Polyolefins stereoblock

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. ... 

The catalyst structures presented in the previous section have been used to prepare many types of polyolefins. The following several sections of this chapter provide an overview of the synthesis of polypropylenes containing various microstructures. These sections describe a selection of the catalysts that have been used to generate isotactic, syndiotactic, hemiisotactic, and stereoblock copolymers. Subsequent sections of this chapter describe catalysts that give rise to various copolymers of ethylene and propylene. Again, representative examples are provided, and the reader should consult other sources for comprehensive coverage of the synthesis and properties of each type of copolymer. ... 

SFM studies of low crystallinity thermoplastic polyolefin elastomers based on polyethylene-alkene copolymers or stereoblock polypropylenes highlighted the importance of the crystalline regions as physical cross-links in these systems (173, 174). Even though crystallinity was very low in these materials, SFM provided sufficient contrast to interrogate the size and distribution of crystallites in the as-prepared and deformed materials (175). As will be discussed in the section under Polymer Crystallization, the crystallization process in such materials was also imaged in real time (176). 

There is a clear trend in current polyolefin research toward the synthesis of novel and well-controlled polymer architectures, leading to materials with special properties and higher added value. Stereoblock polypropylenes can still represent a promising area for development it is up to those who invest in it to take full advantage of the modern experimental and theoretical toolkits, and to transform the traditional, substantially blind approach into a more conscious (and by no means less exciting) scientific adventure. 

For example, see Zhang, Y. Keaton, R. J. Sita, L. R. Degenerative transfer living Ziegler-Natta polymerization Application to the synthesis of monomodal stereoblock polyolefins of narrow poly-dispersity and tunable block length. J. Am. Chem. Soc. 2003, 125, 9062-9069 and references therein. 

Jayaratne, K.C. and Sita, L.R. (2001) Direct methyl group exchange between cationic zirconium Ziegler-Natta initiators and their living polymers Ramifications for the production of stereoblock polyolefins. Journal of the... 

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