Activity pubs.acs.org/jchemeduc
Illustrating Catalysis with Interlocking Building Blocks: Correlation between Structure of a Metallocene Catalyst and the Stereoregularity of Polypropylene Ryo Horikoshi,* Yoji Kobayashi, and Hiroshi Kageyama Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *
ABSTRACT: Catalysis with transition-metal complexes is a part of the inorganic chemistry curriculum and a challenging topic for upper-level undergraduate and graduate students. A hands-on teaching aid has been developed for use during conventional lectures to help students understand these catalytic reactions. A unique method of illustrating the coordination polymerization of propylene with interlocking building blocks is described. Three metallocene block models mimicking real catalysts recognize top and bottom faces of a prochiral propylene monomer block model and subsequently provide three polypropylene structures with different tacticity. The monomer recognition processes of the metallocene block models are based on steric hindrance, similar to those of the real catalysts. KEYWORDS: Upper-Division Undergraduate, Inorganic Chemistry, Polymer Chemistry, Analogies/Transfer, Hands-On Learning, Catalysis, Organometallics, Polymerization
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The use of such moving parts would provide abundant ways to illustrate not only the structure, but also the reactions of materials. This hands-on activity is suitable for upper-level undergraduates or graduate students in learning coordination chemistry or polymer chemistry and allows students to consider the importance of molecular design and structure−property relationships. Polypropylene can be produced by coordination polymerization of propylene with metallocenes as a homogeneous catalyst.5−7 The mechanism of the metallocene catalyst involves a cationic metallocene−alkyl complex with vacant site (Figure 1A). A propylene monomer coordinates to the vacant site and then inserts into the metal−alkyl bond. The propagation of a polymer chain is accomplished by repeating monomer coordination and insertion. Polypropylene can be classified into three categories in terms of tacticity (stereoregularity) (Figure 1B).8−11 The tacticity of polypropylene originates from the prochirality12 of propylene (Figure 1C).13 The methyl groups in isotactic polypropylene are arranged on the same side of the polymer backbone, whereas those in the syndiotactic one are arranged on alternate sides of the polymer backbone. In atactic polypropylene the methyl groups are oriented randomly along the polymer backbone. The tacticity determines much of the polymer’s mechanical properties.11 The isotactic and syndiotactic polypropylenes are crystalline materials, while the
ne possible reason that students have difficulty understanding catalytic properties of transition-metal complexes is that they are not familiar with visualizing the stereochemistry around the metal center in 2-D drawings. This is because some metallocene complexes and their properties encountered in catalysis involve complex geometries. General ball-and-stick model kits cannot construct metal− carbon bonds between the metal center and a π-ligand in a metallocene, and instead, 3-D computer software is required to represent complicated metallocenes accurately. However, implementation of this for all students is not always simple, and still is a type of passive learning. Here, we have utilized interlocking building blocks as a teaching aid to represent the structure and reaction of metallocenes. The interlocking building blocks are less expensive than computer software and more versatile than the ball-and-stick model. Most importantly, the models permit the students to have a handson opportunity to construct the catalyst and polymer chain, ensuring an active learning experience. Interlocking building blocks have been utilized as an educational tool in chemistry and mainly represented various static nanoscale structures.1−3 The blocks possess unique properties, for example, abstract shape, various colors, and connectivity, enabling the same blocks to represent different things in different models. Another characteristic feature of block models is their ability to incorporate moving parts. In interlocking building block packages, there are many rotating parts and hinge blocks to construct movable block models.4 © 2013 American Chemical Society and Division of Chemical Education, Inc.
Published: March 26, 2013 620
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help link propylene monomers in a polymer chain, and the magnet further away from the methyl group enables the monomer to snap into the metallocene piece. Figure 3 depicts
Figure 3. Preparation of metallocene block models 1−3 and corresponding real metallocene catalysts.
the metallocene block models 1−3 along with illustrations of corresponding real catalysts. The red 2 × 2 block, indicating a metal center, is equipped with a magnet, so that the propylene monomers can bond to the metal center using their magnets. Building this model requires about 10 min.
