24 Questions
What is the role of a radical initiator in radical polymerization?
To generate the radical that starts the growth of the polymer chain
What is the result of chain combination in radical polymerization?
Formation of a single chain with a greater molecular weight
What is the propagating site in radical polymerization?
The end of the last unit added to the chain
What is the result of disproportionation in radical polymerization?
One chain is oxidized to an alkene and the other reduced to an alkane
What is the mechanism of radical polymerization that involves the reaction of a monomer radical with another monomer?
Propagation step
What is the type of reaction that occurs when a growing polymer chain reacts with a monomer to form a new subunit?
Head-to-tail addition
What is the result of chain branching in radical polymerization?
Formation of a branch in the polymer chain
What is the type of reaction that occurs when a growing polymer chain reacts with another polymer chain?
Chain transfer reaction
What happens to the concentration of monomer during a polymerization reaction?
It decreases gradually until it is depleted
What type of polymerization is generally associated with step growth?
Condensation polymerization
What is the primary difference between linear and branched polymers?
The presence of short branches connected to the main chain
What is the term for the regions of a polymer where the chains are highly ordered with respect to one another?
Crystallites
What is the primary reason for the decrease in density of a polymer with substituents or branches?
The inability of the chains to pack closely together
What is the primary force responsible for holding individual polymer chains together?
Van der Waals forces
What is the result of a polymer having a more crystalline structure?
It becomes more dense and more resistant to heat
What is the term for the process by which oligomers react together to form longer chains?
Step growth
What occurs when the propagating site removes a hydrogen atom from a chain?
A branch can grow at that point.
What type of branches are more likely to be formed in a polymer chain?
Short branches
What is the result of unbranched chains packing together more closely than branched chains?
The polymer becomes harder.
What is the role of the initiator in cationic polymerization?
To add to the monomer, causing it to become a carbocation.
What is the advantage of using Lewis acids, such as BF3 or AlCl3, as initiators in cationic polymerization?
They do not have accompanying nucleophiles that could act as chain terminators.
What is the result of branching in a polymer chain?
The polymer becomes more flexible.
What is the difference between high-density polyethylene and low-density polyethylene?
High-density polyethylene is more rigid, while low-density polyethylene is more flexible.
What is the rule that governs electrophilic addition reactions in cationic polymerization?
The electrophile adds to the sp2 carbon bonded to the most hydrogens.
Study Notes
Addition Polymers
- Addition or chain-growth polymerization proceeds by three mechanisms: radical polymerization, cationic polymerization, and anionic polymerization.
- Each mechanism has three distinct steps: initiation, propagation, and termination.
Radical Polymerization
- Initiation step: a radical initiator is added to the monomer to generate a radical that starts the growth of the polymer chain.
- Propagation steps:
- A monomer radical reacts with another monomer, converting it into a radical.
- The radical reacts with another monomer, adding a new subunit to the chain.
- The unpaired electron is at the end of the last unit added to the chain (propagating site).
- Termination steps:
- Chain combination: two chains combine at their propagating sites.
- Disproportionation: one chain is oxidized to an alkene and the other reduced to an alkane.
Branching of the Polymer Chain
- Branching occurs when the propagating site removes a hydrogen atom from a chain.
- Branching can occur at different points on the polymer chain, leading to short or long branches.
- Short branches are more likely to be formed than long ones.
- Branching affects the physical properties of the polymer:
- Unbranched chains can pack together more closely than branched chains.
- Linear polyethylene (high-density polyethylene) is a hard plastic, used for artificial hip joints.
- Branched polyethylene (low-density polyethylene) is more flexible, 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.
- The initiator cannot be an acid such as HCl, but rather a Lewis acid, such as BF3 or AlCl3.
- The advantage of these initiators is that they do not have accompanying nucleophiles that could act as chain terminators.
Classification of Polymers
- Polymer growth:
- Chain growth: associated with addition polymerization.
- Step growth: associated with condensation polymerization, where chains have at least two growth points.
- Polymer structure:
- Linear polymers: have a continuous chain of repeating units.
- Branched polymers: have short branches connected to the main chain.
- Cross-linked polymers: have covalently bonded linking units between chains.
- Physical properties:
- Polymers acquire their physical properties as a result of the arrangement of their individual chains.
- The more crystalline (ordered) the polymer is, the denser, harder, and more resistant it is to heat.
- Substituents or branches on the polymer chains can prevent them from packing closely together, reducing the density of the polymer.
This quiz covers the basics of addition polymers, including radical, cationic, and anionic polymerization mechanisms and their steps.
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