What Type of Chemical Reaction Produces a Polymer

Before going direct into the main topic, What type of chemical reaction produces a polymer, let us understand the basics of polymer first.

What is a Polymer

A polymer is a substance consisting of molecules that are arranged as very long sequences of one or more species of atoms or group of atoms linked to each other by covalent bonds. Polymers are considered as macromolecules owing to their massive molecular mass.  The groups of atoms or molecules that interconnect to form a polymer are known as monomers. Thus, monomers are the building blocks of polymers.

The primary bonds found in a polymer are covalent bonds. In addition, there may be van der Waals bonds. Covalent bonds are stronger than van der Waals bonds. Thus, depolymerization of a polymer is quite difficult, and it may involve advanced techniques.

Classification of Polymers

Polymers are classified in various ways.

Based on the types of monomers that build polymers, there are two types of polymers; (a) homopolymers, which are composed by the polymerization of only one type of monomer, and (b) copolymers, which are formed by the polymerization of two or more kinds of monomers.

Based on the nature of the polymers, they can be classified as natural and synthetic polymers. A good example for a natural polymer is natural rubber latex, which is obtained from the tree called Hevea brasiliensis. Synthetic polymers are man-made polymers under controlled condition. Some examples of synthetic polymers include plastic, neoprene rubber, silicone rubber, isoprene rubber, etc.

Based on the structure, there are four types of polymers: linear polymers, cyclic polymers, branched polymers and network polymers.

The most common classification of polymers is based on their chemical and physical characteristics. According to this classification, polymers are grouped into as thermoplastics, elastomers, and thermosets. Thermoplastics are the polymers made up of linear or branched polymers; they soften upon the submission of heat. They can be moulded into any shape by using various moulding techniques. Elastomers are the polymers that have an elastic nature, due to which they can rapidly recover their original dimension when the applied stress is released. Thermosets are the rigid polymers made up of highly cross-linked network of polymers. These polymers cannot be remoulded once formed and degrade upon the application of heat. 

What are Biodegradable Polymers

What is Polymerization

Polymerization is the process which links monomer molecules together to form long chains through a chemical reaction. Homopolymers are formed by homopolymerization, whereas copolymers are formed by copolymerization. For example, ethylene monomers undergo homopolymerization to form polyethene, whereas ethylene and propylene monomers undergo copolymerization to form poly (propylene/ethylene) [PP/PE] copolymer.

What Type of Chemical Reaction Produces a Polymer

The basic requirement for polymerization is the ability of monomers to form bonds with other monomer molecules. There are various types of chemical reactions involved in polymer industry to form polymers. All these reaction types can be categorised into two basic categories called step polymerization and chain polymerization.

Step Polymerization

Step polymerization is a growth reaction. In step polymerization, the growth of polymer chains occurs by step-wise reactions that take place between any two molecular species. During step polymerization, the degree of polymerization increases gradually throughout the reaction as each monomer molecule is transformed into a dimer, then into a trimer and so on until they form the polymeric macromolecules. There are two types of polyreactions under step polymerization: polycondensation and polyaddition. Polycondensation reactions are far more common than polyaddition reactions.

A generic representation of a step-growth polymerization. (Single white dots represent monomers and black chains represent oligomers and polymers)

Chain Polymerization

In chain polymerization, the polymerization reaction occurs only with a monomer attached to a reactive end-group and usually requires an initiator to start the reaction. The monomers that are used for chain polymerization usually contain double bonds, triple bonds or aromatic rings. These reactions can be carried out using anionic mechanisms, cationic mechanisms, free radical mechanisms and coordination mechanisms. The type of mechanism is determined based on the chemical property of the monomer and initiator used. The most common mechanism is free-radical polymerization, where monomers contain carbon-carbon double bonds or vinyl monomers such as ethylene, butadiene, styrene, acrylonitrile, vinyl chloride, etc. Initiators of free radical polymerization can generate radicals by various chemical reactions such as thermal decomposition, redox reactions, etc., in order to initiate the polymerization reaction. Some common examples of such initiators include hydroperoxides, bicarbonate peroxide, peroxyesters, azocompounds, inorganic water-soluble persulfates, hydrogen peroxide, etc.

An example of chain-growth polymerization by ring opening to polycaprolactone

References:

Robert J. Young and Peter A. Lovell, Introduction to polymers (2011), 3rd Edition, CRC Press, USA.

Bruce, R. G, Dalton, W.K., Neely, J.E. and Kibbe, R.R, Modern materials and manufacturing process (2004), 3rd Edition, Repro India Ltd, India.

Image Courtesy:

“Step-growth polymerization” By Chem538grp5w09 – Own work (Public Domain) via Commons Wikimedia

“Polycaprolactone Synthesis” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia

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