A PROCESS FOR THE PREPARATION OF VINYLAROMATIC (CO)POLYMERS GRAFTED ON AN ELASTOMER IN A CONTROLLED WAY The present invention refers to a process for the preparation of vinylaromatic (co)polymers grafted on an elastomer in a controlled way. In detail, the present invention refers to a proc¬ ess for the preparation of (co)polymers of styrene grafted on an elastomer in presence of a living radi- calic polymerization system. The term "living radicalic polymerization", as used in the present description and in the claims, means a conventional radicalic polymerization carried in also in the presence of a chemical substance capable to react in a reversible way with the radical of the growing polymer chain. Such substance consists, for in¬ stance, of stable nitroxides or alkoxy-amines . More de¬ tails on the radicalic polymerization can be found in the U.S. patent 4,581,429, in the European Patent 869,137 or in "Living Free Radical Polymerization", TPoint 2-2002, Notiziario di EniTecnologie (San Donato Milanese), available on the web at: www.enitecnologie.it/tpoint/articoli/FT^petrolchimica/L JvingFree2_2002.pdf. Various processes for the preparation of vinylaro- matic (co)polymers grafted on elastomer in a controlled way are known in literature. For instance, in US Patent 6,262,179 a process to prepare a rubber reinforced vi- nylaromatic polymer is described, characterized by a mono- or bimodal morphology that comprises the polymeri¬ zation of a vinylaromatic monomer solution containing rubber, by means of an initiator system that comprises a stable generator of radicals. At the end of the po- lymerization a product is obtained, consisting of a rigid polymer matrix, in which the rubber particles are dispersed, whose morphology, however, is still linked to the type of rubber used as in the traditional proc¬ esses that use the non stable polymerization initia- tors. US Patent 6,255,402 describes a process for the preparation of a composition consisting of a vinylaro¬ matic matrix polymer, in which particles of rubber are dispersed with morphology different from the one known as "salami", but instead being of the "labyrinth", "on- ion" or, better, "capsule" type, so as to supply a crash resistant end product with improved gloss. The same US patent gives indications about meaning of the terms identifying the above cited morphologic forms. The present process is characterized in that of be¬ ing successful in obtaining the morphologic diversity us¬ ing the homopolymer of the butadiene as rubber which, traditionally, substantially gives the morphology to sa¬ lami . According to said US patent the polybutadienic rubber is dissolved in a solvent in absolute absence of monomer and functionalized with an initiator system consisting of a traditional radical initiator, for in¬ stance a peroxide, and a stable radical initiator, for instance 2, 2, 6, 6-tetramethyl-l-pyperidinyloxyl (com¬ monly known as TEMPO) operating at a temperature com¬ prised between 50 and 1500C, which was stirred for some hours. Finally, the vinylaromatic monomer is added and then its polymerization is started until the desired conversion is obtained. The polymerization system of US Patent 6,255,402 permits on one hand to obtain an end product of variable morphology with a low cost rubber and on the other hand introduces a second disadvantage which reduces, or even runs the risk of canceling, the associate economic ad- vantage of using the polybutadienic rubber. Indeed, in the solution processes, there is the provision for a stage of devolatilization under vacuum to retrieve at the end of the polymerization the solvent and the non reacted monomer which, for economic reasons, must be re¬ cycled. The above implies, for the processes of the current art, one stage to separate the solvent, recycled at the rubber functionalization, from the monomer recy¬ cled at the polymerization reactor, with an increase of the production costs. The Applicant has now found a process for the preparation of vinylaromatic (co)polymers grafted on an elastomer in a controlled way, by means of a catalytic system comprising a stable initiator of radicals, which permits to obtain an end product in which the morphol¬ ogy of the dispersed elastomeric phase does not neces¬ sarily depend on the type of the used elastomer, as the latter could even consist of simple polybutadiene, and in which the not reacted end product, recovered after devolatilization, does not have to be separated in its single constituents (solvent and monomer) but can be used and recycled as such. Therefore, the scope of the present invention is a process for the preparation of vinylaromatic (co)polymers grafted on elastomer in a controlled way which comprises: a) dissolving an elastomer in a liquid phase consisting of a vinylaromatic monomer/polymerization solvent mix¬ ture with a weight ratio comprised between 60/40 and 100/0, preferably between 60/40 and 90/10; b) adding to the ■ solution a catalytic system of polym¬ erization consisting of a free radical initiator (G) , having F functionality, capable of withdrawing a pro¬ ton from the