Chain Growth and Step Growth

Introduction to Polymers: Chain-Growth vs. Step-Growth

Moving from small biological macromolecules into synthetic polymer chemistry, we encounter macromolecules built from repeating structural units. Polymers permeate modern life, forming the plastics, fibers, elastomers, and coatings we use daily. Understanding polymer chemistry begins with basic structural definitions and the two fundamental polymerization mechanisms: chain-growth and step-growth.

1. Basic Polymer Terminology

Before examining how polymers form, we must establish standard structural vocabulary:

• Monomer: The small, reactive or functionalized starting molecule (e.g., ethylene, adipic acid) used to build the polymer.
• Repeating Unit: The structural moiety that repeats continuously along the backbone of the polymer chain.
• Degree of Polymerization (DP): The number of repeating monomer units ($n$) incorporated into an average polymer chain. Molecular weight is directly proportional to the degree of polymerization ($MW = DP \times \text{molecular weight of repeating unit}$).

2. Chain-Growth Polymerization

In chain-growth polymerization (often referred to historically as addition polymerization), monomers add to the growing polymer chain one at a time:

• Mechanism Overview: An active species (such as a free radical, carbocation, or carbanion) reacts with a monomer molecule to initiate a chain.
• Rapid Propagation: Once a chain starts growing, propagation is extremely fast. Monomer molecules add sequentially to the reactive chain end until a termination event stops the process.
• Key Kinetic Characteristic: High molecular weight polymer chains appear almost immediately from the very beginning of the reaction. Throughout the reaction, the mixture consists solely of long polymer chains, unreacted monomer, and initiator—there are few intermediate-sized oligomers.
• Named Example: Polyethylene (formed from the free-radical polymerization of ethylene gas, $\text{CH}_2=\text{CH}_2$).

3. Step-Growth Polymerization

In step-growth polymerization (often referred to as condensation polymerization), polymer growth follows an entirely different statistical logic:

• Mechanism Overview: Any two molecular species present in the reaction mixture—whether two monomers, a monomer and an oligomer, or two growing oligomer chains—can react with each other through their functional groups to combine.
• Gradual Molecular Weight Growth: Because bonds form in independent steps between random pairs of molecules, low-molecular-weight oligomers (dimers, trimers, tetramers) form first.
• High Conversion Requirement: High molecular weight polymers arrive only very late in the reaction, requiring exceptionally high monomer conversion (typically $>99\%$) to achieve long, useful chains.
• Named Example: Nylon-6,6 (formed from the step-growth condensation of hexamethylenediamine and adipic acid with the loss of water).