Q 14-14-44 E - Organic Chemistry (Mcmurry)

Question

1,3-Cyclopentadiene polymerizes slowly at room temperature to yield

a polymer that has no double bonds except on the ends. On heating, the

polymer breaks down to regenerate 1,3-cyclopentadiene. Propose a structure for the product.

Step-by-Step Solution

Answer

Polycyclopentadiene is the product of successive Diels-Alder addition of cyclopentadiene to a growing polymer chain. Strong heat cause depolarization of the chain and reformation of cyclopentadiene monomer units.

The structure of product is given below:

Diels-Alder reaction

The Diels-Alder reaction is an important organic chemical reaction discovered by German chemists Otto Diels and Kurt Alder in the year 1928. 

It is type of reaction in which the reactant include a substituted alkene and conjugated diene. This substituted alkene commonly referred to as dienophile.

The Diels-Alder reaction proceed via concerted mechanism in which bond breakage and bond formation occurs in a single step.

                 Diene                                               dienophile

Polymerization of 1,3-cyclopentadiene

In the first step, 1,3 cyclopentadiene is converted into a dimer and it also act as diene and dienophile. This step occur due to heating of monomer unit. In the further step the dimer adds to double bonds of 1,3-cyclopentadiene and give rise to trimer. This reaction continues to form the final polymeric product.

 

Propose a structure for the product

Polycyclopentadiene is the product of successive Diels-Alder addition of cyclopentadiene to a growing polymer chain.

 In the proposed structure of polycyclopentadiene, molecules have two different kind of double bonds for end group and a complicated backbone of saturated fused rings. The polymerization is reversible and onstrong heat cause depolarization of the chain and reformation of cyclopentadiene monomer units, this final repeating does not contain any double bond.