Technical Field The present invention relates to new derivatives of N-acylurea particularly for preparing polyurethanes, to a polyolefin film that comprises said derivatives, and to the use of said derivatives to produce surface-active films for catalyzing the polyurethane forming reaction. Background Art Currently, in the field of packaging the coupling of two or more films (a process known as lamination) is performed by utilizing the adhesive properties of polyurethane compounds which, as a consequence of a crosslinking phenomenon, are capable of "gluing" together the two or more layers between which they have been interposed. Polyurethanes are polymers that contain the carbamate group, -NHC (O)O-, also known as urethane group, in their main structure. The reaction that leads to the forming of polyurethanes generally involves the polyaddition reaction between a polyisocyanate and a polyol. The crosslinking of the polyurethane adhesive consists substantially of a polyaddition reaction between polyisocyanate and a polyol, which creates a rigid skeleton that "cures" the initial mixture, in which the individual components were instead free to move. The urethane forming reaction, that leads to the curing of the adhesive and is a requisite for the success of the lamination process, is a slow phenomenon and to allow its industrial application it is necessary to catalyze it by using suitable compounds, commonly termed crosslinking promoters. Currently preferred methods apply the crosslinking promoter just after the extrusion of the film to be coupled, where this deposition occurs with various kinds of process. Two examples of these methods are disclosed in DE4229953 (in the name of Morton GmbH) and in WO 00/71326 (in the name of BP Europack). DE4229953 describes wetting the surface of the polymeric film to be coupled or the layer of the polyol-isocyanate mixture with an aqueous solution of at least one crosslinking promoter, while WO 00/71326 describes the deposition of the crosslinking promoter on a specific region of the freshly extruded polymeric film and with a particular timing. However, it has been found that variations in the surface distribution of the active substance anchored to the extruded polymeric film can occur in some cases due to storage. This phenomenon is particularly important for crosslinking promoters that have a low affinity for some widely used polymeric films (such as polyethylene films). Catalysts commonly used in the synthesis of polyurethane adhesives starting from isocyanates and compounds with active hydrogens are for example metalorganic compounds of Sn, Zn, Pb and Zr, which however have the problem of heavy metal disposal. Other known catalysts are alkaline salts of organic acids and phenols, phosphines and phospholine oxides, polyethylene glycol, triphenyl bismuth, and ε-caprolactam. However, many of the known catalysts have a limited affinity for example for polyolefms and therefore are unable to anchor themselves effectively to the polyolefm film to which they are applied. The result is a tendency of the crosslinking promoter to move on the surface of the film on which it is deposited, causing imperfect reproducibility of the final performance. From what has been described, it is therefore evident that the need is felt to have new molecules that allow in particular to improve the crosslinking of polyurethanes. m particular, it would be highly desirable to reduce the crosslinking times of polyurethane adhesives, especially when the packing material to be produced is a multilayer material obtained by coupling different materials, such as plastic films, aluminum sheets or others. Better crosslinking would mean lower lamination costs, reduced storage spaces, and shorter delivery times, in addition to a higher assurance of quality and safety caused by the reduction of the migration of isocyanates and to the consequent reduction of the risk of forming carcinogenic aromatic amines. Disclosure of the Invention Accordingly, the aim of the present invention is to provide a crosslinking promoter particularly for preparing polyurethanes that overcomes the drawbacks of the background art. Within this aim, an object of the invention is to provide a crosslinking promoter that has a high chemical affinity with polyolefins and allows broad versatility of use, maintaining stability and effectiveness in the most disparate conditions of use and in combination with the most disparate urethane precursors and allows to use a polyol-isocyanate mixture with a longer pot life. Another object of the invention is to provide a film of flexible material, preferably of the polyolefin type, to which said new promoter is already added. Another object is to provide the use of a promoter as described above for producing a surface-active film for catalyzing the polyurethane formation reaction, wherein said use occurs in combination with any one of the process methods that are known for this purpose. This aim and these and other objects are achieved by compounds containing formula (I)