polymeric chain of the elastomer and a stable initiator of free radicals comprising the group = N-O' (I) , with molar ratios 1/G1F lower than 4, preferably between 1 and 2, being F equal to the number of functional groups per molecule of the initiator which, by decomposition, produces two free radicals; c) heating, while stirring, the mixture obtained in stage (b) at a temperature comprised between 80 and 1100C, for a time sufficient to obtain the complete functionalization of the elastomer; d) feeding the vinylaromatic monomer to the mixture, containing in solution the functionalized elastomer, polymerizing the thus obtained mixture at a tempera¬ ture greater than or equal to 1200C, preferably between 120 and 2000C; e) recovering the obtained vinylaromatic polymer ob- tained at the end of the polymerization, submitting it also to devolatilization under vacuum, to recover the solvent and the unreacted monomer; and f) recycling the solvent/monomer mixture to stage (a) . According to the present invention, the process of preparation of the vinylaromatic polymer can be real¬ ized via batch processing or via continuous processing. In the first case, the functionalization of the elas¬ tomer and the polymerization of the monomer occur in a single container, for instance in an ' agitated mixer equipped with heating systems from which the polymeri¬ zation mixture is collected to recover the final poly¬ mer to be submitted to the phase of devolatilization when the solid content has reached a level comprised between 60 and 80% in weight. In the second case, in- stead, the functionalization of the elastomer takes place in the stirred mixer, continuously feeding prod¬ ucts and additives, while the polymerization takes place in one or more stirred reactors, chosen among the stirred containers, such as the CSTR (Continuous Stirred Tank Reactor) , and/or the tubular reactors (Plug Flow) , continuously fed with the functionalized solution. Also, in this second case, the recovery of the final polymer takes place through the phase of de- volatilization after the solid content reaches the above mentioned levels. The preferred process accord- ing to the present invention is the continuous process described, for instance, in the European Patent EP 400.479. The term "vinylaromatic (co)polγτaer" , as used in the present description and in the claims, means essen¬ tially a (co)polymer obtained from the (co)polymerization of at least one monomer that corre¬ sponds to following general formula (II) :

in which R is a hydrogen or a methyl group, n is zero or an integer comprised between 1 and 5 and Y is a halogen, like chlorine or bromine, or an alkylic or alkoxy radical having from 1 to 4 carbon atoms . Examples of vinylaromatic monomers having the above identified general formula are: styrene, a- methylstyrene, methylstyrene, ethyl styrene, butyl sty¬ rene, dimethyl styrene, mono -, di -, tri -, tetra- and penta-clorostyrene, bromo-styrene, metoxystyrene, ace- toxystyrene, etc. The preferred vinylaromatic monomers are styrene and/or α-methylstyrene. The vinylaromatic monomers of general formula (I) can be used alone or in a mixture up to 50% in weight with other monomers that can be copolymerizable. Exam- pies of such monomers are the (meth) acrylic acid, the alkyl esters Ci-C4 of (meth)acrylic acid like methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl- methacrylate, isopropyl acrylate, butyl acrylate, the amides and the nitriles of the (meth) acrylic acid like acrylamide, methacrylamide, acrylonitrile, methacry- lonitrile, butadiene, ethylene, divinylbenzene, maleic anhydride, etc. The preferred copolymerizable monomers are acrylonitrile and methylmethacrylate. Any elastomer that can be used as a reinforcing product in a vinylaromatic (co)polymer can be used in the process object of the present invention. However, the preferred product, for its economic convenience, is the homopolymer polybutadiene with a numerical medium molecular weight (Mn) comprised between 50'000 and 350'000 and a medium ponderal molecular weight (Mw) comprised between 100OOO and 500OOO. Other elastomers that can be used in place of polybutadiene or in a mixture with it can be chosen among the homopolymers and the copolymers of 1,3- alkadienes containing 40-100% in weight of the 1,3- alkadiene monomer, for instance butadiene, isoprene or pentadiene, and 0-60% in weight of one or more mono- ethylenically unsaturated monomers chosen among sty- rene, acrylonitrile, a-methylstyrene, methylmetacrylate and ethylacrylate. Co-polymer examples of 1,3-alkadienes are the block co-polymers styrene-butadiene like the bi-block linear elastomers of the S-B type where S represents a poly- styrenic block of medium molecular weight Mw comprised between 5'000 and 80'000 while B represents a polybu- tadienic block of medium molecular weight Mw comprised between 2'00O and 250 '000. In these elastomers the amount of block S is comprised between 10 and 50% in weight with respect to the total of the rubber S-B. The preferred product is the styrene-butadiene copoly¬ mer block having a styrene content