Process for producing random propylene copolymers comprising C4-C12-alpha olefin comonomer units The present invention relates to a process for producing a random copolymer of propylene, optionally ethylene, and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms using a specific class of hafnocene complexes in combination with a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst, preferably in a multistage polymerisation process including a gas phase polymerisation step. Background of the invention Metallocene catalysts have been used to manufacture polyolefins for many years. Countless academic and patent publications describe the use of these catalysts in olefin polymerisation. Metallocenes are now used industrially and polyethylenes and polypropylenes in particular are often produced using cyclopentadienyl based catalyst systems. Metallocene catalysts are used in propylene polymerisation in order to achieve some desired polymer properties. However, there are some problems in using metallocene catalysts on industrial scale especially in multistage polymerisation configurations. Thus, there is room for improving the process and catalyst behaviour in the process. Metallocene catalysts for polypropylene generally show a very steep molecular weight capability response to hydrogen, that is, the melt flow rate of metallocene catalysed polypropylene strongly increases by even a mild increase in hydrogen concentration in the polymerisation medium. On the other hand, the use of hydrogen is needed to reach acceptable catalyst productivities. For this reason, in industrial scale metallocene catalysts are mostly used for the production of high flow polypropylene materials. In addition, in the case of copolymerisation of propylene and higher alpha-olefins (such as butene and hexene) with metallocene-based catalysts, the higher alpha- olefin tends to lower both catalyst activity and copolymer molecular weight. The advantage of using metallocene catalysts for such copolymers is that they have far better butene and hexene incorporation than Ziegler-Natta catalysts. However, for use of propylene-butene and propylene-hexene copolymers in applications such as blown film, BOPP and pipes, high molecular weights (low MFR) are required, without sacrificing catalyst productivity and other properties, thus metallocene catalysts have limited applicability for these uses due to the aforementioned too strong response to hydrogen and the adverse effect of comonomer on activity and molecular weight. The problem was partially solved by using catalysts based on metallocenes as disclosed in WO 2015/014632. Still, catalyst activity was decreased when increasing the hexene content, and still hexene increased the melt flow rate of the copolymer. A further improvement was provided in WO 2019/215122 in which catalysts based on metallocenes are provided with a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst. By means of said metallocene catalyst systems propylene-butene and propylene-hexene copolymers can be polymerized with a higher molecular weight shown in a relatively low melt flow rate of about 2 g/10 min at acceptable catalyst productivity. Such propylene copolymers qualify for applications such as blown films or BOPP but still do not have sufficient molecular weight for pipe applications. Thus it is desired to find metallocene catalyst systems, which have improved performance in the production of propylene copolymers comprising C4-C12-alpha olefin comonomer units, for instance having high activity for high Mw propylene copolymer comprising C ₄ -C ₁₂ -alpha olefin comonomer units products. The desired catalysts should also have improved performance in the production of high molecular weight propylene random copolymers comprising C4-C12-alpha olefin comonomer units, whereby the propylene copolymer comprising C ₄ -C ₁₂ -alpha olefin comonomer units should have higher melting points compared to propylene random copolymers comprising C4-C12-alpha olefin comonomer units produced with metallocene catalyst systems of the prior art. Although a lot of work has been done in the field of metallocene catalysts, there still remain some problems, which relate mainly to the productivity or activity of the catalysts, in particular in multistage polymerisation processes, since the productivity has been found to be relatively low, especially when propylene copolymers of low melt index (MI) (i.e. high molecular weight, Mw) are produced. The inventors have identified a catalyst system composed of a specific class of hafnocene catalysts in combination with a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst having improved polymerisation behaviour, higher catalyst productivity, improved performance in the production of high molecular weight propylene/C4-C12-alpha olefin random copolymers with lower melt flow rates compared to systems known in the art, enabling the production of propylene/C ₄ -C ₁₂ -alpha olefin random copolymers at high Mw, thus being ideal for the production of high molecular weight propylene/C4-C12- alpha olefin random copolymers. The specific hafnocene catalyst system gives a higher flexibility/freedom in the design of propylene/C ₄ -C ₁₂ -alpha olefin random polymers than prior art catalyst systems. Summary of the invention The present invention provides a process for producing a random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms, and optionally ethylene, in the presence of a single-site catalyst comprising (i) a complex of formula (I) wherein each X independently is a sigma-donor ligand in the group R2Si- at least one R is methyl or ethyl, and the other R is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl or isobutyl, pentyl, hexyl, cyclohexyl and phenyl; each R ¹ independently is the same or can be different and are a CH ₂ -R ⁷ group, with R ⁷ being H or linear or branched C1-C6-alkyl group, C3-C8-cycloalkyl group, or C ₆ -C ₁₀ -aryl group; each R ² is independently a –CH=, -CY=, -CH ₂ -‚ -CHY- or -CY ₂ - group, wherein Y is a C1-C6-hydrocarbyl group and where n is 2-6; each R ³ and R ⁴ are independently the same or can be different and are hydrogen, a linear or branched C ₁ -C ₆ -alkyl group, a C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group, C6-C20-aryl group, or an -OY group, wherein Y is a is a C1-C6- hydrocarbyl group; R ⁵ is a linear or branched C ₁ -C ₆ -alkyl group, C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group or C6-C20-aryl group; and R ⁶ is a C(R ⁸ )3 group, with each R ⁸ being independently a linear or branched C1- C ₆ -alkyl group; (A) wherein at least one R ³ per phenyl group and at least one R ⁴ is not hydrogen, and wherein at least one R ³ per phenyl group and at least one R ⁴ is hydrogen; or (B) wherein one R ³ is an -OY group, wherein Y is a is a C ₁ -C ₆ -hydrocarbyl group, in 4-position of each phenyl group and the two other R ³ groups are tert-butyl groups; and/or (C) wherein one R ⁴ is an -OY group, wherein Y is a is a C ₁ -C ₆ -hydrocarbyl group, in 4-position of the phenyl ring and the two other R ⁴ groups are tert- butyl groups; and (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst; and in the presence of hydrogen. The catalyst of the invention can be used in non-supported form or in solid form. The catalyst of the invention may be used as a homogeneous catalyst or heterogeneous catalyst. The catalyst of the invention in solid form, preferably in solid particulate form can be either supported on an external carrier material, like silica or alumina, or, in a particularly preferred embodiment, is free from an external carrier, however still being in solid form. For example, the solid catalyst is obtainable by a process in which (x) a liquid/liquid emulsion system is formed, said liquid/liquid emulsion system comprising a solution of the catalyst components (i) and (ii) dispersed in a solvent so as to form dispersed droplets; and (y) solid particles are formed by solidifying said dispersed droplets. In another aspect the present invention relates to a random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or a random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms obtainable from the process according to the invention as defined above or below, wherein the random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms follows the following relation (A) in behalf of its polymerisation process: metallocene productivity / MFR ₂₁ ≥ 15 [kg/(g‧h) / g/10 min] (A) with metallocene productivity overall productivity of the single site catalyst as kg random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms formed per g metallocene; MFR ₂₁ melt flow rate in g/10 min of the random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms, determined according to ISO 1133 at a temperature of 230°C and a load of 21.6 kg. Finally, the present invention also relates to the use of a single-site catalyst comprising (i) a complex of formula (I) wherein each X independently is a sigma-donor ligand in the group R2Si- at least one R is methyl or ethyl, and the other R is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl or isobutyl, pentyl, hexyl, cyclohexyl and phenyl; each R ¹ independently is the same or can be different and are a CH ₂ -R ⁷ group, with R ⁷ being H or linear or branched C1-C6-alkyl group, C3-C8-cycloalkyl group, or C ₆ -C ₁₀ -aryl group; each R ² is independently a –CH=, -CY=, -CH ₂ -‚ -CHY- or -CY ₂ - group, wherein Y is a C1-C6-hydrocarbyl group and where n is 2-6; each R ³ and R ⁴ are independently the same or can be different and are hydrogen, a linear or branched C ₁ -C ₆ -alkyl group, a C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group, C6-C20-aryl group, or an -OY group, wherein Y is a is a C1-C6- hydrocarbyl group; R ⁵ is a linear or branched C ₁ -C ₆ -alkyl group, C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group or C6-C20-aryl group; and R ⁶ is a C(R ⁸ )3 group, with each R ⁸ being independently a linear or branched C1- C ₆ -alkyl group; (A) wherein at least one R ³ per phenyl group and at least one R ⁴ is not hydrogen, and wherein at least one R ³ per phenyl group and at least one R ⁴ is hydrogen; or (B) wherein one R ³ is an -OY group, wherein Y is a is a C ₁ -C ₆ -hydrocarbyl group, in 4-position of each phenyl group and the two other R ³ groups are tert-butyl groups; and/or (C) wherein one R ⁴ is an -OY group, wherein Y is a is a C ₁ -C ₆ -hydrocarbyl group, in 4-position of the phenyl ring and the two other R ⁴ groups are tert- butyl groups; and (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst for the production of a random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or a random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms as defined above or below. Detailed Description of the Invention The complexes and hence catalysts of the invention are based on formula (I) as hereinbefore defined. The complexes of the invention are asymmetrical. Asymmetrical means simply that the two indenyl ligands forming the hafnocene are different, that is, each indenyl ligand bears a set of substituents that are either chemically different, or located in different positions with respect to the other indenyl ligand. Symmetrical complexes are based on two identical indenyl ligands. The hafnocene complexes of the invention are preferably chiral, racemic bridged bisindenyl C ₁ -symmetric hafnocenes in their anti-configuration. Although the complexes of the invention are formally C ₁ -symmetric, the complexes ideally retain a pseudo-C2-symmetry since they maintain C2-symmetry in close proximity of the metal center although not at the ligand periphery. By nature of their chemistry both anti and syn enantiomer pairs (in case of C ₁ -symmetric complexes) are formed during the synthesis of the complexes. For the purpose of this invention, racemic-anti means that the two indenyl ligands are oriented in opposite directions with respect to the cyclopentadienyl-metal-cyclopentadienyl plane, while racemic-syn means that the two indenyl ligands are oriented in the same direction with respect to the cyclopentadienyl-metal-cyclopentadienyl plane, as shown in the scheme exemplified for zirconocene complexes below. The same approach also counts for the hafnocene compexes of the invention. Racemic Anti Racemic Syn Formula (I), and any sub formulae, are intended to represent complexes in their anti- configurations. The hafnocene complexes of the invention are preferably employed as the racemic- anti-isomers. ldeally, therefore at least 95 mol%, such as at least 98 mol%, especially at least 99 mol% of the hafnocene catalyst complex is in the racemic anti-isomeric form. In the definitions below the term hydrocarbyl group includes alkyl groups, alkenyl groups, alkynyl groups, cycloalkyl groups, cycloalkenyl groups, aryl groups, alkylaryl groups or arylalkyl groups or of course mixtures of these groups such as cycloalkyl substituted by alkyl. In the catalysts of the invention the following preferences apply: Each X independently is a sigma -donor ligand. Preferably, each X is independently a hydrogen atom, a halogen atom, C1-C6-alkoxy group or an R' group, where R' is a C ₁ -C ₆ -alkyl, phenyl or benzyl group. More preferably, X is chlorine, benzyl or a methyl group. Preferably, both X groups are the same. The most preferred options are two chlorides, two methyl or two benzyl groups, especially two chlorides. In the bridging group R2Si- at least one R is methyl or ethyl, and the other R is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl or isobutyl, pentyl, hexyl, cyclohexyl and phenyl. In a preferred formula R ₂ Si- represents Me ₂ Si-, Et ₂ Si- or (cyclohexyl)Me-Si-. Most preferably the bridge is -Si(CH3)2 or Et2Si-. Each R ¹ independently is the same or can be different and are a CH ₃ -R ⁷ group, with R ⁷ being H or linear or branched C1-C6-alkyl group, like methyl, ethyl, n-propyl, i- propyl, n-butyl, i-butyl, sec-butyl and tert-butyl, C3-C8-cycloalkyl group (e.g. cyclohexyl), or C ₆ -C ₁₀ -aryl group (preferably phenyl). Preferably, both R ¹ groups are the same and are a CH ₃ -R ⁷ group, with R ⁷ being H or linear or branched C1-4-alkyl group, more preferably, both R ¹ groups are the same and are a CH ₃ -R ⁷ group, with R ⁷ being H or linear or branched C _1-3 -alkyl group. Most preferably, both R ¹ groups are methyl. Each R ² is independently a -CH=, -CY=, -CH2-‚ -CHY- or -CY2- group, wherein Y is a C ₁ -C ₁₀ -hydrocarbyl group, preferably a C ₁ -C ₄ -hydrocarbyl group and where n is 2-6, preferably 3-4. R ² forms a ring with the atoms of the phenyl ring. ldeally R ² together with the atoms of the phenyl ring forms a five membered ring. It is preferred that R ² is -CH ₂ - and n is 3. Each substituent R ³ and R ⁴ are independently the same or can be different and are hydrogen, a linear or branched C ₁ -C ₆ -alkyl group, a C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ - alkylaryl group, C6-C20-aryl group or an -OY group, wherein Y is a is a C1-C6- hydrocarbyl group. It is required that either: (A) at least one R ³ per phenyl group and at least one R ⁴ is not hydrogen, and wherein at least one R ³ per phenyl group and at least one R ⁴ is hydrogen; or (B) one R ³ is an -OY group, wherein Y is a is a C1-C6-hydrocarbyl group in 4- position of each phenyl group and the two other R ³ groups are tert-butyl groups; and/or (C) one R ⁴ is an -OY group, wherein Y is a is a C1-C6-hydrocarbyl group in 4- position of the phenyl ring and the two other R ⁴ groups are tert-butyl groups. The phenyl rings can therefore be mono, bis or trisubstituted. More preferably, R ³ and R ⁴ are hydrogen or a linear or branched C1-C4 alkyl group or an -OY group, wherein Y is a C1-C4-hydrocarbyl group. Even more preferably, each R ³ and R ⁴ are independently hydrogen, methyl, ethyl, isopropyl, tert-butyl or methoxy, especially hydrogen, methyl or tert-butyl, wherein at least one R ³ per phenyl group and at least one R ⁴ is not hydrogen and wherein at least one R ³ per phenyl group and at least one R ⁴ is hydrogen; or at least one R ³ is a methoxy group in the 4-position of each phenyl group and the two other R ³ groups are tert-butyl groups; and/or at least one R ⁴ is a methoxy group in the 4-position of the phenyl ring and the two other R ⁴ are tert-butyl groups. Thus, in one embodiment, one or two R ³ per phenyl group are not hydrogen and one or two R ³ groups are hydrogen. If there are two non-hydrogen R ³ groups per phenyl group then the R ³ group representing hydrogen is preferably at the 4-position of the ring. If there are two R ³ groups representing hydrogen then the non-hydrogen R ³ group is preferably present at the 4-position of the ring. Most preferably the two R ³ groups are the same. A preferred structure is 3',5'-di- methyl or 4'-tert-butyl for both phenyl groups substituted by R ³ groups. Alternatively, the structure is 3,5-di-tert-butyl-4-methoxyphenyl. For the indenyl moiety, in one embodiment, one or two R ⁴ groups on the phenyl group are not hydrogen. More preferably two R ⁴ groups are not hydrogen. If there are two non-hydrogen R ⁴ groups then the R ⁴ representing hydrogen is preferably at the 4-position of the ring. If there are two R ⁴ groups representing hydrogen then the non-hydrogen R ⁴ group is preferably present at the 4-position of the ring. Most preferably the two R ⁴ are the same like 3',5'-di-methyl or 3',5'-di-tert-butyl. Another option is 3'‚5'-di-tert-butyl-4-methoxyphenyl. R ⁵ is a linear or branched C ₁ -C ₆ -alkyl group such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl and tert-butyl, C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group or C6-C20-aryl group. R ⁵ is a preferably a linear or branched C1-C6-alkyl group or C6-C20-aryl group, more preferably a linear C ₁ -C ₄ -alkyl group, even more preferably a C ₁ or C ₂ -alkyl group and most preferably methyl. R ⁶ is a C(R ⁸ ) ₃ group, with R ⁸ being a linear or branched C ₁ -C ₆ -alkyl group. Preferably each R ⁸ are the same or different with R ⁸ being a linear or branched C ₁ - C4-alkyl group, more preferably with R ⁸ being the same and being a C1 or C2-alkyl group. Most preferably, all R ⁸ groups are methyl. In a preferred embodiment, the invention provides a hafnocene complex of formula (II) with each X is a sigma-donor ligand selected from chloro, benzyl and C1-C6-alkyl; R2Si is Me2Si or Et2Si; each R ³ and R ⁴ are independently the same or can be different and are hydrogen, a linear or branched C1-6-alkyl group or -OY group where Y is a C1-C6-alkyl group; wherein (A) at least one R ³ per phenyl group and at least one R ⁴ is not hydrogen, and at least one R ³ per phenyl group and at least one R ⁴ is hydrogen; or (B) at least one R ³ is an -OY group, wherein Y is a is a C1-C6-hydrocarbyl group, in the 4-position of each phenyl ring and the two other R ³ groups are tert-butyl groups; and/or (C) at least one R ⁴ is an -OY group, wherein Y is a is a C1-C6-hydrocarbyl group in the 4-position of the phenyl ring and the two other R ⁴ groups are tert-butyl groups; R ⁵ is a linear or branched C ₁ -C ₆ -alkyl group; R ⁶ is a -C(R ⁸ )3 group, with R ⁸ being a linear or branched C1 or C2-alkyl group. More preferably, the hafnocene complex of the invention is one of formula (III) with each X is the same and is a sigma-donor ligand selected from chloro, benzyl and C1- C6-alkyl; R ₂ Si is Me ₂ Si or Et ₂ Si; each non-hydrogen R ³ is the same and each non-hydrogen R ⁴ is the same; R ³ is hydrogen, a linear or branched C1-C6-alkyl group; R ⁴ is hydrogen, a linear or branched C ₁ -C ₆ -alkyl group; wherein at least one R ³ per phenyl group and at least one R ⁴ is not hydrogen, and wherein at least one R ³ per phenyl group and at least one R ⁴ is hydrogen, R ⁵ is a linear or branched C ₁ -C ₄ -alkyl group; and R ⁶ is a -C(R ⁸ ) ₃ group, with R ⁸ being a linear or branched C ₁ or C ₂ -alkyl group. In a further preferred embodiment, the invention provides a hafnocene complex of formula (IVa) to (lVd)

