Giorgio Ercolano

Filippo Farina

Sara Cavaliere

Deborah J Jones

Jacques Rozière

Towards ultrathin Pt films on nanofibres by surface-limited electrodeposition for electrocatalytic applications

Novel fuel cell nanofibrous electrodes (NFEs) with higher degree of platinum exploitation and higher durability compared to commercial standards are produced, based on self-standing electrospun carbon nanofiber webs covered by platinum ultrathin nanoislands deposited by high overpotential pulsed electrodeposition.

Introduction

Reduction of the fabrication costs and improved durability are the key to a successful market penetration of proton exchange membrane fuel cells (PEMFC). 1 Platinum is at the centre of oxygen reduction reaction (ORR) electrocatalyst production and research. A more effective and durable utilization of this rare and highly priced metal is paramount to the development of a less expensive and more appealing PEMFC. [2][3][4] The platinum resource being limited, a substantial reduction of its use will offer a key advantage for the success of a sustainable energy technology. One limitation of Pt nanoparticles is that ORR activity is related to the size, low coordination sites and the inevitable presence of less active crystalline planes 5 . Furthermore, the nature of nanoparticles generates a limited carbon-platinum interface and a tendency to migration and agglomeration during duty cycles as a consequence of electrochemical Ostwald ripening. 1,[6][7][8] The deposition of extended Pt surfaces as, for example, in the nanostructured thin film (NSTF) approach developed by 3M [START_REF] Parsonage | US5338430 A[END_REF] , has been shown to produce 10 fold higher surface activity than nanoparticles and better durability than those of conventional carbon-supported nanoparticles. This class of electrocatalysts introduces novel and promising features at the expense of a significantly more complex fabrication process. It opened a new field of research dedicated to thin metal film on various type of support. The percolating/continuous nature of the Pt films reduces the support function to mechanical scaffolding. The extended support-metal interface of the NSTFs has reduced ohmic losses when compared to the high carbon/nanoparticles contact resistance typical of limited interfaces. Their continuous conformal morphology minimises unused platinum e.g. the contribution from coordinatively unsaturated sites including edges and corners of conventional Pt nanoparticulate catalysts. 10 As a consequence, higher ORR specific activity is achieved compared to nanoparticle-based electrodes. [START_REF] Parsonage | US5338430 A[END_REF][11][12][13][14] Furthermore, the stability of the Pt thin film based electrodes is enhanced due to the protective effect of the metal layer on the support and the lower tendency of extended surfaces to electrochemical Ostwald ripening. [15][16][17] Despite these clear advantages, the complexity of the fabrication process used to generate high Pt specific surface area in films is limiting their adoption. [18][19][20][21] The challenge in depositing and controlling conformal morphology and film thickness out of the line of sight on high aspect ratio supports has led to investigation of the potential of gas phase atomic layer deposition 18,22,23 , Cu underpotential deposition-redox replacement 19,21 , spontaneous galvanic displacement 24 and plasma-assisted deposition. 20 Alloying of Pt with transition metals, 25,26 or the de-alloying of PtM (M = transition metal); 27,28 is another powerful strategy used to improve ORR performance.

Here we present a novel class of electrocatalysts that consists of Pt conformally electrodeposited around the carbon surface of a self-standing nanofibrous electrode (NFE). We found that combining two cost-effective and upscalable techniques, electrospinning and electrodeposition, it is possible to obtain an electrocatalyst with outstanding Pt exploitation and high durability while minimizing fabrication complexity. We produced hierarchical 3D electrocatalyst layers with tuneable morphology that could be analysed directly and potentially used as such in a MEA (membrane electrode assembly), allowing the complete exploitation of their highly porous structure, in a similar fashion to NSTF or other 3D macroscopic assembly techniques. [29][30][31] These structured electrocatalyst layers have high electrical conductivity for fast charge transport, and diffused macroporosity for efficient reactant mass transportation. 32

Experimental

Polyacrylonitrile (PAN, Mw=150,000, Sigma Aldrich) derived carbon fibres were prepared following electrospinning with thermal treatments as already reported. [33][34][35][36] In detail 8 % (w/w) PAN solution in N,Ndimethylformamide (DMF, 98 %, Sigma Aldrich) was fed to a metal needle via a syringe pump with a flow rate of 1 ml/h. An electric field of 15 kV was applied between the needle and a metallic rotating drum collector (Spraybase) placed at a distance of 10 cm. As-prepared PAN fibres were collected in the form of non-woven mats. Fibre carbonization was realized by a two-step thermal treatment. First oxidative stabilization was performed at 280 °C for 1 h followed by carbonization for 1 h under N2 at 1500 °C using a heating ramp of 4 °C/min. The resulting carbon electrodes were subsequently treated at 80 °C in 5 M HNO3 solution for 5 hr.

Electrical conductivity was measured using a 2400 Keithley in a 4 wire configuration on a 5 x 40 x 0.05 mm carbon electrode strip in a Fumatech MK3-L cell operated in the current range 0-100 mA.

The ultimate tensile strength was measured in an Instron 5544 on a 25 x 5 x 0.05 mm carbon strip elongated at 0.1 mm/min.

Pt electrodeposition was performed in a 2-electrode setup, where the counter electrode consisted of a 20 x 50 x 0.8 mm graphite foil facing the working electrode and kept 10 mm away; the latter was a 15 x 40 x 0.05 mm carbon nanofibre self-standing mat. Both electrodes were dipped in a N2 saturated 0.1 M H2PtCl6 5H2O solution and contacted using Pt wires. The deposition voltage was alternated between -3.0 V and 0.0 V for the desired number of pulses and duration. Samples were thoroughly washed with deionized water (18 MΩ cm), dried and weighed on a microbalance.

Thermogravimetric analysis was performed in air up to 800 °C (10 °C/min) using a Netzsch STA 409PC thermobalance to determine Pt loading on the diverse carbonaceous supports.

The morphology of the electrospun materials and of the deposited Pt was analysed by field emission-scanning electron microscopy (FE-SEM) using a Hitachi S-4800 microscope and by transmission electron microscopy (TEM) using a JEOL 2200FS (Source: FEG) microscope operating at 200 kV equipped with a CCD camera Gatan USC (16 MP). For TEM analyses, samples were suspended in ethanol and sonicated before deposition onto carbon-coated copper grids. The average diameter and density of Pt structures were determined by measuring 100 selected objects using the ImageJ software.

Powder X-ray diffraction (XRD) patterns were recorded at room temperature in Bragg-Brentano configuration using a PANAlytical X'pert diffractometer, equipped with a hybrid monochromator, operating with CuKα radiation (λ = 1.541 Å), and using a step size of 0.05° 2θ within the 2θ domain from 20 to 70 °.

Electrochemical analyses were carried out in a three-electrode cell comprising a glassy carbon rotating disk electrode (RDE) (working electrode geometric area 0.196 cm 2 ), a reversible hydrogen electrode (RHE, reference electrode) and a platinum wire (counter electrode), and a BioLogic SP300 potentiostat. All the potential values are referred to the RHE.

As a conventional reference, 5 mg of 57 wt.% Pt/C (HiSPEC 9100 from Johnson Matthey) were mixed with 1 mL of DI water, 4 mL of ethanol (Aldrich) and 35 µL of the 5 wt.% Nafion ® solution in alcohols (Aldrich). After 20 min of sonication, 7 µl of ink was deposited onto the electrodes and dried for 10 min to obtain a total Pt loading of 4 µg on the RDE tip.

For the electrodeposited Pt, disk (ϕ = 4 mm) shaped self-standing electrodes were cut from electrodeposited samples using a punch tool and attached to the glassy carbon RDE tip.

Electrochemical measurements were performed in a 0.1 M HClO4 solution saturated with either N2 or O2. In particular ECSA determination was performed by CV in N2 saturated 0.1 M HClO4 cycling between 0.05 and 1.2 V (vs RHE) at a scan rate of 5 mV/s, while linear sweep voltammetry was performed in O2 saturated 0.1 M HClO4 between 0.2 and 1.2 V (vs RHE) at a scan rate of 20 mV/s and RDE rotating speed 1600 RPM. Cyclic voltammetry based accelerated degradation was performed by cycling between 0.6 and 1.2 V at 500 mV/s for 10,000 cycles measuring the ECSA every 100 th cycle at 20 mV/s.

Results and discussion

Carbon nanofibrous electrodes are prepared from polyacrilonitrile (PAN) derived electrospun carbon nanofibres, and have been demonstrated to be a potentially viable alternative carbon support for electrocatalysts. [37][38][39][40][41][42][43] We achieved fine control over electrode thickness, nanofibre diameter, degree of orientation and hydrophilicity. 39,43 Mechanical properties (ultimate tensile strength 2.2 MPa, similar to other inorganic electrospun self-standing mats 44 ) and electrical conductivity (σ = 32 S cm -1 , Figure S1) of the prepared electrodes are more than adequate to perform electrodeposition.

Due to poor nucleation, standard electrodeposition has been shown to produce large (50 -200 nm) and sparse Pt aggregates. Furthermore, diffusion of the noble metal through the porous electrode quickly becomes the limiting factor, generating inhomogeneous deposition. 40,45,46 Self-limited layer-by-layer electrodeposition of thin platinum group metal films has been reported on gold and nickel substrates. [47][48][49] This method could not be transposed as such on the electrodes in this work due to the different nature of the substrates both in terms of their surface chemistry, morphology and more importantly electrodynamic behaviour, but it provided an advantageous starting point for the outcomes in this work.

A pulsed electrodeposition obtained by alternating the voltage between a small overpotential to the Pt deposition in the hydrogen evolution region on Pt (-1.0 V vs SCE for 1 minute) and a resting potential (+0.4 V vs SCE for 2 minutes) as suggested for self-limited Pt deposition methods on metal surfaces 47 , while improving nucleation did not improve homogeneity nor produced thin smooth Pt films. To evaluate the Pt deposition dynamics we prepared a set of NFE using a higher overpotential to improve nucleation [50][51][52][53] (-3.0 V and 0.0 V in a two electrode setup). All samples shared the same total deposition time of 200 s, which was divided into a different number of pulses. Pt loading values were measured by thermogravimetric analysis (TGA) to calculate the Pt deposition rate during a single pulse: the calculated Pt deposition rate is typical for diffusion limited processes and shows a sharp decay during the first 10 seconds of a pulse, after which a severely diffusion limited constant deposition rate is reached (Figure S2). This sharp decay is consistent with an increase mass transfer overpotential induced by an increasingly thicker boundary layer generated by the reactant consumption. The combined effect of pulse duration and number of pulses over the morphology of the deposited Pt was investigated by electron microscopies.

The morphology obtained is strongly dependent on the pulse time range: only pulses ≥ 40 s produce a morphology approaching the targeted Pt extended surface, being composed of a mosaic of flat nanoplatets on the carbon fibre and only few large nanoparticles (>20 nm); with shorter pulses (from 20 to 1 second), a dense distribution of large Pt aggregates is produced (Figure S3), indicating that efficient nucleation requires longer pulse times. [START_REF] Parsonage | US5338430 A[END_REF] The morphology of Pt surfaces obtained with long pulses (200-40 sec) was investigated by scanning electron microscopy. Two-dimensional nanoplatelets all over the fibre surface were distinguishable, as a metal "background" (inset in Figure 1a). HRTEM allowed their observation (Figure 1c) (their crystallinity was further demonstrated by XRD, Figure S4) as well as the determination of their exposed crystal facets (Figure 1f), the (111) (d = 0.23 nm), which are known to have higher initial ORR specific activity than the (100). [54][55][56] From the electrochemical surface area measurements, we can estimate a thickness lower than 1 nm for these platelets (see later in the text). Similar twodimensional objects with a plate-like morphology and thickness of 1.3-1.6 nm were obtained under similar conditions by electrodeposition on Highly Oriented Pyrolytic Graphite (HOPG). 57 In order to better observe the morphology features of the NFE, we increased the deposited Pt amount onto the carbon mats by increasing the number of pulses (while keeping the total deposition time fixed at 200 s). The SEM observations were performed both on the inner and outer layers of the electrodes to investigate the homogeneity of the platinum deposition (Figure 1). Inhomogeneity of Pt growth occurs as the number of pulses increases. A detailed analysis of the formation and morphology of large Pt aggregates formed on increasing the number of pulses, and its effect on the electrochemical properties can be found in the Electronic Supplementary Information (Figure S5 andS6). Indeed, for 1 pulse of 200 s the Pt dispersion appears quite homogenous all over the electrode thickness, while for 5 pulses of 40 s, preferential growth onto existing nanoislands occurs at the external surface of the electrode (thus in direct contact with the electrodeposition solution), while less Pt is deposited in the inner part of the NFE. The experimentally observed higher Pt deposition rate on the NFE external layer compared to the inner layer is typical in diffusion-limited depositions 58 and is in agreement with the calculated Pt deposition rate typical of diffusion limited processes (Figure S2).

