Poly(propene) (polypropene or polypropylene) Chemistry Tutorial

Key Concepts

• Poly(propene) is also known as⁽¹⁾:

  ⚛ polypropene (acceptable alternative IUPAC source-based name)

  ⚛ polypropylene (traditional name)

  ⚛ poly(1-methylethane-1,2-diyl) (IUPAC structure-based name)

• Polypropene is often abbreviated as PP.
• Polypropene can be produced by the addition polymerisation of propene.

  propene	→	polypropene
  nH2C=CH(CH3)	→	-[CH(CH3)−CH2]n-

• The 3-dimensional arrangement of methyl groups (CH₃) along the chain of carbon atoms in the polymer determines whether it is described as:

  ⚛ isotactic (abbreviated as PP-it)

  repeating unit :

  H |     -[ C −CH2 ]-n   | CH3

  ⚛ syndiotactic (abbreviated as PP-st)

  repeating unit :

  H |   CH3 |   -[ C −CH2− C −CH2 ]-n   | CH3 | H

  ⚛ atactic (abbreviated as PP-at)

  no repeating unit: random distribution of CH₃ groups

• Commercially available polypropene is mostly isotactic.
• Commercially available polypropene has the following properties that make it useful and durable:

  ⚛ moderately high melting point (130°C to 171°C)

  ⚛ chemically inert (doesn't react with water, non-oxidising acids, bases nor detergents)

  ⚛ resistant to cracking even under stress

  ⚛ electrical insulator

• Polypropene is commonly used in labelling and packaging while polypropene fibres are used to produce carpets and bags.
• Polypropene is a linear polymer.
• Polypropene is a thermoplastic material, it softens when heated.
• Polypropene can be recycled.

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Synthesis and Naming of Poly(propene) (polypropene)

Polypropene can be produced by the addition polymerisation of the monomer propene, H₂C=CH(CH₃).
The 2-dimensional structural formula of a molecule of propene is shown below:

Propene has a boiling point of −48°C so it is a gas at room temperature and pressure.

The addition polymerisation of propene results in the formation of a white solid at room temperature and pressure. The structural formula of a short section of the polymer is often represented as shown below:

Based on the source of this polymer, that is the name of the monomer used to make this polymer, we can name this polymer poly(propene).
The name of the monomer, propene, is enclosed in round brackets and preceded by the prefix "poly".
Poly(propene) is known as a source-based name, that is, the name of the polymer is based on the name of the monomer used to make it.
It is common for the round brackets to be omitted from the name if the resultant name is not ambiguous, so poly(propene) is also known as polypropene.

Notice the repeating unit in this polymer is −H₂C−CH(CH₃)− (we have highlighted this repeating unit in the diagram below):

The repeating unit in the structure of this polymer is composed of 2 carbon atoms joined by a single covalent bond, ethane.
There is one branch or side-chain which is a methyl group, CH₃. By definition this methyl group is attached to the first carbon of the ethane chain, so we have 1-methylethane.
Since each 1-methylethane unit is joined to another at each end the unit is named as for an alkyl branch with the "yl" suffix and because there are 2 attachment points it is a diyl, the attachments occur at carbons 1 and 2 of the ethane chain hence 1,2-diyl.
The complete name of this polymer based on its structure is therefore poly(1-methylethane-1,2-diyl).

The IUPAC name for this polymer is

• poly(propene) or polypropene (source-based IUPAC name)
• poly(1-methylethane-1,2-diyl) (structure-based IUPAC name)

This polymer is also known as polypropylene. This is not a systematic IUPAC name, it is a traditional name.

The addition polymerisation of propene to produce polypropene requires a catalyst and can be represented by the chemical equation below:

Structure of Polypropene

In order to help us understand the structure of polypropene, we will take a closer look at a "polymer molecule" containing just 4 repeating units as shown in the diagram below:

The polymer produced by the addition polymerisation of propene is a linear polymer, a single linear chain of monomers.

