Polymers are basically large molecules made up of repeating units called monomers. We can make polymers in two ways: addition and condensation. Addition polymers come from monomers that have a C=C double bond. The bond breaks and connects with an adjacent monomer to form a single long polymer chain with a C-C backbone. Meanwhile, condensation polymers come from monomers with two different functional groups. When they join together into a polymer, they lose some of their atoms, which then form a small molecule called a condensate.
Some examples of addition polymers are high and low-density polythene (HDPE and LDPE, respectively), and PVC. Meanwhile, some examples of condensation polymers are Terylene, Nylon, and Kevlar. In this article, we'll explore their structures, properties, and uses. If you're interested in learning more about different types of polymers, check out Polymerisation Reactions.
The first polymer we’ll look at is HDPE. HDPE, properly known as high-density polythene, is a plastic formed from many hundreds of ethene monomers.
HDPE is strong and dense and is used for products such as washing up bowls, plastic pipes, and milk bottles. In order to understand its properties, we must first look at its formation.
HDPE is formed in an addition polymerisation reaction at low temperatures and pressures of about 60℃ and 2-3 atm respectively. A Ziegler-Natta catalyst is used. This consists of a mixture of titanium and aluminium compounds. These conditions result in long, straight hydrocarbon chains with very little random branching.
HDPE chains. Notice how they are predominantly straight with little branching.
Because the hydrocarbon polymer chains of the plastic HDPE are predominantly straight and there are very few branches, the molecules can pack together tightly. This makes HDPE very dense. It also results in greater intermolecular forces, namely van der Waals attraction, between the molecules. (Refresh your memory about this attraction with a look at Intermolecular Forces.) These van der Waals forces mean that HDPE has a high melting point and is very strong.
Did you know? HDPE implants have been used in plastic surgery since 1985 as part of facial augmentation procedures, thanks to their strength and low toxicity.
Now let’s take a look at LDPE, HDPE’s close cousin. LDPE, properly known as low-density polythene, is also a plastic polymer made from ethene monomers. However, it has quite different properties to HDPE. It is relatively weak and flexible and so is used for carrier bags and food packaging.
LDPE is formed in an addition reaction at temperatures of around 200℃ and a pressure of 2000 atm. The reaction mechanism uses free radicals and results in a high proportion of random branching along its hydrocarbon chains.
A free radical is an atom, ion or molecule with an unpaired outer shell electron. They are all extremely reactive.
LDPE chains. Note the high proportion of random branching.
Did you know that the strength and density of LDPE is significantly lower than HDPE due to weaker van der Waals forces between chains? This is because the chains in LDPE are randomly branched and cannot pack together as tightly as in HDPE.
In 2010, Braskem, Latin America’s largest petrochemical company, created the world’s first bio-based plastic made from monomers derived from sugar cane. This bio-based plastic is a type of polythene and is entirely renewable.
Now, let's talk about another addition polymer: polyvinyl chloride, or PVC. It is a type of polymer made from chloroethene monomers and is officially known as poly(chloroethene). PVC is known for being hard and rigid, and is often used in drainpipes, cable insulation, and shoes. In fact, it is the third-most produced plastic in the world.
PVC is formed in an addition polymerisation reaction. It forms long hydrocarbon chains with the chlorine atoms arranged with random orientations.
The polymer of PVC.
Even though the hydrocarbon polymer chains in PVC cannot pack tightly together due to the large size and randomness of their chlorine atoms, the plastic is still hard and strong. This is because of the polar C-Cl bond, which creates permanent dipole-dipole forces between molecules that hold the chains tightly together.
To make PVC more flexible, plasticisers can be added. These small molecules fit between the polymer chains, forcing them apart and allowing them to slip over each other. This reduces the intermolecular forces between the chains and weakens the plastic. PVC that has been treated with plasticisers can be used to create softer products such as imitation leather.
The first condensation polymer we’ll look at is Terylene. Terylene, also known as PET, has the proper name of poly(ethylene terephthalate) and is a polyester polymer. It is used for a variety of purposes, such as clothes and drinks bottles, and accounts for 18 percent of the world’s polymer production.