Formula (IVc) wherein each X is the same and is chloro, benzyl or C1-C6-alkyl, preferably chloro, benzyl or methyl; each R ³ and R ⁴ are independently the same or can be different and are a linear or branched C1-C6-alkyl group. Preferably the R ³ groups are the same. Preferably the R ⁴ groups are the same. Specific hafnocene complexes of the invention include: rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(3’,5’-dime thylphenyl)-1‚5,6‚7- tetrahydro-s-indacen-1-yl] [2-methyl-4-(3’,5’-dimethylphenyI)-5-methoxy-6-tert- butylinden-1-yl] Hafnium dichloride (MC-1), rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(4’-tert-buty lphenyl)-1‚5,6,7- tetrahydro-s-indacen-1-yl][2-methyl-4-(3’,5’-dimethyl-ph enyl)-5-methoxy-6-tert- butylinden-1-yl] Hafnium dichloride (MC-2), rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(4’-tert-buty lphenyl)-1‚5,6,7- tetrahydro-s-indacen-1-yl][2-methyl-4-(4’-tert-butylphenyI )-5-methoxy-6-tert- butylinden-1-yl] Hafnium dichloride (MC-3), rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(3’,5’-dime thylphenyI)-1‚5,6‚7- tetrahydro-s-indacen-1-yl][2-methyl-4-(3’,5’-di-tert-but yl-phenyI)-5-methoxy-6-tert- butylinden-1-yl] Hafnium dichloride (MC-4), rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(3’,5’-dime thylphenyl)-1‚5,6‚7- tetrahydro-s-indacen-1-yl][2-methyl-4-(4’-tert-butylphenyl )-5-methoxy-6-tert- butylinden-1-yl] Hafnium dichloride (MC-5), or For the avoidance of doubt, any narrower definition of a substituent offered above can be combined with any other broad or narrowed definition of any other substituent. Throughout the disclosure above, where a narrower definition of a substituent is presented, that narrower definition is deemed disclosed in conjunction with all broader and narrower definitions of other substituents in the application. The ligands required to form the catalysts of the invention can be synthesised by any process and the skilled organic chemist would be able to devise various synthetic protocols for the manufacture of the necessary ligand materials. WO 2007/116034 discloses the necessary chemistry and is herein incorporated by reference. Synthetic protocols can also generally be found in WO 2002/02576, WO 2011/135004, WO 2012/084961, WO 2012/001052, WO 2011/076780, WO 2015/158790 and WO 2019/179959. The synthesis of the hafnocene complex according to formula (I) is described in detail in WO 2021/058740, which is herein incorporated by reference. Cocatalyst To form an active catalytic species it is normally necessary to employ a cocatalyst as is well known in the art. According to the present invention a cocatalyst system comprising a boron containing cocatalyst as well as an aluminoxane cocatalyst is used in combination with the above defined complex. The aluminoxane cocatalyst can be one of formula (X): where n is usually from 6 to 20 and R has the meaning below. Aluminoxanes are formed on partial hydrolysis of organoaluminum compounds, for example those of the formula AlR3, AlR2Y and Al2R3Y3 where R can be, for example, C ₁ -C ₁₀ alkyl, preferably C ₁ -C ₅ alkyl, or C ₃ -C ₁₀ -cycloalkyl, C ₇ -C ₁₂ -arylalkyl or alkylaryl and/or phenyl or naphthyl, and where Y can be hydrogen, halogen, preferably chlorine or bromine, or C ₁ -C ₁₀ alkoxy, preferably methoxy or ethoxy. The resulting oxygen-containing aluminoxanes are not in general pure compounds but mixtures of oligomers of the formula (X). The preferred aluminoxane is methylaluminoxane (MAO). Since the aluminoxanes used according to the invention as cocatalysts are not, owing to their mode of preparation, pure compounds, the molarity of aluminoxane solutions hereinafter is based on their aluminium content. According to the present invention the aluminoxane cocatalyst is used in combination with a boron containing cocatalyst. Boron based cocatalysts of interest include those of formula (Z) BY ₃ (Z) wherein Y independently is the same or can be different and is a halogen, a halogenated alkylaryl group or a halogenated aryl group, each alkylaryl or aryl group containing from 6 to 20 carbon atoms and at least one fluorine atom as substituent. Preferred examples for Y are p-fluorophenyl, 3,5- difluorophenyl, pentafluorophenyl, 3,4,5-trifluorophenyl and 3,5- di(trifluoromethyl)phenyl. Preferred options are trifluoroborane, tris(4-fluorophenyl)borane, tris(3,5-difluorophenyl)borane, tris(4- fluoromethylphenyl)borane, tris(2,4,6-trifluorophenyl)borane, tris(penta- fluorophenyl)borane, tris(3,5-difluorophenyl)borane and/or tris (3,4,5- trifluorophenyl)borane. Particular preference is given to tris(pentafluorophenyl)borane. However it is preferred that borates are used, i.e. compounds containing a borate anion. Such ionic cocatalysts preferably contain a non-coordinating anion such as tetrakis(pentafluorophenyl)borate. Suitable counterions are protonated amine or aniline derivatives such as methylammonium, anilinium, dimethylammonium, diethylammonium, N- methylanilinium, diphenylammonium, N,N- dimethylanilinium, trimethylammonium, triethylammonium, tri-n-butylammonium, methyldiphenylammonium, pyridinium, p-bromo-N,N- dimethylanilinium or p-nitro- N,N-dimethylanilinium. Preferred ionic compounds which can be used according to the present invention include: tributylammoniumtetra(pentafluorophenyl)borate, tributylammoniumtetra(trifluoromethylphenyl)borate, tributylammoniumtetra(4-fluorophenyl)borate, N,N-dimethylcyclohexylammoniumtetrakis(pentafluorophenyl)bor ate, N,N-dimethylbenzylammoniumtetrakis(pentafluorophenyl)borate, N,N-dimethylaniliniumtetrakis(pentafluorophenyl)borate, N,N-di(propyl)ammoniumtetrakis(pentafluorophenyl)borate, di(cyclohexyl)ammoniumtetrakist(pentafluorophenyl)borate, triphenylcarbeniumtetrakis(pentafluorophenyl)borate, or ferroceniumtetrakis(pentafluorophenyl)borate. Preference is given to triphenylcarbeniumtetrakis(pentafluorophenyl) borate, N,N- dimethylcyclohexylammoniumtetrakis(pentafluorophenyl)borate or N,N- dimethylbenzylammoniumtetrakis(pentafluorophenyl)borate. In particular, triphenylcarbeniumtetrakis(pentafluorophenyl)borate and N,N- dimethylaniliniumtetrakis(pentafluorophenyl)borate are especially preferred. Thus the use of Ph ₃ CB(PhF ₅ ) ₄ and analogues therefore are especially favoured. According to the most preferred embodiment of the present invention, the preferred cocatalysts are alumoxanes, most preferably methylalumoxanes in combination with a borate cocatalyst such as N,N-dimethylammonium- tetrakis(pentafluorophenyl)borate and Ph3CB(PhF5)4. The combination of methylalumoxane and a tritylborate is especially preferred. Suitable amounts of cocatalyst will be well known to the skilled man. The molar ratio of boron to the hafnium ion of the hafnocene may be in the range 0.1:1 to 10:1 mol/mol, preferably 0.3:1 to 7:1 mol/mol, especially 0.5:1 to 5:1 mol/mol. The molar ratio of Al in the aluminoxane to the hafnium ion of the hafnocene may be in the range 1:1 to 2000:1 mol/mol, preferably 10:1 to 1000:1 mol/mol, and more preferably 50:1 to 500:1 mol/mol. The catalyst may contain from 10 to 100 µmol of the hafnium ion of the hafnocene per gram of silica, and 5 to 10 mmol of Al per gram of silica. In the example section it has been found that in multistage polymerisation procedures comprising a gas phase polymerisation stage a higher molar ratio of boron to the hafnium ion results in a higher gas phase split leading to a higher overall productivity while maintaining the same low melt flow rate. Catalyst Manufacture The hafnocene catalyst complex of the present invention can be used in combination with a suitable cocatalyst as a catalyst for the polymerisation of propylene, e.g. in a solvent such as toluene or an aliphatic hydrocarbon, (i.e. for polymerisation in solution), as it is well known in the art. The catalyst of the invention can be used in supported or unsupported form. Preferably, the catalyst system of the invention is used in supported form. The particulate support material used is preferably an organic or inorganic material, such as silica, alumina or zirconia or a mixed oxide such as silica-alumina, in particular silica, alumina or silica-alumina. The use of a silica support is preferred. The skilled man is aware of the procedures required to support a metallocene catalyst. Especially preferably the support is a porous material so that the complex may be loaded into the pores of the support, e.g. using a process analogous to those described in WO 94/14856, WO 95/12622 and WO 2006/097497. The average particle size of the silica support can be typically from 10 to 100 µm. However, it has turned out that special advantages can be obtained if the support has an average particle size from 15 to 80 µm, preferably from 18 to 50 µm. The average pore size of the silica support can be in the range 10 to 100 nm and the pore volume from 1 to 3 mL/g. Examples of suitable support materials are, for instance, ES757 produced and marketed by PQ Corporation, Sylopol 948 produced and marketed by Grace or SUNSPERA DM-L-303 silica produced by AGC Si-Tech Co. Supports can be optionally calcined prior to the use in catalyst preparation in order to reach optimal silanol group content. The use of these supports is routine in the art. In an alternative embodiment, no support is used at all. Such a catalyst can be prepared in solution, for example in an aromatic solvent like toluene, by contacting the hafnocene (as a solid or as a solution) with the cocatalyst, for example methylaluminoxane and/or a borane or a borate salt previously dissolved in an aromatic solvent, or can be prepared by sequentially adding the dissolved catalyst components to the polymerisation medium. In one embodiment, no external carrier is used but the catalyst is still presented in solid particulate form. Thus, no external support material, such as inert organic or inorganic carrier, for example silica as described above is employed, but the solid catalyst is prepared using an emulsion-solidification method. Full disclosure of said method is described in WO 2003/051934, which is herein incorporated by reference. In one embodiment, the preparation of the catalyst system according to the present invention comprises the steps of: a’) reacting a silica support with aluminoxane cocatalyst in a suitable hydrocarbon solvent, such as toluene with optional subsequent washings and drying, to obtain an aluminoxane cocatalyst treated support, b’) reacting the hafnocene complex of formula (I) with a borate cocatalyst and optionally an aluminoxane cocatalyst, in particular methylaluminoxane, in a suitable hydrocarbon solvent, such as toluene or xylene, to obtain a solution of activated hafnocene complex of formula (I), borate cocatalyst and optionally aluminoxane cocatalyst, whereby the borate cocatalyst is added in an amount that a boron/hafnium molar ratio of feed amounts in the range of 0.1:1 to 10:1 is reached, c’) adding the solution obtained in step b’) to the aluminoxane cocatalyst treated support obtained in step a’) wherein the amount of aluminoxane cocatalyst added in step a’) is 75.0 to 100 wt% of the total amount of aluminoxane cocatalyst and the amount of aluminoxane cocatalyst added in step b’) is 0 to 25.0 wt% of the total amount of aluminoxane cocatalyst and d’) optionally drying the so obtained supported catalyst system. In an