Tingzhen Ming

Wei Li

Qingchun Yuan

Philip Davies

Renaud De Richter

Chong Peng

Qihong Deng

Yanping Yuan

Sylvain Caillol

Nan Zhou

Perspectives on removal of atmospheric methane

Keywords: methane removal, photocatalysis, chlorine atoms, hydroxyl radicals, zeolites, methane mitigation, methane remediation, enhanced atmospheric methane oxidation Highlights

 Action on methane is the easiest and faster way to slow down global warming  At COP26 105 countries signed 'the methane pledge' for -30% CH 4 emissions by 2030  CH 4 removal (remediation) is complementary with reducing CH 4 emissions (mitigation)  Atmospheric CH 4 removal methods are described and early costs estimates provided  Co-benefits include: human health, agriculture, economy, and ozone layer recovery.

Introduction A spotlight on methane versus carbon dioxide

While the atmospheric stock of carbon dioxide (CO 2 ) in the atmosphere has increased by about 50% since preindustrial time (417 vs 278 ppm), that of methane (CH 4 ) has more than doubled (1879 vs 722 ppb) [START_REF]Global Monitoring Laboratory -Eath System Research laboratories[END_REF]. Although the importance of CH 4 as a greenhouse gas has been known about for many years, as reflected by the Kyoto protocol of 1997 [START_REF] Zhou | United Nations Framework Convention on Climate Change -The Kyoto protocol[END_REF], until recently public attention focused mainly on CO 2 . Most mitigation and remediation proposals targeted CO 2 .

Recently, however, more attention is being given to CH 4 . Thus, in November 2021, at the UN Climate Change Conference (COP 26) held in Glasgow, 105 participating countries signed "The Global Methane Pledge" committing to a 30% reduction in emissions from oil and gas by 2030 relative to 2020 [3]. Moreover, the new contribution of Working Group 1 to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC WR1 AR6) released on August 2021 [START_REF] Masson-Delmotte | The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] highlights the need to quickly reduce global CH 4 emissions to slow warming [START_REF]Reduce methane or face climate catastrophe, scientists warn[END_REF][START_REF] Cnn | Scientists say this invisible gas could seal our fate on climate change[END_REF] and "buy us time" [START_REF]Climate change: Curbing methane emissions will 'buy us time[END_REF].

Previously, on July 2021, a U.S.-Russia Joint Statement Addressing the Climate Challenge expressed their intent to work together bilaterally to address climate change, including emissions reductions from non-CO 2 greenhouse gases (GHGs), including CH 4 [START_REF]Russia Joint Statement Addressing the Climate Challenge[END_REF]. Methane featured in the agenda of the recent U.S. Leaders' Climate Summit. Meanwhile, the Chinese 14th Five-Year Plan presented in March 2021 was expanded to include CH 4 and other non-CO 2 gases [START_REF] You | China to Reduce Emissions of Methane, a Powerful Greenhouse Gas[END_REF], and China's biggest gas and oil producer is targeting a 50% reduction in CH 4 emission intensity by 2025 [START_REF] Reuters | China's CNPC targets 50% slash in methane emission intensity by 2025[END_REF]. The United Nations Environment Program (UNEP) and the Climate and Clean Air Coalition in their "Global Methane Assessment" [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF] as well as the International Energy Agency [START_REF]IEA calls on companies, governments and regulators[END_REF] are calling for urgent action to cut CH 4 emissions, and scientists and non-governmental organisations (NGOs) are calling for atmospheric CH 4 removal [START_REF]Scientists' Statement on Lowering Atmospheric Methane Concentrations[END_REF] [START_REF]Declaration on Reducing Atmospheric Methane[END_REF].

These calls and decisions are timely. In the absence of further climate action, by the end of the century, global-mean warming due only to CH 4 emissions could contribute to about 0.9°C (±0.2°C), compared to a warming of about 0.5°C (±0.1°C) currently, due to historical CH 4 emissions [START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF], and compared to the Paris agreement target of less than 2°C global warming including all GHGs.

The atmospheric concentration of CH 4 is rising

In 2020, despite Covid-19 shutdowns, the annual increase in atmospheric CH 4 was the largest recorded since 1983, when systematic measurements began [START_REF]Despite pandemic shutdowns, carbon dioxide and methane surged in[END_REF]. Since the preindustrial era, tropospheric concentrations of CO 2 and CH 4 have increased by 47% and 167%, respectively [START_REF]Global Monitoring Laboratory -Eath System Research laboratories[END_REF]. Since 2007, tropospheric CH 4 has been rising rapidly, with an average annual growth rate of 9.3 parts per billion (ppb) (approximately 0.4% year -1 ) between 2014 and 2019 [START_REF]Global Monitoring Laboratory -Eath System Research laboratories[END_REF]. Over a longer timescale, CH 4 concentrations have multiplied 3.28 times from a minimum of 570 ppb, reached 5,000 years ago [START_REF] Ruddiman | plagues, and petroleum: how humans took control of climate[END_REF]. The IPCC [START_REF] Stocker | The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF] predicts that, over the next 10 to 20 years, CH 4

and CO 2 will have similar global warming impacts, as measured by heat absorbed (global warming potential, GWP) and temperature rise (Global Temperature change Potential, GTP). See Figure 1. Besides slowing global warming, CH 4 removal also can help protect the ozone layer, because one of the indirect effects of the rising concentrations of CH 4 is the increasing amounts of water in the stratosphere, which participates in ozone layer depletion [START_REF] Revell | The role of methane in projections of 21st century stratospheric water vapour[END_REF].

Whereas the natural capacity of the atmosphere to remove GHGs remains roughly constant [START_REF] Rigby | Role of atmospheric oxidation in recent methane growth[END_REF][START_REF] Turner | Ambiguity in the causes for decadal trends in atmospheric methane and hydroxyl[END_REF], CH 4 emissions from the major natural and anthropogenic sources are increasing [START_REF]Despite pandemic shutdowns, carbon dioxide and methane surged in[END_REF]. The biogenic sources include tropical wetlands [START_REF] O'connor | Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review[END_REF], lakes [START_REF] Wik | Climate-sensitive northern lakes and ponds are critical components of methane release[END_REF],

ponds, hydroelectric reservoirs [START_REF] Beaulieu | Eutrophication will increase methane emissions from lakes and impoundments during the 21st century[END_REF], and rivers [START_REF] Wilkinson | Continuous seasonal river ebullition measurements linked to sediment methane formation[END_REF]; and human-made sources include fossil fuels (coal mines, oil and gas wells); agriculture (livestock and rice cultivation); landfills; and some of the biomass burning due to intentional wildfires [START_REF] Saunois | The global methane budget 2000-2017[END_REF].

CH 4 emissions from the fossil fuel industry are uncertain but recently shown to be approximately 40% higher than previously estimated [START_REF] Hmiel | Preindustrial 14CH4 indicates greater anthropogenic fossil CH4 emissions[END_REF]. They include venting, flaring, and fugitive emissions of global diesel and gasoline [START_REF] Pieprzyk | Influence of methane emissions on the GHG emissions of fossil fuels[END_REF], as well as leaks in gas distribution and use [START_REF] Schwietzke | Upward revision of global fossil fuel methane emissions based on isotope database[END_REF].

Due to warming oceans and surface air temperatures, several scientists perceive a risk of massive release of CH 4 by destabilization of seafloor methane-hydrates [START_REF] Andreassen | Massive blow-out craters formed by hydrate-controlled methane expulsion from the Arctic seafloor[END_REF][START_REF] Cardoso | Increased methane emissions from deep osmotic and buoyant convection beneath submarine seeps as climate warms[END_REF][START_REF] Archer | Ocean methane hydrates as a slow tipping point in the global carbon cycle[END_REF] and Arctic permafrost thaw [START_REF] Schuur | Climate change and the permafrost carbon feedback[END_REF][START_REF] Holgerson | Large contribution to inland water CO 2 and CH 4 emissions from very small ponds[END_REF]. Some observed CH 4 fluxes from 5 to 24 grams per square meter per day (gm -2 day -1 ) were observed in 2013 [START_REF] Shakhova | The East Siberian Arctic Shelf: towards further assessment of permafrostrelated methane fluxes and role of sea ice[END_REF].

However, some consider such risks to be lower, mainly thanks to microbial CH 4 consumption by methanotrophs [START_REF] Cooper | Limited contribution of permafrost carbon to methane release from thawing peatlands[END_REF][START_REF] Knoblauch | Methane production as key to the greenhouse gas budget of thawing permafrost[END_REF][START_REF] Singleton | Methanotrophy across a natural permafrost thaw environment[END_REF][START_REF] Ruppel | The interaction of climate change and methane hydrates[END_REF] whose populations rise with the increasing abundance of CH 4 . Meanwhile, increasing CH 4 emissions due to warming of wetlands and landfills, eutrophication of lakes [START_REF] Beaulieu | Eutrophication will increase methane emissions from lakes and impoundments during the 21st century[END_REF], and fossil fuel extraction [START_REF] Dalsøren | Discrepancy between simulated and observed ethane and propane levels explained by underestimated fossil emissions[END_REF] adds revenue [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF].

The rapid rise of atmospheric CH 4 concentration requires action to try to limit global warming well below 2°C as targeted by the COP21 Paris Agreement, bearing in mind that the GWP of CH 4 is nearly 28 times higher than that of CO 2 on a 100-year basis, and 84 times higher on a 20-year basis [START_REF] Stocker | The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]. Moreover, because CH 4 depletes the atmospheric reservoir of hydroxyl radicals responsible for removing CH 4 itself, large additions of CH 4 to the atmosphere extends the lifetime and GWP of the CH 4 already in the atmosphere [START_REF] Holmes | Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions[END_REF][START_REF] Voulgarakis | Analysis of present day and future OH and methane lifetime in the ACCMIP simulations[END_REF].

In the next section the importance of focusing in CH 4 reduction is explained. The following sections review the infrastructure needed for large-scale CH 4 removal with the methods and strategies proposed so far, summarizing the advantages and disadvantages of those methods, and their potential costs compared to some CO 2 removal technologies. Then some expected co-benefits are discussed, before the concluding remarks.

2.

