Chapter 2
Recent Developments in Lanthanide Catalysts for 1,3-Diene Polymerization 1
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L. Porri , G. Ricci , A. Giarrusso , N. Shubin , and Z. Lu 1
Department of Industrial Chemistry and Chemical Engineering, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milan, Italy Istituto di Chimica delle Macromolecole, CNR, Via Bassini 15, 20133 Milan, Italy
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Lanthanide catalysts are typical for the cis polymerization of 1,3-dienes. The ternary system AlEt Cl - Nd(OCOR) - Al(iBu) (R = alkyl group) is now used for the commercial production of cis polybutadiene. The effect of catalyst component addition order on activity and heterogeneity has been examined. In the ternary systems, only part of the lanthanide (Ln) is active in the catalysis. Much more active systems are obtained using allyl derivatives of Nd, Pr, Gd, in combination with methylaluminoxane (MAO). Evidence is reported suggesting that Ln catalysts have an ionic structure. Ln catalysts are also active for the polymerization of substituted butadienes and give cis-isotactic polymersfromterminally substituted monomers. 2
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Cis-polybutadiene (cis-PB) is a synthetic elastomer whose properties are very close to those of natural rubber. Since the beginning of the stereospecific polymerization various catalysts have been identified for the synthesis of this polymer (/). The first one, proposed in 1956, was based on iodine containing titanium compounds, T1I4 or T1CI2I2, in combination with aluminum trialkyls, and gave a ca. 93% cis polymer (2,5). More stereospecific systems, capable of yielding a 97-98% cis-PB, were later obtainedfromcobalt (1957) (4,5) and nickel (1963) (6-8) compounds. In thefirsthalf of the 60s, some patents and papers appeared on the use of lanthanide catalysts for the cis polymerization of 1,3-dienes. Catalysts based on cerium (9) were the first used, but were soon abandoned, because the cerium residues catalyze the oxidation of the polymers. Catalysts based on neodymium, praseodymium and gadolinium were studied by Chinese groups (10). In Italy, Eni developed uranium systems, but no 3
On leave from Institute of Macromolecular Compounds RAS, St. Petersburg, Russia. Current address: Department of Applied Chemistry, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan.
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© 2000 American Chemical Society
In Olefin Polymerization; Arjunan, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
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16 practical application ensued, because of the problems connected with the uranium residues in the polymer (1,11-14). The work carried out in the 60s and 70s showed that lanthanide catalysts have some advantages over the Ti, Co, Ni catalysts proposed before. In particular, they give a cis-PB more linear and hence more suitable for use in tires, which is by far the most important application (ca. 80%) for cis-PB. In addition, lanthanide catalysts are free of cationic activity and are more active in aliphatic than in aromatic solvents (75). On the other hand, neodymium-based systems appeared as the most interesting among the lanthanide catalysts. They do not have the disadvantages of Ce or U systems and are more active than Gd or Pr systems. As a consequence, in recent years research work focused on Nd systems. This work has led to the identification of new catalytic compositions and to a better understanding of the nature of these systems. The present paper reports on the most significant results of this work. Conventional Neodymium Systems In their simplest form, Nd catalysts can be obtained by reacting a Nd compound [Nd(acac) ; Nd-2-ethyl-hexanoate; Nd(OCOC Hi )3; NdCb; NdCb complexed with donors] with an aluminum trialkyl. However, catalysts of practical interest are obtained only if CI is present in the Nd compound. Al(iBu)3-Nd(OCOC Hi5)3 is a poor catalyst for butadiene polymerization and gives a polymer consisting of predominantly trans units (16). Catalyst activity and polymer cis content increase in the order: Nd(OCOC Hi )3 < Nd(OCOC Hi ) Cl < Nd(OCOC Hi )Cl < NdCl . Among the aluminum alkyls, Al(iBu)3 is more active than AlEt3 and Α1Μβ3 (17). Various binary systems based on complexes NdCb-nD (D = electron donor) and an aluminum alkyl are reported in patents and papers (1,10,18), but ternary systems based on a Nd carboxylate, a chlorine donor and Al(iBu)3 are more useful commercially. AlEt2Cl or Al Et3Cl3 are the most commonly used chlorine donors, but tert-butyl chloride is reported in some patents and papers (19). Two methods can be used for the preparation of the catalyst, differing for the order of catalyst component addition: 1) Nd(OCOC Hi5) isfirstreacted with the chlorine donor, then with Al(iBu)3 (20): 3
