Comparatively Speaking: Isotactic vs. Syndiotactic vs. Atactic in Polymers

Anthony J. O'Lenick, Jr., asks Thomas O'Lenick, a doctoral candidate in polymer chemistry at the University of Tennessee, to explain the difference between isotactic, syndiotactic and atactic in reference to polymers. Thomas O'Lenick explains:

Tacticity is a term used to describe the way pendent groups on a polymer chain are arranged on a polymer backbone. The tacticity of a polymer is determined by what side of the polymer chain the pendant groups are on. This relative position can have dramatic effects on the physical properties of the polymer. Tacticity only arises when there is an asymmetric carbon in the polymer chain backbone. Typically, this occurs in free radical polymerization of vinyl monomers (CH2CHX). When a non-reacted monomer adds to the end of the polymer chain, the monomer can either join the pendant group on the same side as all of the other pendant groups, or it can join the pendant group on the side away from the nearest pendant group. If the monomer adds to the polymer backbone with the pendant group (X) on the same as the previous pendant group, this is called isotactic. If the monomer adds where the pendant group adds to the opposite side of the polymer backbone, it is called syndiotactic. Isotactic and syndiotactic are illustrated in Figure 1.  If there is no order to the way the pendant group adds, (completely random) the polymer is said to be atactic.

When designing a polymer, a good polymer chemist has to consider the properties he/she wants to the polymer to exhibit. There are many aspects to controlling the physical properties of the polymer. Isotactic and syndiotactic polymers provide long-range order, which leads to higher crystallinity in the polymer chain. Polypropylene is a great example of how tacticity has a dramatic effect on the physical properties of the polymer. Atactic polypropylene has little order in the polymer backbone and is amorphous. Most commonly, amorphous polypropylene is called linear low-density polypropylene (LLDPE). This polymer is clear and used in soda bottles. Isotactic polypropylene (HDPE) has long-range order, which adds mechanical strength and crystallinity. This version of polypropylene is used in laundry detergent bottles and is not transparent like LLDPE. This property of polypropylene was so important that Ziegler and Natta co-invented a catalyst that can be used in the synthesis of isotactic polypropylene. This catalyst changed the way we live our everyday lives and produced a billion dollar industry.

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