube on molecular sieves, under argon. The vinyl acetate (VOAc) conversion was determined by gas chromatography (GC) using a Shimadzu GC 14-A gas chromatograph equipped with a FID detector and a ValcoBond 30 m VB WAX Megabore column or was calculated by the mass ratio before and after the complete removal of solvent and residual monomer by evaporation, until achievement of constant weight. Toluene or pdimethoxybenzene was used as an internal standard for GC. The molecular weight and molecular weight distribution of the PVOAc were measured by gel permeation chromatography (GPC) with PSS columns (styrogel 10 5 , 10 3 , 10 2 Å) and RI detector (molecular weights and polydispersity indices of the PVOAc were determined relative to linear poly(methyl methacrylate) calibration standards using toluene as an internal standard) or with a 300 × 7.5 mm PLgel 5-m Mixed-D column (Polymer Laboratories), equipped with multiangle light scattering (miniDawn Tristar, Wyatt Technology Corp.) and refractive index (RI2000, Soparès) detectors. GPC for the PVOAc was performed using THF as eluent at a flow rate of 1 mL/min at 35 o C Materials. Dinitrogen was purified by passing through a column of anhydrous calcium sulfate. Vinyl acetate (Aldrich, >99%) was passed through a neutral alumina column to remove the stabilizer, dried over calcium hydride, distilled under reduced pressure, and degassed with dinitrogen. Pyridine (Fisher Scientific, 99.9%) and triethylamine (NEt3, Aldrich, 99.5%) were dried over calcium hydride and degassed with dinitrogen or argon. Toluene (Fisher Scientific, >99%) was distilled over sodium/benzophenone and degassed with dinitrogen. Acetone (sds, >99%) was distilled over calcium sulfate and degassed with argon. Heptane (sds, >99,5%) was distilled over calcium hydride and degassed with argon. Bis(acetylacetonate)cobalt(II) (Co(acac)2; Acros, 99%), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70; Wako, 96%), pdimethoxybenzene (Aldrich, 99%), and deuterated benzene (Cambridge Isotope Lab, >99.5%D) were used as received.

Initial addition of electron donors to the Co(acac)2-mediated VOAc

polymerization with V-70. Generally, the polymerizations were conducted using standard Schlenk techniques. The monomer was deoxygenated by dinitrogen bubbling for 30 min prior to addition to the reaction flask. p-Dimethoxybenzene (internal standard for GC, 100 mg), Co(acac)2 (28.0 mg, 1.09  10 -4 mol) , and V-70 (33.5 mg, 1.09  10 -4 mol) were placed in a Schlenk flask (25 mL) equipped with a magnetic stirring bar at room temperature. This flask was evacuated in vacuum with cooling in a liquid nitrogen bath and backfilled with dinitrogen. This process was repeated three times with cooling. To this flask, VOAc (5.0 mL, 5.43  10 -2 mol) and pyridine (0.26 mL, 3.21  10 -3 mol) were added and the resulting mixture was stirred at room temperature (forming an orange solution).

After taking an initial sample for GC, the polymerization was started by heating at 30 o C.

Aliquots were withdrawn periodically by a syringe to follow the kinetics of the polymerization process. After 45 hours, the stirring bar stopped spinning due to the high viscosity of the polymerization medium. The lap samples were diluted with acetone prior to analysis by GC. After the GC measurement, the solvent was changed to THF for GPC. The polymerization in the presence of triethylamine was carried out by an identical procedure, except that NEt3 (0.45 mL, 3.23  10 -3 mol) was added instead of pyridine and deoxygenated toluene (0.2 mL) was used as an internal standard for GC.

Addition of electron donors to the ongoing Co(acac)2-mediated VOAc

polymerization with V-70. The basic procedure was similar to that described in the preceding section. Co(acac)2 (55.8 mg, 2.17  10 -4 mol) and V-70 (66.9 mg, 2.17  10 -4 mol) were placed in a Schlenk flask (25 mL) equipped with a magnetic stirring bar at room temperature. This flask was evacuated under vacuum with cooling in a liquid nitrogen bath and backfilled with nitrogen. This process was repeated three times with cooling. VOAc (10.0 mL, 1.09  10 -1 mol) and toluene (0.5 mL) were added to this flask, and the resulting mixture was stirred at room temperature (purple suspension). After taking an initial sample for GC, the mixture was heated at 30 o C to start the polymerization. Aliquots were withdrawn periodically by a syringe to follow the kinetics of the polymerization process.

After 38 h, two 2.5 mL aliquots of the reaction mixture (Co(acac)2, 5.42  10 -5 mol) were placed into separate deoxygenated Schlenk flasks at room temperature The appropriate amount of electron donor (pyridine (0.13 mL, 1.61  10 -3 mol) or NEt3 (0.23 mL, 1.65  10 - 3 mol)) was then injected by syringe. These two flasks and the initial flask were reheated at 30 o C in the same oil bath in order to continue the polymerization. The lap samples were diluted with acetone prior to analysis by GC. After the GC measurement, the solvent was changed to THF for GPC.

Co(acac)2-mediated VOAc polymerization with PVOAc macroinitiator.

Synthesis of PVOAC End-Capped by Cobalt Complex: Co(acac)2 (26 mg, 1.01  10 -4 mol) and V-70 (186.8 mg, 6.06  10 -4 mol) were placed in a Schlenk flask equipped with a magnetic stirring bar at room temperature. This flask was evacuated under vacuum with cooling in a liquid nitrogen bath and backfilled with argon. This process was repeated three times with cooling. VOAc (4.0 mL, 4.34  10 -2 mol) was added to this flask, and the mixture was heated to 30 o C to start the polymerization. After 16 h (43% conversion), the polymerization was quenched and VOAc was evaporated under vacuum. The polymer was then dissolved in acetone, (re)precipitated with heptane, filtered and dried under reduced pressure. The isolated PVOAc obtained as a pink solid (Mn = 8446; PDI = 1.14). The color of the isolated PVOAc was green when wet acetone was used.

Chain extension:

The PVOAc end-capped with cobalt, prepared as described above (441 mg, 5.2  10 -5 mol), was placed in a Schlenk flask equipped with a magnetic stirring bar at room temperature. This flask was evacuated under vacuum with cooling in a liquid nitrogen bath and backfilled with argon. This process was repeated three times with cooling. VOAc (6.0 mL, 6.50  10 -2 mol) was added to this flask, and the mixture was heated to 30 o C to start the polymerization. Aliquots were withdrawn periodically by a syringe to follow the kinetics of the polymerization process.

Computational details. All geometry optimizations were performed using the B3LYP three-parameter hybrid density functional method of Becke, [22] as implemented in the Gaussian03 suite of programs. [START_REF] Frisch | Gaussian 03, Revision B.04[END_REF] The basis functions consisted of the standard 6-31G** for all light atoms (H, C, O), plus the LANL2DZ function, which included the Hay and Wadt effective core potentials (ECP), [START_REF] Hay | [END_REF] for Co. The latter basis set was however augmented with an f polarization function ( = 0.8) in order to obtain a balanced basis set and to improve the angular flexibility of the metal functions. All geometry optimizations were carried out without any symmetry constraint and all final geometries were characterized as local minima of the potential energy surface (PES) by verifying that all second derivatives of the energy were positive. The unrestricted formulation was used for open-shell molecules. The mean value of the spin of the first-order electron wave function, which is not an exact eigenstate of S 2 for unrestricted calculations on open-shell systems, was considered to identify unambiguously the spin state. The value of <S 2 > at convergence was very close to the expected value of 0.75 for doublets, 2 for triplets, and 3.75 for quartets, indicating minor spin contamination.

Results

1 H NMR investigation of Co(acac)2 in the presence of EDs. As discussed in the introduction, Co(acac)2 is known to be capable to add electron donor (ED) molecules, establishing sometimes complex equilibria depending on the nature of the ED and the added amount. We wished to learn more about the interaction of Co(acac)2 and specific ED molecules under polymerization conditions by using spectroscopic tools. Co II coordination compounds are paramagnetic and could in principle be investigated by EPR spectroscopy.

However, the rapid spin relaxation associated to the quartet state does not allow the detection of a spectrum at room temperature or above. No EPR studies of Co(acac)2(ED)2 complexes have been reported to the best of our knowledge, and toluene solutions of complex Co(acac)2 itself are described as EPR-silent. [25] We have therefore turned to NMR investigations. Indeed, the rapid relaxation of the electron spin magnetization (conditions leading to very broad EPR resonances) favor the observation of relatively sharp (though paramagnetically shifted) NMR resonances. [START_REF] La Mar | NMR of paramagnetic molecules[END_REF] NMR investigations of Co II acetylacetonate complexes are unprecedented, to the best of our knowledge.