Actions are needed against methane emissions and its atmospheric stock Methods have been proposed to limit and reduce CH 4 emissions from several anthropogenic sources [START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF][START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF][START_REF] Johannisson | A Structured Approach for the Mitigation of Natural Methane Emissions-Lessons Learned from Anthropogenic Emissions[END_REF]: examples include better management of landfill (separate biodegradable waste) and coal, oil and gas fields (reduce leakage, recovering instead of flaring, capturing instead of venting, etc.) [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF][START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF], as well as dietary changes to reduce emissions from livestock populations [START_REF] Kebreab | Strategies to Reduce Methane Emissions from Enteric and Lagoon Sources[END_REF]. But few of these can be adapted to natural emissions, which are diffuse over large areas of thousands to millions of square kilometers and/or partly inaccessible. Unfortunately, most CH 4

emissions come from such diffuse natural biogenic sources [START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF], which by top-down estimations represent about 40% of total emissions [START_REF] Saunois | The global methane budget 2000-2017[END_REF]. Biogenic emissions come mainly from wetlands (31%) and a smaller part (7%) from freshwater systems, oceans, estuaries, permafrost, termites, wild animals and vegetation. Other estimates of the natural CH 4 emissions are of about 50% [START_REF] Rosentreter | Half of global methane emissions come from highly variable aquatic ecosystem sources[END_REF], as well as 50% by bottom-up estimates [START_REF] Saunois | The global methane budget 2000-2017[END_REF].

Removal or remediation methods seem even more technologically challenging, because they need to deplete CH 4 already released to the atmosphere, where it has been diluted about 0.5 million times in an air volume of some 1.4 billion cubic kilometers. In addition, the relatively short life cycle of atmospheric CH 4 (about 10 years) means that removal would have to be ongoing to reduce the concentration by a target amount. This contrasts with CO 2 , for which a one-off removal achieves an almost permanent reduction. Further, CH 4 molecules are about 200 times scarcer in the atmosphere than CO 2 molecules. Unlike CO 2 , however, CH 4 and other GHGs can be removed by in-situ oxidation, to products with no GWP or with much lower GWP than CH 4 ; for instance, CO 2 is obtained from CH 4 and nitrogen and oxygen from nitrous oxide (N 2 O) without the need of capture, separation, or storage. The oxidation reaction is exothermic and exergonic, and as such requires no minimum energy input once the activation energy is overcome. This differs from CO 2 , which requires at least 18 kJ mol -1 for its separation from the atmosphere [START_REF] Wilcox | Assessment of reasonable opportunities for direct air capture[END_REF], and more energy still if CO 2 is to be reduced to carbon or organic compounds such as industrial polymers [START_REF] Liu | Highly Efficient Synthesis of Functionalizable Polymers from a CO2/1, 3-Butadiene-Derived Lactone[END_REF].

Removal of CH 4 may be considered an acceleration of the natural oxidation processes, as once in the atmosphere it finally ends-up as CO 2 .

3.

Infrastructure needed for treating large volumes of air

As CH 4 is a well-mixed GHG, and its life expectancy in the troposphere is approximately 10 years, in order to have an impact on global warming, some authors have proposed that at least one-tenth of the atmosphere has to be processed every year, to compete with natural sinks [START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF]. In reality, any process of a portfolio of technologies allowing the atmospheric CH 4 concentration to start decreasing (while the imbalance between sources and sinks is currently increasing) will reduce its direct global warming impacts, as well as its indirect impacts (due to tropospheric ozone generation) on heath, food production, and primary productivity [START_REF] Ainsworth | The effects of tropospheric ozone on net primary productivity and implications for climate change[END_REF][START_REF] Feng | Economic losses due to ozone impacts on human health, forest productivity and crop yield across China[END_REF]. proposed to directly oxidize it in-situ by a variety of possible methods, including bioinspired aqueous-phase catalytic oxidation, bio-reactors containing methanogens, enzymatic systems, and catalysts made of precious metals. But these authors did not provide any specific details about how to process the very large volume of air in the atmosphere, concluding that these ideas were speculative.

Lockley [START_REF] Lockley | Comment on "Review of Methane Mitigation Technologies with Application to Rapid Release of Methane from the Arctic[END_REF] proposed several additional mitigation or removal techniques, such as: ignition of CH 4 at point sources, lake sealing with impermeable covers or with nonbiodegradable foaming agents, ducting CH 4 bubble streams from underwater sources, and others. Again, in the absence of specific details, these ideas seem speculative.

Later, researchers proposed adding to the atmosphere CH 4 depleting agents such as chlorine (Cl) atoms generated by iron salt aerosols in the air [START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF][START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF] or using human made infrastructure devoted to another use. These could include:

 solar updraft chimneys (SUT) [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF], which produce CO 2 -free renewable electricity and each unit can process on the order of 6,000 cubic kilometers (km 3 ) of air per year;

 direct air capture (DAC) systems [START_REF] Keith | A process for capturing CO2 from the atmosphere[END_REF] that are being developed to capture CO 2 from air [START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF][START_REF] Jackson | Methane removal and atmospheric restoration[END_REF].

Existing infrastructure could be used for CH 4 removal. For example, titanium dioxide in self-cleaning windows or other photocatalytic coatings such as paintings on buildings could contribute to CH 4 removal and attract carbon credits and certificates, thus helping to finance future removal infrastructure. Aircrafts, wind turbines, or other structures already in contact with large quantities of air might also provide reaction surfaces to oxidize CH 4 .

Principal methods for enhancing atmospheric methane removal

Enhancing the main natural CH 4 sinks

Currently, in the troposphere, the principal natural CH 4 sinks are the hydroxyl radicals (which remove nearly 90% of the CH 4 ) [START_REF] Naik | Preindustrial to present-day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP)[END_REF], chlorine atoms (which remove about 2.5% of the CH 4 ) [START_REF] Hossaini | A global model of tropospheric chlorine chemistry: Organic versus inorganic sources and impact on methane oxidation[END_REF], minerals in soils and dust [START_REF] George | Heterogeneous photochemistry in the atmosphere[END_REF], soil microbes, plants and trees.

Enhancing those natural sinks can be a strategy to increase atmospheric CH 4 removal.

Enhancing the hydroxyl radical °OH and targeting in majority point sources

Atmospheric natural self-cleansing and volatile organic compound (VOC) removal is mainly due to hydroxyl radicals [START_REF] Rohrer | Maximum efficiency in the hydroxyl-radical-based self-cleansing of the troposphere[END_REF]. Hydroxyl radical generators, as well as ozone generators, are commercially available, and indoor VOC pollution can be efficiently controlled by short-wave ultraviolet (UVC) light in closed systems [START_REF] Johnson | Gas-phase advanced oxidation for effective, efficient in situ control of pollution[END_REF]. Some coauthors of this article are working on methods to enhance the °OH sink of CH 4 and reduce its lifetime, although still more research is needed to be able to provide cost estimates and quantitative estimates of efficacy [START_REF] Ming | Atmospheric methane removal by enhancing the hydroxyl radical sink[END_REF]. Still, the increasing efficiency and lifetime and the decreasing costs of ultra-violet light-emitting diodes (UV-LEDs) are promising. Research conducted at the University of Copenhagen has led to the creation of start-ups such as Infuser and AirLabs, which already apply this technology to point sources [START_REF]Strartups[END_REF]. Several other possible strategies to generate °OH radicals and apply them in the open atmosphere might be possible, based on the numerous and complex mechanisms by which they are produced [START_REF] Zhao | Inter-model comparison of global hydroxyl radical (OH) distributions and their impact on atmospheric methane over the 2000-2016 period[END_REF]. Knowing the intensity of sunlight UV, the °OH radical concentration can be predicted [START_REF] Wennberg | Radicals follow the sun[END_REF]. But care has to be taken not to expose human beings, animals, and plants to dangerous UV radiation and to ozone.

Enhancing the natural chlorine sink of CH 4 -at the molecular level

In 2017, some of the coauthors of this article proposed to deplete the atmospheric CH 4 directly in the lower troposphere with Cl atoms [START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF][START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF], mainly but not exclusively under the marine boundary layer. Recent scientific research from 2015-2017 proved that Cl atoms can be generated in large amounts from the sodium chloride (NaCl) content of natural sea-spray aerosols, thanks to an iron(III)/iron(II) sunlight photo-catalyzed reaction [START_REF] Wittmer | Iron (III)-induced activation of chloride from artificial sea-salt aerosol[END_REF][START_REF] Wittmer | Photochemical activation of chlorine by iron-oxide aerosol[END_REF]. Acidity (pH<3) found over coastal areas is naturally generated from NaCl by acid displacement with biogenic sulfate and nitrate [START_REF] Fong | Effect of relative humidity on HCl formation from the reaction of H 2 SO 4 and HNO 3 with NaCl particles[END_REF]. However, it also can be enhanced by anthropogenic pollution due to combustion sources, where nitrogen oxides and sulfur oxides are further oxidized in the atmosphere into nitric and sulfuric acids that react with sea salt to generate hydrochloric acid and sodium nitrate and sodium sulfate salts. Figure 2 illustrates a possible way by which CH 4 is already being removed by enhancing its chlorine sink.

Over polluted coastal areas, the chlorine sink destroys up to 11% of CH 4 [START_REF] Sommariva | Multiphase halogen chemistry in the tropical Atlantic Ocean[END_REF]. It is worth noting that authors in favor of this method do not propose enhancing acid air pollution, and do not target the enhancement of the Cl atom generation over populated areas or coasts [START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF][START_REF] Ming | A nature-based negative emissions technology able to remove atmospheric methane and other greenhouse gases[END_REF]. As CH 4 is a well-mixed GHG, it can be removed anywhere at atmospheric concentrations (about 1.9 ppm), consequently they propose to do so in remote unpopulated areas. They also plan to address point sources of CH 4 , where it is more concentrated, to deplete it before it becomes diluted and mixed in the global atmosphere, and in this case the Cl atom generation will be carried out in closed systems such as the existing ventilation systems of coal mines, with very low risks.

Enhancing the natural mineral sink of CH 4 -on surfaces

In 2017, it was proposed to perform CH 4 depletion by large scale photocatalysis [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF] using solar updraft chimneys (SUT), which are structures able to process very large volumes of air, as illustrated by Figure 3. As an example, a hypothetical 400-MW SUT would process 38,000 km 3 of air yearly [START_REF] De Richter | An unusual renewable energy device for atmospheric removal of GHGs[END_REF][START_REF] Schindelin | Entwurf eines 1500m hohen Turms eines Solar-Aufwindkraftwerkes Parameteruntersuchung zur Geometrieoptimierung (Design of a 1500m high tower of a Solar Updraft Chimney: Parameter study of the geometry optimization[END_REF].

The proposed photocatalyst for CH 4 depletion is a semiconductor metal oxide: a zinc oxide (ZnO) doped with 0.1% silver (Ag) [START_REF] Chen | Photocatalytic oxidation of methane over silver decorated zinc oxide nanocatalysts[END_REF]. It is expected to remove 50% of the CH 4 from the air processed thanks to sunlight at ambient temperatures [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF]. Then in 2018, DAC devices were proposed to remove atmospheric CH 4 alongside CO 2 , using the same ZnO-0.1%Ag photocatalyst illustrated by Figure 4. The enormous energy costs of fan-driven DAC make it less attractive than passive generation of large airflows by the SUT devices proposed above. Nonetheless, it is anticipated that the DAC technology will develop rapidly to remove CO 2 from the atmosphere. Once DAC plants exist, profiting from this existing infrastructure by upgrading it to also remove CH 4 could enhance the capture yields in terms of CO 2 -eq by 20%. Depending on the CH 4 oxidation yields and costs, it could be advantageous, especially since the removal of other GHGs also seems feasible [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF][START_REF] Ming | Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies[END_REF][START_REF] De Richter | Fighting global warming by GHG removal: destroying CFCs and HCFCs in solar-wind power plant hybrids producing renewable energy with no-intermittency[END_REF] and does not introduce a pressure drop requiring more energy for the fans, and no additional CO 2 capture capacity would be required [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF].

