Lanthanide catalysts stereospecificity

Stereospecific polymerization of 1,3-dienes (10-18) (to butadiene) and isoprene homo- and copolymers), dimerization of propene (19) and recently stereospecific polymerization of acetylene (20) to high cis-content polyacetylene have all been reported using lanthanide catalysts. Sen (21) has reported the preparation of cationic europium systems (which perhaps function as cationic initiators) for polymerization of norbornadiene and 1,3-cyclohexa-diene. 

The most recent developments in catalysts for stereospecific polymerization of dienes have been in the area of the rare earth or Lanthanide catalysts, specifically the neodymium complexes (Hsieh and Yeh, 1985 Kerns et al., 2003). The advantages of these systems are high stereospecificity, high activity, control of molecular weight, and no gel formation (Hsieh and Yeh, 1985). 

Figure 3.9 Stereospecific polymerisation of isoprene (2-methyl-butadiene-l,3). The Russian industry has implemented technologies of c/s-l,4-polyisoprene synthesis with Ti-Al Ziegler-Natta catalysts (synthetic c/s-isoprene rubber SKI-3) and neodymium salts-based lanthanide catalysts SKI-5 grade synthetic rubber (TU 2294-051-16810126-96). SKI-3 rubbers contain up to 93 1% of as-1,4 links, while their content in SKI-5 grade rubber is 96 1%...

In recent years a great number of reports on the stereospecific pol3nn-erization of 1,3-dienes with lanthanide catalysts have been published. ... 

It is thought that polymerization proceeds in the same way as proposed for lanthanide catalysts, and the catalyst systems using non-bridged bis(cy-clopentadienyl)zirconium also afford syndiotactic poly(MMA). Some examples of stereospecific polymerization of MMA are siunmarized in Table 2 and Fig. 16. Complexes with non-bridged indenyl or fluorenyl Hgands, 42 and 43, were ejq)lored,but syndiotacticity did not depend on the hgands [134]. 

The influence of electronic factors on the stereospecific behaviour of metallocene-based catalysts in a-olefin polymerisation is an important aspect of the stereoregulation mechanism. Two phenomena, lanthanide contraction and relativistic effects, that appear in group 4 transition metal series result in... 

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,... 

Recently we reported the stereospecific polymerization of butadiene and isoprene with our newly developed lanthanide coordination catalysts. 

Catalytie systems based on lanthanide compounds are effective catalysts of polymerization of dienes (mainly of isopiene and butadiene). A distinctive feature of these systems is high cis stereospecificity. In particular, the S5mthesized polybutadiene and polyisoprene contain approximately 98% 1,4-cis units [1]. 

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