Dissolution of anhydrous Co(acac)2 in C6D6 yields a homogeneous purple solution which shows only one very broad resonance centered at ca. -1 ppm (w1/2 = 860 Hz), see Figure 1(a). We attribute this resonance to the twelve equivalent CH3 protons of the acac ligand. The signal corresponding to the CH protons could not be identified. We presume that it is either overshadowed by the broad CH3 resonance, or broadened beyond detection and lost in the baseline. Indeed, the metal spin density is probably transmitted more efficiently onto the acac CH proton because it is directly linked to the conjugated Co(acac) system. The addition of VOAc to this solution yields no significant color change and the Insert Figure 1 The addition of py to the Co(acac)2 solution results in an immediate color change from purple to orange, indicating the formation of Co(acac)2(py)x adducts. The color of trans-Co(acac)2(py)2 complex was reported to be orange. [20] The color change was accompanied by dramatic changes in the NMR spectrum, see Figure 1(b-d). The free ligand resonance is already visible after the addition of ca. 1 equiv. On the other hand, the Co(acac)2 resonance immediately disappears after the first py addition and is replaced by new resonances for the py adduct(s). This is consistent with the solution equilibria that have previously been established by the spectrophotometric study, [15] as summarized in Scheme 3. According to that study, the first addition leading to [Co(acac)2]2(py) is essentially quantitative, whereas the equilibrium constant K2 is only ca. 2.0. Thus, a significant amount of free py would remain present after the addition of 1 equivalent, together with a mixture of [Co(acac)2]2(py), Co(acac)2(py) and Co(acac)2(py)2. The resonances of the paramagnetic complexes, as well as that of free py, shift as a function of the added py amount. This indicates that an exchange occurs between the free and coordinated py molecules, and that this is in the slow exchange limit. Note, however, that only one set of resonances is observable for the cobalt complexes. Each resonance of this set becomes sharper and its chemical shift is convergent upon addition of a larger py excess, where the Co(acac)2(py)2 species is predominant. For each proton, the chemical shift difference () between different Co(acac)2(py)x complexes is likely greater than the  between free and coordinated pyridine. Therefore, the failure to observe individual sets of resonances for the individual cobalt complexes cannot be attributed to fast exchange. It is more likely that the resonances of complexes [Co(acac)2]2(py) and Co(acac)2(py) are not detected because they are broader, less intense (especially at the higher py/Co ratios) and more paramagnetically shifted. The last proposition is proven by the direction in which each resonance shifts as a function of the py/Co ratio. Inspection of the relative integrated intensities, paramagnetic shifts, and line widths leads to the resonance assignment given in Figure 1. It is interesting to note how the acac resonance continuously shifts and sharpens from the value of free Co(acac)2 to that of Co(acac)2(py)2. This behavior suggests that the observed resonance for pure Co(acac)2 results from the rapid equilibration between mononuclear and oligonuclear species. The small resonance of the p-py proton is not visible for the py/Co = 1 spectrum, because it is overshadowed by the stronger acac and free py resonances.

Insert Scheme 3

Contrary to the effect of pyridine and similarly to the effect of VOAc, the addition of NEt3 to the Co(acac)2 solution results in no significant color change. The NMR behavior, however, is different from both. After the addition of one equivalent, new broad peaks of relatively small intensity that can be attributed to free NEt3 become visible, as indicated by the black arrows in Figure 2 Co(acac)2 remains unshifted and unbroadened. These results can be rationalized on the basis of the same scheme invoked for the py system (Scheme 3). [15] The observations prove that NEt3, like py, forms adducts with Co(acac)2, but the equilibria are far less in favor of the addition products (i.e. K1, K2 and K3 are smaller). Like for the py system, an exchange occurs between free and coordinated NEt3 molecules. A bis adduct forms to a much lower extent or not at all, or its resonances are much broader because of a faster ligand exchange process. The observation of the unshifted Co(acac)2 resonance at low NEt3/Co ratios shows that this complex is not involved in the amine exchange process. This means that the NEt3 exchange probably involves the different NEt3 adducts, rather than dissociation to yield the Co(acac)2 starting compound.

Insert Figure 2 Since water is a possible contaminant of Co(acac)2 and since the VOAc polymerization was also successfully carried out under suspension and miniemulsion conditions, [4f, 4i] additional NMR experiments were carried out by deliberately adding water.

The addition of water to a C6D6 solution of Co(acac)2 caused the immediate precipitation of a pale solid, presumably Co(acac)2(H2O)2, which is described as sparingly soluble in nonpolar organic solvents. [16,18] In order to avoid the precipitation process, subsequent analyses were carried out in acetone-d6.

First, the spectrum of Co(acac)2 in acetone-d6 was recorded before the addition of water, see Figure 3(a). This shows two broad (w1/2 ca. 400 Hz) resonances at  ca. 6.8 and 9.4 that can be assigned either to the CH3 and CH resonances of the acac ligand in a single compound, or to the CH3 resonances of two different compounds. The latter possibility appears more likely since the CH resonance has not been observed for any of the other spectra (see also below). The nature of the second compound, however, is unclear. The shift and sharpening of the Co(acac)2 resonance from the value observed in C6D6 suggests that an interaction with the acetone solvent has taken place. It is interesting to compare this result with the lack of any interaction between Co(acac)2 and VOAc. Evidently, the replacement of the alkoxo group in VOAc by a methyl group in acetone significantly improves the donor properties of the carbonyl function. The subsequent addition of small amounts of water to this solution caused precipitation, indicating that the acetone ligands have been replaced by water molecules. It is also interesting to analyze the spectral changes observed upon addition of NEt3 in acetone-d6, see Figure 3(b), and to compare them with those observed in C6D6. The broad resonance centered around 6.8 remains visible, even after the addition of 3 equivalents of NEt3, and does not shift significantly with the amount of excess NEt3. At the same time, the free NEt3 resonance is paramagnetically shifted to a lesser extent relative to the corresponding experiment in C6D6 ( 6-7 in acetone-d6, vs. 8-7 in C6D6). This is in agreement with the expected smaller extent of NEt3 coordination to Co(acac)2, since the acetone solvent now competes for the coordination sites on Co II . The broad resonance observed for the Co(acac)2 solution around  9.4, on the other hand, is no longer visible. The addition, to this solution, of small amounts of water led again to the formation of a pale precipitate. However, subsequent addition of pyridine caused the redissolution of this precipitate and the formation of an orange solution, whose 1 H NMR spectrum is identical to that shown in Figure 1.

Insert Figure 3 In conclusion, these NMR experiments indicate a relative affinity of various ED molecules for coordination to Co(acac)2 in the order VOAc (no coordination) < NEt3 < acetone < H2O < pyridine. They also indicate the presence of an exchange between free and coordinated ED molecules, the rate of exchange being faster for the less strongly coordinated ligands. In the presence of excess ED, the cobalt center undergoes less ligand dissociation and exchange, as evidenced by the relatively sharp paramagnetically shifted resonances in the case of pyridine. For the less tightly bound ligands, the NMR spectrum is not visible and no structural information is furnished by the experiment, but equilibria between lower coordinate species, mono-and/or dinuclear, are probably occurring rapidly.

Computational investigation of the Co(acac)2 structure and interactions with

EDs. The adoption of a tetrahedral structure for Co[tBuC(O)CHC(O)tBu]2 [14] suggests an electronic preference for a tetrahedral environment around 4-coordinate Co(II), whereas a trans-octahedral geometry is adopted in the presence of EDs. However, the steric demand of the tBu substituents may be responsible for this choice. Previous spectrophotometric studies have provided evidence that Co(acac)2 is a tetrahedral monomer, like its tBu analogue, in dilute solution. [11] We have recurred to DFT calculations to further confirm this conclusion. Co(acac)2 was optimized in both the tetrahedral and the square planar geometry. In the former case, only the high spin (S = 3/2) configuration was considered, since no low-spin tetrahedral complex for any d 4 -d 7 metal is know with low field ligands. [START_REF] Huheey | Inorganic Chemistry. Principles of Structure and Reactivity[END_REF] The square planar geometry, on the other hand, generates a high-energy dx2-y2 orbital possible resulting in spin pairing, thus both doublet and quartet spin states were considered. The results of the calculations are listed in Table 1. In agreement with the experimental study, the lowest energy is found for the quartet tetrahedral geometry.

Although no symmetry was imposed in the calculation, the geometry converged to an (1.934 Å and 95.4°, respectively). [14b] The square planar geometry gave a stable minimum only in the doublet state, a few kcal mol -1 higher than the lower tetrahedral quartet.

Attempts to optimize a quartet square planar geometry led to the rearrangement to the tetrahedral minimum.

Insert Table 1

The main thrust of our computational investigation concerns the equilibria affecting the radical concentration under polymerization conditions, and how these are modified by the introduction of electron donors. We have therefore studies the binding of ED molecules to the Co(acac)2 catalyst and to the R-Co(acac)2 dormant species. However, we have limited our investigation to mononuclear complexes. The Co III . This phenomenon may be partially responsible for the additional stability of these adducts relative to the other ED's which lack active protons. However, we must also consider that the stabilization energy is calculated relative to isolated Co(acac)2 and H2O or NH3 molecules in the gas phase. In condensed phases (for instance, toluene solution) these protic ED molecules form aggregates, therefore reducing the energetic gain of the Co-H2O or Co-NH3 bond formation. The difference between NH3 and NMe3 could in part be due also to the increased steric demand imposed by the three Me groups. By extrapolation, we can predict that the binding energy of the Co-NEt3 bond will be smaller relative to the Co-py bond, in agreement with the NMR results.