In 2019, other scientists proposed the use of DAC devices dedicated to CH 4 capture by zeolites and then removal of CH 4 using a thermal catalyst [START_REF] Jackson | Methane removal and atmospheric restoration[END_REF]. No published data was found on the pressure drop across the zeolite, making it difficult to know if the DAC device might be attractive to capture CO 2 also. Lackner [START_REF] Lackner | Practical constraints on atmospheric methane removal[END_REF] commented on this proposal by pointing to the extreme dilution of CH 4 in air (200 times more dilute than CO 2 ) which (based on the Sherwood law) may cause a three-order of magnitude energy penalty in using fan-driven DAC systemsthus suggesting that approaches should take advantage of natural air flow and use passive methods. Both the comment and the response [START_REF] Jackson | Reply to: Practical constraints on atmospheric methane removal[END_REF] mention that it would be more interesting to remove N 2 O, the third most important GHG by its radiative forcing, with an atmospheric lifetime estimated to 114 years and a GWP 100 nearly 300 times higher than of CO 2 .

Such a proposal was made in 2016 [START_REF] Ming | Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies[END_REF]. [START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF]. Reproduced from John Bradley [START_REF] Bradley | Environmental Visionaries: The Big Gun[END_REF].

Two proposals [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF][START_REF] De Richter | Iron Salt Aerosol and Photocatalytic Solar Chimneys: Two innovative breakthrough technologies to remove greenhouse gases[END_REF] also suggested removal of other GHGs (like N 2 O, as well as many halogenated GHG gases) with very high global warming potentials, which also damage the stratospheric ozone layer and are included in the Montreal Protocol. A long list of possible photocatalysts was proposed [START_REF] Ming | Fighting global warming by greenhouse gas removal: destroying atmospheric nitrous oxide thanks to synergies between two breakthrough technologies[END_REF][START_REF] De Richter | Fighting global warming by GHG removal: destroying CFCs and HCFCs in solar-wind power plant hybrids producing renewable energy with no-intermittency[END_REF], mainly titanium dioxide derivatives, all acting at ambient temperature and activated by sunlight.

Consequently, not only CH 4 , but almost all non-CO 2 GHGs are targeted by the photocatalytic method [START_REF] De Richter | Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis[END_REF].

In table 1, the principal methods of CH 4 removal are summarized. costs might be reduced, with a target costrange of $100 ton -1 of CO 2 -eq [START_REF]Methane-Action[END_REF].

In case of use of hybrid CO 2 DAC plants upgraded to also capture and oxidize CH4, the costs might be lower, as almost all the infrastructure and the air-flow already exist.

Photocatalysis on surfaces

Point sources such as ventilation systems; and global atmosphere

The estimated cost ton -1 CO 2 -eq is $166

by 2030 with a target of $100 by 2040 [START_REF]Methane-Action[END_REF]. For CH 4 removal using chlorine atoms, directly in the troposphere, mimicking natural processes costs estimates range from $ 54 to as low as $1.7 ton -1 CO 2 -eq [START_REF] Johnson | Methane Removal and Emerging Technologies[END_REF]. A startup targets costs of about $1.6 ton -1 CO 2 -eq [START_REF] Amr | Atmospheric Methane Removal[END_REF].

Those estimations of the costs look very 1 Although it is yet too early in the development process to have accurate cost estimates for CH 4 removal directly in the troposphere, initial estimations have been provided 2 Provided for illustration purposes only, as mitigation is out of the scope of this review and has been reviewed elsewhere [START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF][START_REF] Stolaroff | Review of methane mitigation technologies with application to rapid release of methane from the Arctic[END_REF][START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF][START_REF] Johannisson | A Structured Approach for the Mitigation of Natural Methane Emissions-Lessons Learned from Anthropogenic Emissions[END_REF][START_REF] Kebreab | Strategies to Reduce Methane Emissions from Enteric and Lagoon Sources[END_REF].

Enhancing the sinks by the use of plants, trees, and microbes

Mitigation of landfill CH 4 emissions using soil amendments such as biochar [START_REF] Huang | Enhancement of the methane removal efficiency via aeration for biocharamended landfill soil cover[END_REF], and microbial CH 4 oxidation processes with bio-covers [START_REF] Scheutz | Microbial methane oxidation processes and technologies for mitigation of landfill gas emissions[END_REF] or bio-trickling filters are wellestablished methods [START_REF] Yoon | Feasibility of atmospheric methane removal using methanotrophic biotrickling filters[END_REF]. Adding methanotrophs to flooded paddy soil also mitigates CH 4 emissions [START_REF] Davamani | Mitigation of methane gas emissions in flooded paddy soil through the utilization of methanotrophs[END_REF]. Enhancing methanotrophic activity is among the mitigation methods proposed to prevent CH 4 from reaching the atmosphere [START_REF] Nisbet | Methane mitigation: methods to reduce emissions, on the path to the Paris agreement[END_REF][START_REF] Johannisson | A Structured Approach for the Mitigation of Natural Methane Emissions-Lessons Learned from Anthropogenic Emissions[END_REF]. Similar methods also might be possible for greenhouse gas removal (GGR), as it has been

shown that an important CH 4 sink can be created by cropland reforestation [START_REF] Wu | Afforestation enhanced soil CH4 uptake rate in subtropical China: Evidence from carbon stable isotope experiments[END_REF]. One can probably imagine that for afforestation projects and for the "one trillion trees" initiative, planting trees that absorb tropospheric CH 4 [START_REF] Covey | Methane production and emissions in trees and forests[END_REF] in addition to CO 2 (instead of plants and trees that emit CH 4 [START_REF] Covey | Methane production and emissions in trees and forests[END_REF]), as recently observed in the seasonally flooded Amazon floodplain, is a good ideaespecially if using local trees species, and if biodiversity is preserved or restored, without competing with agricultural land.

Airborne microbes are abundant in the atmosphere [START_REF] Smets | Airborne bacteria in the atmosphere: presence, purpose, and potential[END_REF] and subject to long-range transport [100], so it might be possible to enhance the amount of CH 4 consuming microorganisms (methanotrophs) [101] for instance by enhancing the amount of methanotrophs already present on the bark of tree trunks [102]. Currently, plants, trees, and microbes represent 5%-6% of the sinks for atmospheric CH 4 , but to our knowledge apart a brief mention [START_REF] Boucher | New directions: atmospheric methane removal as a way to mitigate climate change?[END_REF], no large-scale strategy has yet been proposed

to take advantage of these sinks.

Discussion

The mail benefit of returning to CH 4 pre-industrial levels will be to reduce global warming by up to 0. ), many people would consider them significant. When "net-zero" is reached in the second half of this century, renewable energy will be preponderant.

Without knowing much about potential energy costs, and costs of scaling up, it is very difficult to understand and predict how feasible CH 4 removal technologies can be. But in our opinion, even if during the first decade following their invention, a very smallscale effective efficiency is obtained at very high cost, technologies able to avoid GHG emissions or able to remove GHGs already in the atmosphere deserve attention as soon as their scalability and globalization seems possible. Otherwise, the criterion of "at a climatically significant scale" may lead to too many options being dismissed and too few remaining, while the scale of the global warming problem requires a large portfolio of methods and technologies to be developed. A significant scale might be achieved as the sum of many contributions that are not individually very significant.

There are several direct and indirect co-benefits to reduced CH 4 atmospheric concentrations. The rapid climate benefits of reducing the concentration of CH 4 in the atmosphere are significant [START_REF] Ocko | Acting rapidly to deploy readily available methane mitigation measures by sector can immediately slow global warming[END_REF] for agriculture and the economy [109, 110] as the tropospheric ozone burden enhanced by CH 4 will also be reduced [103]. Lower surface ozone concentrations will increase crops yields and global photosynthesis, potentially allowing some CO 2 removal [START_REF] Feng | Economic losses due to ozone impacts on human health, forest productivity and crop yield across China[END_REF]. The co-benefits for human health are numerous [111], as it will reduce hospitalizations, asthma and pulmonary diseases and premature deaths due to the linked ozone pollution [START_REF]Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions[END_REF]. Development of CH 4 removal methods is still in its infancy and requires more research, development and funding [START_REF] Jackson | Atmospheric methane removal: A research agenda[END_REF].

Some of the CH 4 enhanced oxidation methods proposed will have other co-benefits.

As °OH and Cl atoms are very reactive and not very selective, by enhancing their generation several other GHGs and atmospheric pollutants will be removed faster than CH 4 . This includes, for instance VOCs, whose removal will help reduce CH 4 lifetime [112]. Such VOCs include also organo-halogens, human made hydrofluorocarbons and hydrochlorofluorocarbons, as well as natural biogenic halogenated-methane compounds produced mainly by oceanic plankton and bacteria [113]. The latter are not considered to be GHGs, but they participate in the stratospheric ozone layer natural cycle of destruction [114]. By reducing faster the amount of natural biogenic halogenated-methane compounds in the lower troposphere, as well as by reducing the water content of the stratosphere due to CH 4 oxidation [START_REF] Revell | The role of methane in projections of 21st century stratospheric water vapour[END_REF], the ozone layer might recover faster.

Of course, the main benefit of returning to CH 4 pre-industrial levels will be to reduce global warming by up to about 0.5°C [103], which can help reduce a temperature overshoot above 2°C by mid-century [104].

6.

Concluding remarks

This perspective article has discussed different strategies (some already proposed and several new ones) to accelerate the removal of already emitted CH 4 , reducing its radiative forcing by direct and indirect effects. A reduction of the atmospheric CH 4 burden might help the ozone layer to recover faster and will have rapid climate benefits together with significant co-benefits for agriculture, human health and the economy.

