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Review

Homogeneous Group IVB Catalysts of New Generations for Synthesis of Ethylene-Propylene-Diene Rubbers: A Mini-Review

by
Natalia M. Bravaya
*,
Evgeny E. Faingol’d
*,
Evgeny A. Sanginov
and
Elmira R. Badamshina
Institute of Problems of Chemical Physics RAS, Academician Semenov Avenue 1, 142432 Chernogolovka, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(7), 704; https://doi.org/10.3390/catal12070704
Submission received: 6 June 2022 / Revised: 21 June 2022 / Accepted: 25 June 2022 / Published: 27 June 2022
(This article belongs to the Special Issue Novel Catalysts for Polyolefin and Synthetic Rubber)
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Review Reports Versions Notes

Abstract

:
Ethylene-propylene-diene rubbers (EPDM) are one of the most important polyolefin materials widely commercialized and used in various industries in recent years. The production of EPDM is based solely on catalytic coordination polymerization processes. The development of new catalysts and processes for the synthesis of EPDM has expanded the range of products and their manufacturing in terms of energy efficiency, processability, and environmental safety. This mini-review mainly analyzes patented data on the synthesis of EPDM on new-generation single-site catalytic systems based on Group IVB complexes including the systems commercialized by major manufacturers of EPDM. The advantages of these systems are evident in comparison with conventional vanadium systems introduced into production in the 1960s and used to date in the industrial synthesis of EPDM.

1. Introduction

Ethylene-propylene-diene rubbers (EPDM) are ternary copolymers of ethylene, propylene, and non-conjugated dienes, 5-ethylidene-2-norbornene (ENB), dicyclopentadiene (DCPD), and 5-vinyl-2-norbornene (VNB). These elastomeric materials are used as materials showing excellent resistance to heat, air, ozone, and steam [1,2,3,4].
EPDM isproduced using Ziegler-type ion-coordination polymerization catalysts. Homogeneous catalytic systems developed in the early 1960s and widely used up to now include a combination of a vanadium precatalyst (VCl4, V(acac)3, more often VOCl3) and an organoaluminum cocatalyst (AlEt2Cl, AlEt3, Al2Et3Cl3) in slight excess to the precatalyst (Al/V~10 mol/mol), as well as chlorine-containing promoters such as ethyltrichloroacetate, n-butylperchlorocrotonate, etc. [3,5,6,7]. These catalytic systems provide the formation of random, fully amorphous terpolymers with a high ratio of comonomers incorporation showing very good elastomeric properties and operating in a wide temperature range (−50–+130 °C). However, these systems show low activity (80–120 kg of copolymer/molV) and poor catalytic stability even at 20–60 °C. The consequence of low activity is the high residual content of a catalyst and promoters in the polymer, which ishighly toxic and hasa negative effect on the properties of the copolymer. The presence of undesirable impurities requires additional technological operations to remove them.
New opportunities in the synthesis of EPDM are opened by single-site catalytic systems based on metallocene, constrained geometry, half-sandwich, and post-metallocene chelate complexes of Group IVB transition metals. As compared to vanadium ones, these systems have a number of advantages such as high activity, stability at elevated temperatures, single-site nature of active sites, which ensures uniformity of molecular weight characteristics, the microstructure of terpolymers, and, hence, the possibility of creating EPDM with desired properties. It is worth emphasizing that in recent years, there has been a surge of interest in the world’s main elastomer manufacturers (ARLANXEO, Dow Elastomers, ExxonMobil, Mitsui Chemicals, etc.) to new-generation single-site catalytic systems, likelycaused by more stringent requirements for the environmental safety of production and properties of products. A wide range of EPDM properties produced on these systems by the world’s leading manufacturers can be illustrated in Table 1.
In this regard, we have tried to briefly review the current achievements in the development of new promising homogeneous single-site catalytic systems for the EPDM synthesis. In contrast to our earlier review [4], where various aspects of the synthesis of ternary copolymers of ethylene, propylene, and both linear and cyclic dienes have been considered, here we focus on catalysts patented and commercialized by leading manufacturers over the last years.