The above trends remain unchanged upon going to the trans-Co(acac)2(ED)2 systems.

As shown in Table 1, binding the second ED molecule results in each case in a slightly greater energy gain relative to the first one, consistent with the general tendency of Co(acac)2 to form octahedral complexes rather than 5-coordinated ones by the addition of ED's. The optimized coordination geometries are in general quite close to octahedral, with the two acac ligands being approximately coplanar. The H2O and NH3 adducts exhibit a distortion which consists of a flap of the two acac ligands around the O•••O "hinge" axes, in opposite directions. This is particularly evident for the NH3 derivative, where the hinge angle (angle between the acac plane and the CoO2 plane) is 158.85°. The reason for this distortion is not obvious, but we note that this is also experimentally observed for the Co(acac)2(H2O)2 structure [16] (hinge angle of 162.8° vs. 164.0 for the optimized structure).

Upon going from the NH3 to the NMe3 adduct, the two Co-acac 6-member cycles go back to planarity, suggesting that H-bonding may again be the cause of the distortion. The optimized geometry of the pyridine adduct exhibits the two py orthogonal to each other.

This corresponds to the arrangement experimentally observed for trans-Co(acac)2(py)2 [20] and for trans-Co(acac)2(4-CH3py)2. [START_REF] Gulya | [END_REF] The optimized Co-O and Co-N distances (averages are 2.06 and 2.23 Å) are in good agreement with the experimentally observed ones (2.034(3) and 2.19(1) Å). [20] The conclusion of this theoretical investigation is that Co(acac)2 generally forms weak bonds with ED molecules, especially the carbonyl O atom of ester functions. This result is in qualitative agreement with the observation of oxygen-bridged octahedral coordination for tetrameric Co(acac)2 [12] and with the preference of a mononuclear tetrahedral structure for the sterically more encumbered tBu analogue. [14] Amines and pyridine bind more strongly, but the steric bulk of NEt3 should probably reduce the bond formation energy. The calculations overestimate the stability of the H2O and NH3 adducts (indeed, the NMR study proves that H2O binds less strongly than pyridine), because of the incomplete consideration of hydrogen-bonding interactions (neglected for the free ligands).

Computational investigation of the CH3-Co(acac)2 bond strength and interactions with EDs.

The salient results of this study are collected in Table 2. The optimization of 5coordinate CH3-Co(acac)2 molecule was attempted both as a trigonal bipyramid (tbp) and as a square pyramid (sp). The tbp calculation gave a stable minimum only in the triplet state. Attempts to optimize a tbp geometry in the singlet state led to a rearrangement and convergence of the more stable sp structure. Relative to the separate tetrahedral (S = 3/2) Co(acac)2 and CH3 radical, the singlet sp was more stable by 18.50 kcal mol -1 , whereas the triplet tbp was only stabilized by 6.52 kcal mol -1 . The Co III ion is known to be particularly stable in an octahedral coordination environment, thus the presence of ED molecules is expected to lead to dormant species of type Co(acac)2(PVOAc)(ED). The various ED molecules were added to the empty coordination site of the lower-energy sp CH3-Co(acac)2 molecule to yield trans-Co(acac)2(CH3)(ED) adducts. All these molecules were optimized only in the singlet state, as alkylcobalt(III) molecules are known to be diamagnetic. [29] Note, however, that no R-Co(acac)2 or R-Co(acac)2(ED) molecules have been previously

described to the best of our knowledge. Axial ED binding to either CH3-Co III (acac)2 or to Co II (acac)2(ED) results in very similar energetic gains for CH3COOCH3 (4.23 vs. 4.49 kcal mol -1 ), H2O (11.53 vs. 11.47 kcal mol -1 ) and NH3 (13.47 vs. 13.67 kcal mol -1 ). On the other hand, slightly stronger binding to the alkylcobalt(III) occurs for NMe3 (7.41 vs. 5.63 kcal mol -1 ) and especially for py (9.10 vs. 5.63 kcal mol -1 ).

Insert Table 2 The stereoelectronic properties of ED could affect the homolytic bond strength of the Co III -carbon bond (that is, on the energy difference between Co(acac)2(CH3)(ED) and the separate Co(acac)2(ED) and CH3 fragments), as previously demonstrated in the chemistry of vitamin B12. [30] However, the axial ED nature has only a slight effect on the Co III ). In the absence of ED additives, the initial color of the reaction medium was a purple slurry, and then it changed to a brown solution after 1 day, suggesting the formation of cobalt(III) species. A rapid polymerization started after an induction period (see the semilogarithmic plot in Figure 5) and the number average molecular weight increased more or less linearly with conversion, with relatively low polydisperties (see Figure 6). The raw data are also reported in tabular form as Supporting Information. The observed Mn are close to (slightly higher than) those expected for one growing chain per cobalt atom. All these observations are in line with the previous report. [4e] Note that our reaction conditions are identical with those used previously, except that we employed a V-70/Co ratio of 1:1 instead of 6.5:1. Consequently, the only major difference of our results with respect to those previously reported is the longer observed induction time (ca. 30 h).

Insert Figure 5 and Figure 6 For the experiment run in the presence of py (30 equiv), the purple slurry turned immediately to a clear orange solution, as described above. The mixture remained a clear orange solution until the end of polymerization. While this may suggest the persistence of the Co(II) species during the polymerization, note that R-Co(acac)2(ED) compounds have not previously been described, thus the formation of alkylcobalt(III) dormant species that would coincidentally have the same color cannot be excluded. In the case of NEt3, the reaction medium quickly turned to a violet colored slurry, and then it became a mixture of a brown solution and pale purple insoluble materials after 1 day. The semilogarithmic kinetic plots for the monomer consumption are shown in Figure 5. The first striking observation is that, in the presence of py or NEt3, the polymerizations did not show any induction period.

The linearity of the plots suggests that the polymerizations are controlled, which is further confirmed by the evolution of Mn and Mw/Mn versus monomer conversion (see Figure 6).

For both polymerizations, Mn increased with conversion and remained close to the theoretical values, while the polydispersity remained in the 1.3 -1.5 range and all of the GPC curves displayed a monomodal shape (representative chromatograms are available as Supporting Information). Although these values are higher than for the PVOAc prepared in the absence of ED, they are typical of CRP processes. The significance of the Mn growth for ED = NEt3 is limited, since the polymerization was very slow and only low conversions (up to 14%) were achieved. However, the isolated polymers had Mn in close agreement with the theoretical values, under the assumption that only one macromolecular chain is generated per cobalt atom. In the case of py, the Mn of PVOAc exhibited a larger departure from the Mn,th toward greater values, showing that less than one chain per cobalt is generated. The phenomenon is likely resulting from a combination of slow initiation (the half-life of V-70 is ca. 10 h at 30°C) and inefficient trapping of the radicals generated by V-70 by the Co II -py system.

Effect of the addition of electron donors to an ongoing CMRP of VOAc mediated

by Co(acac)2 with V-70. In order to further explore the effect of ED coordination to the dormant species on the Co(acac)2-mediated VOAc polymerization, excess ED was added to the reaction system after the formation of the dormant species. First, a normal CRP of VOAc was conducted at 30 o C to prepare a dormant species having a PVOAc chain

([VOAc]0/[Co(acac)2]0/[V-70]0=500/1/1
). After 43 h, the reaction medium (brown solution)

was split into three equal parts, and 30 equiv of py or NEt3 compared to Co(acac)2 were injected into two of these mixtures. Subsequently, all three mixtures were reheated at 30 o C in the same bath to continue the polymerization. Figure 7 shows the semilogarithmic kinetic plot, whereas Figure 8 reports the evolution of Mn and Mw/Mn versus conversion.

The data are available in tabular form as Supporting Information.

Because the half-life of V-70 in toluene at 30 o C is 10 h, almost of all V-70 should have already decomposed after 43 h. Therefore, the predominant compound in the polymerization system will be the Co(acac)2(PVOAc) complex as a dormant species, with the possible additional coordination of a VOAc monomer molecule. After the addition of 30 equiv of py, the color of reaction medium immediately changed from a brown solution to a clear orange solution, as was the case for initial addition of py to the reaction medium.

Moreover, this mixture became more viscous within 1 h, suggesting the production of polymeric materials. In contrast, no color change was observed in the case of the NEt3 addition. As shown in Figure 7, the semilogarithmic kinetic plot after the addition of excess py displayed the faster rate of polymerization relative to the normal CRP of VOAc. This suggested that the formation of Co(acac)2(PVOAc)(py) complex led to the release of PVOAc radicals from the dormant species, resulting in an acceleration of the polymerization rate. On the other hand, the rate of polymerization in the presence of excess NEt3 was slower than that of the normal CRP, despite no color change. This implies that the coordination of NEt3 to the dormant species suppressed the dissociation of the cobalt-PVOAc bond. The evolution of Mn and Mw/Mn versus conversion (Figure 8) is much the same as when the ED molecule was introduced at the beginning, before the formation of the dormant species (Figure 6), and the molecular weight distribution is once again monomodal (representative chromatograms are available as Supporting Information). These experiments indicate that the addition of the ED molecule changes dramatically the nature of the dormant species but its effect does not depend upon whether the organocobalt(III) compound is generated in its absence or in its presence.

