Leonard-Ionut Atanase

Carlos Larraya

Jean-François Tranchant

Maud Save
email: maud.save@univ-pau.fr

Rational design of tetrahydrogeraniol-based hydrophobically modified poly(acrylic acid) as emulsifier of terpene-in-water transparent nanoemulsions

Keywords: amphiphilic copolymers, RAFT polymerization, Terpenes, renewable resources, nanoemulsion

Amphiphilic copolymers based on renewable resources were involved as emulsifiers to prepare transparent terpene-in-water nanoemulsions. The amphiphilic copolymers are composed of hydrophobically modified poly(acrylic acid) (HMPAA) grafted with different fractions of hydrophobic bio-based tetrahydrogeraniol (THG) side chains. The well-defined PAA were synthesized by reversible addition fragmentation transfer (RAFT) polymerization in order to tune the number-average molar mass of the initial PAA. The self-assembly in aqueous solution of the HMPAA copolymers was investigated through the measurement of their critical aggregation concentration by viscometry, tensiometry, dynamic light scattering and the determination of their aggregation number by static light scattering. Series of oil-in-water nanoemulsions using dihydromyrcenol (DHM) terpene as dispersed phase and PAA-THG as emulsifier were prepared with different PAA-THG/DHM weight ratios and DHM/water weight ratios. The level of transparency of the emulsions was monitored though the transmittance value measured at 600 nm and the measurements of the hydrodynamic diameter of droplets by dynamic light scattering. This study highlights that the structure of the PAAx-THGy is a key parameter to prepare terpene-inwater nanoemulsions with the required high level of transparency. The optimised structure of the emulsifier consists in a moderate degree of polymerization of PAA backbone (DP n ̅̅̅̅̅ ,PAA  180)

along with an intermediate average degree of substitution in hydrophobic THG side chains (13  DS ̅̅̅̅  32).

Introduction

Microemulsions were defined as a system of water, oil and surfactant (also named stabilizer or emulsifier) which is a single optically isotropic and thermodynamically stable system with dispersed domain size varying approximately from 10 to 100 nm, usually 10 to 50 nm. [START_REF] Danielsson | The definition of micro-emulsion[END_REF][START_REF] Slomkowski | Terminology of polymers and polymerization processes in dispersed systems (IUPAC Recommendations 2011)[END_REF] In a microemulsion, the domains of the dispersed phase in the phase diagram are either globular (waterin-oil (W/O) or oil-in-water (O/W) droplets) or interconnected to provide a bicontinuous structure.

A microemulsion mostly involves a quaternary system composed of water, oil, surfactant and a hydrophobe miscible in water like n-butanol [START_REF] Chiappisi | A journey through the phase diagram of a pharmaceutically relevant microemulsion system[END_REF] or acetone [START_REF] Yan | Modular construction of single-component polymer nanocapsules through a one-step surfactant-free microemulsion templated synthesis[END_REF] for instance. In the present work, we will focus our attention on O/W emulsion with droplet structure prepared by applying ultrasonication shear in the absence of additional hydrophobic solvent apart from the oil. The term nanoemulsion is also used for O/W emulsions with droplet size in the order of 100 nm. [START_REF] Mason | Nanoemulsions: formation, structure, and physical properties[END_REF][START_REF] Gutierrez | Nano-emulsions: New applications and optimization of their preparation[END_REF][START_REF] Mcclements | Nanoemulsions versus microemulsions: terminology, differences, and similarities[END_REF][START_REF] Gupta | Nanoemulsions: formation, properties and applications[END_REF] By contrast to the thermodynamically stable microemulsion independent of the preparation process (simple mixing), nanoemulsion is associated to kinetically stable emulsions formulated with a lower amount of surfactant to produce process dependent droplets prepared for instance by external shear. [START_REF] Mason | Nanoemulsions: formation, structure, and physical properties[END_REF] In summary, microemulsion and nanoemulsions have similarities in the final size of droplets but nanoemulsion are kinetically stable emulsions with small droplet sizes (range 20 -100 nm) regardless the method of preparation. [START_REF] Gutierrez | Nano-emulsions: New applications and optimization of their preparation[END_REF][START_REF] Mcclements | Nanoemulsions versus microemulsions: terminology, differences, and similarities[END_REF][START_REF] Gupta | Nanoemulsions: formation, properties and applications[END_REF] Therefore, the term nanoemulsion will be preferred in the present work.

The range of nanoemulsion applications includes drug delivery systems, food industry, and cosmetic industry. [START_REF] Gupta | Nanoemulsions: formation, properties and applications[END_REF][START_REF] Lawrence | Microemulsion-based media as novel drug delivery systems[END_REF][START_REF] Tadros | Colloid Aspects of Cosmetic Formulations with Particular Reference to Polymeric Surfactants[END_REF] Contrary to macroemulsions (often called shortly as an 'emulsion' with droplet size of hundreds of micrometers), no inherent creaming, sedimentation, flocculation, or coalescence is observed for nanoemulsions. [START_REF] Sharma | Nanoemulsion: A new concept of delivery system[END_REF] Another difference concerns their appearance as emulsions are cloudy while nanoemulsions are transparent or translucent. This property is especially relevant for the cosmetic industry to develop alcohol-free transparent formulation of fragrances. In specific applications of cosmetics, different challenges should be simultaneously addressed: preparation of transparent alcohol-free fragrance formulations, use of efficient emulsifier with minimum toxicity and meeting the increasing demand for reducing the dependence on fossil resources by increasing the use of renewable resources. In cosmetics, nanoemulsions are preferred to microemulsion to minimize the toxicological risk of skin irritation induced by a high level of surfactant.

The present work aims to address these challenges by synthesizing amphiphilic copolymers from renewable resources as macromolecular emulsifiers of terpene-based transparent nanoemulsions.

Such macromolecular emulsifiers improve the emulsion stability with respect to the conventional molecular surfactants due to much slower desorption kinetics at the interface of the droplet. [START_REF] Atanase | Block copolymers as polymeric stabilizers in non-aqueous emulsion polymerization[END_REF] Moreover, macromolecular emulsifiers of molar mass above 5 to 10 kDa reduce skin toxicity by limiting diffusion across skin barrier. According to Osipow et al. [START_REF] Osipow | Transparent emulsions[END_REF], a transparent nanoemulsion is favored by matching the stabilizer and the oil phase chemical structure. In addition, the transparent nanoemulsions are characterized by droplet sizes in the range of 20 to 100 nm and by high storage stability against Ostwald ripening. As terpenes are a major component of fragrance formulations, the covalent attachment of hydrophobic terpenes onto a hydrophilic polymer backbone to prepare amphiphilic copolymers is a strategy to match the structure of the dispersed phase while simultaneously integrating renewable resources in the copolymer emulsifier. Amphiphilic copolymers provide attractive surface and interfacial properties resulting from the incompatibility between the chemically different sequences linked together in the same macromolecule. [START_REF] Perrin | Low hydrophobically modified poly(acrylic acid) stabilizing macroemulsions: Relationship between copolymer structure and emulsion properties[END_REF][START_REF] Alves | Terpene and Dextran Renewable Resources for the Synthesis of Amphiphilic Biopolymers[END_REF] The platform terpene molecules (α-and β-pinene) are produced from pine woods via distillation of either turpentine or paper oil, a by-product of the paper-making industry.

The range of synthesized terpenes, which are branched or cyclic C10 and C15 molecules, represent an alternative to oil seeds and palm oil. It is worth to mention that acrylic acid monomer used to synthesize poly(acrylic acid) (PAA) can be produced from renewable resources, ie from 3 hydroxypropionic acid produced from glucose. [START_REF] Meng | Extraction process for separating and recovering 3-hydroxypropionic acid from aqueous mixtures of 3-hydroxypropionic acid and acrylic acid[END_REF][START_REF] Blaschke | Method for producing acrylic acid[END_REF] PAA is among the most common pHresponsive polymer with a pKa of 4.5. [START_REF] Garces | Ionization equilibria and conformational transitions in polyprotic molecules and polyelectrolytes[END_REF][START_REF] Colombani | Ionization Of Amphiphilic Acidic Block Copolymers[END_REF] The level of protonation of the carboxylic groups switches the net hydrophilic-hydrophobic character of the polymer. At pH values above 4.5, the PAA is neutralized to form the water soluble anionic poly(sodium acrylate) (PANa). On the contrary, at pH < 4.5 precipitation occurs in aqueous solutions due to protonation of the carboxylate groups, which renders the polymer only sparsely soluble in water. [START_REF] Fujiwara | Characterization of pH-dependent poly(acrylic acid) complexation with phospholipid vesicles[END_REF] Hydrophobically modified poly(acrylic acid) (HMPAA) have proved to be an interesting class of amphiphilic copolymers with a wide range of properties. HMPAA are extensively used as stabilizers and rheology modifiers in paints, cosmetics, pharmaceuticals, foods, enhanced oil recovery, water treatment and controlled release of bioactive materials. [START_REF] Glass | A perspective on the history of and current research in surfactant-modified watersoluble polymers[END_REF][START_REF] Kroschwitz | Encyclopedia of polymer science and engineering[END_REF][START_REF] Winnik | Associative polymers in aqueous solution[END_REF] The hydrophobic interactions, and thus the solution properties, can be controlled by either the type or the grafting density of hydrophobic groups. [START_REF] Wang | Viscometric behaviour of hydrophobically modified poly(sodium acrylate)[END_REF][START_REF] Xie | Copolymers of N,N-Dimethylacrylamide and 2-(Nethylperfluorooctanesulfonamido)ethyl acrylate in aqueous media and in bulk. Synthesis and properties[END_REF] or by addition of salts and surfactants. [START_REF] Iliopoulos | Association between hydrophobic polyelectrolytes and surfactants[END_REF][START_REF] Biggs | Effect of surfactant on the solution properties of hydrophobically modified polyacrylamide[END_REF] HMPAA have demonstrated stabilizing properties for the preparation of gold nanoparticles [START_REF] Note | Hydrophobically modified polyelectrolytes used as reducing and stabilizing agent for the formation of gold nanoparticles[END_REF] or for direct oil-in-water emulsions. [START_REF] Perrin | Low hydrophobically modified poly(acrylic acid) stabilizing macroemulsions: Relationship between copolymer structure and emulsion properties[END_REF][START_REF] Galindo-Alvarez | Enhanced stability of nanoemulsions using mixtures of non-ionic surfactant and amphiphilic polyelectrolyte[END_REF][START_REF] Perrin | Emulsions stabilized with hydrophobically modified poly(acrylic acid)[END_REF][START_REF] Li | Study on the molecular behavior of hydrophobically modified poly(acrylic acid) in aqueous solution and its emulsion-stabilizing capacity[END_REF][START_REF] Lochhead | An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil-in-water emulsions. 1. Poly(acrylic acids) and hydroxyethyl celluloses[END_REF] Lockhead et al. [START_REF] Lochhead | An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil-in-water emulsions. 1. Poly(acrylic acids) and hydroxyethyl celluloses[END_REF] highlighted that hydrophobically-modified PAA were more efficient stabilizers than non-modified PAA for cyclohexane/water and mineral oil/water emulsions in a semi-dilute regime. Perrin et al. [START_REF] Perrin | Low hydrophobically modified poly(acrylic acid) stabilizing macroemulsions: Relationship between copolymer structure and emulsion properties[END_REF][START_REF] Perrin | Emulsions stabilized with hydrophobically modified poly(acrylic acid)[END_REF] studied the emulsifying properties of a series of HMPAA for n-dodecane/water macroemulsions and concluded to a stronger anchoring of polymers to oil droplets and hence a better stabilization for a substitution degree (DS ̅̅̅̅ ) higher than 10 mol% or a longer alkyl chains. PAA samples of molar mass ranging between 5 and 5000 kg.mol -1 were hydrophobically modified with aliphatic aromatic or fluorocarbon groups by peptidic coupling or by copolymerization. [START_REF] Perrin | Low hydrophobically modified poly(acrylic acid) stabilizing macroemulsions: Relationship between copolymer structure and emulsion properties[END_REF][START_REF] Wang | Viscometric behaviour of hydrophobically modified poly(sodium acrylate)[END_REF][START_REF] Perrin | Emulsions stabilized with hydrophobically modified poly(acrylic acid)[END_REF][START_REF] Li | Study on the molecular behavior of hydrophobically modified poly(acrylic acid) in aqueous solution and its emulsion-stabilizing capacity[END_REF][START_REF] Lochhead | An investigation of the mechanism by which hydrophobically modified hydrophilic polymers act as primary emulsifiers for oil-in-water emulsions. 1. Poly(acrylic acids) and hydroxyethyl celluloses[END_REF][START_REF] Zhou | Hydrophobically modified polyelectrolytes, 4: Synthesis and solution properties of fluorocarbon containing poly(acrylic acid)[END_REF] Up to now, mostly fossil-based resources were involved in the hydrophobic modification of PAA while there is a global need in reducing the dependence on petrochemical resources. [START_REF] Imre | From natural resources to functional polymeric biomaterials[END_REF][START_REF] Llevot | Biobased polyesters and polyamides: Quo vadis[END_REF][START_REF] Smet | MacroRAFT agents from renewable resources and their use as polymeric scaffolds in a grafting from approach[END_REF] In the variety of biomass feedstock, terpenes have known a growing interest in recent studies of polymer chemistry. [START_REF] Wilbon | Progress in Renewable Polymers from Natural Terpenes, Terpenoids, and Rosin[END_REF][START_REF] Sarkar | Synthesis, characterization and properties of a bio-based elastomer: polymyrcene[END_REF][START_REF] Hilschmann | Bio-based polymyrcene with highly ordered structure via solvent free controlled radical polymerization[END_REF][START_REF] Sainz | A facile and green route to terpene derived acrylate and methacrylate monomers and simple free radical polymerisation to yield new renewable polymers and coatings[END_REF][START_REF] Baek | Preparation of biomass-based transparent pressure sensitive adhesives for optically clear adhesive and their adhesion performance[END_REF] The originality of the present work is to prepare amphiphilic copolymers based on tetrahydrogeraniol (THG) terpene and well-defined poly(acrylic acid) in order to stabilize solventfree transparent nanoemulsion of dihydromyrcenol (DHM) in water. In addition to mimicking the structure of the stabilizer side groups with the one of the dispersed oil phase, the use of tetrahydrogeraniol increases the biomass content. At this point it is worth mentioning that the preparation of terpene-in-water nanoemulsions, using exclusively D-limonene as oil phase, was studied until now only in the presence of commercially available stabilizers, generally based on ethylene oxide units [START_REF] Kalaitzaki | Biocompatible colloidal dispersions as potential formulations of natural pyrethrins:a structural and efficacy study[END_REF][START_REF] Li | Process optimization and stability of D-limonene-in-water nanoemulsions prepared by ultrasonic emulsification using response surface methodology[END_REF][START_REF] Lu | Skin permeation of D-limonene-based nanoemulsions as a transdermal carrier prepared by ultrasonic emulsification[END_REF], and mixed surfactants of low molecular weights. [START_REF] Lu | Nanoemulsion of D-limonene in water system prepared by ultrasonic emulsification[END_REF] The major drawback of these surfactants is related not only to their polydispersity in composition and in molar mass but also to their possible irritating effect on skin which makes them undesirable in cosmetic formulations. Moreover, it was stated by Schellekens et al. [START_REF] Schellekens | The immunogenicity of polyethylene glycol: Facts and fiction[END_REF] that the oligo(ethylene oxide) sequences of these products may be immunogenic and that the anti-OEO antibodies induce blood clearance and reduce efficacy of the products.