Furthermore, this phenomenon can be intensified by the strong hydrogen evolution occurring at the high overpotentials imposed. Indeed, gas evolution can promote mass transfer to the external layers reducing the diffusion layer thickness and effectively increasing the deposition rate to these areas. 53 In addition, ohmic effects could also contribute to the deposition gradient, yet the relevance of the two controlling factors cannot be quantified. As expected, increasing the pulse number amplifies the observed difference (Figure S3). We can thus observe Pt growth on existing nanoplatelets, which ultimately leads to a thick continuous layer on the outer part of the electrode beyond 10 electrodeposition cycles.

The existence of an ultrathin diffuse platinum coverage is confirmed by the STEM images taken on fibres with a minimum amount of platinum agglomeration obtained with a 200 s deposition time. In particular the EDX map in Figure 2b provides evidence for the presence of platinum in regions where no particles are visible in the bright field STEM of Figure 2a. In addition, HAADF (Figure 3c) also suggests a diffuse almost complete coverage of platinum showing only slightly darker regions where bare carbon is exposed. An ultrathin diffuse coverage was obtained using the same deposition protocol on a model HOPG surface and observed by AFM, providing additional support for the above conclusions. Electrochemical properties were evaluated directly on disk shaped electrodes mechanically attached on a glassy carbon RDE tip (Figure 3a). Extremely high Pt exploitation was observed: the electrochemical specific surface area (ECSA) was calculated from the hydrogen adsorption charges. 59 ECSA exceeding 120 m 2 g -1 was measured for the sample prepared with a 40 sec pulse; this value is comparable to those obtained in electrocatalysts with Pt nanoparticles with diameter in the range 1-2 nm. 60,61 . Similar values were obtained with the sample deposited with a single 200 s pulse (ECSA ~ 140 m 2 g -1 ). These values are smaller than the theoretical value for a (111) oriented Pt monolayer (265 m 2 g -1 ) yet higher than those expected for a continuous Pt film (around 60 m 2 -1 for a 1 nm thick film), and it is due to the presence of discrete nano-platelets. This also indicates that the Pt utilisation is much higher when Pt is synthesized directly on the fibre surface provided that the agglomeration is kept to a minimum and the dispersion is maximized. The ECSA of ca. 140 m 2 g -1 was used to derive an indirect indication of the platelet thickness of < 1 nm. It is thus evident that the presence of the few large nanoparticles on the fibre surface (Figure 1a andd) has little influence on the overall ECSA values.

All the prepared NFE are active towards the oxygen reduction reaction (ORR). Figure 3b is an example of a typical ORR curve taken at 1600 RPM in oxygen saturated 0.1 M HClO4 solution (the NFE shown was prepared with 40 s pulses). ORR onset potential reaches 1.07 VRHE and at 0.9 VRHE (the typical reference potential at which compare electrocatalytic activities) the reaction is already into the diffusion-limited domain. Due the thickness of the support (~ 50 µm) and the mounting setup it is impossible to calculate reliable mass activity values without modelling the oxygen diffusion through a thick layer and taking into consideration the presence of a limiting kinetic current after the external diffusion effect is corrected. 62 Nevertheless, without considering that the mounting setup (Figure S7) partially masks the electrode from the electrolyte and disturbs a normal laminar flow, and without taking into account the oxygen diffusion through the thick layer, a value of 130 A g -1 at 0.9 VRHE was calculated for the mass activity of the NFE. This mass activity is vastly underestimated due to the impossibility of applying the adequate corrections to the measured currents. Tafel slopes of ~70 mV decade -1 confirm a 4 electron pathway.

A second key factor, as essential as Pt exploitation, is the stability of the catalyst over time. Support corrosion together with Pt dissolution, migration and agglomeration tend to reduce the available electroactive surface. The samples were aged by cycling the potential between 0.6 V and 1.2 V in the range of both carbon and platinum degradation, and the change in ECSA was monitored to evaluate the stability of the catalysts (Figure 3d). ECSA decay was lower for the NFE (-39 %) compared to the commercial electrocatalyst (-55 %). After 10,000 potential cycles, the ECSA of the aged NFEs (77 m 2 g -1 ) is still comparable to that of fresh (non-aged) conventional Pt/C (78 m 2 g -1 ). These values are even higher than those obtained when using highly corrosion resistant metal oxide supports that however, present lower electrocatalytic performance. 35,[63][64][65] Furthermore, the ECSA decay trend of conventional Pt/C is identical to that of carbon nanofibres also bearing Pt nanoparticulate catalysts and submitted to the same accelerated stress test: 66 in both cases only 45 % of ECSA is retained after 10,000 electrochemical cycles. This result demonstrates that combination of Pt morphology and deposition method seems to be more crucial than the morphology of the support itself. Therefore the nature of the Pt/support interface and the presence of thin 2D Pt islands are key factors to explain the increased stability. Electrodeposited Pt tends to form an extended interface with the support that increases the adhesion and the amount of carbon that needs to be removed before a particle is free to migrate; thus increasing the stability of the system. This result is further confirmed by ORR activity evaluation after accelerated stress testing. Figure 3b shows that at the halfway point the total loss was around 10 mV for the NFE and 58 mV for conventional Pt/C. Whilst the ex situ characterisation of the NFE produced is overwhelmingly promising, performance of these electrodes in devices such as fuel cells, where oxygen reduction occurs at or near the 3-phase (electrode surface, electrolyte, gas phase) boundary, may be significantly different, and may not necessarily directly translate to high performance.

The requirements for a good electrocatalyst layer include high gas/ion transport to and from the catalyst: the thickness, low geometric surface areas and anisotropic connectivity of the NFE will have to be addressed to achieve the full potential in MEA operation. We are now pursuing various approaches to adapt the characteristics of the above NFEs to fuel cell MEA application.

Conclusions

We demonstrated that it is possible to produce highly stable and hierarchically structured 3 dimensional nanofibrous electrodes (NFEs) with outstanding Pt exploitation and electrochemical stability combining two simple and cost-effective techniques: electrospinning and electrodeposition. In particular we showed how controlling the pulse length in a pulsed Pt electrodeposition can be used to obtain extended metal surfaces composed of nanoislands on self-standing nanofibrous carbon mats. The ECSA of these novel electrocatalysts reaches 140 m 2 g -1 , higher than that of most highly optimised currently available commercial catalysts. This new class of catalysts intermediate between NTSF and conventional carbon black supported Pt nanoparticles shows outstanding stability, significantly higher than what is achieved with conventional methods and without the introduction of more stable but less conductive metal oxide supports. Support morphology does not define alone the stability of the electrocatalyst, it is paramount to maximise support interface with the Pt catalyst.

High ECSA, durability over time of the prepared catalyst layer combined with the scalability of the adopted process makes NFEs prime candidates as alternative to conventional Pt/C catalysts, as well as to NTSF and Pt/metal oxides or carbides.

Figure 1 .

 1 Figure 1. SEM micrographs of NFE deposited with various pulses times. 1 pulse of 200 s internal (a) and external layers (b) of the electrodes, 5 pulses of 40 sec internal (d) and external layers (e) of the electrodes, (c) HRTEM micrographs of an isolated Pt nanoplatelet and (f) the respective FFT images.

Figure 2 .

 2 Figure 2. STEM micrographs of platinum covered nanofibres: Bright field (a) and relative platinum EDX map (b). (c) HAADF STEM image.

Figure 3 .

 3 Figure 3. (a) Cyclic voltammetry in N2 saturated 0.1 M HClO4, (b) oxygen reduction reaction polarisation curves at 1600 RPM and (c) the corresponding Tafel analysis. (d) Normalised electrochemically active surface area as a function of the number of voltage cycles. The samples compared are conventional Pt/C in black, 40 s pulse in red.

Acknowledgements

The research leading to these results has received funding from the European Research Council (ERC) under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement SPINAM n. 306682. Funding from the European Community's Seventh Framework Programme (FP7/2013-2016) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement CATAPULT n. 325268 is also acknowledged.

Notes and references


Sophie M Guillaume

Advances on the Synthesis of Silyl-Modified Polymers (SMPs)

Silyl-modified polymers (SMPs) are alkoxysilyl functionalized polymers of high importance both academically and industrially. SMPs are being developed for coating, adhesive, sealant and elastomer applications. In this highlight, emphasis is placed first on the different chemical modification approaches available to tackle the synthesis of SMPs by in situ functionalization (ISF) and post-polymerization functionalization (PPF) routes, with their pros and cons being underlined. Recent progress and modern trends towards alkoxysilyl telechelic polyenes and polyolefins by olefin metathesis strategies is next discussed. The state of the art of alkoxysilyl functionalized polymers, especially of telechelic ones, delineated according to these latter two main synthetic pathways, is then covered along with their subsequent curing, and illustrated by selected examples including in particular polyenes, polyolefins and a some other selected relevant polymers.

TOC

Silyl-modified polymers (SMPs) are being synthesized from chemical modification and olefin metathesis strategies.

Introduction

Silyl-modified polymers (SMPs)