Unlike CO 2 removal methods, the CH 4 removal methods described here do not require capture and long-term geological sequestration as for CO 2 , as they only accelerate the natural oxidation processes that will anyway occur with the products remaining in the atmosphere. By returning to CH 4 pre-industrial levels, the increase of atmospheric CO 2 resulting from CH 4 oxidation is small compared to global CO 2 annual emissions, while the reduction of the radiative forcing could be significant. Net

Figure 1 :

 1 Figure 1: Equivalent emissions of the principal GHGs, on a GWP and GTP basis, compared over a time horizon of 10, 20, and 100 years, from the IPCC[START_REF] Stocker | The Physical Science Basis, Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]

Figure 2 :

 2 Figure 2: A container ship powered by bunker fuel mixed with commercially availableiron additives[START_REF] Ergin | An experimental investigation on the effects of fuel additive on the exhaust emissions of a ferry[END_REF][START_REF] Debbarma | Effects of iron nanoparticle fuel additive on the performance and exhaust emissions of a compression ignition engine fueled with diesel and biodiesel[END_REF] which are sold to reduce black carbon and carbon monoxide (CO) emissions and to reduce fuel consumption. In the exhaust plume, the iron compounds react with sea salt to produce iron chloride (FeCl 3 ), which under sunlight generates Cl atoms[START_REF] Wittmer | Photochemical activation of chlorine by iron-oxide aerosol[END_REF] that oxidize CH 4 16 times faster than °OH radicals[START_REF] Sommariva | Multiphase halogen chemistry in the tropical Atlantic Ocean[END_REF].

Figure 3 :

 3 Figure 3: A photocatalytic solar updraft tower concept to remove atmospheric CH 4 , nitrous oxide (N 2 O), and other GHGs, from [55].

Figure 4 :

 4 Figure 4: A hypothetical industrial DAC device with added photocatalyst, proposed to oxidize CH 4 into CO 2 [57]. Reproduced from John Bradley [82].

  favorable but are uncertain prior to demonstration, waiting for field trails which cannot start before a full-scale environmental assessment has been conducted.[START_REF] Oeste | Climate engineering by mimicking natural dust climate control: the iron salt aerosol method[END_REF][START_REF] Ming | A nature-based negative emissions technology able to remove atmospheric methane and other greenhouse gases[END_REF] For CH 4 mitigation at point sources, for instance in ventilation systems of coal mines where CH 4 is more concentrated, costs might even be lower as the infrastructure and the air-flow already exist and the generation of Cl atoms can be made by photolysis of Cl 2 gas, produced by the well-established chlor-alkali industrial process.Possible co-benefit: iron salt aerosols provide iron to depleted oceans, with possible CO 2 capture in the oceans at costs about $1 ton -1 of CO 2[START_REF] Ming | A nature-based negative emissions technology able to remove atmospheric methane and other greenhouse gases[END_REF] based on the "Redfield ratio" of oceanic C-N-P-Fe stoichiometry and assuming 10% sequestration in the bottom of the oceans.

  warming could be reduced by about 0.5°C.Although CH 4 enhanced oxidation methods can be applied both to global tropospheric CH 4 and to some local concentrated sources, removal and mitigation strategies and methods do not necessarily target the same sources and are complementary. In case of an abrupt acceleration of CH 4 emissions from natural sources (e.g. submarine methane-hydrates, or wetlands), the availability of effective and proven techniques would constitute an assurance to avoid a rapid acceleration of global warming.Those innovative methods deserve more attention from the scientific community to help evaluate their potential risks, costs, public acceptability, and societal appropriation. Together with CO 2 and CH 4 mitigation and with CO 2 removal, CH 4 removal methods can help fight climate change, win time by slowing down warming and thus meet the targets of the Paris Agreement with limited temperature overshoot.

Table 1 :

 1 Summary of the methane removal methods, technologies and estimated costs. For comparison, some methane mitigation strategies are also briefly described.

	Method	Principal targets Description, possible costs and	Ref.
			comments
		Both for removal and mitigation of CH 4	1
	Zeolites +	Point sources	For removal, using air moving devices	[85]
	catalysis	such as ventilation	such as Direct CO 2 Air Capture plants
		systems; and	associated with zeolites and a catalyst, if
		global atmosphere	the DAC plant is only devoted to CH 4
			capture and oxidation, costs might be
			slightly higher than for CO 2 capture [80] as
			tropospheric CH 4 is more than 200 times
			more diluted than CO 2 and air handling will
			consume more energy. But if passive
			systems are used and as on zeolites some
			catalysts can oxidize CH 4 at room
			temperature [83] and as no capture, no
			purification, no compression, no transport
			and no long-term storage is required in the
			case of CH 4 compared to CO 2 , then global

  as being of interest, for a global potential of 0.13 Gt CO 2 removal per year and of only 0.02 Gt CO 2 yr -1 in the United States. After a half-century of development, solar photovoltaics and wind turbines are now cost-competitive with other electricity generating technologies. In 2018, PV and wind turbines avoided respectively 0.15 and 0.37 gigatons of CO 2 -equivalent per year (Mt CO 2 -eq yr -1 ) [107, 108], compared to only a few tons per year five decades ago. Even though these savings are still small compared to global CO 2 emissions (currently about 40 gigatons CO 2 -eq yr -1

	5°C [103], which can help reduce a temperature overshoot above
	2°C by mid-century [104].
	The expression "at a climatically significant scale" often appears in discussions about
	GHG removal, but lacks precise definition. Perhaps one benchmark could be the
	amount of GHG removal achieved up to now. According to the International Energy
	Agency [105], after 10 years of development, existing DAC installations captured just
	9,000 tons CO 2 yr -1 in 2019. The report of three U.S. National Academies [106] on
	negative emissions technologies (NETs) considers "coastal blue carbon"
	(mangroves)

Acknowledgement

This research was supported by the National Key Research and Development Plan (Key Special Project of Inter-governmental National Scientific and Technological Innovation Cooperation, Grant No. 2019YFE0197500), National Natural Science Foundation of China (Grant No. 51778511), the European Commission H2020 Marie S Curie Research and Innovation Staff Exchange (RISE) award (Grant No. 871998), Hubei Provincial Natural Science Foundation of China (Grant No.2018CFA029), Key Project of ESI Discipline Development of Wuhan University of Technology (Grant No. 2017001), and the Fundamental Research Funds for the Central Universities (Grant No. 2019IVB082).


Minh-Loan Tran-Do

Jean-Pierre Habas

Bruno Ameduri
email: bruno.ameduri@enscm.fr

Oxygen-Tolerant Alternating Copolymerization of Fluorinated Monomers and Vinyl Ethers at Mild Temperature

Keywords: fluorinated copolymers, radical polymerization, rheology, spectroscopic analyses, thermal properties, water angle angles

 F NMR and 1 H NMR spectroscopy techniques. Thermomechanical properties (T g = -16 to 18 °C) of these polymers were also studied and justified in terms of structure-properties relationships. Their stabilities were investigated as a function of temperature by thermogravimetric analyses. Then, their degradation temperatures related to a weight loss of 5%, (T d5% ) were evaluated as being comprised between 262 and 316 °C while poly(PMVE-alt-EVE) copolymer was characterized by a higher thermostability (T d5% = 382 °C). Their surfaces were found to be hydrophobic, evidenced by water contact angle measured from 95 to 110 °. When soluble in SEC solvent, these fluorinated copolymers reached M n up to 169,200 g mol -1 . Because of these features, such fluorinated copolymers may find applications in paints and coatings or as elastomers.

