Solution: Polyethylene is used in many items, including

Chapter 13, Problem 13.125

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Polyethylene is used in many items, including water pipes, bottles, electrical insulation, toys, and mailer envelopes. It is a polymer, a molecule with a very high molar mass made by joining many ethylene molecules together. (Ethylene is the basic unit, or monomer for polyethylene.) The initiation step is R2 k1 2R ? initiation The R ? species (called a radical) reacts with an ethylene molecule (M) to generate another radical R ? 1 M M1 ? Reaction of M1 ? with another monomer leads to the growth or propagation of the polymer chain: M1 ? 1 M kp M2 ? propagation This step can be repeated with hundreds of monomer units. The propagation terminates when two radicals combine M ? 1 M ? kt MM termination The initiator frequently used in the polymerization of ethylene is benzoyl peroxide [(C6H5COO)2]: [(C6H5COO)2] 2C6H5COO ? This is a first-order reaction. The half-life of benzoyl peroxide at 1008C is 19.8 min. (a) Calculate the rate constant (in min21 ) of the reaction. (b) If the halflife of benzoyl peroxide is 7.30 h, or 438 min, at 708C, what is the activation energy (in kJ/mol) for the decomposition of benzoyl peroxide? (c) Write the rate laws for the elementary steps in the above polymerization process, and identify the reactant, product, and intermediates. (d) What condition would favor the growth of long, high-molar-mass polyethylenes?