Silyl-modified polymers (SMPs), defined as polymers typically featuring alkoxysilyl functional groups (FGs), [START_REF] Da Silva | Eds Handbook of Adhesion Technology[END_REF][START_REF] Mittal | Handbook of Sealant Technology[END_REF][START_REF] Ed | Adhesive Technology Handbook[END_REF][START_REF] Pizzi | Handbook of Adhesive Technology, Revised and Expanded[END_REF] have been known for more than 40 years, being essentially exploited over the past two decades in the coatings, adhesives, sealants and elastomers (CASE) industry. SMPs constitute the main components in solvent-free, poly(vinyl chloride)free, and isocyanate-free sealants, adhesives and coating materials towards architectural, construction or industrial (aerospace, aeronautics) applications. 5 , 6 , 7 , 8 , 9 , 10 , 11 Their good adhesion to a wide range of substrates (e.g. metal, ceramic, plastics, glass…), their good temperature and UV (weather) resistance, and their excellent durability with limited surface treatment (absence of shrinkage or crack over time), are especially valued. In particular, SMPs are widely used to bond directly to glass with minimal surface preparation, and are suitable for large surface areas within adequate working times, efficient handling, while providing actual insulation. They thus represent the adhesives of choice for bonding glass in all types of vehicles and side panels in trailers/trucks, for door and window construction, fixing solar panels, assembly parts, interior elements, container construction, etc. Also, in the biomedical domain, a recent illustration of the potential of SMPs has been provided by the research group of David Haddleton who developed an ibuprofen transdermal skin patch as a novel drug delivery system, from the SMP adhesive produced without solvent by Bostik. [START_REF] Daniels | University of Warwick chemist develops world's first ibuprofen patch[END_REF] SMPs are commercially available hybrid polymers which combine the environmentand user-friendly advantages of the silicon-cure chemistry of silicone sealants (no strong odor, fast and controlled curing process, no bubblingi.e. no CO 2 emission and thus no foaming -, no staining of porous substrates) to the backbone of a polymer (typically polyurethane, polyether, polyisobutylene, polyacrylate), with bond strength, durability and elasticity over a wide range of temperatures and humidity ratios. [START_REF] Lee | Adhesion Science and Technology[END_REF][START_REF] Owen | [END_REF][START_REF] Chruściel | Modification of Thermoplastics with Reactive Silanes and Siloxanes[END_REF] SMPs thus present the advantage of being particularly environmentally and industrially attractive water-based and solvent-free adhesives for which shrinkage, typically encountered with solvent-based adhesives, hardly occurs upon curing. [START_REF] Necasova | [END_REF]17,18,19,20,21,22,23,24,25,[START_REF] Michaud | 1 -CNRS[END_REF] The curing process of SMPs involves hydrolysis of the silyl alkoxide FG(s) which then results in the formation of cohesive siloxane (-Si-O-Si-) bonds, thus insuring the cross-linking of a liquid or gel pre-material into a 3D solid one, and assuring the cohesive adhesive joint providing the resistance of the assembly. [START_REF] Mittal | Handbook of Sealant Technology[END_REF][START_REF] Laferte | [END_REF][START_REF] Okamatsu | [END_REF][START_REF] Lee | Adhesion Science and Technology[END_REF][START_REF] Owen | [END_REF][START_REF] Chruściel | Modification of Thermoplastics with Reactive Silanes and Siloxanes[END_REF]21 Curing is a two-step process involving first the conversion of the alkoxysilane to silanol followed by the condensation of -Si-OH and -Si-OR to form siloxane linkages upon elimination of ROH (or by condensation of two -Si-OH accompanied with H 2 O abstraction) (Scheme 1). The condensation process is promoted by the surrounding moisture arising from the environment and/or the substrate onto which the SMP is applied, and also most often by the presence of an acid or base catalyst. Polysiloxane network formation may even take place at room temperature (RT) and the curing process is advantageously fast. Polymers with Si-OR bonds can also react with silanol groups or metal oxides present at the surface of inorganic materials, thus making them valuable in the preparation of organic-inorganic composite materials. 27 Given the renewed interest in the use of silica as a reinforcing filler for elastomers, trialkoxysilyl functionalized polymers have thus become attractive for their ability to facilitate the dispersion of silica and to promote the polymer interactions with the filler. Finally, SMPs represent an alternative to polyurethanes used in the adhesives, sealants and paint industries. Indeed, one major drawback of uncured polyurethane-based sealants and adhesives is the presence of unreacted isocyanate groups which are very reactive and harmful to humans in various forms, and thus undesirable. In particular isocyanate groups react with H 2 O thus forming CO 2 which induces the formation of voids within the material, thereby promoting cohesion failure. SMPs will also comparatively pick-up less air-borne dust particles due to their lower static charge, induce less streaking, prompt no shrinkage after curing due to the absence of solvent, be paintable, and enable damp substrate bonding. The major manufacturers of SMPs are Bostik, Evonik, Henkel, Kaneka and Wacker.

Within SMPs, the -Si(OR) x moiety may be either an end-capping or a side-chain function. Although commercially relevant SMPs with advanced performances are typically α,ω-di(alkoxysilyl) telechelic polymers, mono-Si(OR) 3 end-capped (i.e. semi telechelic) polymers have also been reported, with successful ensuing curing.

Synthetic routes to SMPs

While the successful end-capping of a polymer still remains somewhat synthetically challenging, the two most conventional synthetic approaches towards end-functionalized polymers include either i) the direct formation of the chain-end functionalized polymer by introduction of the reactive group during the polymerization (in situ functionalization (ISF)) either through the functional initiator (during the initiation), the functional chain-transfer agent (CTA; during the propagation), or the functional terminating agent (during termination) to end-cap the growing polymer chain, or ii) the post-polymerization chemical modification of a preformed polymer (post-polymerization functionalization (PPF)) (Scheme 2). 28,29,30,31,32,33,34 Another possible synthetic route towards SMPs, specific to polyenes and polyolefins (POs), is the metathesis of olefins (Scheme 2). 28,31, 35 , 36 , 37 Living and controlled/living polymerization methods now enable to access well-defined end-functionalized polymers with a high degree of end-functionalization, featuring controlled and tunable molar mass, and controlled narrow dispersity, chain-end fidelity, functionality, microstructure and tacticity, resulting in polymer materials with various physical properties. 28- 37

SMPs

Presynthesized polymer

Post-polymerization chemical modification

Direct polymerization

Monomer

Functional reagent

ISF PPF

Olefin

Metathesis

Scheme 2. Synthetic strategies towards SMPs.

Scope of this highlight

It is the purpose of this highlight to summarize routes and modern trends in the special class of alkoxysilyl functionalized polymers, emphasizing on telechelic ones, and illustrating with selected studies and important milestones ( telechelic polyenes and POs. The chemistry employed to obtain SMPs, including polyenes and POs as well as various selected relevant alkoxysilyl functionalized polymers, is presented along with, when reported, their subsequent curing and the utility of the product materials.

Focus is placed on the chemistry developed towards SMPs, on their macromolecular features (especially molar mass and dispersity) when available, and not on the spectroscopic characteristics nor on the materials themselves. Scientific articles which appeared in the literature in the last decade and a half are the main focus of this highlight, although appropriate earlier examples are also mentioned so as to provide a complete overview. 38

Synthesis of SMPs

Synthesis of SMPs by chemical modification

Tuning of a polymer end-capping group(s) can be achieved through either the use of a functional initiator used in the initiation step (A α-end-group), upon in situ termination of the growing polymer chain with a functional quenching reagent (B ω-end-group), or upon postpolymerization chemical modification of pre-existing A and/or B chain-end group(s), thus affording the desired parent end-functionalized polymer. [28][29][30][31][32][33][34][35][START_REF] Theato | Functional Polymers by Post-Polymerization Modification: Concepts, Guidelines and Applications[END_REF][START_REF] Boaen | [END_REF]76,[START_REF] Chung | Functionalization of Polyolefins[END_REF] In the first functional initiator/in situ termination (in-situ functionalization (ISF)) routes, the functional initiator/termination agent should be compatible with the polymerization process and with the chemical composition of the polymer. These ISF methods may suffer from a poor control of the degree of end-functionalization with mixtures of incomplete mono-and/or difunctionalized polymers being possibly recovered. In the PPF approach, a single parent (synthetic and natural as well) polymer precursor can serve as a template towards the preparation, through chemical modification, of a whole and large family of chain-end functionalized polymers displaying identical molar mass, dispersity, tacticity, and yet different architectures and properties. Another advantage of the PPF towards endfunctionalized analogues is to avoid any catalyst poisoning and any issues in catalyst FGtolerance. Conversely, the limitation of the third PPF method remains the possibly unreactive nature of the polymer requiring sometimes harsh operating conditions and possibly accompanied with a lack of selectivity, which overall may compromise the end-functionalization of the polymer. These latter two points are especially relevant for the synthesis of functionalized polyenes or POs. [START_REF] Boaen | [END_REF][76][START_REF] Chung | Functionalization of Polyolefins[END_REF] Quantitative conversion of chemical moieties is also essential to avoid a time-consuming and costly purification of the polymer from the unreacted reagents. Advances in PPF over the last three decades, especially due to the progress in revisited coupling chemistries such as thiol-ene/yne processes, chemistries based on isocyanates, ketones, aldehydes, epoxides, anhydrides, activated esters, cycloaddition reactions, etc., nowadays afford a reliable strategy towards end-functionalized polymers.

Synthesis of SMPs through an in situ functionalization (ISF) approach

End-functionalization of a polymer upon termination of the polymerization using a functional reagent to react in situ with the growing polymer chain is an easy way to introduce chain-end functionality onto a polymer. Benefiting from earlier developments in anionic polymerization, several SMPs have thus been synthesized following this ISF approach. This method has indeed been more widely exploited than the ISF route using a functional initiator.

However, it remains so far limited to access polybutadiene (PBD), polyisoprene (PIP) and polystyrene (PS) SMPs.

α-Trimethoxysilyl (or triethoxysilyl) end-capped 1,4-PIPs were synthesized upon deactivation (THF, -20 °C for 4 h then RT for 18 h) of poly(isoprene)-yl-lithium (PIPLi; M n = 800 g.mol -1 , Ð M = 9.8) by using tetramethoxysilane, tetraethoxysilane, or 3-chloropropyltrimethoxysilane as reported by Derouet and coworkers (Scheme 3). 40,41,42 On the basis of 1 H and 29 Si 1D and 2D NMR investigations, the nucleophilic substitution reaction of alkoxy groups from the alkoxysilane CTA by the poly(isoprene)-yl ω-carbanions was shown to be effective, and to occur exclusively on the silicon atom and not on the carbon bearing the X substituent (evidenced with X = Cl). 68,000 g.mol -1 , Ð M = 1.2). Cross-linking of the isopropoxysilyl groups was next achieved via hydrolysis in dilute aqueous HCl solution and subsequent condensation of the resulting silanol groups from poly(VP-SiMe 2 (OiPr)) segments. The films therefrom were next investigated for their microstructure.

In a similar approach, a high (80-90%)-1,4-PBD end-capped with a (3-chloropropyl)(dimethoxy)silyl group was prepared by Soutar and coworkers in good yield (up to 85% functionalization) (Scheme 4). 47 A previously formed living poly(butadiene)-yllithium (prepared from the anionic polymerization of 1,3-butadiene initiated by nbutyllithium) was reacted with 3-chloropropyl(trimethoxy)silane (THF, -10 °C for > 5 h and RT for 18 h), with displacement of one or two methoxy groups (M n,NMR ca. 2100 g.mol -1 , Ð M = 1.2). Also, termination of poly(butadien)-yl-lithium with tetramethoxysilane afforded a mixture of silylated PBDs including polymers terminated by -Si(OMe) The alkyl lithium-initiated anionic polymerization of styrene afforded living poly(styryl)lithium anions which could be similarly end-capped with a variety of FGs. PS with an α -Si(OMe) 3 end-capping group was thus synthesized by Ward and coworkers from the anionic polymerization of styrene initiated by sec-butyllithium, and subsequent quantitative end-capping of the living chain ends with p-chloromethylphenyl trimethoxysilane (in excess) (M n,NMR = 2900-11,000 g.mol -1 , Ð M = 1.05-1.3). 48 Subsequent acid catalyzed hydrolysis and condensation (upon heating) of the trimethoxysilane end-groups (THF, HCl, 36 h, 25 °C, quantitative) resulted in star-branched PS, as evidenced by NMR spectroscopy and SEC analysis. This represented the first report of core-functionalized star-shaped polymers as silicon (Si) surface modifiers. Similarly, using butadiene (5 equiv.) or 1,1diphenylethylene (DPE, stoichiometric amount) as end-capping reagent, PS-b-PBDLi or PS-DPELi, respectively, were thus obtained from PSLi, as reported by Quirk and coworkers. 49 Subsequent functionalization upon termination with glycidoxypropyltrimethoxysilane generated the corresponding mono(trimethoxysilyl) functionalized polymers, as evidenced by 1 H, [START_REF] Lee | Adhesion Science and Technology[END_REF] C, and 29 Si NMR spectroscopies. These latter trimethoxysilyl chain-end functionalized PS were hydrolyzed (THF, aqueous HCl, 115 °C) into four-armed star-branched polymers. Also, note that some mono-, di-, tri-alkoxy-substituted styrene monomers were successfully anionically polymerized to provide PS with pendant alkoxysilyl functions.

