Addition Polymers and Branching

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Questions and Answers

What is the primary role of a radical initiator in radical polymerization?

To reduce the chain to an alkane

To terminate the polymer chain

To oxidize the chain to an alkene

To generate the radical that starts the growth of the polymer chain (correct)

What is the result of chain combination in radical polymerization?

Propagation of the polymer chain

Termination of the polymer chain

Formation of an alkene and an alkane

Formation of a single polymer chain (correct)

What is the propagating site in radical polymerization?

The site where the unpaired electron is located (correct)

The site where the polymer chain is terminated

The site where the radical initiator is added

The site where the monomer reacts with another monomer

What is the end result of a termination step in radical polymerization?

The termination of the polymer chain Signup and view all the answers

What is the term for the reaction where a chain transfers a radical to a monomer?

Chain transfer reaction Signup and view all the answers

What is the term for the process of adding monomers to a growing polymer chain in a sequential manner?

Head-to-tail addition Signup and view all the answers

What is the term for the process of forming branches in a polymer chain?

Chain branching Signup and view all the answers

What is the result of disproportionation in radical polymerization?

Formation of an alkene and an alkane Signup and view all the answers

What is the primary reason for head-to-tail addition being favored in radical polymerization?

Stabilization of the radical by electron delocalization Signup and view all the answers

What type of bond is typically cleaved in a chain transfer reaction?

Homolytic bond Signup and view all the answers

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

Chain branching Signup and view all the answers

What is the primary function of a radical initiator in radical polymerization?

To initiate the chain reaction Signup and view all the answers

What is the effect of a substituent like phenyl on the radical formed in radical polymerization?

It stabilizes the radical by electron delocalization Signup and view all the answers

What is the result of chain transfer reactions in radical polymerization?

Initiation of a new chain Signup and view all the answers

What type of molecule can undergo chain transfer reactions in radical polymerization?

All of the above Signup and view all the answers

What is the result of termination steps in radical polymerization?

Chain termination Signup and view all the answers

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

Formation of a long branch 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 difference between high-density polyethylene and low-density polyethylene?

High-density polyethylene is unbranched, while low-density polyethylene is branched Signup and view all the answers

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

Branching decreases the hardness of the polymer Signup and view all the answers

What is the role of the propagating site in branching?

It removes a hydrogen atom from a chain, leading to branching Signup and view all the answers

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

They do not have accompanying nucleophiles that could act as chain terminators Signup and view all the answers

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

Formation of a short branch Signup and view all the answers

What is the difference between the mechanism of cationic polymerization and radical polymerization?

Cationic polymerization involves an electrophile, while radical polymerization involves a nucleophile 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 different polymer chain or the same polymer chain.
Removing a hydrogen atom from a carbon near the end of a chain leads to short branches, whereas 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.

Physical Properties of Polymers

Branching affects the physical properties of the polymer.
Unbranched chains (linear polyethylene) pack together more closely, making them hard and used for producing artificial hip joints.
Branched chains (branched polyethylene) are more flexible and used for trash bags and dry-cleaning bags.

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 often used as initiators.
The initiator does not have an accompanying nucleophile that could act as a chain terminator.

Radical Polymerization

Mechanism

Termination steps may occur by one of the following pathways:
- Chain transfer: the growing chain reacts with a molecule XY, allowing X· to terminate the chain and leaving Y· to initiate a new chain.
- Chain combination: two chains combine at their propagating sites.
- Disproportionation: one chain is oxidized to an alkene and the other reduced to an alkane.

Chain-Growth Polymerization

Head-to-tail addition is favored due to two factors:
- Less steric hindrance at the unsubstituted sp2 carbon of the alkene.
- Radicals formed by addition to the unsubstituted sp2 carbon can be stabilized by the substituent attached to the other sp2 carbon.

Initiators for Radical Polymerization

Any compound with a weak bond that undergoes homolytic cleavage by heat or ultraviolet light can be an initiator.

Important Chain-Growth Polymers and Their Uses

Not specified in the text.

Addition or Chain-Growth Polymerization

Proceeds by one of three mechanisms: radical polymerization, cationic polymerization, and anionic polymerization.
Each mechanism has three distinct steps: initiation, propagation, and termination.

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Description

This quiz covers the basic concepts of addition polymers, including branching of the polymer chain and the role of propagating sites in polymerization.