formula (I) where the substituents R1, R2, R3, R4 are selected independently from the group that consists of: (a) Ci-Cioo alkyl, preferably C6-C25, even more preferably C6-Ci6, optionally substituted with one or more polar groups; (b) a polyolefin with 2000 < Mn < 107, preferably 10000 < Mn < 200000, even more preferably 20000 < Mn < 50000, attached to formula (I) by means of any one of its monomers; (c) -R5-(T-R6)n-(U-R7)p, where it is attached to the molecule having the formula (T) by means of R5 and where: - n and p are whole numbers comprised between 1 and 106, preferably between 1 and 1000, even more preferably between 1 and 10; - T and U are each selected independently from the group that consists of: (i) -O- (ii) -C(=OH (iii) -c(=o)-o-, (iv) -NH-C(=O)-O-; and , - R5 and R6 and R7 are each selected independently among C1-C1n alkyl Cwith m < 104, preferably m < 100, even more preferably m < 10), polyglycols, polyalcohols, where R5, R6 and R7 are optionally substituted with one or more polar groups; (d) aryl, optionally substituted in one or more of the remaining positions with a substituent selected from the group that consists of: (i) hydrogen, Cii) Ci-C50 alkyl, (iii) -NR8-C(=O)OR9, where R8 is selected from the group that consists of hydrogen and Ci-C50 alkyl or is as defined in item (c), and R9 is Ci-Cso alkyl or is as defined in item (c), (iv) -NR8-C(=O)-NR10R11, where R8 is as defined above and R10 and R11 are selected independently from the group that consists of hydrogen and C1-C50 alkyl or are as defined in item (c), (v) polyglycol, (vi) polyalcohol, (vii) -C(=O)OR8, and (viii) -CC=O)-NR10R11, where R10 and R11 are as defined above, and (ix) -R12-O-R8, where R12 is C0-C25 alkyl and R8 is as defined above; where the substituents (ii) to (ix) are bound to the aryl by means of their left end; (e) the substituents R1 and R2 and/or the substituents R3 and R4 are bound together covalently so as to form a saturated cycle, preferably with 6, 7 or 13 members, optionally substituted in one or more of the remaining positions with a substituent chosen from the group that consists of: (i) hydrogen (ii) C1-C50 alkyl, (iii) polyglycol, (iv) polyalcohol, (v) -C(=O)OR8, and (vii) -R12-O-R10, where R12 is C0-C25 alkyl and R10 is as defined above, where substituents (ii) to (vii) are bound to the saturated cycle by means of their left end; f) -CH=CH2 or -C(CHs)=CH2, and g) hydrogen One of the substituents from R1 to R4, defined according to the preceding options, may furthermore have, as a substituent, one or more acylurea groups having the general formula (I). In a preferred embodiment, R2 is H. In another preferred embodiment, R3 and R4 are fused together so as to constitute a saturated cycle according to option (e) cited above. Preferred examples of compounds having the formula I are: lauric acid 2-{2-[4-methyl-3[(2-oxo-azepan-l-carbonyl)-amino]- phenylcarbamoyloxy]-polyethoxy} ethyl ester, lauric acid 2-{(6-[(2-oxo-azepan-l-carbonyl)-amino]-hexylcarbamoyloxy)- polyethoxy} ethyl ester, dodecanoic acid 2-methyl-2-(6-[(2-oxo-azepan-l-carbonyl)-amino]- hexylcarbamoyloxy) ethyl ester, (4-methyl-3-[(2-oxo-azacyclotridecan-l-carbonyl)amino]-pheny l)-carbamic- 2-octadecyloxy ethyl ester acid, (4-(4-(([octadecanoylamino]carbonyl)-arnino)-benzyl)-phenyl) carbamic-2- octadecyloxy ethyl ester acid, octadecanoic acid l-methyl-2-(2-methyl-3-[(2-oxo-azepan-l-carbonyl)- amino] phenylcarbamoyloxy) ethyl ester; acrylic acid 2-(6-[(2-oxo-azepan-l-carbonyl)-amino]hexylcarbamoyloxy) ethyl ester, 2-oxo-azeρan- 1 -(2-[2-(2-ethyl-hexyloxy)-ethoxy]-ethoxy)-ethyl) carboxamide, dodecanoic acid 2-(3-[(2-oxo-azepan-l-carbonyl)-arnino]-propoxy) ethyl ester, octadecanoic acid l-methyl-2-(5-[(2-oxo-azepan-l-carbonyl)amino]-l- carbamoyloxy-naphthyl) ethyl ester, acrylic acid 2-(2-(4-[(2-oxy-azepan-l-carbonyl)-amino]benzyl)- phenylcarbamoyloxy) ethyl ester, l-methyl-2,4-[(2-oxy-azepan-l-carbonyl)amino] -benzene (example with two acylurea groups). The aim and objects of the invention are also achieved by a polyolefin film to which a molecule having the formula (I) is already added. The addition of this molecule can occur according to any one of the methods known for this purpose, such as for example the ones mentioned earlier of wetting or anchoring the active substance on the surface of the film or of the adhesive. The aim and objects of the invention are also achieved by the use of a compound having the formula (I) for the production of a surface-active film for catalyzing the polyurethane forming reaction. Said use occurs with any one of the process methods known for this purpose, as mentioned above. The present invention provides new compounds, which are derived from N-acylurea and which, if added to an isocyanate and to a polyalcohol in a suitable commonly-used solvent, such as for example carbon tetrachloride, are capable of accelerating significantly the reaction for forming the urethane group (for example, the reaction between toluene diisocyanate and n-propanol is accelerated 10-15 times). In the present invention, the term "polyolefin" is preferably used to designate any homo- or heteropolymer of alkenyl units, comprising linear, branched and cyclic portions, or a combination thereof, joined to the formula (I) by means of any one of its carbons. Preferred polyolefms have a molecular weight comprised between 2000 and 107, preferably comprised between 10000 and 200000, even more preferably comprised between 20000 and 50000. In the present invention, the term "alkyl" is used to designate radicals with a saturated, linear or branched hydrocarbon chain. Examples of alkyl radicals include methyl, ethyl, propyl, /-propyl, w-butyl, ter-butyl, isobutyl, n-pentyl, 2-methyl butyl, 2,2-dimethyl propyl, et cetera, preferably lauryl and stearyl. In the present invention, the term "aryl" is used to designate carbocyclic aromatic systems comprising one or more aromatic rings, fused or joined by covalent bonds or methylene bridges (-CH2-), each of which is not substituted or optionally substituted as defined above. Examples of aryl radical include phenyl, naphthyl, anthracyl, pyridyl, furanyl, thiophenyl, pyrroyl, imidazyl, oxazyl, thiazyl, pyrazyl, et cetera, preferably phenyl. In the present invention, the expression "polar group" is used to designate a functional group that has a non-uniform charge distribution on all the individual atoms that constitute said polar group. The asymmetry of the distribution can be due to differences in electπmegativity of the atoms and/or to the presence of delocalizable electrons. Examples of polar group are hydroxide (-OH), polyglycols, polyalcohols, acrylic groups (-OC(=O) CH=CH2), methacrylic groups (-OC(=O)C(CH3)=CH2) and carboxylic acids (-COOH), preferably polyglycols. In the present invention, the term "polyglycols" is used to designate groups having the general formula HO-[(-CH2)m-O-]n-OH, where m is a whole number comprised between 1 and 3, preferably 2, and n is a whole number comprised between 1 and 100, preferably comprised between 3 and 10. In the present invention, the term "polyalcohols" is used to designate alkyls as defined above substituted with two or more hydroxy (-OH) groups. Preferred examples of polyalcohols are polymers of polyvinyl alcohol (PVA) with a molecular weight of 40000 or less and a degree of hydrolysis of over 70%, and preferably with a molecular weight comprised between 15000 and 20000 and a degree of hydrolysis of over 95%. All the compounds having the formula (T) described in the examples have surprisingly proved themselves very active, particularly in the preparation of polyurethanes, and have allowed an unexpected increase in the crosslinking rate of the polyurethane polymer. Moreover, they have the great advantage of having physical and chemical properties that can be modified easily by virtue of the many possible substitutions of the R1 -R12 radicals around the acylurea group. The choice of the R1^-R12 substituents to render the compounds having the formula (I) suitable to each specific situation in terms of solvent, reaction, temperature, reagents and of every other process variable is within the ordinary knowledge of the person skilled in the art. By modulating the various groups from R1 to R12, it has been possible to obtain crosslinking promoters that have an appropriate molecular weight, viscosity, degree of polarity and solubility. The variation of the molecular structure has allowed, in particular, to obtain crosslinking promoters that are compatible and stable on polymeric films and materials with different surface energy, such as for example polyolefins, nylon, PET, EVA, PVDC, aluminum. Likewise, it is possible to achieve solubilization or dispersion of the crosslinking promoters according to the invention in media of variable polar nature, such as for example aliphatic and aromatic hydrocarbons, water, alcohols, ethers, esters, ketones and chlorinated compounds. Varying the volatility by increasing the molecular weight of the substituents allows to apply the crosslinking promoters according to the invention to hot and cold surfaces by virtue of known methods for deposition, spreading or spraying, depending on the particular situation. The same variation of the molecular weight also allows to obtain optimum viscosities for dispersion in any type of polyolefin resin, so as to be able to use the crosslinking promoters according to the invention also with methods that provide for the premixing of resin of the monomer still to be polymerized and crosslinking promoters. The new compounds according to the present invention are therefore very versatile compounds due to the possibility to modulate the N-acylurea function substituting groups. This allows their easy use to produce films with surfaces that are catalytically active toward the polyurethane forming reaction. These surface-active films can be produced by depositing solutions of crosslinking promoter by spraying on the hot film, or by deposition with flexographic or rotogravure techniques of viscous solutions that contain the dissolved and/or dispersed promoter. Furthermore, the active compounds can be deposited on the surface of the film by adsorption from a bath in which the hot film in output from the extruder is immersed. The bath can be a bath of water or of mixtures of other solvents that are incapable of swelling the laminated polymer, particularly polyethylene, have a high boiling point and are not easily flammable. The new compounds according to the present invention, by not containing heavy metals, have a considerably reduced environmental impact linked to their use. Another significantly important aspect is that the reaction kinetics (i.e., the kinetics of the crosslinking catalysis reaction) of the compounds having the formula (J) can be controlled thermally. In this manner, the invention provides compounds that allow to use polyol/polyisocyanate mixtures with a longer pot life, allowing easier management for users of polyurethane resins and allowing accurate control of polymerization by choosing suitable temperature gradients, which must be studied according to the specific case but can be determined