In order to thoroughly investigate the relationship between the macromolecular features of the HMPAA stabilizers, their solution properties and the nanoemulsion features, the present work proposes to control the molar mass and dispersity of the PAA backbone together with the substitution degree of the HPMAA amphiphilic stabilizers.

Experimental

2.1. Materials. The following materials were purchased from Sigma Aldrich and used as received: 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid (97%), acrylic acid (AA, 99%), 4,4′-azobis(4-cyanovaleric acid) (ACVA, 98%), N,N′-dicyclohexylcarbodiimide (DCC, 99%), 4-(dimethylamino)pyridine (DMAP, 95%) and anhydrous ethanol. The terpenes used in this study were supplied by Dérivés Résiniques Terpéniques (DRT Company) and the structures of the THG and DHM are reported in Supporting Information (Figure S1). Anhydrous dimethylsulfoxide (DMSO, Technical grade, VWR) was obtained by using molecular sieves that were previously dried under vacuum at 100°C. A sodium citrate buffer solution at pH 6 (Roth) was used as continuous phase for viscosimetry, surface tension and light scattering measurements as well as continuous phase for emulsions.

Synthesis of poly(acrylic acid) (PAA) by RAFT polymerization.

As a typical example for the synthesis of PAA90 (Table 1), a solution of AA (12.5 g, 0.1736 mol), 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid (0.4843 g, 1.736 × 10 -3 mol), and ACVA (0.0486 g, 1.74 × 10 -4 mol) in absolute ethanol (37.5 g) was degassed with N2 for 120 min. RAFT polymerization of AA was carried out at 80°C, and samples were withdrawn every 30 min under nitrogen flow. After 180 min, the reaction was stopped by cooling followed by exposing the mixture to air. The monomer conversion was calculated by on the basis of the signals corresponding to the vinylic protons of AA monomer at 6.2 ppm and to the aliphatic protons of the methine of PAA backbone at 2.2 ppm. The product was purified by precipitation into cold diethyl ether. This operation was repeated 3 times by dissolving PAA in ethanol prior subsequent precipitation. The polymer was lyophilised to recover a white powder.

Synthesis of hydrophobically modified PAA (HMPAA) by esterification of PAA with THG

In a typical experiment, 1g (4.72 × 10 -5 mol) of PAA294 was dissolved in 5 ml of anhydrous DMSO. Then, 0.1493 g (9.45 × 10 -4 mol) of THG was added and the temperature was set at 60°C. At this temperature, 12 mg (9.84 × 10 -5 mol) of 4-(dimethylamino)pyridine (DMAP) and 0.1946 g (9.45 × 10 -4 mol) of N-N' dicyclohexylcarbodiimide (DCC) (dissolved in 1g of DMSO) were added dropwise into the flask under vigorous stirring. The temperature was maintained at 60°C and the reaction mixture was stirred for 20 h. The system is then cooled down to room temperature and 5 ml of ethanol are added. The dicyclohexylurea crystals are eliminated by filtration and the polymer was precipitated into cold diethyl ether and solubilised into ethanol. This operation was repeated 3 times and then crude product is dissolved in water and finally freeze-dried to obtain a white powder.

Preparation of aqueous solutions of HMPAA and of terpene-in-water emulsions.

The PAA-THG copolymers were directly mixed with sodium citrate buffer solution at pH 6 at the desired concentration, the solutions were stirred overnight at room temperature and filtered through PVDF 0.45 mm filters. Polymer concentrations ranged between 1 to 5 wt% corresponding to 10 to 50 g.L - 1 . The emulsions were prepared by adding the DHM oil phase to the copolymer solutions in sodium citrate buffer and stirred for 5 minutes prior to be emulsified at high shear pressure using a VibraCell 72408 ultrasonicator for 2 min at amplitude of 30%. Emulsion containers were immersed into an ice bath during sonication process and the distance of the tip from the bottom of the tube was the same for all experiments. The total mass of the emulsions was kept equal to 10 g and the weight fractions of DHM versus continuous phase ranged between 0.2 and 2 wt%.

Methods. 1 H, 13 C, and DOSY (diffusion-ordered spectroscopy) nuclear magnetic resonance (NMR) spectra were recorded at 25°C on a Brüker 400 MHz spectrometer in DMSO-d6.

The number average molar mass (M n ̅̅̅̅ ) and dispersity (Đ = M w ̅̅̅̅ /M n ̅̅̅̅ /) of the methylated PAA (= poly(methyl acrylate), PMA) were analyzed by size exclusion chromatography (SEC) running at 30°C with THF as eluent at a flow rate of 1 mL.min -1 . The SEC system was equipped with three Waters Styragel columns HR 0.5, 2, 4 working in series (separation range 1 × 10 2 to 3 × 10 6 g.mol - 1 ), a refractive index detector ERC 7515-A and a MALLS WYATT Optilab detector operating at 633 nm. All polymer samples were prepared at 1 to 5 g.L -1 (0.1 to 0.5 wt-%) concentrations and filtered through PVDF 0.45 mm filters. The M n ̅̅̅̅ of the PMA samples was determined from SEC MALLS using a value of dn/dc value measured at 0.060 mL.g -1 which is close to the one reported in reference [START_REF] Guice | Reversible phase transformations in concentrated aqueous block copolymer solutions of poly(methyl acrylate)-b-poly(hydroxyethyl methacrylate-codimethylaminoethyl methacrylate)[END_REF].

The theoretical value of 𝑀 𝑛 ̅̅̅̅ was calculated according to

M n ̅̅̅̅ = [AA] 0 .M AA . conversion [CTA] 0 +[ACVA] 0 (1-e -kd ) + M CTA
With MAA and MCTA the molecular weight of acrylic acid and of the trithiocarbonate chain transfer agent respectively, kd the dissociation rate constant of ACVA in water at 80˚C (kd = 9 × 10 -5 from reference [START_REF] Brandrup | Polymer Handbook[END_REF].

The derivatized HMPAA samples were analyzed by proton NMR in order to calculate the average number of THG group grafted per polymer chain (NTHG/PAA, Erreur ! Source du renvoi introuvable.) and the average degree of substitution (DS ̅̅̅̅ ) corresponding to a molar percent versus AA monomer units (Erreur ! Source du renvoi introuvable.). The measurements of interfacial tension (IFT) and surface tension of the copolymer solutions were studied from dynamic measurements carried out using a Tracker tensiometer from TECLIS (Longessaigne, France), at 20.0 ± 0.2°C. An oil drop was formed by using a needle, with a diameter of 0.84 mm, was immersed in PAA-THG aqueous solution for IFT measurements while a drop a PAA-THG aqueous solution was formed in air for surface tension measurements. The shape of the drop was followed on a CDD camera and surface tension was deduced from the mathematical analysis of this axial symmetrical shape (Laplacian profile). The drop volume was kept constant by automatic adjustment. Time t = 0 was measured immediately after bubble formation.

NTHG/PAA

UV-Visible spectroscopy was performed with a Shimadzu UV-2450PC spectrophotometer in the 500 to 800 nm range, using a quartz cell with a path length of 1 cm.

Results and discussion

Synthesis of poly(acrylic acid)(PAA) by RAFT polymerization and preparation of

HMPAA.

Among the controlled radical polymerization techniques, reversible addition fragmentation polymerization (RAFT) involving a chain transfer agent (CTA) [START_REF] Loiseau | Synthesis and characterization of poly(acrylic acid) produced by RAFT polymerization. Application as a very efficient dispersant of CaCO3, kaolin, and TiO2[END_REF] and nitroxide mediated free radical polymerization (NMP) [START_REF] Couvreur | First nitroxidemediated controlled free-radical polymerization of acrylic acid[END_REF] proved their efficiency in controlling the polymerization of acrylic acid (AA) monomer. For such monomer, RAFT polymerization allows to perform the polymerization at lower temperature, hence broadening the range of available solvents, while NMP was mainly performed at 110°C in dioxane. In the present work, the polymerization of acrylic acid Step 1:

Step 2: The PAA samples of different molar masses were hydrophobically modified by grafting the tetrahydrogeraniol (THG) monohydroxy-terpene via a Steglich esterification reaction involving DMAP and DCC as catalysts (step 2 in Figure 1). The NMR spectra of PAA-THG and THG are displayed in Figure S4, which highlights the absence of overlap between the methine signals of PAA backbone at 2.2 ppm and the tetrahydrogeraniol signals.

Table 2 summarizes the features of the series of HMPAA synthesized by esterification reactions with different initial fractions of THG versus acrylic acid units of PAA. In the following, these copolymers will be designated as PAAx-THGy with x for DP n ̅̅̅̅̅ , NMR and y for THG molar content. The results of Table 2 show that the degree of substitution (DS ̅̅̅̅ ) of PAA-THG can be tuned by the initial ratio of THG versus AA (r0) with an increase of the esterification efficiency from 38 to 100 % by rising r0.

In order to confirm the efficient grafting of the THG, the purified copolymer was analysed by DOSY NMR (Figure 1). On the basis of the alignment of the diffusion coefficients of the methyl signal of THG moiety at  = 0.8 ppm and the PAA backbone ( = 1.0 -2.3 ppm), one can conclude to an efficient THG grafting and the absence of free THG with lower diffusion coefficient.

Figure 1. DOSY NMR spectrum of PAA90-THG39 copolymer in DMSO-d6.

