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SIMPLIFIED – Functional Groups, Functionality, Molecular Weight, Equivalent Weight

Designing polymers is a fascinating art. It’s much like an elaborate puzzle wherein once you learn the shapes and nature of different pieces, your creativity can explore and express an infinite number of artforms. In creating polymeric art, these pieces or building blocks are the monomers – which are small molecules that can connect with other molecules to make a polymeric Mona Lisa.

In this article, we will try to understand the very fundamental aspects of a monomer with the help of LEGOS. Most of us have played with LEGOS when we were kids, or we have seen children play with them. When you look at a singular LEGO brick- the building block, it may look uninspiring but when multiple bricks are connected in the right order, magic happens!

Each brick of LEGO has the ability to connect to other bricks with the help of the “studs” on them – these are the cylindrical bumps that can link them to the studs of another brick. Similarly, monomers have functional groups or reactive sites that help them to link with other monomers. Just like a LEGO brick can have different number of studs which determine how many links it can make, monomers can have multiple reactive sites which determine how many other monomers they can link to, to form long chains or 3d structures.

Let’s take the example of a LEGO brick with 4 studs:

img1

Each of its 4 studs can link to potentially 4 other bricks:

img2

Using the above example, we can think of functional groups or reactive sites as the studs, and the number of studs as the functionality of that LEGO brick.

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Now, since monomers are tiny and we cannot see them with naked eyes like the above LEGO bricks, we cannot visually estimate their reactive sites. Here, the concept of equivalent weights helps us to work with monomers practically.

In LEGO terms, equivalent weight answers the following question:

What is the weight of the LEGO brick per stud on it?

This can also be thought as

What weight of this particular LEGO brick would yield one stud?

Let’s assume the physical weights (in grams) of the above LEGO bricks as follows:

img4

Thus, for us to obtain one stud of the green LEGO brick, we need to weigh out 1.2 grams of it. This is the equivalent weight of the green LEGO brick. The green brick’s total physical weight of 5 grams is its molecular weight. Understanding these basics is vital to create our polymeric art.

Let us apply this to an example:

If we take 5 grams of green LEGO and 6 grams of yellow LEGO, they would link as follows:

img5

Now, if we take 5 grams of green LEGO and 12 grams of yellow LEGO, they would link as follows:

img6

Thus, completely different structures can be built with the same materials by altering their relative amounts. Being aware of the functionalities, equivalent weights and molecular weights helps us to design and control the structure of the final product and therefore its performance.

It is important to note that functionality is specific to a given chemical reaction. Number of functional groups may not always be the number of available reactive sites.

In our industry, we are often asked to provide with the amine value (or amine equivalent weight) of our epoxy curing agents. These curing agents have amine functional groups, but each amine group can have up to 2 reactive sites (active hydrogens) that can each link to one epoxy group. In other words, the functionality of these curing agents or available linking sites are determined by the active hydrogens available to react with epoxy groups and not by the amine functional groups. This can be thought of as 1 or 2 studs from an amine group, which in turn is a section of the curing agent brick. Thus, the active hydrogen equivalent weight (AHEW) is the more important metric which helps determine how much of the curing agent is required for a given quantity of epoxy resin.

Thinking in terms of functionalities (reactive sites) and equivalent weights for a particular chemical reaction helps formulators to make precise chemical structures intended by the manufacturer so as to obtain optimal properties of the final polymer.

I hope this article helped you to better understand some of the fundamentals of monomeric LEGO bricks to explore your creativity in the polymeric arts.

Further readings:
1. Polymer Practical Analysis to learn more about functional groups in polymers.

2. ScienceDirect for more on equivalent weights.

3. LibreTexts for some more information on molecular weights.

4. Polyurethane Manufacturers Association to learn how to use equivalent weights in calculations.

Author:
Shikhin Nadkarni

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sikhin

Shikhin is currently a PhD Student in Coatings and Polymeric Materials Department at North Dakota State University. He is a member of Dr. Dean Webster’s Research Group and his research focuses on Non-Isocyanate Polyurethanes as well as novel Epoxy systems. He is passionate about incorporating bio-based materials in polymers so as to reduce our dependance on petrochemicals.

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