DETAILED DESCRIPTION

[0048] Various specific embodiments, versions and examples of the invention will now be described, including preferred embodiments and definitions that are adopted herein for purposes of understanding the claimed invention. It is understood, however, that for purposes of assessing infringement, the scope of the “invention” will refer to the appended claims, including their equivalents, and elements or limitations that are equivalent to those that are recited. All references to the “invention” below are intended to distinguish claimed compositions and methods from compositions and methods not considered to be part of this invention. It is understood, therefore, that any reference to the “invention” may refer to one or more, but not necessarily all, of the inventions defined by the claims. References to specific “embodiments” are intended to correspond to claims covering those embodiments, but not necessarily to claims that cover more than those embodiments.

[0049] Definitions and Properties

[0050] Certain terms and properties, some of which appear in the claims, will now be defined, as used in this patent and for purposes of interpreting the scope of the claims. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents.

[0051] As used herein, the term “solvent” is defined broadly, to refer to any liquid medium in which any of the polymerization reactions described herein can take place, but not including any liquid material that is polymerized, such as monomers. The term “solvent” includes diluents, which are preferably inert, and specifically includes the solvents and diluents disclosed in Weng et al., U.S. Pat. No. 6,225,432.

[0052] The definition of the term “reactor system” used herein is any vessel, structure, enclosure, or combinations thereof in which a polymerization reaction is capable of taking place, and also includes any vessel or combination of vessels in which the various polymerization processes described herein take place, in whole or in part. A reactor system can thus be or include a single reactor vessel, or multiple reactor vessels, e.g., series or parallel reactors.

[0053] The term “metallocene” is defined broadly as a compound represented by the formula Cp _m MR _n Xq. The symbol “Cp” refers to either a cyclopentadienyl ring, which may be substituted or unsubstituted, or a cyclopentadienyl ring derivative, such as an indenyl ring, which may also be substituted or unsubstituted. As discussed in greater detail below, a preferred metallocene compound includes two cyclopentadienyl rings, is sometimes referred to as a “bis-cyclopentadienyl” metallocene, and preferred cyclopentadienyl derivatives are bis-indenyl and bis-tetrahydroindenyl metallocene compounds. The symbol “M” refers to a Group 4, 5, or 6 transition metal, for example, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten. The symbol “R” in the general formula above refers to a hydrocarbyl group, e.g., methyl, or a hydrocarboxy group, each of which may have from one to 20 carbon atoms. The symbol “X” refers to a halogen, e.g., chlorine, bromine, or fluorine. The letter “m” can represent 1, 2, or 3; the letter “n” can represent 0, 1, 2, or 3; and the letter “q” can represent 0, 1, 2, or 3. The sum of m+n+q should be equal to the oxidation state of the transition metal. Examples of metallocene compounds are found in U.S. Pat. Nos. 4,530,914; 4,542,199; 4,769,910; 4,808,561; 4,871,705; 4,892,851; 4,933,403; 4,937,299; 5,017,714; 5,057,475; 5,120,867; 5,132,381; 5,155,080; 5,198,401; 5,278,119; 5,304,614; 5,324,800; 5,350,723; 5,391,790; 6,376,410; 6,376,412; 6,380,120; 6,376,409; 6,380,122; and 6,376,413. The portions of those patents describing the metallocene compounds and the ingredients and procedures for making and using such compounds are incorporated herein by reference. As discussed in greater detail below, preferred metallocene catalyst compounds are subsets of the general metallocene category, particularly those that provide the desired combinations of properties, as well as those that have demonstrated remarkably high productivities.

