Ethylene Polymerization Catalysts Research Articles

New Neutral Nickel and Palladium Sandwich Catalysts: Synthesis of Ultra-High Molecular Weight Polyethylene (UHMWPE) via Highly Controlled Polymerization and Mechanistic Studies of Chain Propagation

New neutral nickel and palladium ethylene polymerization catalysts have been prepared that incorporate an anionic (N,O) chelating ligand. Extensive axial shielding is provided by two 3,5-dichloroaryl moieties in a "sandwich" orientation. Such shielding results in an exceptionally slow rate of chain transfer relative to migratory insertion in the nickel catalyst, and thus highly controlled polymerization of ethylene is observed, leading to lightly branched ultra-high molecular weight polyethylene with Mn values up to 4.1 × 106 g/mol. The analogous palladium catalysts provide the means for a detailed mechanistic study of chain propagation in an electronically asymmetric neutral palladium catalyst. Both isomers of the methyl ethylene complex can be generated and observed at low temperatures allowing experimental elucidation of mechanistic details of chain propagation probed in other electronically asymmetric systems only through DFT studies or by examination of model studies. The barrier to migratory insertion in these complexes is ca. 19.2 kcal/mol. Investigation of the equilibration of the methyl ethylene isomers in the presence of excess ethylene showed the isomerization rate is dependent on ethylene concentration. This is the first direct proof that isomerization in these alkyl ethylene intermediates is catalyzed by ethylene. Furthermore, isomer equilibration is much faster than migratory insertion so that the barriers for insertion of individual isomers cannot be determined.

2-Phosphine-pyridine-N-oxide palladium and nickel catalysts for ethylene polymerization and copolymerization with polar monomers

A new family of [P,O] late transition metal catalysts, namely 2-phosphine-pyridine-N-oxide palladium complexes Pd1~Pd4 {[κ2(P,O)-2-Ar2P-6-R-(C5H3NO)]PdMeCl} (Pd1: Ar = Ph, R = H; Pd2: Ar = 2-MeO-C6H4, R = H; Pd3: Ar = Ph, R = tBu; Pd4: Ar = 2-MeO-C6H4, R = tBu) and nickel complexes Ni1~Ni4 {[κ2(P,O)-2-Ar2P-6-R-(C5H3NO)]Ni(allyl)}[BArF] (Ni1: Ar = Ph, R = H; Ni2: Ar = 2-MeO-C6H4, R = H; Ni3: Ar = Ph, R = tBu; Ni4: Ar = 2-MeO-C6H4, R = tBu) and Ni5 {[κ2(P,O)-2-Ph2P-6-tBu-(C5H3NO)]2Ni(allyl)}[BArF], was synthesized and comprehensively characterized by NMR spectroscopy, single-crystal X-ray diffraction, and elemental analysis. These new [P,O] palladium and nickel catalysts were evaluated for the (co)polymerization of ethylene. Among the synthesized catalysts, palladium catalysts Pd2 and Pd4 oligomerized ethylene with high activities of 106 g mol−1 h−1, while the nickel counterparts could polymerize ethylene in higher activities and higher polymer molecular weights, among which Ni4 preferred to afford polyethylene with the highest molecular weight. Moreover, Ni4 could catalyze the copolymerization of ethylene and biorenewable polar comonomer methyl 10-undecenoate, with the activity up to 105 g mol−1 h−1 and the incorporation of 0.4–1.2 mol%.

Study of supported catalysts, prepared via binding of Fe(II) bis(imino)pyridyl complex with silica, modified by alumina: Effect of surface Lewis acidic sites on catalyst composition and activity in ethylene polymerization

In this work we present the data on the formation and composition of new supported catalysts for ethylene polymerization, prepared by interaction of Fe(II) bis(imino)pyridyl complex (LFeCl2) with silica, modified by introduction of alumina compounds (SiO2(nAl)) for generation of Lewis acidic sites (LAS) on the silica surface. Data on the content of LAS with different acidity in SiO2(nAl) supports and on the interaction of LFeCl2 with these sites were obtained. It was found that interaction of LFeCl2 with the most acidic sites of SiO2(nAl) supports leads to formation of the most active catalysts for the ethylene polymerization.