alternative embodiment, the preparation of the catalyst system according to the present invention comprises the steps of: a) reacting a silica support with an aluminoxane cocatalyst in a suitable hydrocarbon solvent, such as toluene with optional subsequent washings and drying, to obtain aluminoxane cocatalyst treated support, b) reacting the hafnocene complex of formula (I) with an aluminoxane cocatalyst in a suitable hydrocarbon solvent, such as toluene, c) adding borate cocatalyst to the solution obtained in step b) to obtain a solution of hafnocene complex of formula (l), borate cocatalyst and aluminoxane cocatalyst whereby the borate cocatalyst is added in an amount that a boron/hafnium molar ratio of feed amounts in the range of 0.1:1 to 10:1 is reached, d) adding the solution obtained in step c) to the aluminoxane cocatalyst treated support obtained in step a) wherein the amount of aluminoxane cocatalyst added in step a) is 75.0 to 97.0 wt% of the total amount of aluminoxane cocatalyst and the amount of aluminoxane cocatalyst added in step b) is 3.0 to 25.0 wt% of the total amount of aluminoxane cocatalyst and e) optionally drying the so obtained supported catalyst system. Catalyst Prepolymerisation (“Off-line prepolymerisation”) The use of the heterogeneous, non-supported catalysts, (i.e. “self-supported” catalysts) might have, as a drawback, a tendency to dissolve to some extent in the polymerisation media, i.e. some active catalyst components might leach out of the catalyst particles during slurry polymerisation, whereby the original good morphology of the catalyst might be lost. These leached catalyst components are very active possibly causing problems during polymerisation. Therefore, the amount of leached components should be minimized, i.e. all catalyst components should be kept in heterogeneous form. Furthermore, the self-supported catalysts generate, due to the high amount of catalytically active species in the catalyst system, high temperatures at the beginning of the polymerisation which may cause melting of the product material. Both effects, i.e. the partial dissolving of the catalyst system and the heat generation, might cause fouling, sheeting and deterioration of the polymer material morphology. In order to minimise the possible problems associated with high activity or leaching, it is preferred to "prepolymerise" the catalyst before using it in polymerisation process. It has to be noted that prepolymerisation in this regard is part of the catalyst preparation process, being a step carried out after a solid catalyst is formed. This catalyst prepolymerisation step is not part of the actual polymerisation configuration, which might comprise a conventional process prepolymerisation step as well. After the catalyst prepolymerisation step, a solid catalyst is obtained and used in polymerisation. Catalyst "prepolymerisation" takes place following the solidification step of the liquid-liquid emulsion process hereinbefore described. Prepolymerisation may take place by known methods described in the art, such as that described in WO 2010/052263, WO 2010/052260 or WO 2010/052264. Preferable embodiments of this aspect of the invention are described herein. As monomers in the catalyst prepolymerisation step preferably alpha-olefins are used. Preferable C ₂ -C ₁₀ olefins, such as ethylene, propylene, 1-butene, 1-pentene, 1- hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-nonene 1-decene, styrene and vinylcyclohexene are used. Most preferred alpha-olefins are ethylene and propylene. The catalyst prepolymerisation may be carried out in gas phase or in an inert diluent, typically oil or fluorinated hydrocarbon, preferably in fluorinated hydrocarbons or mixture of fluorinated hydrocarbons. Preferably perfluorinated hydrocarbons are used. The melting point of such (per)fluorinated hydrocarbons is typically in the range of 0 to 140°C, preferably 30 to 120°C , like 50 to 110°C . Where the catalyst prepolymerisation is done in fluorinated hydrocarbons, the temperature for the prepolymerisation step is below 70°C, e.g. in the range of -30 to 70°C, preferably 0-65°C and more preferably in the range 20 to 55°C. Pressure within the prepolymerisation vessel is preferably higher than atmospheric pressure to minimize the eventual leaching of air and/or moisture into the catalyst vessel. Preferably the pressure is in the range of at least 1 to 15 bar, preferably 2 to 10 bar. The prepolymerisation vessel is preferably kept in an inert atmosphere, such as under nitrogen or argon or similar atmosphere. Prepolymerisation is continued until the prepolymerisation degree (DP) defined as weight of polymer matrix/weight of solid catalyst before prepolymerisation step is reached. The degree is below 25, preferably 0.5 to 10.0, more preferably 1.0 to 8.0, most preferably 2.0 to 6.0. Use of the catalyst prepolymerisation step offers the advantage of minimising leaching of catalyst components and thus local overheating. After prepolymerisation, the catalyst can be isolated and stored. The hafnocene catalysts used according to the present invention possess excellent catalyst activity and good comonomer response. The catalysts are also able to provide propylene copolymers of high weight average molecular weight Mw. Moreover, the copolymerisation behaviour of hafnocene catalysts used according to the invention shows a reduced tendency of chain transfer to ethylene. Polymers obtained with the hafnocenes of the invention have normal particle morphologies. In general therefore the inventive catalysts can provide: - high activity in bulk propylene polymerisation; - very high molecular weight capability; - improved comonomer incorporation in propylene copolymers; - good polymer morphology. Polymerisation The present invention relates to a process for producing a random copolymer of propylene and at least one comonomer selected from alpha-olefins having from 4 to 12 carbon atoms and optionally ethylene using the specific class of hafnocene complexes in combination with a boron containing cocatalyst as well as with an aluminoxane cocatalyst, as defined above or below. In the following “copolymer” means random copolymer and “terpolymer” means random terpolymer. A random copolymer or terpolymer is a copolymer or terpolymer in which the comonomer units are randomly distributed in the polymer chain. A random copolymer or terpolymer is to distinguish from a block copolymer or terpolymer, in which the comonomer units are arranged in comonomer rich blocks within the polymer chain. Usually random copolymers and terpolymers are characterized by a lower comonomer content compared to block copolymers or terpolymers. Random copolymers and terpolymers are usually monophasic, i.e. they do not contain an elastomeric phase dispersed in a matrix phase, like e.g. heterophasic copolymers or terpolymers. A random copolymer or terpolymer of propylene is a copolymer or terpolymer with a molar majority of propylene monomer units, in which the comonomers are randomly distributed in the polymer chain. The terms “copolymer of propylene and at least one comonomer selected from alpha- olefins having from 4 to 12 carbon atoms” and “propylene copolymer” are used equally in the following for defining the polymer of propylene produced by the process of the invention. The term “copolymer of propylene” is also used is the following as abbreviation for the embodiment of the random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms. The at least one comonomer is selected from alpha-olefins having from 4 to 12 carbon atoms, preferably from alpha-olefins having from 4 to 10 carbon atoms, more preferably from alpha-olefins having from 4 to 8 carbon atoms, such as 1-butene, 1- hexene and 1-octene. Especially preferred are 1-hexene and 1-octene, mostly preferred 1-hexene. The propylene copolymer can comprise more than one of said comonomer as defined such as two, three or four different of said comonomer, such as 1-hexene and 1- octene. In one specific embodiment the propylene copolymer includes propylene monomer units, comonomer units selected from at least one, preferably one, alpha-olefin having from 4 to 12 carbon atoms as defined above and ethylene comonomer units. In this embodiment the propylene copolymer is a terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms It is, however, preferred that the propylene copolymer only includes one of said comonomers as defined above. The process can be a one-stage process in which the propylene copolymer is produced in one polymerisation reactor. The process can also be a multistage polymerisation process comprising at least two reactors connected in series preferably including a gas phase polymerisation step. Polymerisation in the process of the invention may be effected in at least two or more, e.g.2, 3 or 4, polymerisation reactors connected in series of which at least one reactor is preferably a gas phase reactor. The process may also involve a prepolymerisation step. This prepolymerisation step is a conventional step used routinely in polymer synthesis and is to be distinguished from the catalyst prepolymerisation step discussed above. Preferably, the process of the invention employs one reactor or two reactors wherein for the latter case at least one reactor of the two reactors is a gas phase reactor. For producing the propylene copolymer the process of the invention preferably employs one reactor, suitably for producing a unimodal propylene copolymer, or two reactors connected in series wherein at least one reactor is a gas phase reactor, suitably for producing a bimodal propylene copolymer. For the case of producing a multimodal propylene copolymer the process according to the invention can also employ three or more reactors connected in series wherein at least one reactor is a gas phase reactor. Ideally the process of the invention for producing the propylene copolymer employs a first reactor operating in bulk and optionally a second reactor being a gas phase reactor. Any optional additional subsequent reactor after the second reactor is preferably a gas phase reactor. The process may also utilise a prepolymerisation step. Bulk reactions may take place in a loop reactor. For bulk and gas phase copolymerisation reactions, the reaction temperature used will generally be in the range 60 to 115°C (e.g.70 to 90°C), the reactor pressure will generally be in the range 10 to 25 bar