3.2.Characterization of the PAA-THG aqueous solutions

With the aim to prepare transparent direct O/W nanoemulsions stabilized by the amphiphilic PAA-THG, the transmittance at wavelength of 600 nm of the HMPAA aqueous solution at pH 6 as well as the hydrodynamic diameters of the copolymers in solution were measured (see Table S1 in ESI). Aqueous solutions of 1 wt% of PAA-THG produced transparent solutions with a value of transmittance above 98 % for the entire series of copolymers except for PAA180-THG45 with the highest degree of substitution (Table S1, T = 600 nm = 62 %). At this point it has to be mentioned that the samples PAA90-THG39 and PAA90-THG51 samples are not completely soluble in the sodium citrate buffer solution at pH 6. The lower transmittance of the aqueous solution of the more hydrophobic PAA180-THG45 copolymer is due to the presence of larger aggregates of mean hydrodynamic diameter (Dh) centered at 190 nm while the analysis of the aqueous solutions of the other PAA-THG copolymers highlights that the main Dh of copolymer assemblies is systematically below 15 nm (Table S1). Dynamic light scattering reveals the presence of larger aggregates (Dh > 200 nm) for all the copolymer solutions but represented as a minor population as the volume distribution shows only one population Dh in the 8 -13 nm range (Table S1 and Figure S6).

Finally, the decrease of the surface tension (γ) as a function of the copolymer concentration also provides CAC values at the first crossover point, with values of CAC also ranging between 3.5 to 4.5 g.L -1 (0.30 to 0.45 wt-%) ( Figure 4). copolymer measured by SEC THG (calculated on the basis of M w ̅̅̅̅ 𝑀 𝑛 ̅̅̅̅ ,SEC and DS ̅̅̅̅ reported in Table 2). The copolymers with a degree of substitution below 15 mol-% tend to self-assemble into dimers (Nagg  1.5 -2.5) except for PAA with the highest PAA chain length (PAA345-THG13, Nagg 

which aggregates into more diffusing objects (see Figure 2). The increase of the grafted hydrophobic tetrahydrogeraniol fraction up to 32 mol-% for PAA180 induces an increase of the aggregation number to trimers (Table S2). These results are in concordance with those obtained by Hao et al. [START_REF] Hao | Kinetically driven intra-and interchain association of hydrophobically and hydrophically modified poly(acrylic acid) in dilute aqueous solutions[END_REF] who investigated the self-assembly of dodecyl-modified PAA copolymers in water for PAA of M w ̅̅̅̅ = 2.5 × 10 5 g.mol -1 . These copolymers self-assembled into either unimolecular chains for a dodecyl content in the range of 10 to 34 wt-%, or to plurimolecular aggregates of four chains for a dodecyl content up to 40 wt-%.

3.3.Characterization of the terpene-in-water direct emulsions

Dihydromyrcenol (DHM) was used as dispersed oil phase for the preparation of different oil-inwater emulsions. The concentration of the amphiphilic PAA-THG copolymer used as stabilizer was varied between 10 to 50 g.L -1 (1 and 5 wt-%) versus the water phase. The fraction of the DHM dispersed phase ranged between 0.2 and 2 wt-% based on the total volume of the emulsion. As described in the experimental section, the emulsions were prepared by the following procedure:

the PAA-THG amphiphilic copolymer at the required concentration is first poured into the aqueous buffer solution at pH 6 prior to add the dihydromyrcenol oil phase. The emulsion is submitted to stirring and ultrasonication high shear.

We first monitored the impact of the amphiphilic PAA-THG copolymer on the DHM/water interfacial tension (IFT), which plays an important role in the emulsification process. It is well known that the interfacial tension is affected by the surfactant concentration and molar mass and A first series of terpene in water direct emulsion stabilized by the terpene-based HMPAA copolymer was prepared with 0.2 wt% of dihydromyrcenol (DHM) and 10 g.L -1 (1 wt%) of PAA-THG based on the water phase at pH 6 (Table 4). The variation of the emulsion transparency, assessed through the transmittance at wavelength at 600 nm, as a function of both the average degree of polymerization of the PAA backbone and the DS ̅̅̅̅ value, is summarized in Table 4. It for DS ̅̅̅̅ 13 mol% (Figure S7 and Figure 5) and the high level of transparency of the copolymer aqueous solutions (Table S1), the increase of DP n ̅̅̅̅̅ of PAAx-THG13 stabilizer (DS ̅̅̅̅ = 13 mol%) induced a decrease of the transmittance of the emulsions (E1 to E4, Table 4). The curvature of the nanoemulsion droplets (Dh  13 nm, see Table 4) is higher than the curvature of the large DHM drop used for IFT measurements. One can suggest that the conformation of the PAA-THG copolymer at the oil/water interface of the small droplets is hindered by increasing the PAA molar mass, thus favouring the formation of larger droplets (Dh > 700 nm) to decrease the curvature. This enhances the light diffusion (35 < T < 65 %, see Table 4) and induces loss of transparency.

Transparent emulsions were obtained with the PAAx-THGy copolymers with the lowest degrees of polymerization (x = 90 and 180) whatever the DS ̅̅̅̅ value (E1, E2 and E5 in Table 4). Thus these HMPAA copolymers were selected as emulsifiers to prepare direct DHM-in-water emulsions with a higher fraction of dispersed DHM phase of 1 wt% (Table 5) and 2 wt% (Table S3 in ESI). As reported in Table 5, the weight ratio between the HMPAA stabilizer and the DMH dispersed phase was tuned between 2 to 5. 5). The efficient PAA90-THG13 and PAA180-THG13 emulsifiers provided the ability to produce transparent emulsions at higher content of DHM dispersed phase (2 wt% based on water phase) for mHMPAA/mDHM ratio of 5 (see Table S3 emulsions E18-E19). However, at such higher concentration of emulsifier based on the aqueous phase at pH 6 ([PAA-THG] = 10 wt% = 100 g.L - 1 ), the more hydrophobic PAA180-THG32 became insoluble and provided cloudy solutions. g.L -1 (1 wt%) reported in the second part of this article and insolubility of PAA180-THG32 at 100 g.L -1 (10 wt%) (Table S3)). These results highlight the interest in implementing controlled radical polymerization of acrylic acid in order to finely tune the DP n ̅̅̅̅̅ of the PAA precursor of the amphiphilic PAA-THG emulsifiers. All the terpene-in-water emulsions with high level of transparency were characterized by dynamic light scattering (DLS) in order to give an insight in their droplet sizes (see Table S4 months for respectively  = 500 nm, 600 nm and 800 nm respectively). It is important to note that the selected E16 nanoemulsion remains transparent even after 3 months of storage at room temperature since the transmittance value remained above 70 %.

As intermediate conclusion

Conclusion

In summary, a series of amphiphilic copolymers based on acrylic acid (AA) and tetrahydrogeraniol (THG), molecules that can be synthesized from renewable resources, have been conveniently

1 DS 2 )

 12 = (Ia/9)/(Ib/𝐷𝑃 𝑛 ̅̅̅̅̅ , NMR) Equation ̅̅̅̅ = (100 × NTHG/PAA)/ DP n ̅̅̅̅̅ , NMR Equation 2 Dynamic viscosities were determined with a rolling-ball AMVn Anton Parr micro-viscometer at 20°C and the results provided in the present work are the average of 5 measurements. The specific viscosity was calculated as follows: ηsp = (η -η0)/ η0 , where η0 is the dynamic viscosity of the buffer solution at pH 6 and η is the dynamic viscosity of the copolymer solution at a given concentration. Dynamic light scattering (DLS) measurements of the micellar sizes and emulsion droplets size were performed with a Vasco-2 Particle Size Analyzer from Cordouan Technologies working at an angle of 135° and a wavelength of 658 nm. Autocorrelation functions were recorded using a multi-acquisition mode and the apparent diffusion coefficients were determined via the Pade-Laplace inverse algorithm. The hydrodynamic diameter was calculated from the Stokes-Einstein equation based on the diffusion coefficient. Both size distributions in intensity and in volume are provided in this work in order to gain a better insight into the droplets sizes characterization. The size distribution is derived from the intensity autocorrelation functions fitted with either single or multiple relaxation time distribution. The volume distribution traduces the volume fractions of particles of a given radius. Static light scattering (SLS) measurements, of HMPAA solutions with concentrations ranging from 10 to 50 g.L -1 (1 to 5 wt-%), were performed with a particle size analyser from Cordouan Technologies at different angles, between 30 and 140°. The root mean square radius, also called the radius of gyration Rg, was determined by using the Zimm equation: 2𝑀 𝑤 𝐴 2 𝑐), where M w ̅̅̅̅ is the average weight molar mass, 𝐾 = 4𝜋 2 𝑛 0 2 ( 𝑑𝑛 𝑑𝑐 ) 2 /𝑁 𝐴 𝜆 4 and 𝑅(𝜃) = 𝑅 𝑇 [(𝐼 -𝐼 𝑠𝑜𝑙 )/𝐼 𝑡𝑜𝑙 ] the Rayleigh ratio of solution, RT is the Rayleigh ratio of toluene and is equal to 1.35x10 -5 cm -1 for a HeNe laser, I, Itol and Isol are the intensity of light scattered at  angle by respectively the polymer solution, toluene and solvent, n0 the refractive index of the solvent, nT the refractive index of the toluene, NA Avogadro's number. The refractive index increment dn/dc characterizes the change of the refractive index n with the concentration c. The dn/dc value of the HMPAA in water at pH 6 was considered equal to 0.15.[49] The scattering vector is 𝑞 = 4𝜋𝑛 0 sin ( 𝜃 /𝜆 with λ the wavelength of the light source, c is the concentration of polymer in solution.

Figure 2 .

 2 Figure 2. Intensity of the scattered light versus logarithmic concentration of HMPAA for PAA90-THG13 (triangles), PAA180-THG13 (diamonds), PAA294-THG13 (circles) and PAA345-THG13 (squares) in buffer solution at pH 6 at 20°C.

Figure 3 .

 3 Figure 3. Specific viscosity versus copolymer concentration at 20°C for PAA90-THG13 (triangles), PAA180-THG13 (diamonds), PAA294-THG13 (circles) and PAA345-THG13

  also by its hydrophilic/lipophilic balance. It should first be mentioned that all PAA-based copolymers are efficient stabilizers as the presence of these copolymer systematically induced a decrease of the interfacial tension in comparison with the initial value of DHM/aqueous buffer solution system showing an IFT value of 9.8 mN/m at 600 s. The impact on the IFT of the degree of polymerization (DP n ̅̅̅̅̅ ) of the initial PAA backbone was investigated for the copolymer series exhibiting a degree of substitution in THG equal to 13 mol-% (PAAx-THG13 with x = DP n ̅̅̅̅̅ , NMR = 180, 294, 345, see Figure S7 in ESI). The perfect overlay of the plots reveals the absence of significant effect of DP n ̅̅̅̅̅ of PAA backbone of the HMPAA on the IFT values. On the other hand, the degree of substitution of PAA180-THGy (x = DS ̅̅̅̅ = 0, 7, 13, 32 see Figure S8 in ESI) noticeably influences the interfacial tension. Indeed, IFT values decreased by increasing the substitution degree from 0 to 32 mol% of hydrophobic THG. As expected, the lowest value of IFT, of 2 mN/m, was obtained for PAA180-THG32 copolymer with the highest fraction of grafted hydrophobic THG. The effect of DP n ̅̅̅̅̅ and DS ̅̅̅̅ of PAA-THG amphiphilic copolymers on interfacial tension between dihydromyrcenol and water is summarized in Figure 5.

Figure 5 . 1 Figure 5

 515 Figure 5. Variation of the interfacial tension (IFT) of DHM/water as a function of the degree of substitution (DS ̅̅̅̅ ) for PAAx-THGDS with DP n ̅̅̅̅̅ ,PAA = x = 90, 180, 294, 345. The copolymer concentration is 10 g.L -1 (1 wt%) based on the aqueous solution at pH 6 (sodium citrate buffer), T = 20°C. The reference IFT value of DHM/water in the absence of polymer is 9.8 mN.m -1

  that the emulsions of the present work were classified in the following three categories, according to the transmittance (T) values at 600 nm as well as visual observations (see pictures in Figure6): transparent emulsions with T ranging between 70 % and 100 %, transluscent emulsions for T values ranging between 50 % and 70 % and cloudy emulsions characterized by a transmittance lower than 50%.

Figure 6 .Table 4 .