Likewise, mono-di-and tri-alkoxysilyl side-chain functionalized PBDs were prepared by Nakahama and coworkers from the anionic polymerization of 2-mono-(di-, tri-)alkoxysilyl-1,3-butadienes using oligo(α-methylstyryl)dipotassium. Similarly, PBD/PIP block copolymers featuring pendant Si(OR) 1-3 groups, were also prepared upon addition of 2-mono-(di-, tri-)alkoxysilyl-1,3-butadienes to anionic living PIP. Subsequent hydrogenation of the resulting poly(2-silyl-substituted-1,3-butadiene)s and corresponding PIP copolyolefins using Pd/C catalyst led to the quantitative saturation of the double bonds regardless of the substituent on silicon atom. Also, Teyssié and coworkers, and Nakahama and coworkers, reported the anionic polymerization of 3-trialkoxysilylpropyl methacrylates using 1,1diphenylhexyllithium in the presence of LiCl, affording polymethacrylates with pendant C 3 H 6 -Si-(OR) 3 groups on each repeating unit (OR = OMe, OEt, OCHMe 2 ). Similarly, the anionic polymerization of N-propyl-N-(3-triisopropoxysilylpropyl)acrylamide gave polyacrylamide chain laterally functionalized with alkoxysilyl groups. In these latter series of works, bulky alkoxide Si-substituent(s) prevented the nucleophilic attack of the Si-O bond during the polymerization. 27 and references therein

Synthesis of SMPs through a post-polymerization functionalization (PPF) approach

More recently, thiol-ene chemistry was used by Boisson, D'Agosto and coworkers to access to a large range of homotelechelic PEs (Scheme 5). Similarly, the radical addition under moisture of 3-mercaptopropyl(triethoxy)silane to the C=C double bonds of a low molar mass dihydroxy telechelic PBD (AIBN, 75 °C) quantitatively afforded the analogous PBD with 1.05 to 7.9 -[S(CH 2 ) 3 Si(OEt) 3 ] 2 pendant moieties per chain, as reported by Bunel and coworkers (Scheme 6). 57 Hydrosilylation also provides another efficient entry towards polymers flanked with an alkoxysilyl group pending on each repeating unit. Thus, as reported by Chauhan and coworkers, the highly selective, mild and clean hydrosilylation of 1,2-PBD with hydromono-(di-)alkoxysilanes (HR' 2or1 Si(OR) 1or2 = HSiMe 2 (OEt), HSiMe(OSiMe 3 ) 2 ) catalyzed by recyclable Pt nanoclusters afforded the parent internally alkoxysilyl functionalized PBDs. [START_REF] Chauhan | [END_REF] Similarly, Rempel and coworkers reported the reaction of PBD with HSi(OEt) 3 in the presence of Rh(Cl)(PPh 3 ) 3 (toluene, 110 °C, 10-20 h), thereby providing internally triethoxysilyl functionalized PBD. 79 Also, the gas solubility and permeability of various cured (saturated water vapor, ca. 100 °C) poly(alkoxysilylbutadiene) films were studied in relation to the bulkiness of the alkoxy groups and cross-linking density. 80 In another work, Lam and coworkers prepared α,ω-triethoxysilyl telechelic polymers through a two-step PPF approach from hydrogenated PBD (HPBD) diols upon reaction with isophorone diisocyanate (stoichiometric ratio NCO/OH = 2, bulk, 80 °C, 4 h, vacuum) followed by the addition of aminosilane (stoichiometric ratio NCO/NH 2 = 1, THF, RT, 1 h) (M n,SEC = 8200-14,000 g.mol -1 , Ð M = 2.2-2.7). Hydrolysis and condensation reactions (H 2 O, triflic acid, RT, bulk or THF) gave different micro-phase separated network materials (Scheme 8). This latter strategy was also applied to the PPF preparation of trialkoxysilyl terminated poly(ethylene glycol) (PEG) (M n,SEC = 1000-1500 g.mol -1 , Ð M = 1.1-1.7; gel time), 61,62 poly(proylene glycol) (PPG) (M n,SEC = 16,200 g.mol -1 , Ð M = 1.41), [START_REF] Okamatsu | [END_REF]22,23 PEG-co-PPG, 22 poly(tetramethylene oxide) (M n,SEC = 9600 g.mol -1 ; mechanical properties), [START_REF] Okamatsu | [END_REF]25,68,69 polybutylacrylate (M n,SEC = 9900 g.mol -1 ), [START_REF] Okamatsu | [END_REF] and poly(dimethylsiloxane) (PDMS) (M n,SEC = 4100-12,500 g.mol -1 , Ð M = 2.1; T g = -113 to -123 °C; mechanical properties), 60 which were subsequently hydrolyzed and polycondensed. In particular, moisture-curable polyurethanes end-capped with trialkoxysilane were thus obtained as precursors to one-component and solvent-free adhesives matrials. [START_REF] Okamatsu | [END_REF]22,23,25 Following again the same approach, triethoxysilyl telechelic polycaprolactone (PCL) was prepared by Liu and coworkers from a PCL diol (M n = 1000-2000 g.mol -1 ) and the -NCO groups of isocyanatopropyltriethoxysilane, in the presence of DBTL (65 °C). 65 Similarly, a blend of triethoxysilyl end-capped PCL and PDMS oligomers was prepared from a mixture of the corresponding polydiols. 66 The resulting PCL elastomer or PCL/PDMS blend were then combined under humidity with a commercial epoxy resin (diglycidyl ether of bisphenol A, DGEBA) to form ternary composites which were ultimately cured using an amine-based curing agent and a sol-gel process (DBTL, RT, 3 weeks, under ca. 60% relative humidity), respectively. 65,66 Silyl-terminated oligomers of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) were synthesized by hydrosilylation of allyl-terminated pre-synthesized polyesters as reported by Kimura and coworkers (Scheme 9). 53 The previously prepared acetylated PLAs were chemically end-modified upon reaction with trimethoxysilane in the presence of Karstedt's catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane; 40 °C, 10 h; 75% hydrosilylation). The resulting α-alkoxysilyl functionalized PLAs were characterized by [START_REF] Da Silva | Eds Handbook of Adhesion Technology[END_REF] H NMR (M n,NMR ca. 2800 g.mol -1 ), SEC (M n,SEC = 3900 g.mol -1 , Ð M ca. 1.37), differential scanning calorimetry (DSC, T g ca. 32 °C, T c ca. 84 °C, T m ca. 127 °C), and thermal gravimetric analysis ((MeO) 3 Si-PLAs showed a higher thermal stability than the parent acetylated PLA). These polyesters were next readily hydrolyzed (aqueous HCl, 48 h) into hyper-branched polymers (M n,SEC (bimodal) ca. 17,300, 5200 g.mol -1 , Ð M ca. 1.14-1.20; T g = 47 °C, T c = 105 °C, T m ca. 125-130 °C; the higher T g and T c values compared to the parent (MeO) 3 Si-PLA were attributed to the cross-linking structure that made the chain mobility significantly lower) with particulate morphology. Introduction of pentafluorobenzene (PFB) as a single end-group in polythiophene (PTP) derivatives was reported by Heeney and coworkers via in situ quenching of the polymerization. 54 PFB is a versatile "click" handle due to its stability towards basic conditions thus avoiding the need for protecting groups, and due to its ability to undergo fast nucleophilic aromatic substitutions without required modification, thereby enabling to quantitatively tether a variety of common thiol, amine, and alcohol nucleophiles to the PTP backbone under mild conditions. PTP end-capped with a single cross-linkable trimethoxysilyl ω-FG was thus obtained via the quantitative reaction of a preformed PTP-PFB with the commercially available reagent (3-mercaptopropyl)trimethoxysilane via the nucleophilic attack at the para-position, as assessed by matrix-assisted laser desorption-ionization time-offight (MALDI-ToF) mass spectrometry (MS) and 19 F NMR analyses (Scheme 10). 54 The resulting -Si(OMe) 3 end-groups were next utilized to form, by spin-coating followed by annealing, a partially retractable thin film on the surface of glass substrates. Note also, that hybrids between conjugated poly(3-hexylthiophene), a benchmark material for organic electronics, and inorganic materials made of silica-core organosilica-shell particles, thus featuring particle-Ο-Si(OR) 2 -PTP bonds, have also been reported by Kiriy and coworkers upon growing the polymer from the particle.

Si

OMe

OMe OMe HS Scheme 10. Post-polymerization α-functionalization of PTP with an α-trimethoxy functional thiol. 54 Also, the solvolysis and condensation reactions of PEG diol with tetraethoxy-or tetramethoxy-silane (H + catalyst, MeOH or THF, 50 °C, 30 min) provided the corresponding trialkoxysilyl end-capped PEG oligomers, ultimately affording gels, as reported by Vincens and coworkers. 63 Reaction of the hydroxyl end groups of three PEG moieties bound to a molybdenum center with Cl(CH 2 ) 3 Si(OEt) 3 afforded the corresponding tris[(triethoxy)silyl] functionalized organometallic complex. This latter was subsequently co-condensed with tetraethoxysilane thus providing by sol-gel technique, organic-inorganic hybrid polymers with different properties. 64 Finally, reaction of a bifunctionalized living poly(N-tert-butylaziridine), prepared by Goethals and coworker by cationic ring-opening polymerization of the N-tert-butylaziridine using triflic anhydride, with 3-aminopropyltriethoxysilane afforded the bis(triethoxysilyl) telechelic polyaziridine. Subsequent hydrolysis (THF, CHCl 3 or MeOH, HCl, 25 °C) induced the formation of polyaziridine networks (M w = 2000-9000 g.mol -1 ) displaying a swelling behavior. 67

Synthesis of alkoxysilyl telechelic polyenes and polyolefins by olefin metathesis

Telechelic polyenes and POs, as functionalized polymers, are valuable materials with respect (but not limited) to, adhesion, toughness, print/paintability, compatibility with diverse more polar materials and rheological properties. 36,37,[START_REF] Chung | Functionalization of Polyolefins[END_REF][START_REF] Hutley | Polyolefin Compounds and Materials -Fundamentals and Industrial Applications[END_REF][START_REF] Yanjarappa | [END_REF] Introduction of reactive groups into these non-polar materials, thereby providing access to functionalized polymers, is highly industrially desirable so as to enlarge their field of applications, but yet remains rather synthetically challenging. Silyl-modified polyenes and polyolefins are a special type of SMPs which are desirable to enable adhesion to olefinic substrates, which is typically difficult due to their non-polar and non-porous surface. Current academic and industrial efforts are thus dedicated to develop such polymers. Although sily-modified POs flanked with alkoxysilyl pendant groups may also be cured, α,ω-di(alkoxysilyl) telechelic polyenes and POs remain the industrially targeted materials.

Strategies for the synthesis of telechelic polyenes and polyolefins

Common strategies for the synthesis of telechelic polyenes and POs basically involve the functional tuning/chemical modification of polymers (ISF and PPF as described above), [28][29][30][31][32][33][34][35] and the more topical metathesis polymerization approaches (Schemes 1, 11). 28,31,[START_REF] Boaen | [END_REF][76][START_REF] Chung | Functionalization of Polyolefins[END_REF][START_REF] Hutley | Polyolefin Compounds and Materials -Fundamentals and Industrial Applications[END_REF][START_REF]It should be noted that radical polymerization of non-polar olefins and polar vinyl monomers also enables the preparation of telechelic polymers; however, it[END_REF] As presented above, the preparation of alkoxysilyl functionalized PE upon PPF of α,ω-divinyl telechelic PE remains the sole example of such chemical modification routes towards alkoxysilyl telechelic polyenes/POs. 50,51 On the other hand, the synthesis of telechelic polyenes/POs through the selective catalytic introduction of the functional endgroup following living olefin metathesis polymerization processes valuably provides popular in situ methods applicable to a large array of monomers and carried out under extremely mild and user-friendly conditions. The olefin metathesis polymerization toolbox of mechanistically related techniques include ring-opening metathesis polymerization (ROMP) of cyclic olefins, and acyclic diene metathesis (ADMET) polymerization, while other related process including ring-opening metathesis (ROM), ring-closing metathesis (RCM), self-metathesis which refers to olefin homodimerization (SM), and cross metathesis (CM) of olefins, may also take place (Scheme 11). [START_REF] Hanik | Telechelic Polymers, in Handbook of Metathesis[END_REF][START_REF] Bielawski | Telechelic Polymers from Olefin Metathesis Methodologies[END_REF][START_REF] Grubbs | [END_REF]87 SM and CM of chain-end functional internal or terminal (poly)enes and (poly)olefins can give telechelic polymers with identical or distinct (heterotelechelic) end-capping FGs.