Insert Figure 7 and Figure 8 Interplay of OMRP and DT mechanisms. The controlled growth of PVOAc for the Co(acac)2-mediated polymerization in the presence of py or NEt3, even though the control is not as good as for the polymerization in the absence of ED, strongly suggests that the cobalt complex interacts with the growing chain, establishing an equilibrium with a dormant species. The dramatic difference in polymerization rate as a function of ED (py >> NEt3) that the cobalt complex is directly implicated in this equilibrium. On It should be mentioned that, according to the literature, [31] NEt3 slows down the free radical polymerization of VOAc because of sacrificial H atom transfer from NEt3 to the growing radical chain. Since the resulting Et2NCHCH3 radical is highly stabilized and is not capable to reinitiate polymerization, this process effectively amounts to a termination event. However, a control experiment carried out under the same conditions used for the Co(acac)2 mediated polymerization (bulk, [VOAc]0/[V-70]0/[NEt3]0 = 500/1/30, 30°C)

showed that the polymerization rate was only halved, relative to the free polymerization in the absence of NEt3. Therefore, the sacrificial H atom transfer from NEt3 may only account for a small fraction of the dramatic polymerization rate decrease on going from py to NEt3.

The NMR and DFT studies also indicate that vinyl acetate does not establish a significant interaction with Co(acac)2. These results allow us to predict that the spin trap in the absence of ED additive is the simple mononuclear tetrahedral Co(acac)2 complex (perhaps in equilibrium with a higher aggregate, according to the literature [11] ).

Consequently, the spin trap should not enjoy any particular energetic stabilization in the absence of ED, the equilibrium should be more displaced toward the organocobalt(III) dormant species, and the polymerization should be slower. This indeed corresponds to the experimental evidence at the initial stages of the polymerization process (during the induction period in the absence of ED), but does not account for the fast and controlled polymerization after the initial period.

A reasonable explanation for the latter phenomenon must therefore be based on a different mechanism. The radical influx from the V-70 initiator (t1/2 = 10 h at 30°) will gradually convert the Co(acac)2 complex to an organocobalt(III) species in an essentially irreversible process. Toward the end of the transformation of the Co II to the Co III species, the new radicals will no longer be efficiently trapped and will therefore start to generate Equation 2

Insert Figure 9 One strong point that was previously put forward in favor of an OMRP mechanism is the fact that the isolated and purified polymer (described as a green material), [4e, 4h] supposedly end-capped with the -Co III (acac)2 fragment, can undergo chain extension upon treatment with fresh VOAc and in the absence of new radical source. [4e] This result can only be achieved for an OMRP mechanism, since the DT mechanism relies on the continuous influx of radicals from an external initiator. We have noted that the reaction mixture maintains a pale brown-pink color during the polymerization procedure and that the polymer, when isolated and purified under rigorous air-free and dry conditions, is pink.

However, the aliquots withdrawn from the reaction mixtures for the purpose of calculating the conversion and analyzing the polymer by SEC, turned green during the drying procedure in air. In addition, when the polymer was purified by dissolution in dry acetone and reprecipitation with dry heptane, it maintained its pink color, whereas when the procedure was carried out in the same manner but without using special precautions (regular acetone and in air), the polymer became green. We advance the hypothesis that the pink organocobalt(III) dormant species, generated in the absence of ED, contains 5coordinate chain ends, R0(VOAc)n-Co(acac)2 and that the contact with moist air changes the chemical structure of the chain end, changing the color of the material to green. The color change may result from water binding to yield a 6-coordinate chain-end, R0(VOAc)n-Co(acac)2(H2O), or from an oxidation process. The second possibility appears more likely, since no color change occurred when degassed water was added under an inert atmosphere.

The chemical nature of this green chromophore is currently unknown and will be investigated at a later time. Since we have demonstrated that ED coordination renders the homolytic dissociation of the organocobalt(III) dormant species easier, and since the DFT calculations indicate that water is able to provide sufficient stabilization of the Co II spin trap, a polymer terminated by -Co(acac)2(H2O) chain-ends would be able to undergo chain extension by the OMRP mechanism. In order to test this hypothesis, we have isolated, under as rigorous conditions as possible, a pink poly(vinyl acetate) obtained by the V-70initiated, Co(acac)2-mediated polymerization in the absence of ED additives (Mn = 18800).

Subsequently, this polymer was treated with additional fresh VOAc in bulk at 35°C and without V-70. No significant polymerization took place over one day (ca. 2% after 1566 min, see Figure 10) while the color of the mixture remained pale pink. At this point, water was deliberately added to the reaction mixture and a faster polymerization process ensued (17% in the subsequent 1300 min). This result demonstrates that the pink Co III (acac)2capped dormant chain is not capable to chain extend in the absence of a fresh radical source but does so after the addition of water. Therefore, it suggests that the previously reported [4e] chain extension experiment indeed occurs via OMRP as the authors suggested, but this involves a chemically modified organocobalt(III) chain end, after exposure to moist air.

A final point of discussion is the possibility that the associative radical exchange involves an intermediate dialkylcobalt(IV) complex, lying in a more or less deep local energy minimum (see Figure 11). If the associative species in only a transition state, characterized by one normal mode of vibration with an imaginary frequency (case a of Figure 11), the mechanism is that of a typical degenerative transfer and the control of polydispersity is only ensured by kinetic factors. [START_REF] Goto | [END_REF] If, on the other hand, a stable intermediate dialkyl Co(IV) species exists (case c of Figure 11), then we fall back into an OMRP mechanism, the Co(IV) complex being the depository of the dormant polymer chain. In an intermediate situation (case b of Figure 11) the Co(IV) intermediate is unstable but has a certain lifetime and will help to insure a better chain grown control. It should be mentioned that at least one sufficiently stable alkylcobalt(IV) complex for isolation and characterization, Co(norbornyl)4, is known, though its stability may be attributed to the severe steric encumbrance of the ligand coordination sphere. [32] For the present system, an aliquot of the reaction mixture did not reveal any EPR signal after quenching to the liquid N2 temperature. Thus, it seems that the dialkyl Co IV species is either a transition state or an intermediate with a too short lifetime for measurement by this technique.

Conclusion

The Schematic representations of the molecular structures found for [Co(acac)2], [13] [Co{tBuC(O)CHC(O)tBu}2], [14a, 33] and [Co(acac)2(ED)2].

resulting 1 H

 1 NMR spectrum is the superposition of the broad Co(acac)2 resonance (with essentially unchanged linewidth and chemical shift) and the sharp resonances of free VOAc. The amount of added VOAc affects only the relative intensity of the free VOAc resonances while it does not significantly affect their chemical shift and linewidth. This observation evidences the absence of a significant Co(acac)2-VOAc interaction.

  (a), while a broad resonance is still present in the region of free Co(acac)2 (white arrow). The free NEt3 resonances, especially the one attributed to the CH2 group, exhibit a paramagnetic shift ( 7.86 for CH2, 0.94 for CH3; cf. 2.50 and 1.06 in the absence of the Co complex). Further addition of a second NEt3 equivalent affords a very large increase of the free NEt3 resonances and a shift to  7.56 (CH2) and 0.97 (CH3), while the broad Co(acac)2 resonance is still present, Figure 2(b). A fourfold amount of NEt3 produces a further resonance shift to  6.66 and 1.01, while the free Co(acac)2 resonance is now essentially disappeared, Figure 2(c). Continued NEt3 addition further shifts the free Et3N resonances toward the diamagnetic position (e.g.  ca. 3.0 for the CH3 protons after the addition of 30 equivalents). The resonances of the coordinated NEt3 and acac ligands in complexes Co(acac)2(NEt3)x are not visible. At the same time, the resonance of free

  essentially perfect D2d symmetry. The optimized Co-O distance (1.945 Å) and O-Co-O angle (95.8°) are quite close to those determined experimentally for Co[(Bu t CO)2CH]2

  systems is most likely present in the 6-coordinate R-Co(acac)2(ED) form.Ligand dissociation to give mononuclear R-Co(acac)2 was considered, but the possible dimerization of the latter was not. The activation process generates the mononuclear Co(acac)2(ED) or Co(acac)2 complexes, which may yield dinuclear or oligonuclear aggregates. We have limited our study to the equilibria of the above two mononuclear Co II complexes with each other and with the octahedral complex Co(acac)2(ED)2. The results of these investigations can at least qualitative address the behavior of the polymerization process under conditions of excess ED. An additional cautionary note concerns the neglect of entropy contributions. A statistical mechanics analysis of the entropic term via the partition function is only possible in the gas phase, whereas the polymerization reactions are carried out in condensed phases.Therefore, the considerations that will be drawn can only be qualitatively based on the bond energy values.Compound CH3COOCH3 was selected as a model for VOAc. Since bonding is established by the carbonyl O atom which is not conjugated with the vinyl function, this replacement is not expected to change the donor properties of the ester function and reduces the computational effort. Pyridine was used as such, whereas NEt3 was modeled with NMe3 and NH3, allowing a partial probing of steric effect. Finally, H2O was also used as an ED. Concerning the Co III spin trap, CH3 was chosen as a model for the R group. Although the resulting R-Co BDE's will be unrealistic, the trends obtained for the different ED molecules should be qualitatively relevant.The addition of a single ED molecule affords Co(acac)2(ED) structures with a trigonal bipyramidal geometry, as shown in Table1. The possibility of the alternative square planar geometry was explored with ED = H2O, but resulted in a higher energy. All investigated molecules converged to similar trigonal bipyramidal structures in the quartet state. The interaction energy increases in the order CH3COOCH3 < N(CH3)3 ~ py < NH3 < H2O. It is interesting to remark the very weak interaction with the carbonyl function of CH3COOCH3, whereas NMe3 and py form Co-N bonds of approximately equal strength. The Co-OH2 and Co-NH3 bonds are stronger still. The last two systems exhibit a significant distortion, with one close contact between one of the ED protons and one of the axial acac O atoms (O-HO = 1.894 Å for ED = H2O, N-HO = 2.232 Å for ED = NH3) suggesting the presence of an H-bonding interaction. In agreement with this hypothesis, the bond angles between the Co-O(N) and O(N)-H bonds are correspondingly smaller than expected (Co-O-H = 91.7° vs. 113.5° for the other O-H bond; Co-N-H = 101.8° vs. 114.0° and 114.2° for the other N-H bonds)