 64 Figure 6. Pictures of different types of emulsions as a function of the transmittance (T) values.

  , the structure of the PAAx-THGy drives the formation of nanoemulsions with the required high level of transparency. The optimised structure of the emulsifier consists in a PAA backbone with a moderate degree of polymerization (DP n ̅̅̅̅̅ ,PAA  180) and an intermediate degree of substitution in hydrophobic THG side chains (13  DS ̅̅̅̅  32). It should be noticed that depending on DP n ̅̅̅̅̅ value, the increase of the hydrophobic THG content faced issues of emulsifier solubility (see insolubility of PAA90-THG39 and PAA90-THG51 at 10

  in ESI). The hydrodynamic diameters (Dh) of the main droplets are systemically below 50 nm so the emulsions can be designated as nanoemulsions. DLS analyses revealed a main population of very small nanodroplets (fast mode of relaxation) with Dh values below 15 nm and a minor second population of larger droplets (slow mode of relaxation) of Dh ranging between 50 and 180 nm depending on the PAA-THG emulsifier structure. The calculated fraction in volume of these larger droplets is obviously negligible (see TableS4in ESI) as the associated scattered light intensity represents only 25 % to 70 % of scattered light depending on their size. Finally, the stability of the transparent nanoemulsions with time was investigated by monitoring the transmittance in the wavelength range of 500 to 800 nm for three months of storage at room temperature. A typical example of the transmittance evolution for the different storage durations is provided in FigureS9of ESI. The results for emulsion E16 showed that transmittance value decreased slowly with time. The decrease of transmittance is more significant for transmittance measured at lower wavelength (21 %, 13 % and 7 % of transmittance loss after 3

  prepared to target different chain lengths of PAA backbone and various degrees of substitution (DS ̅̅̅̅ ) in hydrophobic THG side chains. Reversible addition fragmentation transfer (RAFT) polymerization was first implemented to produce a series of PAA of low dispersity and controlled average degree of polymerization (DP n ̅̅̅̅̅ = 100 to 300). The efficient esterification of PAA with monohydroxy-THG terpene, confirmed by DOSY NMR, produced amphiphilic copolymers with DS ̅̅̅̅ ranging between 7 and 45 mol%. The critical aggregation concentration (CAC) of these amphiphilic copolymers dispersed in aqueous solution at pH 6 was determined by three different methods (viscosimetry, surface tension and dynamic light scattering) which converged towards similar CAC values, from 3.2 to 4.2 g.L -1 (0.32 to 0.42 wt-%) depending on copolymer structure. The hydrophobically modified PAA proved to be efficient polymeric emulsifiers to stabilize terpene-in-water direct emulsions with dihydromyrcenol (DHM) as dispersed phase. The objective of the present work to prepare transparent nanoemulsions stabilized by a bio-based polymeric emulsifier was achieved by tuning the structure of the PAAx-THGy copolymers. The optimised structure of the emulsifier for emulsion transparency consists in a PAA backbone with a moderate degree of polymerization (DP n ̅̅̅̅̅ ,PAA  180) and an intermediate degree of substitution in hydrophobic THG side chains (13  DS ̅̅̅̅  32 mol%). Fragrance-based aqueous formulations of high level of transparency were successfully prepared for varying concentrations of DHM terpene dispersed phase (0.2 to 2 wt-%) by adjusting the copolymer/DHM weight ratio.

Table 1 . Macromolecular features of the PAA series synthesized by RAFT polymerization in ethanol at 80 °C for 3 hours of polymerization

 1 (AA) was mediated by the 4-cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid CTA and carried out in ethanol at 80°C in the presence of 4,4′-azobis(4-cyanovaleric acid) as initiator (see Step 1 in Scheme 1). ̅̅̅̅ can be synthesized by tuning the initial concentration of the trithiocarbonate chain transfer agent. The SEC chromatograms of final PAA are unimodal and symmetrical with dispersity ranging between 1.2 and 1.4 (FigureS3in ESI and Table1), indicating polymers with a rather homogeneous distribution of chain lengths. As expected from literature, 4cyano [(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid is an efficient chain transfer agent to control acrylic acid polymerization by the RAFT process.

Scheme 1. Step 1: RAFT polymerization of AA mediated by 4-cyano [(dodecylsulfanylthiocarbonyl)sulfanyl] pentanoic acid as chain transfer agent (CTA); Step 2: Synthesis of THG hydrophobically modified PAA.

A series of poly(acrylic acid) was prepared by varying the initial [AA]/[CTA] ratio at constant monomer concentration of 2.9 M. The macromolecular features of PAA reported in Table

1

show that polymers with different values of M n , [AA]0 = 2.9 mol L -1 .

  The number average degree of polymerization (𝐷𝑃 𝑛 ̅̅̅̅̅ , PAA, NMR) of PAA was calculated as follows: 𝐷𝑃 𝑛 ̅̅̅̅̅ , PAA, NMR = Ib/(Ia/3); where Ia and Ib are the integral values of respectively the methyl group of PAA chain end from the trithiocarbonate CTA (CH3 = 0.8 ppm) and the methine group of the PAA backbone (CH = 2.2 ppm) in the proton NMR spectra. A typical example of 1 H NMR spectrum is given in ESI (FigureS2). Number-average molar mass and dispersity of methylated PAA determined by SEC-MALLS; c 𝐷𝑃 𝑛 ̅̅̅̅̅ ,PAA,SEC values were calculated as follows: 𝐷𝑃 𝑛 ̅̅̅̅̅ , PAA, SEC = (𝑀 𝑛 ̅̅̅̅ , PMA, SEC, MALLS -MCTA) / 86, as the molar mass of the methyl acrylate monomer unit corresponding to the methylated PAA is 86 g.mol -1 .The average degree of polymerization calculated from the proton NMR spectra (DP

	Sample		[AA] 0	Conv. DP n ̅̅̅̅̅̅ , PAA,	M n ̅̅̅̅ ,PMA, SEC b		Đ b		DP n ̅̅̅̅̅̅ , PAA, SEC c	DP n,NMR ̅̅̅̅̅̅̅̅̅̅̅
				[CTA] 0				NMR	a		(g mol -1 ) b									DP n,SEC ̅̅̅̅̅̅̅̅̅̅
	PAA90			100	0.90		90				11310				1.21				127	0.71
	PAA180			200	0.90		180			17280				1.24				196	0.92
	PAA294			300	0.98		CH 3 CN 294			20770				1.35				236	0.80
	PAA345	C H 3	CH 2 CH 2 CH 2 S n 9 400 0.86	S	345		OH	24450				1.46				280	0.80
	n	CO 2 H			ACVA, 80˚C, ethanol S			O	C H 3	CH 2	n 9	S		S	CH CH 2	CH 3 CN n	OH
																			S		CO 2 H			O
	C H 3	CH 2	n 9	S	S	S	CH CH 2 CO 2 H	n CH 3 CN C	O	OH	OH DMSO, DCC m	C H 3	CH 2	n 9	S	S	S	CO 2 H CH CH 2	n	O -m	O CH CH 2	CH 3 m	CH 3 CN	O	OH
														(THG)											C H 3	CH 3
	DP n ̅̅̅̅̅ , NMR/DP n ̅̅̅̅̅ , SEC ratio ranging between 0.7 and 0.9 indicate that DP n ̅̅̅̅̅ , NMR is underestimated due
	to the partial functionalization of PAA chains by the dodecyl trithiocarbonate end group as DP n ̅̅̅̅̅ ,
	NMR is calculated considering 100 % of dodecyl chain ends. The partial functionalization of PAA
	is the sign of irreversible chain transfer reactions to ethanol solvent. The decrease of the slope of
	M n ̅̅̅̅ ,SEC versus conversion plot together with a slight increase of dispersity with monomer
	conversion (see Figure S4 in ESI) also support the occurrence of irreversible transfer reactions to
	ethanol, as reported by Loiseau et al..[50] It has been previously shown that replacing ethanol by
	1,4-dioxane reduces such side reactions,[50, 52] though ethanol was preferred in this study to

a b n ̅̅̅̅̅ , NMR) evolves according to the initial [AA]0/[CTA]0 ratio and shows comparable values within the range of 10% error limits with the theoretical degree of polymerization (DP n ̅̅̅̅̅ = monomer conversion ×

[AA]0/[CTA]0) (see Table

1

). It should be noticed that the values of DP n ̅̅̅̅̅ , SEC calculated on the basis of the number-average molar mass of the methylated PAA measured by SEC (MALLS detector) are systematically higher than those determined by NMR (Table

1

). The values of the reduce solvent toxicity for cosmetic applications. Despite these transfer reactions, the macromolecular features of the poly(acrylic acid) synthesized in the present work were controlled by the initial ratio of monomer over chain transfer agent to provide a series of PAA with a range of M n ̅̅̅̅ and a narrow molar mass distribution.

Table 2 : Features of HMPAA series synthesized by Steglich esterification with THG at 60°C for 20 h.

 2

	PAA	Samples	r0 (%) a	NTHG/PAA b	DS ̅̅̅̅ c	Efficiency d
					(%)	(%)
		PAA90-THG13	22	12	13	59
	PAA90	PAA90-THG39	44	35	39	89
		PAA90-THG51	55	46	51	93
		PAA180-THG7	11	12	7	64
		PAA180-THG13	22	23	13	59
	PAA180	PAA180-THG32	33	56	32	96
		PAA180-THG45	44	80	45	100
	PAA294	PAA294-THG7 PAA294-THG13	12 20	20 39	7 13	59 65
	PAA345	PAA345-THG5 PAA345-THG13	13 17	18 44	5 13	38 76

a r0 = 100 × nTHG,0/nAA,0 with nAA,0 the number of moles of acrylic acid units in PAA, nAA,0 = mAA,0 /MAA. b Number of THG per PAA chains calculated with Erreur ! Source du renvoi introuvable.. c Degree of substitution calculated with Erreur ! Source du renvoi introuvable.. d Esterification efficiency = DS ̅̅̅̅ /r0.

Table 5 . Transmittance (T) values at 600 nm of DHM-in-water emulsions (E) prepared with 1 wt% of DHM dispersed phase on the basis of water and different copolymer ratios. Emulsion Copolymer mHMPAA/mDHM T = 600 nm (%)

 5

	Visual observation
	for emulsion

Table 5 )

 5 whereas transparency is observed only at a ratio of 5 for PAA180-THG13 emulsifier with higher value of DP n ̅̅̅̅̅ ,PAA and similar DS ̅̅̅̅ (E13). This effect of DP n ̅̅̅̅̅ is in accordance with the conclusions previously drawn from Table4. At a constant DP n ̅̅̅̅̅ ,PAA of 180, the increase of DS ̅̅̅̅ in

	hydrophobic THG (PAA180-THG32) allowed for the preparation of transparent emulsions at
	lower weight ratio of emulsifier versus dispersed phase (3  mHMPAA/mDHM  5, E15 to E17 in
	Table

Acknowledgements

The authors are grateful to Aquitaine Science Transfer for funding PAATER project. Dérivés Résiniques Terpéniques (DRT Company) is acknowledged for kindly supplying THG and DHM terpenes. The Equipex Xyloforest program (ANR-10-EQPX-16 XYLOFOREST) is acknowledged for MALLS detector and NMR probe funding. Dr. A. Khoukh is acknowledged for DOSY NMR analysis and Dr. E. Deniau for support in SLS.

It should be mentioned that the concentrations of PAAx-THGy in water further used for the preparation of the direct o/w nanoemulsions range between 10 and 50 g.LH20 -1 (1 to 5 wt%) which is 3 to 10 times higher than the CAC values (Table 3). In order to characterize the self-assembled aggregates of the HMPAA copolymer in water, we attempted to determine the aggregation number (Nagg) of the PAAx-THGy (Table S2). The aggregation number is the ratio between M w ̅̅̅̅ of the selfassembled PAA-THG in water calculated from the Zimm plot and M w ̅̅̅̅ of the HMPAA single Appendix A. Supplementary material


Laura Etchenausia

Abdel Khoukh

Elise Deniau-Lejeune

Maud Save
email: maud.save@univ-pau.fr

Elise Deniau Lejeune

RAFT/MADIX emulsion copolymerization of vinyl acetate and N-vinylcaprolactam: towards waterborne physically crosslinked thermoresponsive particles

RAFT/MADIX emulsion copolymerization of vinyl acetate and N-vinylcaprolactam: towards waterborne physically crosslinked thermoresponsive particles

INTRODUCTION

Waterborne latex produced by the environmentally friendly and industrially relevant emulsion polymerization process is a class of materials with myriad of applications in paints, adhesives, rubbers, coatings, concrete additive and nonwoven fabric, but also in cosmetics, biomaterials and other high-tech areas. 1 Stimuli-responsive polymers are a class of advanced polymers exhibiting properties tuned by an external stimulus such as temperature, light, pH, organic or gaseous molecules. [2][3][4][5] These polymers have received considerable attention within the last decade, due to their wide range of applications for smart materials like responsive optical devices 6,7 , membrane technology, 8,9 or for biomaterials like drug delivery systems, biosensors or cell culture substrates. [10][11][12][13][14] It is thus relevant to produce stimuli-responsive polymers of controlled macromolecular features as waterborne latex by emulsion polymerization process.

