Addition polymers are all those that are formed through the successive and directed growth of their molecular structures. Its synthesis or manufacturing processes are based on chain growth reaction mechanisms, or on 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, their double bonds are broken 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 synthesis conditions (pressure, temperature, selection of catalysts, etc.). Thus, polyethylene can be used to make chairs or armchairs, bags, milk bottles, utensils, plastic wrap, pipes, garbage containers, among other objects.
Structure of addition polymers
Because addition polymers have directed growth, without the simultaneous formation of small molecules, their structures end up in 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 with polyethylene. Meanwhile, the chain will be heterogeneous if it is made up of more than two different monomers (ABBABA…). In this case we speak of a copolymer.
If the chain is vast and of entirely linear growth, it will tend to intertwine with others to join in a kind of rope. This structure gives strength and toughness to the resulting solid or plastic, since ultimately 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 interlock with others, which will give rise to a low-density solid. If there were branches in the rope mentioned, its fibers would separate due to weak intermolecular interactions and its smaller contact area.
Generally speaking, addition homopolymers have the potential to assemble into stiffer plastics compared to those obtained with addition copolymers.
Regardless of whether the addition polymer formation reaction 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 bonds (C≡C).
The growth of the polymer chain occurs one step at a time, which means that no two monomer molecules will be added at the same time. That is why it is said that training is given successively, one by one, and directed, towards a certain direction.
The locations 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 bonds, without atoms in the form of small molecules being lost in the process.
For example, for polyethylene the first steps would be:
2CH 2 = CH 2 »CH 3 -CH 2 -CH = CH 2
The third ethylene molecule will be added to where the double bond is:
CH 3 -CH 2 -CH = CH 2 + CH 2 = CH 2 »CH 3 -CH 2 -CH 2 -CH 2 -CH = CH 2
And so on until obtaining: CH 3 [CH 2 -CH 2 ] n CH 3 .
A particular case of the formation of these polymers is that which occurs through the breaking of a ring, which originates the monomer required for the synthesis of the polymer.
Characteristics of addition polymers
Based on what has been said so far, some general characteristics that all addition polymers share in common can be listed:
-They have chain-type 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
The main function of addition polymers is to serve as a material for countless household or routine applications. Their lightness or robustness vary depending on the densities of their structures and, consequently, on 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 for the manufacture of garbage containers, utensils, pipes, bulletproof vests, skating rinks or toys.
Of greater hardness, polyvinyl chloride is used to make drain pipes, electrical tapes, garden hoses, plastic wraps, synthetic leather, among other objects.
Polystyrene can be in either a crystalline or expanded form. Therefore, its uses are varied, being used for the production of razors, radio knobs, food containers, insulating cups, CD cases and vehicle batteries.
Used for the manufacture of textile fibers, awnings, felts, sails for boats, or fibers for the reinforcement of cement.
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