4

 4 -CH3 BDE for this system: 21.82 kcal mol -1 for CH3COOCH3, 20.65 kcal mol -1 for NMe3, 21.83 kcal mol -1 for py,22.65 kcal mol -1 for H2O and 21.80 for NH3. All these values are greater than the Co III -CH3 BDE of unsolvated Co(acac)2(CH3) (18.50 kcal mol -1 ).It is interesting to compare two processes of relevance to the radical polymerization controlled by Co(acac)2: on the one side, the homolytic CH3-Co bond rupture in the octahedral complex CH3-Co(acac)2(ED); on the other side, the equilibrium between the resulting 5-coordinate Co(acac)2(ED) complex and the corresponding bis-adduct, Co(acac)2(ED)2. For each ED used in the computational study, the energetics of these two processes are shown on the left and right hand sides of Figure4, respectively. The absolute values are not useful for evaluating the equilibrium positions and the effective polymerization rates for a variety of reasons (CH3 model for the growing polymer chain, neglect of entropy, …). The free energies are in principle less positive for the homolytic bond rupture process and less negative for the ED addition process, relative to the energies shown in Figure4. However, the figure allows the estimation of qualitative trends. Under the hypothesis of identical entropic contributions for the different ED systems, a greater Co-CH3 BDE should slow down the effective polymerization rate, whereas a greater stabilization by the ED addition should accelerate the process. The latter phenomenon results from the reduced availability of the Co(acac)2(ED) radical trap and from the fact that the coordinatively saturated Co(acac)2(ED)2 complex has no trapping ability. Therefore, the kinetic acceleration may also be accompanied by partial or total loss of control.Insert FigureEffect of the initial addition of electron donors on the CMRP ofVOAc mediated by Co(acac)2 with V-70. As demonstrated above, N-donor ligands such as py and NEt3 are able to coordinate to a different extent to Co(acac)2 (py >> NEt3), producing Co(acac)2(ED)x complex. There is no indication, on the other hand, for the coordination of VOAc to Co(acac)2. Therefore, an electron-donating effect originating from the coordinating ED may change the reactivity of cobalt(II) complex towards a VOAc radical. Accordingly, 30 equiv of EDs (py or NEt3) compared to cobalt were initially added to the Co(acac)2-mediated VOAc polymerization initiated by V-70 at 30 o C ([VOAc]0/[Co(acac)2]0/[V-70]0/[ED]0 = 500/1/1/30

  the basis of the solution NMR investigation and the DFT calculations, it is natural to propose that this control is established via an OMRP equilibrium, namely involving the formation of an organometallic dormant species, as shown in Scheme 4 (part b). The reason for the relative polymerization rate in the order py >> NEt3 can be attributed to the different stabilization of the product of Co III -R bond cleavage, the 5-coordinate Co II complex Co(acac)2(ED), upon coordination of a second ED molecule. The NMR investigation indicates that the py addition is more favorable than the NEt3 addition and the DFT calculations confirm this trend, after consideration of the steric factors from the result of the NMe3 model system (Figure4). In addition, the DFT calculations show that the homolytic bond dissociation energy of the Co III -R bond does not greatly change as a function of the ED.

  polymer chains, explaining the onset of a fast polymerization process. The fact that the polymerization remains controlled can be explained by the intervention of a degenerative transfer (DT) process, as indicated in Scheme 4 (part a). The same control mechanism has because the greater radical influx allows a faster conversion of the Co(acac)2 into the organocobalt(III) species. Figure 9 shows that the polymerization rate increases when the radical influx increases. The observed pseudo-1 st order rate constants are 2.24•10 -3 s -1 for [V-70]/[Co II ] = 4 and 1.04•10 -3 s -1 for [V-70]/[Co II ] = 2. Given an estimated initiator efficiency factor of 0.6 for V-70 and the irreversible consumption of 1 equiv of radicals to convert all Co II to Co III , the residual radical/metal ratios are 3.8 and 1.4, respectively. The ratio of the observed rate constants, 2.24•10 -3 /1.04•10 -3 = 2.15, is close to the square root of the radical concentration ratio, (3.8/1.4) 0.5 = 1.65. This is as expected for a DT mechanism and disagrees with an OMRP mechanism. kapp = (kp[VOAc])(kact/kdeact)[R0(VOAc)n-Co III (acac)2(ED)]/[Co II (acac)2(ED)] Equation 1 kapp = (kp[VOAc])(fkd[V-70]/kt) 1/2

  present study has shown that the Co(acac)2-mediated polymerization of VOAc may occur by either one of two fundamentally different mechanism: a degenerative transfer in the absence of ligands capable to establish strong interactions with the cobalt center, i.e. when the R0(VOAc)n-Co III (acac)2 dormant species remains 5-coordinated, and a reversible homolytic cleavage of the dormant organocobalt(III) species in the presence of strongly binding electron donors, i.e. when the dormant species is 6-coordinated, R0(VOAc)n-Co III (acac)2(ED). The second mechanism is accelerated by more strongly binding ED, because of the energetic stabilization of the Co II spin trap due to the formation of 6coordinated Co(acac)2(ED)2. On the other hand, no degenerative transfer is possible in the presence of ED, because the latter blocks the coordination site that is necessary for the associative radical exchange. Future efforts in our laboratories will be directed at the synthesis, isolation and characterization of simple model complexes of the organocobalt(III) dormant chain, and to applying the ED modulation of the Co(acac)2 reactivity to the development of new polymeric architectures.

Figure captions Figure 1 . 1 H

 captions11 Figure captions

Figure 2 . 1 H

 21 Figure 2. 1 H NMR spectra of (a) the mixture of Co(acac)2 with 10 equiv of VOAc, (b) (a)+Py (10 equiv), and (c) (a)+Py (30 equiv), in C6D6 at 30 o C (300 MHz).

Figure 3 . 1 H

 31 Figure 3. 1 H NMR spectra of (a) the mixture of Co(acac)2 with 10 equiv of VOAc, (b) (a)+NEt3 (10 equiv), and (c) (a)+ NEt3 (30 equiv), in C6D6 at 30 o C (300 MHz).

Figure 4 .

 4 Figure 4. Semilogarithmic kinetic plots for the initial addition of electron donors to bulk CMRP of VOAc mediated by Co(acac)2 with V-70 at 30 o C.: (■) No addition, (•) Addition of py (30 equiv), (▲) Addition of NEt3 (30 equiv), [VOAc]0/[Co(acac)2]0/[V-70]0/[ED]0=500/1/1/30.

Figure 5 .

 5 Figure 5. Evolution of Mn and Mw/Mn versus conversion for the initial addition of electron donors to bulk CMRP of VOAc mediated by Co(acac)2 with V-70 at 30 o C.: (■, □) No addition, (•, ○) Addition of py (30 equiv), (▲, ∆) Addition of NEt3 (30 equiv), [VOAc]0/[Co(acac)2]0/[V-70]0/[ED]0=500/1/1/30.

Figure 6 .

 6 Figure 6. GPC traces of PVOAc prepared by the initial addition of electron donors to bulk CMRP of VOAc mediated by Co(acac)2 with V-70 at 30 o C.: (a) Addition of py (30 equiv), (b) Addition of NEt3 (30 equiv), [VOAc]0/[Co(acac)2]0/[V-70]0/[ED]0=500/1/1/30.

Figure 7 .

 7 Figure 7. Semilogarithmic kinetic plots for the middle addition of electron donors to bulk CMRP of VOAc mediated by Co(acac)2 with V-70 at 30 o C.: (■) No addition, (•) Addition of py (30 equiv), (▲) Addition of NEt3 (30 equiv), [VOAc]0/[Co(acac)2]0/[V-70]0/[ED]0=500/1/1/30.

Figure 8 .