Introduction

Fluoropolymers are specialty polymers endowed with exceptional properties which enable them to be involved in many high tech applications. [START_REF] Holding | Technology of Fluoropolymers[END_REF][START_REF] Smith | Handbook of Fluoropolymer Science and Technology[END_REF][START_REF] Drobny | Compounds for Automotive Fuel Systems[END_REF][START_REF] Ameduri | Fluorinated Polymers[END_REF][START_REF] Ameduri | Fascinating Fluoropolymers and Their Applications[END_REF][START_REF] Ameduri | Opportunities for Fluoropolymers[END_REF] They are usually produced by radical copolymerization of fluorinated alkenes initiated by persulfate or peroxide at temperature higher than 50 °C. This polymerization requires the absence of oxygen that is regarded as a radical scavenger that inhibits this reaction. Thus, it was of interest to find out conditions at room temperature and in presence of oxygen to possibly enable the success of such a reaction. As a matter of fact, boranes have been found as being appropriate to initiate polymerization of fluoromonomers. [START_REF] Smith | Handbook of Fluoropolymer Science and Technology[END_REF][START_REF] Sianesi | Polymerization and Copolymerization Studies on Vinyl Fluoride[END_REF][START_REF] Zhang | Reaction Mechanism of Borane/Oxygen Radical Initiators during the Polymerization of Fluoromonomers[END_REF][START_REF] Zhang | Synthesis of Chain End Functionalized Fluoropolymers by Functional Borane Initiators and Application in the Exfoliated Fluoropolymer/Clay Nanocomposites[END_REF][START_REF] Zhang | Preparation of Fluoropolymers for High Energy Density Capacitors[END_REF][START_REF] Lin | Borane Chain Transfer Reaction in Olefin Polymerization Using Trialkylboranes as Chain Transfer Agents[END_REF] However, the experimental conditions may be hazardous, because of their pyrophoricity due to electro-deficient character. Nevertheless, borane BH 3 exhibits a high bond dissociation energy (BDE = 106.6 kcal mol -1 ) to be used in organic reactions. As borane is a Lewis acid, complexing it with a Lewis base such as amine, phosphine [START_REF] Roberts | Polarity-Reversal Catalysis of Hydrogen-Atom Abstraction Reactions: Concepts and Applications in Organic Chemistry[END_REF] or carbene-based [START_REF] Rablen | Accurate Borane Sequential Bond Dissociation Energies by High-Level Ab Initio Computational Methods[END_REF] , efficiently reduces the B-H dissociation energy. In 1966, the first organoborane-amine complexes were reported as being air-tolerant. [START_REF] Mottus | Process for Polymerizing Unsaturated Monomers with a Catalyst Comprising an Organoboron Compound, a Peroxygen Compound and an Amine[END_REF] Recently, Magenau and Wilson described an O 2 -tolerant RAFT polymerization of alkyl methacrylates [START_REF] Wilson | Oxygen Tolerant and Room Temperature RAFT through Alkylborane Initiation[END_REF][START_REF] Timmins | Arm-first Star-polymer Synthesis in One-pot via Alkylborane-initiated RAFT[END_REF] and thiol-ene networks nanocomposites [START_REF] Wilson | Alkylborane-Initiated Thiol-Ene Networks for the Synthesis of Thick and Highly Loaded Nanocomposites[END_REF] at room temperature in the presence of tri-nbutylborane methoxypropylamine (TBB-MOPA) (Erreur ! Source du renvoi introuvable.). This complex could release efficient TBB-MOPA which, successfully initiate a polymerization in presence of oxygen. TBB-MOPA bears an ether moiety, as hydrogen bond accepting group of amine, enhancing then the electron density of the latter complexing with the borane. Its decomplexation requires i) a reasonable heating energy (dissociation temperature at 63 °C) in the absence of a deblocking agent, also called "deblocker", [START_REF] Sonnenschein | Amine Organoborane Complex Polymerization Initiators and Polymerizable Compositions[END_REF] or ii) in the presence of the deblocker (e.g. carboxylic acid or isocyanate), at room temperature, as reported by Magenau's group. [START_REF] Wilson | Oxygen Tolerant and Room Temperature RAFT through Alkylborane Initiation[END_REF][START_REF] Timmins | Arm-first Star-polymer Synthesis in One-pot via Alkylborane-initiated RAFT[END_REF][START_REF] Wilson | Alkylborane-Initiated Thiol-Ene Networks for the Synthesis of Thick and Highly Loaded Nanocomposites[END_REF] Actually, boranes play specific role in polymerization. They can be side groups of monomers promoting living radical polymerization. [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] As a matter of fact, the reactivity of electro-deficient boranes is blocked by the donating amine ligand. [START_REF] Sonnenschein | Amine Organoborane Complex Polymerization Initiators and Polymerizable Compositions[END_REF] The complex dissociation can occur under irradiation, [START_REF] Lalevée | Phototriggered In-Situ Generation of Triethylborane for Polymer Synthesis Under Air[END_REF] heating or in the presence of a deblocker, in order to generate in situ the reactive borane. [START_REF] Sonnenschein | Amine Organoborane Complex Polymerization Initiators and Polymerizable Compositions[END_REF] On the other hand, the acceptor-donor copolymerization of electron withdrawing highly fluorinated olefins (FO), such as chlorotrifluoroethylene, hexafluoropropylene, tetrafluoroethylene and perfluoromethyl vinylether with electron donating vinyl ethers (VE), are known to lead to alternating poly(FO-alt-VE) copolymers. [START_REF] Kojima | Fluororesin for Solvent-Soluble Paint[END_REF][START_REF] Boutevin | Studies of the Alternating Copolymerization of Vinyl Ethers with Chlorotrifluoroethylene[END_REF][START_REF] Takakura | Modern Fluoropolymers[END_REF][START_REF] Gaboyard | Photoinitiated Alternating Copolymerization of Vinyl Ethers with Chlorotrifluoroethylene[END_REF][START_REF] Valade | Synthesis and Modification of Alternating Copolymers Based on Vinyl Ethers, Chlorotrifluoroethylene, and Hexafluoropropylene[END_REF][START_REF] Tayouo | New Fluorinated Polymers Bearing Pendant Phosphonic Acid Groups. Proton Conducting Membranes for Fuel Cell[END_REF][START_REF] Alaaeddine | Synthesis and Characterization of Original Alternated Fluorinated Copolymers Bearing Glycidyl Carbonate Side Groups[END_REF][START_REF] Alaaeddine | An Efficient Method to Synthesize Vinyl Ethers (VEs) That Bear Various Halogenated or Functional Groups and Their Radical Copolymerization with Chlorotrifluoroethylene (CTFE) to Yield Functional Poly(VE-Alt-CTFE) Alternated Copolymers[END_REF][START_REF] Puts | Radical Copolymerisation of Chlorotrifluoroethylene with Isobutyl Vinyl Ether Initiated by the Persistent Perfluoro-3-Ethyl-2,4-Dimethyl-3-Pentyl Radical[END_REF][START_REF] Quan | Visible-Light-Enabled Organocatalyzed Controlled Alternating Terpolymerization of Perfluorinated Vinyl Ethers[END_REF] As a matter of fact, fluoroolefins, such as perfluoromethyl vinyl ether (PMVE) and hexafluoropropylene (HFP), have an electron-acceptor character (e values for PMVE and HFP are 3.01 and 1.50, [START_REF] Ameduri | Well-Architectured Fluoropolymers: Synthesis, Properties And Applications[END_REF][START_REF] Moggi | Emulsion Polymerization of the Vinylidene Fluoride-Hexafluoropropene System[END_REF] respectively). Meanwhile, vinyl ethers are electron-donating (-2.0 < e < -1.5). [START_REF] Kojima | Fluororesin for Solvent-Soluble Paint[END_REF][START_REF] Boutevin | Studies of the Alternating Copolymerization of Vinyl Ethers with Chlorotrifluoroethylene[END_REF][START_REF] Gaboyard | Photoinitiated Alternating Copolymerization of Vinyl Ethers with Chlorotrifluoroethylene[END_REF] . Furthermore, both these families of monomers cannot be homopolymerized. A higher yield is observed for the poly(PMVE-alt-EVE) copolymer when the feed molar ratio of comonomers is 1 : 1. This confirms previous studies on the alternating copolymer of CTFE with vinyl ether [START_REF] Tabata | Radiation-Induced Copolymerization of Chlorotrifluoroethylene with Ethyl Vinyl Ether[END_REF] or propylene [START_REF] Takakura | Modern Fluoropolymers[END_REF][START_REF] Ameduri | Well-Architectured Fluoropolymers: Synthesis, Properties And Applications[END_REF] for which the rate of copolymerization is the highest. [START_REF] Ameduri | Well-Architectured Fluoropolymers: Synthesis, Properties And Applications[END_REF][START_REF] Tabata | Radiation-Induced Copolymerization of Chlorotrifluoroethylene with Ethyl Vinyl Ether[END_REF] In that context, industrial productions of Lumiflon® [START_REF] Takakura | Modern Fluoropolymers[END_REF] and Zeffle®, [START_REF] Hoshino | Chapter 5. Fluorinated Specialty Chemicals -Fluorinated Copolymers for Paints and Perfluoropolyethers for Coatings[END_REF] used in resin coating with excellent chemical resistance have been achieved from radical copolymerization of vinyl ethers with chlorotrifluoroethylene or tetrafluoroethylene, respectively. Inspired from such above commercially available alternating copolymers, [START_REF] Takakura | Modern Fluoropolymers[END_REF] our group has studied, since 2009, the copolymerization procedures of fluoroolefins with vinyl ethers using tert-butylperoxypivalate [START_REF] Valade | Synthesis and Modification of Alternating Copolymers Based on Vinyl Ethers, Chlorotrifluoroethylene, and Hexafluoropropylene[END_REF][START_REF] Tayouo | New Fluorinated Polymers Bearing Pendant Phosphonic Acid Groups. Proton Conducting Membranes for Fuel Cell[END_REF][START_REF] Alaaeddine | Synthesis and Characterization of Original Alternated Fluorinated Copolymers Bearing Glycidyl Carbonate Side Groups[END_REF][START_REF] Alaaeddine | An Efficient Method to Synthesize Vinyl Ethers (VEs) That Bear Various Halogenated or Functional Groups and Their Radical Copolymerization with Chlorotrifluoroethylene (CTFE) to Yield Functional Poly(VE-Alt-CTFE) Alternated Copolymers[END_REF] or a persistent highly branched perfluoroalkyl radical [START_REF] Puts | Radical Copolymerisation of Chlorotrifluoroethylene with Isobutyl Vinyl Ether Initiated by the Persistent Perfluoro-3-Ethyl-2,4-Dimethyl-3-Pentyl Radical[END_REF] . These initiators require heating conditions (68 and 95 °C, respectively), leading to poly(FO-alt-VE) alternating copolymers with noteworthy thermal properties.

This study aims at focusing on the use of TBB-MOPA/CH 3 COOH system affording the desired poly(FOalt-VE) copolymer in O 2 -tolerant and room temperature (Scheme 1). Nuclear magnetic resonance (NMR) spectroscopy. The 13 C, 1 H and [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F NMR spectra were recorded on Bruker Avance 400 instruments, using CDCl 3 as deuterated solvent, the signal of which was locked at 7.26 ppm. DMSO-d 6 probe was also used when the copolymers were not soluble in usual deuterated solvents. Hexafluorobenzene was used as internal standard for [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F and 1 H NMR analyses, its signal being calibrated at -164.9 ppm. Coupling constants and chemical shifts are given in Hertz (Hz) and parts per million (ppm), respectively. The experimental conditions for recording 1 H [or [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F] NMR spectra were as follows: angle 90° [or 30°], acquisition time 4.5 s [or 0.7 s], pulse delay 2 s [or 5 s], number of scans 36 [or 64], and a pulse width of 5 μs for [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F NMR. [START_REF] Holding | Technology of Fluoropolymers[END_REF] H NMR decoupling was applied on all [START_REF] Rablen | Accurate Borane Sequential Bond Dissociation Energies by High-Level Ab Initio Computational Methods[END_REF] C NMR spectra and on Attached Proton Test mode ( 13 C{ 1 H}-APT).

Scheme 1: Radical copolymerization of perfluoroolefins (FO) with vinyl ethers (VE) in presence of trinbutylborane methyloxypropylamine (TBB-MOPA).

Materials and Methods

Chemicals

For 2D NMR spectroscopy, HeteroCOSY 1 H- [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F spectra were obtained with 2048 points in the F1 ( 1 H) dimension and 5350 points in F2 ( 19 F) one, a spectral width of 5.2 kHz in F1 and 5.3 kHz in F2.

Acquisition times were 0.20 s in F1 and 0.05 s in F2. Thirty two scans and a relaxation delay of 1 s were applied.

Thermogravimetric analysis (TGA).

The thermogravimetric analysis of the copolymers (ca. 10 mg) was carried out under air with a TA Instruments TGA 51 apparatus. The polymer samples were heated from 20 to 550 °C at a heating rate of 10 °C min -1 . The apparent degradation temperature was defined at the point where the relative weight loss was about 5% and consequently named T d5% .

Fourier transform infrared (FTIR) spectroscopy. FTIR analyses of the copolymers were performed using a PerkinElmer Spectrum 1000 in ATR mode, with an accuracy of ±2 cm -1 and by the accumulation of 32 scans.

Size exclusion chromatography (SEC) measurements. Number average molar masses (M n,sec ) and dispersities (Ɖ) of the polymers were determined using a triple-detection GPC from Agilent Technologies with its corresponding Agilent software, dedicated to multi-detector GPC calculation.

THF was used as the eluent at a flow rate of 0.8 mL min -1 , while polystyrene standards were used for the calibration. The system used two PL1113-6300 ResiPore 300 x 7.5 mm columns. The detector suite was composed of a PL0390-0605390 LC light scattering detector with two scattering angles (15° and 90°), a PL0390-06034 capillary viscometer, and a 390-LCPL0390-0601 refractive index detector.

The entire SEC-HPLC system was thermostated at 35 °C. The typical sample concentration was 1 mg mL -1 .

Differential scanning calorimetry (DSC)

. DSC analyses of the different copolymers (ca. 10 mg) were carried out under N 2 atmosphere using a TA instruments Q100 instrument controlled with DSC 200 F3 software. The instrument was calibrated with noble metals and checked before analysis (indium sample for which T m = 156.6 °C). After its insertion into the DSC apparatus, the polymer sample was cooled down from 20 °C to -100 °C and stabilized at the latter temperature for 10 min. Then, the first scan was performed at a heating rate of 10 °C min -1 from -100 °C up to 100 °C. This thermal cycle was repeated two more times. The glass transition temperature, T g , was evaluated from the average value of the temperature taken at the half-height of the heat capacity drop using the method of tangents for each run.