for gas phase reactions with bulk polymerisation operating at higher pressures. The residence time will generally be 0.25 to 8 hours (e.g.0.5 to 4 hours). The gas used will be the monomer optionally as mixture with a non-reactive gas such as nitrogen or propane. It is a particular feature of the invention that polymerisation takes place at temperatures of at least 60°C. Generally the quantity of catalyst used will depend upon the nature of the catalyst, the reactor types and conditions and the properties desired for the polymer product. As is well known in the art hydrogen can be used for controlling the molecular weight of the polymer. Splits between the various reactors can vary. When two reactors are used, splits are generally in the range of 30 to 70 wt% to 70 to 30 wt% bulk to gas phase, preferably 40 to 60 to 60 to 40 wt%. Where three reactors are used, it is preferred that each reactor preferably produces at least 20 wt% of the polymer, such as at least 25 wt%. The sum of the polymer produced in gas phase reactors should preferably exceed the amount produced in bulk. In one embodiment of the present invention the process comprises the following steps: a) introducing propylene monomer units, alpha-olefin comonomer units having from 4 to 12 carbon atoms, optionally ethylene comonomer units, and hydrogen into a polymerisation reactor; b) polymerizing the propylene monomer units, optional ethylene comonomer units, and alpha-olefin comonomer units having from 4 to 12 carbon atoms to form a copolymer of propylene and at least one comonomer selected from alpha-olefins having from 4 to 12 carbon atoms in the presence of the single-site catalyst. This embodiment is especially suitable for the production of a unimodal propylene copolymer. In another embodiment the process may further comprise the following steps: c) transferring the polymerisation mixture from process step b) comprising the copolymer of propylene and at least one comonomer selected from alpha-olefins having from 4 to 12 carbon atoms and the single site catalyst into a second polymerisation reactor; d) introducing propylene monomer units, optionally alpha-olefin comonomer units having from 4 to 12 carbon atoms and hydrogen into said second polymerisation reactor; e) polymerizing the propylene monomer units and optionally alpha-olefin comonomer units having from 4 to 12 carbon atoms, optionally ethylene comonomer units and optionally hydrogen to form a second polymer of propylene which is selected from a propylene homopolymer or a copolymer of propylene and at least one comonomer alpha-olefin having from 4 to 12 carbon atoms in the presence of the single-site catalyst and the copolymer of propylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms of process step b) in the presence of the single-site catalyst. Said embodiment is especially suitable for the production of a bimodal or multimodal propylene copolymer. Thereby, in the second polymerisation reactor a propylene homopolymer can be polymerized so that the propylene copolymer polymerized according to the process of said embodiment comprises a copolymer component of propylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms and a propylene homopolymer component. It is, however, preferred that in the second polymerisation reactor a copolymer component of propylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms is polymerized so that the propylene copolymer polymerized according to the process of said embodiment comprises two copolymer components of propylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms. The two copolymer components of propylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms can comprise the same comonomer or different comonomers. The two copolymer components of propylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms can differ in their molecular weight, such as their weight average molecular weight Mw and their melt flow rate MFR2. In the embodiment of a terpolymer of propylene, ethylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms the process as described above can be adjusted as such that in one of the two process steps b) or e) the alpha-olefin comonomer units having from 4 to 12 carbon atoms are replaced with ethylene monomer units so that in said polymerisation stage a copolymer of propylene and ethylene is produced. Alternatively, in the embodiment of propylene, ethylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms the process as described above can be adjusted as such that in one of the two process steps b) or e) the alpha-olefin comonomer units having from 4 to 12 carbon atoms and ethylene monomer units are added to the polymerisation mixture so that in said polymerisation stage a terpolymer of propylene, ethylene and at least one comonomer selected alpha-olefins from having from 4 to 12 carbon atoms is produced. During the polymerisation process hydrogen is added. Hydrogen can be added to each polymerisation stage of a multistage polymerisation process. Thereby, the amount of hydrogen can be individually adjusted for each polymerisation stage. It has been found that despite the presence of hydrogen during the polymerisation process propylene copolymers with a high weight average molecular weight and a low melt flow rate can be produced. Additionally, a high amount of hydrogen increases the catalyst activity and productivity. During polymerisation the single site catalyst preferably has an overall catalyst productivity, determined with respect to the catalyst, preferably of at least 20.0 kg of propylene polymer per g of the catalyst (kg/g _catalyst ), more preferably at least 20.5 kg/gcatalyst, most preferably at least 21.0 kg/gcatalyst. Usually the overall catalyst productivity does not exceed 50.0 kg/gcatalyst. The overall catalyst productivity is determined over all polymerisation stages. During polymerisation the single site catalyst preferably has an overall metallocene productivity, determined with respect to the hafnocene catalyst complex, preferably of at least 700 kg of propylene polymer per g of the metallocence (kg/g _metallocene ), more preferably at least 800 kg/gmetallocene, most preferably at least 850 kg/gmetallocene. Usually the overall catalyst productivity does not exceed 2500 kg/gmetallocene. The overall metallocene productivity is determined over all polymerisation stages. Polymer The present invention also relates to a random copolymer of propylene and at least one comonomer selected from alpha-olefins having from 4 to 12 carbon atoms or a random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms obtainable from the process according to the invention as described above and below. Thereby, the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms follows the following relation (A) in behalf of its polymerisation process: metallocene productivity / MFR21 ≥ 15 [kg/g / g/10 min] (A) with metallocene productivity overall productivity of the single site catalyst as kg copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms formed per g metallocene; MFR21 melt flow rate in g/10 min of the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms, determined according to ISO 1133 at a temperature of 230°C and a load of 21.6 kg. Preferably the copolymer of propylene follows the following relation (A1) in behalf of its polymerisation process: metallocene productivity / MFR21 ≥ 20 [kg/g / g/10 min] (A1) More preferably the copolymer of propylene follows the following relation (A2) in behalf of its polymerisation process: metallocene productivity / MFR21 ≥ 24 [kg/g / g/10 min] (A2) The relation (A), preferably relation (A1) more preferably relation (A2) shows that a propylene copolymer with a rather low melt flow rate MFR21, which translates to a high molecular weight, can be produced at high metallocene productivity. A corresponding correlation can preferably be observed when comparing the weight average molecular weights Mw of the propylene copolymers with the metallocene productivity during polymerisation. Preferably, the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms follows the following relation (B) in behalf of its polymerisation process: Mw [(kg/mol) ^-1 ] ≥ -0.3 ∙ metallocene productivity [(kg/g) ^-1 ] + 805 (B) with Mw weight average molecular weight of the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms in kg/mol, determined by GPC metallocene productivity overall productivity of the single site catalyst as kg copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms formed per g metallocene. Preferably the copolymer of propylene follows the following relation (B1) in behalf of its polymerisation process: Mw [(kg/mol) ^-1 ] ≥ -0.3 ∙ metallocene productivity [(kg/g) ^-1 ] + 815 (B1) More preferably the copolymer of propylene follows the following relation (B2) in behalf of its polymerisation process: Mw [(kg/mol) ^-1 ] ≥ -0.3 ∙ metallocene productivity [(kg/g) ^-1 ] + 830 (B2) The relation (B), preferably relation (B1) more preferably relation (B2) shows that a propylene copolymer with high weight average molecular weight Mw can be produced at high metallocene productivity. The at least one comonomer is selected from alpha-olefins having from 4 to 12 carbon atoms, preferably from alpha-olefins having from 4 to 10 carbon atoms, more preferably from alpha-olefins having from 4 to 8 carbon atoms, such as 1-butene, 1- hexene and 1-octene. Especially preferred are 1-hexene and 1-octene, mostly preferred 1-hexene. The propylene copolymer can comprise more than one of said comonomer as defined such as two, three or four different of said comonomer, such as 1-hexene and 1- octene. In one specific embodiment the propylene copolymer includes propylene monomer units, comonomer units selected from at least one, preferably one, alpha-olefin having from 4 to 12 carbon atoms as defined above and ethylene comonomer units. In this embodiment the propylene copolymer is a terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms. It is, however, preferred that the propylene copolymer only includes one of said comonomers as defined above. Thus, it is particularly preferred that the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms is a copolymer of propylene and 1-hexene or a copolymer of propylene and 1-octene. Preferably the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms has a comonomer content of from 0.1 to 5.0 mol%, more preferably of from 0.2 to 4.0 