Terylene is formed in a condensation polymerisation reaction between benzene-1,4-dicarboxylic acid and ethane-1,2-diol. It consists of hydrocarbon chains based around the ester functional group, -COO-. A small molecule is released in the process. In this case, that small molecule is water. Its structure is shown below.
The structure of Terylene.
Terylene has polar bonds and so experiences permanent dipole-dipole forces between chains. Its properties can vary, from flexible to rigid, which is reflected in its variety of uses. For example, you’ll find it in clothing under the generic term polyester, in microfibre towels and cleaning cloths, and in plastic drinks bottles.
Nylon is a polyamide polymer that was first synthesised in 1935. It is used in products such as clothes, toothbrushes, food packaging and electrical equipment, and was the first example of a thermoplastic polymer. These are polymers that melt at high temperatures and resolidify once cooled, instead of becoming brittle and snapping.
Nylon is commonly formed in a condensation polymerisation reaction between an amine and a carboxylic acid. Water is released in the reaction. For example, Nylon-6,6 is made industrially by the reaction between 1,6-diaminohexane and hexane-1,6-dicarboxylic acid, as shown below.
The two reactants first form a salt, which is heated at 350℃ under suitable pressure. Nylon can also be formed in the reaction between an amine and an acyl chloride. This uses hexanediol dichloride, takes place at room temperature, and is a much faster reaction. In this case, the small molecule released is .
Because Nylon contains the amide linkage group, -CONH-, it experiences hydrogen bonding between polymer chains. This occurs between the hydrogen atoms bonded to nitrogen, and the nitrogen on an adjacent chain. This increases Nylon’s strength dramatically.
Did you know? Nylon’s first commercial use was as toothbrush bristles in 1938. Its popularity rose during the Second World War. After the war was over, nylon parachutes were commonly recycled into women’s dresses.
The final polymer we’ll investigate is Kevlar. Kevlar is also a polyamide. It is extremely strong and light and can withstand high temperatures, making it suitable for use in bulletproof vests, lightweight mountaineering ropes, and oven gloves.
Kevlar is made from the condensation polymerisation reaction between benzene-1,4-diamine and benzene-1,4-dicarboxylic acid. Due to its benzene rings, it forms long chains that are predominantly planar.
Like Nylon, Kevlar contains the amide linkage group and so experiences hydrogen bonding between chains. Because its chains are rigid and mostly planar, they can pack together tightly, increasing the strength of the intermolecular forces.
Thank you for providing the table. Here is the information on how polymers are disposed of:
Polymers can be disposed of in several ways, including recycling, landfilling, and incineration. Recycling involves breaking down the polymer into its constituent monomers or reusing the plastic in another product. Landfilling involves burying the waste in a designated area, while incineration involves burning the plastic to generate energy. However, incineration can release harmful chemicals into the environment, and landfilling can lead to the accumulation of non-biodegradable waste. Therefore, it is important to reduce plastic waste by using less and disposing of it responsibly.
Polymers such as polyethene are long-chain hydrocarbons, containing only nonpolar bonds. This makes them unreactive and resistant to attack. Their C-H and C-C bonds are very strong and cannot be broken down by hydrolysis or other common reactions. Although they can be burnt, this releases carbon dioxide and other harmful pollutants like carbon particles, carbon monoxide, and styrene vapours. Therefore, there is no simple way to dispose of these plastics.
Alternative solutions to polymer disposal also include biodegradable plastics made from natural materials such as starch or cellulose. These plastics can be broken down by microorganisms in the environment, reducing pollution. Additionally, reducing plastic consumption through the use of reusable products and implementing waste reduction policies can help decrease the amount of plastic waste generated in the first place.
How does cross-linking affect the properties of polymers?
Cross-linking makes polymers much harder and stronger.
What are the properties of synthetic polymers?
The properties of synthetic polymers depend on their structures. For example, HDPE consists of straight chains packed together densely and is hard and strong. On the other hand, LDPE consists of branched chains, and is much softer and more flexible.
How does the structure of a polymer affect its properties?
The structure of a polymer determines which intermolecular forces are present between polymer chains. This affects the polymer's properties. For example, Nylon contains the amide linkage group. This means that it can form hydrogen bonds between chains, making it hard and strong.
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