However, these routes require to adequately select the olefinic substrate so as to optimize the (stereo)selectivity in the reaction, thus limiting their use. 88,89,90,91,92 [M] ROMP and ADMET are the two most common and versatile routes to telechelic polyenes/POs. ADMET, as well as CM, are olefin metathesis polycondensation reactions involving linear symmetrically functional α,ω-diene or alkene monomers, respectively, which are thus driven by the removal of the small byproduct (usually ethylene gas) typically by an argon/nitrogen flow. ADMET is actually the most widely used olefin CM polymerization technique. 93 , 94 , 95 These latter step-growth polymerizations only give high molar mass polymers at high degrees of monomer conversion, consistent with Carothers equation. ROMP of cyclic alkene is a chain-growth polymerization thermodynamically driven by the release of the cycloalkene monomer ring-strain. 36, 96 , 97 , 98 The major shortcoming of such metathesis approaches is however the required chemical tolerance of the metal alkylidene catalyst towards FGs to be introduced at the chain ends. Developments over the last decades to overcome these synthetic challenges now offer FG-tolerant commercially available catalysts, and robust and reliable metathesis techniques. The nature of the catalyst being of foremost importance in metathesis, latest advances have established ruthenium-based catalysts (in particular, the commercially available Grubbs ruthenium carbenes are the most widely used)

as the most competitive in terms of activity and FG tolerance. [START_REF] Bielawski | Telechelic Polymers from Olefin Metathesis Methodologies[END_REF] When performed in the presence of a difunctional acyclic symmetric internal alkene CTA, ADMET and ROMP/CM straightforwardly provide access to the desired telechelic polyenes/POs (Scheme 11). Note that these metathesis reactions towards telechelic polyenes/POs require the prior synthesis of a library of CTAs. Given that metathesis polymerization preserves carbon double bonds along the backbone during the polymerization, an ultimate hydrogenation of the resulting polyalkenamers (polyenes) is required to get the corresponding POs without any double bonds in the backbone (Scheme 11). Indeed, post-metathesis polymerization hydrogenation has been successfully reported. 29,37,[START_REF] Boaen | [END_REF]88,[START_REF]For instance, hydroxyl-terminated poly(ethylene-co-butylene) (Krasol HLBH-P 3000; Cray Valley SA) is industrially produced via hydrogenation of hydroxyl terminated polybutadiene[END_REF]100,101,102,103,104,105,106,107,108 One can thus prepare telechelic polyenes/POs with finely controlled molar mass and dispersity, and a high degree of selective end-functionalization by ADMET and ROMP.

Thus, ADMET polymerization and depolymerization methods have been used for the synthesis of telechelic polyenes/POs oligomers. Metathesis depolymerization and degradation of 1,4-PBD, cis-1,4-PIP, their copolymers or other polyenes, carried out in the presence of various difunctional monoene CTAs, gave the corresponding telechelic polymers, such as borane, diester, diimide, epoxy, or trithiocarbonyl end-capped polyenes/POs. 104,105,106, 109 , 110 , 111 , 112 Alcohol or amine groups being detrimental to the metathesis catalysts, polyenes/POs chain-end functionalized with such groups were prepared by ADMET using FGs (such as acetoxy or phtalimide)-protected α-olefin CTAs, followed by a post-polymerization deprotection step. 107,108 Also, in a ROMP process, end-capping polyenes/POs with FGs can be implemented in three different ways. First, a functional initiator can be used in the initiation step. Even though each initiated chain is then ensured to bear the FG, this necessitates the prior synthesis of the organometallic complex bearing the suitable functional carbene, thereby requiring some expertise and skills in this field, and thus limiting the scope of this pathway. Besides, only semitelechelic polymers are thus formed. Functionalities such acetate, hydroxyl, halide have been reported from this approach. 113,114 Secondly, introduction of the desired functionality through the use of a functional quenching reagent during the termination step requires a long life-time and stable living species. Within this pathway, functional vinyl ethers such as azide, fluorescein, biotin derivatives, have been most commonly used. 115 , 116 Termination upon addition of carbonyl groups (especially aldehydes), lactones, oxygen, or a functional acyclic olefin such as acrylates (by CM), provide aldehyde, carboxylic acid, or acrylate chain-end functionalized polyenes/POs. 117,118 Finally, addition of a functional acyclic alkene as CTA has become by far the most simple, effective and largely applied (metathesis) approach towards the preparation of telechelic polyenes/POs. 28,31,36,96 The process involves the CM of the acyclic olefin CTA with the active chain-end. This route is metal-efficient since each catalytically active species generates multiple polymer chains, all end-capped by the desired FG. Adjusting the monomer and CTA stoichiometry nicely enables to control and tune the molar mass of the resulting end-functionalized polymer. Cis-alkenes are generally preferred CTAs because of their higher reactivity, while activated double bonds usually favor the CM.

Thus, both protected and unprotected reactive FGs successfully gave acetoxy, amino, borane, (thio)carbonate, carboxylate, halide, pseudo-halide, hydroxyl, and cross-linkable epoxide or methacrylate telechelic polyenes/POs. In particular, commercially valuable hydroxyl telechelic polyenes/POs were largely synthesized from acetoxy protected CTAs followed by a post-polymerization deprotection step. 31,36,119,120,121,122,123,124,125,126,127,128,129,130 Sacrificial synthesis of telechelic polyenes/POs, a more recently established approach, involves the ROMP formation of a block copolymer composed by a first segment made of the desired monomer units end-capped by a first FG, and by an additional cleavable block issued from the polymerization of a readily degradable co-monomer bearing a protected FG.

Subsequent deprotection then liberates this latter chain-end FG (Scheme 11). Hydroxyl-, carboxyl-, thiol-, amine functionalized ROMP polymers have thus been successfully prepared. 28,31,36,[START_REF] Bielawski | Telechelic Polymers from Olefin Metathesis Methodologies[END_REF]96,131,132,133,134,135 Besides from not being atom-economic, this method is limited to a few FGs and more suitable for short telechelic polymers, and it has thus not been extensively developed. To the best of my knowledge, no example of SMP has been reported from this approach.

Nowadays, chemical modification techniques and metathesis pathways have thus become equally viable routes towards the preparation of well-defined and finely tuned telechelic polyenes/POs. However, contemporary metathesis pathways remain more straightforward and convenient, and subsequently more explored. The most relevant alkoxysilyl telechelic polyenes/POs synthesized from olefin metathesis strategies are gathered in Table 1.

Synthesis of alkoxysilyl telechelic polyenes and polyolefins by ADMET

ADMET chemistry has been not so often used towards the preparation of alkoxysilyl telechelic polyenes/POs. A first example involved the synthesis of di(methoxydimethylsilane) telechelic polycyclooctene (PCOE) oligomers using Grubbs' ruthenium Ru(Cl 2 )(CH=Ph)(PCy 3 ) 2 (G1, Figure 1) or Schrock's molybdenum Mo(=CHCMe 2 Ph)(N- Wagener and coworkers have described the synthesis, characterization, and thermal analysis of unsaturated ''latent reactive'' oligoether/carbosiloxane copolymers prepared via ADMET polymerization (Scheme 13). 44,45,46 Dimethoxymethylsilyl end-capped linear unsaturated polymers described as alternating (AB) n microblock copolymers of oligo(oxyethylene) and oligo(methylene), were prepared using Grubbs' 1 st generation catalyst Me OMe OMe m Scheme 13. ADMET synthesis of bis(dimethoxysilyl) telechelic polyether/carbosiloxane copolymers. 44,45 These chain-end functionalized copolymers just described offered "latent reactive" methoxy-substituted carbosiloxane moieties randomly incorporated throughout the polycarbosiloxane backbone. 136,137,138 Hydrolysis of the chain internal methoxysilyl FGs in the presence of moisture and condensation led, in the absence of a curing catalyst, to the formation of stable siloxane linkages affording hybrid silicone thermoset materials. 46,139,140

Synthesis of alkoxysilyl telechelic polyenes and polyolefins by ROMP/CM

Using the ROMP/CM synthetic routes, the straightforward introduction of reactive alkoxysilyl FGs towards the preparation of alkoxysilyl end-functionalized polyenes/POs was effectively achieved through direct metathesis using alkoxysilylalkene CTAs. The approach is quite versatile, provided the metal catalyst remains stable in the presence of the alkoxysilylalkene CTA, affording a series of alkoxysilyl telechelic (co)polymers. 27,28,36,141,142,143,144 Using a set of mono(trimethoxysilylalkene

)
CTA x CTA FG n/x FG n-1/x FG n/x n-1/x R R R R R R R R R (+ BZQ)
Scheme 14. Ruthenium catalyzed tandem ROMP/CM of 3-RCOEs using monofunctional trimethoxysilyl-alkene CTAs, showing the structural diversity of the PCOEs. 70 Note that the degree of polymerization of each PCOE is only indicative, given the possible formation of side macromolecular species.

The similar ROMP/CM of COE using symmetrical bis(trialkoxysilyl)alkenes as CTAs then selectively (> 90wt%) provided α,ω-bis(trialkoxysilyl) telechelic PCOE along with a minor amount of cyclic non-functionalized PCOE (M n,NMR = 2100-16,400 g.mol -1 , Ð M = 1.4-1.9), as evidenced by NMR, MALDI-ToF MS, SEC analyses and fractionation experiments (Scheme 15). 71 telechelic copolyenes. 73 Note that the degree of polymerization of each PCOE is only indicative, given the possible formation of side macromolecular species.

Conclusion

Well-defined end-functionalized polymers are of major relevance for applied research.

Whereas the modern metathesis approaches enable to straightforwardly access to alkoxysilyl telechelic polyenes/POs, SMPs have been traditionally synthesized from chemical modification routes (either ISF or PPF pathways). In this highlight we have provided an overview of the various strategies available to access to SMPs, emphasizing on the I would like to express my thanks to the CNRS and to my (former) research associates who contributed to these studies.

Scheme 1 .

 1 Scheme 1. Curing (hydrolysis and polycondensation) process of SMPs.

Scheme 5 .

 5 Scheme 5. Synthesis of bis(trimethoxysilyl) telechelic PE by thiol-ene chemistry.50

( 7 .

 7 Scheme 7. Synthesis of bis(triethoxysilyl) telechelic PBD from dihydroxy telechelic PBD, and subsequent cross-linking.58,59

2 HPBDScheme 8 .

 28 Scheme 8. Synthesis of bis(triethoxysilyl) telechelic HPBD from dihydroxy telechelic HPBD, and subsequent cross-linking.60,61

Scheme 9 .

 9 Scheme 9. Synthesis of α-trimethoxysilyl PLA, and subsequent hydrolysis.53

Scheme 11 .

 11 Scheme 11. General synthetic routes towards telechelic polyenes and POs.

Figure 1 .

 1 Scheme 12. ADMET synthesis of di(methoxysilyl) telechelic PCOE, and subsequent copolymerization with PDMS-OH.39

(

  G1) under mild conditions (40-55 °C, 3 days). This involved a series of oligo(oxyethylene) α,ω-dienes monomers with different lengths of oligo(oxyethylene) and methylene spacers between the carbon-carbon double bond and the oligo(oxyethylene) unit, along with either the one symmetrical ''chain-internal'' cross-linker, di(4pentenyl)dimethoxysilane or 6,6,9,9-tetramethoxy-6,9-disilatetradeca-1,13-diene.Characterization of the resulting random copolymers (30,700 < M n,SEC < 51,750 g.mol -1 , Ð M = 1.6-2.1) revealed a wide spectrum of properties (e.g.: T g = -79.0 to -50.4 °C; T m = -35.4 to 57.1 °C; T d[START_REF] Laferte | [END_REF] (degradation temperature at 5% weight loss) = 227 to 314 °C) 44 dependent on both the length of oxyethylene segments and also on the length of carbon chains in between them. Exposure to moisture triggered the hydrolysis of methoxy groups, which underwent condensation reaction to provide chemically cross-linked materials that led to the formation of thermosets. The thermoset's mechanical response was varied from flexible plastic to purely elastic behavior, depending on the ratio of oligooxyethylene and carbosiloxane monomers within the different compositions.44,45,46

Scheme 15 .Scheme 16 .Scheme 17 .

 151617 Scheme 15. Ruthenium catalyzed tandem ROMP/CM of COEs using difunctional trialkoxysilyl-alkene CTAs towards the formation of bis(trialkoxysilyl) telechelic PCOE.71

Table

Table 1 .

