Stereoregular atactic polymers

Atactic polymer (Section 7 15) Polymer characterized by ran dom stereochemistry at its chirality centers An atactic polymer unlike an isotactic or a syndiotactic polymer is not a stereoregular polymer... 

In order to generate stereoregular (usually isotactic) polymers, the polymerization is conducted at low temperatures ia nonpolar solvents. A variety of soluble initiators can produce isotactic polymers, but there are some initiators, eg, SnCl, that produce atactic polymers under isotactic conditions (26). The nature of the pendant group can influence tacticity for example, large, bulky groups are somewhat sensitive to solvent polarity and can promote more crystallinity (14,27). 

Non-crystalline polymers are those which include high levels of irregularity within their structure. Typical sources of such irregularity are copolymerisation with significant amounts of at least two co-monomers and also complete absence of stereoregularity, i.e. atactic polymers. 

In atactic polymers, side groups are irregularly positioned on either side of the chain, as illustrated schematically in Fig. 1.8 c). A truly atactic polymer would comprise a random distribution of steric centers. In practice, atactic polymers typically show some preference for either meso or racemic placement The tendency towards stereoregularity is due to the fact that polymerization catalysts often contain steric centers, which tend to direct the incoming monomers and the growing chain into preferred configurations. 

The interaction may not be quite as strong as in the case of 2,1 insertion discussed above, but there will always be a tendency of the growing chain to arrive at an isotactic stereochemistry when 1,2 insertion occurs. One example of chain-end control leading to isotactic polymer was reported by Ewen [13] using Cp2TiPh2/alumoxane as the catalyst. The stereoregularity increased with lower temperatures at -45 °C the isotactic index as measured on pentads amounted to 52 %. The polymer contains stereoblocks of isotactic polymer. At 25 °C the polymerisation gives almost random 1,2 insertion and an atactic polymer is formed. 

The calculations reported here are concerned with the use of alkoxyalkylsilanes as external electron donors. When used as external electron donors, alkoxyalkylsilanes improve the stereoregularity of polypropene, reducing the level of atactic polymer... 

Commercial polystyrene, one of the most representative general-purpose resins, is an amorphous atactic polymer with no melting point (softening temperature ca 100 °C, glass transition temperature ca 70-100 °C). Since the discovery of Ziegler-Natta catalysts, many efforts to produce stereoregular polystyrenes have been made. 

Stereochemistry also affects the crystallinity of a polymer. Stereoregular isotactic and syndiotactic polymers are generally more crystalline than atactic polymers. By careful choice of catalysts, we can make a linear polymer with either isotactic or syndiotactic stereochemistry. 

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