Dynamic mechanical analyses (DMA). DMA analyses were carried out on a Metravib Dynamic

Mechanical Analyser (DMA 25®) driven with Dynatest 6.8 software. Solid samples were tested in uniaxial tension mode at a frequency of 1 Hz. First experiment consisted in determining the linear viscoelastic domain of each sample, i.e. the strain range for which the dynamic mechanical properties are independent of the strain imposed to the material. In such a way, a strain value of 0.01%, taken down in the linear domain, was retained for the conduction of consecutive thermomechanical test. In this analysis, the complex Young modulus E* = E' + j E" was measured as a function of temperature from ca. T g -100 °C up to T g + 50 °C, at a heating rate of 3 °C min -1 . The real component E' is usually named "storage modulus" and is specific of the elastic contribution of the sample. In other words, it is proportional to the mechanical rigidity of the polymer. The imaginary part E" is named "loss modulus". It relates to the material ability for dissipating the mechanical energy.

Water contact angle (WCA). WCA measurements were carried out on Contact Angle System OCA-Data Physics. The water sessile drop method was used for the static contact angle (CA) measurements at ambient temperature. The probe liquid was water (θ H2O ) and the CA value was determined from average of five different drops of ca. 4.0 μL deposited on the same sample.

Poly(PMVE-alt-EVE) and poly(HFP-alt-EVE) copolymers were prepared by pressing (Figures S5 andS6). Due to their softness, poly(PMVE-alt-FAVE6) and poly(HFP-alt-FAVE6) copolymers films were i) frozen in liquid nitrogen; ii) broken into pieces which were placed on glass; iii) warmed up to room temperature; iv) pressed by another glass to form a film; v) frozen in liquid nitrogen again to remove the second glass easily and vi) finally warmed up to room temperature.

Procedure:

Poly(PMVE-alt-EVE) copolymer. In a 50 mL autoclave, was introduced a mixture composed of ethyl vinyl ether (EVE, 8.00 g, 111 mmol, 48 mol%), TBB-MOPA (933 mg, 3.4 mmol, 1.5 mol%) and beforehand O 2 -purged CH 3 CN (30 mL). The autoclave was then closed and cooled down to -30 °C. The PMVE (20.00 g, 120 mmol, 52 mol%) was transferred into the autoclave (P = 1 bar) at that temperature. Under stirring, the mixture was warmed up to 0 °C (P = 2 bar) and the CH 3 COOH (413 mg, 6.9 mmol, 2.9 mol%) diluted in CH 3 CN (5 mL) was added into the mixture via a high pressure pump. After 16 h (T = 18 °C, P = 4 bar), the reaction was stopped. The copolymer, insoluble in CH 3 CN (Figure S1a,b), was dissolved in Et 2 O and precipitated from MeOH (Figure S1c). After drying at 60 °C under high-vacuum over 4 h, the poly(PMVE-alt-EVE) copolymer was obtained as a yellow and translucent waxy elastomer (10.95 g, 39 % yield) (Figure S1d).

Poly(PMVE-alt-FAVE6) copolymer.

In the same conditions as above, a 50 mL autoclave containing FAVE6 (20.05 g, 51.8 mmol, 49 mol%), TBB-MOPA (542 mg, 2 mmol, 2 mol%) and O 2 -purged CH 3 CN (20 mL) was cooled down to -30 °C. Then, PMVE (9.2 g, 54.2 mmol, 51 mol%) was transferred in it (P = 1 bar). The mixture was stirred and warmed up to 0 °C (P = 2 bar) and CH 3 COOH (240 mg, 4 mmol, 4 mol%) diluted in CH 3 CN (5 mL) was injected. The reaction was stopped after 17 h (T = 24 °C, P = 2 bar). The copolymer (Figure S2a,b) was dissolved in 1,1,1,3,3-pentafluorobutane (C 4 H 5 F 5 ) and then precipitated from pentane (Figure S2c). After recovering the waxy product and drying at 60 °C under high-vacuum over 4 h, the poly(PMVE-alt-FAVE6) copolymer was obtained as a dark brown wax (12.36 g, 43 % yield) (Figure S2d).

Poly(HFP-alt-EVE) copolymer. In the same conditions as above, a 50 mL autoclave containing ethyl vinyl ether (EVE, 8.00 g, 111 mmol, 36 mol%), TBB-MOPA (1.2 g, 4.44 mmol, 1.4 mol%), O 2 -purged CH 3 CN (30 mL) was sealed and cooled to -30 °C. Then HFP (30 g, 200 mmol, 64 mol%) was transferred. After stirring and warming up to 0 °C (P = 3 bar), CH 3 COOH (533 mg, 8.88 mmol, 2.8 mol%) diluted in CH 3 CN (5 mL) was added into the mixture. After 3 h (T = 27 °C, P = 5 bar), the reaction was stopped. The copolymer (Figure S3a), insoluble in CH 3 CN, was dissolved in Et 2 O and precipitated from MeOH. After drying at 60 °C under high-vacuum over 4 h, the poly(HFP-alt-EVE) copolymer was obtained as a colorless amorphous material (3.35 g, 9 % yield) (Figure S3b).

Poly(HFP-alt-FAVE6) copolymer. In the same conditions as above, the autoclave containing FAVE6 (20.15 g, 51.8 mmol, 36 mol%), TBB-MOPA (542 g, 2 mmol, 1.4 mol%) and O 2 -purged CH 3 CN (20 mL).

was cooled down to -30 °C. After transferring HFP (14 g, 93 mmol, 64 mol%), the mixture was warmed up to 0 °C (P = 1 bar) and CH 3 COOH (240 mg, 4 mmol, 2.8 mol%) diluted in CH 3 CN (5 mL) was added. After 6.5 h (T = 27 °C, P = 2 bar), the reaction was stopped. The copolymer (Figure S4a), partially soluble in CH 3 CN, was dissolved in C 4 H 5 F 5 and precipitated from MeOH. After drying at 60 °C under vacuum over 4 h, the poly(HFP-alt-FAVE6) copolymer was obtained as a yellowish and translucent soft wax (5.63 g, 17 % yield) (Figure S4b).

Results and discussion

Though the literature reports many studies on the radical copolymerization of chlorotrifluoroethylene (CTFE) [START_REF] Boutevin | Studies of the Alternating Copolymerization of Vinyl Ethers with Chlorotrifluoroethylene[END_REF][START_REF] Takakura | Modern Fluoropolymers[END_REF][START_REF] Gaboyard | Photoinitiated Alternating Copolymerization of Vinyl Ethers with Chlorotrifluoroethylene[END_REF][START_REF] Valade | Synthesis and Modification of Alternating Copolymers Based on Vinyl Ethers, Chlorotrifluoroethylene, and Hexafluoropropylene[END_REF][START_REF] Tayouo | New Fluorinated Polymers Bearing Pendant Phosphonic Acid Groups. Proton Conducting Membranes for Fuel Cell[END_REF][START_REF] Alaaeddine | Synthesis and Characterization of Original Alternated Fluorinated Copolymers Bearing Glycidyl Carbonate Side Groups[END_REF][START_REF] Alaaeddine | An Efficient Method to Synthesize Vinyl Ethers (VEs) That Bear Various Halogenated or Functional Groups and Their Radical Copolymerization with Chlorotrifluoroethylene (CTFE) to Yield Functional Poly(VE-Alt-CTFE) Alternated Copolymers[END_REF][START_REF] Puts | Radical Copolymerisation of Chlorotrifluoroethylene with Isobutyl Vinyl Ether Initiated by the Persistent Perfluoro-3-Ethyl-2,4-Dimethyl-3-Pentyl Radical[END_REF] or tetrafluoroethylene (TFE) [START_REF] Ameduri | Well-Architectured Fluoropolymers: Synthesis, Properties And Applications[END_REF] with VEs, to the best of our knowledge, only one patent claims the use of perfluoromethyl vinyl ether (PMVE) [START_REF] Shimada | A Coating Composition, and a Coated Film and Glass Each Having a Coating Layer Comprised Thereof[END_REF] and one article that of hexafluoropropylene (HFP). [START_REF] Alaaeddine | Synthesis and Characterization of Original Alternated Fluorinated Copolymers Bearing Glycidyl Carbonate Side Groups[END_REF] In addition, terpolymerization of fluorinated olefins (FOs) with two different vinyl ethers (VEs) can be achieved either in presence of a radical initiator [START_REF] Valade | Synthesis and Modification of Alternating Copolymers Based on Vinyl Ethers, Chlorotrifluoroethylene, and Hexafluoropropylene[END_REF] or under visible-light irradiation. [START_REF] Quan | Visible-Light-Enabled Organocatalyzed Controlled Alternating Terpolymerization of Perfluorinated Vinyl Ethers[END_REF] In this work, mild polymerization conditions were studied using TBB-MOPA as initiator with commercially available fluoroolefins (such as PMVE and HFP) and vinyl ethers [such as ethyl vinyl ether (EVE) and 2-(perfluorohexyl)ethyl vinyl ether (FAVE6)] monomers.