Enhancing Chain Initiation Efficiency in the Cationic Allyl-Nickel Catalyzed (Co)Polymerization of Ethylene and Methyl Acrylate.

Improving the efficiency of chain initiation is highly important and also highly challenging in the development of olefin polymerization catalysts. A series of 2-methylallyl-based nickel complexes supported by aryl-N-bridged diphosphazane monoxide (PNPO) ligands containing different electronic and steric substituents were prepared and characterized. These nickel complexes are highly active single-component catalysts for ethylene polymerization and copolymerization with methyl acrylate (MA). 2-Methylallyl substituent on the μ-allyl catalysts led to an increase in the efficiency of chain initiation compared with the corresponding allyl-based analogues, improving the catalytic performances with high activity (up to 4.02 × 106 g PE (mol Ni)-1 h-1). Linear polyethylenes with high molecular weight, narrow PDI values, and high melting temperatures were generated. Most importantly, these 2-methylallyl nickel catalysts can promote ethylene-MA copolymerization to afford functionalized polyethylenes with MA incorporation of up to 7.0 mol %. The current work demonstrates that the change of initiating units can lead to enhancement in catalyst performances. This provides an alternative, simple, and potentially general strategy to improve the properties of different catalyst systems.

New family of single-component Ni catalysts for ethylene polymerization to high and ultrahigh molecular weight polyethylene

New family of single-component Ni catalysts for ethylene polymerization to high and ultrahigh molecular weight polyethylene

Simple and Strong Dative Attachment of α-Diimine Nickel (II) Catalysts on Supports for Ethylene Polymerization with Controlled Morphology

In this article, preparation of novel spherical MgCl2 supported α-diimine nickel (II) catalysts for ethylene polymerization in slurry phase is reported. α-Diimine ligands were synthesized by condensation reaction of 2, 6-disubstituted alkyls or aryls anilines and Ace naphthoquinone Which have hydroxyl functionality in their para-position. Hydroxyl functionalized α-diimine attached strongly on to the spherical MgCl2 support surface by dative bonding. No linker was needed to attach the complexes onto the support surface and the amount of loaded Nickel was controllable to improve morphology and especially bulk density of polymer powder. A significant reduction in catalysts activity has happened when homogeneous catalysts were supported onto silica but this reduction was decreased when they were supported onto thermally treated spherical MgCl2. As homogeneous bis(N,N′-(4-(3-hydroxyl-propyl)-2,6-di[(4-tert-butyl-phenyl)-phenyl) amino] Ace naphthoquinone Nickel dibromide(d) showed the highest activity among other evaluated homogeneous catalysts, its MgCl2 supported catalyst (d/S-MgCl2) has shown the highest activity among MgCl2 supported catalysts too. These MgCl2 supported catalysts were pre-polymerized in presence of ethylene monomer in the mild polymerization condition to yield a pre-polymerized catalyst with polymer/catalyst weight ratio equal to six. Ethylene polymerization was carried out to make spherical particles of polyethylene without reactor fouling by these pre-polymerized catalysts. Clearly, it is shown in SEM images that the spherical morphology of MgCl2 support is replicated in the produced polymer. The molecular weight and molecular weight distribution of produced polymer with MgCl2 supported catalysts were higher than those produced by homogeneous catalysts. α–Diimine nickel (II) complexes have hydroxy functionality where produce strong dative bonding onto spherical MgCl2. This bonding is strong enough that these catalysts are suitable for slurry polymerization of ethylene without reactor fouling due to catalyst leaching from support. The chemical structure of MgCl2 leads to high active supported catalysts. The molecular weight and polydispersity index of produced polymrers using these supported catalysts are higher than those produced by equivalent homogeneous catalysts and are controllable by selection of appropriate ligand for used α–diimine nickel (II) complex or hydrogen concentration in ethylene polymerization.

Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

AbstractIn various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β‐hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain‐growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra‐high‐molecular‐weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.

Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization.

In various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β‐hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain‐growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra‐high‐molecular‐weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.

Paradox of Late Transition-Metal Catalysts in Ethylene Polymerization

Paradox of Late Transition-Metal Catalysts in Ethylene Polymerization

Probing the effect of ortho-cycloalkyl ring size on activity and thermostability in cycloheptyl-fused N,N,N-iron ethylene polymerization catalysts.

The syntheses of six bis(imino)-5,6,7,8-tetrahydrocycloheptapyridine-iron(ii) chloride complexes, [2-{(Ar)NCMe}-9-{N(Ar)}C10H10N]FeCl2 (Ar = 2-(C5H9)-6-MeC6H3Fe1, 2-(C6H11)-6-MeC6H3Fe2, 2-(C8H15)-6-MeC6H3Fe3, 2-(C5H9)-4,6-Me2C6H2Fe4, 2-(C6H11)-4,6-Me2C6H2Fe5, 2-(C8H15)-4,6-Me2C6H3Fe6), are reported in which the ring size of the ortho-cycloalkyl group has been varied as has the type of para-substituent. The molecular structures of Fe3 and Fe6 reveal square pyramidal geometries at iron while the ortho-cyclooctyl rings adopt boat-chair conformations. On treatment with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all six complexes showed optimal activities at 80 °C [up to 1.9 × 107 g of PE per mol Fe per h for Fe5/MMAO] for ethylene polymerization forming linear polyethylene (Tm's > 126 °C). Notably, the catalytic activities showed a marked correlation with the ring size of the ortho-cycloalkyl substituent: cyclohexyl (Fe2 and Fe5) > cyclooctyl (Fe3 and Fe6) > cyclopentyl (Fe1 and Fe4) for either para-substituent, H or Me. Furthermore, this family of iron catalysts exhibited remarkable thermostability by remaining highly active even at temperatures as high as 100 °C (1.1 × 107 g of PE per mol Fe per h); the wide variation in polymer molecular weights (Mw: 2.4-166 kg mol-1), influenced through choice of precatalyst and co-catalyst as well as by temperature and pressure, further highlights the versatility of these catalysts.

Indole-bridged bisphosphine-monoxide palladium catalysts for ethylene polymerization and copolymerization with polar monomers

Novel indole-based C–N-bridged bisphosphine-monoxide palladium catalysts enable ethylene polymerization with completely different activities and molecular weights and copolymerization with polar comonomers.

Bis-cycloheptyl-fused bis(imino)pyridine-cobalt catalysts for PE wax formation: positive effects of fluoride substitution on catalytic performance and thermal stability.

The α,α'-bis(imino)-2,3:5,6-bis(pentamethylene)pyridyl-cobalt(ii) chlorides, [2,3:5,6-{C4H8C(N(2-R1-4-R3-6-R2C6H2))}2C5HN] CoCl2 (R1 = Me, R2 = R3 = CH(p-FPh)2Co1; R1 = Et, R2 = R3 = CH(p-FPh)2Co2; R1 = i-Pr, R2 = R3 = CH(p-FPh)2Co3; R1 = Cl, R2 = R3 = CH(p-FPh)2Co4; R1 = F, R2 = R3 = CH(p-FPh)2Co5; R1 = F, R2 = R3 = CHPh2Co5'', R1 = R2 = Me, R3 = CH(p-FPh)2Co6; R1 = R3 = Me, R2 = CH(p-FPh)2Co7), have been synthesized by a one-pot template reaction of α,α'-dioxo-2,3:5,6-bis(pentamethylene)pyridine, cobalt(ii) chloride and the respective aniline in n-butanol. By contrast, the mixed cobalt(ii) chloride/acetate complex, [2,3:5,6-{C4H8C(N(2-F-4,6-(CH(p-FPh)2)2C6H2))}2C5HN]CoCl(OAc) (Co5'), was isolated when the corresponding template reaction was carried out in acetic acid. Structural characterization of Co4, Co5 and Co5'' revealed distorted square pyramidal geometries while six-coordinate Co5', incorporating a chelating acetate ligand, exhibited a distorted octahedral geometry. On activation with either MAO or MMAO, 2-fluoride-4,6-bis{di(p-fluorophenyl)methyl}-substituted Co5 showed maximum catalytic activity for ethylene polymerization at a high operating temperature of 60 °C (up to 2.1 × 107 g (PE) mol-1 (Co) h-1), producing highly linear (Tms > 121 °C), low molecular weight polyethylene waxes (Mw range: 1.5-5.0 kg mol-1) with narrow dispersity (Mw/Mn range: 1.7-2.9). End-group analysis of the waxes reveals β-H elimination as the dominant chain transfer process.

