A polymer is a very large molecule that is built from monomers (the repeating units that make up a polymer).
Synthetic polymers are human-made polymers. They can be encountered almost in every material used in the daily life.
Two different types of synthetic polymers are addition polymers and condensation polymers.
1. Synthetic Addition Polymers. Plastics and Elastomers.
- Function of plasticizers
- Polymer cross-linking
- Synthetic plastic polymers
Synthetic Addition Polymers. Plastics and Elastomers.
Function of plasticizers
In addition to the polymer itself, many plastic products include plasticizers.
Plasticizers are substances added to increase flexibility, making the plastics easier to produce and shape into useful objects.
Plasticizer molecules are inserted between polymer chains, keeping the chains spaced apart. This slightly weakens the forces that hold the molecules rigidly in place.
However, some plasticizers, such as phthalates, have been linked to negative health effects.
Depending on functional groups attached to the monomers, individual chains of some polymers can link together.
Chemical bonds can form between separate polymer strands in a process called cross-linking.
Cross-linking binds multiple polymer chains together to form networks. These networks may be two-dimensional or three-dimensional structures.
In general, the more cross-links there are, the more tightly the chains are held together and the more rigid and inflexible the polymer.
Dienes are alkenes with two carbon–carbon double bonds. Incorporated into polymer molecules, dienes form strong cross-links from one chain to another.
The more dienes that are added to a polymer chain, the more cross-linking will occur.
A diene used for cross-linking polymers is 1,4-diethenylbenzene, also called divinylbenzene or DVB. Incorporated into polystyrene, 1,4-diethenylbenzene makes the resulting plastic stronger.
Some cross-linking agents are inorganic. Sulfur is able to form two covalent bonds to connect two different polymer chains.
A plastic is a synthetic polymer that can be molded into shape (often under heat and pressure) and will then retain its shape when cooled.
Examples of addition polymers and their uses
Properties of synthetic plastics
Most plastics are chemically unreactive. Some plastics, however, are weakened or even dissolved by non-polar organic solvents.
The properties of a polymer depend on a number of factors including:
- the functional groups present,
- the number of monomers in the polymer molecule,
- the degree of cross-linking that occurs.
Not all plastics are suitable for all uses.
Low-density and high-density polyethene
Polyethene plastics are classified according to their density.
The most common forms are low-density polyethene (LDPE) and high-density polyethene (HDPE).
Low-density polyethene is made by adding a small amount of compounds, such as butadiene, that have two or more double bonds. The butadiene results in branches in the polymer chains. These branches prevent the molecules from packing as tightly as those in high-density polyethene, which consist of mostly straight-chain molecules.
The branched polymer, therefore has a lower density.
Low-density polyethene is mostly used for packaging materials.
The major use of high-density polyethene is for blow-moulded products.
Because polypropene has more carbon atoms per monomer than polyethene, it tends to be harder and less flexible.
Thermoplastics and thermoset polymers
Some plastics can withstand more heat than others.
Some plastics will melt or soften when heated and hold their shape when cooled. These plastics are called thermoplastics and they can be molded.
Thermoplastic polymers do not contain many cross-links.
A thermoplastic polymer – polytetrafluoroethylene
Highly cross-linked plastics will not soften at high temperatures because the cross-links, which are strong covalent bonds, are not overcome by heating. These plastics are called thermoset polymers.
Elastomers are as they sound—elastic.
Elastomers are polymers with a limited amount of cross-linking, which allows them to stretch and then snap back to their original shape.
Most elastomers are carbon-based, such as neoprene and rubber.
Silly Putty is a type of silicone-based elastomer. It is bouncy and stretchy, and it floats.
Condensation polymer - a very long organic molecule formed as a result of condensation reactions between monomers with two functional groups.
The two most common types of condensation polymers have either ester linkages or amide linkages.
Esters have a carboxyl functional group joining the monomers (R–COOR');
amides have a carbonyl group, –C=O, connected to a nitrogen atom, –N–, joining the monomers (R–CONR').
A polyester is a synthetic condensation polymer formed by a series of esterification reactions between monomers.
Monomers with two functional groups, –OH (hydroxyl) group of alcohols and –COOH (carboxyl) group of carboxylic acids, form chains by undergoing condensation reactions at both ends.
As each monomer molecule is added to the chain, the condensation reaction produces a small by-product molecule, such as water.
Polyesters can also be formed from two different monomers.
One monomer must have two hydroxyl groups (a diol) and the other must have two carboxylic acid groups (a dicarboxylic acid).
In this case, the polyester is a copolymer formed with alternating monomers diol and dicarboxylic acid.
A copolymer has different types of monomers combined to form the polymer chain.
Dacron is the brand name for polyethene terephthalate (PET).
It is a condensation copolymer formed by the reaction of ethane-1,2-diol (ethene glycol) and benzene- 1,4-dicarboxylic acid (terephthalic acid).
Ester linkages join the monomers together.
Polyamide - a polymer formed by condensation reactions resulting in amide linkages between monomers.
The carboxyl groups allow many hydrogen bonds to form between polymer chains.
Together, these hydrogen bonds make polyamides exceptionally strong. Polyamides are also resistant to damage from insects and heat.
Kevlar is a polyamide used in body armour.
Nylon is a synthetic condensation copolymer, with two different types of monomers.
One common form of nylon is produced when hexane-1,6- diamine (hexamethylenediamine) and hexanedioyl dichloride (adipoyl chloride) react, resulting in the formation of a C–N bond and a hydrogen chloride molecule.
There are several types of nylon, depending on the particular diamine and dicarboxylic acid or dicarbonyl chloride used.
Polymers and their properties