Polymerization 101: Radical and Condensation Routes to Polymers

Overview

In this MIT OpenCourseWare lecture, two main polymerization routes are introduced: radical (addition) polymerization and condensation polymerization. The instructor walks through how radicals form and how double bonds enable chain growth to produce long polymer chains, using methane, chlorine, and ethene as leading examples. The discussion connects to familiar polymers such as polyethylene and polystyrene, and then switches to condensation polymerization, where two different monomers join together with the loss of a small molecule to form strong links, exemplified by nylon. The talk also weaves in real-world packaging and environmental considerations to show the broad impact of polymer chemistry on daily life.

Introduction to Polymers and Two Production Routes

The lecture begins by explaining what polymers are and why radicals matter in polymer chemistry. A radical is a molecule with unpaired electrons, which behaves like a broken bond eager to bond again. The instructor uses a Lewis-dot perspective to illustrate radical formation, and then shows how radicals interact with double bonds to initiate polymerization. This section culminates in the identification of two main polymerization strategies: radical chain polymerization (addition polymerization) and condensation polymerization, highlighting that the two approaches produce very different types of polymer structures and properties.

Radical Polymerization: Initiation to Propagation

In radical polymerization, a radical initiator (R dot) attacks a double-bonded monomer such as ethene (ethylene). The double bond’s two electrons allow the radical to take one electron, forming a new radical at the end of the growing chain while keeping the original bond intact. This process repeats as the radical encounters more ethene molecules, creating a growing chain composed of repeating monomers and a new radical at the chain end. The chain can continue this way until termination, giving rise to chain-growth polymers. The speaker emphasizes the idea of chain propagation and the fact that the radical remains active until it finds another monomer (or another radical terminates it). This is often called addition polymerization or radical polymerization, and it explains why polymers can be extremely long with molecular weights ranging from thousands to millions of grams per mole.

“The double bond allowed a radical to come in, take an electron, make a new radical out of the whole thing and remain stable.” — MIT OpenCourseWare

Polymer Notation and How We Describe Polymers

The talk then notes how chemists describe polymers with a simple notation: the monomer inside parentheses becomes a repeating unit, and an N after the parentheses indicates the number of repeats. For ethene, a polymer is shown as (CH2-CH2)n, which is often written in shorthand as poly(ethylene). This notation communicates the idea that many repeating units form a single macromolecule, a polymer, whose length is described by the degree of polymerization. The discussion helps students connect the structural ideas to properties such as molecular weight and viscosity, and it sets up the contrast with condensation polymerization later in the talk.

“This is the polymer. The chain grows by turning double bonds into single bonds as the radical keeps propagating.” — MIT OpenCourseWare

Typical Polymers and Properties

The lecturer points to polyethylene as one of the simplest, most common polymers, and shows how changing the monomer (for example, replacing a hydrogen with a phenyl group to yield polystyrene) dramatically alters the material’s properties and applications, from packaging to consumer goods. The molecular weight and degree of polymerization are discussed as key factors that determine whether a polymer is crystalline or amorphous, and how chain entanglements influence mechanical behavior, processability, and optical properties. The characterization concepts mirror earlier discussions of glassy and crystalline solids, drawing parallels between long polymer chains and other extended materials.

Industrial Context: Packaging, Recyclability, and Environmental Impact

The talk includes a glance at the commercial scale of polymer use, using a Pepsi advertisement as a segue into how plastics enable lightweight, shatter-resistant bottles. The lecturer cites global bottle production and recycling challenges, noting that a large share of plastic packaging is single-use and that a relatively small fraction is recycled. Real-world data is shared: the majority of plastic bottles are not recycled, and plastics that accumulate in the environment pose long-term ecological risks through fragmentation into microplastics and potential toxicity.

“Packaging is 40% of plastic produced today, and 91% of those bottles are not recycled.” — MIT OpenCourseWare

“In 2050, the plastic in the ocean will equal the weight of all fish.” — MIT OpenCourseWare

The Second Route: Condensation Polymerization and Nylon

Switching gears, the lecture introduces condensation polymerization, where two different monomers join to form a large polymer and release a small molecule such as water. The example uses a dicarboxylic acid and a diamine to form polyamides (nylons). The box-like segments in the monomer units illustrate how the end groups control polymer growth, and the reaction forms an amide linkage as the condensation product. Nylon demonstrates the tunability of this approach: the “box” can be any hydrocarbon chain, and by choosing different boxes, polymers with a range of properties can be synthesized. The famous nylon-66 example arises when the box contains six carbons on each monomer arm.

“In condensation polymerization, the goal is also to make these super long strands, but you do it with two different starting molecules and no initiator.” — MIT OpenCourseWare

Nylon Demonstration and Applications

A short video shows nylon rope being drawn from an interface between two liquids containing the diamine and the dicarboxylic acid, illustrating how polymer chains form and extend at the boundary as more monomers react. This demonstration emphasizes the practical lure of condensation polymerization: you can engineer chains with specific functional groups and tailor properties for fibers, parachutes, and other important applications, including everyday textiles.

“Nylon can't be made that way.” — MIT OpenCourseWare

Concluding Thoughts and Next Steps

The lecturer foreshadows how other properties such as crystallinity, miscibility, and processing can be engineered by controlling monomer choice, chain length, and architectural design. A brief nod to the broader societal and environmental context concludes the talk, while the promise of more polymer science, processing, and engineering is framed as a key area for future exploration in the course.

Quotes to Reflect On

“Pepsi Cola's new 2 liter plastic bottle is 25% lighter than glass.” — MIT OpenCourseWare

“Packaging is 40% of plastic produced today.” — MIT OpenCourseWare

“In condensation polymerization, the goal is also to make these super long strands, but you do it with two different starting molecules and no initiator.” — MIT OpenCourseWare

“The double bond allowed a radical to come in, take an electron, make a new radical out of the whole thing and remain stable.” — MIT OpenCourseWare

“In 2050, the plastic in the ocean will equal the weight of all fish.” — MIT OpenCourseWare