In the early 2000s, different research groups have made tremendous efforts to transpose the control of radical polymerization from homogeneous systems (bulk, solution) to heterogeneous systems and especially to the industrially relevant emulsion polymerization process performed in aqueous dispersed media. [15][16][17] Indeed, the emulsion polymerization process presents many advantages compared to polymerizations performed in bulk or in organic solution. In fact, the use of water as continuous phase facilitates heat transfer of the exothermic polymerization, decreases the viscosity of the final product which facilitates polymer recovery from the reactor at the end of the polymerization, and is more environmentally safe by limiting the amount of volatile organic compounds. As reported in several reviews, [15][16][17] most of the investigations focused on controlling the polymerization in aqueous dispersed media of (meth)acrylic or styrenic monomers, while very few studies concerned less activated monomers (LAM) such as vinyl acetate (VAc) [18][19][20][21][22][23][24][25] or vinyl chloride (VC). 26 Note that, despite the interesting features of the biocompatible thermoresponsive poly(N-vinylcaprolactam) (PVCL), 27 controlled radical polymerization of VCL by polymerization in aqueous dispersed media has never been investigated up to now. Among the different strategies investigated to implement controlled radical polymerization in aqueous dispersed media, the insitu formation of amphiphilic diblock copolymers simultaneously to the particle growing via emulsion polymerization process, in a concept known as "polymerization-induced self-assembly" (PISA), has emerged as one of the most efficient method. 28 PISA simultaneously addresses several challenges. The use of the reactive hydrophilic polymer producing the initial block copolymer micelles overcame the issue of diffusion of the hydrophobic control agent to the loci of polymerization, hence offering the opportunity to implement controlled radical polymerization in a batch ab initio emulsion polymerization process. 29,30 Moreover, it is a straightforward method to synthesize stable hairy particles at high solids content by surfactant-free emulsion polymerization.

Finally, it is an alternative route to produce self-assembled block copolymer nano-objects of various morphologies dispersed in water by a one-step method. 28,31,32 The PISA concept applied with thermoresponsive growing block was recently termed polymerization-induced thermal selfassembly (PITSA). 33 The pioneering works of using a reactive macromolecular stabilizer for the design of thermoresponsive particles dealt with chemically crosslinked particles. [34][35][36] Delaittre et al. reported the benefits provided by PISA to straightforwardly synthesize physically crosslinked poly(N,N-diethylacrylamide)-based thermoresponsive spherical particles via a reactive poly(acrylic acid) macroalkoxyamine. 37 Physically crosslinked microgels are a class of interesting supramolecular particles linked via reversible non-covalent interactions to create dynamic particles. 38 Among the stimuli-responsive polymers, thermoresponsive polymers are a class of relevant polymers which undergo a conformation phase transition in response to a change of temperature. 12 The most extensively studied thermoresponsive polymer is the poly(N-isopropylacrylamide) (PNIPAAm) with a LCST of 32°C close to the physiological temperature. Over the last ten years, poly(oligo(ethylene glycol) (meth)acrylates) (P(OEG)(M)A) [39][40][41] or poly(N-vinylcaprolactam) (PVCL), 27,42 which also present a LCST near to physiological temperature, have emerged as biocompatible thermoresponsive polymers of interest. Poly(N-vinylcaprolactam) is a 7-membered ring type polymer which is biocompatible, water-soluble, non-ionic and exhibits a LCST in water ranging between 30 and 40°C. 27, 42-44 42, 45-47 As for other thermoresponsive polymers, the temperature in which the change of PVCL chain solvation state occurs can be tuned by introducing co-monomers in the polymer chains. [48][49][50][51] Among the different co-monomers that can be used to tune the phase transition temperature of PVCL chains in water, vinyl acetate (VAc) is an attractive co-monomer to simultaneously promote non-covalent hydrophobic interactions. Moreover, poly(vinyl acetate) (PVAc) is used in a vast number of applications, the most common being binder for adhesive formulation, for the paper industry and for latex paints. 52,[START_REF] Amann | Synthetic Biodegradable Polymers[END_REF] Particularly, PVAc is approved by Food and Drug Administration (FDA) and it is a precursor of poly(vinyl alcohol) (PVA) which is a biocompatible and biodegradable hydrophilic polymer that is used in many technical (textile, papermaking) or food applications. [START_REF] Amann | Synthetic Biodegradable Polymers[END_REF] PVA is for instance used as efficient thickener, protective colloids, component of water-soluble film useful for packaging. [START_REF] Amann | Synthetic Biodegradable Polymers[END_REF] Nowadays, the raw materials for vinyl acetate monomer are produced from fossil resources. In the context of sustainable development, it can be mentioned that bioethanol produced from renewable resource could substitute the feedstock for these raw materials. Therefore, if the "vinyl acetate circle" can be closed by the important steps of hydrolysis and complete biodegradation of vinyl ester-based polymers to carbon dioxide, the whole production of PVAc could be switched to a renewable, sustainable and CO2-neutral production process based on bioethanol. [START_REF] Amann | Synthetic Biodegradable Polymers[END_REF] Herein, we present the first example of emulsion copolymerization of VCL and VAc mediated by a xanthate-functionalized poly(ethylene glycol) (PEG-X) as the reactive macromolecular chain transfer agent to follow the PISA mechanism. Our previous work demonstrated that xanthate-mediated RAFT/MADIX copolymerization of VAc/VCL was wellcontrolled in bulk with a close reactivity of both monomers (rVAc = 0.33 and rVCL = 0.29). [START_REF] Etchenausia | [END_REF] These P(VAc-co-PVCL) amphiphilic copolymers of homogeneous distribution exhibited tunable phase transition temperature (PTT) and self-assembly behavior depending on the fraction of hydrophobic VAc. Indeed, static and dynamic light scattering measurements highlighted that P(VAc-co-VCL) copolymer with FVAc = 0.53 had the capability to self-assemble in water into trimer aggregates at a temperature below the cloud point temperature, by means of hydrophobic interactions. The dual objective of the present work is to synthesize the amphiphilic thermoresponsive PEG-b-P(VAc-co-VCL) block copolymers of controlled molar mass directly in aqueous dispersed media while providing the opportunity to prepare thermoresponsive particles physically crosslinked by hydrophobic interactions. Insight into the control of RAFT/MADIX emulsion copolymerization of VAc and VCL as well as the features of the synthesized thermoresponsive colloidal particles, in relationship with the initial VAc monomer feed ratio is provided. Moreover, as poly(vinyl alcohol) (PVA) is a biocompatible polymer of interest for a wide field of applications, thermoresponsive PVA-based statistical copolymers are prepared by hydrolysis of the PEG-b-P(VAc-co-VCL) precursor copolymers.

EXPERIMENTAL PART

Materials

Vinyl acetate (VAc, Sigma Aldrich, 99 %+) was mixed with inhibitor removers (Sigma Aldrich, 0.1 g of inhibitor remover for 50 mL of VAc) for 30 min prior to be filtered and used for polymerization. N-vinylcaprolactam (VCL, Sigma Aldrich, 98 %), Poly(ethylene glycol) methyl ether (PEG-OH, Mn = 2 000 g.mol -1 , Sigma Aldrich), 2-bromopropionyl bromide (Sigma Aldrich, 97 %), anhydrous dichloromethane (Sigma Aldrich, 99,8 %), triethylamine (Sigma Aldrich, 99 %), potassium ethyl xanthogenate (Sigma Aldrich, 96 %), tris(hydroxymethyl)aminomethane hydrochloride buffer (TRIZMA, Sigma Aldrich, 99 %), potassium hydroxide (Sigma Aldrich, 90 %), methanol (Sigma Aldrich, 99,8 %) and 2,2'-Azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (ADIBA or VA-044, Wako, 99 %) were used as received. For the emulsion copolymerizations double deionized water was used as continuous medium.

Preparation of xanthate-terminated PEG (PEG-X) macro-chain transfer agent (macro-CTA).

The xanthate-terminated PEG macro-CTA was synthesized in a two steps procedure from commercially available PEG-OH, according to an already described procedure (see Scheme 1 in Supporting Information). 25,55 In the first step the hydroxyl end-group of PEG-OH was esterified with 2-bromopropionyl bromide, using triethylamine (TEA) as a base. The second step involved the nucleophilic substitution of potassium ethyl xanthogenate with PEG-Br leading to the xanthateterminated PEG macro-CTA named as PEG-X throughout this work. The proton NMR analysis of PEG-OH, PEG-Br and PEG-X (Figure S1) confirms the successful functionalization of the PEG chain-end. The perfect overlay of the UV-visible (λ = 355 nm) and RI traces of the SEC chromatograms of PEG-X also corroborates a high level of chain-end functionalization with the presence of the dithiocarbonate chain-end over the entire molar mass distribution (Figure S2).

General procedure for the RAFT/MADIX emulsion copolymerization of VAc and VCL mediated by a xanthate-terminated PEG macro-CTA.

The reactions were carried out at 10 wt-% of initial solids content, with different initial molar fractions of VAc (fVAc,0 = 0.2, expt 1 in Table 1 and fVAc,0 = 0.5, expt 2 in Table 1). For both experiments, the [Monomer]0/[PEG-X]0 and [PEG-X]0/[ADIBA]0 ratios were respectively set at 430 and 3.2. As example of experiment 2 in Table 1, 1,3,5-trioxane (0.40 g, 4.4 × 10 -3 mol), TRIZMA buffer (0.15 g, 9.5 × 10 -4 mol), PEG-X (0.30 g, 1.0 × 10 -4 mol), N-vinylcaprolactam (3.10 g, 2.2 × 10 -2 mol), vinyl acetate (1.90 g, 2.2 × 10 -2 mol) and deionized water (48.00 g) were introduced in a 100 mL round-bottom flask and the mixture was purged with nitrogen for 30 min in an ice-bath. A sample was withdrawn under nitrogen at time t = 0. The round-bottom flask was placed into an oil-bath previously heated to 65°C. The cationic initiator was added under nitrogen flow at 65°C (10.40 mg of ADIBA dissolved in 2 g of deionized water, 3.2 × 10 -5 mol of ADIBA).

The polymerization reaction was then allowed to continue with 300 rpm stirring for 5 h 30 min at 65°C. At the end of the reaction, a sample was withdrawn and kept at 65°C for DLS analysis. The reaction mixture was subsequently cooled to 25°C and oxygen was introduced in the reaction mixture. The same protocol was followed for the free radical emulsion copolymerization of VAc and VCL using PEG-OH as stabilizer (expt 3 in Table 1). The final copolymers were dialyzed against a 3.5 kDa cut-off membrane and freeze-dried prior SEC and turbidimetry analyses.

Alkaline hydrolysis of the PEG-b-P(VAc-co-VCL) copolymers

In a typical experiment, the dialyzed (Mw cut-off: 3500 Da) and freeze-dried PEG-b-P(VAc0.17-co-VCL0.83), (expt 1 in Table 1, 0.10 g, 6.2  10 -6 mol of copolymer) was introduced in a 10 mL round-bottom flask and dissolved in 2.9 g of methanol. A solution of 38.1 mg of KOH (6.8  10 - 4 mol, [KOH] = 1.4  10 -1 mol.L -1 ) dissolved in 1.0 g of methanol was then added to the flask. The mixture was stirred at room temperature for 18 h. Subsequently, the solution was dialyzed against distilled water through a Spectra/Pore membrane (6-8 kDa cut-off membrane) for 7 days and the PEG-b-P(VA-co-VCL) copolymer was recovered by freeze-drying. The same procedure was followed for the alkaline hydrolysis of PEG-b-P(VAc0.47-co-VCL0.53) copolymer (expt 2 in Table 1).

Characterization method

Nuclear Magnetic Resonance (NMR) spectroscopy. NMR spectra were recorded using a Brüker 400 MHz spectrometer, at 25°C, except for the characterization of PEG-b-P(VA-co-VCL) copolymers which was performed at 80°C. 1 H measurements were performed at frequencies of 400 MHz and 13 C measurements at 100 MHz. 13 C quantitative spectra are taken without Nuclear Overhauser Effect and with a long relaxation delay (10 s). This decoupling mode is referred to as inverse-gated (zgig as 1D Bruker pulse sequence).

