Polymers and Polymerization in Organic Chemistry

What are Polymers and Monomers?

A polymer is a large molecule (known as a macromolecule) constructed of multiple repeating subunits. These simple subunits that make up each polymer are known as monomers. In fact, the term polymer means many monomers! Monomers are the building blocks for each polymer. The formation of a polymer is called polymerization.

As polymers are formed from monomer subunits, creating a polymer chain involves combining multiple monomer units (also known as propagation) until the desired polymer forms.

With a chain-like structure, polymers are found naturally within cells and can also be synthesized artificially. Essential to everyday life, polymers can be found with any structure, mass (molecular weight), or function.

What is Polymerization?

Polymerization is the process of linking monomers to form polymer chains. Based on the polymerization process, monomers link through functional group reactions or by creating reactive reaction intermediates (free radical propagation).

Polymers are chains of repeating monomer groups often created from thousands of singular monomers. With multiple kinds of polymerization, the polymerization process can form macromolecules as numerous monomers bond.

Different Types of Polymerization

  1. Chain-Reaction (Addition) Polymerisation

Addition polymerisation is when a monomer and catalyst react with each other in a three-step process. The three steps are initiation, propagation, and termination.

The first step, initiation, involves the monomer undergoing an alkene addition reaction. For addition polymerisation, monomers usually have one carbon-carbon double bond. The catalyst is usually a free-radical peroxide.

The Polymerization of Vinyl Chloride to Polyvinyl Chloride. The alkenes in vinyl chloride react to form the PVC polymer.
Polymerisation of Vinyl Chloride to Polyvinyl Chloride (PVC)

For example, propagating the ethylene molecule with a catalyst forms polythene/polyethylene.

2. Step-Reaction (Condensation) Polymerization

Instead of the free-radical propagation step, condensation polymerization involves monomers with two reactive functional or end groups that directly react with each other. These reactions often create bi-products such as water, ammonia, or HCl.

After the functional groups react, the two monomers form bonds and continue bonding with other functional groups.

Compared to addition polymerization, condensation polymerization requires higher temperatures, resulting in polymers with lower molecular weight.

For example, monomers in polyesters (a polymer category) are linked by the ester functional group.

Two functional groups (an amine and ketone) react to form nylon. Continuously adding monomers propagates this polymer chain.
Nylon Synthesis

Monomers to Polymers

When forming polymers from monomers, polymerisation is usually a multi-step process involving multiple reactions that propagate the polymer’s chain length.

For example, Addition Polymerisation begins with initiation, where the monomer’s double bond reacts with the catalyst, breaking the double bond and forming a free radical (valence election). The initiation step can also form a free radical with heat absorption and light irradiation

Then, the monomer undergoes the propagation step. Monomers continually bond with the free radical formed in the initiation step, thus creating the polymer chain. Propagation repeats the initiation step, maintaining the free radical’s formation and reacting with more monomers.

The final step to complete the polymer is the termination step, which deactivates the free radical propagation. Either with a free radical byproduct reacting with the polymer’s end or two incomplete polymer chains reacting with each other, the termination step halts free radical propagation and produces a complete polymer.

The Addition Polymerization process involves three steps: initiation, propagation, and termination. The Thiol Michael reaction is an example of the Addition Polymerisation process.
Thiol Michael Polymerisation

Polymer Structural Categories

All polymers cannot be categorized under the same system due to their diverse characteristics and behaviors. As polymers are defined as any chain consisting of repeating monomer groups, polymers include various macromolecules with different purposes.

Therefore, there are many different classifications of polymers – from natural availability, withstanding of high temperatures, to molecular forces. Listed below are polymer categories based on their monomer chain structures.

  • Linear Polymers

Per the name, linear polymers are long, straight polymer chains held together by weak Van Der Waals and hydrogen bonding.

These polymers pack densely together, with high boiling and melting points. Some examples of linear polymers include polyvinyl chloride (PVC), Teflon, and polystyrene.

An example of a linear polymer: linear polyethylenimine.
Linear PEI
  • Branched-Chain Polymers

Branched-chain polymers are linear polymers with secondary polymer chains attached. Instead of a straight polymer chain, branched-chain polymers resemble tree branches, with other chains extending from the primary backbone.

Due to variations in branch length, branched-chain polymers have lower density and melting/boiling points. Some examples include starch and glycogen.

An example of a branched polymer: branched polyethylenimine.
Branched PEI
  • Cross-Linked Polymers

Cross-linked polymers are groups of polymer chains that are linked together with covalent bonding groups. These polymers often resemble a ladder. Many malleable polymers undergo chemical cross-linking (the addition of an anhydride hardener) that joins polymer chains with a covalent bond.

After the cross-linking process, the molecule now has a more rigid and stable structure that maintains its form. Some examples include vulcanized rubber, fiberglass, epoxy, and acrylate resins.

Polymerization and Polymers in Everyday Life

Some examples of frequently-used polymers include polyvinyl chloride (PVC), low-density polyethylene (LDPE), nylon, Teflon, and poly methyl methacrylate (acrylic). Polymers are prevalent in almost every everyday setting – whether it be kitchens, factories, classrooms, or construction sites.

With the invention of polymers in 1869, the polymer industry boomed, manufacturing plastics, vulcanized rubber, and other substances to improve daily life. All plastics, synthetic fabric, and acrylic manufacture depend on the polymerization process, making polymers a crucial staple in modern life.

Z-Blade kneaders and one heating agitator. Devices used in polymerization
Polymer Manufacturing