An Introduction to Polymers: Types, Classifications, and Applications
Polymers
1. Definition
In chemistry, polymers are large molecules (macromolecules), usually organic, formed by the joining of smaller molecules called monomers.
2. Classification
2.1. Depending on their Origin
- Natural polymers: Many polymers occur naturally, and biomolecules that make up living things are polymeric macromolecules. For example: proteins, nucleic acids, polysaccharides (such as cellulose and chitin), rubber or natural rubber, lignin, etc.
- Semi-synthetic polymers: These polymers are obtained by processing natural polymers. For example: vulcanized rubber, nitrocellulose, etc.
- Synthetic polymers: Many polymers are obtained industrially from monomers. For example: nylon, polystyrene, polyvinyl chloride (PVC), polyethylene, and so on.
2.2. According to their Mechanism of Polymerization
- Condensation polymers: The polymerization reaction involves the formation of a low molecular weight molecule, e.g., water, at every step.
- Addition polymers: Polymerization does not involve the release of any compound of low molecular mass. This polymerization occurs when a “catalyst” starts the reaction. This catalyst breaks the carbon double bond in the monomers, then those monomers are joined by others due to free electrons, and thus are united one by one until the reaction ends.
The following table differentiates these two types of polymerization:
| Feature | Addition Polymerization | Condensation Polymerization |
|---|---|---|
| Monomer reaction | Only adds to the growing chain | Any two molecular species present can react |
| Monomer concentration | Decreases continuously during the reaction | Monomer disappears early in the reaction |
| Polymer formation | Formed rapidly | Polymer molecular weight increases continuously over time |
| Molecular weight change | Molecular weight changes little during the reaction | Long reaction times are essential for high molecular weight |
| Reaction time effect | Reaction times lead to higher yields but have little effect on molecular weight | Long reaction times are essential for high molecular weight |
These two methods are also known by the names:
- Polymers formed in stages: (condensation) An oligomer may react with others, such as a dimer with a trimer, etc.
- Polymers formed by chain reaction: (addition) Each chain grows one by one, forming first dimers, then trimers, tetramers, etc.
2.3. According to Chemical Composition
- Organic polymers: Have carbon atoms in the main chain.
- Organic vinyl polymers: The main chain of molecules is composed exclusively of carbon atoms.
Among them can be distinguished:
- Polyolefins: Formed by the polymerization of olefins. Examples: polyethylene and polypropylene
- Styrenic polymers: Include styrene between their monomers. Examples: polystyrene and styrene-butadiene rubber
- Halogenated vinyl polymers: Include halogen atoms (chlorine, fluorine, etc.) in their composition. Examples: PVC and PTFE
- Acrylic polymers: Examples: PMMA
- Non-vinyl organic polymers: In addition to carbon atoms, these polymers have oxygen or nitrogen in their main chain. Some sub-categories of importance:
- Polyurethanes
- Polyamides
- Polyesters
- Inorganic polymers: Among others:
- Based on sulfur. Example: polysulfides
- Based on silicon. Example: silicone
2.4. According to their Applications
Based on their properties and end uses, polymers can be classified into:
- Elastomers: Materials with very low modulus of elasticity and high extensibility, i.e., they deform significantly under stress but regain their original shape when the stress is removed. In each cycle of expansion and contraction, elastomers absorb energy, a property called resilience.
- Plastics: Polymers that, given a sufficiently intense force, deform irreversibly and are unable to return to their original shape. It should be stressed that the term “plastic” is sometimes incorrectly applied to refer to all polymers.
- Fibers: Have high modulus and low extensibility, allowing the manufacture of cloth whose dimensions remain stable.
- Coatings: Substances, usually liquids, which adhere to the surface of other materials to give them some property, such as abrasion resistance.
- Adhesives: Substances that combine high adhesion and high cohesion, which allows them to connect two or more bodies in contact.
2.5. According to their Behavior When Heated
To classify polymers, a simple empirical method is to heat them above a certain temperature. Depending on whether the material melts and flows or does not, we differentiate two types of polymers:
- Thermoplastics: Flow (go to a liquid) when heated and harden again (return to a solid-state) upon cooling. Their molecular structure has few (or no) crosslinks. Examples: polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC)
- Thermosets: Do not flow when heated; instead, they decompose chemically. This behavior is due to a structure with many junctions, which prevent relative displacements of the molecules.
The thermoplastic/thermoset classification is independent of the elastomer/plastic/fiber classification. Some plastics exhibit thermoplastic behavior, and others behave as thermosets. This is, in fact, the main branch of the group of plastics, and often when speaking of “thermosets,” it actually refers only to “thermosetting plastics.” But this should not obscure the fact that elastomers are also divided into thermosets (most) and thermoplastics (a minority but with very interesting applications).
2.6. According to their Monomers
Homopolymers: Macromolecules formed by the repetition of identical monomer units.
Examples: cellulose, rubber, polyethylene, PVC, etc.- Copolymers: Macromolecules consisting of two or more different monomer units.
Examples: Silk, Bakelite, etc.
Among the copolymers, we can distinguish the following sub-classification:Random copolymers: Monomers are grouped randomly.
Alternating copolymers: Monomers alternate.
Block copolymers: Monomers are grouped in blocks.
Grafted copolymers: Ramifications of another monomer are added to a main chain of a monomer.
2.7. According to their Molecular Structure
Linear: Long-chain macromolecules with no branching (e.g., polyethylene, silk, thermoplastics).
Branched: The main chain is connected laterally with strings.
Crosslinked: Adjacent linear chains are bound across in various positions by covalent bonds (e.g., elastomers).
Networked: Macromolecules are formed by chains and branches intertwined in three dimensions (e.g., bakelite, epoxy, thermosets).
3. Polymerization Methods
- Suspension polymerization: The reactants are placed in water. The monomers are insoluble and therefore create a suspension in water. To prevent the polymer from agglomerating, a bit of polyvinyl alcohol is dissolved in the water.
- Emulsion polymerization: Also performed in water, but this time the monomers are soluble in water but not in an emulsifier (soap, detergent). Under these conditions, the monomer is emulsified, i.e., droplets are created that are stabilized by the emulsifier. This forms a kind of latex. Then, the polymer is precipitated, breaking the emulsion.
- Mass polymerization: In this type, no solvent is used; only the monomers react.