WO2008087119A1

WO2008087119A1 - Cosmetic composition comprising a diblock copolymer

Info

Publication number: WO2008087119A1
Application number: PCT/EP2008/050348
Authority: WO
Inventors: Jean-Thierry Simonnet; Florence L'alloret; Christophe Hadjur
Original assignee: L'oreal
Priority date: 2007-01-19
Filing date: 2008-01-14
Publication date: 2008-07-24
Also published as: FR2911502A1; FR2911502B1

Abstract

The present invention relates to a composition comprising, in a physiologically acceptable medium: at least one (blockA)-(blockB) diblock copolymer in which: -blockA comprises at least units deriving from styrene; -blockB comprises at least (a) units deriving from acrylic acid in free or salified form and (b) at least units deriving from a C1-C4 alkyl acrylate. The diblock copolymer has moisturizing properties for keratin materials. Use for caring for and making up keratin materials.

Description

Cosmetic composition comprising a diblock copolymer

The present invention relates to a cosmetic composition comprising a block copolymer based on styrene and such a copolymer as moisturizers for keratin materials, and more particularly for the skin.

The stratum corneum is the thin cornified layer which forms the interface between the organism and the dehydrating external environment. It is formed from the upper epithelial tissue: the epidermis, and from the connected tissue located below: the dermis.

The stratum corneum, which is approximately 10 to 15 μm thick, is composed of vertically stacked corneocytes surrounded by a matrix formed from lipids organized in a lamellar liquid crystal phase. Thus, it is two- compartment system which can be compared to a brick wall, composed of anucleating cells (the "bricks") and intercellular lamellar liquid crystal phases (the "cement") .

The function of the stratum corneum is in particular to regulate flows of water which enter and leave the skin and therefore to delay the excessive loss of water originating from the deeper layers of the epidermis. The stratum corneum also protects against mechanical attack and the passage of chemical products and foreign microorganisms. It also constitutes the first line of defence against UV radiation.

In general, a moisturizer is defined as a substance which reduces the impression of dry skin by influencing the water content of the skin by addition or retention.

In general, corneometry is the method used to measure the moisturization provided by humectants such as glycerol, urea, and its derivatives. However, this method cannot be used to demonstrate the moisturizing properties of polymers, and it has therefore been necessary to develop a suitable method for demonstrating the moisturizing properties of polymers. Various methods based on spectroscopic techniques (infrared, Raman, NMR, etc.) have been developed over the past few years with the aim of improving knowledge of the skin, in particular in the medical field. These techniques have the advantage of not requiring any specific preparation of the sample and are non-invasive and non-destructive. These methods are in particular described in:

1. K. Wichrowski, G. Sore, and A. Khaiat, "Use of infrared spectroscopy for in vivo measurement of the stratum corneum moisturization after application of cosmetic preparations," Int. J. Cos. Sci. 17, 1-11 (1995) .

2. Querleux B, Richard S, Bittoun J, Jolivet O, Idy- peretti I, Bazin R, Leveque JL: In vivo hydration profile in skin layers by high-resolution magnetic resonance imaging. Skin Pharmacol 1994; 7: 210-216.

3. B. Schrader, B. Dippel, S. Fendel, S. Keller, T. Lochte, M. Riedl, R. Schulte, and E. Tatsch, "NIR FT Raman spectroscopy - a new tool in medical diagnosis, " J. MoI. Struct. 408/409, 23-31 (1997). In the context of the present application, the method selected for measuring the amount of water provided by solutions of polymers at 1% by weight is a laser confocal microscopy combined with Raman spectroscopy.

Raman spectroscopy makes it possible to provide information on the molecular composition and the structure of the sample tested. When combined with confocal microscopy, it offers the advantage of providing information on the stratum corneum from the surface of the skin to a depth of a few tens of microns. Furthermore, the method used in the present application has the advantage of being non-invasive and non-destructive . This method had made it possible to establish that the polymers used in accordance with the present application are such that a solution at 1% by weight of these polymers makes it possible to provide a water content at least equal to that provided by a solution at 7% by weight of glycerol.

In order to implement this method, isolated stratum corneum, compacted stratum corneum squame blocks, arti- ficial skin (Episkin, Matek, Skinethic, etc.) or skin derived from aesthetic surgery can be used as evaluation support. These examples are not limiting; all supports which model the skin may be used.

In the context of the present application, compacted plantar squame blocks, weighing from 80 to 100 mg, more than 100 μm thick and with a surface area of approximately 1 cm², are used as a stratum corneum model; 5 measurements are carried out on each plantar squame block and 5 blocks are used for each polymer solution, e.g. 25 measurements for each polymer solution .

The amount of polymer solution applied to each block is approximately 50 μl .

A LabRam Raman laser confocal microscope (Jobin-Yvon Horiba) equipped with a He:Ne laser (λ = 633 nm and power approximately 8.4 mW) and with a CCD detector is used to carry out the measurements. The laser beam is focused in the skin with a X50 objective and the Raman spectra are recorded using a point-by-point mode, from the surface of the sample to the depth. The spectral acquisition parameters are: hole and slit: 200 μm, acquisition time: 30 s over a frequency of 2800 to 4000 cm^"1. The microscope in question was subjected to a technical modification essential for measuring water, with the incorporation of a chamber for controlling, during the acquisitions, the temperature and the humidity: the parameters are 25°C and 50% relative humidi ty .

The water quantification is determined by the ratio of the water OH bands/protein CH₃ bands, by Raman spectrography, as described in:

1 Chrit L, Hadjur C, Morel S, Sockalingum G. D., Lebourdon G, Leroy F, Manfait M: In vivo chemical investigation of Human skin using a confocal Raman Optic microprobe. J. Biomed Opt 2005. 2 Caspers PJ, Lucassen GW, Carter EA, Bruning HA, Puppels GJ: In vivo confocal Raman microspectroscopy of the skin: non invasive determination of molecular concentration profiles. Journal of Investigative Dermatology 2001; 116(3): 434-442.

The water/protein ratios in the stratum corneum were determined by Raman spectrography, by calculating the ratio between the intensity of the water OH bands (area under the curve) integrated between 3350 cm^"1 and 3550 cm^"1 and the intensity of the protein CH₃ bands (area under the curve) integrated between 2910 cm^"1 and 2965 cm^"1.

The water quantification is calculated using the following equations:

W/P = (m_w/m_p)R

Water content (%) : m_w/ (m_w+m_p) = 100% (W/P) / (W/ (P+R) )

m_w and m_p being respectively the masses of water and of proteins in the volume of the sample tested;

W being the intensity of the integrated Raman signal for the water (integrated water OH bands (area under the curve) ) ;

P being the intensity of the integrated Raman signal for the proteins (integrated protein CH₃ bands (area under the curve) ) ;

R is a proportionality constant expressing the ratio between the Raman signals for the water and for the proteins in a solution at 50%. The water content is expressed in grams of water per 100 g of wet tissue.

These functionalities are integrated in the LabSpec software from Jobin-Yvon Horiba.

To evaluate the moisturizing efficiency, the measurements are carried out at T=O, before treatment, and at T+l hour after treatment, from the surface to a depth of 20 μm.

The problem stated by the present application was that of proposing families of compounds that can be used as moisturizers for keratin materials, and more particularly for the skin.

The inventors of the present application have shown that this problem can be solved by means of specific styrene-based diblock copolymers.

The applicant has discovered, surprisingly and unexpectedly, that this objective can be achieved using at least one (block A) - (block B) diblock copolymer in which : block A comprises at least units deriving from styrene; block B comprises at least (a) units deriving from acrylic acid in free or salified form and (b) at least units deriving from a Ci-C₄ alkyl acrylate.

More specifically, the subject of the present invention is a cosmetic composition comprising, in a physiologically acceptable medium: a (block A) - (block B) diblock copolymer in which: block A comprises at least units deriving from styrene; block B comprises at least (a) units deriving from acrylic acid in free or salified form and (b) at least units deriving from a Ci-C₄ alkyl acrylate. The subject of the invention is also a nontherapeutic cosmetic treatment process for caring for or making up keratin materials, in particular the skin, comprising the application to the keratin materials, in particular to the skin, of a cosmetic composition as defined above .

The cosmetic process is in particular a cosmetic process for moisturizing the skin.

A subject of the present invention is also the use of a diblock copolymer as defined above, as a moisturizer for keratin materials, in particular for the skin.

The term "physiologically acceptable medium" is intended to mean a nontoxic medium that can be applied to the skin, the lips, the hair, the eyelashes, the eyebrows and the nails. The composition of the invention may in particular constitute a cosmetic or dermatological composition.

The diblock copolymers in accordance with the invention are advantageously linear.

More preferably, the proportion by weight of block B relative to the copolymer is greater than or equal to 50%.