mol%, still more preferably of from 0.3 to 3.0 mol% and most preferably of from 0.5 to 2.5 mol%, based on the total weight of the copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms. For the embodiment of the terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms the total comonomer content of comonomer selected from alpha olefins having from 4 to 12 carbon atoms and ethylene is preferably in the range of from 0.1 to 5.0 mol%, more preferably of from 0.2 to 4.0 mol%, still more preferably of from 0.3 to 3.0 mol% and most preferably of from 0.5 to 2.5 mol%, based on the total weight of the terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms. The copolymer of propylene is preferably monophasic. This excludes heterophasic propylene copolymers, which comprise a matrix phase and an elastomeric phase dispersed in said matrix phase. The propylene copolymer preferably has a melt flow rate MFR2 of from 0.01 to 0.50 g/10 min, more preferably in the range of 0.02 to 0.45 g/10 min, more preferably in the range of 0.05 to 0.40 g/10 min. The propylene copolymer preferably has a melt flow rate MFR ₂₁ of from 1.0 to 50 g/10 min, more preferably in the range of 2 to 45 g/10 min, more preferably in the range of 5 to 40 g/10 min. Further, the propylene copolymer preferably has a weight average molecular weight Mw of at least 500 kg/mol, preferably at least 550 kg/mol and more preferably of at least 600 kg/mol up to 2000 kg/mol, preferably up to 1500 kg/mol and more preferably up to 1000 kg/mol, depending on the use and amount of hydrogen used as Mw regulating agent. Still further, the polydispersity index (PDI; Mw/Mn as measured with GPC) of the propylene copolymer can be relatively broad, i.e. the Mw/Mn can be up to 7.0. Preferably the Mw/Mn is in a range of from 2.0 to 7.0, more preferably from 2.3 to 6.5 and even more preferably from 2.5 to 6.0. Further the propylene copolymer preferably has a melting temperature Tm of from 135 to 145°C, more preferably from 137 to 143°C. Still further, the propylene copolymer preferably has a crystallization temperature Tc of from 95 to 110°C, more preferably from 98 to 107°C. It is preferred that the propylene copolymer has a content of 2,1 regiodefects of from 0.10 to 0.50 mol%, more preferably from 0.15 to 0.40 mol%. The propylene copolymers are suitable for applications which are in need for propylene copolymers with high molecular weights, such as pipe applications. Use The present invention further relates to the use of a single-site catalyst comprising (i) a complex of formula (I) wherein each X independently is a sigma-donor ligand; in the group R ₂ Si- at least one R is methyl or ethyl, and the other R is selected from methyl, ethyl, n-propyl, isopropyl, n-butyl or isobutyl, pentyl, hexyl, cyclohexyl and phenyl; each R ¹ independently is the same or can be different and are a CH ₂ -R ⁷ group, with R ⁷ being H or linear or branched C1-C6-alkyl group, C3-C8-cycloalkyl group, or C ₆ -C ₁₀ -aryl group; each R ² is independently a –CH=, -CY=, -CH ₂ -‚ -CHY- or -CY ₂ - group, wherein Y is a C1-C6-hydrocarbyl group and where n is 2-6; each R ³ and R ⁴ are independently the same or can be different and are hydrogen, a linear or branched C ₁ -C ₆ -alkyl group, a C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group, C6-C20-aryl group, or an -OY group, wherein Y is a is a C1-C6- hydrocarbyl group; R ⁵ is a linear or branched C ₁ -C ₆ -alkyl group, C ₇ -C ₂₀ -arylalkyl, C ₇ -C ₂₀ -alkylaryl group or C6-C20-aryl group; and R ⁶ is a C(R ⁸ )3 group, with each R ⁸ being independently a linear or branched C1- C ₆ -alkyl group; (A) wherein at least one R ³ per phenyl group and at least one R4 is not hydrogen, and wherein at least one R ³ per phenyl group and at least one R ⁴ is hydrogen; or (B) wherein one R ³ is an -OY group, wherein Y is a is a C ₁ -C ₆ -hydrocarbyl group, in 4-position of each phenyl group and the two other R ³ groups are tert-butyl groups; and/or (C) wherein one R ⁴ is an -OY group, wherein Y is a is a C1-C6-hydrocarbyl group, in 4-position of the phenyl ring and the two other R ⁴ groups are tert- butyl groups; and (ii) a cocatalyst system comprising a boron containing cocatalyst and an aluminoxane cocatalyst for the production of a random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms or a random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms as defined above or below. Thereby, the single-site catalyst, the random copolymer of propylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms and the random terpolymer of propylene, ethylene and at least one comonomer selected from alpha olefins having from 4 to 12 carbon atoms include all embodiments as described above or below. The invention will now be illustrated by reference to the following non-limiting Examples Examples Analytical tests Measurement methods: Al, Zr and Hf determination (ICP-method) In a glovebox, an aliquot of the catalyst (ca.40 mg) was weighed into glass weighting boat using analytical balance. The sample was then allowed to be exposed to air overnight while being placed in a steel secondary container equipped with an air intake. Then 5 mL of concentrated (65 %) nitric acid was used to rinse the content of the beat into the Xpress microwave oven vessel (20 mL). A sample was then subjected to a microwave-assisted digestion using MARS 6 laboratory microwave unit over 35 minutes at 150 °C. The digested sample was allowed to cool down for at least 4 h and then was transferred into a glass volumetric glass flask of 100 mL volume. Standard solutions containing 1000 mg/L Y and Rh (0.4 mL) were added. The flask was then filled up with distilled water and shaken well. The solution was filtered through 0.45 pm Nylon syringe filters and then subjected to analysis using Thermo iCAP 6300 lCP-OES and iTEVA software. The instrument was calibrated for AI, B, Hf, Mg, Ti and Zr using a blank (a solution of 5 % HNO3) and six standards of 0.005 mg/L, 0.01 mg/L, 0.1 mg/L, 1 mg/L, 10 mg/L and 100 mg/L of Al, B, Hf, Mg, Ti and Zr in solutions of 5 % HNO3 distilled water. However, not every calibration point was used for each wavelength. Each calibration solution contained 4 mg/L of Y and Rh standards. Al 394.401 nm was calibrated using the following calibration points: blank, 0.1 mg/L, 1 mg/L, 10 mg/L and 100 mg/L. Al 167.079 nm was calibrated as Al 394.401 nm excluding 100 mg/L and Zr 339.198 nm using the standards of blank, 0.01 mg/L, 0.1 mg/L, 1 mg/L, 10 mg/L and 100 mg/L. Curvilinear fitting and 1/concentration weighting was used for the calibration curves. Hf 264.141 nm was calibrated using the standards of blank, 0.01 mg/L, 0.1 mg/L, 1 mg/L, 10 mg/L and 100 mg/L. Immediately before analysis the calibration was verified and adjusted (instrument reslope function) using the blank and a 10 mg/L Al, B, Hf, Mg, Ti and Zr standard which had 4 mg/L Y and Rh. A quality control sample (QC: 1 mg/L AI, Au, Be, Hg & Se; 2 mg/L Hf & Zr, 2.5 mg/L As, B, Cd, Co, Cr, Mo, Ni, P, Sb, Sn & V; 4 mg/L Rh & Y ; 5 mg/L Ca, K, Mg, Mn, Na & Ti; 10 mg/L Cu, Pb and Zn; 25 mg/L Fe and 37.5 mg/L Ca in a solution of 5 % HNO ₃ in distilled water) was run to confirm the reslope for AI, B, Hf, Mg, Ti and Zr. The QC sample was also run at the end of a scheduled analysis set. The content for Zr was monitored using Zr 339.198 nm {99} line. Content of Hf was monitored using line Hf 264.141 nm {128}. The content of aluminium was monitored via the 167.079 nm {502} line, when Al concentration in test portion was under 2 wt% and via the 394.401 nm {85} line for Al concentrations above 2 wt%. Y 371.030 nm {91} was used as internal standard for Zr 339.198 nm and A| 394.401 nm and Y 224.306 nm {450} for Al 167.079 nm. The reported values were back calculated to the original catalyst sample using the original mass of the catalyst aliquot and the dilution volume. GPC: Molecular weight averages, molecular weight distribution, and polydispersity index (M _n , M _w , M _w /M _n ) Molecular weight averages (Mw, Mn), Molecular weight distribution (MWD) and its broadness, described by polydispersity index, PDI= Mw/Mn (wherein Mn is the number average molecular weight and Mw is the weight average molecular weight) were determined by Gel Permeation Chromatography (GPC) according to ISO 16014-4:2003 and ASTM D 6474-99. A PolymerChar GPC instrument, equipped with infrared (IR) detector was used with 3 x Olexis and 1x Olexis Guard columns from Polymer Laboratories and 1,2,4- trichlorobenzene (TCB, stabilized with 250 mg/L 2,6-Di tert butyl-4-methyl-phenol) as solvent at 160 °C and at a constant flow rate of 1 mL/min.200 μL of sample solution were injected per analysis. The column set was calibrated using universal calibration (according to ISO 16014-2:2003) with at least 15 narrow MWD polystyrene (PS) standards in the range of 0,5 kg/mol to 11500 kg/mol. Mark Houwink constants for PS, PE and PP used are as described per ASTM D 6474-99. All samples were prepared by dissolving 5.0 – 9.0 mg of polymer in 8 mL (at 160 °C) of stabilized TCB (same as mobile phase) for 2.5 hours for PP or 3 hours for PE at max.160°C under continuous gentle shaking in the autosampler of the GPC instrument Quantification of copolymer microstructure by ¹³ C-NMR spectroscopy Comonomer content (ethylene) Quantitative ¹³ C { ¹ H} NMR spectra were recorded in the solution-state using a Bruker Advance III 400 NMR spectrometer operating at 400.15 and 100.62 MHz for ¹ H and ¹³ C respectively. All spectra were recorded using a ¹³ C optimised 10 mm extended temperature probehead at 125 °C using nitrogen gas for all pneumatics. Approximately 200 mg of material was dissolved in 3 ml of 1,2-tetrachloroethane-d2 (TCE-d2) along with chromium-(III)-acetylacetonate (Cr(acac)3) resulting in a 65 mM solution of relaxation agent in solvent as described in G. Singh, A. Kothari, V. Gupta, Polymer Testing 2009, 28(5), 475. To ensure a homogenous solution, after initial sample preparation in a heat block, the NMR tube was further heated in a rotatory oven for at least 1 hour. Upon insertion into the magnet the tube was spun at 10 Hz. This setup was chosen primarily for the high resolution and quantitatively needed for accurate ethylene content quantification. Standard single-pulse excitation was employed without NOE, using an optimised tip angle, 1 s recycle delay and a bi-level WALTZ16 decoupling scheme as described in