por The addition polymers They are all those that are formed through the successive and directed growth of their molecular structures. Its synthesis or manufacturing processes are based on mechanisms of chain growth reactions, or polyadditions.
The best way to differentiate this type of polymer from condensation polymers is that they do not originate any molecule as a product; that is, no water, ammonia, carbon dioxide, or any other small molecule is released. Therefore, the number of atoms before and after addition polymerization is said to be the same.
Polyolefins, such as polyethylene, represent an example of addition polymers. Ethylene polymerizes without its molecules losing atoms. Instead, its double bonds break to make way for the progressive joining of ethylene molecules, which end up assembling into chains.
The uses of addition polymers depend on their molecular structure and density, which in turn are subject to the synthesis conditions (pressure, temperature, catalyst selection, etc.). Thus, polyethylene can be used to make chairs or armchairs, bags, milk bottles, utensils, plastic wrap, pipes, garbage containers, among other objects.
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Structure of addition polymers
Because addition polymers have directed growth, without the simultaneous formation of small molecules, their structures end up taking the form of a chain.
The chain will be homogeneous if it is a homopolymer; that is, if all its monomers are identical (AAAA…), as is the case with polyethylene. Meanwhile, the chain will be heterogeneous if it is formed by more than two different monomers (ABBABA…). In this case one speaks of a copolymer.
If the chain is vast and grows entirely linearly, it will tend to intertwine with others to assemble itself into a kind of rope. This structure gives strength and hardness to the resulting solid or plastic, since after all the ropes are characterized by their ability to support a lot of weight.
On the other hand, if the chain has branches, it will hardly be able to intertwine with others, which will give rise to a low-density solid. If there were ramifications in the aforementioned rope, its fibers would separate due to weak intermolecular interactions and their smaller contact area.
In general terms, addition homopolymers have the potential to assemble into stiffer plastics compared to addition copolymers.
Training
Regardless of whether the reaction for the formation of the addition polymers proceeds following a chain growth or polyaddition mechanism, the basis of the process is due to the unsaturations present in the molecular structure of the monomers. That is, there must be double (C=C) or triple (C≡C) bonds.
Polymer chain growth occurs one step at a time, which means that two monomer molecules will not be added at the same time. That is why it is said that the formation is given successively, one by one, and directed, towards a certain direction.
The location of the C=C and C≡C bonds represent the potential regions where the polymer will continue to grow. Other monomers are added or added to these links, without losing atoms in the form of small molecules in the process.
For example, for polyethylene, the first steps would be:
2CH2=CH2 ” CH3-CH2-CH=CH2
The third ethylene molecule will add to where the double bond is located:
CH3-CH2-CH=CH2 + CH2=CH2 ” CH3-CH2-CH2-CH2-CH=CH2
And so on until you get: CH3[CH2-CH2]nCH3.
A particular case of formation of these polymers is the one that occurs through the breaking of a ring, which originates the monomer required for the synthesis of the polymer.
Characteristics of addition polymers
From what has been said so far, some general characteristics that all addition polymers share in common can be listed:
They have chain-like structures.
-Its monomers have double or triple bonds
-No by-products are generated, be it water, ammonia, hydrogen chloride, nitrogen or carbon dioxide
-Polymers grow following a chain reaction or polyaddition
-The resulting molar mass is a multiple of the molar mass of the monomers. For example, polyethylene can have an average mass of 200,000 times or more than the molar mass of ethylene.
-They are inert, and therefore are not biodegradable due to their strong CC bonds
Function
The main function of addition polymers is to serve as a material for countless domestic or routine applications. Their lightness or robustness vary depending on the densities of their structures and, consequently, their synthesis parameters.
Thus, having the function of being a material, many uses derive from them for each addition polymer separately (or together). Some of these polymers are found in natural materials. For example, the polymer poly-cis-isoprene is part of the sap of rubber trees.
Examples and uses
Finally, mention will be made of some addition polymers with their respective uses.
low density polyethylene
Because it is light and moldable, it is used to make plastic bags, plastic wrap, flexible lids, and shampoo or juice bottles.
High density polyethylene
Being more robust and opaque, it is used to make garbage containers, utensils, pipes, bulletproof vests, skating rinks or toys.
Polyvinyl chloride
With greater hardness, polyvinyl chloride is used to make drainage pipes, electrical tapes, garden hoses, plastic wrap, synthetic leather, among other objects.
Polystyrene
Polystyrene can be in either crystalline or expanded form. Therefore, its uses are varied, being used to make shavers, radio knobs, food containers, insulating glasses, CD casings and vehicle batteries.
polyacrylonitrile
Used for the manufacture of textile fibers, awnings, felts, sails for boats, or fibers for reinforcing cement.
References
Graham Solomons TW, Craig B. Fryhle. (2011). organic chemistry. (10th ed.). WileyPlus.
Carey F. (2008). organic chemistry. (Sixth edition). Mc Graw Hill.
Morrison and Boyd. (1987). organic chemistry. (Fifth edition). Addison-Wesley Iberoamericana.
Wikipedia. (2020). Addition polymer. Retrieved from: en.wikipedia.org
Virtual Chembook. (June 5, 2019). Addition Polymers. Chemistry LibreTexts. Retrieved from: chem.libretexts.org
Byju’s. (nd). Difference Between Addition and Condensation Polymerization. Recovered from: byjus.com
Polymer Science Learning Center. (2020). Polyethylene. Retrieved from: pslc.ws