In this lesson, learn about the process of polymerization. Explore the two major types, common polymers, and some of the chemistry involved with this biologically and commercially important chemical reaction.
‘Polymer’ comes from the Greek, meaning ‘many parts.’ A polymer is a long molecule consisting of many identical or similar building blocks linked by covalent bonds – like how a train consists of a chain of cars. Most large molecules, or macromolecules, are polymers. The repeating units that serve as the building blocks of a polymer are small molecules called monomers.
How are these monomers put together? Polymerization is the process of connecting these monomers together and creating large macromolecules of different sizes and shapes. Polymerization is similar to constructing a large building out of the same type of Lego blocks. The blocks can be connected in various ways to create a larger, more intricately shaped structure than the original Lego block on its own.
The two major types of polymerization are addition polymerization and condensation polymerization.
Polymerization that occurs through the coupling of monomers using their multiple bonds is called addition polymerization. The simplest example involves the formation of polyethylene from ethylene molecules. In this reaction, the double bond in each ethylene molecule opens up, and two of the electrons originally in this bond are used to form new carbon-carbon single bonds with two other ethylene molecules.
Some common commercial addition polymers are:
- Polyethylene – films, packaging, bottles
- Polypropylene – kitchenware, fibers, appliances
- Polyvinyl chloride – pipe fittings, clear film for meat packaging
Condensation Polymerization and Hydrolysis
The chemical mechanism that cells use to make and break polymers are basically the same in all cases. Monomers are connected by a reaction in which two molecules are covalently bonded to each other through loss of a water molecule; this is called a condensation polymerization because the lost molecule is water. When a bond forms between two monomers, each monomer contributes part of the water molecule that is lost; one molecule provides a hydroxyl group, while the other provides a hydrogen. To make a polymer, this reaction is repeated as monomers are added to the chain one by one.
Polymers are disassembled to monomers by hydrolysis, a process that is essentially the reverse of the dehydration reaction. ‘Hydrolysis,’ from Greek, means to ‘break with water.’ Bonds between monomers are broken by the addition of water molecules, a hydrogen from the water attaching to one monomer and a hydroxyl attaching to the adjacent monomer.
The process of digestion in our bodies is an example of hydrolysis. The bulk of the organic material in our food is in the form of polymers that are much too large to enter our cells. Hydrolysis helps to break these polymers into absorbable bits within the digestive tract.
Some common polymers in life are:
- Disaccharides and polysaccharides like maltose, sucrose, and glycogen
- All proteins made from amino acids
- Nucleic acids, like DNA and RNA, made from nucleotides
Some common commercial condensation polymers are:
- Polyethylene terephthalate (a polyester)
- Nylon 6,6
Each cell has thousands of different kinds of macromolecules. These molecules are constructed from only 40 to 50 common monomers and some others that occur rarely. Building an enormous variety of polymers from such a limited list of monomers is analogous to constructing an infinite amount of words from only 26 letters of the alphabet. The key is arrangement, or variation in the linear sequence the units follow. But the alphabet-word building analogy actually falls well short because most words are not very long. Macromolecules can consist of hundreds of letters in one word and can be constructed more like a Scrabble board game instead of just straight lines from left to right.
Polymerization is the process of forming larger macromolecules from simpler sub-units known as monomers. The two major types of polymerization are addition polymerization and condensation polymerization.
In addition polymerization, electrons from a double-bond are used to form bonds between other monomers, like in the process used to make polyethylene. In condensation polymerization, water is released as a result of bonding between monomers, when one monomer gives up a hydroxyl group and the other monomer gives up a hydrogen in the process, forming a covalent bond between monomers to make a polymer.
Some of the more common chemicals formed by polymerization include plastics, polyurethane, nylon, proteins, DNA, and RNA. Without this chemical reaction occurring, life as we know it would not exist.
- Polymerization: the process of connecting small molecules together and forming larger ones
- Monomers: small molecules that make up the blocks of polymers
- Macromolecules: the larger cells created by polymerization
- Addition Polymerization: occurs through the coupling of monomers using their multiple bonds
- Condensation Polymerization: monomers are connected by a reaction in which two molecules are covalently bonded to each other through loss of a water molecule
Thoroughly study the above details in order to subsequently:
- Contrast polymers and monomers
- Interpret the process of polymerization and recognize the two main types
- Present an example of addition polymerization
- Analyze condensation polymerization and recognize the usefulness of hydrolysis
- Discuss the variety of the macromolecules contained in cells