Cédric Hamelin
email: hamelin@ipgp.fr

Laure Dosso

Barry B Hanan

Manuel Moreira

Andrew P Kositsky

Marion Y Thomas

Geochemical portray of the Pacific Ridge: New isotopic data and statistical techniques

Keywords: oceanic basalts, Pacific-Antarctic Ridge, mantle heterogeneity, Principal Component Analysis, Sr Nd Pb Hf isotopes

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Introduction

Mid-oceanic ridge basalts (MORB) are the result of continuous melting of the ambient upper mantle beneath oceanic ridges. The MORB-source mantle is generally thought to have been depleted ~2 Gy ago by extraction of the continents. Although the range of geochemical variations in oceanic basalts is mostly attributed to the influence of Ocean Island Basalt (OIB), significant geochemical heterogeneity in MORB has been recognized (Hoffman et al., 2003 and reference therein;[START_REF] Rudge | A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt[END_REF]. This heterogeneity has been revealed using radiogenic isotope ratios (Sr, Nd, Pb and Hf) in areas devoid of plume influence. Numerous studies have attempted to model the mixing relationship between enriched and depleted domains within the mantle to reproduce the range of isotopic variations observed in oceanic basalts [START_REF] Albarède | Radiogenic ingrowth in systems with multiple reservoirs: applications to the differentiation of the mantle-crust system[END_REF][START_REF] Meibom | The statistical upper mantle assemblage[END_REF][START_REF] Rudge | A theoretical approach to understanding the isotopic heterogeneity of mid-ocean ridge basalt[END_REF][START_REF] Kellogg | Modeling lead isotopic heterogeneity in mid-ocean ridge basalts[END_REF]. The range of Sr, Nd and Pb isotopic compositions depends not only on the endmember compositions but also on the volume of mantle sampled during the melting relative to the length scale of heterogeneities [START_REF] Kellogg | Modeling lead isotopic heterogeneity in mid-ocean ridge basalts[END_REF]. Therefore, the range of MORB geochemical variations reflects the size, the spatial distribution, and the difference of fusibility of heterogeneities within the upper mantle. In addition to this intrinsic heterogeneity of the MORB mantle, radiogenic isotope studies have led to the definition of distinct broad mantle isotopic domains, such as the archetypal DUPAL anomaly located in the south hemisphere [START_REF] Dupré | Pb-Sr isotope variation in Indian Ocean basalts and mixing phenomena[END_REF]Castillo, 1988). Boundaries between these domains can be (i) extremely sharp as seen at the Antarctic Australian-Antarctic Discordance (ADD), with isotopic ratios (Sr, Nd, Pb and Hf) abruptly changing from Indian to Pacific values [START_REF] Pyle | Resolving an isotopic boundary within the Australian-Antarctic Discordance[END_REF][START_REF] Hanan | Contrasting origins of the upper mantle revealed by hafnium and lead isotopes from the Southeast Indian Ridge[END_REF][START_REF] Meyzen | Isotopic portrayal of the Earth/'s upper mantle flow field[END_REF][START_REF] Cooper | Distribution of recycled crust within the upper mantle: Insights from the oxygen isotope composition of MORB from the Australian-Antarctic Discordance[END_REF] or (ii) more gradual as seen in the transition from Southwest Indian Ridge to South Atlantic MORB [START_REF] Meyzen | New insights into the origin and distribution of the DUPAL isotope anomaly in the Indian Ocean mantle from MORB of the Southwest Indian Ridge[END_REF].

Another example of these large geochemical provinces is given by the two sub-pacific mantle domains [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF]. The distinctive isotopic properties of these large-scale geochemical domains suggest a long-term isolation of these mantle provinces, each with their own convective histories involving various amounts of melting residues and recycled components. On the basis of an incomplete dataset, a boundary between these two pacific provinces has been located at the latitude of the Easter Island microplate [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF]. But in order to have a complete geochemical view of the southern Pacific Ridge from 66 to 10°S, it was necessary to fill the sampling gap between 53°S and 41°S. This became one of the main objectives of the PACANTARCTIC2 cruise which took place in 2004-05 [START_REF] Dosso | The Pacific-Antarctic Ridge between 41°15'S and 52°45'S: Survey and sampling during the PACANTARCTIC 2 cruise[END_REF][START_REF] Klingelhoefer | New structural and geochemical observations from the Pacific-Antarctic Ridge 52°45'S and 41°15[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF]. Analyses of Sr, Nd, Pb, Hf and He isotopic compositions of these new samples from the Pacific Antarctic Ridge (PAR) are compiled here together with published data from 66°S to 53°S and from the East Pacific Rise (EPR). Therefore it becomes possible to have a picture of the geochemical variations along the entire Pacific Ridge from 66°S to the 10°S. Adapting a Principal Component Analysis (PCA) to incomplete datasets, we show a detailed portray of the geochemical variability of the Pacific Ridge.

Data selection and analytical methods.

The Pacific Ridge database from 10 to 66°S.

Along the Pacific Ridge, the Chile Triple Junction at 35°S/110°W separates the PAR from the EPR. The two pacific ridges show different geological settings: the full spreading rate increases along the PAR from 54 mm/yr at 65°S to 100 mm/yr at 35°S whereas it decreases along the EPR from 158 mm/yr at 35°S to 146 mm/yr at 10°S. In conjunction, the ridge axis morphology changes from a valley to a dome north of 60°S along the PAR [START_REF] Ondréas | Variations in axial morphology, segmentation, and seafloor roughness along the Pacific-Antarctic Ridge between 56°S and 66°S[END_REF], whereas the EPR is characterized by a uniform dome shaped morphology. Three plume-ridge interactions generate abnormal morphological structures along this section: Foundation (~38°S), Easter Island (~26°S) and 17°S. They are recognized by a high ridge cross-sectional area (Klingelhöfer et al., 2006) and the presence of intense off-axis volcanic activity. Compared to the EPR, the Pacific Antarctic Ridge still remains geochemically poorly known. A large portion of this plate boundary has been previously surveyed [START_REF] Lonsdale | Geomorphology and structural segmentation of the crest of the southern (Pacific-Antarctic) East Pacific Rise[END_REF][START_REF] Cande | Geophysics of the Pitman fracture zone and Pacific-Antarctic plate motions during the Cenozoic[END_REF] but the northern part has only recently been sampled [START_REF] Dosso | The Pacific-Antarctic Ridge between 41°15'S and 52°45'S: Survey and sampling during the PACANTARCTIC 2 cruise[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF].

To generate a coherent database along the pacific ridges, on-axis samples analyzed for one or more isotopic ratios (Sr-Nd-Pb-Hf-He) are selected from 66°S to 10°S using our new isotope data (table 1) completed with the petrological database of the Lamont-Doherty Earth Observatory (http://www.petdb.org). In most cases when the reference values of the standards are reported, the data are normalized to the values of NBS987, 0.71025 for Sr, of JNdi-1, 0.512104 and La Jolla, 0.511852 for Nd, NBS981, 206 Pb/ 204 Pb=16.9373, 207 Pb/ 204 Pb=15.4925, 208 Pb/ 204 Pb=36.7054, for Pb and JMC475, 0.282162 for Hf (see http://georem.mpchmainz.gwdg.de/ for details and references).:

Analytical methods.

Double Spike Pb analyses along the PAR

New high-resolution Pb analyses were carried out for samples collected during PACANTARCTIC1 (PAC1) and PACANTARCTIC2 (PAC2) cruises along PAR segments from 66°S to 56°S and from 53°S to 41°S respectively (Fig. 1, Table 1). Small chips from the inner part of the pillow lavas were handpicked to avoid altered surfaces that could be a potential source of Pb contamination. Powdered samples were leached with 6M HCl at 140°C

for an hour and then rinsed up to 6 times with ultrapure water prior to dissolution. Lead separation was then performed on an anionic exchange resin. Pb analyses were performed at Ifremer (Centre de Brest) on a Finnigan MAT 26x multicollector instrument (MAT261 upgraded by Spectromat), using the double spike technique with the calibrated Southampton-Brest 207/204 spike [START_REF] Ishizuka | Fluid-mantle interaction in an intra-oceanic arc: constraints from high-precision Pb isotopes[END_REF]. Replicate analyses of the Pb isotope standard NBS981 gave an average of 16.9432±0.0027 and 15.5004±0.0029 and 36.7326±0.0086 for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, respectively. Pb blanks measured using this procedure were < 100 pg, and thus negligible relative to the amount of sample analyzed.

Hf measurements along the PAR

Hafnium isotopic compositions were analyzed along the PAC1 ridge segments at SDSU on splits from the same samples [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF]. Hf was separated using the protocol of Blichert-Toft et al. 1997 with a negligible blank of less than 25 pg. Hf isotope ratios were measured at SDSU using the Nu Plasma. The 176 Hf/ 177 Hf was normalized for mass fractionation relative to 179 Hf/ 177 Hf=0.7325. The JMC-475 Hf standard 176 Hf/ 177 Hf gave 0.282160 ±0.000010 (2σ) during this study. The standard was run alternately with samples to monitor machine performance.

He measurements along the PAR

Helium isotopic compositions have been measured on PAC1 and PAC2 samples at IPGP (Institut de Physique du Globe de Paris). Fresh pieces of glass were cleaned with distilled water, ethanol and acetone using an ultrasonic bath. Some samples were also cleaned with hydrogen peroxide in order to remove some Mg crust. Analytical procedure is identical to previous studies from our laboratory and can be found in [START_REF] Moreira | A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51-52°S)[END_REF]. Samples were crushed under vacuum with analytical blanks of 7±1 nccSTP 4 He. This corresponds to 0.02 to 0.4% of the samples. Helium concentrations and isotopic composition were measured using the ARESIBOII mass spectrometer [START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF].

New results compared to published MORB data along this ridge section.

These results include samples from 66°S to 56°S (PAC1) and from 53°S to 41°S (PAC2) (Fig. 1). New high resolution Pb analyses as well as Hf and He analyses are presented here in Table 1. The Sr, Nd and Hf analyses on the same samples are found in [START_REF] Vlastélic | Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56°S and 66°S[END_REF] and [START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF].

Binary isotopic correlations

With 7 isotopic ratios, the number of possible binary isotope diagrams is 21. As an example, we choose here to describe 4 such binary plots, selecting some of the most commonly discussed (Fig. 2A-D). In all diagrams, pacific MORB samples devoid of plume influence define linear correlations that in most cases have been previously described in the literature. This result is in good agreement with the expected coherence in behavior between Rb-Sr, U-Pb, Th-Pb, Sm-Nd and Lu-Hf isotopic systems compared to each other during magmatic processes. When plume-ridge interaction samples are included in our dataset, they define elongated fields overlapping PAR-EPR array except in Figure 2C where He-Ɛ Nd isotopic ratios define sub-parallel trends which emerge from the PAR-EPR array and point towards higher 3 He/ 4 He (R/Ra) ratios. More than 30 years of He isotopes systematic of oceanic basalts worldwide have shown that MORB samples are characterized by a narrow range of composition compared to OIB samples. However even within this limited range, our data show a linear correlation between 3 He/ 4 He (R/Ra) values and Ɛ Nd (Fig. 2C). The correlation between He and other isotopic systems will be discussed further below ( §4.4.2).

In contrast with the decoupling of Hf and Nd isotope compositions previously documented in MORB (e.g., [START_REF] Patchett | Hafnium isotope variations in oceanic basalts[END_REF][START_REF] Salters | Hf isotope constraints on mantle evolution[END_REF][START_REF] Chauvel | A hafnium isotope and trace element perspective on melting of the depleted mantle[END_REF][START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF], a rather good correlation between these two isotopic systems is observed along our studied are (Fig. 2B). [START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF] have suggested that a distinctive behavior of Hf during disequilibrium melting along their studied ride area (Atlantic ridge 22-35°N) could explain a decoupled behavior of 176 Hf/ 177 Hf with respect to other isotopic ratios. The linear correlations observed in our dataset, show that the hypothesis of a specific behavior of Hf does not apply along the pacific ridges. Recently, good correlations between Hf and Nd isotopic ratios have been reported from other ridge segments such as Mohns Ridge [START_REF] Blichert-Toft | Geochemical segmentation of the Mid-Atlantic Ridge north of Iceland and ridge-hot spot interaction in the North Atlantic[END_REF] and the entire mid-Atlantic Ridge [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF]. Helium isotopes do not fit this Sr-Pb/Hf-Nd coherent behavior. At the latitude of the Foundation-Ridge intersection (38°S), the bell-shaped He curve shows radiogenic 3 He/ 4 He ratios. But on the other hand, hot-spot influenced samples form negative "plume anomalies" with very unradiogenic signatures (high 3 He/ 4 He ratios, up to 12 R/Ra). The amplitudes of the PIB and the anomalies attributed to the plume effect are of the same order of magnitude for Nd, Hf and Pb. For He and Sr, the PIB has much smaller amplitude than the hotspot anomalies. Additionally, local MORB variability is expressed as spikes, which are likely related to transform faults (Eltanin System).

MORB variability along the south

Discussion

Statistical definition of mantle reference lines

In most binary plots of radiogenic isotopic ratios, representative points of mantle derived material define linear trends. Historically, these trends have been used to define reference lines such as the Northern Hemisphere Reference Line (NHRL) in Pb-Pb plot [START_REF] Hart | A large scale isotope anomaly in the Southern Hemisphere mantle[END_REF] or the "Mantle Array" in Sr-Nd plot [START_REF] Depaolo | Sm-Nd age of the Stillwater complex and the mantle evolution curve for neodymium[END_REF] and the Nd-Hf plot [START_REF] Vervoort | Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time[END_REF]. These reference lines are arbitrary and depend on the abundance of data available at the time of their definition. However, the data colinearity in these plots justifies these convenient choices. The NHRL has been convenient to quantify the Dupal anomaly using the Δ7/4 and Δ8/4 [START_REF] Hart | A large scale isotope anomaly in the Southern Hemisphere mantle[END_REF]. In 1999, new MORB data from the south Pacific lead to the definition of the Pacific Reference Line (PRL) which proved convenient to compare two sub-pacific mantle domains, using the δ(Nd-Sr) and δ(Sr-Pb) notations [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF]. With the recent advance in analytical MC-ICPMS techniques, the number of isotopic data increases dramatically. It becomes possible to look at these reference lines in multidimensional space from a statistical point of view. Principal Component Analysis (PCA) is a powerful mathematical tool to study data sets such as a geochemical database including Sr-Nd-Pb-Hf analyses. The purpose of PCA analysis is to reduce the number of dimensions in a data set by keeping those characteristics that contribute most to its variance. This technique has been used in previous mantle heterogeneity studies (e.g. [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF][START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF] The PCA method used with our geochemical dataset has been initially developed for low-rank matrix approximations [START_REF] Srebro | Weighted low-rank approximations[END_REF] and was recently adapted for tectonic problems using incomplete geodetic times series [START_REF] Kositsky | Inverting geodetic time series with a principal component analysis-based inversion method[END_REF]. The main difference with traditional PCA methods is that the singular value decomposition is replaced by a more sophisticated decomposition, which appropriately takes into account samples with a missing isotope measurement. This technique is particularly suitable to geochemical data as it allows computing the principal components using the whole dataset, increasing therefore the accuracy of the calculation (see supplementary material for a more detailed discussion about the concept and the limits of our calculation). Although more recently ICA (Independent Component Analysis) has been chosen by some authors (Iwamori andAlbarède, 2008, Iwamori et al., 2010), the preferred PCA method used here has the advantage of dealing with missing isotopic data, assuming that decorrelation is still a good assumption of independence in our dataset.

Because the PCA method is an orthogonal linear transformation, it assumes the linearity of the data co-variations. In most binary isotopic diagrams, mixing processes are expressed by hyperboles whose curvatures depend on the elemental concentration ratios of the involved end-members. But in the case of MORB, pseudo-linear correlations are observed (Fig. 2) indicating that denominator elements are in approximately constant proportions in the mixing components. It is very difficult to evaluate the extent of non-linear relationships concealed within the analytical noise. However, it is worth noting that the geographical variations of the components calculated with our method are approximately the same as those computed in the 3-dimensional space of Pb isotopes, in which relationships are linear. In order to minimize the correlations induced by the 204 Pb analytical noise (it represents only about 1.4% of the total Pb), the computation has been made in the 204 Pb/ 206 Pb, 207 Pb/ 206 Pb and 208 Pb/ 206 Pb space.

Considering the problematic of our study, this observation suggests that the curvature can be neglected. One drawback of PCA in general stems from the fact that PCA is a projection method, and sometimes low-dimensional visualization can lead to erroneous interpretations.

Distribution of the variance among the principal components.

The most striking result of the PCA is that the first principal component accounts for more than 70% of the total variance. This result is remarkable considering the number of geochemical parameters involved in the very large number of samples used in this analysis. It indicates the strong coherence of all isotopic ratios within the depleted mantle domain. These correlations in binary plots of isotopic systems are the result of two antagonistic processes that took place over time: chemical fractionation events leading to the existence of enriched and depleted mantle reservoirs and mechanical mixing of theses reservoirs during mantle convection. The chemical fractionation between parent and daughter isotopes is controlled by distribution coefficients. Even if these coefficients can be modified by multiple parameters, they remain consistent from one element to another. Since these processes are approximately linear, the resulting dispersion is located along a line corresponding to PC1 (the first Principal Component).

While the importance of the first component is indisputable, one challenge with PCA is to establish the number of significant components needed to explain the observed data variance.

A classical way to illustrate the number of relevant components is to study the residual variance (χ 2 ) obtained for each component (e.g. [START_REF] Kositsky | Inverting geodetic time series with a principal component analysis-based inversion method[END_REF]. This value drops abruptly after the third component (see supplementary material). At first, we can infer that the information brought by the fourth to the sixth principal components is statistically insignificant and can safely be ignored. Using a PCA computed in the 3-dimensional space of Pb isotopes, the principal components account for 91.7%, 8.2% and 0.1% of the variance for PC1, PC2 and PC3 respectively. This observation suggests that the variance expressed by principal components over the order of 2 is the result of a randomly distributed error. Also the absence of coherent geographical variation for the third component suggests that this variance could either be a very local variation or an artifact of our data compilation (i.e. sampling bias, data and error normalization between different laboratories). This justifies to limit the discussion of the south pacific mantle heterogeneity to the PC1 and PC2 characteristics.

Mapping geochemical heterogeneities in the mantle using a PCA method.