The diblock copolymers in accordance with the invention are more particularly characterized in that they are (block A) - (block B) diblock copolymers in which: block A comprises at least 90% by weight of units deriving from styrene, relative to the total weight of block A; block B is a random block comprising, relative to the total weight of block B:

(i) from 34% to 95% by weight of units deriving from acrylic acid in acid form or in salified form;

(ii) from 5% to 66% by weight of units deriving from

Ci-C₄ alkyl acrylate. In the present application, the term "diblock copolymer" relates to a block copolymer architecture being constituted of two blocks, and having substan- tially no other sequence of blocks.

In the present application, the term "unit deriving from a monomer" denotes a unit which may be obtained directly from said monomer by polymerization. Thus, for example, a unit deriving from an acrylic or methacrylic acid ester does not cover a unit of formula -CH₂-CH(COOH)- or -CH₂-C(CH₃) (COOH)-, obtained for example by polymerizing an acrylic or methacrylic acid ester and then by hydrolysing. Thus, the terminology "unit deriving from a monomer" relates only to the final constitution of the polymer and is independent of the polymerization process used to synthesize the polymer .

The ratio by weight between the blocks corresponds to the ratio between the masses of the monomers (or mixtures of monomers) used to prepare the blocks

(taking into account the variations in mass related to subsequent hydrolysis) . The proportions by weight of the blocks are the proportions relative to the total diblock copolymer, and correspond to the proportions by weight of the monomers (or mixtures of monomers) used to prepare the blocks, relative to all the monomers used to prepare the diblock copolymer (taking into account the variations in mass related to subsequent hydrolysis) .

The masses and ratios associated with the blocks are indicated in acid equivalents (units deriving from an acrylic acid in acid form, as opposed to a salt form of sodium acrylate type) .

The Ci-C₄ alkyl acrylate monomer is preferably ethyl acrylate . According to a preferred form of the invention, block A and/or block B comprise (s) up to 10% by weight (in particular from 0.1% to 10% by weight), and preferably up to 5% by weight (in particular from 0.1% to 5% by weight) of an additional ionic or nonionic, hydrophilic comonomer, relative to the weight of block A or of block B containing said hydrophilic monomer.

The term "hydrophilic monomer" is intended to mean a monomer which has affinity for water, and which typically is not capable of forming a two-phase macroscopic solution in distilled water at 25°C, at a concentration of 1% by weight.

Among the additional ionic or nonionic, hydrophilic comonomers, mention may, for example, be made of acrylamide, hydroxyethyl (meth) acrylate, methacrylic acid (MMA) and salts thereof. Use is preferably more particularly made of methacrylic acid or a salt thereof. Block A may also comprise, as additional hydrophilic monomer, acrylic acid and salts thereof.

A first family of diblock copolymers in accordance with the invention that is particularly preferred is constituted of (block A) - (block B) diblock copolymers of type (1) in which the proportion by weight of block B relative to the copolymer is between 50% and 85%, and preferably between 50% and 75%.

A second family of diblock copolymers in accordance with the invention which is particularly preferred is constituted of (block A) - (block B) diblock copolymers of type (2) in which the proportion by weight of block B relative to the copolymer is greater than or equal to 85%.

Among these type (2) diblock copolymers, two types of copolymers advantageously stand out: Type (2a) : where the proportion by weight of block B relative to the copolymer is greater than or equal to 87%, in particular greater than or equal to 87% and less than 94%.

Type (2b) : where the proportion by weight of block B relative to the copolymer is greater than or equal to 94%, in particular ranging from 94% to 97%.

Among these type (2a) diblock copolymers, two types of copolymers advantageously stand out:

Type (2al) : where, in block B: - the proportion by weight of units deriving from acrylic acid in free or salified form is between 64% (obtained, for example, by hydrolysis to a degree of T=O.7) and 75% (obtained, for example, by hydrolysis to a degree of T=O.8), and - the proportion by weight of units deriving from the Ci-C₄ alkyl acrylate is between 25% (obtained, for example, by hydrolysis to a degree T=O.8) and 36% (obtained, for example, by hydrolysis to a degree of T=O.7) .

Type (2a2) : where, in block B: the proportion by weight of units deriving from acrylic acid in free or salified form is between 75% (obtained, for example, by hydrolysis to a degree of T=O.8) and 95% (obtained, for example, by hydrolysis to a degree of T=O.96), and the proportion by weight of units deriving from the Ci-C₄ alkyl acrylate is between 5% (obtained, for example, by hydrolysis to a degree T=O.96) and 25% (obtained, for example, by hydrolysis to a degree of T=O.8) .

The (block A) - (block B) diblock copolymers used in the context of the invention may be obtained by means of a process comprising the following steps:

I) a (block A) - (block B') diblock copolymer is prepared by means of a process comprising the following intermediate steps Ia) and Ib) :

Ia) a first block A is prepared by bringing the following together: n_τ mol of a transfer agent comprising a single transfer group; n_A mol of styrene or of a mixture of monomers comprising at least 90% by weight of styrene and where n_A/n_T > 5 and preferably < 5000; and, optionally, a free-radical initiator;

Ib) a second block B' is prepared, in order to obtain a (block A) - (block B') diblock copolymer, by bringing the following together: the block A obtained in the preceding step; - n_B mol of a Ci-C₄ alkyl acrylate or of a mixture of monomers comprising at least 90% by weight of a Ci-C₄ alkyl acrylate such that n_B/n_T > 5 and preferably < 5000; and, optionally, a free-radical initiator;

II) hydrolysis of block B' is then performed, to a degree T by mol of between 0.4 and 0.96, so as to obtain said (block A) - (block B) diblock copolymer;

III) optionally, deactivation of transfer groups borne by macromolecular chains and/or purification of the

(block A) - (block B) diblock copolymer and/or destruction of by-products from hydrolysis and/or deactivation is/are carried out, during and/or after step II), and preferably: - T is between 0.4 and 0.96, preferably between 0.7 and 0.8, preferably approximately 0.75 or approximately 0.90, n_A/n_T > 5, and preferably n_A/n_T < 5000, n_B/n_T > 5, and preferably n_B/n_T < 5000. The terminologies of the type (block A) - (block B') do not however exclude the presence of chemical groups that are useful (transfer groups or residues) for the polymerization, in particular at chain ends. Thus, the diblock copolymer may in reality have a formula of the type R- (block A) - (block B ' ) -X (for example, X is a transfer group of formula -S-CS-Z- or a residue of such a group) .

In the present application, the degree of hydrolysis T is defined as the ratio between the number of units deriving from acrylic acid or an acrylic acid salt, and the number of units deriving from the Ci-C₄ alkyl acrylate, present in a copolymer before hydrolysis. The number of units deriving from the Ci-C₄ alkyl acrylate is considered to be equal to the amount by number of alkyl acrylate monomer used to prepare the copolymer. The number of units deriving from acrylic acid or from an acrylic acid salt can be determined by any known method, in particular by potentiometric acid-base titration of the number of -COONa groups using a strong acid, for example hydrochloric acid.

In the present application, the molar mass M_A of a mixture of monomers Ai and A₂ of respective molar masses M_AI and M_A2, present in respective numbers n_Ai and n_A2, denotes the number-average molar mass M_A = M_Ai n_Ai/ (n_Ai + n_A2) + M_A2 n_A2/ (n_Ai + n_A2) . The molar mass of a mixture of units in a macromolecular chain or a part of a macro- molecular chain (a block, for example) is similarly defined, with the molar masses of each of the units and the number of each of the units.

In the present application, the measured average molecular mass of a first block or of a copolymer denotes the number-average molecular mass in polystyrene equivalents of a block or of a copolymer, measured by size exclusion chromatography (SEC), in THF, with calibration using polystyrene standards. The measured average molecular mass of the same block in a copolymer comprising n blocks is defined as the difference between the measured average molecular mass of the copolymer and the measured average molecular mass of the copolymer comprising (n-1) blocks from which it is prepared.

In the present application, the term "transfer agent" is intended to mean an agent capable of inducing controlled radical polymerization in the presence of unsaturated monomers and, optionally, of a source of free radicals. Such agents are known to those skilled in the art, and include in particular compounds comprising a transfer group -S-CS-, for the implementation of polymerization processes known as RAFT and/or MADIX. Such processes and agents are described in detail below.

The polymerization process as described above is in particular described in document WO 01/16187.