equal to 40% in weight and a viscosity in solution, measured at 230C in a 5% solution in weight of styrene, comprised between 35 and 50 CPS. Other elastomer examples that can be used in the process object of the present invention are those de¬ scribed in the European Patent 606.931. The previously described elastomers are dissolved in the liquid phase containing the monomer and a polym¬ erization solvent. The preferred solvent, according to the present invention, is ethyl benzene, but other aro¬ matic solvents can be used, like toluene or xylenes, or the aliphatic solvents, such as hexan or cyclohexan. The catalytic system of polymerization is added to the solution prepared in said manner, in an amount com¬ prised between 0.02 and 0.5% in weight with respect to the total. This system consists of a free radical ini¬ tiator and of a stable initiator of free radicals with the previously indicated ratios. Surprisingly, there is not a substantial formation of polymer which, if formed, does not exceed 2% in weight, and no reticulation of the elastomer is observed during the functionalization phase. The free radical initiators capable of withdrawing one proton from the polymer chain of the elastomer are chosen among the azo-derivates, such as the 4,4' -bis- (diisobutirronitryle) , 4,4' -bis (4-cianopentanoic acid) , 2 , 2 -azobis (2-amidinopropane) dihydrochloride, etc, or among the peroxides, the hydroperoxides, the percarbon- ates, the peresters and the persalts, for instance the persulphates such as the potassium persulphate or the ammonium persulphate. In general, the preferred free radicals initiators are the peroxides chosen among t- butil isopropyl monoperoxycarbonate, t-butil 2- ethylesil monoperoxycarbonate, dicumil peroxide, of-t- butil peroxide, 1, 1-di (t-butilperoxy) cyclohexan, 1,1- di (tbutilperoxy) -3,3, 5-trimethylcyclohexan, t- butilperoxyacetate, cumil t-butil peroxide, t-butil per- oxybenzoate and t-butil peroxy-2-ethylhexanate. The stable initiator of free radicals characterized by the group =N-0 is chosen among those of general formula ( III ) :

RJ N - O

Rs Re

where the groups Ri, R2, R5 and Re, equal or different from each other, are straight or branched alkyl radi¬ cals, substituted or unsubstituted, containing from 1 to 20 atoms of carbon or alkylaromatic radicals, in which the alkyl group contains from 1 to 4 carbon atoms while the R3 and R4 groups, equal or different from each other, are equal to Rl, R2, R5 and R6, or R3-CNC-R4 is part of a cyclic structure, for instance with 4 or 5 carbon atoms, possibly fused with an aromatic ring or with a saturated ring containing from 3 to 20 carbon atoms in which at least a hydrogen atom of the cyclic structure can be replaced by a hydroxyl group. According to a further embodiment of the present invention, the group =N-0' is replaced by the group group =N-0-R' wherein R' is a Ci-C8 alkyl or C7-Ci2 arylalkyl radical, possibly containing at least one heteroatom. Examples of radical R' are 2-phenylethyl or 2-methyl-2-cyanopropyl radical. Initiators of general formula (III) and their preparation are described in US Patent 4,581,429. Examples of particularly preferred initiators of general formula (III) that can be used in the process object of the present invention are the 2, 2, 5, 5-tetramethyl-l- pyrrolidinyloxyl, known with the trade name PROXYL, the 2, 2, 6, 6-tetramethyl-l-pyperinedinyloxyl, known with the trade name TEMPO, and the 4-hydroxy- 2,2,6,6- tetramethyl-1-pyperinediniloxyl, known with the trade name 4OH-TEMPO. Other examples of stable initiators that can be used in the process object of the present inven¬ tion and that are comprised in the general formula (III) are described in the above mentioned US Patent 4.581.429. At the end of the functionalization of the elas¬ tomer, the process of polymerization of vinylaromatic (co)polymers grafted on elastomer proceeds like the traditional process of the known technique, by feeding the monomer and starting the polymerization reaction by increasing the temperature in one or more stages. At the end of the polymerization, the polymer is submit¬ ted to a devolatilization process to recover the not re- acted monomer and the solvent, which are found to be in such a ratio as to allow their recycling to the mixer without having to separate one from the other. If a co- monomer is present, it can be recovered, e.g. by distil¬ lation, before the recycling. To better understand the present invention and to put it in practical use some non limitative examples are illustrated below. EXAMPLE 1 (SALAMI) In a 1 liter flat bottom autoclave, complete with a temperature recorder and a stirring system consisting of an anchor and of a turbine with six slanted blades (the distance between the walls of the autoclaves and the anchor being 5.5 millimeters) , at ambient tempera¬ ture, 60 g of ethylbenzene (ET) , 160 g of styrene mono- mer (S) (ratio S/ET = 73/27) and 50 g of polybutadiene INTENE 60 (Mw = 308'600, polydispersivity 2.13) are in¬ troduced. The stirring speed is set at 100 rpm. The temperature of the system is raised to 900C in an hour and maintained constant for another hour. After that, 0.4840 g of benzoil peroxide (BPO) and 0.5160 g of 4- 2, 2, 6, 6-tetramethyl-l-pyperidinyloxyl (40H-TEMPO) are added. The temperature is raised to 1050C in three hours and maintained constant for another two hours . 