Question

What is polystyrene? The following processes result in a stronger polystyrene polymer. Explain why in each case.

a. addition of catalyst to form syndiotactic polystyrene

b. addition of 1,3-butadiene and sulfur

c. producing long chains of polystyrene

d. addition of a catalyst to make linear polystyrene

Short answer

(a) More efficient packing results in stronger London dispersion forces

(b) Covalent bonds can be created between neighbouring polymer chains.

(c) London dispersion forces occur through the whole length of the polymer chain. Therefore, longer chain will be stronger

(d) Linear polystyrene has higher packing efficiency that results with stronger London dispersion forces.

Step-by-step solution

Step 1: Addition of catalyst to form syndiotactic polystyrene

Syndiotactic polystyrene is shown in the picture below. As we can see, the difference between polystyrene and syndiotactic polystyrene is that polystyrene has benzene rings on one side while syndiotactic polystyrene has alternating benzene rings.

In terms of physical properties, syndiotactic polystyrene is much stronger because benzene rings from two parallel chains of polymer are placed in gaps between two benzene rings of neighbouring benzene rings. See the picture below (red and blue rings are neighbouring polymers). This type of packing increases London dispersion forces between neighbouring polymers and the stability of the polymer.

Hence More efficient packing results in stronger London dispersion forces

Addition of 1,3-butadiene and sulfur

The picture on the left shows a copolymer made of polystyrene and butadiene. As we can see, it contains a double bond that is able to react with sulfur atoms creating a polymer on the right. This type of polymer is able to form crosslinks with parallel neighbouring chains. That covalent bond is the main reason for the stronger polymer.

Hence covalent bonds can be created between neighbouring polymer chains.

Producing long chains of polystyrene

Between two neighbouring parallel polymer chains occur London dispersion forces. This attraction force is proportional to the length of the polymer because attractive forces happen through the whole polymer length. Therefore, the longer polymer will mean more intermolecular forces and chains will be stronger than shorter ones.

Hence London dispersion forces occur through the whole length of the polymer chain. Therefore, a longer chain will be stronger.

Addition of a catalyst to make linear polystyrene

Linear polystyrene has a more organized structure than non-linear polystyrene that contains branches. This more organized structure implies that two neighbouring polymer chains are in contact and between them, London dispersion forces occur. In the case of nonlinear chains, they have reduced packing efficiency that results with empty space between chains. Because of that empty space, neighbouring polymers have less contact and less London dispersion forces. Furthermore, they are not as strong as linear polystyrene.

Hence Linear polystyrene has higher packing efficiency that results with stronger London dispersion forces