During step I) described above, it is possible to carry out the preparation of a first block from monomers or a mixture of monomers, initiators and/or agents for promoting control of the polymerization (transfer agents comprising -S-CS- groups, nitroxides, etc.), and then the growth of a second block on the first block so as to obtain a diblock copolymer with monomers different from those used for the preparation of the preceding block, and optionally with the addition of initiators and/or agents for promoting control of the polymerization. These processes for preparing block copolymers are known to those skilled in the art. It is mentioned that the copolymer may have, at the end of the chain, a transfer group or a residue of a transfer group, for example a group comprising an -S-CS- group (for example derived from a xanthate or from a dithio- ester) or a residue of such a group. During step II), the units deriving from the hydrolysable monomers of block B' are partially hydrolysed, so as to form a block B comprising units deriving from acrylic acid or from a salt (hydrolysed units) and units deriving from the alkyl acrylate monomer (nonhydrolysed units) . These two types of units are distributed randomly in block B; it may therefore be considered that block B is a block in the form of a random copolymer comprising units deriving from alkyl acrylate and units deriving from acrylic acid or from an acrylic acid salt. Naturally, block B may comprise other units, in minimal amounts, if a mixture of monomers is used during the implementation of step Ib) .

Block A comprises units derived from styrene. Block A may be obtained from a mixture of monomers comprising at least 90% by weight, preferably at least 95%, of styrene ("St") and from a hydrophilic comonomer or several hydrophilic comonomers . Block A may thus be a random copolymer comprising at least 90% (in particular from 90% to 99.9% by weight), preferably at least 95% by weight (in particular from 95% to 99.9% by weight) of units deriving from styrene, and up to 10% by weight (in particular from 0.1% to 10% by weight), preferably up to 5% by weight (in particular from 0.1% to 5% by weight) of other units, deriving from hydrophilic comonomer (s) .

Block B' comprises units derived from a Ci-C₄ alkyl acrylate. Block B' may be obtained from a mixture of monomers comprising at least 90% (in particular from 90% to 99.9%), preferably at least 95% (in particular from 95% to 99.9%), by weight of a Ci-C₄ alkyl acrylate and from one or more hydrophilic comonomer (s) . Block B' may thus be a random copolymer comprising at least 90%

(in particular from 90% to 99.9%), preferably at least

95% (in particular from 95% to 99.9%), by weight of units deriving from the Ci-C₄ alkyl acrylate and up to 10% (in particular from 0.1% to 10%), preferably up to 5% (in particular from 0.1% to 5%), by weight of other units, deriving from hydrophilic comonomer(s) .

Block B obtained from block B' after hydrolysis comprises units deriving from the hydrolysable Ci-C₄ alkyl acrylate, units deriving from acrylic acid or a salt, and optionally units deriving from a hydrophilic comonomer used during step Ib) for growth of block B', for example units deriving from acrylic acid. The acrylic acid is generally present in block B in the form of a salt. This form is generally the result of the conditions under which the hydrolysis is performed and the reactants used. It is generally an alkali metal, such as sodium or potassium, salt. Consequently, block B generally comprises units deriving from acrylic acid in the form of sodium acrylate or of potassium acrylate .

Among the hydrophilic comonomer (s) that may be of use for the preparation of block A and/or block B', mention is made of the hydrophilic comonomer (s) capable of stabilizing an emulsion of monomers and/or of stabilizing the polymer obtained by emulsion polymeri- zation. Mention may in particular be made of ionic or nonionic, hydrophilic comonomers such as acrylamide, hydroxyethyl (meth) acrylate, methacrylic acid (MMA) and salts thereof. Use is preferably made of methacrylic acid or salts thereof. Methacrylic acid is not sensitive to hydrolysis. It can, however, be salified during hydrolysis. For the preparation of block A, acrylic acid and salts thereof may also be used as hydrophilic comonomer.

Among the hydrolysable C1-C4 alkyl acrylates, mention may in particular be made of ethyl acrylate (EA or AE or AEt) .

According to a specific embodiment, block A and/or block B' or B comprise (s) from 0.1% to 10%, preferably from 0.1% to 5%, by weight of hydrophilic comonomer, in particular methacrylic acid or a salt thereof, relative to the total weight of block A or of block B or B' containing said hydrophilic comonomer.

Thus, during step Ia) , use may be made of a mixture of monomers comprising at least 90%, preferably at least 95%, by weight of styrene, and up to 10%, preferably up to 5%, by weight of methacrylic acid.

During step Ib) , use may be made of a mixture of monomers comprising at least 90%, preferably at least 95%, by weight of Ci-C₄ alkyl acrylate such as ethyl acrylate, and up to 10%, preferably up to 5%, by weight of methacrylic acid or a salt thereof.

Some features of the process for preparing the copolymers of the invention are described in detail below.

Step I)

The copolymers according to the invention can be obtained by any known method, whether by controlled or noncontrolled radical polymerization, by ring-opening polymerization (in particular anionic or cationic) , by anionic or cationic polymerization, or else by chemical modification of a polymer.

Preferably, for polymerization step I), methods of radical polymerization referred to as living or controlled are carried out, and particularly preferably, controlled or living radical polymerization methods using a transfer agent comprising a transfer group of formula -S-CS-, in particular known as RAFT or MADIX, are carried out.

By way of examples of "living" or "controlled" poly- merization processes, reference may in particular be made : to the processes of applications WO 98/58974, WO 00/75207 and WO 01/42312, which implement controlled radical polymerization using control agents of xanthate type, to the controlled radical polymerization process using control agents of dithioester or trithiocarbonate type, of application WO 98/01478, - to the controlled radical polymerization process using control agents of dithiocarbamate type, of application WO 99/31144, to the controlled radical polymerization process using control agents of dithiocarbazate type, of application WO 02/26836, to the controlled radical polymerization process using control agents of dithiophosphoroester type, of application WO 02/10223, to the process of application WO 99/03894, which implements polymerization in the presence of nitroxide precursors, or the processes using other nitroxides or nitroxide/alkoxyamine complexes, to the process of application WO 96/30421, which uses atom transfer radical polymerization (ATRP) , - to the controlled radical polymerization process using control agents of iniferter type according to the teaching of Otu et al., Makromol. Chem. Rapid, Commun., 3, 127 (1982), to the controlled radical polymerization process by degenerative transfer of iodine according to the teaching of Tatemoto et al . , Jap. 50, 127, 991 (1975),

Daikin Kogyo Co Ltd Japan and Matyj aszewski et al . ,

Macromolecules, 28, 2093 (1995), to the controlled radical polymerization process using tetraphenylethane derivatives, disclosed by D. Braun et al . in Macromol . Symp. Ill, 63 (1996), or alternatively, to the controlled radical polymerization process using organocobalt complexes, described by Wayland et al . , in J . Am . Chem . S oc . 1 1 6 , 7 973 ( 1 994 ) , to the controlled radical polymerization process using diphenylethylene (WO 00/39169 or WO 00/37507) .

The polymerizations may be carried out in an emulsion in water ("latex" process). These processes may use emulsifiers, most commonly surfactants. Without wishing to be bound to any theory, it is thought that preparation processes in an emulsion lead to the formation of nodules of blocks A, that may influence the physico- chemical properties of the copolymer.

The polymerizations can be carried out in the presence of free-radical initiators, known to those skilled in the art. Sodium persulphate may, for example, be used. Amounts of initiators of from 5% to 50% by number relative to the amount of transfer agent may typically be used.

Step II)

During step II), the respective amounts of the various units in block B are controlled by the degree of hydrolysis. The composition of block A may remain unchanged during the hydrolysis, if block A does not comprise hydrolysable units. It is not, however, out of the question for block A to be slightly modified during the hydrolysis step.

Preferably, the hydrolysis step II) is carried out by the addition of a strong base such as sodium hydroxide or potassium hydroxide. Typically, a proportion by number of base relative to the amount of hydrolysable monomer used during step Ib) , corresponding approxi- mately to the intended degree of hydrolysis, is added, with optionally an excess of a few %. For example, an amount of sodium hydroxide of 75% by number of the amount of hydrolysable ethyl acrylate used during step Ib) is introduced. The process is preferably carried out by homogeneous hydrolysis, gradually adding the sodium hydroxide to the copolymer.

The hydrolysis step can in particular bring about deactivation and/or cleavage of certain transfer groups or of other groups attached to the macromolecular chains. Step II) may thus generate by-products which it is desirable to eliminate, or generate groups on the macromolecular chains that it is desirable to chemi- cally modify. Such operations can be carried out during step III) .

Step III)

Step III) is a step for deactivation of transfer groups borne by macromolecular chains, and/or for purification of the (block A) - (block B) diblock copolymer and/or for destruction of by-products from hydrolysis and/or deactivation .

During optional step III), the block copolymers obtained or the by-products from hydrolysis may be subjected to a purification reaction or a reaction to destroy certain species, for example by means of processes of hydrolysis, oxidation, reduction, pyroly- sis, ozonolysis or substitution type. An oxidation step with aqueous hydrogen peroxide is particularly suitable for treating sulphur species. It is mentioned that some of these reactions or processes can be carried out entirely or partly during step II) . In this case, for these reactions or processes, the two steps are simultaneous .