[0054] The term “mixed metallocene catalyst system,” as used herein, is defined as two or more different metallocene compounds, in combination with other ingredients, such as co-catalysts, activators, and other compounds that further aid the catalyst in promoting polymerization; but the term does not include polymerizable materials, e.g., monomers or macromers, or inert solvents. One metallocene is considered to be “different” from another metallocene when the two metallocenes have different chemical structures, except that metallocene compounds that are enantiomers of each other are not considered to be different from each other. Preferably, the mixed metallocene catalyst system includes the first and second metallocenes described below, and more preferably having the specific structures indicated below and being contacted with the polymerization medium in the proportions and manner described below. The two or more metallocene compounds of the mixed metallocene catalyst system can be introduced to a reactor system in a manner to cause the formation of a branched polypropylene (BCPP) discussed herein. The reactor system may be a single reactor vessel in which the processes described herein may be conducted in a continuous manner, so that each metallocene compound may be added to that reactor simultaneously or separately, e.g., at different times and even in different locations, e.g., by being introduced via separate catalyst feed streams. Alternatively, a reactor system that includes multiple reactor vessels may be used, in which case each metallocene compound may be added separately to the separate reactors. In one or more specific embodiments, each of the at least two metallocene compounds simultaneously contact the polymerization medium at some point during polymerization, and preferably contact propylene monomers, and more preferably contact both propylene monomers and polypropylene macromers. In one or more embodiments of the processes described herein, both metallocene compounds are supported, e.g., on silica or alumina particles; while in other embodiments, the metallocene compounds are not supported.

[0055] An important feature of one or more specific embodiments of the invention relates to the formation of crystalline polymers that are “branched,” at least to some degree. Various procedures have been published, and either are or will be available to identify whether a polymer is branched or not, and a polymer is regarded herein as being “branched” to the extent branching can be detected, regardless of the method or equipment used for such detection. Preferably, the crystalline polypropylene is branched to a degree that it can be measured quantitatively, and even more preferably expressed in terms of a branching index. A well known branching index for monodisperse polymers is used herein, referred to herein as “Branching Index,” also known as g′, which is defined as the ratio of intrinsic viscosities of the branched to linear molecules, i.e., g′=[η] _br /[η] _lin . The term “η” stands for intrinsic solution viscosity. The term “[η] _br ” is the intrinsic viscosity for the branched polymer molecule, and the term “[η] _lin ” is that for a linear polymer molecule of equal molecular weight. For polydisperse samples the Branching Index is an average branching index, <g′> _avg , defined as: 1 $\text{<}g^{\prime }{\text{>}}_{\mathrm{avg}}=\frac{{\left[\eta \right]}_{\mathrm{branched}}}{{\left[\eta \right]}_{\mathrm{linear}}}=\frac{\sum _{i=1}^NC_i\times {\left[\eta \right]}_i}{\sum _{i=1}^NC_i\times \left[k\times M_i^{\alpha }\right]}$

[0056] Here, the index i refers to a given polymer fraction, M _i is the molecular weight of that fraction as measured by light scattering, [η] is the intrinsic viscosity of that fraction measured by viscometry, C _i is the concentration of that fraction, and “k” and “a” are the Mark Houwink coefficients for a linear polymer of the same chemical species. These quantities are measured by a GPC setup with online light scattering, viscometer, and concentration detectors. A polymer sample having branching will have intrinsic viscosity that deviates from that of a linear polymer. If a polymer sample is linear, the branching index, g′, will be 1.0 (+/−0.01). If a polymer sample is branched, the average branching index will be less than 1. A lower branching index indicates more branching. In practice, average deviation levels can be calculated from GPC-3D method involving three different detectors on line—LALLS, Viscometry, DR1 —to measure, respectively, the molecular weights, viscosity, and concentration of the polymer solution. First, the GPC-LALLS data is used to measure molecular weight averages (M _w , M _z ). The respective intrinsic viscosity of the polymer solution, “1”, is obtained from the viscometer data while the concentration at each data point is provided by the DR1 technique. Finally the “η” is related to absolute molecular weight. Weight-average values of g′ are to be calculated from the data points that fall in the range of from the characteristic M _w of the polymer examined to the upper limit of 2,000,000 Daltons. For any case in which some values of M _w are below 100,000 Daltons, the weight average is calculated using only those points between 100,000 Daltons and 2,000,000 Daltons. To calculate the branching index for polypropylene that includes at least some ethylene monomer units, the following equations should be used: g′ =1.18w, where “w” is the weight fraction of ethylene.

[0057] Melting and crystallization temperatures of the polymers (Tm and Tc) are measured on a DuPont DSC-912 with thin molded film samples, scanning at 10° C./min. The melting temperatures described herein are obtained from the second melt.

[0058] As used herein, the term “polypropylene” means a polymer made of at least 50% propylene units, preferably at least 70% propylene units, more preferably at least 80% propylene units, even more preferably at least 90% propylene units or 95% propylene units, and most preferably essentially 100% propylene units, which polypropylene is referred to as a “homopolymer.” In one or more specific embodiments described herein, a “polypropylene” referenced herein may have 65 wt % or more propylene; or 80 wt % or more propylene; or 90 wt % or more propylene; or 97 wt % or more propylene.