The experimental conditions of radical copolymerization of fluoroolefins with vinyl ethers, the molar masses, polydispersities as well as the degradation and glass transition temperatures of the obtained materials are summarized in Table 1. [START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F NMR spectroscopy c Molar masses and polydispersities assessed from size exclusion chromatography (SEC) in THF calibrated with polystyrene standards. d nd = not determined since the copolymer was not soluble in THF or warm DMF Prior describing the resulting fluorinated copolymers, the low yields could be explained by the probable excess of oxygen. Actually, Chung et al. reported that the monooxidation of trialkylborane in catalytic oxygen content is essential in forming the active radicals for initiating the polymerization. [START_REF] Zhang | Reaction Mechanism of Borane/Oxygen Radical Initiators during the Polymerization of Fluoromonomers[END_REF][START_REF] Zhang | Synthesis of Chain End Functionalized Fluoropolymers by Functional Borane Initiators and Application in the Exfoliated Fluoropolymer/Clay Nanocomposites[END_REF][START_REF] Zhang | Preparation of Fluoropolymers for High Energy Density Capacitors[END_REF] Though we have not quantified the O 2 content in the medium, it can be expected that the excess of oxygen not involved in that monooxidation might bring some inhibition of the copolymerization, hence making lower the yields. In addition, the difference in yields from the radical copolymerization of FO monomers arises from two features: i) the 1:1 stoichiometry in PMVE:VE feed (runs 1-2) led to the best copolymerization rate (in contrast to an excess of HFP in runs 3-4) and ii) the comparison of high e values of comonomers in absolute value (e EVE = -1.80 versus that of PMVE to favor the alternance (e PMVE = 3.01 [START_REF] Ameduri | Well-Architectured Fluoropolymers: Synthesis, Properties And Applications[END_REF] >> e HFP = 1.50 [START_REF] Moggi | Emulsion Polymerization of the Vinylidene Fluoride-Hexafluoropropene System[END_REF] ). A low yield was also reported for a copolymerization of HFP with another vinyl ether. [START_REF] Valade | Synthesis and Modification of Alternating Copolymers Based on Vinyl Ethers, Chlorotrifluoroethylene, and Hexafluoropropylene[END_REF]

NMR

Poly(PMVE-alt-EVE) copolymer. After reaction and purification, the copolymer was characterized by 1 H NMR spectroscopy, the spectrum of which (Figure 1a) shows: i) the absence of the signals around 6 ppm that evidences the good reactivity of EVE; ii) a singlet at 1.18 ppm, attributed to CH 3 in EVE unit; iii) large signals ranging from 2.37-2.99, 3.55-3.92 and 4.06-4.55 ppm, assigned to CH 2 , O-CH 2 and CH, [START_REF] Valade | Synthesis and Modification of Alternating Copolymers Based on Vinyl Ethers, Chlorotrifluoroethylene, and Hexafluoropropylene[END_REF][START_REF] Tayouo | New Fluorinated Polymers Bearing Pendant Phosphonic Acid Groups. Proton Conducting Membranes for Fuel Cell[END_REF][START_REF] Alaaeddine | Synthesis and Characterization of Original Alternated Fluorinated Copolymers Bearing Glycidyl Carbonate Side Groups[END_REF][START_REF] Alaaeddine | An Efficient Method to Synthesize Vinyl Ethers (VEs) That Bear Various Halogenated or Functional Groups and Their Radical Copolymerization with Chlorotrifluoroethylene (CTFE) to Yield Functional Poly(VE-Alt-CTFE) Alternated Copolymers[END_REF][START_REF] Carnevale | Multinuclear Magnetic Resonance and DFT Studies of the Poly(Chlorotrifluoroethylene-Alt -Ethyl Vinyl Ether) Copolymers[END_REF] respectively. The 19 F NMR spectrum (Figure 1b) exhibits a characteristic singlet at -52.20 ppm attributed to CF 3 [START_REF] Boyer | Telechelic Diiodopoly(VDF-Co -PMVE) Copolymers by Iodine Transfer Copolymerization of Vinylidene Fluoride (VDF) with Perfluoromethyl Vinyl Ether (PMVE)[END_REF]

= x = x (1)
where [IS] and stand for the internal standard concentration and the integral of the NMR signal assigned to CX 2 ranging from i to j ppm, respectively. ppm, respectively, while the complex signal from -107.9 to -121.4 ppm is attributed to CF 2 . Similarly to the case of poly(PMVE-alt-EVE) copolymer, the HFP : EVE ratio was determined from the use of hexafluoroxylene as internal standard in both NMR spectra (Equation 2), and is 45 : 55. All these vibrations have stretching nature. The absence of C=C bands, that should be noted at ca. 1600 cm -1 , evidences the absence of the monomers.

Figure 5: Expansion of FTIR spectra of (a) poly(PMVE-alt-EVE), (b) poly(PMVE-alt-FAVE6), (c) poly(HFPalt-EVE) and (d) poly(HFP-alt-FAVE6) copolymers (the full spectra being supplied in the Supporting information).

Size Exclusion Chromatography (SEC)

As poly(PMVE-alt-FAVE6) and poly(HFP-alt-FAVE6) copolymers are not soluble in THF or DMF, only molar masses of poly(PMVE-alt-EVE) and poly(HFP-alt-EVE) copolymers could be determined by size exclusion chromatography (SEC) (Entries 1 and 3, Table 1, Figure S14) with reasonable molar masses (169,200 and 76,900 equivalent polystyrene, respectively, Table 1), especially when the comonomer feed was close to 1:1, as in the cases of the copolymers involving PMVE. As expected, for a conventional radical copolymerization, high dispersity values, Ɖ = 2.42 and 1.89, (even quasi bimodal for the former one, Figure S14), as well as molar mass, M n = 70,000 and 40,700 g mol -1 , were observed for poly(PMVE-alt-EVE) and poly(HFP-alt-EVE) copolymers, respectively.

Thermogravimetry analysis (TGA)

The thermogravimetric analyses of the synthesized copolymers were performed under air (Figure 6).

It is known that fluoroelastomers are a class of synthetic rubbers that exhibit resistance above 200 °C. [START_REF] Drobny | Compounds for Automotive Fuel Systems[END_REF][START_REF] Drobny | Fluoroelastomers Handbook[END_REF]39 Such a behavior is observed here with our copolymers. Indeed, the thermal degradation of poly(PMVE-alt-FAVE6), poly(HFP-alt-EVE) and poly(HFP-alt-FAVE6) copolymers occurs at relatively high temperatures. The apparent degradation temperatures, T d5% , are close to 316, 282 and 262 °C, respectively. The highest thermostability is obtained with the poly(PMVE-alt-EVE) copolymer with an apparent degradation temperature close to T d5% = 385 °C, probably due to its higher molar mass. This

Figure 6 : TGA thermograms of poly(PMVE-alt-EVE) (red orange), poly(PMVE-alt-FAVE6) (blue), poly(HFP-alt-EVE) (yellow) and poly(HFP-alt-FAVE6) (green) copolymers under air. The dotted line

shows the 5 wt% thermal decomposition of the copolymer.

Differential scanning calorimetry (DSC)

The calorimetric profiles of the four synthesized copolymers were evaluated by DSC in dynamic mode under inert atmosphere (N 2 ). The thermograms specific of poly(PMVE-alt-EVE) and poly(HFP-alt-EVE) copolymers are shown in Figures S7 and S9 while those characteristic of poly(PMVE-alt-FAVE6) and poly(HFP-alt-FAVE6) copolymers are represented in Figures S8 andS10, respectively. DSC thermograms of these copolymers revealed purely amorphous materials because no melting zone was observed. The glass transition temperature of each copolymer is reported in Table 1. The order of magnitude of these different T g values is consistent with the data described in the literature with similar macromolecular systems. [START_REF] Carnevale | Multinuclear Magnetic Resonance and DFT Studies of the Poly(Chlorotrifluoroethylene-Alt -Ethyl Vinyl Ether) Copolymers[END_REF][START_REF] Friedman | High Performance Films: Review of New Materials and Trends[END_REF] On an increasing T g scale, the different materials can be classified as follows: poly(PMVE-alt-FAVE6) < poly(HFP-alt-FAVE6) < poly(PMVE-alt-EVE) < poly(HFP-alt-EVE). Such a hierarchy can be explained using structure-properties relationships. Indeed, the highest T g value (18 °C) is observed with the most compact and most rigid chemical backbone based on HFP and EVE units . Keeping the EVE sequence, the replacement of HFP by PMVE induces a reduction of the T g value to -1 °C (i.e. for poly(PMVE-alt-EVE) copolymer) due to the increase of the space between macromolecular chains.

Indeed, the CF 3 side group present in HFP chemical structure is shorter than of the -O-CF 3 unit present in PMVE. Then, in this latter copolymer, the macromolecular cooperativity is lower and the presence of the oxygen atom brought some softness inducing a reduction of the T g value compared to that of poly(HFP-alt-EVE). The same tendency is observed between poly(PMVE-alt-FAVE6) and poly(HFP-alt-FAVE6) copolymers. Steric considerations can also be used to explain the reduction in T g that is detected when the EVE sequence is replaced by the much larger FAVE6 sequence. Then, the lowest T g (-16 °C) is registered with the copolymer characterized by the most branched chemical backbone, i.e. the largest intermolecular space, for poly(PMVE-alt-FAVE6) copolymer. Nevertheless, the difference between both FAVE6-based copolymers is quite reduced because the length of the O-C 2 H 4 -C 6 F 13 side group hides the possible influence of the much shorter dangling unit present in HFP or PMVE sequence. Such a situation is summarized in Figure 7.

The influence of macromolecular size on the copolymer T g value is identified as being much less important than the macromolecular free volume. Indeed, as discussed above (SEC section), the poly(PMVE-alt-EVE) copolymer is characterized by a M n = 169,200 g mol -1 but its T g is much reduced than that characteristic of poly(HFP-alt-EVE) copolymer with a M n = 76,900 g mol -1 . In our study, the distance between macromolecular chains is the most important parameter for influencing the copolymer T g value. Poly(PMVE-alt-EVE) and poly(HFP-alt-EVE) copolymers exhibit a rubbery appearance at room temperature due to their low T g . Their handling is quite easy for the production of rectangular samples as required for DMA tests (Figure S6) [START_REF] Shimada | A Coating Composition, and a Coated Film and Glass Each Having a Coating Layer Comprised Thereof[END_REF] . The other copolymers based on FAVE6 sequence present a pasty appearance at room temperature. This can be easily understood by the fact that they have an even lower T g and especially are not crystalline.

After determination of the linear viscoelastic domain (Figure S11), the thermomechanical profile of poly(PMVE-alt-EVE) and poly(HFP-alt-EVE) copolymers was studied from -100 °C up to 50 °C. Figure 8a shows as example the data registered for the poly(PMVE-alt-EVE) copolymer. At low temperatures, the copolymer is in the glassy state with a high mechanical rigidity (E' > 10 9 Pa). The elastic character is predominant. Then, for T > 0 °C, the E' value sharply decreases in a limited temperature range. In the same area, the loss modulus E" describes a peak commonly called "α peak" that is characteristic of the main mechanical relaxation of macromolecular chains during the glass transition. Taken at its maximum, the T α temperature is evaluated close to 7 °C. This temperature gives a rough evaluation of the copolymer T g . Indeed, the T α value depends on the frequency value used in the DMA test. But, in our case, the discrepancy between both temperatures remains limited. Above the glassy transition, the sample is in a rubbery state. The storage modulus E' is still predominant, but its values are two decades lower than that in the glassy state. Similar phenomena can be observed in the thermomechanical profiles of poly(HFP-alt-EVE) copolymer (Figure 8b). The T α temperature, taken at the maximum of the main mechanical relaxation peak is evaluated as being close to 29 °C. Once more, it is slightly higher than the T g value (18 °C). It is noted that just above the glass transition temperature, Tg, the main relaxation peak observed in E" curve intersects the discontinuity registered in the E' curve. In other words, no crystalline plateau is detected but just a rubbery plateau. This characteristic is typical of a purely amorphous polymer.