The final overall molar monomer conversion (Xm) was calculated from nuclear magnetic resonance spectra in acetone-d6 using 1,3,5-trioxane as internal standard (Equation 1 and Figure S3a). in Figure S3a).

The compositions of the P(VAc-co-VCL) blocks (FVAc and FVCL) were calculated on the basis of the integrals of the protons of each polymer unit of 1 H NMR spectra (Equation 2and Figure S3b)

and on the basis of the integrals of the well-resolved carbonyl signals at 170 -180 ppm of each polymer unit of the P(VAc-co-VCL) copolymer (Equation 3and Figure S4).

𝐹 VAc = 𝐼 IV /3 𝐼 𝐼𝐼𝐼,𝐶 Equation 2 𝐹 VAc = 𝐼 C=O,PVAc 𝐼 C=O,PVAc + 𝐼 C=O,PVCL Equation 3
The copolymers synthesized by emulsion polymerization were characterized by DOSY NMR spectroscopy in DMSO-d6. The DOSY NMR experiments were performed using the bipolar longitudinal eddy current delay pulse sequence (BPLED). The spoil gradients were also applied at the diffusion period and the eddy current delay. Typically, a value of 2 ms was used for the gradient duration (δ), 150 ms for the diffusion time (Δ), and the gradient strength (g) was varied from 1.67 G.cm -1 to 31.88 G.cm -1 in 32 steps. Each parameter was chosen to obtain 95 % signal attenuation for the slowest diffusion species at the last step experiment. The pulse repetition delay (including acquisition time) between each scan was larger than 2s. Data acquisition and analysis were performed using the Bruker Topspin software (version 2.1). DOSY spectra were generated by using the program GIFA software, 56 the DOSY Module of NMR Notebook from NMRTEC, using inverse Laplace Transform driven by maximum entropy to create 2-D spectra with NMR chemical shifts along one dimension and the calculated diffusion coefficients along the other. The diffusion coefficients were extracted from T1/T2 analysis module of Topspin 2.1.

Size Exclusion Chromatography (SEC).

The SEC system operates in THF, at 30°C (flow rate:

1mL.min -1 ). The SEC apparatus is equipped with a Viscotek VE 5200 automatic injector, a pre- The experimental Mn are compared to the theoretical number-average molar masses of the copolymers calculated from Equation 4.

𝑀𝑛 𝑡ℎ𝑒𝑜. = 𝑀 PEG-X + 𝑋 𝑚 [M] 0 × (𝑓 VAc,0 × 𝑀 VAc + 𝑓 VCL,0 × 𝑀 VCL ) [[PEG-X] 0 + [Initiator] 0 (1-𝑒 -𝑘 𝑑 ×𝑡 )]
Equation 4

Where MPEG-X corresponds to the molar mass of the PEG-X macromolecular chain-transfer agent (Mn, PEG-X, MALLS = 2 240 g.mol -1 ), Xm is the final overall molar conversion calculated from NMR spectra (see Equation 1), kd is the initiator dissociation constant (kd, ADIBA, water, 70°C 57 = 6.7 × 10 -4 s - Turbidimetry analysis. Prior analysis samples were dialyzed against distilled water through a Spectra/Pore membrane (Mw cut-off: 3500 Da) for 48 h and recovered by freeze-drying.

Turbidimetry measurements of the (co)polymers solutions (0.3 wt-% in water) were performed with a Shimadzu UV-2450PC spectrophotometer, from 13°C to 33°C (heating rate: 0.5°C/min).

Transmittance at 500 nm was plotted versus the temperature. The minimum of the first-order derivative fit of transmittance versus temperature was considered as the phase transition temperature (PTT). °C to 10 °C. The measurements were performed every to 2°C with 10 minutes of equilibrium time at each temperature. The analysis of P(VAc-co-VA-co-VCL) copolymers dispersed in deionized water were carried out at a concentration of 5 g.L -1 from 10°C to 48°C, every 2°C.

Dynamic Light Scattering (DLS). The hydrodynamic diameters (

Fourier transform Infra-red (FTIR) spectroscopy. Spectra of the dialyzed and freeze-dried PEG-b-P(VAc-co-VCL) copolymers before and after hydrolysis were recorded on a FTIR NICOLET Magna-IR560, at 25°C. Attenuated total reflectance (ATR) mode was used.

RESULTS AND DISCUSSION

In this work, the xanthate-functionalized poly(ethylene glycol) (PEG-X) is involved as macromolecular chain transfer agent (macro-CTA) in the RAFT/MADIX emulsion copolymerization of VAc and VCL carried out at 65°C, using ADIBA as water-soluble initiator (Table 1). On the basis of the results obtained in our previous study concerning the bulk copolymerization of VAc and VCL, the thermoresponsive P(VAc-co-VCL) statistical copolymers are insoluble in water at 65°C. [START_REF] Etchenausia | [END_REF] Moreover, both monomers are water insoluble at a concentration of 10 wt-% based on water, hence forming monomer droplets at the initial state. It is thus expected that the mechanism will follow an emulsion polymerization process. 13 C NMR signal integrals (Equation 3 and Figure S4); c Molar fraction of VAc in the P(VAc-co-VCL) block, determined from 1 H NMR signal integrals (Equation 2 and Figure S3b); d Xm is the overall monomer molar conversion at 5h30 min of polymerization determined by 1 H NMR (Equation 1); e Theoretical number-average molar mass from Equation 4; f Values obtained from the SEC analysis with a MALLS detector using the dn/dc values given in experimental part; g Number-average hydrodynamic diameter of PEG-b-P(VAc-co-VCL) copolymer dispersions (0.5 g.L -1 ), measured at 55 °C at the end of the reaction (without cooling), corresponding to 98 % and 91 % of the intensity signal for Expt 1 and Expt 2 respectively.

For the sake of comparison, emulsion copolymerization of VAc and VCL with an initial molar fraction of VAc equals to 0.5 (fVAc,0 = 0.5) was carried out using a non-reactive PEG-OH hydrophilic polymer as the sole stabilizer (expt 3 in Table 1). The growing polymer spontaneously precipitated during the emulsion copolymerization of VAc and VCL to produce a large coagulum representing 97 wt-% of the polymer (Figure S5). These results highlighted the crucial role played by the reactive xanthate chain-end of PEG-X during the emulsion polymerization process, ensuring the colloidal stability of the particles by means of the in-situ synthesis and self-assembly of amphiphilic PEG-b-P(VAc-co-VCL) block copolymers. Indeed, as displayed in Figure 1 and 2).

The chromatograms of the final block copolymers display a shoulder that can be ascribed to an incomplete consumption of the initial macromolecular PEG-X chain transfer agent as suggested by Binauld et al. 25 for RAFT/MADIX emulsion polymerization of VAc mediated by a xanthatefunctionalized PEG. The incomplete consumption of the macromolecular RAFT agent is at the origin of lower values of experimental Mn compared to theoretical Mn of the block copolymers as the residual low molar mass PEO is accounted in the average experimental Mn (Table 2). The low efficiency of xanthate was also observed for bulk VAc/VCL copolymerization mediated by a molecular xanthate agent. [START_REF] Etchenausia | [END_REF] In order to assess the effective formation of the PEG-b-P(VAc-co-VCL) block copolymers by emulsion polymerization, the final dialyzed samples were analyzed by DOSY NMR spectroscopy. This technique allows for the identification of different components in the mixture by means of the acquisition of two-dimensional spectra which correlate the diffusion coefficient of each component with their corresponding chemical shifts. 58 As depicted in Figure 2 and Figure S7, the signals corresponding to PEG and P(VAc-co-VCL) blocks are aligned with one diffusion coefficient, which confirms that both blocks are part of the block copolymer structure. DOSY NMR spectra also show the presence of residual unreacted PEG homopolymer with higher diffusion coefficient of 86 µm 2 .s -1 . The diffusion coefficient of PEG-b-P(VAc0.47-co-VCL0.53) is slightly lower (D = 36 µm 2 .s -1 ) than the PEG-b-P(VAc0.17-co-VCL0.83) (D = 45 µm 2 .s -1 ) according to the higher Mn of the more hydrophobic copolymer (see Table 2). 2).

In comparison with the low dispersity values of the statistical P(VAc-co-VCL) copolymers synthesized in bulk (Ð < 1.3), [START_REF] Etchenausia | [END_REF] the dispersity values of the PEG-b-P(VAc-co-VCL) copolymers synthesized by emulsion copolymerization are higher (Ð = 1.6 -2.0) (see Table 2). The presence of unreacted PEG is at the origin of the higher dispersity values. Also, the higher concentration of VCL water solubility limit in comparison with VAc ([VCL]limit, water, 25°C = 41 g.L -1 , [VAc]limit, water, 20°C = 23 g.L -1 ), 1,59 might induce the enrichment in VCL for the initial growing oligomer chains produced in the water phase. It was concluded from our previous work carried out in bulk that dispersity values of P(VAc-co-VCL) statistical copolymers increases with increasing the initial VCL feed ratio. [START_REF] Etchenausia | [END_REF] The final average composition of the P(VAc-co-VCL) block was calculated by proton and carbon NMR (Figure S3 and Figure S4). The determination of the copolymer composition by comparing the integrals of both carbonyls of the 13 C NMR spectrum corresponding to each polymer units is of interest because of the good level of resolution. These compositions match those determined from the integrals of 1 H NMR spectrum of the PEG-b-P(VAc-co-VCL) (Table 2). The 13 C NMR analysis of both PEG-b-P(VAc0.17 The 13 C NMR spectra of the PEG-b-P(VAc-co-VCL) copolymers synthesized by emulsion polymerization were compared with the ones of their P(VAc-co-VCL) counterparts of similar composition synthesized in bulk. The signals of both PVAc and PVCL units exhibiting a similar shape between bulk and emulsion copolymers of similar composition (CHII', CHB' in Figure S8), the distribution of the co-monomer along the chain probably follows a similar statistic. The additional small peaks observed for the PEG-b-P(VAc-co-VCL) copolymers in the range of 50 to 75 ppm arise from the chain end signals of the PEG-xanthate (A", D", C") and from the rotational bands of the main PEO peak (B").

The pictures of the dispersion of PEG-b-P(VAc-co-VCL) particles displayed in Figure 4 show a macroscopic observation of the latex at different temperatures (T = 5°C, 20°C and at 50°C). The latex produced at 65°C was cooled at 5 °C followed by a gradual heating up to 20°C and 50°C. The PEG-b-P(VAc0.47-co-VCL0.53) copolymer synthesized by emulsion polymerization is a white dispersion of diffusing particles in the whole range of temperatures while translucent aqueous solutions of PEG-b-P(VAc0.17-co-VCL0.83) were observed at 5°C (Figure 4). This suggests that a sufficient level of hydrophobic interactions between the VAc units ( 47 Turbidimetry is a suitable method to monitor the thermoresponsiveness of the PEG-b-P(VAc-co-VCL) block copolymers synthesized by emulsion copolymerization. The evolution of transmittance versus temperature of the different copolymers is displayed in Figure 5 and the experimental values of the phase transition temperature are gathered in Table 3.

Table 3. Phase transition temperatures of PEG-b-P(VAc-co-VCL) copolymers synthesized by emulsion polymerization.

Expt

Copolymer FVAc PTT, UV-vis a °C 1 PEG-b-P(VAc0.17-co-VCL0.83) 0.17 The solution of PEG-b-P(VAc0.17-co-VCL0.83) copolymer allows for 100 % of light transmittance at low temperatures, which is in accordance with the absence of large diffusing objects below the phase transition temperature (Figure 4 and Figure 5). On the other hand, the 28 % transmittance of the aqueous dispersion of PEG-b-P(VAc0.47-co-VCL0.53) copolymer observed at low temperature is consistent with the presence of stable diffusing particles even at low temperature by means of sufficient hydrophobic interactions between VAc units (Figure 4 and Figure 5). The more hydrophobic PEG-b-P(VAc0.47-co-VCL0.53) exhibits a lower phase transition temperature than the PEG-b-P(VAc0.17-co-VCL0.83) as the enhanced hydrophobic interactions promotes the polymer collapse (Table 3). Note that the hydrophobic xanthate chain-end of the PEG-X does not impart any phase transition temperature to the low molar mass PEG as no variation of the transmittance was observed in the temperature range of 20 to 75°C (Figure S9). The presence of the hydrophilic PEG sequence in the PEG-b-P(VAc0.17-co-VCL0.83) copolymer influences the thermal response of the P(VAc0.17 copolymers with higher VAc fraction traduces a predominant effect of these hydrophobic units on the collapse of the thermoresponsive copolymer chains. The hydrodynamic diameters of the particles synthesized by VAc/VCL emulsion copolymerization carried out in the presence of the PEG-X stabilizer were measured at 55 °C directly at the end of the polymerization process, without cooling the reaction medium (Table 2). The average hydrodynamic diameter of the PEG-b-P(VAc0.47-co-VCL0.53) particles (Dh = 95 nm) is lower than the one of PEG-b-P(VAc0.17-co-VCL0.83) particles (Dh = 150 nm), thus indicating a higher number of particles for the highest initial fraction of hydrophobic monomer (fVAc,0 = 0.5). The increasing hydrophobicity likely results in the early self-assembly of the PEG-b-P(VAc-co-VCL) copolymer synthesized during the nucleation step, hence producing a higher number of particles. We further investigated the thermoresponsive behavior of the PEG-b-P(VAc-co-VCL) copolymers synthesized by emulsion copolymerization.