Z. Zhou, R. Kuemmerle, X. Qiu, D. Redwine, R. Cong, A. Taha, D. Baugh, B. Winniford, J. Mag. Reson.187 (2007) 225 and V. Busico, P. Carbonniere, R. Cipullo, C. Pellecchia, J. Severn, G. Talarico, Macromol. Rapid Commun.2007, 28, 1128. A total of 6144 (6 k) transients were acquired per spectra. Quantitative ¹³ C { ¹ H}NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals. All chemical shifts were indirectly referenced to the central methylene group of the ethylene block (EEE) at 30.00 ppm using the chemical shift of the solvent. This approach allowed comparable referencing even when this structural unit was not present. With characteristic signals corresponding to 2,1 erythro regio defects observed (as described in L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev.2000, 100 (4), 1253, in Cheng, H. N., Macromolecules 1984, 17, 1950, and in W-J. Wang and S. Zhu, Macromolecules 2000, 331157) the correction for the influence of the regio defects on determined properties was required. Characteristic signals corresponding to other types of regio defects were not observed. Characteristic signals corresponding to the incorporation of ethylene were observed (as described in Cheng, H. N., Macromolecules 1984, 17, 1950) and the comonomer fraction calculated as the fraction of ethylene in the polymer with respect to all monomer in the polymer. The comonomer fraction was quantified using the method of W-J. Wang and S. Zhu, Macromolecules 2000, 331157, through integration of multiple signals across the whole spectral region in the ¹³ C { ¹ H} spectra. This method was chosen for its robust nature and ability to account for the presence of regio-defects when needed. Integral regions were slightly adjusted to increase applicability across the whole range of encountered comonomer contents. Comonomer content (1-hexene) Quantitative ¹³ C{ ¹ H} NMR spectra recorded in the molten-state using a Bruker Avance III 500 NMR spectrometer operating at 500.13 and 125.76 MHz for ¹ H and ¹³ C respectively. All spectra were recorded using a ¹³ C optimised 7 mm magic-angle spinning (MAS) probehead at 180°C using nitrogen gas for all pneumatics. Approximately 200 mg of material was packed into a 7 mm outer diameter zirconia MAS rotor and spun at 4 kHz. This setup was chosen primarily for the high sensitivity needed for rapid identification and accurate quantification.(Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys.2006;207:382., Parkinson, M., Klimke, K., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys.2007;208:2128., Castignolles, P., Graf, R., Parkinson, M., Wilhelm, M., Gaborieau, M., Polymer 50 (2009) 2373). Standard single-pulse excitation was employed utilising the NOE at short recycle delays of 3s (Klimke, K., Parkinson, M., Piel, C., Kaminsky, W., Spiess, H.W., Wilhelm, M., Macromol. Chem. Phys.2006;207:382., Pollard, M., Klimke, K., Graf, R., Spiess, H.W., Wilhelm, M., Sperber, O., Piel, C., Kaminsky, W., Macromolecules 2004;37:813.). and the RS-HEPT decoupling scheme (Filip, X., Tripon, C., Filip, C., J. Mag. Resn. 2005, 176, 239., Griffin, J.M., Tripon, C., Samoson, A., Filip, C., and Brown, S.P., Mag. Res. in Chem.200745, S1, S198). A total of 16384 (16k) transients were acquired per spectra. Quantitative ¹³ C{ ¹ H} NMR spectra were processed, integrated and relevant quantitative properties determined from the integrals. All chemical shifts are internally referenced to the methyl isotactic pentad (mmmm) at 21.85 ppm. Characteristic signals corresponding to the incorporation of 1-hexene were observed and the comonomer content quantified in the following way. The amount of 1-hexene incorporated in PHP isolated sequences was quantified using the integral of the αB4 sites at 44.2 ppm accounting for the number of reporting sites per comonomer: H = IαB4 / 2 The amount of 1-hexene incorporated in PHHP double consecutive sequences was quantified using the integral of the ααB4 site at 41.7 ppm accounting for the number of reporting sites per comonomer: HH = 2 * IααB4 When double consecutive incorporation was observed the amount of 1-hexene incorporated in PHP isolated sequences needed to be compensated due to the overlap of the signals αB4 and αB4B4 at 44.4 ppm: H = (IαB4 – 2 * IααB4) / 2 The total 1-hexene content was calculated based on the sum of isolated and consecutively incorporated 1-hexene: Htotal = H + HH When no sites indicative of consecutive incorporation observed the total 1-hexeen comonomer content was calculated solely on this quantity: Htotal = H Characteristic signals indicative of regio 2,1-erythro defects were observed ( Resconi, L., Cavallo, L., Fait, A., Piemontesi, F., Chem. Rev.2000, 100, 1253). The presence of 2,1-erythro regio defects was indicated by the presence of the Pαβ (21e8) and Pαγ (21e6) methyl sites at 17.7 and 17.2 ppm and confirmed by other characteristic signals. The total amount of secondary (2,1-erythro) inserted propene was quantified based on the αα21e9 methylene site at 42.4 ppm: P21 = Iαα21e9 The total amount of primary (1,2) inserted propene was quantified based on the main Sαα methylene sites at 46.7 ppm and compensating for the relative amount of 2,1- erythro, αB4 and ααB4B4 methylene unit of propene not accounted for (note H and HH count number of hexene monomers per sequence not the number of sequences): P12 = ISαα + 2*P21 + H + HH / 2 The total amount of propene was quantified as the sum of primary (1,2) and secondary (2,1-erythro) inserted propene: Ptotal = P12 + P21 = ISαα + 3* Iαα21e9 + (IαB4 – 2 * IααB4) / 2 + IααB4 This simplifies to: Ptotal = I _S αα + 3* Iαα21e9 + 0.5*IαB4 The total mole fraction of 1-hexene in the polymer was then calculated as: fH = Htotal / ( Htotal + Ptotal) The full integral equation for the mole fraction of 1-hexene in the polymer was: fH = (((IαB4 – 2 * IααB4) / 2) + (2 * IααB4)) /((I _S αα + 3* Iαα21e9 + 0.5*IαB4 ) + ((IαB4 – 2 * IααB4) / 2) + (2 * IααB4)) This simplifies to: fH = (IαB4/2 + IααB4) / (I _S αα + 3* Iαα21e9 + IαB4 + IααB4) The total comonomer incorporation of 1-hexene in mole percent was calculated from the mole fraction in the usual manner: H [mol%] = 100 * fH The total comonomer incorporation of 1-hexene in weight percent was calculated from the mole fraction in the standard manner: H [wt%] = 100 * ( fH * 84.16) / ( (fH * 84.16) + ((1 - fH) * 42.08) ) Melt Flow Rate (MFR) The melt flow rate (MFR) or melt index (MI) is measured according to ISO 1133. Where different loads can be used, the load is normally indicated as the subscript, for instance, MFR2 which indicates 2.16 kg load, or MFR21 which indicates 21.6 kg load. The temperature is selected according to ISO 1133 for the specific polymer, for instance, 230 °C for polypropylene. Thus, for polypropylene MFR ₂ is measured at 230 °C temperature and under 2.16 kg load and MFR21 is measured at 230 °C temperature and under 21.6 kg load. DSC analysis DSC analysis was measured With a Mettler TA Instrument Q2000 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 11357 / part 3 /method C2 in a heat / cool / heat cycle with a scan rate of 10 °C/min in the temperature range of -30 to +225°C. Crystallization temperature (Tc) is determined from the cooling step, while main melting temperature (Tm) and heat of melting (Hm) are determined from the second heating step. The Sealing Initiation Temperature (SIT) was predicted by analyzing the second heating scan according to the following procedure: the first limit for integration was set at 16°C, the second limit at Tm+20°C, and the total melting enthalpy was registered. The temperature T1 is defined as the temperature at which 19% of this melting enthalpy with the abovementioned limits for integration was obtained. The parameter SIT is finally calculated as: SIT=1.0596 × T1 + 3.8501 Catalyst Activity The catalyst activity was calculated on the basis of following formula: amount of polymer produced (kg) Catalyst Activity (kg-PP/g-Cat/h) = catalyst loading (g) × polymerisation time (h) Productivity Overall productivity was calculated as amount of polymer produced (kg) Catalyst Productivity (kg-PP/g) = catalyst loading (g) For both the catalyst activity and the productivity the catalyst loading is either the grams of prepolymerized catalyst or the grams of metallocene present in that amount of prepolymerized catalyst. Prepolymerisation degree (DP): weight of polymer /weight of solid catalyst before prepolymerisation step The composition of the catalysts (before the off-line prepolymerisation step) has been determined by ICP as described above. The metallocene content of the prepolymerized catalysts has been calculated from the ICP data as follows: Equation 1 Equation 2 Equation 3 Equation 4 Examples Catalysts IC1 Inventive catalyst system IC1 comprises the hafnocene complex Anti- dimethylsilanediyl[2-methyl-4‚8-bis(3‚5-dimethylphenyl)- 1‚5,6‚7-tetrahydro-s- indacen-1-yl][2-methyl-4-(3‚5-dimethylphenyl)-5-methoxy-6- tert-butyl-1H-inden-1- yl] hafnium dichloride (MC-1). The synthesis of the hafnocene complex is disclosed in the example section of WO 2021/058740 as MC1. IC1 is prepared as described in detail in the example section of WO 2021/058740 as catalyst IE1a. IC1 comprises MAO and trityl tetrakis(pentafluorophenyl)borate cocatalyst system. IC2 Inventive catalyst system IC2 differs from IC1 in a higher weight amount of hafnocene complex in the catalyst system. IC3 Inventive catalyst system IC3 differs from IC1 in a higher boron to transition metal (Hf) molar ratio in the catalyst system. CC1 Comparative catalyst system CC1 was prepared as described in detail in WO 2015/011135 A1 (metallocene complex MC1 with methylaluminoxane (MAO) and borate resulting in Catalyst 3 described in WO 2015/011135 A1) with the proviso that the surfactant is 2,3,3,3-tetrafluoro-2-(1,1,2,2,3,3,3-heptafluoropropoxy)-1- propanol. The metallocene complex (MC1 in WO 2015/011135 A1) was prepared as described in WO 2013/007650 A1 (metallocene E2 in WO 2013/007650 A1). CC2 Comparative catalyst system CC2 comprises the zirconocene complex Anti- dimethylsilanediyl[2-methyl-4,8-di(3,5-dimethylphenyl)-1,5,6 ,7-tetrahydro-s- indacen-1-yl][2-methyl-4-(3,5-dimethylphenyl)-5-methoxy-6-te rt-butylinden-1-yl] zirconium dichloride (MC-6) as disclosed in WO 2019/179959 A1 as MC-2 CC2 comprises MAO and trityl tetrakis(pentafluorophenyl)borate cocatalyst system, and was prepared as follows: In a nitrogen filled glovebox, a solution of MAO 0.2 mL (30% wt in toluene, AXION 