Alkenes like ethene can undergo addition polymerisation.

They can join end to end …

… to make poly(ethene), the simplest addition polymer.

Ethene is the monomer for poly(ethene), also called polythene.

This is the repeating unit for poly(ethene).

Propene is the monomer for poly(propene), also called polypropylene.

This is the repeating unit for poly(propene).

Chloroethene (vinyl chloride) is the monomer for poly(chloroethene), also calledpolyvinylchloride or PVC.

This is the repeating unit for poly(chloroethene).

Polymer molecules can grow outwards from the surface of a Ziegler–Natta catalyst like a growing hair.

Let's look at poly(ethene), a polymer made from ethene monomers like this one.

An intermediate structure forms.

The ethene molecule bonds to the polymer chain and the catalyst.

The ethene molecule bonds to the polymer chain and the catalyst.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

Another monomer molecule is ready to join the growing polymer chain.

An intermediate structure forms.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

The conformation changes ready for the next ethene monomer molecule.

COPOLYMERSPolymers which are formed by combining two differentmonomers in alternating fashion are called copolymers.

A B A B A B A B+ + +

styrene-butadienerubber (SBR)

styrene

butadiene

Most addition polymers are thermoplastics.

Thermoplastics can be softened or melted by heatand reformed (molded) into another shape.

THERMOPLASTICS

The polymer chains are held together by weakinteractions (noncovalent bonds) such as :

van der Waal’s forces,London dispersion forces and Dipole-dipole attractions.

These interactions are disrupted by heating, allowingthe chains to become independent of each other.

Heating and reforming can be repeated indefinitely(if degradation doesn’t occur). This allows recycling.

THERMOSET PLASTICSThermoset plastics melt initially, but on furtherheating they become permanently hardened.

Once formed, thermoset plastics cannot be remolded, and they cannot be recycled.

On heating, thermoset plastics become cross-linked(covalent bonds form between the chains). The cross-linked chains form a rigid network

heat

cross-linkedlinear