Commercial polymer stereoselective polymerization

The simple homoleptic complex Sn(Oct)2 catalyzes lactide polymerization both in solution and in the melt at temperatures >130°C and is the most widely used catalyst for lactide polymerization industrially. As previously mentioned, commercially available lactide with >90% L-LA polymerized with Sn(Oct)2 will generate -90% isotactic PLA. While this material has commercial applications, its thermal and mechanical properties are not suitable for many applications where polyolefins are typically used. One way to improve the polymer properties is by increasing the isotacticity or forming isotactic stereoblock PLA through stereoselective polymerization of L- and D-LA mixtures. However, this cannot be achieved with Sn(Oct)2 and other simple initiators. ... 

There is a large amount of literature and many patents in this area, as well as many good reviews and books [8,9,10,11,12,13,14,15,16,17,18,19]. The recent review by Coates [10] describing stereoselective polymerization overlaps considerably with this chapter, and is recommended for consultation. In this chapter, metallocene-catalyzed olefin polymerization is discussed, focusing on the synthesis of stereoregulated polymers. The aim of this review is not to be a complete survey the outline and some recent topics in polymerization of propylene, higher a-olefins, styrene, acrylate esters such as methyl methacrylate (MMA), 1,3-butadienes, and cycloolefins will be described. Polyethylene is one of the most important commercially manufactured polymers. The homopolymer, as well as the copolymer with ethylene and other olefins, is an important subject in the polyolefin industry. However, it will be only briefly mentioned because the stereochemistry is less involved. 

Historically, heterogeneous polymerization catalysts have been the workhorse of the polymer industry. Although these catalysts offer many important advantages over their homogeneous counterparts in commercial production, they also have a significant number of drawbacks. For example, hetereogeneous catalysts typically have multiple active sites, each of which has its own rate constants for monomer enchainment, stereoselectivity, comonomer incorporation, and chain transfer. Therefore a substantial amount of empirical optimization of these catalysts is necessary before polymers of relatively uniform molecular weights, composition, and stereochemistry can be produced. 

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