The average molecular masses of the (block A) - (block B') diblock copolymer before hydrolysis, or of each of the blocks, typically depend on the relative amounts of the monomers and of transfer agent used during step a) . Of course, the average molecular masses of the (block A) - (block B) diblock copolymer after hydrolysis, or of each of the blocks, depend on these same relative amounts and also on the degree of hydrolysis, for example depend on the amount of reactant, generally a base, introduced for this hydrolysis.

In the interest of simplicity, it is common to express the average molecular masses of the blocks as "theoretical" or "targeted" average molecular masses, considering a complete and perfectly controlled polymerization. In this case, one macromolecular chain forms per transfer agent; to obtain the molecular mass, it is sufficient to multiply the average molar mass of the units of a block by the number of units per block (amount by number of monomer per amount by number of transfer agent) . In these calculations, the differences induced by small amounts of comonomers such as methacrylic acid can be ignored. The theoretical or targeted average molecular masses of block B are expressed by considering complete hydrolysis (the masses are expressed with a degree of hydrolysis of 1 being imagined) .

The theoretical average molecular mass M_bi_ock of a block is typically calculated according to the following formula :

M_block = Y_JM_ι precursor

where M₁ is the molar mass of a monomer i, n_± is the number of moles of the monomer i, n_precursor is the number of moles of functions to which the macromolecular chain of the block will be linked. The functions may come from a transfer agent (or a transfer group) or an initiator, a previous block, etc. If this is a previous block, the number of moles may be considered as the number of moles of one compound to which the macromolecular chain of said previous block has been linked, for example a transfer agent (or a transfer group) or an initiator. In practice, the theoretical average molecular masses are calculated from the number of moles of monomers introduced and from the number of moles of precursor introduced.

The theoretical or targeted average molecular mass of a block copolymer is considered to be the sum of the average molecular masses of each of the blocks, considering hydrolysis to be total (the masses are expressed with a degree of hydrolysis of 1 being imagined), if such a hydrolysis has been carried out.

The targeted or theoretical total mass of a block is defined as the mass of the macromolecular chain, taking polymerization to be complete and perfectly controlled. To obtain the total mass, it is sufficient to multiply the molar mass of one unit of a block by the number per block of this unit, and to add the masses thus obtained for each type of unit in the block. In these calculations, the differences induced by small amounts of comonomers such as methacrylic acid can be ignored. The theoretical or targeted total masses of block B are expressed considering the effect of a partial hydrolysis (for this descriptor, a degree of hydrolysis of 1 is not imagined) , if such a hydrolysis has been carried out.

Thus : - the theoretical or targeted total mass of block A is M_An_A; the theoretical or targeted average molecular mass of block A is: M_An_A/n_T, the theoretical or targeted total mass of block B' is M_Bn_B; the theoretical or targeted average molecular mass of block B' is: M_Bn_B/n_T, the theoretical or targeted total mass of block B is T M_M n_B + (1-T) M_B n_B; the theoretical or targeted average molecular mass of block B is: NU_A n_B/n_T (since

T=I for the theoretical or targeted average molecular mass ) , the theoretical or targeted total mass of the block copolymer is

M_An_A + T M_AA n_B + (1-T)M_B n_B; the theoretical or targeted average molecular mass of the (block A) - (block B) block copolymer is n_A/n_T M_A + M_M n_B/n_T, where :

M_A is the molar mass of styrene or of the mixture of monomers comprising styrene, used in step Ia), NU_A is the molar mass of acrylic acid, M_B is the molar mass of the Ci-C₄ alkyl acrylate or of the mixture of monomers comprising the Ci-C₄ alkyl acrylate, used in step Ib) .

By way of references, the following correspondences are given : n_A/n_T = 5 corresponds to a theoretical average molecular mass of block A of approximately 500 g/mol, - n_A/n_T = 5000 corresponds to a theoretical average molecular mass of block A of approximately 500000 g/mol, n_B/n_T = 5 corresponds to a theoretical average molecular mass of block B' of approximately 500 g/mol, - n_B/n_T = 5000 corresponds to a theoretical average molecular mass of block B' of approximately 500000 g/mol, n_A/n_T M_A + M_AΛ n_B/n_T = 13000 g/mol (respectively 2000, respectively 8000, respectively 20000, respec- tively 50000) corresponds to a theoretical average molecular mass of the (block A) - (block B) diblock of approximately 13000 g/mol (respectively 2000, respectively 8000, respectively 20000, respectively 50000), taking hydrolysis to be total and for the case where the Ci-C₄ alkyl acrylate is ethyl acrylate.

The ratios by weight between the blocks are defined as the ratios between the theoretical or targeted total masses (for this descriptor, a degree of hydrolysis of 1 is not imagined) . Thus:

M_An_A < T M_AA n_B + (1-T) M_B n_B indicates that the ratio by weight (block B) / (block A) is > 1. This is a feature of the copolymer used according to the invention .

M_An_A/ [M_An_A + T M_M n_B + (1-T) M_B n_B] indicates the amount by weight of block A in the (block A) - (block B) diblock copolymer, i.e. the proportion of block A.

[T M_M n_B + (1-T) M_B n_B]/[M_An_A + T M_M n_B + (1-T) M_B n_B] indicates the amount by weight of block B in the (block A) - (block B) diblock copolymer, i.e. the propor- tion of block B.

It is mentioned that the use and the adaptation of other preparation processes that produce substantially identical diblock copolymers would not depart from the context of the invention. In particular, it is possible to envisage using transfer agents comprising several transfer groups (for example, trithiocarbonates Z-S-CS-S-Z) resulting in telechelic copolymers of R- [ (block B') -(block A) ] _w type (for example, (block A)- (block B') -R- (block B') -(block A)), and then cutting up

("cleaving") the telechelic copolymers so as to obtain

(block A) - (block B') diblock copolymers. The cutting up may take place at the time of the hydrolysis, in which case (block A) - (block B) diblock copolymers are directly obtained. In such cases, those skilled in the art will adjust the implementing conditions so as to target average molecular masses equivalent to those indicated, for example by multiplying the amounts of monomers introduced by the number of transfer groups included in the transfer agent.

Preferably, the (block A) - (block B) linear diblock copolymers of type (1), in which the proportion by weight of block B relative to the copolymer [T M_M n_B + (1-T) M_B n_B]/[M_An_A + T M_AA n_B + (1-T) M_B n_B] is between

50% and 85%, preferably between 50% and 75%, generally have a theoretical average molar mass (n_A/n_T M_A + M_M n_B/n_T) of less than or equal to 13000 g/mol, and in particular of between 8000 and 13000 g/mol.

Preferably, the (block A) - (block B) linear diblock copolymers of type (2), in which the proportion by weight of block B relative to the copolymer [T M_M n_B + (1-T) M_B n_B]/[M_An_A + T M_M n_B += (1-T) M_B n_B] is greater than or equal to 85% (BOL 44 and 55 and 64), generally have a theoretical average molecular mass (n_A/n_T M_A + M_M n_B/n_T) of greater than or equal to 13000 g/mol.

Among these copolymers of type (2), those specific of type (2a) , where the proportion by weight of block B relative to the copolymer is greater than or equal to 87%, in particular greater than or equal to 87% and less than 94%, generally have a theoretical average molecular mass of between 13000 and 20000 g/mol, those specific of type (2b) , where the proportion by weight of block B relative to the copolymer is greater than or equal to 94%, in particular ranging from 94% to 97%, generally have a theoretical average molecular mass of greater than or equal to 20000 g/mol, and preferably of between 20000 and 50000 g/mol.

The diblock copolymer (s) in accordance with the inven- tion may be present in the composition according to the invention at a content ranging from 0.01% to 10% by weight, relative to the total weight of the composition, preferably ranging from 0.05% to 10% by weight, and preferentially ranging from 0.1% to 5% by weight.

The cosmetic composition according to the invention contains a physiologically acceptable medium, i.e. a medium compatible with cutaneous tissues such as the skin and the scalp. This physiologically acceptable medium may more particularly be constituted of water and optionally of a physiologically acceptable organic solvent chosen, for example, from lower alcohols containing from 1 to 8 carbon atoms, and in particular from 1 to 6 carbon atoms, such as ethanol, isopropanol, propanol or butanol; polyethylene glycols having from 6 to 80 ethylene oxide units and polyols such as propylene glycol, isoprene glycol, butylene glycol, glycerol and sorbitol.

The compositions that can be used may be in all the galenical forms conventionally used for topical application, and in particular in the form of aqueous or aqueous-alcoholic solutions, oil-in-water (O/W) or water-in-oil (W/O) or multiple (triple: W/O/W or O/W/O) emulsions, aqueous gels, or dispersions of a fatty phase in an aqueous phase by means of spherules, it being possible for these spherules to be polymeric nanoparticles such as nanospheres and nanocapsules, or lipid vesicles of ionic and/or nonionic type (liposomes, niosomes, oleosomes) . These compositions are prepared according to the usual methods .