For that purpose, the aqueous dispersions of the amphiphilic copolymers were cooled down at room temperature after the polymerization process to be subsequently analyzed by dynamic light scattering. A heating and cooling cycle was applied in the range of 10°C to 55°C, at a polymer concentration of 0.05 g.L -1 (Figure 6). The temperature-dependence of the hydrodynamic diameters confirms the occurrence of a thermoresponsive phase transition for both types of copolymer particles but with opposite profiles (Figure 6). Figure 6. Hydrodynamic diameters as a function of the temperature for the PEG-b-P(VAc-co-PVCL) dispersions synthesized by emulsion polymerization at 10 wt-% of initial solids content for different initial monomer feed ratio (a) FVAc = 0.17 (expt 1 in Table 1) and (b) FVAc = 0.47 (expt 2 in Table 1). () 1 st heating cycle, () cooling cycle of polymer dispersions at 0.05 g.L -1 .

Indeed, the Dh versus temperature profile for PEG-b-P(VAc0.17-co-VCL0.83) copolymer (expt 1 in Table 1) (Figure 6a) is close to the one of the (PVAc-co-PVCL) statistical copolymers synthesized in bulk (see reference [START_REF] Etchenausia | [END_REF] ) in the sense that Dh increases by rising temperature. The very low average count rate measured at low temperature, quantifying the diffused light intensity, suggests that PEGb-P(VAc0.17-co-VCL0.83) copolymer chains disassembled into very small objects of 50 nm in water at a temperature below 20°C. The presence of objects with diameter of ca 600 nm above the phase

transition temperature, together with the hysteresis observed between heating and cooling cycles, both support the formation of polydisperse aggregates of the PEG-b-P(VAc0.17-co-VCL0.83) copolymer chains by increasing temperature up to 55°C (Figure 6a). Note that the self-assembly induced by dehydration of the hydrophobic PVCL block has been previously investigated for PEGb-PVCL diblock copolymers. 55 Despite the balanced compositions between the hydrophilic and hydrophobic blocks, these PEG114-b-PVCLx diblock copolymers self-assembled into large aggregates (Dh  300 to 550 nm) instead of well-defined micelles. 55 Contrary to PEG-b-P(VAc0.17 42,[60][61][62] due to the presence of the VAc hydrophobic units. Indeed, as the hydrophobic composition of thermoresponsive copolymers is increased, the hydrogen bonding effect between the copolymer chains and water is lowered while hydrophobic interactions are enhanced, thus lowering the required energy to collapse. 50,51 It should be noticed that the Dh of the collapsed particles measured after applying a cooling/heating cycle to the PEG-b-P(VAc0.47-co-VCL0.53) 4 particle dispersion (Dh, 55°C = 250 nm) is higher than the one measured for the kinetically-trapped particles recovered at the end of the polymerization at 55 °C in the absence of any cooling step (Dh, 55°C = 95 nm) (Table 2). After cooling the latex, some PEG-b-P(VAc0.47-co-VCL0.53) copolymer chains might exchange between the swollen particles formed by hydrophobic interactions, thus producing particles of larger size.

Such difference between the Dh measured at high temperature at the end of polymerization or after a cooling step has already been observed for physically crosslinked thermoresponsive particles synthesized by PISA. 37 In a recent study, in situ time-resolved SAXS measurements monitoring particles synthesized by RAFT PISA revealed an increase of the mean aggregation number as a function of monomer conversion for the kinetically-trapped spheres. 63 As perspective, the presence of the hydrophobic PVAc within the physically crosslinked particles might be of interest to enhance the loading capacity of hydrophobic molecules.

Alkaline hydrolysis of the PEG-b-P(VAc-co-VCL)

Poly(vinyl alcohol) (PVA) is prepared upon alkaline hydrolysis of PVAc under mild conditions (transesterification with methanol) leading to PVA with different degrees of saponification. [START_REF] Amann | Synthetic Biodegradable Polymers[END_REF]64 It has been shown that a degree of saponification of 88 % was optimum to provide a water soluble polymer as the hydrophobic acetyl neighborhood limits the hydrogen bonding between poly(vinyl alcohol) chains. [START_REF] Amann | Synthetic Biodegradable Polymers[END_REF] Therefore, the P(VAc-co-VCL)-based copolymers directly synthesized in water by emulsion polymerization are potential precursors of P(VA-co-VCL) copolymers. This is a strategy to synthesize by an environmentally friendly process some biocompatible thermoresponsive statistical copolymers with a tunable phase transition. The efficiency of the alkaline hydrolysis of PEG-b-P(VAc-co-VCL) copolymers was monitored by 1 H NMR analysis of the copolymers before and after hydrolysis (Figure 7 and Figure S10). For both copolymers, the effective hydrolysis was attested by the appearance of the PVA characteristic signals at 3.8 -4.5 ppm (N and M protons) together with the disappearance of PVAc signals at 4.8 ppm (III methyne proton) and 1.8 ppm (IV methyl protons) (Figure 7 and Figure S10). 1) copolymer (blue) and of the corresponding copolymer after hydrolysis (black).

The yield of hydrolysis of the copolymers can be calculated on the basis of the integrations of the characteristic NMR peaks of PVA and PVAc units ( Equation 5). I1HPVAc, before hydrolysis and I1HPVAc, after hydrolysis were obtained from the integral of IV protons of PVAc before and after hydrolysis and Ireference corresponds to the integral of the signal between 1.0 and 1.8 ppm (Figure 7). The hydrolysis yields calculated from Equation 5 are given in Table 4. The hydrolysis of the PVAc units of the PEG-b-P(VAc-co-VCL) copolymers was also monitored by Fourier Transform Infra-Red (FTIR) spectroscopy via the disappearance of the characteristic ester band of the PVAc at 1730 cm -1 (Figure 8 and Figure S11). Through a normalization with the signal of the amide bond of PVCL units of copolymers before and after hydrolysis, the yield of hydrolysis was calculated by comparing the absorbance of the ester band of PVAc (1730 cm -1 ) before and after the hydrolysis step. The range of hydrolysis yield calculated by FTIR is slightly lower (77-85 %) in comparison with the yield calculated by NMR (87-94 %) but in a similar range to provide an average yield of hydrolysis (see Table 4). The shift of the PVCL amide resonance frequency from 1623 cm -1 in the initial PEG-b-P(VAc0.47-co-VCL0.53) copolymer to 1604 cm -1 in the hydrolyzed copolymer traduces the surrounding change from PVAc to PVA neighbor units (Figure 8). The normalization of the absorption band at 2850 -2950 cm -1 characteristic of the aliphatic carbon stretching shows a constant intensity of the amide band of PVCL (1604 -1623 cm -1 ) before and after hydrolysis, which excludes any concomitant PVCL hydrolysis.

C=O PVCL C=O PVAc

These results show that is it possible to produce thermoresponsive P(VAc-co-VA-co-VCL) copolymers of controlled molar mass from the PEG-b-P(VAc-co-VCL) copolymers synthesized by surfactant-free emulsion polymerization process. The covalent linkage between the reactive PEG-xanthate and the P(VAc-co-VCL) block being an ester group, the analysis of the polymer after hydrolysis by DOSY NMR was required to provide information on the stability of the covalent bonding. Only one diffusion coefficient correlated to the 1 H NMR signals of the PEG block (Ib'' at 3.5 ppm) is observed in Figure S12 at D = 86 µm 2 s -1 corresponding to the free PEG (Figure S7). A better alignment of PEG and P(VAc-co-VCL) blocks was observed for the PEG-b-P(VAc-co-VCL) block copolymer precursors (Figure 2 and Figure S7). Therefore, DOSY-NMR analyses evidence the hydrolysis of the covalent linkage between both PEG and P(VAc-co-VA-co-VCL) blocks. Note that an average weight fraction of 6 wt-% of residual PEO versus the total mass of polymer was calculated by proton NMR for both copolymers (see example in Figure 7).

The thermoresponsive behavior of the P(VAc-co-VA-co-PVCL) copolymers in water was investigated by means of DLS measurements. The increase of the hydrodynamic diameter by increasing the temperature confirms the thermoresponsive behavior in water of both P(VAc-co-VA-co-VCL) copolymers with a phase transition temperature ranging between 26°C and 30°C (Figure 9). As a perspective of this work, as poly(vinyl alcohol) is known to be a biodegradable polymer under suitable conditions, [START_REF] Amann | Synthetic Biodegradable Polymers[END_REF] it would be of interest to investigate how the incorporation of PVA units into the biocompatible PVCL copolymer particles might provide degradable linkages in order to produce more attractive thermoresponsive degradable PVCL-based materials. 65

CONCLUSION

  corresponds to the integral of one proton of 1,3,5-trioxane (5.1 ppm, 6 H, i protons in Figure S3a) used as internal standard and I(1H,VAc + 1H,VCL) corresponds to the integral of the vinylic protons of both VCL monomer and VAc monomer at 7.1-7.4 ppm (I7.1-7.4 ppm = I1H,VAc + I1H,VCL, protons 3+c

  column and two columns (Styragels HR 5E and 4E (7.8  300 mm)) working in series, a Wyatt Heleos II Multi Angle Laser Light Scattering detector (MALLS, 18 angles, λ0 = 664.4 nm), a viscosimeter detector (Wyatt Viscostar II), a refractive index (RI) detector Viscotek VE 3580 and a Viscotek VE 3210 UV-visible detector. Toluene was used as flow marker. All (co)polymer samples were prepared at 3 g.L -1 concentrations. The number-average molar mass (Mn) and molar mass distribution (Đ) were obtained from MALLS data using the Mi value of each slice of the chromatogram (Debye plot).

  1 ), t is the polymerization time, [Xanthate]0, [Initiator]0 and [M]0 are the initial concentrations of respectively the chain-transfer agent, the initiator and monomer ([M]0 = [VAc]0 + [VCL]0). fVAc,0 and fVCL,0 are the initial monomer feed ratios. MVAc and MVCL are the molar masses of each monomer (MVCL = 139 g.mol -1 and MVAc = 86 g.mol -1 ).The refractive index increments (dn/dc) in THF of the PEG-b-P(VAc-co-VCL) copolymers were measured: (dn/dc)PEG-b-P(VAc0.17-co-VCL0.83) = 0.099 mL.g -1 and (dn/dc)PEG-b-P(VAc0.47-co-VCL0.53) = 0.088 mL.g -1 .

  Dh) of the latex particles synthesized by emulsion copolymerization of VAc and VCL mediated by xanthate-terminated PEG macromolecular chain-transfer agent were measured at 55°C directly after withdrawing the latex at the end of the polymerization, without cooling step. Before analysis, samples were diluted by a factor 100 in warm distilled water and were characterized with a Vasco DL135 Cordouan Technologies equipment at λlaser = 657 nm. The evolution of the final average hydrodynamic diameters of aqueous solutions of PEG-b-P(VAc-co-VCL) copolymers before and after hydrolysis were measured as a function of the temperature, by dynamic light scattering (DLS), using a Zetasizer Nano ZS instrument (Malvern Instruments). A He-Ne 4.0mW power laser was used, operating at a wavelength of 633 nm. For evaluation of the data, the DTS (Nano) program was used. The hydrodynamic diameters were calculated from the diffusion coefficient using the Stokes-Einstein equation 𝐷 ℎ = 𝑘 B 𝑇 3𝜋𝜂𝐷where T is absolute temperature, the viscosity of the solvent, kB the Boltzmann constant and D the diffusion coefficient. For the analysis of PEG-b-P(VAc-co-VCL), measurements were carried out at a solids content of 0.005 wt-% from 10°C to 55 °C and from 55

Figure S6 ,

 S6 Figure S6, the shift of the SEC chromatograms towards lower elution volumes suggests that the PEG-X was chain extended by the P(VAc-co-VCL) block.