 8 Figure 8. Evolution of Mn and Mw/Mn versus conversion for the later addition of electron donors to bulk CMRP of VOAc mediated by Co(acac)2 with V-70 at 30 o C.: (■, □) No

Figure 1 . 1 H

 11 Figure 1. 1 H NMR spectra in C6D6 of the mixture of Co(acac)2 with py: (a) no added ligand; (b) 1 equiv; (c) 2 equiv; (d) 3 equiv.

Figure 2 . 1 H

 21 Figure 2. 1 H NMR spectra in C6D6 of the mixture of Co(acac)2 with NEt3: (a) 1 equiv; (b) 2 equiv; (c) 4 equiv.

Figure 3 . 1 HCH 3 - 3 Figure 4 .

 31334 Figure 3. 1 H NMR spectra of Co(acac)2-NEt3 mixtures in acetone-d6: (a) before NEt3 addition; (b) after the addition of 1-3 equivalents.

Figure 5 .

 5 Figure 5. Semilogarithmic kinetic plots for the initial addition of electron donors (EDs) to bulk CRP of VOAc mediated by Co(acac)2 with V-70 at 30 o C ([VOAc]0/[Co(acac)2]0/[V70]0/[ED]0 = 500/1/1/30): (■) No addition, (•) Addition of py (30 equiv), (▲) Addition of NEt3 (30 equiv).

Figure 6 .

 6 Figure 6. Evolution of Mn and Mw/Mn versus conversion for the initial addition of electron donors to bulk CRP of VOAc mediated by Co(acac)2. Conditions are the same as in Figure 5. (■, □) No addition, (•, ○) Addition of py (30 equiv), (▲, ∆) Addition of NEt3 (30 equiv).

Figure 9 .

 9 Figure 9. Semilogarithmic kinetic plots for the bulk CRP of VOAc mediated by Co(acac)2 in the presence of different amounts of V-70 at 30 o C ([VOAc]0/[Co(acac)2]0/[ED]0 = 500/1/30): (■) [V-70]0/[Co(acac)2]0 = 2; (•)[V-70]0/[Co(acac)2]0 = 4. The values in parentheses indicate the polydispersity index of the polymer samples.

Figure 10 .

 10 Figure 10. Semilogarithmic kinetic plots for the bulk CRP of VOAc mediated by pink R0(VOAc)n-Co(acac)2 (Mn = 18800) at 40 o C ([VOAc]0/[Co III ]0 = 5400/1).Water ([H2O]/[Co III ]0= ca. 32) was added after 1566 min.

Table 1 .

 1 DFT-optimized energies (in kcal mol -1 ) of Co(acac)2(ED)n (n = 0, 1, 2), relative to [Co(acac)2 + n(ED)].

	n	ED	Geometry a	S	E
	0	-	tet	3/2	0
		-	sp	1/2	8.80
	1	CH3COOCH3	tbp	3/2	-0.91
		N(CH3)3	tbp	3/2	-5.26
		py	tbp	3/2	-5.77
		H2O	tbp	3/2	-7.38
		H2O	spy	3/2	-3.77
		NH3	tbp	3/2	-10.17
	2	CH3COOCH3	oct	3/2	-5.40
		N(CH3)3	oct	3/2	-10.89
		py	oct	3/2	-11.40
		H2O	oct	3/2	-18.85
		NH3	oct	3/2	-23.84

a tet = tetrahedral; sp = square planar; tbp = trigonal bipyramidal; spy = square pyramidal; oct = octahedral.

Table 2 .

 2 DFT-optimized energies (in kcal mol -1 ) of CH3-Co(acac)2(ED)n (n = 0, 1),

	relative to [Co(acac)2 + CH3 + n(ED)].
	n	ED	Geometry a	S	E
	0	-	sp	0	-18.50
		-	tbp	1	-6.52
	1	CH3COOCH3	oct	0	-22.73
		N(CH3)3	oct	0	-25.91
		py	oct	0	-27.60
		H2O	oct	0	-30.03
		NH3	oct	0	-31.97

Scheme

Mechanism of ligand (ED) addition and exchange on [Co(acac)2].

Acknowledgments

KM greatly appreciates the financial support from the National Science Foundation (DMR-0549353 and CHE-0405627), Mitsui Chemicals, Inc. and CRP Consortium at

been proposed recently for the polymerization of methyl acrylate, acrylic acid, and vinyl acetate by a cobalt porphyrin system. [START_REF] Ab | ACS National Meeting[END_REF] Note that no degenerative transfer mechanism may take place in the presence of ED, because ED blocks the coordination site on the organocobalt(III) dormant species that is necessary for the associative radical exchange.

The ED dissociation process from the dormant species (indicated with dashed arrows in Scheme 4) is expected to be slow, because of the well known inertness of low-spin d 6 coordination compounds. Thus, a DT mechanism may only take place when the ED does not strongly bond the Co III dormant species, leaving a significant amount of 5-coordinate R0(VOAc)n-Co III (acac)2 at equilibrium. Conversely, d 7 complexes are kinetically labile, thus the ED addition/dissociation equilibrium between Co(acac)2(ED) and Co(acac)2(ED)2 is kinetically competent to stabilize the Co II spin trap and thereby accelerate the polymerization by the OMRP mechanism.

Insert Scheme 4

In order to find supporting evidence for the presence of a DT process, we have carried out additional polymerizations with different V-70/Co ratios. If the control mechanism were of OMRP type, the apparent rate of polymerization, kapp, should follow Equation 1 and thus be independent on the radical influx from the initiator. For a DT mechanism, on the other hand, the polymerization rate should follow an expression like Equation 2 as in free radical polymerization, the rate of exchange kexch affecting only the degree of control. [START_REF] Goto | [END_REF] The P n -X

(3) Scheme 1. General Mechanisms for Three Controlled/"Living" Radical Polymerizations.


Jean-Christophe P Gabriel

Franck Camerel

Bruno J Lemaire

Hervé Desvaux

Patrick Davidson

Patrick Batail

Bruno J Lemaire³

Herve  Desvaux

Patrick Davidson³

Swollen liquid-crystalline lamellar phase based on extended solid-like sheets

Ordering particles at the nanometre length scale is a challenging and active research area in materials science. Several approaches have so far been developed, ranging from the manipulation of individual particles 1,2 to the exploitation of self-assembly in colloids 3 . Nanometre-scale ordering is well known to appear spontaneously when anisotropic organic moieties form liquidcrystalline phases; this behaviour is also observed for anisotropic mineral nanoparticles 4,5 resulting in the formation of nematic 4±7 , smectic 8 and hexagonal 9,10 mesophases. Here we describe a lyotropic liquid-crystalline lamellar phase comprising an aqueous dispersion of planar solid-like sheets in which all the atoms involved in a layer are covalently bonded. The spacing of these phosphatoantimonate single layers can be increased 100-fold, resulting in one-dimensional structures whose periodicity can be tuned from 1.5 to 225 nanometres. These highly organized materials can be mechanically or magnetically aligned over large pH and temperature ranges, and this property can be used to measure residual dipolar couplings for the structure determination of biomolecules by liquid-state NMR. We also expect that our approach will result in the discovery of other classes of mineral lyotropic lamellar phases.

Lyotropic liquid-crystalline lamellar phases not only show longrange orientational order but also one-dimensional long-range positional order. They are classically formed (Fig. 1a±c) by selfassembly into layers of organic amphiphilic molecules, blockcopolymers or nanorods such as viruses or b-FeOOH colloids 8,11 . The inorganic layered materials that are used for intercalation purposes 12 display limited swelling (that is, their lamellar period can be increased) upon solvent addition. But these phases rarely show a lamellar period larger than a few nanometres, are hardly ¯uid and their positional order is lost very quickly upon further swelling. Therefore, such intercalation compounds cannot be considered as forming lamellar liquid-crystalline phases. With large swelling, concentrated aqueous gels of montmorillonite (smectite) clays may still display some long-range orientational ordering, as was recently demonstrated, but no positional long-range order was observed 13,14 .

In our quest for a lyotropic liquid-crystalline lamellar phase comprised of purely inorganic solid-like sheets, we need to consider low-dimensional solid-state structures built from highly charged covalent single layers that can be swollen. These layers must be chemically stable upon swelling and have the smallest possible counter-ions. This is because electric charges are needed to produce long-range electrostatic repulsions and a small ion size is known to enhance solubility and swelling properties 5 . These criteria are met for the series of solid acids H n M n Z 2 O 3n+5 (with for example M Sb, Nb, Ta; Z P, As; n 1; 3). Of these, we have chosen to study the layered phase, H 3 Sb 3 P 2 O 14 (ref. 15; Fig. 1d). This phase is easily made in a three-step synthesis (the detailed, experimental procedure is given in the Supplementary Information), involving ®rst the high-temperature solid-state synthesis of K 3 Sb 3 P 2 O 14 , then the exchange of potassium cations by protons in concentrated nitric acid 15 , and ®nally, a thorough rinsing and dialysis of the obtained solid to remove remaining nitrate ions. This leads to homogeneous, transparent, colourless suspensions of H 3 Sb 3 P 2 O 14 in water. Their viscoelastic properties depend on the overall mineral volume fraction, f (f Mc=r; where c is the concentration in mol l -1 , M is the molecular mass in g mol -1 , and r 2:4 g cm [START_REF] Desvaux | First use of a mineral liquid crystal for NMR measurement of residual dipolar couplings of a non-labeled biomolecule[END_REF] is the density of H 3 Sb 3 P 2 O 14 ), with dilute suspensions being very ¯uid.

