Ch.8 - Reactions of Alkenes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch.8 - Reactions of Alkenes

Problem 74

Chapter 8, Problem 74

The cationic polymerization of isobutylene (2-methylpropene) is shown in Section 8-16A. Isobutylene is often polymerized under free-radical conditions. Propose a mechanism for the free-radical polymerization of isobutylene.

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Identify the three main stages of free-radical polymerization: initiation, propagation, and termination. These stages will guide the mechanism for the polymerization of isobutylene.

In the initiation step, a free radical is generated. This is typically achieved by using an initiator such as benzoyl peroxide or AIBN (azobisisobutyronitrile). The initiator decomposes under heat or UV light to form free radicals. Write the decomposition reaction of the initiator to show the formation of free radicals.

In the propagation step, the free radical generated in the initiation step reacts with the double bond of isobutylene (2-methylpropene). This reaction forms a new carbon-centered radical at the terminal carbon of the isobutylene molecule. Represent this step using a curved arrow mechanism to show the movement of electrons from the radical to the double bond and the formation of the new radical.

Continue the propagation step by showing how the newly formed radical reacts with another molecule of isobutylene. This step repeats, leading to the growth of the polymer chain. Use a general structure to represent the growing polymer chain and show the addition of another isobutylene unit.

In the termination step, two growing polymer radicals combine to form a stable, non-radical product. This can occur through coupling (where two radicals directly combine) or disproportionation (where one radical abstracts a hydrogen atom from another radical). Write the possible termination reactions and explain how they lead to the end of the polymerization process.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Free-Radical Polymerization

Free-radical polymerization is a method of polymer synthesis that involves the formation of free radicals, which are highly reactive species with unpaired electrons. This process typically occurs in three main steps: initiation, propagation, and termination. In the initiation step, a radical initiator generates free radicals that react with monomers, leading to the growth of polymer chains through repeated addition of monomer units.

Cationic Polymerization

Cationic polymerization is a type of chain-growth polymerization that involves the formation of a positively charged cation as an active center. This mechanism is characterized by the addition of monomers that can stabilize a positive charge, allowing for the growth of the polymer chain. While the question focuses on free-radical conditions, understanding cationic polymerization provides insight into alternative mechanisms and the behavior of isobutylene under different conditions.

Isobutylene Structure and Reactivity

Isobutylene, or 2-methylpropene, is an alkene with a branched structure that influences its reactivity in polymerization processes. The presence of the double bond makes it susceptible to addition reactions, which are fundamental in polymerization. Understanding the structure of isobutylene helps predict how it will behave under free-radical conditions, including the formation of stable radical intermediates during the polymerization process.

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