 1 SMPs obtained by chemical modification or metathesis routes.

	Chain-end functionality	Synthetic route	Polymer	Ref.
	Me
	Si	OMe	ADMET a	Polycyclooctene	39
	Me
	Me
	Si	OiPr
	Me
	Si OMe OMe OMe	Chemical ISF a modification -	Polyisoprene-based triblock copolymer Polyisoprene	40,41, 42,43
	OEt
	Si	OEt
	OEt
	Me
	Si	OMe	ADMET	Polyether/carbosiloxane segments	44,45,46
	OMe
	(CH2)3 Cl
	Si	OMe
	OMe OMe	Chemical modification -ISF	Polybutadiene	47
	Si	OMe
	OMe
	Si OMe OMe OMe	Chemical modification -ISF	Polystyrene Polystyrene-b-Polybutadiene	48,49
	OMe	Chemical	Polyethylene	50,51,52
	Si	OMe	modification -	Polylactide	53
	OMe	PPF a	Polythiophene	54,55
					56,57,58,59
				Polybutadiene	60,61
				Hydrogenated Polybutadiene	22,61,62,63,64
	Si OEt OEt	OEt	Chemical modification -PPF	Poly(ethylene glycol) Polycaprolactone Polycaprolactone/Polydimethylsiloxane blend Polydimethylsiloxane	65,66 60
				Polybutylacrylate	7
				Polyaziridine	67
	OMe
	Si OMe OMe Si OEt OEt	Chemical modification -PPF	Poly(propylene glycol) Poly(tetramethylene oxide) Poly(ethylene glycol-co-propylene glycol)
	OEt

  Subsequent quaternization of the terminal tertiary amino group of the resulting α,ω-heterofunctionalized polymer (MeO) 2 [(CH 2 ) 3 Cl]Si{C 4 H 6 } n (CH 2 ) 3 NMe 2 with methyl iodide next quantitatively produced the parent ionic difunctionalized polymer (MeO) 2 [(CH 2 ) 3 Cl]Si{C 4 H 6 } n (CH 2 ) 3 NMe 3 + I -. 47 This work further showed that the two

	methoxy	groups	on	the	silicon	of	the	α-monofunctionalized	PBD
	(MeO) Cl		(CH2)3 Cl Si OMe			(CH2)3 Cl Si OMe	HCl or
	1 or 2		OMe -10 to -70 °C, 5 h		or	OMe (CH2)3 Cl	n -Bu 4 NOH THF/H 2 O
		PBDLi	RT,18 h				Si OMe 2

3

, among other polymer materials resulting from further coupling by elimination of methoxy groups such as the coupling of two chains through Si(OMe) 2 linkages. Likewise, the anionic polymerization of 1,3-butadiene initiated by 3-dimethylaminopropyllithium gave the living αdimethylaminopropyl poly(butadien)-yl-lithium, which was next reacted with 3chloropropyl(trimethyoxy)silane (> 80% termination). 2 [(CH 2 ) 3 Cl]Si{C 4 H 6 } n Bu were not readily hydrolyzed by water, but that they could be removed in acidic (HCl) or basic (n-Bu 4 NOH) conditions as evidenced by 1 H and 13 C NMR and FTIR spectroscopies (disappearance of Si-OMe bond). Interestingly, the terminal methoxysilyl group of this latter PBD coupled to the surface of silica gel (80 °C, 24 h) through Si-O-Si linkages. 47 -PBD-Si-O-Si-PBDn Bu Li n Bu n Bu Scheme 4. Synthesis of α-methoxysilyl end-capped PBD from PBDLi, and subsequent hydrolysis. 47

  The influence of the catalyst (CF 3 SO 3 H, acetic acid, or best: DBTL), temperature (best: 80 °C), moisture and curing time (5-24 h) were next evaluated by swelling measurements and determination of the mechanical properties (storage modulus E' at the rubbery plateau, maximum of tan δ) of the cross-linked materials.56

			OEt
	OCN	Si	OEt
	HO X = 0.78, y = 0.22, n = 50 x y OH n	OEt DBTL, 50 °C, 45 min to 2 h		EtO	Si OEt OEt	H N	O	O		x	O n y	O	N H	Si OEt OEt OEt
												DBTL, 80 °C,
												Moisture
					OEt
		HS			Si	OEt
					OEt		n			OEt
	n		AIBN, 75	°C				S		Si	OEt
											OEt
										DBTL, 80 °C,
										H 2 O
							n			OEt
								S		Si	O	Si
										O
										Si
	Scheme 6. Synthesis of triethoxysilyl functionalized PBD and subsequent cross-linking. 56,57
	Bunel and coworkers have also prepared low molar mass bis(triethoxysilyl) telechelic
	PBD from the PPF of the corresponding α,ω-dihydroxy functionalized PBD using
	3-isocyanatopropyl(triethoxy)silane, a quasi-quantitative reaction catalyzed by DBTL (50 °C,
	45 min-2 h) (Scheme 7). 58 Subsequent cross-linking was carried out under moisture with
	DBTL at 80 °C, and the swelling and mechanical properties of the resulting materials were
	modulated according to the number of alkoxysilyl groups per polymer chains (1.2 to 2.4),
	whereas the glass transition temperature remained constant. 59

  2,6-iPr 2 C 6 H 3 )(OCMe(CF 3 ) 2 ) 2 catalysts in the ADMET polymerization of 1,9-decadiene, as reported by Wagener and coworkers (Scheme 12).39 Having first evidenced that the Si-OCH 3 bond remained stable under typical ADMET conditions in the presence of the Ru or Mo catalyst, the 5-hexenylmethoxydimethylsilane monofunctional reagent was then used to terminate the propagation reaction (in bulk, 50 h, 60 °C), thus affording (MeO)Si(Me) 2 (CH 2 ) 4 {CH=CH(CH 2 ) 6 } n (CH 2 ) 4 Si(Me) 2 (OMe) oligomers, as characterized by FTIR, 1 H,[START_REF] Lee | Adhesion Science and Technology[END_REF] C, and29 Si NMR spectroscopies. The initial monomer-to-chain limiter ratio was shown to control the molar mass of the polymer as determined by NMR and SEC analyses (1200 < M n,NMR < 3500 g.mol -1 , Ð M = 1.6-2.0). Subsequent condensation of these O{SiMe 2 O} n SiMe 3 , gave ABA triblock copolymers, thus demonstrating the difunctionality of these SMP oligomers.

	methoxysilyl	terminated	PCOEs	with		hydroxyl	end-capped	PDMS,
	+ HOSiMe 2 Si Me	OMe
									Me
					Catalyst
		MeO	Si Me Me		50 h, 60 °C, bulk	Si Me Me	OMe
						Me		Me	Me
				MeOH	Me	Si Me	O	Si Me	O	Si Me	OH

  The influence of several parameters (nature of the CTA, substituent on COE, catalyst, and to a lesser extent of the solvent) on the rate and selectivity of the reaction was studied. As anticipated, addition of 1,4-benzoquinone (BZQ) completely inhibited the isomerization process upon involving the

	Ru-hydride species (arising from the degradation of the Ru-alkylidene catalyst) accountable
	of this side reaction, in its reduction into 1,4-hydroquinone. Also, bulkier substituents on
	3-RCOE monomers promoted the selective formation of monofunctionalized polymers, and
	Grubbs' 2 nd -generation catalyst (G2) afforded the best compromise in terms of productivity
	(turnover numbers (TONs) up to 2,000 mol(COE).mol(Ru) -1 and 200 mol(CTA).mol(Ru) -1 ),
	reactivity and selectivity (in the presence of BZQ, CH 2 Cl 2 at 40 °C, 24 h) in comparison to
	functional CTAs (vinyl trimethoxysilane, Hoveyda-Grubbs 2 nd -generation catalyst (HG2), Zhan's catalyst (Zhan) (Figure 1), or some
	allyl trimethoxysilane, and 3-(trimethoxysilyl)propyl acrylate), the Ru-catalyzed ROMP/CM other specific Ru-carbene-alkylidene catalysts from Omega Cat Systems Company (Rennes,
	of cyclooctene (COE) and other 3-alkyl-substituted COEs (3-RCOEs, R = ethyl, n-hexyl) France).
	gave a statistical distribution of end-capped polymers (M n,NMR = 1300-39,000 g.mol -1 , Ð M = R FG
	n
	1.25-1.85), including linear nonfunctionalized, isomerized linear nonfunctionalized, cyclic 3-RCOE (R = H, ethyl, n -hexyl) Si(OMe)3
	[Ru catalyst] nonfunctionalized, α-monofunctionalized, isomerized α-monofunctionalized, and/or CH 2 Cl 2 O Si(OMe)3 Si(OMe)3
	α,ω-difunctionalized PCOE, as reported by Guillaume and Carpentier and coworkers (Scheme 40 °C, 24 h O
	n/x 14). 70 A two-stage mechanism was proposed to involve first, the formation of alkoxysilyl FG
	monofunctionalized and non-functionalized (linear and cyclic) macromolecules through n-1/x
	ROMP/CM and RCM processes. Next, the C=C isomerization combined with a second CM
	process would form the isomerized (linear non-functionalized and monofunctionalized) and
	difunctionalized macromolecules. The trimethoxysilyl chain-end functionalized polymers
	(mono-, isomerized mono-, di-functionalized) were recovered in larger amounts than the non-
	functionalized polymers (linear, isomerized linear, and cyclic), while the bis(trimethoxysilyl)

functionalized PCOE remained a minor component, as demonstrated by 1 H,

[START_REF] Lee | Adhesion Science and Technology[END_REF]

C NMR, SEC and MALDI-ToF MS analyses and fractionation experiments.

  Two methoxysilyl functional CTAs (namely [(MeO) 3 SiCH 2 CH=] 2 and [(MeO) 3 Si(CH 2 ) 3 NHC(O)OCH 2 CH=] 2 ) were identified as the most efficient in terms of reactivity, catalyst productivity and selectivity (ca. 93%) towards the formation of the bis(trialkoxysilyl)telechelic PCOE, with G2 and HG2 catalysts giving the best compromise (vs. Grubb's third generation catalyst (G3), Zhan and Omega Cat System Company's catalysts) in terms of selectivity and productivity (TONs up to 95,000

mol(COE).mol(Ru) -1 and 5000 mol(CTA).mol(Ru) -1 ).

Acknowledgments

opportunities provided by both ISF/PPF and metathesis reactions. Each of these general pathways described herein has its own advantages and limitations.

While PPF of already isolated/purified and well-defined polymers enables a good/easy control of the macromolecular structure, metathesis functionalization routes also allow to achieve a good control over the microstructure of the polyenes/POs. Besides, metathesis as well as ISF are beneficial one-pot methodologies easy to implement. The selectivity of the reactions is a key parameter to assess the formation of only alkoxysilyl telechelic polymers.

Whereas, the quantitative conversion of chemical end-moieties through ISF or PPF procedures is essential to avoid the formation of a mixture of various alkoxysilyl endfunctionalized polymers, the use of a difunctional alkene CTA in a tandem ROMP/CM appears as a valuable, mild and rather highly selective route to alkoxysilyl telechelic polyenes/POs. However, regardless of the approach, the stability of the polymer backbone and/or of its pending moieties (and of the catalyst as well in metathesis approaches) towards the FG to be introduced, is a major (if not the most prevalent) factor for the success of the alkoxysilyl end-functionalization. Relatively few scientific papers mention the hydrolysis/polycondensation of the alkoxysilyl end-groups of polymers. Yet this ultimate curing step from such alkoxysilyl functionalized pre-polymers towards CASE applications still remains to be more largely considered. In spite of the recent advances in the field of SMPs, investigations of their thermo-mechanical properties and their cured analogues, still remains to be further developed.

Conflicts of interest

There are no conflicts to declare.