Using the dataset available at the time, [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF] identified a large geochemical variation in the south Pacific depleted mantle. Based on an incomplete sampling of the ridge, they suggested the existence of a sharp boundary located at the latitude of the Easter Island Microplate (27°S). As an interpretation, they proposed that the Pacific Superswell divided the mantle into two domains each with their own convective histories, producing slight differences in their average isotopic signatures. More recently, the origin of these two domains has been challenged by a model based on plate kinetics [START_REF] Small | Plate-kinematic explanation for mid-oceanic-ridge depth discontinuities[END_REF]. Small and Danyushevsky proposed that geochemical discontinuities result from variations of the asthenosphere consumption, which corresponds to the ratio between the accretion rate and the spreading center migration relative to plumes. Their model predicted that the fast spreading, slowly migrating East Pacific Rise should have higher average melting degrees compared to the slower spreading, rapidly migrating Pacific-Antarctic Rise. In order to identify the two mantle domains along the Pacific Ridge, [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF] used δ(Sr-Pb) and δ(Nd-Sr), which are defined as the vertical deviations from references lines in Pb vs Sr and Sr vs Nd isotopic ratio plots respectively. These reference lines (see §4.1) were drawn intuitively in the greatest variance of their dataset. They can be directly compared to our first principal component. By construction, whatever the number of dimensions considered in the PCA, the projection of PC1 in a binary diagram resembles the relevant reference line (Fig. 2). Because PC2 is orthogonal to the greatest variance, sample values along this component are correlated to delta notations such as δ(Sr-Pb), δ(Nd-Sr) or Δ8/4Pb. But compared to delta notations, PC2 has the advantage of being rigorously and statistically determined in a multidimensional isotopic space. The application of the PCA method to our data compilation (excluding He isotopes) reveals a geochemical profile of the Pacific Ridge. The plot of PC1 versus latitude summarizes all the characteristics noted with the different isotopic systems (Fig. 4). The short scale geochemical variations associated with hotspots as well as the large-scale variation are clearly visible. 17°S and 25°S (Easter Island) are well defined by sharp anomalies superimposed on the bell shaped curve. A plot of PC2 versus latitude shows a very different picture (Fig. 4): only the large scale variation is expressed by this component, the geochemical variations associated with hotspots are flattened and the corresponding samples are projected along the bell shape curve defining the Pacific Isotopic Bump. This PIB is also shown on plots of delta versus latitude, illustrating the equivalence of these parameters and PC2 (Fig. 4). At the Juan Fernandez/Foundation latitude (36°S), the isotopic variation curve reaches an extreme which corresponds to a less "depleted" isotopic signature. In contrast with the conclusion of [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF], we proposed that geochemical variations along the Pacific Ridge are not the result of two separated mantle domains but should rather be seen as a progressive variation of the isotopic composition of the sub-Pacific depleted mantle. Since the asthenosphere consumption varies abruptly at the Chile triple junction and is almost constant along the PAR, the plate kinematic model of [START_REF] Small | Plate-kinematic explanation for mid-oceanic-ridge depth discontinuities[END_REF] is also inconsistent with the observed progressive variation of the MORB depleted matrix.

4.4.

Mixing relationship within the depleted mantle compared to ridge-hotspot interactions.

Asthenospheric versus hotspots signals as illustrated by Sr-Nd-Pb-Hf.

Except in figures involving He isotopes, geochemical variations related to ridge/hotspot interactions are consistent with variations of samples devoid of plume influence. At first, it is difficult to distinguish the two types of variations in binary isotopic plots (Fig. 2).

Nevertheless, a PCA computed with all heavy radiogenic isotopes clearly illustrate a difference: hotspot signatures are exclusively visible with PC1, while the variance related to the large scale variation (PIB) is illustrated by both PC1 and PC2 versus latitude (Fig. 4). In PC1 versus PC2 space, the MORB field extends from the depleted end member of the mantle (DMM) toward a recycled oceanic crust end member with a HIMU affinity (Fig. 5). The samples identified as resulting from a plume-ridge interaction are not part of this "depleted trend". Rather their data field extends from the depleted trend towards more enriched-type end members such as C and/or EM. This observation supports the idea that the large scale variation in the depleted Pacific mantle is unrelated to ridge/hotspot interactions. This variation is therefore equivalent to the intrinsic variability of MORB recognized in other areas (e.g. [START_REF] Dosso | The age and distribution of mantle heterogeneity along the Mid-Atlantic Ridge (31-41°N)[END_REF], Donnelly et al., 2004;[START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF][START_REF] Hémond | Origin of MORB enrichment and relative trace element compatibilities along the Mid-Atlantic Ridge between 10° and 24[END_REF]. It has to be noted that PCA calculations assume a linearity criteria which is not satisfied when dealing with isotopic compositions of the mantle end-members. It is thus not possible to determine more precisely the nature of the different mantle components responsible for the observed trends. Nevertheless, the PCA calculations establish that the progressive geochemical change of the depleted matrix of the Pacific mantle is not the result of hotspot material being diluted into the depleted mantle. Using a different approach, [START_REF] Meyzen | Isotopic portrayal of the Earth/'s upper mantle flow field[END_REF] successfully unscrambled the hotspot and asthenospheric signals. These authors have proposed that the geochemical variations unrelated to ridge/hotspot interactions along the South West Indian ridge and the South Atlantic ridge are related to a broad lower mantle upwelling in this area. Similarly, a broad lower mantle input could be a plausible cause of the Pacific Isotopic Bump associated with the high spreading rates in the vicinity of the Chile Ridge Triple junction.

It is interesting to compare the results of our PCA calculation along the Pacific ridge with the spectral analysis performed along the Atlantic ridge by [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF]. These authors found two contrasting types of spectra along their study area. The first type is associated with ridge-hotspot interactions and is seen in the first principal component. The second type is illustrated by the continuous power decrease with the decreasing wavelength of the second principal component. [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF] have interpreted this second type of spectra, unrelated to hotspots, as being the result of the continuous size reduction of mantle heterogeneities upon stretching and refolding of the convecting mantle. Despite the differences in geological settings between the Pacific ridge and the Atlantic ridge, our observations are in good agreement with the statistical analysis of [START_REF] Agranier | The spectra of isotopic heterogeneities along the mid-Atlantic Ridge[END_REF]. We view the PIB as the consequence of a progressive change in the relative proportions of the marble-cake components present in the Pacific upper mantle. In this model, hotspot anomalies are superimposed on the intrinsic mantle heterogeneity expressed in MORB.

He isotopes in the depleted mantle compared to hotspot signals.

Unlike other isotopic systems, helium shows a first order discrepancy between the hotspot and the depleted mantle signals in our studied area (Fig. 3 and Fig. 6): the large scale variation associated to the depleted mantle is characterized by an increase towards more radiogenic (low 3 He/ 4 He) compositions, whereas the short scale variation associated to the mantle plumes points towards high 3 He/ 4 He values as previously noted in [START_REF] Hanan | Lead and Helium Isotope Evidence from Oceanic Basalts for a Common Deep Source of Mantle Plumes[END_REF]. The origin of the large range of elevated 3 He/ 4 He values in OIB lavas is a long-lived controversial question [START_REF] Kurz | Helium isotopic systematics of oceanic islands and mantle heterogeneity[END_REF][START_REF] Allègre | Constraints on evolution of the earth's mantle from rare gas systematic[END_REF][START_REF] Meibom | Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components? Earth Planet[END_REF]Moreira et al., 2004).

Historically, the requirement of a high 3 He/ 4 He reservoir for the OIB lead to the idea that plumes are tapping a deep, undegassed lower mantle, isolated from the upper mantle convection [START_REF] O'nions | Phase-transition modulated mixing in the mantle of the Earth -Discussion[END_REF][START_REF] Allègre | Limitation on the mass exchange between the upper and lower mantle: the evolving convection regime of the Earth[END_REF]. Numerous alternative models have tried to solve the apparent inconsistency between the high 3 He/ 4 He ratio in plumes and the requirement of a source previously processed through partial melting (e.g. [START_REF] Parman | Helium solubility in olivine and implications for high 3He/4He in ocean island basalts[END_REF][START_REF] Class | Evolution of helium isotopes in the Earth's mantle[END_REF]Purtika, 2008;[START_REF] Albarède | Rogue mantle Helium and Neon[END_REF]Davies, 2010).

Compared to plume-influenced samples, systematics of He isotopes in depleted MORB samples have shown a very restricted range of 3 He/ 4 He with a peak of distribution around 8±1 (R/Ra) [START_REF] Moreira | A primitive plume neon component in MORB: The Shona ridge-anomaly, South Atlantic (51-52°S)[END_REF]. Within the uncontaminated depleted mantle sampled along our studied area, 3 He/ 4 He varies from 6 to 9.5 (R/Ra). Samples devoid of plume influence fall along a negative trend toward low 3 He/ 4 He values and high 206 Pb/ 204 Pb. This variation of He isotopes in our samples devoid of plume influence is clearly related to the PIB identified along the pacific ridge using PCA with Sr, Nd, Pb and Hf isotopes. It is interesting to note that the most enriched samples from this correlation show the lowest 3 He/ 4 He values (more radiogenic). In order to reconcile the 3 He/ 4 He variations with variations of Sr, Pb, Nd and Hf isotopes, new models take into account the physical specificities of He compared to heavy radiogenic isotopes: its diffusivity in mantle conditions and its capacity to be outgassed from melts at sub-surface pressure (e.g. [START_REF] Hart | Scale length of mantle heterogeneities: Constraints from helium diffusion[END_REF][START_REF] Albarède | Rogue mantle Helium and Neon[END_REF][START_REF] Gonnermann | Preserving noble gases in a convecting mantle[END_REF]. We propose that the melting of a marble-cake upper mantle, unpolluted by plumes, produces the good correlation observed between He and Pb isotopes in our depleted samples. The less depleted component of the mantle assemblage is characterized by a high Pb isotopic ratio and a low 3 He/ 4 He ratio. We suggest that this component was derived from the recycling of an extensively outgassed oceanic crust. When the oceanic crust is subducted back into the mantle, it contains negligible concentration of mantle-derived He [START_REF] Staudacher | Recycling of oceanic crust and sediments: the noble gas subduction barrier[END_REF]. With time, this very low 3 He/(U+Th) in subducted slabs is expected to produce the end-member with high Pb isotopes value and low 3 He/ 4 He ratio (Fig. 6).

