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How do polymers form?

How do polymers form?

Polymers are formed through a process called polymerisation, where monomers react together to form a polymer chain. Two of the types of polymerisation reactions are addition polymerisation and condensation polymerisation.

Definition: Polymerisation

Polymerisation is a process of bonding monomers, or single units together to form longer chains called polymers.

Addition polymerisation

In this type of reaction, monomer molecules are added to a growing polymer chain one at a time. (No small molecules are eliminated in the process).

Four major examples of addition polymers are polyethylene, polypropylene, polyvinylchloride (PVC) and polystyrene (see table below). All four of these organic polymers are also plastics.

Polymer

name

Production

(metric tons)

Some uses

polyethylene

\(> 80\) million

plastic bags

bottles

plastic films

polypropylene

\(> 45\) million

labelling

textiles

stationery

polyvinyl

chloride

\(> 30\) million

construction

clothing

insulation

polystyrene

\(> 2\) million

packaging

molds

cutlery

Table: Common uses of four major polymers formed through addition reactions.

  1. Polyethene (polyethylene)

    How do polymers form?

    A collection of products made from polyethylene: plastic bags, a syringe, pipes, a plastic tree guard.

    Earlier in this tutorial we looked at the structure of a group of hydrocarbons called the alkenes. One example is the molecule ethene. The structural formula of ethene is shown in the figure below. When lots of ethene molecules bond together, a polymer called polyethene (commonly called polyethylene) is formed. Ethene is the unsaturated monomer which, when joined to other ethene molecules through an addition reaction, forms the saturated polymer polyethene.

    Polyethene is the most common plastic with over 80 million metric tons produced each year. It is commonly known as polyethylene. It is cheap and is used to make squeeze bottles, plastic bags, films, toys and molded objects as well as electric insulation. It has a recycling number 4 which means that it is easy to process, has strength, toughness, flexibility, is easy to seal and has a barrier to moisture.

    How do polymers form?

    The polymerisation of an ethene monomer to form a polyethene polymer. The repeat unit is highlighted in blue.

    A polymer may be a chain of thousands of monomers, and so it is impossible to draw the entire polymer. Rather, the structure of a polymer can be condensed and represented as shown in the figure below. The monomer is enclosed in brackets and the n represents the number of repeating units (the saturated form of the monomer) in the polymer, where n is any whole number. What this shows is that the monomer is repeated an indefinite number of times in a molecule of polyethene.

    How do polymers form?

    A simplified representation of polyethene.

    Optional Video: What is HDPE?

  2. Polypropene (polypropylene)

    How do polymers form?

    A collection of products made from polypropylene: a lamp cover, computer parts, and shopping trolleys.

    Another example of a polymer is polypropene (see figure below). Polypropene (commonly known as polypropylene) is also a plastic, but is stronger than polyethene and is used to make crates, fibres and ropes as well as being used in textiles, stationery and car parts. In this polymer, the monomer is the alkene called propene.

    How do polymers form?

    How do polymers form?

    (a) The polymerisation of a propene monomer to form a polypropylene polymer. The repeat unit is highlighted in blue. (b) A simplified representation of polypropylene.

    Optional Video: What is PP?

  3. Polyvinyl chloride (PVC)

    How do polymers form?

    A collection of products made from polyvinyl chloride: pipes, electrical tape, and car parts.

    Polyvinyl chloride or PVC (see figure below) is formed from the monomer chloroethene, which is commonly known as vinyl chloride. PVC is used in construction, especially plastic piping. With the addition of a plasticiser it is also used in clothing and upholstery and to replace rubber. The role of the plasticiser is to increase the ability of a material to change shape without breaking.

    How do polymers form?

    How do polymers form?

    (a) The polymerisation of a chloroethane monomer to form a polyvinyl chloride polymer. The repeat unit is highlighted in blue. (b) A simplified representation of polyvinyl chloride.

    Optional Video: What is PVC?

  4. Polyvinyl acetate

    How do polymers form?

    Glowing slime made with polyvinyl acetate.

    Polyvinyl acetate or PVA (see figure below) is formed from the monomer ethenyl ethanoate, which is commonly known as vinyl acetate. PVA is used in various glues and adhesives (such as wood glue).

    Tip:

    Ethenyl is not the same as ethyl. Ethyl is an ethane molecule bonded to another compound (\(\color{red}{\textbf{R}}\)). Ethenyl is an ethene molecule bonded to another compound (\(\color{red}{\textbf{R}}\)).

    How do polymers form?How do polymers form?

    How do polymers form?

    How do polymers form?

    (a) The polymerisation of an ethenyl ethanoate monomer to form a polyvinyl acetate polymer. The repeat unit is highlighted in blue. (b) A simplified representation of polyvinyl acetate.

  5. Polystyrene

    How do polymers form?

    A collection of products made from polystyrene: a cup, a guitar case, polystyrene carvings, and floating pool noodles.

    Polystyrene is made from the monomer styrene which is a liquid petrochemical. Styrene consists of a benzene ring (a six membered ring with three double bonds) bonded to an ethene chain. Polystyrene is an aromatic polymer and has many uses including protective packaging, in trays, as plastic lids and bottles.

    Fact:

    Benzene can be represented with three double bonds:

    How do polymers form?

    or with a circle inside the ring:

    How do polymers form?

    How do polymers form?

    How do polymers form?

    (a) The polymerisation of a styrene monomer to form a polystyrene polymer. The repeat unit is highlighted in blue. (b) A simplified representation of polystyrene.

    Optional Video: Expanded Polystyrene Foam Products

It is interesting to note that the polymerisation of a monomer leads to different physical properties of the polymer (see table below). For the polymers the melting points are dependent on the polymer grade.

Fact:

Remember that the IUPAC name of:

  • ethylene is ethene

  • propylene is propene

  • vinyl chloride is chloroethene

Name

Melting point (℃)

Boiling point (℃)

Phase (at 25℃)

ethylene

\(-\text{169.2}\)

\(-\text{103.7}\)

gas

polyethylene

\(\text{105}\)\(\text{130}\)

does not boil

solid

propylene

\(-\text{185.2}\)

\(-\text{47.6}\)

gas

polypropylene

\(\text{130}\)\(\text{171}\)

does not boil

solid

vinyl chloride

\(-\text{153.8}\)

\(-\text{13.4}\)

gas

polyvinyl chloride

\(\text{100}\)\(\text{260}\)

does not boil

solid

vinyl acetate

\(-\text{93}\)

\(\text{72.7}\)

liquid

polyvinyl acetate

\(\text{60}\)

does not boil

solid

styrene

\(-\text{30}\)

\(\text{145}\)

liquid

polystyrene

~\(\text{240}\)

does not boil

solid

Table: Different physical properties of addition monomers and polymers.

Optional Activity: Development of addition polymers

Polyethene was discovered by accident, twice. Look into the history of this commonly used plastic as well as other addition polymers.

  • Who discovered them?

  • How were they discovered?

  • What difference have they made to modern life?

Optional Activity: Building polymers

Using atomic model kits, jelly tots, or playdough and toothpicks build four ethene monomers, four propene monomers and four vinyl chloride monomers.

  • Join three of each type of monomer to form polyethene, polypropene and polyvinyl chloride.

  • Identify the repeating unit in each polymer.

  • List the differences and similarities between the monomers and the repeating unit.

Note that the monomers all contain double bonds, while the repeating unit has only a single bond.

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