2. Metallocene Catalysts

Metallocene catalysts (MC) are a broad class of Group IVB complexes in which a transition metal is bounded by π-bonds to two cyclopentadienyl rings of substituted or unsubstituted ligands: Cyclopentadienyl (Cp), indenyl (Ind), and fluorenyl (Flu) [13,14]. This class of complexes of different symmetry (C2v, C2, Cs, C1) and different structures has been widely investigated for 80years as catalysts for homo- and copolymerization of ethylene and α-olefins. Since the mid-1990s, these complexes have also beenstudied as catalysts for the synthesis of EPDM using linear and cyclic dienes. In general, it should be noted that, on the one hand, the copolymerization of olefins with linear nonconjugated dienes on metallocene catalytic systems significantly expands the possibilities of creating new polymer hybrid structures, which cannot be produced by conventional vanadium-based catalysts. On the other hand, the characteristics of the resulting EPDM are unsatisfactory in most cases due to the occurrence of side reactions such as cyclization or crosslinking. As regards copolymerization with cyclic dienes, one can generally note a high response of the system activity, the ability to insert a diene, and the molecular weights of copolymers in the nature of the metallocene precatalyst and diene concentration. In general, it can be noted that the efficiency of incorporation of ENB or VNB into the chain of the macromolecule is much lower as compared to propylene mainly due to the steric hindrance from both diene and metallocene. Usually, an increase in the concentration of the diene is accompanied by a decrease in the activity and molecular weight of the formed copolymers.
Since, generally, different catalytic systems and conditions are described for EPDM synthesis, it is difficult to compare and generalize available data on their preparation. Thus, it is interesting to compare the experimental data given in the patent [15] since the process protocol is the same for metallocene catalytic systems of various symmetry and structure and various dienes. The bridging unsubstituted bisindenylzirconocenes of C2 symmetry, namely, rac-Me2SiInd2ZrCl2 and rac-EtInd2ZrCl2 activated by polymethylalumoxane (MAO), the most widely used activator, exhibit very high activity (up to 26,000 and 34,700 kg copolymer/(molZr h atm)) in ternary copolymerization with ENB providing the incorporation of a diene up to 5–7 wt%. Copolymers show high molecular weights of Mw = 164 and 142 kDa, respectively. However, the Mooney viscosity (ML 1 + 4 (125 °C)) of copolymers is low. The corresponding hafnocenes exhibit significantly lower activity, which, nevertheless, is higher than the activity of vanadium systems (210 and 1160 kg of copolymers/(molHf h atm)), but form EPDM with a high molecular weight (Mw = 162 and 514 kDa) with a diene content of 4 and 12 wt%. The ML 1 + 4 (125 °C) value for the copolymer prepared with rac-EtInd2HfCl2 is 70. The activity of the systems in ternary copolymerization with DCPD is 2–3 times lower than that with ENB. At the same time, the differences in the characteristics of ternary copolymers obtained with ENB and DCPD are insignificant. A good combination of properties was shown by catalytic systems based on ansa-zirconocene of Cs symmetry Ph2CCpFluZrCl2. The activity of systems in ternary copolymerization