[0059] A polypropylene polymer made according to the processes described herein is considered distinguishable from polymers that are sometimes described in the scientific or patent literature as “polypropylene” but which contain undesirably high levels of ethylene. It has been recognized that even relatively small amounts of ethylene monomer can have a significant or substantial effect on final polymer properties. Accordingly, as used herein, the term “polypropylene” refers to a polypropylene polymer with no more than 3.0 wt % ethylene; or no more than 2.5 wt % ethylene. Preferably, the polypropylenes described herein have no more than 2.0 wt % ethylene; or no more than 1.5 wt % ethylene; or no more than 1.0 wt % ethylene.

[0060] As used herein, the term “linear polypropylene” means a polypropylene having no detectable branching (quantitatively or qualitatively), preferably a Branching Index of 1.0 (+/−0.02).

[0061] As used herein, the term “branched polypropylene” (BCPP) means a polypropylene that is branched (detected quantitatively or qualitatively), and preferably has a Branching Index, based on measured data, of less than 1.0 (+/−0.02).

[0062] As used herein, the term “polymerization medium” includes at least the monomers that form the polypropylene polymer and optionally a solvent. The term “polymerization medium” does not include a catalyst system, e.g., catalyst compounds or activators. After polymerization has begun, the polymerization medium may also include products of polymerization, e.g. macromers and other polymers.

[0063] As used herein, the term “slurry polymerization” means a polymerization process in which particulate, solid polymer is formed in a liquid or vapor polymerization medium.

[0064] As used herein, the term “bulk process” means a polymerization process in which the polymerization medium consists entirely of or consists essentially of monomers and any products of polymerization that has taken place, e.g. macromers and polymers, but does not include solvent.

[0065] As used herein, the term “macromer” is defined as a polymeric structure that contains monomers, e.g., propylene monomer units. A macromer is a polymer with a relatively low molecular weight, in contrast with the fully formed polymer. For example, a macromer can be a polymer having a weight average molecular weight (M _w ) of 150,000 or less, or more narrowly 100,000 or less, or 80,000 or less. Narrower preferred ranges for the macromer are from 1,000 to 100,000, or from 10,000 to 80,000. Although the macromers described herein are types of polypropylene polymers, they are not considered to be fully formed polypropylene polymers, e.g., they do not necessarily have the desired properties, structures, or molecular weights, e.g., as those of the final polymer product. The macromers described herein preferably have only a small amount of branching or no branching. Preferably, however, they are crystalline, being either isotactic or syndiotactic. In at least certain embodiments, during polymerization a macromer is incorporated into the branched crystalline polypropylene polymer that is formed according to processes described herein.

[0066] As used herein, the terms “unimodal” and “unimodal molecular weight distribution” are defined as any molecular weight distribution of a polymer composition that is neither bimodal nor multimodal, and broadly encompasses any polymer composition of which a GPC-3D curve can be taken, and the derivative of such GPC-3D curve demonstrate no inflection point. Preferably, a GPC-3D curve is prepared using the viscometer test procedures, conditions and equipment set forth in the article, “Effect of Short Chain Branching on the Coil Dimensions of Polyolefins in Dilute Solutions,” by T. Sun, P. Brant, R. Chance and W. Graessley, Macromolecules 2001, Vol. 34 (No. 19), pages 6812-6820, which is incorporated herein by reference.

[0067] The amount of vinyl chain ends is determined by ¹ H NMR as set forth in the literature, specifically in Weng et al., Macromol. Rapid Commun. 2000, 21, 1103-07.

[0068] The terms “molecular weight” (M _n and M _w ) and “polydispersity” (Mw/Mn) are intended to broadly encompass molecular weights that are obtained, measured and/or calculated using any published procedure, except to the extent a particular procedure is specified herein. Preferably, the molecular weights are measured in accordance with the procedure described in the article by T. Sun et al., cited above.

[0069] The “melt flow rate” (MFR) is measured in accordance with ASTM D-1238 at 230° C. and 2.16 kg load.