Observation with the naked eye of a series of test-tubes containing suspensions of decreasing volume fraction between crossed polarizers shows that: (1) samples of high volume fraction (f . 1:78%) form birefringent gels (Fig. 2a); (2) for 0.75% , f ,1.78%, ¯uid birefringent suspensions are observed (Fig. 2b); and

(3) for f , 0:75% suspensions are biphasic with a well-de®ned interface between a denser birefringent ¯uid phase (bottom) and an isotropic, though ¯ow birefringent, phase (top) (Fig. 2c andd). The existence of suspensions that are both very ¯uid and permanently birefringent unambiguously indicates the thermodynamic nature of the liquid-crystalline order. When examined in natural light the bottom phase shows interference colours ranging from blue to red as f decreases (Fig. 2g). This already shows that there is some positional ordering at the length scale of fractions of a micrometre. This phase behaviour (Fig. 2) is that usually expected for the swelling of a lyotropic lamellar phase: once the maximum swelling is reached, water molecules can no longer be inserted into the interlamellar space and excess water is expelled, leading to phase separation. However, the ¯ow birefringence of the top phase indicates that it contains some anisotropic particles in suspension. This can arise from an equilibrium between the lower and upper phases, which may be viewed as a colloidal `evaporation/condensation' process, in analogy to a liquid/gas interface.

We established the phase diagram of H 3 Sb 3 P 2 O 14 suspensions versus volume fraction and NaCl concentration (Fig. 3). Flocculation appears at large NaCl molarities (.0.1 mol l -1 ). The maximum lamellar period decreases with increasing salt concentration, inducing a shift of the lamellar phase boundary towards a higher volume fraction. Similarly, moving up a vertical line of the phase diagram in the biphasic domain, the volume of the lower birefringent phase decreases, showing that more water is expelled from the interlamellar space as the salt concentration increases. This is strong evidence that electrostatic repulsive interactions are needed to stabilize this lyotropic liquid-crystalline phase. Indeed, such an effect arises when additional salt ions screen the electrostatic forces, reducing their range.

The structure of these aqueous H 3 Sb 3 P 2 O 14 suspensions has been further investigated by small-angle X-ray scattering (SAXS) on the ID2 high brilliance beamline at the European Synchrotron Radiation Facility (ESRF) 16 . Experiments performed on very dilute isotropic suspensions showed an X-ray scattering intensity, I, that monotonously decreases with scattering vector modulus q (q 4p sin v=l, where 2v is the scattering angle) as I ~q22 in all the range of concentrations studied (0:033% , f , 0:18%). This scattering law is typical of two-dimensional planar objects, and its q-range shows that sheets are of considerable lateral spatial extension (at least 300 nm). These experiments prove that the layers remain ¯at in suspension and do not crumple or fold. At higher volume fractions, typical diffraction patterns of unoriented samples of birefringent suspensions (Fig. 4a) display many sharp peaks (up to 12) that can be indexed to the 001 re¯ections of a lamellar phase.

The small width of these re¯ections proves that long-range positional ordering does take place, as expected for a lyotropic lamellar phase. The numerous higher-order re¯ections observed indicate that there is very little positional ¯uctuation [START_REF] Sa®nya | Steric interaction in a model multimembrane system: a synchrotron study[END_REF][START_REF] Salditt | Specular and diffuse scattering of highly aligned phospholipid membranes[END_REF] and that the lamellar positional order is very high. Even at maximum swelling, quite a large number (up to 7) of lamellar re¯ections are still detected. This contrasts with more usual swollen lamellar phases comprised of liquid-like layers, and suggests that this lamellar phase comprised of solid-like sheets has very different elastic constants.

The evolution of the lamellar period d of the birefringent phase upon dilution is shown in Fig. 4b. A ®rst regime, f . 0:75%, is observed in which d ~1=f, as expected for the one-dimensional swelling of a pure lamellar phase. For f < 0:75%, a crossover to a second regime occurs as d levels off and reaches a maximum of about 225 nm. This regime corresponds to the expulsion of excess water in the biphasic domain of the phase diagram. The layer thickness d < 1:05 6 0:05 nm can be extrapolated from the ®rst regime, a value that compares very well with that derived from the crystallographic structure, d < 1:10 6 0:01 nm. This proves that the layers are perfectly exfoliated. Moreover, as expected, wide-angle Xray scattering experiments (on samples of volume fractions f . 1:78%) showed the existence of fairly thin diffraction lines that arise from the long-range two-dimensional atomic positional order within the covalent layers (Fig. 4a,inset).

The major role of the electrostatic interactions was further assessed by SAXS experiments (Fig. 4c) in which the lamellar period was measured as a function of salt concentration, namely along the dashed line superimposed on the phase diagram (Fig. 3). d strongly decreases at large ionic strength beyond NaCl < 10 22 mol l 21 when the electrostatic interactions are screened, as expected by the DLVO theory of colloidal stability [START_REF] Deryagin | Theory of the stability of strongly charged lyophobic sols and of the adhesion of strongly charged particles in solutions of electrolytes[END_REF][START_REF] Verwey | Theory of the Stability of Lyophobic Colloids[END_REF] . Thus, at f 1:9%, d falls from 40 nm in pure water down to 23 nm at NaCl 0:1 mol l [START_REF] Sa®nya | Structure of membrane surfactant and liquid-crystalline smectic lamellar phases under ¯ow[END_REF] . The lamellar structure of these suspensions was con®rmed by X-ray scattering studies of single domains. Aligned samples could easily be produced by mechanical shearing with a Couette cell mounted on the ID2 beamline in both radial and tangential geometries (Fig. 5a), at shear rates, g Ç , ranging from 30 to 1,000 s -1 . At low volume fraction (near the biphasic domain), a strong alignment was induced even at very small shear rate (Fig. 5b). The three possible types of orientation [START_REF] Sa®nya | Structure of membrane surfactant and liquid-crystalline smectic lamellar phases under ¯ow[END_REF] were observed. However, we can conclude from the difference in scattering intensities that the intuitive c orientation in which the layers orient parallel to the shearing surfaces, is very strongly dominant. The detection of some a and b orientations is very probably due to a few topological defects. No relaxation was observed when the shearing was stopped, which proves that the aligned domain is stable (Fig. 5b). Similarly, samples aligned by sucking them into capillary tubes still showed high orientation weeks after their preparation. Comparable results were also obtained with samples of larger volume fractions. However, the degree of alignment of the sheared samples decreased as their concentration increased, probably because of their larger viscoelasticity, which hinders alignment. Single domains of aqueous suspensions of H 3 Sb 3 P 2 O 14 could even be grown in magnetic ®elds classically used for NMR. Indeed, a SAXS study of these aligned samples showed mosaic spreads as low as 208 (full-width at half-maximum, FWHM) (Fig. 5c) and proved that the layers align their normals parallel to the magnetic ®eld. Also, 2 H NMR spectra obtained from suspensions of H 3 Sb 3 P 2 O 14 in deuterated water, D 2 O, exhibit a well-resolved residual quadrupolar splitting of the water signal. This property of magnetic alignment, observed in the whole temperature range useful for biomolecules (4±50 8C), could easily be extended at any pH in the range 2.5±9.5 by using a dilute base (c , 0:3 mol l 21 ) with a bulky counter-ion (to avoid ¯occulation) such as tetramethylammonium hydroxide or tetra(n-butyl)ammonium hydroxide (Fig. 2e,f). This medium induces a partial alignment of the dissolved biomolecules, as illustrated in Fig. 6 on an oligosaccharide, leading to residual dipolar couplings, which provide long-range structural restraints that help to determine the structure of the biomolecule by solutionstate NMR [START_REF] Tjandra | Direct measurement of distances and angles in biomolecules by NMR in a dilute liquid crystalline medium[END_REF] . The alignment tensor is signi®cantly different from that induced by vanadium pentoxide suspensions [START_REF] Desvaux | First use of a mineral liquid crystal for NMR measurement of residual dipolar couplings of a non-labeled biomolecule[END_REF] , so the simultaneous use of these two mineral liquid crystals is very useful for structure determination [START_REF] Ramirez | Modulation of the alignment tensor of macromolecules dissolved in a dilute liquid crystalline medium[END_REF] (see Supplementary Information). By taking advantage of the slow kinetics of the phase separation (a few days), we could even prepare single domains, as observed optically and by NMR, at volume fractions inside the biphasic domain. This procedure could be used to tune the degree of alignment of biomolecules over an even larger range of concentrations. Finally, the possibility of lyophilizing the whole system over a wide range of pH, and the fact that tetramethylammonium hydroxide is commercially available in perdeuterated form, provides a 1 H/ 13 C/ 15 N-free liquid crystal that seems particularly promising for the structural determination by liquid-state NMR of non-labelled biomolecules.