with ENB and DCPD reaches 10,800 kg copolymer/(molZr h atm), the diene content is ~7 wt%, Mw≈230 kDa, Mw/Mn = 2.0, and ML 1 + 4 (125 °C) ≈ 20. From the data given in the patent, it can be concluded that in ternary copolymerization with ENB, the steric characteristics of zirconocenes are manifested to a greater extent than in copolymerization of ethylene with propylene. For example, the zirconocene, which has methyl substituents at the 2-position of the indenyl system, exhibits very low activity at the same composition of the comonomer mixture and does not provide diene incorporation.
Commercial interest incatalysts of Cs symmetry can be demonstrated by the patent data of Mitsui Chemicals [16,17,18,19,20]. They claim a new effective class of hafnocene and zirconocene complexes of type 1 (Figure 1) containing different substituents in both the bridging group and the peripheral positions of the Flu ligand. When activated with CPh3B(C6F5)4 (4 and 10 equiv.), the complexes effectively provide ternary copolymerization of E/P/ENB at 80–120 °C. Hafnocenes 1 ensure the incorporation of ENB at a level of 7–11 wt% and form elastomers with high molecular weights (Mw = 1000–2000 kDa) [16,17,18]. The compounds with R1 = 4-MePh, 4-MeOPh, 4-Me2NPh and R2 = Me show the highest activity up to a 10,000–20,000 kg copolymer/(molHf h atm)). Zirconocenes 1 show high efficiency in the synthesis of well-vulcanizing VNB terpolymers with a low degree of branching [19]. Such copolymers cannot be obtained by using conventional vanadium and other metallocene systems. The content of VNB in this EPDM reaches 10 wt%. The molecular weights range from 50 to 600 kDa.
Another promising type of MC catalysts for the synthesis of EPDM is a new group of complexes of type 2 (Figure 1) developed by Lotte Chemical [20]. Their activity upon activation with MAO in the ternary copolymerization of ethylene, propylene, and ENB reaches 155,000–190,000 kg copolymer/(molZr h) at 80 °C. Terpolymers with Mw = 163–209 kDa, Mw/Mn = 5.6–9.3, and ENB content up to 7.1–8.5 wt% have been obtained.
In the vast majority of publications, MAO (or modified MAO) and perfluoroaryl borates are reported to be used to activate metallocene precatalysts in the synthesis of polyolefins [13,14,21,22,23], including EPDM [4,15,16,17,18,19,24,25,26]. However, MAO is too expensive, unstable during storage, and is used in large molar excess to the precatalyst. The borates are extremely sensitive to impurities and demonstrate unstable polymerization kinetics. Thus, the development of new effective inexpensive activators of MC is an actual task thatshould also be noted.
Among promising alternative activators, the following can be noted. Thus, researchers from the Uniroyal Chemical Comp successfully employ a mixture of boraryl compounds LiB(C6F5)4 and B(C6F5)3, which provide higher process stability even at elevated temperatures [15]. Other new effective activators for the synthesis of EPDM based on isobutylaluminumoxanes (-O-Al(Bui)-)n and isobutylaluminum aryl oxides (Al(Bui)3−kOark) have been proposed [27,28,29,30,31,32]. These compounds effectively activate MC precatalysts at low molar excess (200–300 molar equiv.) and, unlike conventional MAO-based systems, allow the production of copolymers with a low content of propylene blocks, which determines their good elastomeric properties.