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Nd(OCOC Hi ) + 3Affit Cl -> NdCl + 3Affit (OCOC Hi ) 7
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NdCb + 30 Al(iBu)3 -> active species (b) 2) Nd(OCOC His)3 is first reacted with Al(iBu)3, then the chlorine donor is added (19): 7
Nd(OCOC Hi ) + 30 Al(iBu) -> Nd-Al complex 7
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Nd-Al complex + AlEt Cl - » active species (d) We have examined the influence of the order of catalyst component addition on both heterogeneity and activity of the system. 1. When AlEt Cl is gradually introduced into a stirred heptane solution of Nd(OCOC Hi5) at room temperature, a precipitate immediately forms, consisting of NdCl or NdCl (OCOC Hi ), depending on the Cl/Nd ratio. At the end of the 2
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In Olefin Polymerization; Arjunan, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
17 addition of AlEt2Cl, stirring is continued for ca. 15 minutes, then Al(iBu>3 is introduced (Al/Nd molar ratio about 30) into the stirred suspension. In our runs, the final Nd concentration was about 2Η4Χ10" M. Reaction (a) is rapid and gives a finely subdivided precipitate consisting of the Nd-chloride. Reaction (b) is slow, as shown by the fact that the activity of the catalytic suspension increases with time, at room temperature (Figure 1; heterogeneous system). 2. The reaction between Nd(OCOC7Hi5>3 and Al(iBu>3 in heptane ([Nd] about 210" M) does not produce a precipitate, at room temperature. Nothing is known about the product of this reaction, but it has been reported (21) that the reaction between AlMe3 and Nd(0-*Bu)3 gives a bimetallic complex of structure [^(μ-0 Βυ)3(μ-Μβ)3(Α1Μβ2)3]. It is plausible that analogous Al-Nd complexes are formed in reaction (c). However, this reaction is slow, as shown by the fact that addition of AlEt2Cl gives different results depending on how long Nd(OCOC7His) and Al(iBu)3 are let to react. 2
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i) If AlEt2Cl is added to the stirred solution of reaction (c) 1-2 hours after its beginning, a precipitate is slowly formed in a few hours; however, the precipitate appears after days for longer reaction times of (c). ii) If reaction (c) is kept at room temperature for more than about 10 days, the subsequent addition of AlEt2Cl does not produce a precipitate, even after weeks. A soluble catalyst is formed by this procedure. However, the formation of the active species in reaction (d) is also a slow process. Figure 1 (homogeneous system) shows the variation of activity with time of a catalyst prepared by adding AlEt2Cl to a solution of reaction (c) kept for ten days at room temperature. Time is measured startingfromthe addition of AlEt2Cl. Polymer cis content (> 96%) and intrinsic viscosity were found to be practically independent of catalyst aging time. The soluble catalysts gave polymers having molecular weights lower than the heterogeneous ones ([η] ca. 7.5 dL/g vs. ca. 10 dL/g). Neodymium 2-ethyl-hexanoate and Nd-salts of versatic acids (branched fatty acids) are most commonly used for the catalyst preparation, as mentioned before. Some differences have been observed between thefreshlyprepared catalysts obtained from each compound, but the activity of the two systems tends to level off with aging. Catalysts Based on Nd-allyl Compounds The number of active centers for the heterogeneous system AlEt2ClNd(OCOC7His)3-Al(iBu)3 has been determined by Russian authors and found to be rather low. About 7-8% of the Nd is active infreshlyprepared catalysts (22). The data of Figure 1 indicate that aging the heterogeneous catalysts for aboutfiveweeks enhances significantly the activity, by a factor of about 3. The fact that polymer molecular weight is independent of catalyst aging time suggests that activity enhancement is mainly due to an increase in the number of active centers. However, the percentage of the metal active in catalysis ( 10
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