Polypropylene synthesis stereoblock polypropylenes

Figure 8 Metallocenes for iso-, syndic-, and stereoblock polypropylene synthesis.

Collins has reported the synthesis of a related class of metallocenes (43), some of which form elastomeric, stereoblock polypropylene when activated by The elastomeric properties of the polymer formed using 43 (M = Hf, X = SiMe2 7)xn = 25 °C) were far superior to those formed by the other metallocenes in the study. The polymers made using 42 and 43 have similar microstructures, as well as physical and mechanical properties. However, after detailed microstructural analysis of the polymer the authors proposed an alternate mechanism of stereocontrol to Chien s site epimerization model (Scheme... 

Rieger has recently reported another class of Ci catalysts (45) for the synthesis of stereoblock polypropylenes. However unlike the polymers formed using 42—44, polymers ranging from flexible plastics (with melting points as high as 134 °C) to thermoplastic elastomers are produced. Similar metallocenes... 

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. ... 

A conceptually different route to the synthesis of stereoblock polypropylenes entails the repeated exchange of growing polymer chains between active species of different stereoselectivity. Two pathways are conceivable ... 

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. 

These results show that using catalyst 9a for polypropylene synthesis results in isotactic polypropylenes not in the manner of stereoblock polymers, but as iPPs with statistically distributed stereoerrors. 

Cai, Z.G., Nakayama, Y, and Shiono, T. (2008b) Facile synthesis of tailor-made stereoblock polypropylenes via successive variation of monomer pressure. Macromolecules, 41,6596-6598. 

Collette, J. W. Tullock, C. W. MacDonald, R. N. Buck, W. H. Su, A. C. L. Harrell, J. R. Mulhaupt, R. Anderson, B. C. Elastomeric polypropylenes from alumina-supported tetraalkyl group IVB catalysts. 1. Synthesis and properties of high molecular weight stereoblock homopolymers. Macromolecules 1989, 22, 3851-3858. 

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