In addition, the compositions that can be used according to the invention may be more or else fluid and have the appearance of a white or coloured cream, an ointment, a milk, a lotion, a serum, a paste or a foam. They may be optionally applied to the skin in the form of an aerosol. They may also be in solid form, and for example in the form of a stick.

When the composition that can be used according to the invention comprises an oily phase, the latter preferably contains at least one oil. It may also contain other fatty substances.

As oils that can be used in the composition of the invention, mention may, for example, be made of: hydrocarbon-based oils of animal origin, such as perhydrosqualene; hydrocarbon-based oils of plant origin, such as liquid triglycerides of fatty acids containing from 4 to 10 carbon atoms, for example triglycerides of heptanoic acid or octanoic acid or alternatively, for example, sunflower oil, corn oil, soybean oil, marrow oil, grapeseed oil, sesame oil, hazelnut oil, apricot oil, macadamia oil, arara oil, castor oil, avocado oil, triglycerides of caprylic/capric acids such as those sold by the company Stearineries Dubois or those sold under the names "Miglyol 810", "812" and "818" by the company Dynamit Nobel, jojoba oil, shea butter oil; synthetic esters and ethers, in particular of fatty acids, such as oils of formulae R1COOR2 or R1OR2 in which Rl represents the residue of a fatty acid containing from 8 to 29 carbon atoms, and R2 represents a linear or branched hydrocarbon-based chain containing from 3 to 30 carbon atoms, for instance Purcellin oil, isononyl isononanoate, isopropyl myristate, 2-ethyl- hexyl palmitate, 2-octyldodecyl stearate, 2-octyl- dodecyl erucate, isostearyl isostearate; hydroxylated esters such as isostearyl lactate, octyl hydroxy- stearate, octyldodecyl hydroxystearate, diisostearyl malate, triisocetyl citrate; fatty alcohol heptanoates, octanoates and decanoates; polyol esters, such as propylene glycol dioctanoate, neopentyl glycol dihepta- noate and diethylene glycol diisononanoate; and pentaerythritol esters such as pentaerythrityl tetraisostearate; - linear or branched hydrocarbons of mineral or synthetic origin, such as volatile or non-volatile liquid paraffins, and derivatives thereof, petroleum jelly, polydecenes, hydrogenated polyisobutene such as parleam oil; - fatty alcohols containing from 8 to 26 carbon atoms, such as cetyl alcohol, stearyl alcohol and the mixture thereof (cetylstearyl alcohol), octyldodecanol, 2-butyloctanol, 2-hexyldecanol, 2-undecylpentadecanol, oleyl alcohol or linoleyl alcohol; partially hydrocarbon-based and/or silicon-based fluoro oils such as those described in document JP-A-2- 295912; silicone oils such as volatile or non-volatile polydimethylsiloxanes (PDMSs) containing a linear or cyclic silicon chain, which are liquid or pasty at ambient temperature, in particular cyclopolydimethyl- siloxanes (cyclomethicones) such as cyclohexasiloxane; polydimethylsiloxanes comprising alkyl, alkoxy or phenyl groups, pendent or at the end of a silicone chain, these groups containing from 2 to 24 carbon atoms; phenylsilicones such as phenyl trimethicones, phenyl dimethicones, phenyltrimethylsiloxydiphenyl- siloxanes, diphenyl dimethicones, diphenylmethyldi- phenyltrisiloxanes, 2-phenylethyl trimethylsiloxysili- cates, and polymethylphenylsiloxanes; mixtures thereof.

The term "hydrocarbon-based oil" in the list of oils mentioned above is intended to mean any oil predominantly comprising carbon and hydrogen atoms, and optionally ester, ether, fluoro, carboxylic acid and/or alcohol groups.

The other fatty substances that may be present in the oily phase are, for example, fatty acids containing from 8 to 30 carbon atoms, for instance stearic acid, lauric acid, palmitic acid and oleic acid; waxes such as lanolin, beeswax, carnauba wax or candelilla wax, paraffin wax, lignite wax, microcrystalline wax, ceresin or ozokerite, synthetic waxes such as polyethylene waxes and Fischer-Tropsch waxes; silicone resins such as trifluoromethyl-Cl-4-alkyl dimethicone and trifluoropropyl dimethicone; and silicone elastomers such as the products sold under the names "KSG" by the company Shin-Etsu, under the names "Trefil", "BY29" or "EPSX" by the company Dow Corning or under the name "Gransil" by the company Grant Industries. These fatty substances may be chosen in a varied manner by those skilled in the art in order to prepare a composition having the desired properties of, for example, consistency or texture.

The emulsions generally contain at least one emulsifier chosen from amphoteric, anionic, cationic or nonionic emulsifiers, used alone or as a mixture, and optionally a coemulsifier . The emulsifiers are suitably chosen according to the emulsion to be obtained (W/0 or 0/W) . The emulsifier and the coemulsifier are generally present in the composition in a proportion ranging from 0.3% to 30% by weight, and preferably from 0.5% to 20% by weight, relative to the total weight of the composition .

For the W/O emulsions, emulsifiers that may be mentioned include, for example, dimethicone copolyols, such as the mixture of cyclomethicone and of dimethicone copolyol, sold under the name "DC 5225 C" by the company Dow Corning, and alkyl dimethicone copolyols, such as the lauryl methicone copolyol sold under the name "Dow Corning 5200 Formulation Aid" by the company Dow Corning and the cetyl dimethicone copolyol sold under the name "Abil EM 90^®" by the company Goldschmidt. Use may also be made, as surfactant of W/O emulsions, of a solid crosslinked elastomeric organopolysiloxane comprising at least one oxyalkylenated group, such as those obtained according to the protocol of Examples 3, 4 and 8 of document US- A-5,412,004 and of the examples of document US-A- 5,811,487, in particular the product of Example 3 (synthesis example) of Patent US-A-5, 412, 004, and such as that sold under the reference KSG 21 by the company Shin-Etsu .

For the O/W emulsions, emulsifiers that may be mentioned include, for example, nonionic emulsifiers, such as oxyalkylenated (more particularly polyoxy- ethylenated) fatty acid esters of glycerol; oxyalkylenated fatty acid esters of sorbitan; oxyalkylenated (oxyethylenated and/or oxypropylenated) fatty acid esters; oxyalkylenated (oxyethylenated and/or oxypropylenated) fatty alcohol ethers; sugar esters such as sucrose stearate; and mixtures thereof such as the mixture of glyceryl stearate and PEG-40 stearate .

In a known manner, the cosmetic or dermatological composition that can be used according to the invention may also contain adjuvants that are customary in the cosmetics or dermatological field, such as gelling agents, film-forming polymers, preserving agents, solvents, fragrances, fillers, UV screens, bactericides, odour absorbers, dyestuffs, plant extracts and salts. The amounts of these various adjuvants are those conventionally used in the field in question, and are, for example, from 0.01% to 20% of the total weight of the composition. Depending on their nature, these adjuvants may be introduced into the fatty phase and/or into the aqueous phase.

The cosmetic composition according to the invention may comprise an additional active cosmetic or dermatological agent.

By way of such an additional active cosmetic agent, mention may in particular be made of active agents that act on the barrier function of the skin, active agents that promote moisturization of the skin and desquamating agents.

The term "desquamating agent" is intended to mean any compound capable of acting: either directly on desquamation by promoting exfoliation, such as β-hydroxy acids, in particular salicylic acid and its derivatives (including 5-n-octanoylsalicylic acid) ; CC-hydroxy acids, such as glycolic acid, citric acid, lactic acid, tartaric acid, malic acid or mandelic acid; urea; gentisic acid; oligofucoses; cinnamic acid; extract of Saphora japonica; resveratol; or on the enzymes involved in desquamation or degradation of corneodesmosomes, such as glycosidases, stratum corneum chymotryptic enzymes (SCCE), or even other proteases (trypsin, chymotrypsin-like) . Mention may be made of agents that chelate mineral salts: EDTA;

N-acyl-N, N ' , N ' -ethylenediaminetriacetic acid; aminosulphonic compounds, and in particular

N- (2-hydroxyethyl) piperazine-N ' -2-ethanesulphonic acid

(HEPES); derivatives of 2-oxothiazolidine-4-carboxylic acid (procysteine) ; derivatives of alpha-amino acids of glycine type (as described in EP-O 852 949, and also the sodium methyl glycine diacetate sold by BASF under the trade name "Trilon M"); honey; sugar derivatives such as O-octanoyl-6-D-maltose and N-acetylglycosamine .