3. “Constrained” Geometry Complexes

Special attention in the synthesis of EPDM is paid to bridged monocyclopentadienyl complexes, the so-called “constrained” geometry complexes (CGCs). CGCs contain aη5-Cp-ligand linked by a bridging group with a donor ligand (Don). The bridging group reduces the Cp–M–Don angle by approximately 20–30° as compared to Cp–M–Cp, which ensures high availability of a transition metal in the active site and, thus, a high degree of comonomer incorporation [33,34,35]. The donor is linked to the transition metal by a σ-bond and ensures high stability of catalysts of this type at elevated temperatures (up to 160–180 °C). Variations in any part of the CGC ligands, as well as the type of transition metal, affect catalytic properties of the complexes, such as activity, the ability to insert comonomer(s), and molecular weight characteristics of the resulting polymers.
CGCs 3 (Figure 2) were commercialized by DOW in the mid-1990s for homogeneous polymerization processes used for the production of linear low-density polyethylene [35,36,37,38]. Complexes of this type are also effective for the synthesis of EPDM [37,38], which allowed DOW to launch a wide range of elastomers (NordelTM IP). CGCs provide the production of EPDM with a wide range of Mw (up to 500 kDa), composition (up to 50 wt% of propylene and 7.5 wt% of ENB), and Mooney viscosity (ML 1 + 4 (125 °C) = 18–85).
In recent years, one can note increasing interest incatalysts of this type from several leading EPDM manufacturers. From the late 1990s to the present, active developments in the field of CGC molecular design have been carried out at Sumitomo Chemical, Kumho Polychem, and LG Chem, which have made it possible to significantly expand the range of promising catalysts for the synthesis of EPDM [39,40,41,42,43,44,45].
In the late 1990s and early 2000s, Sumitomo developed a new type of CGC, which received the general name PHENICS (phenoxy-induced complex of Sumitomo). The key element in catalysts of this type is the presence of a side donor phenoxy group with a bulky substituent in the ortho position. Precatalyst 4 (Figure 2) was found to be promising for the synthesis of EPDM [39]. When activated by CPh3B(C6F5)4 in the presence of TIBA at 60 °C and a molar ratio of comonomers E/P/ENB = 0.76/2.08/0.016, it provides a highly active system with the activity of 53,200 kg copolymer/(molTi h) that forms a copolymer with a molar composition of 66.7/30.8/2.5.
New effective CGCs based on bimetallic complexes (complex 5 (Figure 2), Y=–C6H4–, –CH2–C6H4–; R = tBu, cyclohexyl (Cy)) were developed by Kumho Polychem, one of the largest EPDM manufacturers in the world [40,41]. These catalysts are capable of carrying out the copolymerization of ethylene with propylene and the ternary copolymerization of ethylene, propylene, and ENB with high activity in a wide temperature range (up to 150 °C) and produce rubbers with high molecular weight and easily controlled molecular weight characteristics (Mw = 220–400 kDa, Mw/Mn = 2.5–5.1) due to their binuclear structure. The compounds show a high ability to incorporate both propylene and dienes. The content of propylene and ENB in the formed copolymers reaches up to 50 wt% and 20 wt%, respectively.
Binary catalytic systems based on tetrahydroquinolinyl- and trihydroindolinyl-cyclopentadinyl titanium complexes (complexes 6 (Figure 2)) were developed by LG Chem [42,43,44,45]. When Me2NHPhB(C6F5)4 is used as an activator, they provide high activity (up to 18,300–49,000 kg copolymer/(molTi h)) and are able to operate at temperatures up to 160 °C in a flow reactor. It is claimed that these systems make it possible to vary the composition of the resulting long-chain branched terpolymers over a wide range (E/P/ENB = (50–70)/(24–48)/(5–12) wt%) to obtain terpolymers with a high molecular weight (Mw = 150–300 kDa, Mw/Mn = 2.7–3.2) and ML 1 + 4 (125 °C) = 30–80. Theelastic terpolymers produced are capable of satisfying excellent processability and elasticity (flexibility) at the same time.

4. Half-Sandwich Titanium Complexes

Another promising class of catalysts for the synthesis of EPDM are half-sandwich titanocenes 7–11 (Figure 3) containing N-donor ligands [46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64]. The development of such compounds began with the discovery by Nova Chemical of phosphinimide (7) and ketimide (8) half-sandwich titanium catalysts in 2001 [46]. In 2007, DSM Elastomers (later Lanxess, now ARLANXEO) announced that it had entered a licensing agreement with Nova Chemical to obtain exclusive rights to manufacture EPDM using catalytic technology based on complexes of 7 and 8 [65]. This new technology was called the Keltan Advanced Catalyst Elastomer (Keltan ACE™). Later, the molecular design of the N-donor ligand within the framework of Keltan ACETM technology made it possible to significantly expand the effective series of catalysts due to guanidinate [33,51,52], iminoimidazolinate [33,47,48,49,50,51], and k1-amidinate [33,51,52,53,54,55,56,57,58,59,60,61,62,63,64] titanium complexes (structures 9–11, respectively).
Keltan ACETM catalysts are characterized by high catalytic activity (up to 1200 tons copolymer/(molTi h)) in the ternary copolymerization of olefins and dienes and make it possible to obtain copolymers with a high content of dienes (up to 15 wt% of ENB, 5 wt% of VNB, and 20 wt% of DCPD). Most of these catalysts are stable in copolymerization processes at temperatures up to 120 °C and form copolymers with high Mw values up to 2000 kDa. A variety of developed complexes of this type makes it possible to obtain EPDM with a wide range of properties [52,53,54,55,56,57,58,59,60,61,62,63,64].
On the basis of Keltan ACE™ technology, large-scale production of EPDM has been organized at the plants in Geleen, Netherlands, and Changzhou, China, each with a listed annual capacity of 160,000 tons, along with 40,000 tons per year in Triunfo, Brazil [51,66,67].