High molecular weight polyethylenes of narrow dispersity promoted using bis(arylimino)cyclohepta[b]pyridine-cobalt catalysts ortho-substituted with benzhydryl & cycloalkyl groups.

A one-pot template strategy has been utilized to synthesize sterically enhanced bis(imino)cyclohepta[b]pyridine-cobalt(ii) chlorides, [2-{(Ar)N[double bond, length as m-dash]CMe}-9-{N(Ar)}C10H10N]CoCl2 (Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2Co1, 2-(C6H11)-4,6-(CHPh2)2C6H2Co2, 2-(C8H15)-4,6-(CHPh2)2C6H2Co3, 2-(C12H23)-4,6-(CHPh2)2C6H2Co4, 2,6-(C5H9)2-4-(CHPh2)C6H2Co5). All five complexes have been characterized by a combination of FT-IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of Co1, Co3 and Co5 highlight the substantial steric hindrance imparted by the 2-cycloalkyl-6-benzhydryl or 2,6-dicyclopentyl ortho-substitution pattern; distorted square pyramidal geometries are exhibited in each case. On activation with methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the complexes (apart from Co4/MAO) were active ethylene polymerization catalysts (up to 3.70 × 106 g PE per mol (Co) per h for Co5/MMAO), operating effectively at temperatures between 50 °C and 60 °C, producing polyethylenes with high molecular weights (up to 589.5 kg mol-1 for Co3/MAO). Furthermore, all polymers were highly linear (Tm > 130 °C) with narrow dispersities (Mw/Mn range: 2.0-3.0). The coexistence of two chain termination pathways, β-H elimination and transfer to aluminum, has been demonstrated using 13C/1H NMR spectroscopy.

Pentafluorophenyl Groups as Remote Substituents in Ni(II) Polymerization Catalysis

Perfluoroaromatic molecules have been studied intensively as versatile electron-poor π systems. We demonstrate the use of pentafluorophenyl substituents in remote positions of Ni(II) salicylaldiminato catalysts for ethylene polymerization. With their strongly electron-withdrawing character, high molecular weights of Mn > 105 g/mol and simultaneously low degrees of branching are accessible due to an efficient suppression of β-hydride elimination. This can be ascribed to suppressed nickel–aryl interactions and reduced electron densities at the metal center, as concluded from X-ray crystal structure analysis and electrochemical studies. The pentafluorophenyl-substituted catalyst is stable under aqueous conditions for direct polymerization to dispersions of well-defined nanocrystals.

XAS Analysis of Reactions of (Arylimido)vanadium(V) Dichloride Complexes Containing Anionic NHC That Contains a Weakly Coordinating B(C6F5)3 Moiety (WCA-NHC) or Phenoxide Ligands with Al Alkyls: A Potential Ethylene Polymerization Catalyst with WCA-NHC Ligands.

