Stereospecific polymerizations conjugated diene

The first stereoregular 1,4-polybutadiene was obtained by Morton and his associates by using an Alfin catalyst in 1947 (34) and had a predominantly trans structure. The structure of this polymer was determined by careful X-ray and IR-excimination (61). A few years later the first synthesis of 1,4-cis-polyisoprene was accomplished (62) by using a Li metal dispersion. In the same period, Ziegler-type catalysts were reported to polymerize stereospecifically to 1,4-trans- or 1,4-cis-polymers a number of conjugated dienes. More recently, new types of catalysts for the stereospecific polymerization of dienes were prepared starting with TT-allyl derivatives of transition metals. Some catalysts for the stereospecific 1,4-polymerization of butadiene are indicated in Scheme 12. 

The catalysis of the stereospecific polymerization of conjugated dienes is of considerable interest from both the scientific and the industrial points of view [1,2]. From butadiene and isoprene, as the industrially most important 1,3-dienes, in comparison with the polymerization of olefins many more structurally different stereoregular polymers can be derived cf the structures of the stereoregular polybutadienes and polyisoprenes given in Scheme 1 [106]. 

The lanthanide coordination catalysts are known to be highly stereospecific for producing high-cis polybutadiene and high-cis polyisoprene as well as high-cis copoljnnerization of the two monomers. In addition, the polymerizations of other conjugated dienes such as trans-piperlene, 2,4-hexadiene,... 

Nakayama, Y Baba, Y Yasuda H. Kawakita, K. Ueyama, N. Stereospecific polymerizations of conjugated dienes by single site iron complexes having chelating NJ, N-donor ligands. Macromolecules 2003, 36, 7953-7958. 

It has been found that germylmercurate of praseodymium in combination with Et3Al catalyses the stereospecific polymerization of conjugated dienes [37]. 

Stereospecific polymerization of mono- or 1 1-disub-stituted ethylenes. Historical developments Stereospecific polymerization of mono- or l l-disub-stituted ethylenes. Stereoregulating factors Stereospecific polymerization of conjugated dienes to ly4-polymers... 

Stereospecific polymerization of substituted conjugated dienes. Stereoregular polytactic polymers have been obtained from a number of substituted dienes, including one optically active 1,3-substituted propadiene, and various 1- or 1,4-substituted butadienes. (R)-penta-2,3-diene has been polymerized by means of 7T-allyl-Ni-iodide to an optically active polymer, to which an interesting stereoregular structure has been attributed (Scheme 26) (224). Some of the stereoregular polymers... 

Abstract Metallocene complexes that serve as stereoselective olefin polymerization catalysts are described. The polymerization of propylene, styrene, methyl methacrylate, 1,3-dienes, non-conjugated dienes and cycloolefins is discussed. The stereochemistry of monomer insertion is governed by the chiral steric environment of catalysts derived from a ligand structure (catalytic-site control) or a chiral center in the polymer chain (chain-end control). The mechanism of formation of isotactic and syndiotactic polymers in each monomer and catalyst is explained. Non-metallocene catalysts for stereospecific polymerization are also mentioned. 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

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