Figure 1. (A) Mechanism for olefin polymerization, (B) tacticity of polypropylene, and (C) prochirality of metal adducted propylene monomer.
atactic polypropylene is a viscous liquid.12 Most metallocene catalysts are rationally designed such that a monomer coordinates to the metal center in a set fashion; hence, the catalysts are able to produce such stereoregular polymers.13−15
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ACTIVITY DETAILS AND DISCUSSION Videos S1−S3 (Supporting Information) show the polymerization of propylene monomer blocks by the metallocene models 1−3 that instructors can also use to demonstrate how to make polymer models with different stereoregularity. The total time required for the three polymerization activities is about 20 min. During the polymerization activity, the block models must behave according to the following rules: (1) the propylene monomers coordinate to the red 2 × 2 block of the metallocene models with their 2 × 3 block (i.e., alkene coordination) and (2) the metallocene models always rotates 180°. The metallocene models 1 and 2 force the monomer to adopt a specific orientation by steric hindrance between the 1 × 8 or 1 × 10 blue blocks around the metal center and the methyl moiety of the propylene monomer model (Figure 4, right and center). In real catalysts, the blue block is constructed by bulky organics such as indene or fluorene derivatives.5,6,13 Metallocene model 1 chooses propylene monomers with the same face upward (i.e., same prochiral direction of attack) to produce an isotactic polypropylene block model in which methyl groups are located one side of the polymer backbone (Figure 4, right). In contrast, metallocene model 2 forces each sequential monomer model block to flip over to show its bottom face, constructing a syndiotactic polypropylene model in which methyl groups are located right and left of the polymer backbone regularly (Figure 4, center; see video files in the Supporting Information). The metallocene model 3, on the other hand, cannot discriminate between the top or bottom faces of the propylene monomers because the two blades in 3 have no significant steric hindrance with the methyl group of propylene (Figure 4, left). Therefore, the metallocene model 3
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PREPARATION Figure 2 (top) shows the assembly of a propylene monomer model in which the 2 × 2 block (2 pegs wide and 2 pegs long)
Figure 2. (Top) Assembly of a propylene monomer model. (Bottom) Positions of two magnets.
and a 2 × 3 block represent a methyl group and a vinyl group, respectively, of a propylene monomer. The 2 × 3 block is equipped with two magnets (neodymium magnet, 4ϕ × 2 mm, ca. 300 mT). One of the two magnets is fitted into the side further from the methyl group, while the other is fitted into the side near the methyl group (Figure 2, bottom). Both magnets 621
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polymerization of propylene is the differentiation between the two prochiral surfaces of the monomer by the metallocene catalyst, which has been visualized by the block models effectively. However, the block models are just a teaching aid for lectures and cannot replace 3-D computer visualizations. Illustrating chemical reactions on catalysis with interlocking building block models is a subject for more future studies.
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ASSOCIATED CONTENT
S Supporting Information *
Demonstrations of coordination polymerization of propylene monomer block models by metallocene block models 1−3. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Hayao Nakayama Foundation for Science & Technology and Culture. We thank an anonymous reviewer for his or her helpful discussion and suggestions. R.H. wishes to express his thanks to Dr. N. Ishihara (Idemitsu Kosan Co. Ltd.) and Prof. Dr. T. Mochida (Kobe University) for their helpful discussion and continued encouragement.
Figure 4. Monomer recognition of metallocene block models 1-3 and polypropylene block models with different tacticity.
produces an atactic polypropylene block model in which methyl groups point right and left of the polymer backbone randomly. In the classroom, the similarities and differences between the block models and genuine molecular catalysts can be emphasized. The monomer recognition processes of the metallocene block models using steric effects are somewhat similar to those of real metallocene catalysts.13 On the other hand, the block model cannot completely illustrate the realistic mechanism. The real catalysts do not need to rotate during polymerization. The angle of interaction between the monomer and polymer chain is less than 180°. Although the metal center of real metallocene catalysts forms a metal−alkyl bond and holds both polymer chain and monomer during polymerization, those of the metallocene block models release the polymer chain. The block system cannot illustrate the insertion process of monomer into the metal−alkyl bond precisely. These are the main differences between the block models and actual systems, so when the students feel comfortable with the basic concepts via this model, it may help to also explain the mechanism in terms of pure traditional means with electron-pushing arrows. One advantage of the activities with interlocking building blocks includes their ease of modification. For example, the relatively expensive parts of these models (swiveling blocks and magnets) can be omitted if cost or time of preparation is an issue. A simpler version using only 2 × 2 and 2 × 4 blocks is shown in the Supporting Information as suggested by a reviewer. Here, the propylene monomer model is composed of blocks turned on their side, which allows them to be linked directly together. This model is suitable for lectures with a great number of students; however, it cannot represent the prochirality of propylene monomer effectively.
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REFERENCES
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CONCLUSION The interlocking building block models described herein provide students with a hands-on analogy to experience coordination polymerization by metallocene catalysts. Despite their simplicity, the block models can represent steric effects in catalytic cycles effectively. The key step of the stereoregular 622
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