Question

Identify the chain initiation step of the polymerisation of ethene in the presence of benzoyl peroxide initiator, from the following?

(a) C6H5• + C2H4 = C6H5-CH2-CH2•

(b) C6H5-CH2-CH2• + C2H4 = C6H5-CH2-CH2-CH2-CH2•

(c) C6H5-CH2-CH2-CH2-CH2• = C6H5-(-CH2-CH2-)n-CH2•

(d) C6H5-(-CH2-CH2-)n-CH2• + C6H5-(-CH2-CH2-)n-CH2• = C6H5-(-CH2-CH2-)n-C6H5

I got this question in an interview for internship.

Enquiry is from Types of Polymerisation Reactions in division Polymers of Chemistry – Class 12

Answer 1

Right answer is (a) C6H5• + C2H4 = C6H5-CH2-CH2•

To elaborate: Benzoyl peroxide undergoes homolytic fission to from phenyl free radical which acts an initiator. The first step is the addition of this radical to the ethene double bond, thus generating a new and larger free radical. This is called the chain initiating step.