Among the active agents that act on the barrier function of the skin or promote moisturization of the skin, mention may be made of: either compounds that act on the barrier function for the purpose of maintaining the moisturization of the stratum corneum, or occlusive compounds, in particular ceramides, sphingoid-based compounds, lecithins, glycosphingolipids, phospholipids, cholesterol and its derivatives, phytosterols (stigmasterol, β-sitosterol, campesterol) , essential fatty acids, 1, 2-diacylglyce- rol, 4-chromanone, pentacyclic triterpenes such as ursolic acid, petroleum jelly and lanolin; or compounds which directly increase the water content of the stratum corneum, such as threalose and its derivatives, hyaluronic acid and its derivatives, glycerol, pentanediol, sodium pidolate, serine, xylitol, sodium lactate, poly (glycerol acrylate) , ectoin and its derivatives, chitosan, oligosaccharides and polysaccharides, cyclic carbonates, N-lauroyl- pyrrolidonecarboxylic acid, and N-CC-benzoyl-L-arginine; or compounds that activate the sebaceous glands, such as steroid derivatives (including DHEA) and vitamin D and its derivatives. The compositions may show their effectiveness as a nontherapeutic treatment for maintaining the skin, i.e. as a preventive treatment. They may also be used by way of a nontherapeutic treatment for the skin after a manifestation of skin moisturization problems.

The composition may be in the form of a care and/or make-up product.

The make-up product may be chosen from foundations, face powders, eye shadows, concealers, body make-up, lip make-up products.

The (block A) - (block B) diblock copolymer block copolymers as defined above or blends thereof may also be used for the preparation of a dermatological composition for use in moisturizing the skin, and more particularly in the treatment of dryness of the skin or in the treatment of dry skin.

The following examples are given by way of illustration of the present invention.

Synthesis examples

Example 1 :

Preparation of a polystyrene-block-poly (ethyl acrylate- stat-acrylic acid sodium salt) diblock copolymer of type (2b) by synthesis of a polystyrene-block- poly (ethyl acrylate) diblock copolymer of targeted Mns 2000-block-42000 (g/mol) and then 75% hydrolysis of the ethyl acrylate ester groups.

Step Ia: Preparation of a first block of polystyrene of theoretical molecular mass of approximately 2000 g/mol

3000 g of water, 17.6 g of sodium dodecyl sulphate and 0.290 g of sodium carbonate Na2CO3 are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 30 minutes under nitrogen. The temperature is then increased to 75°C, and a mixture 1 comprising the following is then added: - 10.00 g of styrene (St),

0.200 g of methacrylic acid (MAA), and

10.42 g of xanthate (CH₃) (CO₂CH₃)CH-S(C=S)OCH₂CH₃.

The mixture is brought to 85°C, and then a solution of 1.19 g of sodium persulphate Na₂S₂Os solubilized in 20.0 g of water is introduced.

After 5 minutes, the addition of a mixture 2 comprising the following is begun: - 90.0 g of styrene (St) and

1.80 g of methacrylic acid (MAA).

The addition is continued for 60 minutes. After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

A sample (5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 2000 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecularity index Mw/Mn is equal to 2.0.

An analysis of the sample by gas chromatography reveals that the monomer conversion is greater than 99%.

Step Ib: Growth of a second block of poly (ethyl acrylate) of theoretical molecular mass of approxi- mately 42000 g/mol in order to obtain a polystyrene- block-poly (ethyl acrylate) diblock copolymer

The starting material is the copolymer in emulsion obtained previously in step Ia, after having removed 5 g thereof for analysis and without interruption of the heating. 1.19 g of sodium persulphate Na2S2θs diluted in 50.O g of water are added continuously for three hours . Simultaneously, for three hours, a mixture 3 comprising the following is added at 85°C:

200.0 g of water,

2.20 g of sodium carbonate Na2CO3, and

4.40 g of sodium dodecyl sulphate.

A mixture 4 comprising the following is simultaneously added:

2100 g of ethyl acrylate (EA), and

42.0 g of methacrylic acid (MAA).

After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour. 4.4O g of tert-butylbenzylperoxide are then introduced in one step and the addition of a mixture 5 comprising the following is begun:

2.20 g of erythorbic acid,

50.0 g of water.

The addition is continued for 60 minutes. After complete addition of the various ingredients, the emulsion is cooled to ~25°C for one hour. A sample (5 g) is then removed and analysed by size exclusion chromatography (SEC) in THF. Its measured number- average molecular mass Mn is equal to 41000 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecularity index Mw/Mn is equal to 6.

An analysis of the sample by gas chromatography reveals that the monomer conversion is greater than 99.8%. The product obtained is a dispersion in water of the copolymer (latex) , having a dry extract of approximately 41%. Step II: Partial (targeted 75%) hydrolysis of the poly- (ethyl acrylate) block of the copolymer obtained above in step Ib in order to obtain the polystyrene-block- poly (ethyl acrylate-stat-acrylic acid sodium salt) diblock of type (2b)

750 g of water, 250 g of 2-propanol and 1347 g of copolymer in emulsion (c.f. 550 g of copolymer on a dry basis) obtained above in step Ib are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 15 minutes. The temperature is then increased to 75°C, and 678 g of sodium hydroxide (solution in water at 23.2% by mass) are then added continuously over one hour. 30 minutes after the beginning of the addition of sodium hydroxide, the continuous addition, over one hour, of 12 g of aqueous hydrogen peroxide (solution at 30%) is begun. After complete addition of the various ingredients, the copolymer solution obtained is maintained at 75°C for four hours, and then cooled to 25°C for one hour.

The product recovered at the end of the reaction is a translucent gel in water having a dry extract of approximately 20%.

The copolymer thus obtained has the following characteristics : theoretic average molecular mass of block A: 2000 g/mol - theoretic average molecular mass of block B: 30000 g/mol proportion by weight of block B: 96% proportion by weight of block A: 4% amount by weight of units deriving from ethyl acrylate in block B: 31%.

Example 2 :

Preparation of a polystyrene-block-poly (ethyl acrylate- stat-acrylic acid sodium salt) diblock copolymer by synthesis of a polystyrene-block-poly (ethyl acrylate) diblock copolymer of targeted Mns 5000-block-7000 (g/mol) and then 75% hydrolysis of the ethyl acrylate ester groups.

Step Ia: Preparation of a first block of polystyrene of theoretical molecular mass of approximately 5000 g/mol

1000 g of water, 6.50 g of sodium dodecyl sulphate and 0.30 g of sodium carbonate Na2CO3 are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 30 minutes under nitrogen. The temperature is then increased to 75°C, and a mixture 1 comprising the following is then added:

- 83.7 g of styrene (St),

1.67 g of methacrylic acid (MAA), and

- 17.4 g of xanthate (CH₃) (CO₂CH₃)CH-S(C=S)OCH₂CH₃.

The mixture is brought to 85°C, and then a solution of 2.0O g of sodium persulphate Na₂S₂Os solubilized in 20.0 g of water is introduced.

After 5 minutes, the addition of a mixture 2 comprising the following is begun:

- 334.7 g of styrene (St) and

6.69 g of methacrylic acid (MAA) .

The addition is continued for 60 minutes. After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour. A sample (5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 5800 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecu- larity index Mw/Mn is equal to 1.9.

Step Ib: Growth of a second block of poly (ethyl acrylate) of theoretical molecular mass of approxi- mately 7000 g/mol in order to obtain a polystyrene- block-poly (ethyl acrylate) diblock copolymer

The starting material is the copolymer in emulsion obtained above in step Ia, after having taken 5 g thereof for analysis and without interruption of the heating.

2.0O g of sodium persulphate Na2S2θs diluted in 50.O g of water are introduced continuously for three hours. Simultaneously, for three hours, a mixture 3 comprising the following is added at 85°C: - 200.0 g of water,

1.00 g of sodium carbonate Na2CO3, and 2.00 g of sodium dodecyl sulphate.

Simultaneously, a mixture 4 comprising the following is added:

581.6 g of ethyl acrylate (EA), and 11.63 g of methacrylic acid (MAA) .

After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

2.00 g of tert-butylbenzylperoxide are then introduced in a single step and the addition of a mixture 5 comprising the following is begun: 1.00 g of erythorbic acid, 50.0 g of water.

The addition is continued for 60 minutes. After complete addition of the various ingredients, the emulsion is cooled to ~25°C for one hour. A sample (5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 12700 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecularity index Mw/Mn is equal to 1.9.

An analysis of the sample by gas chromatography reveals that the monomer conversion is greater than 99.8%.

The product obtained is a dispersion in water of the copolymer (latex) , having a dry extract of approximately 44%.

Step II: Partial (targeted 75%) hydrolysis of the poly- (ethyl acrylate) block of the copolymer obtained above in step Ib in order to obtain the polystyrene-block- poly (ethyl acrylate-stat-acrylic acid sodium salt) diblock of type (Ib)

638 g of water, 212 g of 2-propanol and 1485 g of copolymer in emulsion (c.f. 650 g of copolymer on a dry basis) obtained above in step Ib are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 15 minutes. The temperature is then increased to 75°C, and 488 g of sodium hydroxide (solution in water at 23.2% by mass) are then added continuously over one hour.