Indeed, in the case of a semi-crystalline polymer, the existence of the crystalline plateau, prevents the two curves of E' and E" from approaching each other in the glass transition region. Similar observation was noted with all copolymers studied here and that can be qualified as purely amorphous as evidenced by DSC experiments.

Water contact angle (WCA)

The surface properties of the synthesized copolymers were evaluated by means of water contact angle (WCA) measurements (Surprisingly, the contribution of FAVE6 is not so effective as it could have been anticipated, since the WCAs of poly(FO-alt-EVE) are slightly higher than those of poly(FOalt-FAVE6) copolymers. S1). A hydrophobic character was observed for the four copolymers with relatively similar measured WCAs, ranging from 95 to 110 °. This result is interesting because if the fluorinated chains are known to be hydrophobic, the presence of ether moieties likely disadvantage this characteristic properties. [START_REF] Delucchi | Fluoropolyether Coatings: Relationships of Electrochemical Impedance Spectroscopy Measurements, Barrier Properties, and Polymer Structure[END_REF] Beside the nature of the chemical groups present in each chemical skeleton, the influence of the average intermolecular distance is likely to being an influencing parameter as discussed previously for explaining the hierarchy in T g values. Indeed, the highest contact angle is observed with the poly(HFP-alt-EVE) copolymer that also presents the highest T g value. The reverse situation is described with poly(PMVE-alt-FAVE6) copolymer characterized by the largest dangling molecular units and the lowest T g value. In this latter case, the higher intermolecular distance (or higher free volume) makes it possible a slightly higher diffusion of water in the polymer matrix and consequently a "better" affinity with water. This peculiarity is likely at the origin of the lowest registered contact angle value. Surprisingly, the contribution of FAVE6 is not so effective as it could have anticipated, since the WCAs of poly(FO-alt-EVE) are slightly higher than those of poly(FO-alt-FAVE6) copolymers.

This study reports the first oxygen-tolerant radical copolymerization of fluorinated and gaseous comonomers at room temperature, i.e. under non-conventional conditions, by using TBB-MOPA/CH 3 COOH system. Different fluoroolefins and vinyl ethers, which do not homopolymerize under radical initiation, were successfully involved in this process, leading to various poly(FO-alt-VE) alternating copolymers in up to 43 % yield, clearly evidenced by IR and NMR spectroscopies. These fluorinated amorphous copolymers exhibit low T g (-16 to 18 °C), hydrophobic surface (contact angle comprised between 95 and 110 °) and high thermostability (T d5% up to 382 °C). The highest thermal stability is observed for poly(PMVE-alt-EVE) copolymer even if it does not present the highest T g value that is observed for poly(HFP-alt-EVE) copolymer. The first characteristic is mainly governed by the polymer size while the second one is driven by the macromolecular stiffness. Our different data demonstrate the occurrence of an interesting copolymerization procedure in term of energy and easiness to handle, with simple oxygen introduction. Though the yields deserved to be increased by a better control of the oxygen content, all these points are of first importance for supporting a possible transfer to industrial scale. Further studies i) to improve the copolymerization yields, ii) to expand to other fluorinated monomers (e.g. CTFE and VDF) and iii) involving more complex procedures (i.e. controlled radical copolymerization as RAFT and iodine transfer polymerization) are under study.

Such fluorinated copolymers are potential candidates in paints and coatings or elastomers.

.

  Perfluoromethylvinyl ether (PMVE) was purchased at SynQuest, Florida, USA. Hexafluoropropene (HFP) was kindly offered by Arkema (Pierre-Bénite, France); 2-(perfluorohexyl)ethyl vinyl ether (FAVE6) by Unimatec Co., Ltd, Ibaraki, Japan; while tri-nbutylborane methyloxypropylamine (TBB-MOPA) by CALLERY TM and 1,1,1,3,3-pentafluorobutane C 4 H 5 F 5 by Solvay, Tavaux, France. Ethyl vinyl ether (EVE), hexafluoroxylene, acetic acid, acetonitrile and diethyl ether were purchased from Aldrich Sigma, Apollo Scientific and VWR, and used without any further purification.

Figure 1 : 1 H

 11 Figure 1 : 1 H (a) and 19 F (b) NMR spectra of poly(PMVE-alt-EVE) copolymer in CDCl 3 with hexafluoroxylene as the internal standard.

Figure 2 :

 2 Figure 2 : HeteroCOSY 1 H-19 F NMR spectrum and its expansion (right) of poly(PMVE-alt-EVE) copolymer in CDCl 3 . The one-dimensional spectra are plotted on the horizontal top ( 19 F) and vertical left ( 1 H) of the 2D plots.The HeteroCOSY 1 H-19 F spectrum (Figure2) displays a correlation (purple) between CH 2 (2.37-2.99 ppm) and CF 3 (-52.20 ppm), which should be a through-space coupling with the presence of oxygen atom in OCF 3 group. The CH 2 is also correlated to both CF (blue lines in different shades) and CF 2 (red line) signal groups, spread from -130 to -110 ppm. The aligned (green) signals correlation of CH (4.25 ppm) with CF 2 was also observed. It is worth noting that CH 2 /CF (blue) correlations have a much stronger intensity than other ones. These assignments allow us to distinguish CF 2 and CF signals that was not possible from the[START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F NMR 1D spectra. Though to the best of our knowledge, such a HeteroCOSY spectrum of this copolymer has never been reported, deeper work is required to explain the tacticity in this structure.

Figure 3 : 19 F

 319 Figure 3 : 19 F NMR spectrum and its expansion in various ppm zones (top) of poly(PMVE-alt-FAVE6) copolymer in 1,1,1,3,3-pentafluorobutane with a DMSO-d 6 probe

  -alt-FAVE6) copolymers. As above, poly(HFP-alt-FAVE6) copolymer was dissolved in C 4 H 5 F 5 and characterized by NMR spectroscopy with a DMSO-d 6 probe. Though the absence of ethylenic protons of FAVE-6 monomer were absent, the 1 H NMR spectrum could not help in that characterization because of the intense signals of C 4 H 5 F 5 . Similarly, the composition of that copolymer was assessed by the[START_REF] Jäkle | New Polymeric Materials Based on Element-Blocks[END_REF] F NMR spectrum (Figure4) that shows the characteristic signals of both monomer units. As above, related to HFP units, the signal centered at -76 ppm is characteristic of CF 3 while the broad signals ranging from -110 to -121 ppm and from -182 to -186 pm were assigned to CF 2 and CF groups, respectively. The former broad signal comes from the non-equivalent fluorine atoms in alpha position about both asymmetric carbons in bulky CF(CF 3 ) and CH(OC 2 H 4 C 6 F 13 ) groups. Beside the signals of FAVE6 units located at -114.98, -123.1, -124.1, -125.1 and -127.6 ppm attributed to CF 2 moieties, that of the CF 3 group is low field shifted to -82.8 ppm. Based on the integrals of both CF 3 signals in both comonomers units, the calculated HFP : FAVE6 molar ratio was 1 : 1.

Figure 4 : 19 F

 419 Figure 4: 19 F NMR spectrum and its expansion of poly(HFP-alt-FAVE6) copolymer in 1,1,1,3,3pentafluorobutane with a DMSO-d 6 probe Fourier transform infrared (FTIR) spectroscopy Each FTIR spectrum of the four synthesized copolymers (Figure 5 and S12) displays three wide bands centered at ca. 2980, 2920 and 2850 cm -1 , corresponding to C(sp 3 )-H bond. The C-H bands characteristic of poly(HFP-alt-EVE) copolymer present the highest amplitude, probably because of

  is not surprising since PFA (perfluoroalkoxy polymer), a copolymer based on tetrafluoroethylene (TFE) and PMVE, is more thermally stable than FEP [poly(TFE-co-HFP) copolymer]

Figure 7 :

 7 Figure 7: Comparison of structure-properties relationships between PMVE and HFP (top) and EVE and (bottom) Dynamic Mechanical Analyses (DMA)

Figure 8 :

 8 Figure 8 : DMA thermograms of poly(PMVE-alt-EVE) (graph a) and poly(HFP-alt-EVE) (graph b) copolymers with E' (blue curve) and E'' (orange curve) versus the temperature.

Table 1 :

 1 Feed composition, molar mass and thermal properties of poly(FO-alt-VE) copolymers produced from radical copolymerizations a of fluorinated olefins (FOs) with vinyl ethers (VEs) Polymerization conditions : TBB-MOPA (ca. 2 mol%), CH 3 COOH (ca. 4 mol%), CH 3 CN, 0 °C to 25 °C

	Entry	FO	VE	Feed compo-	Copolymer compo-	Yield (%)	M n (g mol	-1 ) c		M w	Ɖ ( )	c	T g (°C)	T d5% (°C)
				sition	sition
				(FO (%) : VE (%)) a	(FO (%) : VE (%)) b
	1	PMVE EVE	52 : 48	45 : 55	39	70,000	169,200	2.42	-1	385
	2	PMVE FAVE6 51 : 49	50 : 50	43	nd	d		nd	d	nd	d	-16	316
	3	HFP	EVE	64 : 36	45 : 55	9	40,700	76,900	1.89	18	282
	4	HFP	FAVE6 64 : 36	51 : 49	17	nd	d		nd	d	nd	d	-15	262
	a

b

Copolymer composition was assessed from

  and broad signals assigned to CF and CF 2 ranging from -130 to -110 ppm. The complexity of the latter is due to 2 chiral carbon centers -CF 2 -C*F(OCF 3 )-CH 2 -CH*(OEt)-, making both protons in CH 2 and 2 F atoms in CF 2 non-equivalent. The PMVE : EVE ratio was determined with the use of hexafluoroxylene as internal standard, quantitatively relating the 1 H and 19 F NMR signal intensities of perfluorinated and non-fluorinated monomers units. This allows to get PMVE : C 6 H 4 (CF 3 ) 2 and EVE : C 6 H 4 (CF 3 ) 2 ratios from the integrals of their characteristic signals in the 1 H and 19 F NMR spectra, respectively. By deduction, PMVE : EVE ratio was determined from Equation 1 as 45 : 55.

Acknowledgments

The authors thank the Arkema (Pierre-Bénite, France), Unimatec (Ibaraki, CALLERY TM Borane (Bellevue, WA, USA) and Solvay (Tavaux, France) Companies for supplying free samples of hexafluoropropene (HFP), 2-(perfluorohexyl)ethyl vinyl ether (FAVE6), tri-nbutylborane methyloxypropylamine (TBB-MOPA) and 1,1,1,3,3-pentafluorobutane, respectively.

Conflicts of interest

The authors declare that there is no conflict of interest.

Supporting Information

IR and NMR spectra; pictures of films used for DMA; DSC thermograms of poly(FO-alt-VE) copolymers; tables of WCA values; SEC chromatograms of poly(PMVE-alt-EVE); these items can be found in https://pubs.acs.org/journal/aapmcd.

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