In our model, the variation of He isotopes along the pacific ridge could be interpreted as:

(1) a smaller contribution of the refractory layers (less radiogenic), corresponding to a lower 3 He/ 4 He ratio at the PIB but because the melting rate is expected to be higher near the Chile Ridge Triple Junction, this hypothesis seems very unlikely, or (2) a higher volume of the recycled end-member in this area. This hypothesis is in agreement with the conclusions derived from the PCA based on heavy radiogenic isotopes ( §4.4.1). The intrinsic geochemical variation of the depleted upper mantle comforts the image of a marble cake mantle composed of a refractory component and a recycled oceanic crust component. A broad volume of recycled component associated with the high spreading rates in the vicinity of the Chile Ridge Triple junction is therefore a plausible cause of the Pacific Isotopic Bump. The correlation between He and Pb isotopes revealed by our new data (Fig. 6) suggests that the depleted mantle is the outcome of a mixing of a different nature that the one involved in the plumeridge interaction.

Conclusion

Analyses of this new sampling of the PAR complete the dataset of the Pacific Ridge.

Therefore it becomes possible to have a picture of the geochemical variations from 10 to 70°S. Our data show a clear geographical evolution of isotopic characteristics along the Pacific Ridge. In binary isotopic plots, this large-scale variation is expressed by correlations between each isotopic dimension. This observation holds true even for the Nd-Hf system previously reported as decoupled for MORB samples [START_REF] Debaille | Geochemical component relationships in MORB from the Mid-Atlantic Ridge, 22-35[END_REF]. Even more significant in this study is the linear correlation shown in the isotopic He-Nd and 3 He/ 4 He -206 Pb/ 204 Pb plots. In these isotopic spaces, the samples affected by plume-ridge interactions depart clearly from the linear correlations displayed by the ridge samples coming from the depleted mantle.

Despite the paucity of combined Sr-Nd-Pb-Hf isotope data on individual samples, the PCA algorithms used here allow us to portray the variation along a huge section of the Pacific Ridge from a statistical point of view. Compared to previous studies of the sub-Pacific mantle, the application of PCA reveals a Pacific Isotopic Bump, which can be seen as a progressive geochemical variation of the depleted upper-mantle matrix rather than a sharp frontier between two mantle domains. Combining PCA results with the information given by He isotopes, we suggest that the Pacific Isotopic Bump is unrelated to plume-ridge interactions. This geochemical variation in the upper mantle reservoir is the result of a marble-cake mantle assemblage composed of a residual mantle component and a recycled oceanic crust component. [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF], in the Nd/Hf plot (B), the mantle array [START_REF] Vervoort | Evolution of the depleted mantle: Hf isotope evidence from juvenile rocks through time[END_REF], in the Pb/Pb plot (D), the North Hemisphere Reference Line (NHRL) [START_REF] Hart | A large scale isotope anomaly in the Southern Hemisphere mantle[END_REF]. Since no reference line had yet been recognized in plots involving He isotopes, we show here a regression line ( 3 He/ 4 He = 0.63  Nd + 1.74, r = 0.89) (C). The projection of the two first principal components (PC1 and PC2) calculated in our study are shown (solid black and grey lines, see text for additional information). Data references can be found in supplementary materials. samples. In italic font, previously published data [START_REF] Vlastélic | Large-scale chemical and thermal division of the Pacific mantle[END_REF][START_REF] Vlastélic | Chemical systematics of an intermediate spreading ridge: The Pacific-Antarctic Ridge between 56°S and 66°S[END_REF][START_REF] Moreira | Helium isotopes on the Pacific-Antarctic ridge (52.5-41.5°S)[END_REF][START_REF] Hamelin | Sr-Nd-Hf isotopes along the Pacific Antarctic Ridge from 41 to 53°S[END_REF].

Figure captions

  Pacific Ridge. Latitudinal isotopic variations of south Pacific Ridge basalts are shown in Figure 3. An important geochemical feature from 66 to 10°S is a large scale (spanning approximately 5000km) and coherent variation of all isotopes shown by a bell shape grey line. The extremum of this variation defining the Pacific Isotopic Bump (PIB), is located at the latitude of Foundation (38°S). It reveals a less depleted component, characterized by more radiogenic values for Sr, Pb isotopic ratios, and correlatively less radiogenic values for Nd and Hf. Three shorter wavelength variations of the order of 200 to 700km are seen as isotopic anomalies superimposed on the otherwise bell shaped curve of the isotope signature along the ridge. In a way similar to the Pacific Isotopic Bump, these variations are towards more radiogenic Sr and Pb values and coherently less radiogenic Nd and Hf values. They indicate the influence of enriched materials due to plume-ridge interactions.

Figure 1 :

 1 Figure 1: Map of the south pacific basin showing the location of the PACANTARCTIC1 and

Figure 2 :

 2 Figure 2: Examples of binary plots showing correlations within the MORB field. Samples

Figure 3 :

 3 Figure 3: Geochemical variations along the PAR and the EPR from 66°S to 10°S. Dashed

Figure 4 :

 4 Figure 4: Plot of the two first principal components along the Pacific Ridge. The similarity

Figure 5 :

 5 Figure 5: Plot of PC2 versus PC1 for a principal component analysis computed using Sr, Nd,

Figure 6 :

 6 Figure 6: Binary plot of 3 He/ 4 He vs 206 Pb/ 204 Pb. The dashed regression line ( 3 He/ 4 He = -2.33 206 Pb/ 204 Pb + 51.0) is defined by samples devoid of plume influence. Plume-ridge interactions

Table 1 :

 1 Complete Sr, Nd, Pb, Hf and He isotope data table for PACANTARCTIC 1 and 2

Acknowledgements

The Pb isotope work was funded by CNRS/INSU. The Hf isotope work was supported by NSF grants to B.B. Hanan. We thank Joan Miller for technical assistance at SDSU. We acknowledge Rick Carlson for the editorial handling, Francis Albarède and an anonymous reviewer for constructive comments. Cedric Hamelin thanks the Caltech Tectonic Observatory for its hospitality during his stay in Pasadena. We gratefully acknowledge Neus Sabater for her valuable comments.

Supplementary Material

New algorithm to compute a PCA:

The PCA method is a linear transformation that converts the data into a new coordinate system in which the direction along which the greatest data variance is expressed, becomes the first axis (PC1).

The direction along which the greatest data variance is expressed orthogonal to PC1 becomes the second axis (PC2), and so on. The first step, called the whitening, consists in normalizing the data. This is done by subtracting the mean value from each data point and dividing the result by the standard deviation of each variable. The correlation matrix (Pearson's correlation matrix) of the reduced variables can then be calculated. The next step consists into diagonalizing the correlation matrix in order to find the eigenvalues, which correspond to the dimensions that have the strongest correlation in the data set. The principal components are the coordinates of the data points in the eigenvector referential.

The PCA method used with our geochemical dataset has been initially developed for low-rank matrix approximations [START_REF] Srebro | Weighted low-rank approximations[END_REF] and was recently adapted for tectonic problems using incomplete geodetic times series [START_REF] Kositsky | Inverting geodetic time series with a principal component analysis-based inversion method[END_REF]. The algorithm used to compute our principal components has been written specifically to address the major problem of a geochemical database: missing data in the matrix. Traditionally, for a given sample, it is necessary to acquire as many different isotope measurements as the number of dimensions used in the PCA. The paucity of combined Sr-Nd-Pb-Hf isotope data in individual samples is therefore an important issue for standard PCA algorithms. The more dimensions we use in the PCA, the fewer samples meet the required conditions: from 210 samples for 2-dimension (Sr-Nd) space to only 99 samples for a 6-dimension space. The main difference with a classical PCA is that we have replaced the standard Singular Value Decomposition (SVD) with a more sophisticated decomposition proposed by [START_REF] Srebro | Weighted low-rank approximations[END_REF]. In this approach the data matrix X is decomposed into components U, S, V, with each data point weighted according to the square of its standard error. This decomposition is particularly adapted to take into account individual measurement errors and deal with missing data points. It allows us to compute the Principal Components on the entire data set, even if some isotopic ratios are missing.

Each missing data point is weighted with infinite standard error. We want to emphasize that, consequently, no interpolation is made, which could erase some local effect. This technique is particularly suitable to geochemical data as it allows us to complete the principal components using the whole dataset, increasing therefore the accuracy of the calculation.