550 g of styrene monomer are added to the reaction mixture and the mixture is then heated to 1250C in 45 minutes . The temperature is maintained at this value for six hours. After that, the reaction mixture is transferred into an appropriate container and the polym¬ erization is completed by heating the mixture to 1500C for four hours. Finally, the obtained polymer is devola- tilized at 23O0C under vacuum to eliminate the ethylben- zene and the not reacted styrene. The ethylbenzene and the styrene in an S/ET weight ratio equal to 73/27 are recycled to the 1 liter autoclave without having to be separated. The properties of the obtained polymer are indi¬ cated in Table 1. EXAMPLE 2 (LABYRINTH) In a flat bottom 1 liter autoclave, complete with a temperature recorder and a stirring system consisting of an anchor and a turbine with six slanted blades (the distance between the walls of the autoclaves and anchor being 5.5 millimeter), at ambient temperature, 54 g of etilbenzene, 126 g of styrene monomer (S/ET = 70/30) and 45 g of polybutadiene INTENE 60 (Mw = 308'600 polidis- persita 2.13) are introduced. The stirring speed is set at 100 rpm. The temperature of the system is taken to 9O0C in an hour and maintained constant for another hour. Afterwards, 0.8704 g of benzoilperoxyde (BPO) and 0.9280 g of 40H-TEMPO are added. The temperature is in- creased to 1050C in three hours and maintained constant for further two hours . 500 g monomer styrene are added to the reaction mixture and heated to 125°C in 45 minutes. The tempera- ture is maintained at said value for six hours. The re¬ action mixture is then transferred into a suitable con¬ tainer and the polymerization is completed by heating the mixture to 15O0C for four hours. Finally, the ob¬ tained polymer is devolatilized at 2300C under vacuum to eliminate the ethylbenzene and the not reacted styrene. These last ones in a weight ratio S/ET equal to 70/30 are recycled to the 1 liter autoclaves without having to be separated. The properties of the obtained polymer are shown in Table 1. EXAMPLE 3 (GIANT ONIONS) In a 1 liter flat bottom autoclave, complete with a temperature recorder and a stirring system consisting of an anchor and one turbine with six slanted blades (with distance between the walls of the autoclaves and the an¬ chor of 5.5 millimeter), at ambient temperature, 55 g of ethylbenzene, 127 g of styrene monomer (S/ET = 70/30) and 45 g of polybutadiene INTENE 40 (Mw = 225 '548 polydispersivity 2.17) are introduced. The stirring speed is set at 100 rpm. The temperature is increased to 900C in an hour and maintained constant for another hour. Afterwards 1.1910 g of benzoilperoxyde (BPO) and 1.2697 g of 40H-TEMPO are added. The temperature is raised to 1050C in three hours and maintained constant for further two hours . 500 g of styrene monomer are added to the reaction mixture and heated to 1250C in 45 minutes. The tempera¬ ture is maintained at this value for six hours . After¬ wards the reaction mixture is transferred into a suitable container and the polymerization is completed by heating the mixture to 1500C for four hours. Finally, the ob¬ tained polymer is devolatilized at 2300C under vacuum to eliminate the ethylbenzene and the not reacted styrene. These last ones in a weight ratio S/ET equal to 70/30 are recycled to the 1 liter autoclave without having to be separated. The properties of the obtained polymer are shown in Table 1. EXAMPLE 4 (CAPSULES) In 1 liter flat bottom autoclave complete with a temperature recorder and a stirring system consisting of an anchor and a turbine with six slanted blades (with distance between the walls of the autoclaves and the an¬ chor of 5.5 millimeter), at ambient temperature, 55 g of ethylbenzene, 127 g of styrene monomer (S/ET = 70/30) and 45 g of polybutadiene INTENE 40 (Mw = 225'548 polydispersivity 2.17) are introduced. The stirring speed is set at 100 rpm. The temperature of the system is increased to 9O0C in an hour and maintained constant for another hour. Afterwards 0.5955 g of benzoilperoxyde (BPO) and 0.6349 g of 4OH-TEMPO are added. The tempera¬ ture is increased to 1050C in three hours and maintained constant for further two hours . 500 g of styrene monomer are added to the reaction mixture and heated to 125°C in 45 minutes. The tempera¬ ture is maintained at this value for six hours . After¬ wards the reaction mixture is transferred into a suitable container and the polymerization is completed by heating the mixture at 1500C for four hours. Finally, the ob- tained polymer is devolatilized at 2300C under vacuum to eliminate the ethylbenzene and the not reacted styrene. These last ones in an S/ET weight ratio equal to 70/30 are recycled to the 1 liter autoclave without having to be separated. The properties of the obtained polymer are shown in Table 1. TABLE 1