[0070] A property that can be used to characterize the branched crystalline polypropylenes described herein is its heat of fusion. As used herein, the “heat of fusion” is measured using Differential Scanning Calorimetry (DSC), using the ASTM E-794-95 procedure. About 4 mg to about 10 mg of a sheet of the polymer pressed at approximately 200° C. to 230° C. is removed with a punch die and is annealed at room temperature for 48 hours. At the end of this period, the sample is placed in a Differential Scanning Calorimeter (Perkin Elmer 7 Series Thermal Analysis System) and cooled to about −50° C. to −70° C. The sample is heated at about 10° C./min to attain a final temperature of about 180° C. to about 200° C. The thermal output is recorded as the area under the melting peak of the sample which is typically at a maximum peak at about 30° C. to about 175° C. and occurs between the temperatures of about 0° C. and about 200° C. The thermal output is measured in Joules as a measure of the heat of fusion. The melting point is recorded as the temperature of the greatest heat absorption within the range of melting temperature of the sample.

[0071] The term “isotatic” as used herein is defined as referring to a polymer sequence in which more than 50% of adjacent monomers having groups of atoms that are not part of the backbone structure are located either all above or all below the atoms in the backbone chain, when the latter are all in one plane.

[0072] The term “syndiotactic” as used herein is defined as referring to a polymer sequence in which more than 50% of adjacent monomers which have groups of atoms that are not part of the backbone structure are located in some symmetrical fashion above and below the atoms in the backbone chain, when the latter are all in one plane.

[0073] The branched polypropylene polymers described herein are characterized as being “crystalline.” The crystallinity of a polymer can be expressed in terms of percent crystallinity, usually with respect to some reference or benchmark crystallinity. The crystallinities of the polypropylenes described herein are expressed as a percentage of the crystallinity of isotactic polypropylene homopolymer, which is defined herein to be 190 J/g. Thus, in one or more specific embodiments, crystalline polypropylene compositions described herein have a crystallinity of from 30% of the crystallinity of an isotactic polypropylene homopolymer, preferably from 40% to 50%. Preferably, heat of fusion is used to actually measure crystallinity for purposes of comparing to isotactic polypropylene homopolymer. Thus, for example, based on a heat of fusion for a highly crystalline polypropylene homopolymer of 190 J/g, a branched crystalline polypropylene having a heat of fusion of 95 J/g will have a crystallinity of 50%.

[0074] The term “melting point” for a material as used herein is defined as the highest peak among principal and secondary melting peaks as determined by Differential Scanning Calorimetry (DSC), discussed above.

[0075] As used herein, the term “productivity” is defined as the weight of polymer produced per weight of the catalyst used in the polymerization process per 1 hour of polymerization time (e.g., grams polymer/gram catalyst/hr). Note that the term “catalyst” may actually refer to a mixed catalyst system, which includes at least two different catalyst compounds, in which case the productivity value refers to the productivity of the combined catalysts.

[0076] Specific Embodiments of Processes

[0077] Certain specific embodiments of the invention will now be discussed. As described in greater detail below, at least certain embodiments of the process result in crystalline branched polypropylene, and yet avoid the necessity for using diene comonomers, particularly those diene comonomers that result in gel formation, or for using hydrogen during polymerization, and yet provide a branched polypropylene that is crystalline and has high melt strength and other desirable properties. An advantage to certain embodiments is unexpectedly high productivities. Furthermore, although two different catalysts are used, the compositions in certain preferred embodiments unexpectedly have a unimodal composition.

[0078] In one or more specific embodiments, a process of preparing a polymer composition that includes branched crystalline polypropylene is described, which process includes: combining two or more different metallocene catalyst compounds with a polymerization medium that includes propylene, for a time sufficient to provide branched crystalline polypropylene that has from 0.0 wt % to 2.0 wt % ethylene and a heat of fusion of 70 J/g or more.

[0079] In one or more specific embodiments, a process of preparing a unimodal polymer composition that includes branched crystalline polypropylene is described, such process including combining two or more different metallocene catalyst compounds with propylene monomers in a polymerization medium having less than 30 volume percent diluent, or more preferably less than 25 volume or 20 volume percent diluent; conducting polymerization of the propylene monomers in the polymerization medium at a reaction temperature of 75° C. or less to form branched crystalline polypropylene; and recovering a branched crystalline polypropylene that has (a) from 0.0 wt % to 2.0 wt % ethylene; (b) a heat of fusion of 70 J/g or more; and (c) a unimodal molecular weight distribution.