(Arylimido)vanadium(V) dichloride complexes containing anionic N-heterocyclic carbene (NHC) ligands that contain weakly coordinating B(C6F5)3 moieties (WCA-NHC) of the type [V(NAr)Cl2(WCA-NHC-Ar′)] (5, Ar = 2,6-Me2C6H3, Ar′ = 2,6-iPr2C6H3) showed significant catalytic activity for ethylene polymerization in the presence of Al cocatalysts (MAO and AliBu3); the activity by the 5–MAO catalyst (19 500 kg-PE/mol-V·h; TOF 11 600 min–1) is the highest among those reported using the other (imido)vanadium(V) complexes in the presence of MAO, and the 5–AliBu3 catalyst showed higher activity (66 000 kg-PE/mol-V·h; TOF 39 200 min–1). The V K-edge X-ray absorption near-edge structure (XANES) analyses (in toluene) strongly suggest the formation of certain vanadium(III) species by reduction with AliBu3 accompanying structural changes; the EXAFS analysis suggests the presence of the arylimido ligand and one V–Cl bond (2.34 ± 0.04 Å), which is longer than those [2.1901(8)–2.2462(8) Å] in the reported (imido)vanadium(V) complexes. The XANES analysis of [V(NAr)Cl2(OAr)] strongly suggests the formation of the other vanadium(III) species by reduction with Me2AlCl or Et2AlCl, and the EXAFS analysis suggests the presence of the arylimido ligand and two V–Cl bonds (2.45 ± 0.03 Å). The XANES spectra showed no significant changes in both the pre-edge peak(s) and the edge peak when these complexes were treated with MAO, suggesting that the basic geometry and the high oxidation state were preserved under these conditions.

Synthesis of Half-Titanocenes Containing Anionic N-Heterocyclic Carbenes That Contain a Weakly Coordinating Borate Moiety, Cp′TiX2(WCA-NHC), and Their Use as Catalysts for Ethylene (Co)polymerization

Synthesis and structural analysis of half-titanocenes containing anionic N-heterocyclic carbenes with a weakly coordinating borate [B(C6F5)3] moiety (WCA-NHC) of the type, [Cp′TiX2(WCA-NHC)] [Cp′ =...

Amidine/Phosphine‐Oxide‐Based Nickel Catalysts for Ethylene Polymerization and Copolymerization

AbstractNickel catalyzed copolymerization of olefins with polar co‐monomers remains a great challenge in the field of olefin polymerization. In this contribution, a series of amidine/phosphine‐oxide‐based ligands were synthesized and investigated for nickel catalyzed ethylene polymerization and copolymerization with polar co‐monomers. These nickel catalysts exhibited high catalytic activities (homopolymerization: up to 2.6×106 g (mol of Ni)−1 h−1; copolymerization: up to 1.0×105 g (mol of Ni)−1 h−1;) and good thermal stabilities at 80 °C, generating high‐molecular‐weight polyethylenes (Mn up to 8.45×104 g mol−1) as well as copolymers (Mn up to 2.19×104 g mol−1). The Ni center exhibited square planar geometry. The N atom between the imine and phosphine‐oxide moieties exhibited trigonal planar geometry, indicating electron‐donating conjugate effect of the sp2 hybrid N‐atom linker. This electronic effect can increase the electron density on the nickel center and result in enhanced catalytic performance.

Co‐catalyst effects on the thermal stability/activity of N,N,N‐Co ethylene polymerization Catalysts Bearing Fluoro‐Substituted N‐2,6‐dibenzhydrylphenyl groups