Limits of PCA:

Classical limits related to PCA calculation:

Because the PCA method is an orthogonal linear transformation, it assumes the linearity of the data co-variations. In most binary isotopic diagrams, mixing processes are expressed by hyperboles whose curvatures depend on the elemental concentration ratios of the involved end-members. But in the case of MORB, pseudo-linear correlations are observed (Fig. 2) indicating that denominator elements are in approximately constant proportions in the mixing components. It is very difficult to evaluate the extent of non-linear relationships concealed within the analytical noise. However, it is interesting to note that the geographical variations of the components calculated with our method are approximately the same as those computed in the 3-dimensional space of Pb isotopes, in which relationships are linear. In order to minimize the correlations induced by the 204 Pb analytical noise (it represents only about 1.4% of the total Pb), the computation has been made in the 204 Pb/ 206 Pb, 207 Pb/ 206 Pb and 208 Pb/ 206 Pb space.

Considering the problematic of our study, this observation suggests that the curvature can be neglected.

Limits related to our new algorithm:

Compared to traditional PCA algorithms, principal components are computed here altogether rather than separately, because the number of components affects the subspace in which these components reside. A limit to this method appears if one variable is represented by fewer samples than compared to other variables. In this particular case, a sample with a value corresponding to this "rare" variable is artificially given more weight. In return, each variable is also weighed in proportion to its number of samples. Because the number of He isotope analyses along the studied ridge section is too low compared to other isotopes, the information provided by this parameter has not been included in the PCA calculation.

Distribution of the variance among the principal components:


Santhosh Kumar

Dr Yugang Li

Yves Gnanou
email: gnanou@enscpb.fr

Dr Ulrich Baisch

Dr Yohan Champouret

Rinaldo Poli
email: rinaldo.poli@lcc-toulouse.fr

Prof. W Kiyoshi C D S Robson

W Stephen Mcneil

Santhosh Kumar

W Stephen Mcneil

Dr Santosh Kumar

Prof R Poli

Mcneil

Electronic and Steric Ligand Effects in the Radical Polymerization of Vinyl Acetate Mediated by β-Ketoiminate Complexes of Cobalt(II)

Keywords: controlled radical polymerization, OMRP, poly(vinyl acetate), cobalt, ketoiminato ligands

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In recent years, increasing attention has been paid to controlled radical polymerization (CRP). [1][2][3][4] Novel polymeric materials with well-defined molecular parameters such as average molecular weight, molecular-weight distribution, chain-end functions, and topological-structure have been accessible through various strategies. A current challenge of CRP is to control the radical polymerization of monomers that generate more reactive radicals such as vinyl acetate, vinyl chloride, vinylidene chloride, and ethylene. A variety of methods have been devoted to mediate the polymerization of vinyl acetate with a certain success, such as degenerative chain transfer with alkyl iodides, [5] dithiocarbamates and xanthates (RAFT/MADIX), [6][7][8] ATRP, [9,10] and cobalt-mediated radical polymerization (CMRP). [11,12] It has been argued that the use of metal-mediated CRP may allow controlling difficult monomers. [13,14] The advantage of using metal complexes is the variety of available transition metals and ligands, which can be tailored to mediate the polymerization of the monomer of choice. In recent contributions, it has been shown that the control exerted by acetylacetonatocobalt(II), Co(acac)2, on the VAc polymerization occurs by degenerative transfer (DT) when no additional Lewis bases are present, [15] but it switches to reversible deactivation (RD) and more specifically to a process that we have termed "organometallic radical polymerization" (OMRP) [13] in the presence of donor ligands such as pyridine, NEt3, or water, see <Scheme 1. [15,16] Indeed, the bond established between Co(acac)2 and the growing PVAc chain is too strong to release a sufficient amount of radical chains through homolytic bond cleavage under typical polymerization conditions. However, coordination of Lewis bases stabilizes the Co II radical trapping species by a greater extent, through formation of a bis(adduct) Co(acac)2(L)2, than the Co III dormant species, through formation of a mono(adduct) (L)(acac)2Co-PVAc. Thus, the homolytic bond breaking process becomes thermodynamically more favoured. At the same time, coordination by L blocks the open coordination site necessary for the associative radical exchange, shutting down the DT pathway.

Porphyrin-based cobalt systems have also been shown to function either by the OMRP mechanism [17][18][19][20] or by degenerative transfer, [21- 25] depending on conditions and on the nature of the monomer. <Scheme 1 here> Following the original report of the Co(acac)2-mediated polymerization, [11] Matyjaszewski has reported the VAc polymerization mediated by fluorinated -diketonato analogues of Co(acac)2, namely Co(CF3C(O)CHC(O)CH3)2, Co(acac-F3)2, and Co(CF3C(O)CHC(O)CF3)2, Co(acac-F6)2. [26] The study shows that Co(acac-F3)2 can control the polymerization, whereas Co(acac-F6)2 cannot. Diblock copolymers of VAc with N-vinylpyrrolidone, [27] styrene, [28] and acrylonitrile [29,30] have also been obtained by use of Co(acac)2. More recently, we have shown that an increase of steric bulk in the -diketonate ligand without a major change in the electronic properties, namely going to the 2,2,6,6tetramethylheptane-2,5-dionato (tmhd) system, Co[tBuC(O)CH-C(O)tBu]2, results in a slight weakening of the Co III -PVAc bond and renders this system operational also in the OMRP of VAc, even in the absence of Lewis base additives. [31] <Scheme 2 here>

In this paper, we report the use of Co[OC(Ph)CHC(Me)NAr]2 [Ar = Ph, 1; o,o'-C6H3(CH3)2 (Xyl), 2; p-C6H4CF3, 3] (<Scheme 2) in the radical polymerization of vinyl acetate initiated by 2,2'-azobis(4-methoxyl-2,4dimethylvaleronitrile) (V-70) at 30°C. -Ketiminato ligands are isoelectronic with -diketonato ligands, but the metal-bound nitrogen group permits a greater versatility in both steric and electronic changes in close proximity to the metal centre, [32] which can lead to dramatic variations in properties such as the geometry, spin state, and metal-ligand bond strengths. [33] As will be shown, the polymerization rate is indeed sterically activated by the bulk of the NAr group, although the electronic difference between the NAr group of these ligands and the O atom in the diketonate ligand has a more important effect. It will also be demonstrated that the positive effect of steric activation for the dormant Co III -PVAc chain is associated with a negative effect of slower trapping for the active PVAc radical chain.

Results and Discussion

(a) Polymerization of vinyl acetate in the absence of external base

In order to probe for an OMRP mechanism, polymerization experiments of vinyl acetate (VAc) initiated by V-70 were initially conducted with a small amount (0.8 equiv per Co) of initiator in toluene solution (50% v/v) at 30°C. Under these conditions, the initiator half life is ca. 10 h. With each of the compounds 1-3, polymerization took place, although rather slowly, and continued well beyond the time needed to completely decompose the initiator, namely ca. 50-60 hours (5-6 half-lives), see Table 1. Without cobalt complex, a more rapid polymerization takes place under these conditions, which stops after all initiator has been consumed and yields higher molecular weights, as also reported in Table 1. This proves that the growing PVAc chains are trapped by the cobalt complex and that they are subsequently released reversibly and continuously into solution according to the OMRP mechanism. [a] Conditions: VAc/Co/V-70 = 500:1:0.8 (500:0:6.0 for the experiment without cobalt); VAc/Toluene = 1:1 (v/v); T = 30°C.

The monomer consumption follows first order kinetics reasonably well in all cases after then end of the initiator decomposition (> ca. 60 h), see Figure 1, in agreement with the presence of a persistent radical effect. The effective polymerization rate constant varies in the order 3 (9.04•10 -4 h -1 ) < 1 (1.56•10 -3 h -1 ) < 2 (4.02•10 -3 h -1 ). The rate change on going from 3 to 1 (rate ratio of ca. 1.7) is less important than on going from 1 to 2 (rate ratio of ca. 2.6). The difference between compounds 2 and 1 can be easily rationalized on the basis of the steric effect of the aryl o-Me substituents in 2, which is expected to weaken the Co III -PVAc bond. Similar steric effects have previously been noted for Co[OC(R)CHC(R)O]2 (R = tBu vs. Me) [31] and for CpCr[ArNC(Me)CHC(Me)NAr (Ar = 2,6-C6H3Me2, 2,6-C6H3iPr2). [14] Note that the electronic effect due to the greater electron-donating power of the xylyl group relative to Ph, as confirmed by the easier oxidation of 2 (Ep,c = +0.91 V) relative to 1 (Ep,c = +1.02), should make the formation of the Co III -PVAc thermodynamically more favorable and thus the radical dissociation less extensive (i.e. slow down the polymerization process) for the former complex. Thus, the faster polymerization observed in the presence of 2 relative to 1 shows that the steric effect overrules the electronic one. <Figure 1 here> Figure 1. Time dependence of ln([M]0/[M]) for the polymerization of vinyl acetate at 30°C (data from Table 1). The dotted line corresponds to the data obtained in the absence of cobalt.