[0080] Additionally described is a process of preparing a polymer composition that includes branched crystalline polypropylene, comprising: conducting polymerization of propylene monomers in the presence of a first metallocene catalyst compound and a second metallocene catalyst compound at a temperature of 75° C. or less to provide a composition that includes branched crystalline polypropylene containing from 0.0 wt % to 2.0 wt % ethylene, wherein: (a) the first metallocene catalyst compound is capable of producing polypropylene macromers; and (b) the second metallocene catalyst compound is capable of producing crystalline polypropylene having a weight average molecular weight of 100,000 Daltons or more.

[0081] Also described herein is a process of preparing a branched crystalline polypropylene composition, which process includes: contacting a polymerization mixture that includes propylene monomers with a first metallocene catalyst compound and a second metallocene catalyst compound; and conducting polymerization of the propylene monomers for a time sufficient to form a branched crystalline polypropylene composition having a heat of fusion of 70 J/g or more, wherein: the first metallocene compound is an alkyl bridged metallocene compound that has at least two indenyl rings or derivatives of indenyl rings, each ring being substituted at one or both of the 4 and 7 positions; and the second metallocene compound is a bridged metallocene compound that has at least two indenyl rings or derivatives of indenyl rings, each ring being substituted at the 2 and 4 positions.

[0082] Further described is a process of preparing a branched crystalline polypropylene composition, which includes contacting a polymerization mixture that includes propylene monomers with a first metallocene catalyst compound and a second metallocene catalyst compound; and conducting polymerization of the propylene monomers for a time sufficient to form a branched crystalline polypropylene composition having a heat of fusion of 70 J/g or more, wherein: the first metallocene compound is an alkyl bridged metallocene compound that has at least two indenyl rings or derivatives of indenyl rings, each ring being substituted at one or both of the 4 and 7 positions; the second metallocene compound is different from the first metallocene compound; and the molar amount of the second metallocene compound contacting the polymerization mixture is greater than the molar amount of the first metallocene compound contacting the polymerization mixture.

[0083] Also described is a process of preparing a unimodal branched crystalline polypropylene composition, including: combining a mixed metallocene catalyst system that includes at least a first metallocene compound and a second metallocene compound with a polymerization mixture that includes propylene monomers in a reactor system, and carrying out polymerization of the propylene monomers in the reactor system for a time sufficient to form a branched crystalline polypropylene having a unimodal molecular weight distribution. In such a process, the first metallocene compound is preferably represented by the formula: 6

[0084] wherein: M is a metal of Group 4, 5, or 6 of the Periodic Table, for example titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum and tungsten, preferably, zirconium, hafnium and titanium, most preferably zirconium and hafnium;

[0085] R ¹ and R ² are identical or different, preferably identical, and are one of a hydrogen atom, a C ₁ -C ₁₀ alkyl group, preferably a C ₁ -C ₃ alkyl group, a C ₁ -C ₁₀ alkoxy group, preferably a C ₁ -C ₃ alkoxy group, a C ₆ -C ₁₀ aryl group, preferably a C ₆ -C ₈ aryl group, a C ₆ -C ₁₀ aryloxy group, preferably a C ₆ -C ₈ aryloxy group, a C ₂ -C ₁₀ alkenyl group, preferably a C ₂ -C ₄ alkenyl group, a C ₇ -C ₄₀ arylalkyl group, preferably a C ₇ -C ₁₀ arylalkyl group, a C ₇ -C ₄₀ alkylaryl group, preferably a C ₇ -C ₁₂ alkylaryl group, a C ₈ -C ₄₀ arylalkenyl group, preferably a C ₈ -C ₁₂ arylalkenyl group, or a halogen atom, preferably chlorine; or a conjugated diene which is optionally substituted with one or more hydrocarbyl, tri(hydrocarbyl)silyl groups or hydrocarbyl, tri(hydrocarbyl)silylhydrocarbyl groups, said diene having up to 30 atoms not counting hydrogen;

[0086] R ³ and R ⁴ may be as defined for R ¹ and R ² but are preferably hydrogen atoms;

