Syndiotactic Polymerization and Syndiotacticity

The physical and mechanical properties of a polymeric material critically depend on many factors, one of which is stereochemistry. Polymers that have chiral centers in the repeated unit can exhibit two structures of maximum order, isotactic and syndiotactic [27]. Sequential stereocenters of isotactic polymers are of same relative stereochemistry whereas those of syndiotactic polymers are of opposite relative configuration. Due to their stereoregularity, isotactic and syndiotactic polymers are typically crystalline, which is an important feature for many applications. Isotactic polymers are used in a wide range of applications. Typical examples include isotactic polyolefins and almost all natural polymers. In contrast, syndiotactic polymers have limited applications mainly due to their hard productivity and inherently alternating stereochemistry. The properties of syndiotactic polymers are usually similar to or in some cases better than isotactic counterparts according to the studies on syndiotactic polystyrene and other syndiotactic polyolefins [28]. Syndiotactic PLA is expected to be a versatile polymer with controllable stereochemistry. 

Syndiotactic PLA has been synthesized by syndiospecific polymerization using meso-LA. Three mechanisms are involved in the synthesis of syndiotactic polymers. One is the solid-phase polycondensation for A-B-type monomer with appropriate stereochemistry for instance, syndiotactic polypeptides are produced by this approach. The second route to syndiotactic polymerization is the chain-end control 

FIGURE 6.6 Syndiotactic polymerization of meso-. K. Reproduced from Ref. 29 with permission from American Chemical Society. 

The catalytic systems for syndiotactic polymerization of meso- K proposed and investigated in recent years are summarized in Table 6.2. Although the approach has only been applied successfully in olefin s syndiotactic polymerization, the SCM route can be used in some other monomers syndiotactic polymerizations with a high degree of stereochemical integrity. Since the chain-end control mechanism cannot accurately explain stereochemistry in metal alkoxide initiating LA polymerizations [18, 19], many studies have focused on the initiators related to the site control mecha-nism[13,14,16,17,29-31]as shown in Table 6.2. The chain-end control mechanism has also been reported [15]. 

TABLE 6.2 Progress of the Catalyst Systems in Syndiotactic Polymerization with LA 

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