The comparison between compounds 1 and 3 cannot be easily rationalized, because the two systems should be quite similar in terms of steric influence; the p-CF3 substituents in 3 are located far from the Co center. On the other hand, the electronic effect should favour a faster polymerization in the presence of 3, since this complex is less easily oxidized (Ep,c = +1.11 V), contrary to the experimental result. However, the apparent rate constant (kapp, <Scheme 3) depends not only on the equilibrium constant of the reversible radical dissociation process (kd/ka), which is related to the relative stability of Co II and Co III species, but also on the concentration ratio of Co III and Co II species. If the residual amount of Co II is greater in the case of 3 (for instance, because of a less efficient radical trapping by this compound), then the apparent polymerization rate constant may become lower even if (kd/ka) is higher. The evolution of Mn for each polymerization in Table 1 is represented graphically in Figure 2. It can be noted that the initial Mn values are greater than expected (on the basis of one polymer chain per Co atom) and there is an evolution with conversion in each case, but the data deviate from the theoretical line in the order 2 > 1 > 3, namely the same order as the apparent polymerization rate constants. The polydispersity index is in all cases below 1.4 and as low as 1.15 in the case of 3. Thus, compounds 1-3 appear to have a certain degree of controlling ability for the VAc radical polymerization by the OMRP mechanism. The larger initial Mn value is evidence that these Co II complexes are not very efficient free radical scavengers, at least when the radicals are produced in large amounts at the beginning of the polymerization process. This low trapping efficiency, compared for instance with that of Co(acac)2, [16] may be due to the larger steric crowding around the metal for the -diketiminato system. The slow trapping rate is further confirmed by the results obtained at higher temperatures with compound 3 (vide infra). <Figure 2 here> Figure 2. Dependence on conversion of Mn and Mw/Mn for the VAc polymerization mediated by compounds 1-3 (data from Table 1). The straight line represents the theoretical Mn evolution.

In comparison with the OMRP mediated by compound Co(tmhd)2 the polymerizations mediated by 1-3 are rather slow (a 60% conversion in 91 h was observed in the presence of 0.7 equiv of V-70 under the same experimental conditions of temperature, solvent and monomer dilution). [31] This slow rate indicates a rather strong Co III -PVAc bond for the dormant chain and is opposite to the expected trend on steric grounds, since the encumbrance of the NAr group is far larger than that of the carbonyl group. Thus, this bond strengthening must have an electronic origin. The imine N atom of the -ketiminato ligand is a stronger electron donor than the carbonyl O atom of the -diketonato ligand, resulting in an electronic stabilization of the higher oxidation state and hence a greater energy cost for the homolytic rupture of the Co III -PVAc bond. This proposition is further confirmed by the comparison of the oxidation potentials. The three -ketiminato complexes reported here are easier to oxidize than Co(acac)2, for which no oxidation wave was reported in previous electrochemical studies carried out in DME, DMF of MeCN/THF. [34,35] We have reinvestigated the electrochemical behaviour of Co(acac)2 in CH2Cl2, under identical conditions as those used for the three -ketiminato complexes, and find an irreversible oxidation at +1.46 V vs. SCE, namely ca. 0.35 mV more positive than the less easily oxidized -ketiminato complex 3. Thus, the comparison of the very similar compounds 1 and 2 shows that the steric effect dominates relatively to the (weak) electronic effect, whereas in the comparison between compounds 1-3 with an electronically very different -diketonato complex, [31] the most important effect appears to be the electronic one. At least under the present conditions, the polymerization does not follow the degenerative transfer pathway, which was previously demonstrated for the Co(acac)2 and Co(tmhd)2-mediated polymerizations, [15,16,30,31] because a DT polymerization needs the continuous injection of new free radicals with respect to the dormant species. [36,37] In order to probe the possible action of this cobalt system as a transfer agent for the DT mechanism, an additional experiment was carried out using compound 1 with a larger V-70 excess (6 equiv relative to Co). The results (Table 2 andFigure 3) show that a faster polymerization occurs relative to the experiment with a smaller amount of V-70, and the resulting polymer has a rather narrow molecular weight distribution (PDI ca. 1.2). Furthermore, the initial Mn is greater than expected (though only by a factor of less than 2) and then converges to values in closer agreement with theory. Thus, all evidence is in favor of the existence of a relatively efficient control by degenerative transfer, the initial discrepancy between calculated and theoretical molecular weights being related to the initial slow trapping of the PVAc radical chains by the Co II complex. By all evidence, the associative radical exchange appears to be faster than the radical addition to Co II (<Scheme 4). According to our recent study on the Co(tmhd)2controlled polymerization, [31] which was carried out under the same experimental conditions (1:1 in toluene at 30°C), the V-70 initiator is capable to inject new radicals into solution until ca. 60 hours (ca. 6 half-lives), following which time any further polymer growth must be attributed to the OMRP mechanism. The fall of Mn below the theoretical value at long polymerization times, on the other hand, may be attributed to a catalytic chain transfer (CCT) activity of the Co II complex that can only accumulate in solution when an excess of radicals is no longer present because the irrelersible terminations are no longer compensated by an influx of new radicals. Thus, as already demonstrated for the Co(tmhd)2-controlled polymerization, [31] three mechanistic pathways (OMRP, DT and CCT) cohabitate for the polymerization mediated by compound 1. 2), 0.8 (diamonds, Table 1). <Scheme 4 here> Next, since compound 3 appeared to afford the best results at 30°C (observed Mn in closer agreement with theory from the very beginning of the polymerization; linear growth with conversion; lower polydispersities), additional experiments were carried out with this compound at higher temperatures (60 and 80°C). The results are collected in Table 3 and the time evolution of the conversion is graphically shown in Figure 4 in comparison with the 30°C results (Table 1).

(v/v).
As Figure 4 clearly shows, the conversion increases rather rapidly at the beginning of the polymerization at 60°C and even more at 80°C, but then it slows down to a rate near that observed for the better controlled polymerization at 30°C. This abrupt change of conversion rate coincides approximately with the time necessary to completely consume the initiator. The presence of an initial faster conversion only at the higher temperatures indicates that compound 3 can efficiently trap the growing radical chains only when the release of the primary radicals is sufficiently slow. When the growing radical concentration is too high, irreversible terminations become favoured, which entails a discrepancy between the targeted molar masses and those measured. The rate of primary radical production is higher at 60°C (V-70; t1/2 of 10 min) than at 80°C (AIBN; t1/2 of 70 min), [START_REF] Brandrup | Polymer Handbook[END_REF] but the initial rate of monomer consumption is slower at 60°C and in both cases the polymerization is slower than free radical polymerization (i.e. without Co complex) under the same conditions. For instance, only slightly more than 5% conversion is observed in 2 h in the presence of complex 3 vs. 26% without Co complex at 60°C. The observed polymerization rate is obviously a combination of the rate of primary radical generation with ensuing free radical polymerization and the rate of radical trapping with ensuing OMRP. As can be seen in Table 3, the polydispersities are quite low even in this initial period of faster monomer consumption. These data also suggest that a greater number of dead polymer chains are generated during the 60°C experiment, because of the faster primary radical generation. Therefore, a smaller [Co III -PVAc]/[Co II ] ratio should result after the end of the initiation phase for the 60°C experiment, giving rise to a slower OMRP according to the equation in <Scheme 3. A smaller kapp (apparent first order rate constant for monomer consumption) value at 60°C relative to that observed for the 30°C experiment should result. The results shown in Figure 4 are not inconsistent with this proposition.

It is to be noted that the polydispersities remain low throughout the polymerization even though the growing radical chains are slowly trapped. The molar masses are initially higher than the theoretical values for the above mentioned reasons. Note that discrepancy is initially smaller at 80°C, in agreement with the smaller amount of dead chains generated at this temperature (smaller primary radical production). However, the Mn subsequently drop below the theoretical values for the 80°C experiment. This result is explained, as already mentioned above, by the catalytic activity of the Co II complex for the chain transfer reaction to monomer.

(b) Polymerization of vinyl acetate in the presence of pyridine

In our previous studies on the VAc polymerization mediated by Co(acac)2 or Co(tmhd)2 [15,16,31] we have shown that the addition of external donor ligands, particularly pyridine, facilitates the OMRP process and shuts down the associative exchange (degenerative transfer), see Introduction. We therefore wondered whether the same effect could operate for the -ketoiminato complexes, particularly those yielding a less active dormant species.

Therefore, polymerization experiments were carried out in the presence of compounds 1 and 3 in the presence of 30 equiv of pyridine. The results are shown in Table 4, while Figure 5 compares the first order kinetics in the presence and absence of pyridine, under the same experimental conditions, for the experiments carried out at 30°C. The monomer consumption follows first order kinetics in all cases. However, the effect of pyridine is strikingly different for the two different compounds. When using compound 1, the apparent rate constant is essentially unchanged by the presence of pyridine (1.92•10 -3 h -1 with py vs. 1.84•10 -3 h -1 without py), whereas the rate quadruples (from 9.04•10 -4 h -1 to 3.63•10 -3 h -1 ) in the presence of pyridine for compound 3. The polymerization rate for 3 with py becomes even greater than that measured in the presence of compound 1. The presence of pyridine also decreases the level of control for both compounds, as indicated by the greater discrepancy between observed and calculated Mn (cf. Table 4 and Table 1). The polydispersities, however, remain low. 4) and absence (Table 1) of pyridine (30 equivalents): compound 1 (with py, triangles; without py, diamonds); compound 3 (with py, circles; without py, squares).

