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Handbook of Polymer Synthesis Second Edition Episode 5

Tham khảo tài liệu 'handbook of polymer synthesis second edition episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 5 Polymeric Dienes Walter Kaminsky and B. Hinrichs University of Hamburg Hamburg Germany I. INTRODUCTION Homopolymers of conjugated dienes such as 1 3-butadiene isoprene chloroprene and other alkylsubstituted 1 3-butadienes as well as copolymerzs with styrene and acrylonitrile are of great economical importance 1-3 . The conjugated dienes can polymerize via 1 4 or 1 2 linkage of monomeric units. In addition to this 3 4 linkage occurs with butadienes bearing substituents in the 2-position. In the case of 1 4 linkage the polymer chain can exist as cis or trans type ---H2C CHzr 1-3 H I Kat---CH c--- I I CH H ch2 1 2-linkage 1 2 Linkage yields a tertiary carbon atom thereby making it possible to form isotactic syndiotactic and atactic polybutadiene 3 in analogy to polypropene. The rare 3 4 linkage also gives isotactic syndiotactic or atactic configuration. This applies only to high stereoselectivities. Further isomeric structures are formed when next to head-to-tail linkages head-to-head and tail-to-tail linkages also occur. The polymerization of dienes can be initiated ionically by coordination catalysts or by radicals 4-10 . Copyright 2005 by Marcel Dekker. All Rights Reserved. II. POLYBUTADIENE Polybutadiene belongs to the most important rubbers for technical purposes. In 1999 more that 2 million tons were produced worldwide that is about 20 of all synthetic rubbers 11 12 . The cis type made by 1 4-addition is economically the most important polybutadiene 13 14 . Trans- as well as isotactic syndiotactic or atactic 1 2-polybutadiene can also be synthesized in good purity with suitable catalysts. For anionic polymerization with butyllithium or the coordinative process with Ziegler catalysts 1 3-butadiene must be carefully purified from reactive contaminants such as acetylene aldehydes or hydrogen sulfide. A. Anionic Polymerization Metal alkyls preferably of alkali metals are used as initiators. The polarization of the catalyst exerts a strong influence on the .