Environmental Engineering Reference
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will have a limiting value of P o . Then at a given average molecular weight
Mn , the property P will have a value:
where k is a constant.
3.2.2 Tacticity
Chain-like geometry also allows for different stereoregular arrangement of
the repeat units. The regularity in structure or the “tacticity” of the polymer
therefore strongly influences the degree to which it has rigid, crystalline
long-range order or flexible, amorphous disorder and therefore the bulk
properties of the polymer (Stevens, 1999; Chang, 2011). New catalysts are
continually being developed to increase the stereoregularity of plastics such
as PP with better mechanical properties.
With vinyl polymers such as PP or PVC, the substituent group Z (Z = -CH 3
and -Cl, in PP and PVC, respectively) might be expected to be randomly
oriented along the polymer chain. This yields a polymer with no
stereoregularity in its structure or an atactic polymer. Using special
catalysts (Ziegler-Natta catalysts or the new metallocenes) to facilitate the
polymerization reaction, it is also possible to synthesize the very same
polymer with the same molecular weight but as a stereoregular or isotactic
polymer, where all the substituents are located on the same side 6 of the
carbon chain backbone. Where the substituent groups have alternative
positions on the chain, a syndiotactic polymer is obtained. These structures
are illustrated in Figure 3.5 .
 
 
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