by the person skilled in the art with laboratory experiments. The expression "pot life" is used to designate the period of time during which the mixture remains liquid enough to be used. The compounds having the formula (I) have also shown a considerable and surprising stability in stressful chemical conditions, thus allowing their use also in processes that entail particularly drastic conditions. In this regard, although one does not wish to be bound to particular theories, it is believed that the high stability of the compounds having the formula (I) can be ascribed to the forming of intramolecular hydrogen bonds. The new compounds having the formula (I) are very versatile and can be used for various fields and preferably in the production of surface-active films for the catalysis of the polyurethane forming reaction. These films, "functionalized" by applying, by known methods, the compounds according to the invention can then be used effectively to produce polylaminated films, with particular attention in the industry of flexible packaging, in which polyurethane adhesives are widely used to mutually bond normally incompatible layers, such as for example polyethylene/PET, polypropylene/PET, polyethylene/Nylon, polypropylene/Nylon, and others. Therefore, in a further embodiment, the present invention relates to a polyolefin film that has already received the addition of a compound having the formula (J), in which the addition of said compound can occur according to any method known for this purpose. For example, the compound can be deposited on the surface, and this deposition can occur according to methods that provide, for example, for wetting the film (in accordance to what is disclosed in DE4229953) or for the addition of the crosslinking promoter during the extrusion process. In particular, the particular chemical and physical properties of the compounds having the formula (I) make them unexpectedly compatible with all polyolefins, thus bypassing the known incompatibility that is peculiar to currently widely used molecules. In another embodiment, the present invention relates to the use of a compound having the formula (I) as defined above to produce surface-active films for the catalysis of the polyurethane forming reaction, where this production can occur according to the criteria of any one of the methods known for this purpose. For example, the present invention relates to the use of compounds having the formula (T) in combination with "bubble" or "cast" extrusion processes or with methods that comprise wetting at least one of the parts to be coupled (film and/or adhesive). In particular, in view of their compatibility with polyolefins, the incidence of the phenomenon of surface migration of the compounds is reduced significantly. Further characteristics and advantages of the present invention will become better apparent from the detailed description of the following preferred embodiments, given merely by way of non-limiting example. Likewise, although the text has described only some preferred embodiments of the invention, the person skilled in the art will understand immediately that it is in any case possible to obtain other equally advantageous and preferred embodiments. Example 1 N-hexyl-2-oxo-azepan-l-earboxyamide was prepared according to the reaction described schematically below. 2 g (0.0157 moles) of 1-hexyl-isocyanate and 2.4 g (0.0212 moles) of ε-caprolactam were placed in 10 ml of CCl4. The solution, placed in an atmosphere of N2, was kept under agitation at ambient temperature for 8 days. The solvent was then eliminated from the solution, and the solution was analyzed by FT-IR. The spectrum still showed the presence of VN=oo (stretching of the isocyanate group). The mixture was then diluted in CCl4, placed in countercurrent with the solvent, in an inert atmosphere, for 8 hours so as to complete the reaction. The solvent was removed from the solution again and the residue was dissolved hot in 50 ml of petroleum ether, and the insoluble part was eliminated by filtration. The filtrate was then extracted with 3x50 ml of a mixture of H2OZCH3OH (90/10, V/V), dried on Na2SO4 and dried. 2.6 g (0.0108 moles, 69% yield) of product in the form of clear oil were obtained. This oil was found to have good solubility in common organic solvents such as chloroform, ethyl acetate, methanol, ethanol, petroleum ether. It was instead found to be insoluble in H2O both hot and cold. Solubility in ethylene glycol was tested by preparing a 1% solution (weight/volume). The product of the reaction was found to be scarcely cold- soluble. Solubilization occurred by heating, but after cooling the product again precipitated in the form of oil, thus indicating a limited tendency to remain in solution. In order to check its catalytic activity, a solution in carbon tetrachloride was prepared of TDI 0.015 M, n-propyl alcohol 0.03 M, and N-hexyl-N'-(cyclohexylacyl)urea 0.085 10"3 M. The disappearance rate of the band at 2274 cm'1 related to the stretching of the isocyanate group of TDI was then monitored by FT-IR spectroscopy. This rate was compared with the rate related to the same reaction in the absence of the catalyst by comparing the half-lives, which were found to be 21.5 h for the non- catalyzed reaction and 2.5 h for the reaction performed in the presence of the catalyst. Example 2 By means of standard synthesis methods, the following molecules comprising the N-acylurea functional group were prepared: a) lauric acid 2-{2-[4-methyl-3[(2-oxo-azepan-l-carbonyl)-amino]- phenylcarbamoyloxy]-polyethoxy} ethyl ester;

