Welcome to another blog post on the applications of stereochemistry! This blog post will focus on exploring tacticity and crystallinity. Tacticity and crystallinity are important in determining the properties of polymers.
Tacticity refers to the orderliness of succession of configurational repeating units in the main chain of a macromolecule [1]. This is an application of stereochemistry as it concerns the spatial arrangement of substituent groups. Dependent on the arrangement of the pendent groups, there will then be polymers of different tacticity ranging from isotactic, to syndiotactic and also atactic polymers [2]. An example can be seen in the case of polypropylene in Figure 1 below. When we write the carbon chain on polypropylene as a zigzag line of atoms in the plane of the paper, we will see that there are three different ways in which the side groups can attach [3]. The first way would be when all the pendent groups are occupying the same side of the carbon chain, resulting in an isotactic polypropylene [2]. Secondly, the pendent groups can also be on alternating sides of the chain and the polymer is then syndiotactic polypropylene [2]. Lastly, the pendent groups could be arranged randomly and the polymer would then be known as atactic polymers [2].
Tacticity will also affect the physical characteristics of the structure in terms of crystallinity. The tacticity will affect how well the atoms can pack together. Crystals have regular structures and polymers that are isotactic or syndiotactic will be able to solidify into crystalline or semicrystalline networks [1]. In contrast, atactic polymers have a random arrangement of the repeating units so they will not be able to form crystals and will usually form amorphous glasses instead [1].
When polymers have crystalline structure, the intermolecular forces of attraction are at a maximum, so the polymer created is denser and stronger. This can be seen in the case of polypropylene [4]. Atactic polypropylene will be soft and sticky, making it suitable for making adhesives. On the other hand, isotactic polypropylene can form stronger crystals and can be made into structural components.
Tacticity and crystallinity is important in the polymer synthesis industry. Different polymerisation conditions such as temperature and choice of solvent will affect the tacticity of the polymer formed [4]. If a polymer of high strength is required, there is a need to first create isotactic or syndiotactic polymers [4]. If rubber is required, there is a need to use tactic polymers instead [4]. Therefore, there is a need to match the polymerisation conditions to the type of polymer that is required.
To sum up, we can see how stereochemistry plays a role in the properties of polymers by considering tacticity and crystallinity. It is hence important to consider such factors when creating polymers for use.
References
- Quirk, R.P., Stereochemistry and macromolecules: Principles and applications. Journal of Chemical Education, 1981. 58(7): p. 540.
- Ziegler–Natta Catalysts and Polymer Stereochemistry. 2015; Available from: https://chem.libretexts.org/Textbook_Maps/Organic_Ch emistry/Map%3A_Organic_Chemistry_(Smith)/Chapter_31% 3A_Synthetic_Polymers/31.4%3A_Ziegler%E2%80%93Natt a_Catalysts_and_Polymer_Stereochemistry
- Introduction to Polymers. Available from: https://www.open.edu/openlearn/science-maths-technology/science/chemistry/introduction-polymers/content-section-2.3.4
- Fried, J.R.. Introduction to Polymer Science. 2014; Available from: http://www.informit.com/articles/article.aspx?p=2235827&seqNum=2