Figure 1 .

 1 Figure 1. Size exclusion chromatograms in THF: PEG-X (full line) and PEG-b-P(VAc0.47-co-VCL0.53) copolymer synthesized by RAFT/MADIX emulsion polymerization (expt 2 in Table2).

Figure 2 .

 2 Figure 2. DOSY NMR spectrum in DMSO-d6 of the dialyzed PEG-b-P(VAc0.47-co-VCL0.53) copolymer synthesized by RAFT/MADIX emulsion polymerization (expt 2 in Table2).

  -co-VCL0.83) and PEG-b-P(VAc0.47-co-VCL0.53) copolymers also shows that the shapes of the characteristic methine signals of both PVAc (CHII', 65 -70 ppm) and PVCL units (CHB', 45 -49 ppm) are influenced by the copolymer composition (Figure3). Indeed, it is known that the probability of the different co-monomer triads in a statistical copolymer, depending among other factors to the initial monomer feed ratio, influences the chemical environment of each monomer unit.

Figure 3 .

 3 Figure 3. Overlay of the 13 C NMR spectra of the precipitated PVAc, PVCL, PEG-b-P(VAc0.17-co-VCL0.83) and PEG49-P(VAc0.47-co-VCL0.53) (co)polymers in CDCl3.

  mol-%) is enable to preserve the integrity of the PEG-b-P(VAc0.47-co-VCL0.53) particles even at low temperature. The dispersion of PEG-b-P(VAc0.17-co-VCL0.83) copolymer in water becomes gradually cloudier by increasing the temperature (LE 245, Figure 4), which is the sign of thermo-induced increasing size of the diffusing objects.

Figure 4 .

 4 Figure 4. Pictures taken at different temperatures (5°C, 20°C and 50°C) of the final polymer dispersion obtained by PEG-X mediated emulsion copolymerization of VAc and VCL. For each picture, left sample corresponds to: PEG-b-P(VAc0.17-co-VCL0.83) copolymer (Expt 1 inTable 1, LE245) and right sample corresponds to PEG-b-P(VAc0.47-co-VCL0.53) copolymer (Expt 2 in Table 1, LE246).

Figure 5 .

 5 Figure 5. Transmittance at λ = 500 nm versus temperature for aqueous solution of PEG-b-P(VAc-co-VCL) copolymers synthesized by RAFT/MADIX emulsion polymerization (plain lines, present work): PEG-b-P(VAc0.17-co-VCL0.83) (blue) and PEG-b-P(VAc0.47-co-VCL0.53) (black). Comparison with transmittance of P(VAc-co-VCL) copolymers with similar compositions synthesized by RAFT/MADIX bulk polymerization from reference 54 (dotted lines): P(VAc0.19-co-VCL0.81) (blue) and P(VAc0.53-co-VCL0.47). Polymer concentration of 3 g.L -1 (heating cycle).

a

  Phase transition temperature, see Figure 5.

  -co-VCL0.83) sequence as a shift towards a higher phase transition temperature (PTT = 32 °C) is observed in comparison to the P(VAc0.19-co-VCL0.81) statistical copolymer of close composition (PTT = 27°C). On the other hand, the similar values of the phase transition temperatures (PTT = 22°C) of both PEG-b-P(VAc0.47-co-VCL0.53) and P(VAc0.53-co-VCL0.47)

  co-VCL0.83) copolymer, emulsion copolymerization of VAc/VCL performed with 50 mol-% of VAc in the initial monomer feed (expt 2 in Table1) produced stable particles at low temperature by means of sufficient hydrophobic interactions between the VAc units (Dh, 10-20°C = 550 nm) (Figure6b). The collapse of the P(VAc0.47-co-VCL0.53) blocks within the particles by increasing temperature induced a decrease of the particle diameter down to 250 nm at 55°C. The reversible swelling-to-collapse transition of the PEG-b-P(VAc0.47-co-VCL0.53) particles with almost no hysteresis emphasizes the formation of well-defined physically crosslinked thermoresponsive particles of P(VAc0.47-co-VCL0.53) stabilized by the PEG block (Figure 6b). The volume phase transition temperature (VPTT) of the PEG-b-P(VAc0.47-co-VCL0.53) particles, determined as the minimum of the derivative of the plot of Dh as a function of the temperature, is equal to 26°C at 0.05 g.L -1 . The slightly higher value of VPTT measured by DLS at 0.05 g.L -1 (Figure 6) in comparison with the phase transition temperature of PEG-b-P(VAc0.47-co-VCL0.53) copolymer (PTT = 22 °C, Figure 5) measured by turbidimetry at 3 g.L -1 shows a concentration effect on the transition temperature. The VPTT of the PEG-b-P(VAc0.47-co-VCL0.53) particles is lower than the one measured for chemically crosslinked PVCL-based nanogels (~32°C),

Figure 7 . 1 H

 71 Figure 7. 1 H NMR spectra in DMSO-d6 at 80°C of the dialyzed and freeze-dried PEG-b-P(VAc0.47-co-VCL0.53) (expt 2 in Table1) copolymer (blue) and of the corresponding copolymer after hydrolysis (black).

Figure 8 .

 8 Figure 8. FTIR spectra before (blue) and after hydrolysis (black) of PEG-b-P(VAc0.47-co-VCL0.53) copolymer.

Figure 9 .

 9 Figure 9. Evolution of the hydrodynamic diameter of the P(VAc-co-VA-co-VCL) copolymers a function of the temperature: () P(VAc0.03-co-VA0.14-co-VCL0.83) copolymer, () P(VAc0.05-co-VA0.42-co-VCL0.53) copolymer at 5 g.L -1 in water (heating cycle).

  In conclusion of this work, the RAFT/MADIX batch emulsion copolymerization of vinyl acetate (VAc) and N-vinylcaprolactam (VCL) mediated by a xanthate-terminated PEG macromolecular chain-transfer agent was successfully performed at 10 wt-% of solids content, with two different initial monomer feed ratios. DOSY NMR and SEC analyses both support the successful chain extension of the PEG-X macro-CTA enabling the direct synthesis and self-assembly of thermoresponsive PEG-b-P(VAc-co-VCL) block copolymers in aqueous dispersed media. The present study is the first example of synthesis of PVCL-based amphiphilic copolymers by controlled radical emulsion polymerization. It is worth to mention that the PEG-X efficiently acted as both steric stabilizer and macromolecular chain-transfer agent during the RAFT/MADIX emulsion copolymerization of VAc and VCL to produce well-defined physically crosslinked thermoresponsive particles via hydrophobic interactions between the self-assembled PEG-b-P(VAc-co-VCL) copolymers. The integrity of the particles at a temperature below the phase transition temperature is maintained by means of hydrophobic interactions with 47 mol-% of hydrophobic VAc units in the P(VAc-co-VCL) sequence of the block copolymer. The well-defined physically crosslinked PEG-b-P(VAc0.47-co-VCL0.53) particles interestingly behaved as thermoresponsive colloids able to undergo a reversible swelling-to-collapse transition upon increasing the temperature. A lower fraction of VAc in the copolymer (17 mol-%) was not sufficient to prevent disassembling of the PEG-b-P(VAc0.17-co-VCL0.83) polymer chains in the aqueous phase at low temperature (T < 20°C). The PEG-b-P(VAc0.17-co-VCL0.83) copolymer chains self-assemble into large polydisperse aggregates by rising the temperature. Finally, the VAc/VCL statistical copolymers of controlled molar mass were successfully hydrolyzed into promising thermoresponsive biocompatible statistical copolymers based on vinyl alcohol and Nvinylcaprolactam units.

Table 1 .

 1 Experimental conditions for the emulsion copolymerization of VAc and VCL performed at 65°C with 10 wt-% of initial solids content in water. fVAc,0 is the VAc molar fraction in the initial monomer feed.It should be mentioned that stable latex particles were formed during the course of polymerization in the absence of coagulum or phase separation for both syntheses carried out at different initial monomer feed ratios (see Table1and Table2, expt 1 and 2). This suggests that the PEG-X efficiently acts as steric stabilizer during the emulsion copolymerization of VAc and VCL. Note that 80-85 % of monomer conversion is reached by emulsion copolymerization (

	Expt fVAc,0 a [ADIBA]0 mol.L -1 H2O	Stabilizer [Stabilizer]0 g.L -1 H2O	Stabilizer wt-% based on	[PEG-X] 0 [ADIBA] 0	[VAc] 0 +[VCL] 0 [PEG-X] 0
						monomers
	1	0.2	5.4 × 10 -4	PEG-X	4.9	5.4	3.2	430
	2	0.5	5.9 × 10 -4	PEG-X	5.4	6.0	3.2	430
	3	0.5	5.9 × 10 -4 PEG-OH	5.4	6.0	-	-

a

Table 2

 2

) while the monomer conversion was limited to 50-60 % in bulk for a similar 6 h of polymerization (see Table

2

and reference

[START_REF] Etchenausia | [END_REF]

Table 2 .

 2 Results of emulsion copolymerization of VAc and VCL mediated by PEG-X macromolecular chain transfer agent.

a

Expt FVAc

b FVAc c Xm d %

  10 wt-% of initial solids content and T = 65°C; b Molar fraction of VAc in the P(VAc-co-VCL) block, determined from

			Mn,theo e	Mn,SEC f	Đ f	Dh, 55°C g	Copolymer
			g.mol -1	g.mol -1		nm
	1	0.17	0.17 80 32 520 16 700 1.6	150	PEG-b-P(VAc0.17-co-VCL0.83)
	2	0.47	0.52 85 31 490 19 790 2.0	95	PEG-b-P(VAc0.47-co-VCL0.53)

a

Table 1 ,

 1 LE245) and right sample corresponds to PEG-b-P(VAc0.47-co-VCL0.53) copolymer (Expt 2 in

Table 1 ,

 1 LE246).

Table 4 .

 4 Hydrolysis yields (in %) of PEG-b-P(VAc-co-VCL) copolymers.

	% hydrolysis	FTIR 1 H NMR	Average % of hydrolysis
	PEG-b-P(VAc0.17-co-VCL0.83)	77	87	82 ± 7
	PEG-b-P(VAc0.47-co-VCL0.53)	85	94	90 ± 6

ACKNOWLEDGEMENTS

The French ministry of research, the University of Pau & Pays Adour and the University of the Basque Country UPV/EHU are acknowledged for funding L.E.'s PhD work performed under a joint doctorate program. The Equipex Xyloforest program (ANR-10-EQPX-16 XYLOFOREST) is acknowledged for MALLS detector and NMR probe funding. Prof. P. Stepanek is acknowledged for giving us the opportunity to perform measurements with the Zetasizer Nano ZS instrument at the institute of macromolecular chemistry (Prague). Electronic supplementary information (ESI) available: Preparation of PEG-X macro-chain transfer agent and 1 H NMR spectra; Overlay of the UV-visible (λ = 355 nm) and refractometer (RI) traces of the SEC chromatograms in THF for the PEG-X polymer; 1 H NMR and 13 C NMR spectra of the dialyzed and freeze-dried PEG-b-P(VAc-co-VCL) diblock copolymers; Pictures of the coagulum and of the dispersion recovered at the end of the emulsion copolymerization of VAc and VCL using non-reactive PEG-OH as stabilizer; Size exclusion chromatograms of PEG-X and PEGb-P(VAc0.17-co-VCL0.83) copolymer; DOSY NMR spectra of the dialyzed PEG-b-P(VAc0.17-co-VCL0.83) copolymer and initial PEG-OH homopolymer; Comparison of 13 C NMR spectra of the VAc/VCL copolymers synthesized either by bulk polymerization or by emulsion polymerization;

Transmittance at λ=500 nm versus temperature for aqueous solution of PEG-X; 1 H NMR spectra before and after hydrolysis of PEG-b-P(VAc0.17-co-VCL0.83) copolymer; FTIR spectra before and after hydrolysis of PEG-b-P(VAc0.17-co-VCL0.83) copolymer; DOSY NMR spectrum of PEG-b-P(VAc0.03-co-VA0.14-co-VCL0.83) and PEG-b-P(VAc0.05-co-VA0.42-co-VCL0.53) copolymers after hydrolysis.