b) lauric acid 2-{(6-[(2-oxo-azepan-l-carbonyl)-amino]- hexylcarbamoyloxy)-polyethoxy} ethyl ester;

c) dodecanoic acid 2-methyl-2-(6-[(2-oxo-azepan-l-carbonyl)-amino]- hexylcarbamoyloxy) ethyl ester;

d) (4-methyl-3-[(2-oxo-azacyclotridecan- l-carbonyl)amino]-phenyl)- carbamic acid-2-octadecyloxy ethyl ester acid;

e) (4-(4-(([octadecanoylamino] carbonyl)~arnino)-benzyl)-phenyl) carbamic-2-octadecyloxy acid ethyl ester;

f) octadecanoic acid l~methyl-2-(2-methyl-3-[(2-oxo-azepan-l- carbonyl)-amino] phenylcarbamoyloxy) ethyl ester;

g) acrylic acid 2-(6-[(2-oxo-azepan-l-carbonyl)-amino] hexylcarbamoyloxy) ethyl ester;

h) 2-oxo-azeρan-l-(2-[2-(2-ethyl-hexyloxy)-ethoxy]-ethoxy)-eth yl) carboxamide;

i) dodecanoic acid 2-(3-[(2-oxo-azepan-l-carbonyl)-amino]-propoxy) ethyl ester;

1) octadecanoic acid l-methyl-2-(5-[(2-oxo-azepan-l-carbonyl) amino]-l-carbamoyloxy-naphthyl) ethyl ester;

m) acrylic acid 2-(2-(4-[(2-oxy-azepan-l-carbonyl)-amino]benzyl)- phenylcarbamoyloxy) ethyl ester; Molecules a) and b) were obtained as mixtures of compounds in which the polyethoxylene chain had an average value of n equal to 14. Molecules a) to m) were then tested regarding their effectiveness in promoting and accelerating the urethane polymerization process of an industrial adhesive. 8.85 10"5 moles of catalyst were added to 1 g of 1:5 mixture of Adoxene 91R/Adoxene AD25 (produced by Mydrin Findley) and the decrease of the IR band at 2274 cm"1, due to the stretching of the isocyanate group, was monitored over time until complete disappearance occurred. By comparison, a test was performed without the catalyst according to the invention. The table below lists the disappearance times of the band at 2274 cm1 related to the stretching of the isocyanate group of TDI by FT-IR sectrosco related to each exeriment.

Example 3 n) l-methyl-2,4-[(2-oxy-azepan-l-carbonyl)amino]-benzene

The effectiveness of the catalyst was tested on the reaction between 2,4-toluene diisocyanate (TDI) and n-propyl alcohol (n-PrOH) in CCl4, 5 μl of the catalyst in a solution of CCl* (0.03 M) were added to 1 ml of a 1:1 mixture of TDI/n-PrOH (0.033/0.072 M) in CCl4. The reaction was followed over time by IR spectroscopy (Figure 1), following the decrease of the isocyanate band at 2265 cm"1. The half-life of the band is tm = 128 minutes. The time for 98% disappearance of the signal is hs% = 21500 minutes. In the absence of the catalyst, the reaction has a tm = 1290 minutes. The disclosures in Italian Patent Application No. PD2004A000138 from which this application claims priority are incorporated herein by reference.