These results can be compared with those of the Co(tmhd)2-and Co(acac)2-mediated polymerizations, where OMRP is significantly accelerated by pyridine (as well as by other ligands). [15,31] The acceleration factor is particularly large for Co(acac)2 (essentially no OMRP takes place in the absence of added Lewis bases, kapp < 1.4•10 -4 h -1 in 50% toluene, [31] vs. 2.25•10 -2 h -1 in the presence of 30 equiv of pyridine in bulk [15] ) which establishes the stronger interaction with py and this was experimentally indicated by 1 H NMR studies and supported by DFT calculations. [31] Thus, the essential equivalence of the polymerization rate in the presence and absence of pyridine for compound 1 shows that this Lewis base does not significantly affect the OMRP equilibrium with this compound. On the other hand, weak pyridine bonding seems to occur for compound 3. As will be shown in the next section, the NMR investigation confirms these conclusions.

The initial molar mass, like in the corresponding experiment without pyridine, is greater than Mn,theo, but increased with monomer conversion in a more pronounced manner. Figure 6 shows the progression of the Mn measured by SEC. <Figure 6 here> Figure 6. Size-exclusion chromatograms for PVAc initiated at 30°C by V-70 in the presence of 30 equiv of pyridine (the analyzed polymers where those at 8.4, 14.3 and 24.8% conversion in Table 4).

Repeating the experiment at higher temperatures resulted in a faster polymerization (see Table 4). All the higher temperature studies were carried out with 0.8 equiv per Co of the AIBN initiator. At 60°C (initiator t1/2 = ca. 20 h), a 51.3% conversion was reached after 44 h and the initial Mn was much greater than theory, further increasing very little with conversion (by a factor of 1.24 from 30.5% to 51.3%), but remaining higher than the theoretical value. Note, however, that the polydispersity index is still relatively low (in the 1.3-1.5 range). At 80°C (initiator t1/2 = ca. 70 min), a 45.2% conversion was obtained in only 4 h. Once again, the polydispersity is relatively narrow and Mn is greater than theory, but less than the Mn obtained at 60°C. Essentially no increase of Mn with conversion occurs in this experiment. Finally, at 90°C (initiator t1/2 = ca. 20 min) the polymerization is faster still (45.4% conversion in 3 h, 79.5% in 7 h), and the initial Mn is smaller than in the lower temperature experiments. In this case, the Mn value falls below the theoretical value at the highest conversion, suggesting the presence of catalyzed chain transfer, as was the case for the experiment without pyridine (vide supra). Comparison of the results at the four different temperatures (especially the two extreme temperatures) shows that the catalyzed chain transfer process becomes more favorable at higher T, as already concluded for the experiments with compound 3 in the absence of pyridine (vide supra).

(c) NMR study of complexes 1 and 3 in the presence of pyridine

Complexes 1 and 3 were investigated by 1 H NMR in the absence and presence of up to 3 equivalents of pyridine. The spectrum of 1, see Figure 7, exhibits relatively broad and shifted resonances, as expected from the compound paramagnetism (S = 3/2). Upon addition of pyridine, the resonances due to both compound 1 and pyridine do not change position and do not broaden, while the free pyridine resonance grows in intensity proportionally to the py/Co ratio. This means that binding to the cobalt atom is not sufficiently strong. On the other hand, pyridine addition has a profound effect on the NMR spectrum of compound 3, see Figure 8, where both the Co complex and pyridine resonances are significantly broadened and slightly shifted. The behavior of compound 3 qualitatively resembles that of complexes Co(acac)2 [15] and Co(tmhd)2, [31] where weak metal binding and rapid chemical exchange between coordinated and free pyridine lead to a change of chemical shift and linewidth for the free pyridine resonances as a function of py/Co ratio. Like compound 3, the two -diketonato complexes of Co II experience a large accelerating effect on the VAc polymerization rate by the addition of pyridine. This accelerating effect, as detailed in previous contributions, [12,15,16,30,31] is associated with the energetic stabilization of the Co II species by ligand coordination, thereby shifting the OMRP equilibrium toward the active radical chain. The observed difference in pyridine binding affinity for complexes 1 and 3 must be attributed to purely electronic effects associated to the electron withdrawing power of the CF3 groups, since the steric control of pyridine coordination should be identical. As a result of this electronic effect, the cobalt center is electronpoorer in complex 3 and thus a greater Lewis acidity is expected.

A semi-quantitative comparison between the behavior of 3, Co(tmhd)2 and Co(acac)2 shows that the free pyridine resonance are broadened and shifted to an increasing degree in the order 3 < Co(tmhd)2 < Co(acac)2, reflecting an increasing binding affinity in the same order. Conversely, the cobalt ligand resonances are broadened for 3 and not at all for the -diketonato derivatives. This phenomenon must be related to a faster ligand exchange in the case of 3, to which the pyridine ligand is less tightly bonded. This is consistent with the expected steric and electronic effects. The above comparisons demonstrate the profound effect that the nature of the anionic bidentate ligand ( -diketonate or -ketiminate) has on the ability by external Lewis bases to coordinate the Co II complex, which in turn affects the performance of the Co complexes as controlling agents in radical polymerization. Like previously investigated bis(-diketonato)cobalt(II) complexes, these ketiminato complexes are able to reversibly trap the growing PVAc radical chains in a controlled OMRP mechanism and to catalyze chain transfer. The CCT activity of these Co(II) complexes plays a significant role only at high temperatures. The initially high Mn values indicate a relatively slow radical trapping process by the Co II complexes, slowing down in the order 3 > 1 > 2. Furthermore, the resulting PVAc-cobalt(III) dormant species are able to function as transfer agents for a controlled polymerization by degenerative transfer. Replacement of a carbonyl function in bis(-diketonato) systems by a bulkier C=NAr function has two effects: the cobalt center becomes electron richer (NAr is less electronegative than O), stabilizing the higher oxidation state complex in the organocobalt(III) dormant chain, but also introduces a greater steric bulk in the metal proximity, weakening the PVAc-Co bond. The relative importance of these two opposing effects depends on the precise nature of the ligand substituents. The NAr group strongly affects the ability of the cobalt(II) system to coordinate Lewis bases (as exemplified by pyridine in this study). Hence, addition of pyridine to system 1 under OMRP conditions results in no significant rate change, whereas a slight accelerating effect results upon addition of pyridine to complex 3, but the accelerating factor (and the pyridine binding ability as qualitatively indicated by NMR) is lower for 3 than for complexes Co(acac)2 and Co(tmhd)2.

Experimental Section

Materials. Co(O2CCH3)2•4H2O (reagent grade), 1-phenyl-1,3butanedione (99%), aniline (99%), 2,6-dimethylaniline (99%), and 4-(trifluoromethyl)aniline (99%) (all Sigma-Aldrich) were used as received. Vinyl acetate (>99%, Aldrich), N-vinylpyrrolidone (Alfa-Aesar), and anisole (Acros organics) were dried over calcium hydride and freshly distilled under reduced pressure prior to use. Toluene was distilled from sodium benzophenone ketyl. 2,2'-Azobis(4-methoxy-2,4-dimethyl valeronitrile) (V-70; Wako) and azobis(isobutyrronitrile) (AIBN; Acros organics) were used as received. The -ketoimine proligands were prepared by condensation of 1-phenyl-1,3-butanedione with the appropriate aniline in refluxing toluene, using a catalytic amount of ptoluenesulfonic acid and a Dean Stark apparatus. [START_REF] Parks | [END_REF] Characterizations. 1 H NMR spectra were recorded on a Bruker AC400 NMR spectrometer. Size exclusion chromatography (SEC) of poly(vinyl acetate) was carried out in filtered THF (flow rate: 1 ml/min) at 35°C on a 300x7,5 mm PL gel 5 micrometer mixed-D column (polymer laboratories), equipped with multiangle light scattering (minidawn Tristar, Wyatt Technology Corporation) and refractive index (RI2000, Sopares) detectors or with a waters column pack (300x7,5 mm, ultrastyragel 104,103,100 Å), equipped with multiangle light scattering (miniDawn Tristar,Wyatt Technology Corp.) and refractive index (waters 410) detectors. Cyclic voltammetric measurements of compounds 1-3 were conducted with an EG&G Princeton Applied Research model 263A potentiostat using ~4 mM solutions in anhydrous anaerobic CH2Cl2 with 0.1 M nBu4NClO4, using glassy carbon disk working, Pt wire auxiliary, and Ag/AgCl pseudoreference electrodes. Potentials at scan rates of 100 mV/s are reported relative to the saturated calomel electrode (SCE), and were measured vs internal ferrocene standard, taken as +480 mV vs SCE under these conditions. [40] Complexes 1 and 2 exhibited irreversible oxidations, while that of complex 3 showed some quasi-reversibility. None exhibited any observable reduction features up to ~ -1.2 V.

Syntheses of the cobalt complexes. Complexes 1-3 were prepared in a modification of the previously published procedure for 2, [41] reported below in detail. a) Synthesis of [Co(OC(Ph)CHC(CH3)NC6H5)2] 3.005 g (12.66 mmol) of yellow phenyl proligand HOC(Ph)CHC(CH3)NC6H5 was dissolved in 60 mL MeOH in a 125 mL Erlenmeyer flask to yield a yellow solution. The solution was stirred and heated. 0.711 g (12.7 mmol) KOH was dissolved in 5 mL MeOH, and added dropwise over 2 minutes to the reaction flask. 1.566 g (6.287 mmol) of Co(OAc)2•4H2O was added to the reaction flask, causing an orange solid to precipitate, which was recovered by filtration and washed with 5 mL cold MeOH. The orange solid was recrystallized from CH2Cl2/hexanes to give deep red block crystals to yield 1.875 g (71.87 %). 1 General procedure for the radical polymerization of vinyl acetate. The polymerizations were conducted in standard Schlenk glassware. In a typical procedure, VAc (2 mL, 0.014 mol), toluene (2 mL), AIBN (5.1 mg, 0.031 mmol), and compound 1 (21.0 mg, 0.039 mmol) were placed in a Schlenk flask equipped with a magnetic stirring bar. The mixture was degassed by three freezevacuum-thaw cycles and then immersed into an oil bath thermostated at 80°C. At the desired time, the Schlenk flask was rapidly cooled to room temperature by immersion into iced water before sample withdrawal.