30 minutes after the beginning of the addition of sodium hydroxide, the continuous addition of 37 g of aqueous hydrogen peroxide (solution at 30%) , over one hour, is begun.

After complete addition of the various ingredients, the copolymer solution obtained is maintained at 75°C for four hours and then cooled to ~25°C for one hour.

The product recovered at the end of the reaction is a translucent gel in water having a dry extract of approximately 18%.

The copolymer thus obtained has the following characteristics : - theoretic average molecular mass of block A: 5000 g/mol theoretic average molecular mass of block B: 5000 g/mol proportion by weight of block B: 57% - proportion by weight of block A: 43% amount by weight of units deriving from ethyl acrylate in block B: 31%.

Example 3:

Preparation of a polystyrene-block-poly (ethyl acrylate- stat-acrylic acid sodium salt) diblock copolymer by synthesis of a polystyrene-block-poly (ethyl acrylate) diblock copolymer of targeted Mns 2000-block-20000 (g/mol) and then 75% hydrolysis of the ethyl acrylate ester groups.

2150 g of water, 6.00 g of sodium dodecyl sulphate and 0.650 g of sodium carbonate Na2CO3 are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 30 minutes under nitrogen. The temperature is then increased to 75°C, and a mixture 1 comprising the following is then added: 18.2 g of styrene (St),

0.360 g of methacrylic acid (MAA), and - 18.9 g of xanthate (CH₃) (CO₂CH₃)CH-S(C=S)OCH₂CH₃.

The mixture is brought to 85°C, and then a solution of 2.16 g of sodium persulphate Na₂S₂Os solubilized in 20.0 g of water is introduced. After 5 minutes, the addition of a mixture 2 comprising the following is begun:

163.6 g of styrene (St) and

3.30 g of methacrylic acid (MAA). The addition is continued for 60 minutes. After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

A sample (~5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 2000 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecularity index Mw/Mn is equal to 2.1.

Step Ib: Growth of a second block of poly (ethyl acrylate) of theoretical molecular mass of approximately 20000 g/mol in order to obtain a polystyrene- block-poly (ethyl acrylate) diblock copolymer

The starting material is the copolymer in emulsion obtained above in step Ia, after having taken 5 g for analysis and without interruption of the heating.

2.16 g of sodium persulphate Na2S2θs diluted in 50.0 g of water are introduced continuously for three hours. Simultaneously, for three hours, a mixture 3 comprising the following is added at 85°C: - 200.0 g of water,

4.00 g of sodium carbonate Na2CO3, and - 8.00 g of sodium dodecyl sulphate.

Simultaneously, a mixture 4 comprising the following is added:

1818 g of ethyl acrylate (EA), and 33.4 g of methacrylic acid (MAA). After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour.

4.00 g of tert-butylbenzylperoxide are then added in a single step and the addition of a mixture 5 comprising the following is begun:

2.00 g of erythorbic acid, - 50.0 g of water.

The addition is continued for 60 minutes. After complete addition of the various ingredients, the emulsion is cooled to ~25°C for one hour.

A sample (5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 17500 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolec- ularity index Mw/Mn is equal to 2.9.

Step II: Partial (targeted 75%) hydrolysis of the poly- (ethyl acrylate) block of the copolymer obtained above in step Ib in order to obtain the polystyrene-block- poly (ethyl acrylate-stat-acrylic acid sodium salt) diblock of type (2al)

900 g of water, 300 g of 2-propanol and 1563 g of copolymer in emulsion (c.f. 700 g of copolymer on a dry basis) obtained above in step Ib are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 15 minutes. The temperature is then increased to 75°C, and 822 g of sodium hydroxide (solution in water at 23.2% by mass) are then added continuously over one hour.

30 minutes after the beginning of the addition of sodium hydroxide, the continuous addition of 25 g of aqueous hydrogen peroxide (solution at 30%) , over one hour, is begun.

The product recovered at the end of the reaction is a translucent gel in water having a dry extract of approximately 17%.

The copolymer thus obtained has the following characteristics: theoretic average molecular mass of block A: 2000 g/mol theoretic average molecular mass of block B: 14000 g/mol - proportion by weight of block B: 90% proportion by weight of block A: 10% amount by weight of units deriving from ethyl acrylate in block B: 31%.

Example 4 :

Preparation of a polystyrene-block-poly (ethyl acrylate- stat-acrylic acid sodium salt) diblock copolymer by synthesis of a polystyrene-block-poly (ethyl acrylate) diblock copolymer of targeted Mns 2000-block-20000 (g/mol) and then 90% hydrolysis of the ethyl acrylate ester groups.

2150 g of water, 6.00 g of sodium dodecyl sulphate and 0.650 g of sodium carbonate Na2CO3 are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 30 minutes under nitrogen. The temperature is then increased to 75°C, and a mixture 1 comprising the following is then added:

- 18.2 g of styrene (St), - 0.360 g of methacrylic acid (MAA), and

- 18.9 g of xanthate (CH₃) (CO₂CH₃)CH-S(C=S)OCH₂CH₃.

The mixture is brought to 85°C, and then a solution of 2.16 g of sodium persulphate Na₂S₂Os solubilized in 20.0 g of water is introduced.

After 5 minutes, the addition of a mixture 2 comprising the following is begun:

163.6 g of styrene (St) and

3.30 g of methacrylic acid (MAA).

A sample (5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 2000 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecularity index Mw/Mn is equal to 2.1.

Step Ib: Growth of a second block of poly (ethyl acrylate) of theoretical molecular mass of approximately 20000 g/mol in order to obtain a polystyrene- block-poly (ethyl acrylate) diblock copolymer The starting material is the copolymer in emulsion obtained above in step Ia, after having taken 5 g for analysis and without interruption of the heating.

2.16 g of sodium persulphate Na2S2θs diluted in 50.O g of water are introduced continuously for three hours. Simultaneously, for three hours, a mixture 3 comprising the following is added at 85°C: - 200.0 g of water,

4.00 g of sodium carbonate Na2CO3, and 8.00 g of sodium dodecyl sulphate.

Simultaneously, a mixture 4 comprising the following is added:

1818 g of ethyl acrylate (EA), and 33.4 g of methacrylic acid (MAA).

After complete addition of the various ingredients, the copolymer emulsion obtained is maintained at 85°C for one hour. 4.00 g of tert-butylbenzylperoxide are then added in a single step and the addition of a mixture 5 comprising the following is begun: 2.00 g of erythorbic acid, - 50.0 g of water.

The addition is continued for 60 minutes. After complete addition of the various ingredients, the emulsion is cooled to 25°C for one hour.

A sample (5 g) is then taken and analysed by size exclusion chromatography (SEC) in THF. Its measured number-average molecular mass Mn is equal to 17500 g/mol in polystyrene equivalents (calibration using linear polystyrene standards) . Its polymolecu- larity index Mw/Mn is equal to 2.9.

An analysis of the sample by gas chromatography reveals that the monomer conversion is greater than 99.8%. The product obtained is a dispersion in water of the copolymer (latex) , having a dry extract of approximately 44%.

Step II: Partial (targeted 90%) hydrolysis of the poly- (ethyl acrylate) block of the copolymer obtained above in step Ib in order to obtain the polystyrene-block- poly (ethyl acrylate-stat-acrylic acid sodium salt) diblock of type (2a2)

1209 g of water, 206 g of 2-propanol and 1670 g of copolymer in emulsion (c.f. 700 g of copolymer on a dry basis) obtained above in step Ib are introduced into the reactor at ambient temperature. The mixture obtained is stirred for 15 minutes. The temperature is then increased to 70⁰C, and 1038 g of sodium hydroxide (solution in water at 23.2% by mass) are then added continuously over one hour.

30 minutes after the beginning of the addition of sodium hydroxide, the continuous addition (step c) of 27 g of aqueous hydrogen peroxide (solution at 30%) , over one hour, is begun.

After complete addition of the various ingredients, the copolymer solution obtained is maintained at 70⁰C for four hours. The reaction mixture is then cooled to ~25°C for one hour.

The copolymer thus obtained has the following characteristics : theoretic average molecular mass of block A: 2000 g/mol theoretic average molecular mass of block B: 14000 g/mol proportion by weight of block B: 89% proportion by weight of block A: 11% amount by weight of units deriving from ethyl acrylate in block B: 13%.

