Q26E - Organic Chemistry (Mcmurry)

Question

1,3-Cyclopentadiene undergoes thermal polymerization to yield a polymer that has no double bonds in the chain. Upon strong heating, the polymer breaks down to generate cyclopentadiene. Propose a structure of the polymer.

Step-by-Step Solution

Answer

The structure of the polymeric product can be given as:

Step1: Addition polymerization

Addition polymerization is also termed as chain-growth polymerization. In this process, the initiator incorporates the carbon-carbon double bond and produces a reactive monomer (intermediate). This intermediate further reacts with the monomer to generate the polymeric product.

Step 2: Proposing a structure of the polymer

The monomeric unit of the final polymeric product is 1,3-cyclopentadiene.

Monomeric unit of the final polymeric product 

 

The polymerization reaction happens by the thermal polymerization of 1,3-cyclopentadiene.

The mechanism for the reaction can be given as:

Addition reaction mechanism

In the first step, 1,3-cyclopentadiene is transformed into a dimer and it functions as a diene and a dienophile. This step happens due to the heating of the monomer unit.

In the next step, the dimer is added to the double bond of the 1,3-cyclopentadien to produce a trimer. These reactions continue to result in a final polymeric product.

In polycyclcopentadiene, the molecules comprise two kinds of double bonds for the end groups. The polymerization is reversible; and on heating, the polymer reverts to cyclopentadiene. The final repeating unit does not have a double bond.

The structure of the polymeric product can be given as:

Structure of the polymeric product