5. Post-MetalloceneChelate Catalysts

The most promising catalysts of this class for the synthesis of EPDM are aryloxyether 12, bis(phenolate) 13, pyridyldiamido 14, and quinolinyldiamido 15 complexes of hafnium and zirconium (Figure 4). The compounds were discovered using the approaches of Combinatorial Chemistry and High-Throughput Screening [68,69,70,71,72] and have been actively developed by Dow Elastomers and ExxonMobil for the last 15 years [37,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90].
Aryloxyether complexes 12 have the most unique catalytic characteristics and by the sum of properties surpass many known classes of catalysts for the production of EPDM [37,72,73,74,75,76,77,78,79,80]. The advantages of the catalysts are the ability to be effectively activated by low molar amounts of the activator (AlMAO/M = 10–200, Bborate/M up to 5 mol/mol), and high activity in copolymerization processes in high-temperature solution polymerization conditions (120–180 °C). The advantage of a high-temperature process carried out in the solution polymerization (the polymer is soluble in the reaction medium) is the formation of amorphous copolymers with broad molecular weight distribution. A feature of these systems is also their ability to form copolymers with high molecular weights (Mw = 100–1000 kDa) and low content of the gel fraction [37]. Such catalysts were used to obtainrubbers with a high content of propylene (up to 50 wt%) and dienecomonomers of various types (ENB, VNB, DCPD, etc., up to 16 wt%). Aryloxyether complexes are the basis of highly efficient Advanced Molecular Catalyst technology (AMC) developed in 2015 for obtaining new NordelTM rubber grades with improved properties [37]. Based on this technology, a plant with an annual capacity of 200,000 tons of EPDM was launched in Plakimin (USA) in 2018 [81].
Bis(phenolate) catalysts 13 structurally similar to aryloxyether complexes have been actively developed in recent years by ExxonMobil [82,83,84,85]. The compounds show high catalytic efficiency in the copolymerization of ethylene or propylene with ENB at high temperatures (100 °C). They are able to form copolymers with high molecular weights Mw = 100–1000 kDa and ENB content up to 20 wt%. Undoubtedly, the further development of these complexes will ensure the creation of new effective catalysts for EPDM synthesis.
Other promising candidates of chelate catalysts for the synthesis of EPDM are pyridyldiamido 14 and quinolinyldiamido 15 hafnium complexes [86,87,88,89,90]. The use of these complexes in the production of branched and bimodal EPDM is described in patent literature. Branched copolymers are produced on catalysts in the presence of an organoaluminum chain transfer agent. Such EPDM contains 35–70 wt% of ethylene, 20–64 wt% of propylene, and up to 10 wt% of diene, and hasa viscosity of ML 1 + 4 (125 °C) = 30–120. Bimodal EPDM are produced on pyridyldiamido 14 and quinolinyldiamido 15 catalysts in the presence of a hafnocene complex of Cs symmetry. These EPDMs contain a high molecular weight fraction (Mw = 2000–3000 kDa) of up to 20 wt% and a low molecular weight fraction (100–2000 kDa) above 80 wt%. The content of comonomers in the minor fraction is 40–80 wt% of ethylene, 20–60 wt% of propylene, and 1–7 wt% of diene. The composition of the other fraction is 10–50 wt% of ethylene, 50–90 wt% of propylene, and 1–7 wt% of diene. Bimodal EPDM shows excellent elasticity, toughness, and processibility.
Post-metallocene chelate complexes of hafnium and zirconium are among the most advanced catalysts for the synthesis of EPDM. A wide variety of compound structures makes it possible to tune the resulting properties of terpolymers. The potential for molecular design of the catalysts in this field is enormous, and we expect great success in the development of new unique elastomeric materials with unique properties and a wide range of characteristics.

6. Conclusionsand Perspectives

In conclusion, it should be noted that despite the fact that EPDM have being produced by the industry for more than 50 years, there is a high potentialin both improving the technologies for the synthesis of EPDM and expanding the range of their grades. This potential is ensured by the development of new-generation catalytic systems based on Group IVB complexes, which are alternatives to low-activity, unstable, and toxic vanadium catalysts. In particular, this is evidenced by high research activity over the past 10–15 years of the world’s leading manufacturers of EPDM in the development of new systems and their active commercialization. The main advantage of modern systems compared to vanadium ones is their environmental safety, higher activity, and thermal stability, with the possibility of producing EPDM with different contents of comonomers and microstructures, including highly branched and bimodal terpolymers. The use of these catalytic systems has already made it possible to bring to market new grades of EPDM with a wide range of properties (for example, Keltan® (ARLANXEO), NordelTM IP (Dow Elastomers), VistalonTM (Exxon), Mitsui EPTTM (Mitsui), etc.).
The huge potential for tuning the structures of catalytic systems allows us to count on the further creation of new efficient systems for the synthesis of EPDM and, accordingly, the production of elastomers with a high content of diene(s) and the creation of new polymer microstructureswith an even wider range of properties.