[0087] R ⁵ and R ⁶ are identical or different, preferably identical, are one of a hydrogen atom, a halogen atom, preferably a fluorine, chlorine or bromine atom, a C ₁ -C ₁₀ alkyl group, preferably a C ₁ -C ₄ alkyl group, which may be halogenated, a C ₆ -C ₁₀ aryl group, which may be halogenated, preferably a C ₆ -C ₈ aryl group, a C ₂ -C ₁₀ alkenyl group, preferably a C ₂ -C ₄ alkenyl group, a C ₇ -C ₄₀ arylalkyl group, preferably a C ₇ -C ₁₀ arylalkyl group, a C ₇ -C ₄₀ alkylaryl group, preferably a C ₇ -C ₁₂ alkylaryl group, a C ₈ -C ₄₀ arylalkenyl group, preferably a C ₈ -C ₁₂ arylalkenyl group, a —NR ₂ ¹⁵ , —SR ¹⁵ , —OR ¹⁵ , —OSiR ₃ ¹⁵ or —PR ₂ ¹⁵ radical, wherein: R ¹⁵ is one of a halogen atom, preferably a chlorine atom, a C ₁ -C ₁₀ alkyl group, preferably a C ₁ -C ₃ alkyl group, or a C ₆ -C ₁₀ aryl group, preferably a C ₆ -Cg aryl group;

[0088] R7 is 7

[0089] —B(R ¹⁴ )—, —Al(R ¹⁴ )—, —Ge—, —Sn—, —O—, —S—, —SO—, —SO ₂ —, —N(R ¹⁴ )—, —CO—, —P(R ¹⁴ )—, or —P(O)(R ¹⁴ )—;

[0090] wherein: R ¹⁴ , R ¹⁵ and R ¹⁶ are identical or different and are a hydrogen atom, a halogen atom, a C ₁ -C ₂₀ branched or linear alkyl group, a C ₁ -C ₂₀ fluoroalkyl or silaalkyl group, a C ₆ -C ₃₀ aryl group, a C ₆ -C ₃₀ fluoroaryl group, a C ₁ -C ₂₀ alkoxy group, a C ₂ -C ₂₀ alkenyl group, a C ₇ -C ₄₀ arylalkyl group, a C ₈ -C ₄₀ arylalkenyl group, a C ₇ -C ₄₀ alkylaryl group, or R ¹⁴ and R ¹⁵ , together with the atoms binding them, form a cyclic ring;

[0091] preferably, R ¹⁴ , R ¹⁵ and R ¹⁶ are identical and are a hydrogen atom, a halogen atom, a C ₁ -C ₄ alkyl group, a CF ₃ group, a C ₆ -C ₈ aryl group, a C ₆ -C ₁₀ fluoroaryl group, more preferably a pentafluorophenyl group, a C ₁ -C ₄ alkoxy group, in particular a methoxy group, a C ₂ -C ₄ alkenyl group, a C ₇ -C ₁₀ arylalkyl group, a C ₈ -C ₁₂ arylalkenyl group, or a C ₇ -C ₁₄ alkylaryl group;

[0092] M ² is carbon, silicon, germanium or tin;

[0093] R ⁸ and R ⁹ are R ⁸ and R ⁹ are identical or different, preferably identical and have the meanings stated for R ⁵ and R ⁶ ;

[0094] R ¹⁰ , R ¹ , R ¹² and R ¹³ are identical or different and have the meanings stated for R ⁵ and R ⁶ ;

[0095] preferably, R ¹² and R ¹¹ are hydrogen and at least one of R ¹³ and R ¹⁰ , preferably both, are identical or different, preferably identical, and are one of a hydrogen atom, a halogen atom, preferably a fluorine, chlorine or bromine atom, a C ₁ -C ₁₀ alkyl group, preferably a C ₁ -C ₄ alkyl group, which may be halogenated, a C ₆ -C ₁₀ aryl group, which may be halogenated, preferably a C ₆ -C ₈ aryl group, a C ₂ -C ₁₀ alkenyl group, preferably a C ₂ -C ₄ alkenyl group, a C ₇ -C ₄₀ arylalkyl group, preferably a C ₇ -C ₁₀ arylalkyl group, a C ₇ -C ₄₀ alkylaryl group, preferably a C ₇ -C ₁₂ alkylaryl group, a C ₈ -C ₄₀ arylalkenyl group, preferably a C ₈ -C ₁₂ arylalkenyl group, a —NR ₂ ¹⁵ , —SR ¹⁵ , —OR ¹⁵ , —OSiR ₃ ¹⁵ or —PR ₂ ¹⁵ radical, wherein: R <