In discussions of polypropylene and other polymer-based plastics, polymerization is the industrial chemical process in which monomers, small molecular units, chemically bond to create larger, more complex polymer structures.
This article of TLD Vietnam examines this process within chemical processes, outlining its types, representative examples, and underlying mechanisms.
What Is Polymerization?
To fully understand the polymerization, you have to know the definition of a monomer first. A monomer is defined as a simple molecule with two or more binding sites. Then, the polymerization is a chemical process where multiple monomer molecules react to form a polymer, resulting in macromolecules with linear, branched, or intricate three-dimensional network structures.
Several main categories of polymerization reactions exist, the most prominent being:
- Step-growth polymerization
- Chain-growth polymerizationÂ
- Condensation polymerization
A polymer comprises large macromolecules formed by the repeated linkage of monomer units. Through polymerization, these monomers chemically bond to create the macromolecular structure of the polymer. For instance, styrene undergoes polymerization to produce polystyrene, as depicted in standard reaction schemes.
The mechanism’s complexity depends on the functional groups within the monomers. The simplest polymerizations often involve alkenes undergoing free-radical reactions, with ethylene polymerization to form polyethylene being a key commercial example.
Polymerizations involving a single monomer type are termed homopolymerizations, while those with two or more different monomers are called copolymerizations. Fundamentally, polymerization is the chemical process transforming small monomer units into high-molecular-weight polymers, or macromolecules.
How Polymerization Happens
Chain-reaction polymerization proceeds through three primary stages: initiation, propagation, and termination.
Initiation
Initiation occurs when a free-radical catalyst reacts with a monomer’s carbon-carbon double bond, starting chain formation. The double bond breaks, enabling the monomer to bond with the radical, transferring the unpaired electron to the chain’s terminal carbon.
Propagation
During propagation, the chain grows as additional monomer units sequentially bond to the reactive site. This step is thermodynamically favored, as the polymer’s single bonds are more stable than the monomers’ double bonds, resulting in lower overall energy.
Termination
Termination happens when a free radical, often from the initial catalyst, reacts with the chain’s terminal unit or when two active chains combine, forming complete polymer molecules. Other mechanisms, such as chain transfer, may also terminate the reaction.
For example, polyethylene polymerization can occur rapidly, often in under 0.1 seconds, yielding high-molecular-weight chains that may exhibit branching or cross-linking.
Types Of Polymerizations
Polymerization reactions are primarily categorized into two types based on the reaction mechanism. Below is a detailed explanation:
- Condensation Polymerization: This involves monomers with functional groups reacting, often releasing byproducts like water or hydrogen chloride. For instance, nylon, a polyamide, forms through the reaction of diamine and dicarboxylic acid monomers, creating a long chain with amide bonds.
- Addition Polymerization: This mechanism features monomers with double or triple bonds forming polymers without byproduct loss. The reaction advances through the creation of new single bonds. Common examples include polyethylene, polystyrene, and polyacrylate, derived from monomers with unsaturated bonds
Polymerization and Polymer Classification
Polymerization is a process of creating polymers. Polymers are categorized based on the source, reaction, force, and structure. The kinds of polymers are as follows:
Based on the structure
Based on the structure, the polymer is divided into three types: linear, branched, and cross-polymers.
- Linear Polymers: These consist of long, straight chains of monomers. Polyvinyl chloride (PVC) is a typical example.
- Branched Polymers: These are linear polymers with side branches. Low-density polyethylene (LDPE) exemplifies this category.
- Cross-linked or Network Polymers: These have interconnected chains, often formed from bi- or tri-functional monomers, creating rigid, three-dimensional structures. Examples include Bakelite and melamine.
Based on the source
Polymers are divided into natural, semi-synthetic, and synthetic types based on their origin.
- Natural Polymers: Sourced from plants or animals, examples include starch, rubber, and resins.
- Semi-synthetic Polymers: These are chemically modified natural polymers, such as cellulose acetate and cellulose nitrate.
- Synthetic Polymers: Fully man-made, these include materials like polyethylene, nylon 6,6, and synthetic rubber.
Based on the reaction
As previously described, polymerization reactions are classified into two types: addition and condensation.
- Addition Polymers: Formed by the successive addition of monomers with double or triple bonds. Homopolymers arise from identical monomers, while copolymers are formed from different monomers.
- Condensation Polymers: Produced through repeated condensation reactions of bi- or tri-functional monomers, often releasing byproducts such as water or hydrogen chloride.
Based on force
Polymers are also categorized by the strength of intermolecular forces, influencing their physical properties.
- Elastomers: These rubber-like polymers exhibit elasticity due to weak intermolecular forces, allowing stretching. Cross-links between chains enable the polymer to return to its original shape after deformation. Examples include natural and synthetic rubbers.
- Fibers: These polymers possess strong intermolecular forces, such as hydrogen bonding, resulting in closely packed, crystalline structures. Examples include polyamides and polyesters.
- Thermoplastic Polymers: Linear or slightly branched, these polymers soften upon heating and harden upon cooling due to intermediate intermolecular forces. Examples include polyvinyls and polystyrene.
- Thermosetting Polymers: Heavily branched or cross-linked, these polymers can be molded during heating but cannot revert to their original form, making them non-recyclable. Bakelite is a typical example.
Examples Of Polymerization In Everyday Applications
Polymerization is integral to producing materials encountered in daily life and industrial applications. Some key examples include:
- Olefins: Prevalent in petroleum and natural gas processing, olefin polymerization affects fuel characteristics.
- Synthetic Rubbers: Produced by copolymerizing styrene and butadiene, these materials are used in tires and other elastic products.
- PVC: Available in rigid forms (e.g., water pipes) and flexible forms (e.g., infusion tubes), PVC is produced via polymerization.
- Synthetic Fibers: Materials like nylon and polyester, used in apparel, are created through polymerization processes.
- LDPE: Derived from ethylene polymerization, LDPE is used in flexible pipes, toys, and squeeze bottles.
Conclusion
Polymerization is a fundamental chemical process that transforms small monomer molecules into large, high-molecular-weight polymers with diverse structures and properties. By understanding the mechanisms, manufacturers can tailor materials for specific applications ranging from packaging and construction to textiles and automotive components.
