To initiate the reaction of styrene to yield polystyrene in the presence of a cationic initiator, we can follow these three steps: initiation, propagation, and termination.
1. Initiation:
In this step, the cationic initiator, such as a Lewis acid or a strong acid, initiates the reaction by generating a carbocation from the styrene molecule. For example, let's consider the use of a Lewis acid like boron trifluoride (BF3). BF3 reacts with a molecule of styrene, abstracting a proton from the benzene ring and forming a carbocation:
Styrene + BF3 → Styrene-BF3 complex
Styrene-BF3 complex → Styrene carbocation + BF3-
2. Propagation:
The propagation step involves the repeated addition of styrene molecules to the growing chain, which is initiated by the carbocation formed in the initiation step. The carbocation attacks another styrene molecule, leading to the formation of a new carbocation and extending the polymer chain. This process is repeated multiple times to create a long chain of styrene units. Here's an example of the propagation step:
Styrene carbocation + Styrene → Polystyrene chain
Polystyrene chain + Styrene → Longer Polystyrene chain
3. Termination:
To terminate the reaction, a nucleophile (Nu:-) is added to react with the carbocation, resulting in the formation of a stable product. The nucleophile donates a pair of electrons to the carbocation, neutralizing the positive charge and terminating the polymerization process. This addition of the nucleophile can occur at any point along the growing chain. Here's an example of termination with the nucleophile Nu:−:
Polystyrene chain + Nu:- → Polystyrene-Nu product
In conclusion, the reaction mechanism for the synthesis of polystyrene from styrene in the presence of a cationic initiator involves initiation, propagation, and termination steps. The initiation step forms a carbocation, the propagation step extends the polymer chain, and the termination step involves the addition of a nucleophile to stop the polymerization process and yield the final polystyrene product.