Author Contributions

Conceptualization, N.M.B. and E.R.B.; investigation, N.M.B., E.E.F., E.A.S. and E.R.B.; writing—original draft preparation, N.M.B. and E.E.F.; writing—review and editing, N.M.B., E.E.F., E.A.S. and E.R.B.; visualization, E.E.F. and E.A.S.; supervision, N.M.B.; project administration, E.R.B.; funding acquisition, N.M.B. and E.R.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education of the Russian Federation (State Task, No. registration AAAA-A19-119041090087-4).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Catalysts 12 00704 g001 550
Figure 1. Chemical structures of MCs 1 and 2.
Figure 1. Chemical structures of MCs 1 and 2.
Catalysts 12 00704 g001
Catalysts 12 00704 g002 550
Figure 2. Chemical structures of CGCs 3–6.
Figure 2. Chemical structures of CGCs 3–6.
Catalysts 12 00704 g002
Catalysts 12 00704 g003 550
Figure 3. Chemical structures of half-sandwich titanium complexes 7–11.
Figure 3. Chemical structures of half-sandwich titanium complexes 7–11.
Catalysts 12 00704 g003
Catalysts 12 00704 g004 550
Figure 4. Chemical structures of post-metallocene chelate complexes 12–15.
Figure 4. Chemical structures of post-metallocene chelate complexes 12–15.
Catalysts 12 00704 g004
Table
Table 1. EPDM characteristics of leading manufacturers.
Table 1. EPDM characteristics of leading manufacturers.
EPDM
Manufacturer		Polymer GradeEthylene Content, wt%Diene (ENB) Content, wt%Mooney Viscosity
ML1+4(125 °C) 1		Polymer StructuresRef.
ARLANXEOKeltan®44–710–11.022–92LCB 2,
ND 3, MD 4, BD 5		[8]
DOW ElastomersNordelTM50–850–8.518–85ND, MD, BD[9]
ExxonMobilVistalonTM54–770–10.016–82LCB,
ND, MD, BD		[10]
Mitsui ChemicalMitsui EPTTM41–720–14.040–78LCB,
ND, MD, BD		[11]
KumhoPolychemKEP®55–710–10.023–95No data[12]
1 Mooney viscosity is the routine industry standard correlated with molecular weight of EPDM. High Mooney viscosities are preferred. Abbreviation according to ASTM D1646 is ML 1+4 (125 °C), where M—Mooney units, L—large rotor, 1—the sample preheat time (min), 4—the sample test time (min) at 125 °C. 2 Long Chain Branching. 3 Narrow molecular weight distribution.4 Medium molecular weight distribution. 5 Broad molecular weight distribution.
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Bravaya, N.M.; Faingol’d, E.E.; Sanginov, E.A.; Badamshina, E.R. Homogeneous Group IVB Catalysts of New Generations for Synthesis of Ethylene-Propylene-Diene Rubbers: A Mini-Review. Catalysts 2022, 12, 704. https://doi.org/10.3390/catal12070704

AMA Style

Bravaya NM, Faingol’d EE, Sanginov EA, Badamshina ER. Homogeneous Group IVB Catalysts of New Generations for Synthesis of Ethylene-Propylene-Diene Rubbers: A Mini-Review. Catalysts. 2022; 12(7):704. https://doi.org/10.3390/catal12070704

Chicago/Turabian Style

Bravaya, Natalia M., Evgeny E. Faingol’d, Evgeny A. Sanginov, and Elmira R. Badamshina. 2022. "Homogeneous Group IVB Catalysts of New Generations for Synthesis of Ethylene-Propylene-Diene Rubbers: A Mini-Review" Catalysts 12, no. 7: 704. https://doi.org/10.3390/catal12070704

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