Kolle E Thomas

Laura J Mccormick

Daniel Carrie

Hugo Vazquez-Lima

Gerard Simonneaux
email: gerard.simonneaux@univ-rennes1.fr

Abhik Ghosh
email: abhik.ghosh@uit.no

Daniel Carrié

Gérard Simonneaux

Conformational Dynamics of the Inherently Chiral Copper Corrole Chromophore

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

Introduction. The discovery of a new inherently chiral chromophore is a relatively uncommon occurrence. The concept, first introduced by Moscowitz and others over a half-century ago, [START_REF] Moscowitz | Theoretical Aspects of Optical Activity. Part 1: Small Molecules[END_REF][START_REF] Kwit | Applications in Stereochemical Analysis of Synthetic Compounds, Natural Products, and Biomolecules[END_REF] refers to molecular systems whose chirality cannot be ascribed to classic stereogenic elements, in particular stereogenic centers. [START_REF] Mislow | Stereoisomerism and Local Chirality[END_REF] Of particular interest from the point of view of chiroptical properties are inherently chiral chromophores with curved π-systems such as helicenes [START_REF] She | Synthesis and Applications[END_REF] and certain fullerenes (e.g. D 2 -C 76 , D 3 -C 78 , D 2 -C 80 , and D 2 -C 84 ). [START_REF] Thilgen | Chirality in Fullerene Chemistry[END_REF] In the course of our own research on the coordination chemistry of corroles, [START_REF] Ghosh | Electronic Structure of Corrole Derivatives: Insights from Molecular Structures, Spectroscopy, Electrochemistry, and Quantum Chemical Calculations[END_REF] we have discovered two different classes of inherently chiral chromophores. [START_REF] Gross | N-Substituted Corroles: A Novel Class of Chiral Ligands[END_REF] The first of these to be discovered, copper corroles, owe their chirality to an electronically driven saddling of the macrocycle, which allows an otherwise symmetry-forbidden Cu(d x2-y2 )-corrole(π) orbital interaction. [START_REF] Wasbotten | Electronic Absorption, Resonance Raman, and Electrochemical Studies of Planar and Saddled Copper(III) Meso-Triarylcorroles. Highly Substituent-Sensitive Soret Bands as a Distinctive Feature of High-Valent Transition Metal Corroles[END_REF][START_REF] Ou | Spectroelectrochemical and ESR Studies of Highly Substituted Copper Corroles[END_REF][START_REF] Bröring | Revisiting the Electronic Ground State of Copper Corroles[END_REF][START_REF] Thomas | Synthesis and Molecular Structure of Gold Triarylcorroles[END_REF]12 Steric crowding due to peripheral substituents can accentuate this saddling, but steric effects alone, in the absence of the electronic imperative, do not bring about saddling of the macrocycle. [START_REF] Thomas | Molecular Structure of a Gold β-Octakis(trifluoromethyl)-meso-triarylcorrole: An 85° Difference in Saddling Dihedral Relative to Copper[END_REF] To our disappointent, so far we have not been able to resolve Cu corroles; chiral HPLC experiments have failed to resolve even highly saddled copper β-octabromo- [START_REF] Alemayehu | Noninnocent, and Chiral: A Strongly Saddled Metallocorrole[END_REF][START_REF] Thomas | Ligand Noninnocence in Coinage Metal Corroles: A Silver Knife-Edge[END_REF] and β-octakis(trifluoromethyl)- [START_REF] Thomas | Copper β-Octakis(Trifluoromethyl)Corroles: New Paradigms for Ligand Substituent Effects in Transition Metal Complexes[END_REF][START_REF] Thomas | A Metallocorrole with Orthogonal Pyrrole Rings[END_REF] mesotriarylcorroles. [START_REF] Schies | Metallocorroles as Inherently Chiral Chromophores: Resolution and Electronic Circular Dichroism Spectroscopy of a Tungsten Biscorrole[END_REF] In contrast, a second class of inherently chiral metallocorroles, Mo [START_REF] Alemayehu | Relativistic Effects in Metallocorroles: Comparison of Molybdenum and Tungsten Biscorroles[END_REF] and W [START_REF] Alemayehu | Tungsten Biscorroles: New Chiral Sandwich Compounds[END_REF] biscorroles, have been successfully resolved and the enantiomers of a W biscorrole have been chracterized via electronic circular dichroism spectroscopy. [START_REF] Schies | Metallocorroles as Inherently Chiral Chromophores: Resolution and Electronic Circular Dichroism Spectroscopy of a Tungsten Biscorrole[END_REF] Herein, we have revisited the challenge of synthesizing copper corroles in scalemic (i.e., enantiomerically enriched) form by appending chiral substituents on the corrole periphery -a strategy that proved successful. Toward that end, we synthesized the corrole analogue of a Halterman porphyrin via the oxidative condensation of enantiopure 1,2,3,4,5,6,7,8-octahydro-1:4,5:8-dimethanoanthracene-9-carboxaldehyde (hereafter referred to as -CHO, where refers to the Halterman substituent of either chirality) and pyrrole. [START_REF] Halterman | Catalytic Asymmetric Epoxidation of Unfunctionalized Alkenes Using the First D 4 -Symmetric Metallotetraphenylporphyrin[END_REF][START_REF] Halterman | Synthesis of D 2 -Symmetric 5,10,15,20-Tetraarylporphyrins from C 2 -Symmetric Benzaldehydes and Achiral Aryldipyrromethanes[END_REF] A modified version of Gryko's water-methanol method, [START_REF] Koszarna | Efficient Synthesis of meso-Substituted Corroles in a H 2 O-MeOH Mixture[END_REF] employing higher dilutions (to solubilize the relatively nonpolar aldehyde) and longer reaction times (16-18 h), led to good yields (~40%) of the Halterman corroles (Figure 1). Copper was then inserted and the Cu complexes β-octabrominated with elemental bromine via standard protocols. [START_REF] Wasbotten | Electronic Absorption, Resonance Raman, and Electrochemical Studies of Planar and Saddled Copper(III) Meso-Triarylcorroles. Highly Substituent-Sensitive Soret Bands as a Distinctive Feature of High-Valent Transition Metal Corroles[END_REF] Single-crystal X-structures were successfully obtained for three different Cu[ 3 Cor] (Cor = corrole) derivatives. As discussed below, the structures clearly showed that the Halterman substituents in conjunction with β-octabromosubstitution are unable to lock the Cu-corrole conformation into a given chirality, which may be described by a P/M nomenclature as illustrated in Figure 2. The chemical implications of these findings are also discussed below. Results and discussion. Table 1 presents crystal and refinement data for the three singlecrystal structures reported herein and Tables 2-4 present selected distances, angles, and torsion angles. In general, these structural parameters are similar to those of related Cu corrole structures reported in the literature. Some of the more notable aspects of the structures are as follows.

The X-ray structure of enantiopure Cu[ + 3 Cor] (Figure 3 and Table 2) shows that all corrole units are identical and may be described as relatively mildly saddled, with only one moderately large saddling dihedral at ~38°. The + substituents have led to a uniform P configuration of the Cu-corrole ring system. In contrast, the enantiopure β-octabrominated complex Cu[Br Cor] were also characterized via electronic circular dichroism (ECD) spectroscopy (Figure 6). The key spectral features clearly correspond to the Soret and Q bands of the complexes, indicating that they arise largely from the Cu-corrole moiety and not from the substituents. Observable ECD spectra also strongly suggest that the P and M conformations are present in unequal amounts in solution, in contrast to the 1:1 P/M ratio found in the X-ray structure of Cu[Br 8 -3 Cor], implying a dynamic equilibrium between the P and M conformations in solution. Such a conclusion is also in accord with DFT calculations (Table 4), which indicate exceedingly small energy differences between the P and M conformations for all the molecules studied.

Conclusion.

In summary, the first Halterman corrole ligand has been synthesized in reasonably good yield and derivatized to the corresponding Cu complexes as well as to the βoctabrominated Cu complexes. Access to the inherently chiral Cu-corrole chromophore in scalemic form has allowed us to record its electronic circular dichroism spectrum for the first time. Single-crystal X-ray structures were obtained for three different Cu complexes. Somewhat to our surprise, the structures proved that the Halterman substituents, even in conjunction with βoctabromo-substitution, are unable to lock the Cu-corrole conformation into a given chirality.

While in one sense this finding may be considered disappointing, in another sense it sheds valuable light on the dynamics of copper corroles. Together with our inability to resolve copper β-octabromoand β-octakis(trifluoromethyl)meso-triarylcorroles, the present results underscore the facile interconversion of the P and M conformations of sterically hindered Cu corroles.

Although we have not yet conclusively located a transition state for this process, we may speculate that saddling inversion provides for a plausible pathway. Finally, there can be no doubt that the present synthesis of Halterman corrole derivatives will open up the study of chiral metallocorroles as asymmetric catalysts, especially for group transfer [START_REF] Halterman | Synthesis and Catalytic Reactivity of D 4 -Symmetric Dinorbornabenzene-derived Metallotetraarylporphyrins[END_REF][START_REF] Wang | Enantioselective Intramolecular Carbene C-H Insertion Catalyzed by a Chiral Iridium(III) Complex of D 4 -Symmetric Porphyrin Ligand[END_REF][START_REF] Thu | Highly Selective Metal Catalysts for Intermolecular Carbenoid Insertion into Primary C-H Bonds and Enantioselective C-C Bond Formation[END_REF][START_REF] Nicolas | Chiral Recognition of Amines and Amino Acid Derivatives by Optically Active Ruthenium Halterman Porphyrins in Organic Solvents and Water[END_REF] reactions such as hydroxylation/epoxidation, [START_REF] Halterman | Synthesis and Catalytic Reactivity of Sterically and Electronically Modified D 4 -Symmetric Metallotetraarylporphyrins[END_REF][START_REF] Amiri | Nitration of Halterman Porphyrin: a New route for Fine-tuning Chiral Iron and Manganese Porphyrins with Application in Epoxidation and Hydroxylation reactions using Hydrogen Peroxide as Oxidant[END_REF][START_REF] Srour | Enantioselective Manganese-Porphyrin-Catalyzed Epoxidation and C-H hydroxylation with Hydrogen Peroxide in Water/Methanol Solutions[END_REF] aziridination, [START_REF] Jones | Cobalt-Catalyzed Asymmetric Olefin Aziridination with Diphenylphosphoryl Azide[END_REF][START_REF] Subbarayan | Highly Asymmetric Cobalt-Catalyzed Aziridination of Alkenes with Trichloroethoxysulfonyl Azide (TcesN 3 )[END_REF][START_REF] Jiang | Asymmetric Radical Bicyclization of Allyl Azidoformates via Cobalt(II)-Based Metalloradical Catalysis[END_REF] and cyclopropanation, [START_REF] Lai | Alkene Cyclopropanation Catalyzed by Halterman Iron Porphyrin: Participation of Organic Bases as Axial Ligands[END_REF][START_REF] Che | Asymmetric Inter-and Intramolecular Cyclopropanation of Alkenes Catalyzed by Chiral Ruthenium Porphyrins. Synthesis and Crystal Structure of a Chiral Metalloporphyrin Carbene Complex[END_REF][START_REF] Nicolas | Synthesis of Chiral Water-soluble Metalloporphyrins (Fe, Ru,): New catalysts for Asymmetric Carbene Transfer in Water[END_REF][START_REF] Nicolas | Asymmetric Intermolecular Cyclopropanation of Alkenes by Diazoketones Catalyzed by Halterman Iron Porphyrins[END_REF][START_REF] Ruppel | Cobalt-Catalyzed Asymmetric Cyclopropanation of Electron-Deficient Olefins[END_REF] an area where chiral metalloporphyrins already play a significant role. [START_REF] Rose | Enantioselective epoxidation of olefins with chiral metalloporphyrin catalysts[END_REF][START_REF] Simonneaux | Recent Advances in Catalysis by Water-Soluble Metalloporphyrins[END_REF][START_REF] Simonneaux | Metalloporphyrin Symmetry in Chiral Recognition and Enantioselective Catalysis[END_REF] Experimental Section Materials and Instruments. All reagents and solvents were used as purchased except pyrrole, which was purified by passing through a pad of basic aluminum oxide 60 (Activity I, 0.063-0.200 mm particle size, Merck Millipore). The Halterman aldehydes (1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1:4,5:8-dimethanoanthracene-9-carboxaldehyde and its enantiomer as well as the corresponding racemate were synthesized as reported. [START_REF] Halterman | Catalytic Asymmetric Epoxidation of Unfunctionalized Alkenes Using the First D 4 -Symmetric Metallotetraphenylporphyrin[END_REF] Free-base Halterman corroles were synthesized according to a modified version of Gryko's method employing sterically hindered dipyrromethane. [START_REF] Koszarna | Efficient Synthesis of meso-Substituted Corroles in a H 2 O-MeOH Mixture[END_REF] Copper complexes were prepared as previously described. [START_REF] Wasbotten | Electronic Absorption, Resonance Raman, and Electrochemical Studies of Planar and Saddled Copper(III) Meso-Triarylcorroles. Highly Substituent-Sensitive Soret Bands as a Distinctive Feature of High-Valent Transition Metal Corroles[END_REF] Cor], its enantiomer, and its racemate. To the Halterman aldehyde (480 mg, 2 mmol) thoroughly dissolved in MeOH (600 mL) was added pyrrole (279 µL, 4 mmol), followed by a solution made from 37% HCl (16.5 mL) diluted in distilled water (300 mL). The resulting mixture was stirred overnight for about 16-17 h. The orange suspension obtained was extracted with CHCl 3 and the organic phase was washed twice with distilled water. The olive-green organic phase was dried with anhydrous Na 2 SO 4 and filtered. The filtrate was diluted with CHCl 3 to a volume of 450 mL and refluxed for 1 h with p-chloranil (991 mg, 4 mmol). The suspension obtained was concentrated to a minimum volume and chromatographed on a silica gel column with 1:1 n-hexane/CH 2 Cl 2 as eluent, yielding the free-base corrole as the first brown band. Yield: 235 mg (38.2%). UV-Vis λ max (nm), ε x 10 -4 (M -1 cm -1 ): 388 (8.83), 399 (8.12), 419 (6.51), 548 (0.80), 703 (0.40). [START_REF] Moscowitz | Theoretical Aspects of Optical Activity. Part 1: Small Molecules[END_REF] Column chromatography on silica gel with 13:7 n-hexane/CH 2 Cl 2 resulted in brown bands, which were combined and evaporated to yield the crude product, which was further purified by crystallization from 1:1 CH 3 OH/CHCl 3 . Yield: 30 mg (62%). UV-Vis λ max (nm), ε x 10 -4 (M -1 cm - 1 ): 411 (5.54), 457 (7.52), 547 (1.47), 651 (0.72). X-ray structure determination. X-ray data for all three crystalline materials were collected on beamline 11.3.1 at the Advanced Light Source, employing a Bruker D8 diffractometer equipped with a PHOTON100 CMOS detector operating in shutterless mode. The crystal was coated in protective oil prior to being mounted on a MiTeGen ® kapton micromount and placed under a nitrogen stream at 100(2) K provided by an Oxford Cryostream 800 Plus low temperature apparatus. Diffraction data were collected using synchrotron radiation monochromated with Si(111) to a wavelength of 0.7749(1)Å. An approximate full-sphere of data was collected using a combination of phi and omega scans; multiple spheres of data were collected for Cu[ + 3 Cor] and Cu[Br 8 -