The monomer conversion was determined gravimetrically after removal of the unconverted monomer under reduced pressure and the resulting residue was used for SEC characterization. For the purpose of polymer isolation, the reaction mixture was added to an excess of heptane and the resulting polymer precipitate was collected by vacuum filtration. The product was then vacuum-dried at room temperature for 24 h.

Figure 3 .

 3 Figure 3. Time dependence of the conversion for the polymerization of vinyl acetate at 30°C mediated by compound 1; V-70/Co = 6 (triangles, Table2), 0.8 (diamonds, Table1).

Figure 4 .

 4 Figure 4. Conversion as a function of time for the polymerization of vinyl acetate in the presence of compound 3. Temperatures: 30° (triangles), 60°C (diamonds), 80°C (squares). Data are fromTable 3 at 60 and 80°C and from Table 1 at 30°C.

Figure 5 .

 5 Figure 5. Time dependence of ln([M]0/[M]) for the polymerization of vinyl acetate at 30°C in the presence (Table4) and absence (Table1) of pyridine (30 equivalents): compound 1 (with py, triangles; without py, diamonds); compound 3 (with py, circles; without py, squares).

Figure 7 . 1 HFigure 8 . 1 H

 7181 Figure 7. 1 H NMR spectrum of compound 1 in the presence of variable amounts of free pyridine in CDCl3. <Figure 8 here> Figure 8. 1 H NMR spectrum of compound 3 in the absence and presence of pyridine (5 equiv) in CDCl3.

  H NMR (400 MHz, 298K, CDCl3): 19.35 (4H), 14.80 (2H), 12.77 (s, 4H), 10.07 (4H), -21.3 (6H), -26.89 (2H), -33.21 (2H), -49.83 (4H). Magnetic moment (Evans' Method, CDCl3): 4.1 B. Anal. Calcd for C32H28N2O2Co: C, 72.31; H, 5.31; N, 5.27. Found: C, 71.94; H, 5.29; N, 5.28. Cyclic voltammetry: Ep,c (vs. SCE) = + 1.02 V. b) Synthesis of [Co(OC(Ph)CHC(CH3)N(2,6-Me2C6H3)2] (2). 2.150 g (8.632 mmol) Co(OAc)2•4H2O was dissolved in 10 mL MeOH and stirred and heated to give a purple solution. 4.573 g (17.23 mmol) of the yellow xylyl proligand HOC(Ph)CHC(CH3)N(2,6-Me2C6H3) was added to the solution to give a suspension. 0.945 g (16.8 mmol) KOH was dissolved in 8 mL MeOH and added dropwise to reaction flask over 8 minutes, causing an orange solid to precipitate. The orange solid was collected by filtration and washed with 5 mL cold MeOH, and recystallized from CH2Cl2 as deep red needles. Yield: 2.459 g (79.4%). 1 H NMR (CDCl3) 17.27 (4H), 14.32 (2H), 11.21 (4H), 9.64 (4H), 3.32 (12H), -16.39 (6H), -25.81 (2H), -34.53 (2H). Magnetic moment (Evans' Method, CDCl3): 4.0 B. Anal. Calcd for C36H36N2O2Co: C, 73.58; H, 6.18; N, 4.77. Found: C, 73.20; H, 6.20; N, 4.81. Cyclic voltammetry: Ep,c (vs. SCE) = + 0.91 V. c) Synthesis of [Co(OC(Ph)CHC(CH3)N(p-C6H4CF3)2] (3). 0.282 g (0.924 mmol) of the yellow p-CF3 proligand HOC(Ph)CH-C(CH3)N(p-C6H4CF3) was dissolved in 15 mL MeOH, stirred and heated. 0.051 g (0.91 mmol) KOH was dissolved in 2 mL MeOH and added dropwise to the reaction flask. 0.117 g (0.470 mmol) Co(OAc)2•4H2O was added to the reaction flask, which caused precipitation of an orange solid, which was collected by filtration and recrystallized from CH2Cl2/hexanes to give deep red block crystals. Yield: 0.145 g (68.1 %). 1 H NMR (CDCl3) 20.10 (4H), 15.25 (2H), 13.48 (4H), 10.27 (s, 4H), -23.65 (6H), -35.24 (2H), -50.23 (4H). Magnetic moment (Evans' Method, CDCl3): 4.2 B. Anal. Calcd for C34H26N2O2F3Co: C, 61.17; H, 3.93; N, 4.20. Found: C, 61.08; H, 4.07; N, 4.53. Cyclic voltammetry: Ep,c (vs. SCE) = + 1.11 V.

Table 1 .

 1 Polymerization of VAc initiated by V-70 in the presence of complexes 1-3. a

	Complex	Time(h)	Conv(%)	Mn,SEC	Mn,theo	PDI
	1	64	12.3	15000	5300	1.17
	1	96	17.1	18700	7400	1.20
	1	160	24.6	21300	10600	1.36
	2	25	22.9	31300	9900	1.37
	2	49	33.8	33900	14600	1.34
	2	72	40.2	35300	17300	1.34
	2	114	49.1	37600	21200	1.30
	3	16	3.3	-		-
	3	38.75	4.8	-		-
	3	63	5.2	-		-
	3	134.5	12.3	8700	5290	1.15
	3	212	18.6	10700	8000	1.19
	3	303	22.7	12800	9770	1.22
	-	3.25	8.9	33800		1.23
	-	6	12.8	25300		1.22
	-	21.5	27.9	25100		1.21
	-	30.5	38.8	24100		1.19
	-	46	49	20600		1.28
	-	117.5	56.8	15600		1.47

Table 2 .

 2 Polymerization of VAc in the presence of complex 1 initiated by V-70 (6 equiv).

	Time (h)	Conversion (%)	Mn,SEC	Mn,theo	PDI
	17	27.6	22300	12000	1.21
	41	43.3	25400	18800	1.21
	65.5	53.7	26800	23300	1.19
	88.5	57.5	-	25000	-
	161.5	72.8	28700	31600	1.20

Table 3 .

 3 Polymerization of VAc in the presence of complex 3 at 60 and 80°C. a

	T Initiator Time(h)	Conv(%)	Ln([M]0/[M])	Mn,SEC Mn,theo PDI
			0.25	3.5	0.036		1507
			0.75	4.1	0.042		1765
			1.5	5.3	0.054	6320	2281	1.11
	60	V-70	6	11	0.117		4735
			8.75	11.8	0.126	15460	5079	1.32
			32.25	18.6	0.206	17700	8006	1.33
			146	25.1	0.289		10804
			4	26	0.301	15800 11192 1.29
			22.5	44.2	0.583		19026
	80 AIBN	49	47.5	0.644	18050 20446 1.28
			92.75	51.2	0.717		22039
			117	55.4	0.807	22900 23847 1.29

a Conditions: VAc/Co/Initiator = 500:1:0.8; VAc/Toluene = 1:1

  Table 3 at 60 and 80°C and from Table 1 at 30°C.

Table 4 .

 4 VAc polymerization in the presence of pyridine. a

	T	Complex Initiator	Time (h)	Conv(%)	Mn,SEC	Mn,theo	PDI
				18	3.3	-	-	-
				42	8.5	15000	3660	1.19
	30	1	V-70	68.25	14.3	17500	6160	1.11
				91.25	18.7	-	-	-
				163.5	24.8	20600	10680	1.23
				21.5	8.3	18000	3573	1.14
	30	3	V-70	65 90	22.7 30.2	21700 -	9771 13000	1.27 -
				113.5	31.1	22200	13387	1.34
				24.5	30.5	27200	10000	1.29
	60	1	AIBN	33	33.6	30800	11100	1.33
				44	51.3	33700	16900	1.49
	80	1	AIBN	1.5 4	28.3 45.2	23000 21800	9300 14900	1.24 1.27
				1	32.1	16600	10600	1.50
	90	1	AIBN	3	45.4	17200	15000	1.44
				7	75.9	19500	24800	1.36

a VAc/Toluene = 1:1 (v/v). VAc/Co/initiator/py = 500:1:0.8:30.

<Figure 4 here>

Acknowledgements

We are grateful to the Agence Nationale de la Recherche (contract ANR No. NT05-2_42140) for financial support. RP also thanks the Institut Universitaire de France for additional support. WSM is grateful for financial support from the Natural Sciences and Engineering Research Council of Canada and UBC Okanagan.

Entry for the

Bis(-ketoiminato) complexes of cobalt(II) display a controlling ability in the radical polymerization of vinyl acetate; while an increased steric encumbrance of the ligand coordination sphere labilizes the Co III -PVAc bond, it also makes the Co II complex a poorer trap for the active radical chain.