From a more fundamental perspective, the symmetry properties of this mineral lyotropic lamellar mesophase are very unusual: it has orientational order and exhibits a periodic one-dimensional stacking of covalent solid-like layers. The strong covalent bonds that make up the layers of this mineral lamellar phase are reminiscent of the L b phase of amphiphilic molecules [START_REF] Janiak | Nature of the thermal pretransition of synthetic phospholipids: Dimyristoyl-and Dipalmitoyllecithin[END_REF] . However, one important difference is that the layers of the L b phase form through the crystallization of self-assembled amphiphilic molecules and melt on heating. In contrast, the mineral lamellar phase, even when swollen, is not sensitive to temperature (up to 70 8C where chemical degradation starts taking place within days), owing to the strength of the covalent bonds. This lamellar phase of extended single layers is also an interesting experimental system for testing theoretical predictions describing the properties of periodic stacks of tethered membranes. Indeed, the statistical physics of membranes made up from strong covalent bonds are expected to be very different from those of liquid-like pattern of a vertical capillary ®lled with a suspension of H 3 Sb 3 P 2 O 14 (f 0:75%), obtained 20 min after its alignment using a 18.7-T magnetic ®eld applied along its long axis. The vertical localization of the interference peaks (d 215 nm) indicates that the layers are oriented perpendicular to the magnetic ®eld (in this case, as no mechanical shear is applied, the sample relaxes back to a powder within 1±2 hours once it has been removed from the magnetic ®eld). membranes formed by self-assembly [START_REF] Toner | New phase of matter in lamellar phases of tethered, crystalline membranes[END_REF] . The elastic constants and the diffraction peak pro®les of single domains of this new phase should be carefully examined and compared to these predictions.

From the point of view of materials chemistry, the approach described here should lead to the discovery of a whole new class of mineral lamellar liquid crystalline phases. Indeed, from current work, we expect that similar phase behaviour will be found not only in other related members of the series H 3 M 3 Z 2 O 14 (with for example M Sb, Nb, Ta; Z P; As), but also in phases such as HM 2 Nb 3 O 10 (M Pb; Ca), HTiNbO 5 or Zr(HPO 4 ) 2 ×H 2 O that can all be fully exfoliated using tetra(n-butyl)ammonium hydroxide [START_REF] Fang | Layer by layers growth and condensation reactions of niobate and titanoniobate thin ®lms[END_REF][START_REF] Kaschak | Chemistry on the edge: a microscopic analysis of the intercalation, exfoliation, edge functionalization and monolayer surface tiling reactions of a-zirconium phosphate[END_REF] . All of these phases, because of the robustness of their layers, could be used for polymerization of soluble species under tunable twodimensional con®nement conditions.

M

Understanding long-term variability in the occurrence of tropical cyclones that are of extreme intensity is important for determining their role in ecological disturbances 1±5 , for predicting present and future community vulnerability and economic loss 6 and for assessing whether changes in the variability of such cyclones are induced by climate change 7 . Our ability to accurately make these assessments has been limited by the short (less than 100 years) instrumented record of cyclone intensity. Here we determine the intensity of prehistoric tropical cyclones over the past 5,000 years from ridges of detrital coral and shell deposited above highest tide and terraces that have been eroded into coarse-grained alluvial fan deposits. These features occur along 1,500 km of the Great Barrier Reef and also the Gulf of Carpentaria, Australia. We infer that the deposits were formed by storms with recurrence intervals of two to three centuries 8±11 , and we show that the cyclones responsible must have been of extreme intensity (central pressures less than 920 hPa). Our estimate of the frequency of such `super-cyclones' is an order of magnitude higher than that previously estimated (which was once every several millennia 12±14 ), and is suf®ciently high to suggest that the character of rainforests and coral reef communities were probably shaped by these events.

In tropical, intraplate settings that are largely free from the effects of sizeable earthquakes, tsunami, volcanic eruptions and landslides, tropical cyclones rank as one of the main hazards affecting both natural and human communities. In natural communities, disturbance by hazards is one of the most important mechanisms affecting the evolution and diversity of species 1±5 . Because of the short timescales over which direct and continuous observation of change within ecological communities is possible 5 , it has been dif®cult to assess the in¯uence of various scales of disturbance, and whether other mechanisms such as recruitment limitation 15 are

Figure 1 Figure 2

 12 Figure 1 Possible architectures for a liquid crystalline lamellar phase. In a swollen lamellar phase (centre), the lamellae of thickness d are regularly stacked with a period d and separated by the solvent. The lamellae are usually made from the self-assembly of amphiphilic molecules, such as surfactants (a) or block copolymers (b), or rod-shaped molecules or colloids (c). In contrast, the lamellae considered in this Letter are extended phosphatoantimonate covalent single layers made of at least 400 successive unit cells along any direction within the planes (300 nm) (d). The projection of the structure of H 3 Sb 3 P 2 O 14 is also shown in d (dark tetrahedra represent PO 4 groups and hatched octahedra represent SbO 6 groups).

Figure 3

 3 Figure 3 Phase diagram of H 3 Sb 3 P 2 O 14 suspensions versus volume fraction and salt concentration. Upon decreasing the volume fraction, the suspensions ®rst form a lamellar gel phase (L g ), then a lamellar ¯uid phase (L f ) and ®nally enter a biphasic regime (B). The system ¯occulates (F) at high salt molarity.

Figure 4

 4 Figure4SAXS study of `powder' samples. a, An example of scattered intensity versus scattering vector modulus q, showing 10 orders of re¯ection due to the lamellar period. Inset, the thin diffraction lines due to the two-dimensional positional order within the covalent layers (f 2:0%). b, Variation of the lamellar period, d, with inverse volume fraction (including two data points at very small d extracted from ref.16). The dashed straight line represents the one-dimensional swelling behaviour of the lamellar phase d d=f with d 1:05 nm. A crossover from this law to a plateau is observed when entering the biphasic regime. c, Variation of the lamellar period d with salt molarity at constant volume fraction (f 1:9%); along the vertical dashed line in Fig.3.

Figure 5

 5 Figure 5 Small-angle X-ray scattering studies of aligned samples. a, Scheme of the Couette shear cell illustrating the radial and tangential geometries with respect to the X-ray beam (¯at arrows). We also depict within the cell the three different types of lamellar orientations (a, b and c) as seen by the X-rays in both geometries (the layers are sketched as circular disks). b, SAXS two-dimensional scattering pattern obtained with a sample aligned in a Couette shear cell, in the tangential geometry, showing that the layers (d 175 nm) mostly belong to the c orientation. c, SAXS two-dimensional scattering

Figure 6

 6 Figure 6 Superposition of three non-decoupled 13 C-1 H NMR sub-spectra 29 of the glucosamine (Nag) C4±H4 region of the pentasaccharide containing the Lewis X motif acquired in a suspension of V 2 O 5 (a), in isotropic D 2 O solution (b) and in a suspension of H 3 Sb 3 P 2 O 4 in D 2 O, f 0:75% (c). In a and c, the residual dipolar coupling D 15:7 Hz and D9 2 5:2 Hz superimposes to the scalar J coupling, con®rming the induced alignment of the biomolecule. The different D values obtained using these two media illustrate the different induced alignments (see Supplementary Information).

² Present address: Covalent Materials Inc., 1295 A 67th Street, California 94608, Emerville, USA.© 2001 Macmillan Magazines Ltd

© 2001 Macmillan Magazines Ltd

Acknowledgements

We thank J. A. Sauls, J. Moreno, R. Wortis and K. Machida for discussions. This work was supported by the Science and Technology Center for Superconductivity, the Materials Research Center at Northwestern Unviersity, and the National High Magnetic Field Laboratory supported by the National Science Foundation and the State of Florida. M.E.

Acknowledgements

We thank the LURE and ESRF synchrotron radiation facilities for the award of beamtime, C. Bourgaux, P. Panine and T. Narayanan for technical support at D24 (LURE) and ID2 (ESRF), Y. Piffard for supplying us with 1g of H 3 Sb 3 P 2 O 14 powder at the start of this work, C. Chaudemanche and X. Leguevel for their participation in the synthesis of large amounts of H 3 Sb 3 P 2 O 14 , F. Alvarez for pH measurements, S. Grolleau for TGA measurements, P. Berthault and D. Jeannerat for their help in NMR measurements and processing of the spectra, P. Sinay È and Y. Zhang for the gift of the pentasaccharide and J. P. Simorre for giving us access to a 18.7 T magnet while we were performing our SAXS experiments at the ESRF. Financial support from the Ministry of Education (PhD fellowship for F.C.), the Ecole Normale Supe Ârieure and the Ecole Nationale des Ponts et Chausse Âes (PhD fellowships for B.L.), the Re Âgion Pays de Loire and the GDR-CNRS 690 FORMES (for D 2 O) is gratefully acknowledged. Correspondence and requests for materials should be addressed to J.-C.P.G. (e-mail: jeang@covalentmaterials.com) or P.D. (e-mail: davidson@lps.u-psud.fr). ................................................................. High frequency of `super-cyclones' along the Great Barrier Reef over the past 5,000 years Jonathan Nott* & Matthew Hayne²

Supplementary information is available on Nature's World-Wide Web site (http://www.nature.com) or as paper copy from the London editorial of®ce of Nature.