The unsymmetrical bis (arylimino)pyridines, 2‐[CMeN{2,6‐{(4‐FC6H4)2CH}2–4‐t‐BuC6H2}]‐6‐(CMeNAr)C5H3N (Ar = 2,6‐Me2C6H3 L1, 2,6‐Et2C6H3 L2, 2,6‐i‐Pr2C6H3 L3, 2,4,6‐Me3C6H2 L4, 2,6‐Et2–4‐MeC6H2 L5), each containing one N‐aryl group bedecked with ortho‐substituted fluorobenzhydryl groups, have been employed in the preparation of the corresponding five‐coordinate cobalt (II) chelates, LCoCl2 (Co1 – Co5); the symmetrical comparator [2,6‐{CMeN(2,6‐(4‐FC6H4)2CH)2–4‐t‐BuC6H2}2C5H3N]CoCl2 (Co6) is also reported. All cobaltous complexes are paramagnetic and have been characterized by 1H/19F NMR spectroscopy, FT‐IR spectroscopy and elemental analysis. The molecular structures of Co3 and Co6 highlight the different degrees of steric protection given to the metal center by the particular N‐aryl group combination. Depending on the aluminoxane co‐catalyst employed to activate the cobalt precatalyst, distinct variations in thermal stability and activity of the catalyst towards ethylene polymerization were exhibited. In particular with MAO, the resultant catalysts reached their optimal performance at 70 °C delivering high activities of up to 10.1 × 106 g PE (mol of Co)−1 h−1 with Co1 > Co4 > Co2 > Co5 > Co3 >> Co6. On the other hand, using MMAO, the catalysts operate most effectively at 30 °C but are by comparison less productive. In general, the polyethylenes were highly linear, narrowly disperse and displayed a wide range of molecular weights [Mw range: 18.5–58.7 kg mol−1 (MAO); 206.1–352.5 kg mol−1 (MMAO)].

Mechanistic Study on the Dominant Promotion Effect of Al-/Ti-/Zr-modifications over the VOx/SiO2 UHMWPE Catalysts

Recently, we reported the first VOx/SiO2 ethylene polymerization catalyst for making Cl-free UHMWPE, and found the dominant promotion effects of Al-/Ti-/Zr-modifications over this catalyst system (Macromol. Chem. Phys. 2017, 218, 1600443). In this work, density functional theory is applied to investigate the underlying mechanism of this remarkable promotion effect of Al-/Ti-/Zr-modifications on a molecular and atomic level. The cluster model with V(III) is found to be the most possible active site due to its lowest overall energy barrier for monomer insertion, though the process of C2H4 coordination and the subsequent formation of transition state are most energy favored for V(II) species. By modifying one of or both V-O-Si in the active model with V-O-M (M = Al, Ti, or Zr), the energy barrier for the binding of the upcoming C2H4 gets lower (particularly for Al- and Zr-modified catalysts), and the transition state also becomes more stable. Generally, the insertion process of C2H4 gets easier after support Al-/Ti-/Zr-modifications. This dominant promotion effect is partially ascribed to the more enriched positive charge distribution on or nearby the V center, and the narrower energy gap between the LUMO of model catalysts and the HOMO of C2H4 for these modified catalysts also contributes much. In addition, the decreased steric hindrance around the V center should be taken into account for the modified models as well. Furthermore, the Bronsted acidity of the catalysts is investigated by introducing a pendent hydroxyl group to the model catalysts, which has a close contact with the V center. Similar promotion effect of support modification by Al, Ti, and Zr could still be observed.

Facile synthesis of sterically demanding SHOP‐type nickel catalysts for ethylene polymerization

The P,O‐chelated shell higher olefin process (SHOP) type nickel complexes are practical homogeneous catalysts for the industrial preparation of linear low‐carbon α‐olefins from ethylene. We describes that a facile synthetic route enables the modulation of steric hindrance and electronic nature of SHOP‐type nickel complexes. A series of sterically bulky SHOP‐type nickel complexes with variable electronic nature, {[4‐R‐C6H4C(O) = C‐PArPh]NiPh (PPh3); Ar = 2‐[2′,6′‐(OMe)2C6H3]C6H4; R = H (Ni1); R = OMe (Ni2); R = CF3 (Ni3)}, were prepared and used as single component catalysts toward ethylene polymerization without using any phosphine scavenger. These nickel catalysts exhibit high thermal stability during ethylene polymerization and result in highly crystalline linear α‐olefinic solid polymer. The catalytic performance of the SHOP‐type nickel complexes was significantly improved by introducing a bulky ortho‐biphenyl group on the phosphorous atom or an electron‐withdrawing trifluoromethyl on the backbone of the ligand, indicating steric and electronic effects play critical roles in SHOP‐type nickel complexes catalyzed ethylene polymerization.