Addition Polymers Branching

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Questions and Answers

What is the site at the end of the last unit added to the chain called?

Disproportionation site

Termination site

Chain combination site

Propagation site (correct)

What is the outcome of chain combination in termination steps?

A new chain is initiated and the growing chain is terminated

Two chains combine at their propagating sites (correct)

One chain is oxidized to an alkene and the other reduced to an alkane

The growing chain reacts with a molecule XY to form a new chain

What is the result of disproportionation in termination steps?

One chain is oxidized to an alkene and the other reduced to an alkane (correct)

One chain is terminated and the other chain is initiated

Two chains combine at their propagating sites

The growing chain reacts with a molecule XY to form a new chain

What is the role of X· in chain transfer?

To initiate a new chain Signup and view all the answers

What happens to the propagating sites during the repetition of propagation steps?

They are destroyed Signup and view all the answers

What is the primary mechanism by which branching occurs in polymer chains?

The propagating site removes a hydrogen atom from a chain Signup and view all the answers

What is the characteristic of linear polyethylene that makes it a hard plastic?

It can pack together more closely than branched chains Signup and view all the answers

What is the role of the initiator in cationic polymerization?

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

What type of polyethylene is used for the production of trash bags and drycleaning bags?

Low-density polyethylene Signup and view all the answers

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

It affects the ability of the polymer chains to pack together Signup and view all the answers

What is the characteristic of short branches in polymer chains?

They are more likely to form than long branches Signup and view all the answers

What is the reason why elastomers return to their original shape after stretching?

The chains return to their random shapes due to weak van der Waals forces Signup and view all the answers

What type of polymer is formed from ethylene or substituted ethylenes?

Vinyl polymer Signup and view all the answers

How many distinct steps are involved in each mechanism of addition polymerization?

3 Signup and view all the answers

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

To react with the monomer to form a radical Signup and view all the answers

What is the result of the first propagation step in radical polymerization?

The conversion of a monomer into a radical Signup and view all the answers

What is a characteristic of elastomers, such as rubber and spandex?

They are able to stretch and return to their original shape Signup and view all the answers

Study Notes

Radical Polymerization Mechanism

The unpaired electron is at the end of the last unit added to the chain, known as the propagating site.
The propagating site can react with multiple monomers, repeating the propagation steps hundreds or thousands of times.
The propagation steps are terminated in a termination step, which can occur through one of three pathways.

Termination Steps

Chain combination: two chains combine at their propagating sites.
Disproportionation: one chain is oxidized to an alkene, and the other is reduced to an alkane.
Chain transfer: the growing chain reacts with a molecule XY, terminating the chain and initiating a new one.

Branching of the Polymer Chain

The propagating site can remove a hydrogen atom from a chain, leading to branch growth.
Branching can occur through the removal of a hydrogen atom from a different polymer chain or from the same polymer chain.
Short branches are more likely to be formed than long ones, depending on the location of the removed hydrogen atom.
Branching affects the physical properties of the polymer, with unbranched chains packing together more closely than branched chains.

Physical Properties of Polymers

Linear polyethylene (high-density polyethylene) is a hard plastic, used for producing artificial hip joints.
Branched polyethylene (low-density polyethylene) is a more flexible polymer, used for trash bags and drycleaning bags.

Cationic Polymerization

The initiator is an electrophile that adds to the monomer, causing it to become a carbocation.
The initiator cannot be an acid such as HCl.
Elastomers, such as rubber and spandex, are formed through cationic polymerization.

Chain-Growth Polymerization

The monomers commonly used are ethylene (ethene) and substituted ethylenes (CH2=CHR).
Polymers formed from ethylene or substituted ethylenes are called vinyl polymers.
Chain-growth polymerization proceeds by one of three mechanisms: radical, cationic, and anionic polymerization.
Each mechanism has three distinct steps: initiation, propagation, and termination.

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Description

This quiz covers the concept of branching in addition polymers, including the removal of hydrogen atoms from polymer chains and the growth of branches. It also explores the differences between short and long branches. Test your understanding of polymer chemistry!