Example 5 :

The moisturizing efficiencies of the following solutions were tested:

Solution A Distilled water 93^! Glycerol

Solution B

Distilled water 99%

Diblock polymer of Example 1 1% AM

Solution C

Distilled water 99%

Diblock polymer of Example 2 1% AM

Solution D Distilled water 99%

Diblock polymer of Example 3 1% AM

Solution E Distilled water 99% Diblock polymer of Example 4 1% AM

Significantly, the diblock copolymers in solution in water at 1% have a moisturizing efficiency, measured by Raman laser confocal microscopy, greater than that of glycerol at 7% in water.

Example 6 :

The following cosmetic moisturizing composition was prepared:

Phase Al :

Phase A2 :

Phase B:

Phase C :

The two phases Al and B are brought to 65°C before being combined (B in Al) with very vigorous stirring (rotor-stator) . After verification that complete dispersion has been obtained, it is possible, optionally, to homogenize the dispersion at between 2 x 10⁶ Pascal and 60 x 10⁶ Pascal, so as to obtain a dispersion in which the size of the oil globules is less than 500 nm. Phase A2 is dispersed, at ambient temperature, in the first dispersion, with vigorous stirring. Phase C, prepared beforehand, is then dispersed in order to gel the suspension.

A moisturizing emulsion suitable for dry skin is obtained. The diblock copolymer contributes to this moisturizing activity.

Claims

1. Cosmetic composition comprising, in a physiologically acceptable medium: a (block A) - (block B) diblock copolymer in which: block A comprises at least units deriving from styrene; block B comprises at least (a) units deriving from acrylic acid in free or salified form and (b) at least units deriving from a Ci-C₄ alkyl acrylate .

2. Composition according to Claim 1, where said diblock copolymer is linear.

3. Composition according to Claim 1 or 2, where the proportion by weight of block B relative to the copolymer is greater than or equal to 50%.

4. Composition according to any one of Claims 1 to 3, where the copolymer is characterized in that: block A comprises at least 90% by weight of units deriving from styrene, relative to the total weight of block A; - block B is a random block comprising, relative to the total weight of block B:

(i) from 34% to 95% by weight of units deriving from acrylic acid in acid form or in salified form; (ii) from 5% to 66% by weight of units deriving from Ci-C₄ alkyl acrylate, the proportion by weight of block B relative to the copolymer being greater than or equal to 50%.

5. Composition according to any one of Claims 1 to 4, where the Ci-C₄ alkyl acrylate is ethyl acrylate.

6. Composition according to any one of Claims 1 to 5, characterized in that block A and/or block B comprise (s) up to 10% by weight, and preferably up to 5% by weight of an additional ionic or nonionic, hydrophilic comonomer.

7. Composition according to Claim 6, where the hydrophilic comonomer is methacrylic acid or a salt thereof .

8. Composition according to any one of Claims 1 to 7, where the (block A) - (block B) diblock copolymer is of type (1) in which the proportion by weight of block B relative to the copolymer is between 50% and 85%.

9. Composition according to Claim 8, where the copolymer is of type (Ib) in which the proportion by weight of block B is between 50% and 75%.

10. Composition according to any one of Claims 1 to 7, where the (block A) - (block B) diblock copolymer is of type (2) in which the proportion by weight of block B relative to the copolymer is greater than or equal to 85%.

11. Composition according to Claim 10, where the copolymer is of type (2a) in which the proportion by weight of block B relative to the copolymer is greater than or equal to 87% and less than 94%.

12. Composition according to Claim 10, where the copolymer is of type (2b) in which the proportion by weight of block B relative to the copolymer is greater than or equal to 94%.

13. Composition according to Claim 10, where the copolymer is of type (2b) in which the proportion by weight of block B relative to the copolymer ranges from 94% to 97%.

14. Composition according to any one of Claims 1 to 13, where the (block A) - (block B) diblock copolymer can be obtained by means of a polymerization process comprising at least the following steps:

Ia) a first block A is prepared by bringing the following together:

- n_τ mol of a transfer agent comprising a single transfer group;

- n_A mol of styrene or of a mixture of monomers comprising at least 90% by weight of styrene and where n_A/n_T > 5 and preferably < 5000;

- and, optionally, a free-radical initiator;

Ib) a second block B' is prepared, in order to obtain a (block A) - (block B') diblock copolymer, by bringing the following together:

- the block A obtained in the preceding step;

- n_B mol of a hydrolysable Ci-C₄ alkyl acrylate or of a mixture of monomers comprising at least 90% by weight of a Ci-C₄ alkyl acrylate such that n_B/n_T > 5 and preferably < 5000;

- and, optionally, a free-radical initiator;

II) hydrolysis of block B' is then performed, to a degree T by mole of between 0.4 and 0.96, so as to obtain said (block A) - (block B) diblock copolymer .

15. Composition according to the preceding claim, characterized in that an additional step III) of deactivation of transfer groups borne by macro- molecular chains and/or of purification of the (block A) - (block B) diblock copolymer and/or of destruction of by-products from hydrolysis and/or deactivation is carried out during and/or after step I I ) .

16. Composition according to Claim 14 or 15, characterized in that step I) is carried out by water- borne emulsion polymerization.

17. Composition according to any one of Claims 14 to 16, characterized in that step I) is carried out by controlled radical polymerization with a transfer agent comprising a transfer group of formula -S-CS-.

18. Composition according to one of Claims 14 to 17, characterized in that, in step Ia) , use is made of a mixture of monomers comprising at least 90% by weight of styrene and up to less than 10% by weight of methacrylic acid, preferably at least 95% by weight of the Ci-C₄ alkyl acrylate and up to less than 5% by weight of an ionic or nonionic, hydrophilic comonomer.

19. Composition according to one of Claims 14 to 18, characterized in that, in step Ib) , use is made of a mixture of monomers comprising at least 90% by weight of the Ci-C₄ alkyl acrylate and up to less than 10% by weight of methacrylic acid, preferably at least 95% by weight of the Ci-C₄ alkyl acrylate and up to less than 5% by weight of an ionic or nonionic, hydrophilic comonomer.

20. Composition according to Claim 18 or 19, where the hydrophilic comonomer is methacrylic acid or a salt thereof.

21. Composition according to one of Claims 14 to 20, characterized in that the Ci-C₄ alkyl acrylate is ethyl acrylate.

22. Composition according to one of Claims 14 to 21, characterized in that the (block A) - (block B) diblock copolymer is of type (1) in which: the proportion by weight of block B relative to the copolymer is between 50% and 85%, and - its theoretical average molecular mass is less than or equal to 13000 g/mol.

23. Composition according to the preceding claim, characterized in that the (block A) - (block B) diblock copolymer is of type (1) in which: the proportion by weight of block B relative to the copolymer is between 50% and 75% by weight, and its theoretical average molecular mass is between 8000 and 13000 g/mol.

24. Composition according to one of Claims 14 to 21, characterized in that the (block A) - (block B) diblock copolymer is of type (2) in which: - the proportion by weight of block B relative to the copolymer is greater than or equal to 85%, and its theoretical average molecular mass is greater than or equal to 13000 g/mol.

25. Composition according to the preceding claim, characterized in that the (block A) - (block B) diblock copolymer is of type (2a) in which: the proportion by weight of block B relative to the copolymer is greater than or equal to 87%, and - its theoretical average molecular mass is between 13000 and 20000 g/mol.

26. Composition according to Claim 24, characterized in that the (block A) - (block B) diblock copolymer is of type (2b) in which: the proportion by weight of block B relative to the copolymer is greater than or equal to 94%, and its theoretical average molecular mass is greater than or equal to 20000 g/mol, and preferably between 20000 and 50000 g/mol.

27. Composition according to any one of Claims 1 to 26, where the diblock copolymer (s) is (are) present in an amount ranging from 0.01% to 10% by weight, relative to the total weight of the composition, preferably ranging from 0.05% to 10% by weight, and preferentially ranging from 0.1% to 5% by weight.

28. Composition according to any one of the preceding claims, also comprising at least one cosmetic adjuvant chosen from fatty substances, organic solvents, ionic or nonionic, hydrophilic or lipo- philic thickeners, demulcents, humectants, opacifiers, stabilizers, emollients, silicones, antifoams, fragrances, preserving agents, anionic, cationic, nonionic, zwitterionic or amphoteric surfactants, active agents, fillers, polymers, propellants, and basifying or acidifying agents.

29. Nontherapeutic cosmetic treatment process for caring for or making up keratin materials, in particular the skin, comprising the application to the keratin materials, in particular to the skin, of a cosmetic composition according to any one of the preceding claims.

30. Process according to the preceding claim, charac- terized in that it is a cosmetic process for moisturizing the skin.

31. Cosmetic use of a (block A) - (block B) diblock copolymer in which: - block A comprises at least units deriving from styrene; block B comprises at least (a) units deriving from acrylic acid in free or salified form and (b) at least units deriving from a C1-C4 alkyl acrylate ; as defined in any one of Claims 1 to 26, as a moisturizer for keratin materials, preferably for the skin.