1330 CA Lanxess) in dry toluene (2.3 mL) was added to 75 mg of metallocene MC-6. The mixture was stirred for 30 minutes at room temperature. Then, 75 mg of trityl tetrakis(pentafluorophenyl)borate (TB) was added and the mixture was stirred for an additional 30 min. Next, 2.0 g of MAO treated silica prepared as described above, was placed in a glass vial. The above solution of metallocene, TB and MAO in toluene was then slowly added to the support over the course of 5 minutes with gentle mixing. The resulting mixture was allowed to rest overnight. The resulting solid was dried under vacuum for 2 h and 40 min to yield the catalyst as a red, free flowing powder. CC3 Comparative catalyst system CC3 differs from CC2 in a lower weight amount of zirconocene complex in the catalyst system, and was prepared as follows: In a nitrogen filled glovebox, a solution of MAO 0.2 mL (30% wt in toluene, AXION 1330 CA Lanxess) in dry toluene (2.3 mL) was added to 50 mg of metallocene MC-6. The mixture was stirred for 30 minutes at room temperature. Then, 52 mg of TB was added and the mixture was stirred for an additional 30 min. Next, 2.0 g of MAO treated silica prepared as described above, was placed in a glass vial. The above solution of metallocene, TB and MAO in toluene was then slowly added to the support over the course of 5 minutes with gentle mixing. The resulting mixture was allowed to rest overnight. The resulting solid was dried under vacuum for 2 h and 40 min to yield the catalyst as a red, free flowing powder. CC4 Comparative catalyst system CC4 comprises the same zirconocene complex as CC2 and was prepared according to ICS3 of WO2020/239602. Comparative catalyst system CC4 differs from CC3 and CC2 in a lower weight amount of zirconocene complex in the catalyst system. In Table 1 the Al (wt.%), metallocene MC (wt.%), B (wt.%), Al/TM (transition metal) ratio and B/TM (transition metal) ratio for the catalysts IC1, IC2, IC3, CC1 and CC2 are shown. Table 1: based on ICP * metallocene content in catalyst before the offline prepolymerisation step (see below) Off-line prepolymerisation procedure The catalyst CC1 was offline prepolymerized according to the following procedure: The pre-polymerisation experiment was done in a 125 mL pressure reactor equipped with gas-feeding lines and an overhead stirrer. Dry and degassed perfluoro-1.3- dimethylcyclohexane (15 cm ³ ) and the desired amount of the catalyst to be pre- polymerized were loaded into the reactor inside a glove box and the reactor was sealed. The reactor was then taken out from the glove box and placed inside a water cooled bath kept at 25 °C. The overhead stirrer and the feeding lines were connected and stirring speed set to 450 rpm. The experiment was started by opening the propylene feed into the reactor. The total pressure in the reactor was raised to about 5 barg and held constant by propylene feed via mass flow controller until the target degree of polymerisation of 6.5 was reached. The reaction was stopped by flashing the volatile components. Inside glove box, the reactor was opened and the content poured into a glass vessel. The perfluoro-1,3-dimethylcyclohexane was evaporated until a constant weight was obtained to yield the pre-polymerized catalyst. Polymerisation Examples Monomers Ethylene was purified in columns filled with molecular sieves 3A EPG 1/16, PolyMax 301 T-4427B and Selexsorb COS. Hydrogen (quality 6.0) was supplied by Air Liquide Propylene: quality 2.3; purified via columns filled with PolyMax301 T-4427B (60°C; Cu/CuO), MS13X-APG 1/16 and Selexsorb COS. 1-Hexene: supplier INEOS alpha olefins; purified by purging with N2 and dried over 3Å molecular sieves (Type MB-KOL-MT2-350) in the M-Braun solvent station. The GC-FID and GC-MS analysis of 1-hexene (N15089) gave a purity of 99.4 %, main impurities being unreactive hexene isomers. Hexene feed Hexene was purified in the M-Braun solvent station, transferred via fixed line to the syringe pump, and then fed via the syringe pump into the reactor at 71 °C. Polymerisations All bench scale experiments were performed in a stirred autoclave equipped with a ribbon stirrer and a total volume of 21 dm³. 1. 1-step propylene/hexene copolymerisation (liquid monomer) A stainless-steel reactor equipped with a ribbon stirrer, with a total volume of 20.9 dm³ containing 0.2 bar-g propylene, was filled with additional 4.45 kg propylene and the chosen amount of 1-hexene. Triethylaluminium (0.8 ml of 0.62 molar solution in n-heptane) was added using a stream of 250 g propylene, then H ₂ was added via mass flow controller. The reactor temperature was stabilized at 20 °C (HB-Therm) and the solution was stirred and 250 rpm for at least 20 min. Then the catalyst was injected as described in the following. The desired amount of solid, prepolymerised catalyst was loaded into a 5 ml stainless steel vial and a second 5 ml vial containing 4 ml n-heptane was added on top inside a glovebox. Then the vial on top was pressurized with 10 bars of nitrogen. This dual feeder system was mounted on a port on the lid of the autoclave. The valve between the two vials was opened and the solid catalyst was contacted with n-heptane under N2 pressure for 2 s, and then flushed into the reactor with 250 g propylene. Stirring speed was kept at 250 rpm and pre-polymerisation was run for 10 minutes at 20 °C. Then the polymerisation temperature was increased to 75 °C. When the internal temperature has reached 71 °C, the chosen amount of hexene was fed via the syringe pump in one minute. The reactor temperature was kept constant at 75 °C throughout the polymerisation. The polymerisation time was measured starting when the temperature is 2 °C below the set polymerisation temperature. When the polymerisation time of 60 min had lapsed, the reaction was stopped by injecting 5 ml ethanol, cooling the reactor and simultaneously flashing the volatile components. After purging the reactor 3 times with N2 and one vacuum/N2 cycle, the reactor was opened, the polymer powder was taken out and dried overnight in a fume hood.100 g of the polymer was additivated with 0.5 wt% Irganox B225 (dissolved in acetone) and then dried overnight in a fume hood and additionally one hour in a vacuum drying oven at 60°C. 2. Two- step propylene/hexene copolymerisation (liquid monomer) + propylene/hexene/ethylene copolymerisation (gas phase) After the liquid monomer polymerisation step was completed as described above for the 1-step copolymerisation, instead of stopping the reaction as described above the stirrer speed was reduced to 50 rpm and the pressure was reduced to 18 bar-g by venting the monomer. Afterwards the stirrer speed was set to 180 rpm, the reactor temperature to 70 °C and a certain batch amount of ethylene was added to reach the desired transition ratio of 0.11. No C6 feed was conducted during the transition because the left over amount from bulk should be enough. After the batch amount of ethylene the reactor pressure was increased to 21 bar-g by feeding a C3/C6 gas mixture of composition defined by: C6/C3 is the weight ratio of the two monomers and R is their reactivity ratio, determined experimentally. In the present experiments, R=1.70. C2/C3 is the weight ratio of the two monomers and R is their reactivity ratio, determined experimentally. In the present experiments, R=0.49. The calculation accordingly as described above. After that 0.3 NL hydrogen (except 0.5 NL used in IE5) was added via flow controller in one minute. The temperature was held constant by thermostat and the pressure of 21 bar-g is kept constant by feeding via mass flow controller a C2/C3 and C6/C3 gas mixture of composition corresponding to the target polymer composition and, until the set duration for this step had lapsed. Then the reactor was cooled down to about 30°C and the volatile components flashed out. After purging the reactor 3 times with N2 and one vacuum/N2 cycle, the product was taken out and dried overnight in a fume hood.100 g of the polymer was additivated with 0.5 wt% Irganox B225 (solution in acetone) and dried overnight in a hood followed by one hour in a vacuum drying oven at 60°C. The polymerisation results of the liquid monomer polymerisation experiments are shown in Table 2 and Table 3. Table 2: polymerisation settings, productivities and MFR in 1-step copolymerisation * productivity based on catalyst before offline prepolymerisation From the results of Table 2 it can be seen that all three inventive catalysts give a higher productivity and/or melt flow rate (MFR ₂₁ is shown here, being more precise than the MFR ₂ measurement) than both CC1 and CC2. The relation of productivity to melt flow rate MFR21 is shown in Figure 1. Table 3: composition, molecular weight and thermal properties Table 3 shows that all copolymers contain about the same amount of hexene comonomer and therefore have very similar crystallisation and melting temperatures, meaning that the results can be used to compare the MFR capability of the catalysts in a liquid loop polymerisation stage. As mentioned above, the negative effect of hexene on MFR and productivity is seen more strongly in the gas phase. Since for pipe applications between 40 and 60 wt% of the product needs to be produced in the gas phase reactor, a catalyst must be able to produce the target low MFR also in gas phase. The results of C3/C6 and C2/C3/C6 two-step copolymerisation experiments are listed in tables 4-7. Table 4: Polymerisation settings for the liquid monomer polymerisation step xene g 02 02 02 13 12 13 Table 5: Polymerisation settings for the gas phase step se C6 feed g 8.7 18.3 40.9 10.2 7.3 21.4 Table 6: Polymerisation results and polymer analysis MFR21 10min 167 121 72 35 48 30 Comparing the inventive catalysts to the comparative catalysts in terms of the productivity-MFR balance, it can be seen that only the inventive catalysts can reach MFR ₂ below 0.5, or MFR ₂₁ below 50 at similar metallocene productivity. The relation of productivity to melt flow rate MFR2 is shown in Figure 2. The relation of productivity to melt flow rate MFR21 is shown in Figure 3. In addition, by using a higher borate/hafnium ratio (IC3 in IE6), the gas phase split increases, leading to a higher overall productivity while maintaining the same low MFR. Table 7: Polymer analysis The inventive catalysts produce polymers with a higher weight average molecular weight compared to the comparative catalysts.