Addition Polymers: Cationic Polymerization

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Questions and Answers

What type of polymerization is common for alkene monomers?

Cationic polymerization

Condensation polymerization

Anionic polymerization (correct)

Radical polymerization

Which mechanism involves a carbocation intermediate?

Anionic polymerization

Radical polymerization

Cationic polymerization (correct)

Ring-opening polymerization

Why does methyl α-cyanoacrylate undergo anionic polymerization?

Because it is a naturally occurring compound

Because it has a high molecular weight

Because it is a ring-shaped monomer

Because it has two electron-withdrawing groups (correct)

What is the role of surface-absorbed water in anionic polymerization?

It initiates the polymerization reaction Signup and view all the answers

Which type of polymerization is characterized by a living polymer?

Anionic polymerization Signup and view all the answers

What is the initiating step in anionic polymerization?

Initiating step Signup and view all the answers

What type of compounds undergo ring-opening polymerization?

Epoxides Signup and view all the answers

What is the function of a nucleophile in anionic polymerization?

It initiates the polymerization reaction Signup and view all the answers

What is the primary role of substituents in monomers during cationic polymerization?

To stabilize the positive charge at the propagating site by hyperconjugation or resonance Signup and view all the answers

What is the nature of the carbocation intermediate formed during cationic polymerization?

Secondary carbocation Signup and view all the answers

What is the primary reason for the requirement of a strong nucleophile as an initiator in anionic polymerization?

To facilitate nucleophilic attack on the alkene Signup and view all the answers

What is the result of a 1,2-hydride shift in the carbocation intermediate during cationic polymerization?

Formation of a more stable tertiary carbocation Signup and view all the answers

What is the mechanism of stabilization of the positive charge at the propagating site in cationic polymerization?

By hyperconjugation and donating electrons by resonance Signup and view all the answers

What is the primary characteristic of the initiator in anionic polymerization?

It is a strong nucleophile Signup and view all the answers

What is the role of electron-withdrawing substituents in the alkene during anionic polymerization?

To decrease the electron density of the double bond Signup and view all the answers

What is the type of polymerization that involves the formation of a carbocation intermediate?

Cationic polymerization Signup and view all the answers

What is the effect of branching on the physical properties of a polymer?

It increases the flexibility of the polymer Signup and view all the answers

What is the mechanism of cationic polymerization?

The initiator is an electrophile that adds to the monomer Signup and view all the answers

What is the advantage of using a Lewis acid as an initiator in cationic polymerization?

It does not have an accompanying nucleophile that could act as a chain terminator Signup and view all the answers

What is the difference between high-density polyethylene and low-density polyethylene?

High-density polyethylene is used for artificial hip joints while low-density polyethylene is used for trash bags Signup and view all the answers

What is the role of the initiator in polymerization?

It adds to the monomer to form a carbocation Signup and view all the answers

What is the rule that governs electrophilic addition reactions?

The electrophile adds to the sp2 carbon bonded to the most hydrogens Signup and view all the answers

What is the effect of removing a hydrogen atom from a carbon near the end of a chain?

It leads to the formation of short branches Signup and view all the answers

What is the difference between the propagation site and the initiator?

The propagating site is the site where the polymer chain grows Signup and view all the answers

Study Notes

Cationic Polymerization

The mechanism involves a chain-initiating step and chain-propagating steps
Carbocation intermediates formed during cationic polymerization can undergo rearrangement by 1,2-hydride shift or 1,2-methyl shift to form a more stable carbocation
The unrearranged propagating site is a secondary carbocation, while the rearranged propagating site is a more stable tertiary carbocation
The best monomers have substituents that stabilize the positive charge at the propagating site through hyperconjugation or resonance
Examples of alkenes that undergo cationic polymerization are given

Anionic Polymerization (Living Polymers)

The initiator is a nucleophile that reacts with the monomer to form an anion propagating site
Nucleophilic attack on an alkene requires a very good nucleophile, such as sodium amide or butyllithium, and an alkene with electron-withdrawing substituents
Examples of radical initiators are given
Living polymers are most common in anionic polymerization because the chains cannot be terminated by proton loss or chain combination/disproportionation

Branching of the Polymer Chain

Branching occurs when the propagating site removes a hydrogen atom from a chain
The propagating site can remove a hydrogen atom from a different polymer chain or from the same polymer chain
Removing a hydrogen atom from a carbon near the end of a chain leads to short branches, while removing a hydrogen atom from a carbon near the middle of a chain leads to long branches
Short branches are more likely to be formed than long ones
Branching affects the physical properties of the polymer, with unbranched chains packing more closely together than branched chains

Properties of Polymers

Linear polyethylene (high-density polyethylene) is a hard plastic, used for production of artificial hip joints
Branched polyethylene (low-density polyethylene) is more flexible and used for trash bags and dry-cleaning bags

Ring-Opening Polymerizations

Some compounds can polymerize well, such as epoxides, which undergo chain-growth polymerization
If the initiator is a nucleophile, polymerization occurs by an anionic mechanism
The nucleophile attacks the less sterically hindered carbon of the epoxide

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Description

This quiz covers the mechanism of cationic polymerization, including the chain-initiating step, chain-propagating steps, and termination. It also discusses the carbocation intermediates formed during cationic polymerization.