Notice the fourth carbon atom from the left in the chain, C. There are four different substituents on this carbon atom:

• H atom
• CH₃ group
• CH₂-CH(CH₃)-CH₃ group
• CH₂-CH(CH₃)-CH₂-CH₂-CH₃ group

This carbon atom has an unsymmetrical arrangement of substituents ⁽²⁾ so the location of these substituents in 3-dimensions will be important to the overall 3-dimensional shape of the polymer.

So let's go back and look at the three dimensional structure of propene.
With the C=C in the plane of the page, we have two options for the location of the methyl group; either

(a) coming out of the plane of the page (a solid black wedge in the diagram below)
or
(b) going behind the plane of the page (indicated by broken lines making up the wedge in the diagram below)

(a)		(b)

Notice that these two structures are identical for a molecule of propene, we can rotate the first molecule around the C=C to make the second structure.
However, when molecules of propene react to form a polymer the orientation of the methyl groups becomes important.

Imagine the polymerisation of propene in which one propene molecule adds to another in a "head-to-tail" reaction to produce a dimer, then another propene molecule adds to this dimer in a "head-to-tail" reaction, and so on.
In a 2-unit section of the polymer chain there are 2 possible positions in 3-dimensions for the methyl groups:

(i) both methyl groups coming out of the plane of the page (which is the same as both methyl groups going behind the page because you can rotate one molecule to make the other)
or
(ii) one methyl group coming out of the plane of the page and the other going behind the plane of the page

(i)		(ii)

If we continue with this ordered arrangement of "head-to-tail" additions of propene to the existing chain we end up with 2 polypropene chains with different arrangements of methyl groups in 3-dimensions.

• If all the methyl groups extend out of the plane of the page we call this isotactic polypropene ⁽³⁾
• If the methyl groups alternate, the first comes out of the plane, the second behind the plane, and this pattern repeats, then we refer to this as syndiotactic polypropene ⁽⁴⁾

As the polymer chain gets longer than 2 repeating units, a third possibility emerges for the location of the methyl groups, that is, instead of an ordered arrangement in which all the methyl groups are behind the plane of the carbon chain or the ordered arrangement in which the methyl groups alternate between behind then in front of the plane, there may be a completely random distribution of the methyl groups along the carbon backbone.
A random arrangement of methyl groups results in a polymer known as atactic polypropene. ⁽⁵⁾

Ordered arrangments of methyl groups result in polymer chains that can pack together well, maximising the weak intermolecular forces acting between the polymer chains and resulting in a material that is highly crystalline.
The random distribution of bulky methyl groups in atactic polypropene means that the polymer chains can not pack together neatly, the weak intermolecular forces acting between chains are not maximised, so the resulting material is said to have low crystallinity.

These three types of polypropene are summarised in the table below:

Type	Structural formula	Repeating unit	Trend in Crystallinity
isotactic polypropene		H |     -[ C −CH2 ]-n   | CH3	most crystalline
syndiotactic polypropene		H |   CH3 |   -[ C −CH2− C −CH2 ]-n   | CH3 | H	↓
atactic polypropene		none	least crystalline

Properties of Polypropene

Most commercial polypropene is isotactic polypropene which is an opaque material with a melting point⁽⁶⁾ between 160 and 166°C.
Isotactic polypropene is essentially linear and highly crystalline. The hardness, stiffness and high tensile strength of isotactic polypropene is due to its high crystallinity.
Isotactic polypropene is the lightest major plastic having a density of 0.905 g cm^-3.
Isotactic polypropene is a poor conductor of electricity, it is an electrical insulator.
Isotactic polypropene is chemically inert and therefore resistant to corrosion. It does not react with non-oxidising acids and bases, it is resistant to fats and almost all organic solvents except for strong oxidants. At higher temperatures polypropene can be dissolved in some non-polar solvents.

Polypropene is a linear polymer, that is it does NOT contain cross-links to other polymer chains.
Linear polymers are thermoplastic, that is, they soften with heated.
Because polypropene is a thermoplastic it can be recycled. After the polypropene is collected for recycling it is cleaned then cut or shredded into small pieces. These small pieces can be heated and moulded, extruded, or blown into new shapes.

The properties of polypropene can be used to identify an unknown polymer as polypropene:

Uses for Polypropene

Polypropene has many uses in our society. Some of these uses have been summarised in the table below: