Addition Polymers Branching

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Questions and Answers

What is the mechanism by which a growing chain reacts with a molecule XY, resulting in termination of the chain and initiation of a new chain?

Head-to-tail addition

Chain combination

Chain transfer (correct)

Disproportionation

What is the result of head-to-tail addition in radical polymerization?

Initiation of a new chain

Stabilization of the growing end of the chain (correct)

Formation of a stable radical

Termination of the growing chain

What is the role of the substituent attached to the other sp2 carbon in radical polymerization?

To terminate the growing chain

To initiate a new chain

To undergo homolytic cleavage

To stabilize the radical formed by addition (correct)

What is the result of head-to-head addition and some tail-to-tail addition in radical polymerization?

Decreased stabilization of the growing end of the chain Signup and view all the answers

What is required for a compound to act as an initiator in radical polymerization?

A weak bond that undergoes homolytic cleavage Signup and view all the answers

What is the mechanism by which two radicals combine to form a single, non-radical species?

Chain combination Signup and view all the answers

What is the result of disproportionation in radical polymerization?

Formation of a non-radical species Signup and view all the answers

What is the condition required for a molecule XY to undergo chain transfer in radical polymerization?

The molecule XY must have a bond that can readily be cleaved homolytically Signup and view all the answers

What is the role of the initiator in anionic polymerization?

It is a nucleophile that reacts with the monomer to form an anion propagating site Signup and view all the answers

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

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

What is the characteristic of a good monomer in cationic polymerization?

It has substituents that stabilize the positive charge at the propagating site Signup and view all the answers

What is the role of hyperconjugation in cationic polymerization?

It stabilizes the positive charge at the propagating site Signup and view all the answers

What type of carbocation is formed initially in cationic polymerization?

Secondary carbocation Signup and view all the answers

What is the requirement for the alkene to undergo anionic polymerization?

It must have an electron-withdrawing substituent Signup and view all the answers

What is the characteristic of the initiator in anionic polymerization?

It is a very good nucleophile Signup and view all the answers

What is the result of the rearrangement of the propagating site in cationic polymerization?

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

What occurs when the propagating site removes a hydrogen atom from a chain?

A branch can grow chain at that point Signup and view all the answers

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

Unbranched chains can pack together more closely Signup and view all the answers

What type of initiator is often used in cationic polymerization?

A Lewis acid, such as BF3 or AlCl3 Signup and view all the answers

What is the rule that governs electrophilic addition reactions in cationic polymerization?

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

What is the difference between linear polyethylene and branched polyethylene?

Linear polyethylene is harder, whereas branched polyethylene is more flexible Signup and view all the answers

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

Short branches are formed Signup and view all the answers

What is the advantage of using Lewis acid initiators in cationic polymerization?

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

What occurs when the propagating site removes a hydrogen atom from a different polymer chain?

A branch can grow chain at that point Signup and view all the answers

Study Notes

Branching of the Polymer Chain

Branching occurs when the propagating site removes a hydrogen atom from a chain, allowing a new branch to grow.
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 it from a carbon near the middle of a chain leads to long branches.
Short branches are more likely to be formed than long ones.

Physical Properties of Polymers

Branching affects the physical properties of the polymer.
Unbranched chains can pack together more closely than branched chains, making them harder and less flexible.
Linear polyethylene (high-density polyethylene) is a hard plastic used for artificial hip joints, while branched polyethylene (low-density polyethylene) is more flexible and used for trash bags and dry-cleaning bags.

Cationic Polymerization

In cationic polymerization, the initiator is an electrophile that adds to the monomer, causing it to become a carbocation.
Lewis acids, such as BF3 or AlCl3, are commonly used as initiators.
The initiator cannot be an acid such as HCl.
The reaction follows the rule that governs electrophilic addition reactions.

Radical Polymerization

Termination steps may occur by chain transfer, where the growing chain reacts with a molecule XY to terminate the chain.
Molecule XY can be a solvent, a radical initiator, or any molecule with a bond that can readily be cleaved homolytically.
Chain-growth polymerization of substituted ethylenes can occur via three different ways.
Head-to-tail addition is favored due to less steric hindrance at the unsubstituted sp2 carbon of the alkene and stabilization of radicals formed by addition to the unsubstituted sp2 carbon.
When Z is a small substituent, head-to-head addition and some tail-to-tail addition occurs.

Initiators for Radical Polymerization

The initiator for radical polymerization can be any compound with a weak bond that undergoes homolytic cleavage by heat or ultraviolet light.

Cationic Polymerization Mechanism

Chain-initiating step: the initiator reacts with the monomer to form a carbocation.
Chain-propagating steps: the carbocation reacts with more monomers to form a polymer chain.
Termination: the carbocation is neutralized, stopping the polymerization reaction.

Rearrangement of Carbocation Intermediates

Carbocation intermediates formed during cationic polymerization can undergo rearrangement by either a 1,2-hydride shift or a 1,2-methyl shift if rearrangement leads to a more stable carbocation.
The unrearranged propagating site is a secondary carbocation, whereas the rearranged propagating site, obtained by a 1,2-hydride shift, is a more stable tertiary carbocation.

Anionic Polymerization

The initiator is a nucleophile that reacts with the monomer to form an anion propagating site.
Nucleophilic attack on an alkene does not occur readily because alkenes are electron-rich.
Therefore, the initiator must be a very good nucleophile, such as sodium amide or butyllithium, and the alkene must contain electron-withdrawing substituents to decrease the electron density of the double bond.

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Description

This quiz covers the concept of branching of the polymer chain in addition polymers, including the removal of hydrogen atoms from chains and the growth of new branches.