Syndiotactic structures conjugated dienes

The polyinsertion reaction of conjugated dienes can proceed in three modes which yield three different isomers 1,2-polymers, 3,4-polymers and 1,4-polymers [297]. The situation is more complicated as the 1,4-isomers either exhibit as-1,4 or trans-, 4 configuration. In addition, the 1,2-/3,4-polymers can have an atactic, isotactic or syndiotactic structure (Scheme 1 in Sect. 1.2) [623]. The various moieties are either randomly distributed along the polymer chain or are aligned block wise. 

The reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

In addition to the configurational isomerism encountered in polymers derived from asymmetric olefins, geometric isomerism is obtained when conjugated dienes are polymerized, e.g., (CH2=CX—CH=CH2). Chain growth from monomers of this type can proceed in a number of ways, illustrated conveniently by 2-methyl-1,3-butadiene (isoprene). Addition can take place either through a 1,2-mechanism or a 3,4-mech-anism, both of which could lead to isotactic, syndiotactic, or atactic structures, or by a 1,4-mode leaving the site of unsaturation in the chain. 

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. 

---