3 Cor] to ensure that the absolute configuration could be reliably determined from anomalous scattering. The structures were solved by intrinsic phasing (SHELXT) [START_REF] Sheldrick | SHELXT -Integrated Space-Group and Crystal-Structure Determination[END_REF] and refined by full-matrix least squares on F 2 (SHELXL-2014). [START_REF] Sheldrick | Crystal Structure Refinement with SHELXL[END_REF] All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were geometrically calculated and refined as riding atoms. Some disorder of the pendant groups was observed for Cu[ + 3 Cor] and Cu[Br 8 5,15-± 2 -10-∓ Cor], and these have been modelled over multiple sites. SQUEEZE analysis was used to determine the solvent content of the voids in Cu[Br 8 5,15-± 2 -10-∓ Cor], as the solvent of crystallization was too disordered for modeling attempts. Additional crystallographic information has been summarized in Tables 1 to 4 and full details can be found in the crystallographic information files provided as Supporting Information.

Figure 1 .

 1 Figure 1. Synthesis of a Halterman corrole, where + denotes the (1S,4R,5R,8S)-1,2,3,4,5,6,7,8octahydro-1:4,5:8-dimethanoanthracen-9-yl substituent, which occurs in the (+)-isomer of the Halterman aldehyde; denotes the same substituent of either chirality.

Figure 2 .

 2 Figure 2. Definition of saddling torsion angles (χ 1 -χ 4 ). The inherent chirality of Cu corroles is defined here in terms of the sign of the C 8 -C 9 -C 11 -C 12 (χ 3 ) torsion angle.

N 8 - 2 Figure 3 .

 823 Figure 3. Top and side views of the X-ray structure of Cu[ + 3 Cor]. Thermal ellipsoids for this and all subsequent figures are shown at 50% probability.

Figure 4 .-Figure 5 .- 3

 453 Figure 4. X-ray structure of Cu[Br 8 -

Figure 6 . 7 *

 67 Figure 6. Circular dichroism spectra of Cu[ + 3 Cor]/Cu[ - 3 Cor] and Cu[Br 8 + 3 Cor]/ Cu[Br 8 -

  Silica gel 150 (35-70 μm particle size, Davisil) was generally used for column chromatography. Silica gel 60 preparative thin-layer chromatography plates (20 x 20 cm; 0.5 mm thick, Merck) were used for final purification of all compounds. UV-vis spectra were recorded on an HP 8453 spectrophotometer at room temperature in CH 2 Cl 2 . Proton NMR spectra were recorded on a Mercury Plus Varian spectrometer at 400 MHz in CDCl 3 and referenced to residual CHCl 3 at δ = 7.26 ppm. High-resolution electrospray ionization (ESI) mass spectra were recorded on an LTQ Orbitrap XL spectrometer. Common synthetic procedure for 5,10,15-tris[(1S,4R,5R,8S)-1

  H NMR: δ -0.54 to 0.75 (br); 0.92 -1.19 (br); 1.26 -1.45 (m, br); 1.74 -2.20 (m, br). HR-MS (ESI + , major isotopomer): [M + H] + = 921.4901 (expt), 921.4891 (calcd). Elemental analysis found (calcd): C, 86.68 (87.16); H, 6.80 (6.77), N, 6.08 (6.07). yl]corrole, its enantiomer, and its racemate, Cu[ 3 Cor]. Copper acetate (4 equiv, 43 mg) was added to a solution of the free-base (50 mg, 0.054 mmol) in 5 mL pyridine. After stirring for 20 min, the suspension was evaporated to a brown residue which was dissolved in a minimum volume of CHCl 3 and eluted through a silica gel column with 7:3 nhexane/CH 2 Cl 2 . The copper corrole was eluted as brown bands. Yield: 47 mg (88.5 %). UV-Vis λ max (nm), ε x 10 -4 (M -1 cm -1 ): 411 (7.84), 541 (0.90), 653 (0.20). 1 H NMR: δ 0.81 -0.87 (m, 3 H, CH 2 ); 0.92 -1.01 (m, 3 H, CH 2 ); 1.15 -1.21 (m, 6 H, CH 2 ); 1.35 (d, 2 H, CH 2 ); 1.39 (d, 2 H, CH 2 ); 1.44 (d, 2 H, CH 2 ); 1.62 -1.73 (m, 9 H, CH 2 ); 1.77 -1.88 (m, 9 H, CH 2 ); 2.98 (br s, 2 H, CH); 3.09 (br s, 2 H, CH); 3.27 (br s, 2 H, CH); 3.37 (br d, 4 H, CH); 3.42 (br d, 2 H, CH); 7.02 (d, 2 H, β-H); 7.04 (s,1 H, p-H); 7.09 (s, 2 H, p-H); 7.22 (br s, 2H); 7.39 (br d, 2 H, β-H); 8.01 (br s, 2 H, β-H). HR-MS (ESI + , major isotopomer): [M] + = 982.4044 (expt), 982.4030 (calcd). Elemental analysis found (calcd): C, 81.10 (81.80); H, 5.86 (6.04); N, 5.32 (5.70). dimethanoanthracen-9-yl]corrole, Cu[Br 8 3 Cor]. The Cu[ 3 Cor] starting material (30 mg, 0.03 mmol) was dissolved in CHCl 3 (15 mL) and to the solution was added Br 2(2.44 mmol, 125µL, 80 equiv), dissolved in CHCl 3(10 mL), in a dropwise manner over 20 min.The resulting suspension was stirred for 1 h, following which pyridine (203 µL, 2.52 mmol, 84 equiv) dissolved in CHCl 3 (10 mL) was added dropwise over 20 min. The resulting mixture was again stirred for 1 h and then shaken with an equal volume of 20% w/v aqueous Na 2 S 2 O 7 . The organic phase was then dried over anhydrous Na 2 SO 4 , filtered, and evaporated to a minimum.

8 - Table 1 .

 -1 Crystal and refinement data for Cu Halterman corroles. 69 H 63 Cl 4 CuN 4 C 73.75 H 66.56 Br 8 Cl 0.41 CuN 4 C 77.74 H 75.31 Br 8 Cl 1.71 CuN 4

	Sample	Cu[ +	3 Cor]	Cu[Br 8	-	3 Cor]	Cu[Br 8 -5,15-±	2 -10-
							∓ Cor]
	Chemical Formula
	Formula mass	1153.57	1726.20	1828.90
	Crystal system	Orthorhombic	Monoclinic	Monoclinic
	Space group	P2 1 2 1 2 1	C2		C2/c
	λ (Å)	0.7749	0.7749	0.7749
	a (Å)	11.8437(6)	11.5580(6)	11.4727(6)
	b (Å)	17.9042(9)	23.9229(12)	23.9890(12)
	c (Å)	26.3407(13)	25.9436(13)	25.9406(13)
	α (°)	90	90			90
	β (°)	90	91.359(2)	91.002(3)
	γ (°)	90	90			90
	Z	4		4			4
	V (Å 3 )	5585.6(5)	7171.4(6)	7138.2(6)
	Temperature (K)	100(2)	100(2)	100(2)
	ρ (g/cm 3 )	1.372	1.599	1.702
	Measured reflections	386855	428802	40786
	Unique reflections	23535	28900	8921
	Parameters	703	843		391
	Restraints	0		122		84
	R int	0.0549	0.0618	0.0713

C Abs. struct. parameter 0.0300

(14)

0.028(3) θ range (°) 1.499 to 38.123 0.856 to 37.450 1.712 to 31.276 R1, wR2 all data 0.0475, 0.1322 0.0397, 0.0929 0.0713, 0.1341 S (GooF) all data 1.085 1.052 1.109 Max/min res. Dens. (e/Å 3 ) 1.242/-1.653 1.114/-1.463 1.459/-1.119 Table 2. Selected distances (Å), angles (°), and torsion angles (°) for Cu[ + 3 Cor]. Distances N(1)-Cu(1) 1.8681(19)

  1 H NMR: δ 1.07 -1.18 (m, 6H, CH 2 ); 1.21 -1.36 (m, 8 H, CH 2 ); 1.39 (d, 2 H, CH 2 ); 1.42 -1.48 (m, 4 H, CH 2 ); 1.57 -1.65 (m, 3 H, CH 2 ); 1.74 -1.94 (m, 11 H, CH 2 ); 2.03 (d, 2 H, CH 2 ); 2.65 (bd, 2 H, CH); 2.83 (bd, 2 H, CH); 3.20 (br d, 2 H, CH); 3.27 (br d, 2 H, CH); 3.33 (br d, 2 H, CH); 3.36 (br d, 2 H, CH); 7.15 (s,1 H, p-H); 7.21 (s, 2 H, p-H). HR-MS (ESI + , major isotopomer): [M] + = 1613.6810 (expt), 1613.6805 (calcd). Elemental analysis found (calcd) : C 49.41 (49.83), H, 2.98 (3.18), N 3.66 (3.47).

Cor] was found to contain two unique Cu[Br 8 -3 Cor] units, one with M and the other with P configuration, both of which exhibit a strongly saddled Cu-corrole core with saddling dihedrals in the

range 47-68°. There is some positional disorder involving the 10--

Cor] unit; however, the enantiopurity of the molecule is not in question.

Cor] containing an equimolar distribution of the P (left) and M (right) conformations. The thermal ellipsoids have been drawn at 50% probability.

Cor].

Acknowledgements. The above work was supported by grants 231086 and 262229 of the Research Council of Norway. This research also used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. We thank Dr. Soizic Chevance for stimulating discussions and recording the CD spectra.