24 Questions
What is the purpose of a radical initiator in radical polymerization?
To break homolytically into radicals and start the growth of the polymer chain
What is the result of chain combination in termination steps?
Two chains combine at their propagating sites
What is the name of the site where the unpaired electron is located in the propagation steps?
Propagation site
What happens to the concentration of monomer over time?
It decreases gradually until it is depleted
What is the result of disproportionation in termination steps?
One chain is oxidized to an alkene and the other reduced to an alkane
What type of polymerization is associated with step growth?
Condensation polymerization
What is the maximum number of subunits that can be added to the chain in the propagation steps?
Thousands
What is the term for regions of a polymer where the chains are highly ordered with respect to one another?
Crystallites
What is the type of reaction that occurs when a radical reacts with a monomer?
Radical addition reaction
What type of polymer has a continuous chain of repeating units?
Linear polymer
What is the process by which the chain grows in the propagation steps?
The monomer radical reacts with another monomer, adding a new subunit to the chain
What is the result of substituents or branches on polymer chains?
Decreased density of the polymer
What is the role of the monomer radical in the propagation steps?
To react with another monomer, adding a new subunit to the chain
What is the term for the process by which oligomers react to form longer chains?
Step growth
What type of forces hold individual polymer chains together?
Van der Waals forces
What is the result of a polymer being highly crystalline?
It becomes more resistant to heat and more dense
What is the result of the propagating site removing a hydrogen atom from a different polymer chain?
Chain branching
What type of initiators are used in cationic polymerization?
Electrophiles
What is the difference between short and long branches in a polymer chain?
Short branches are formed near the end of the chain, while long branches are formed near the middle
What is the effect of branching on the physical properties of a polymer?
Branching makes the polymer more flexible and soft
What is the advantage of using Lewis acids as initiators in cationic polymerization?
They do not have accompanying nucleophile that could act as a chain terminator
What is the result of the propagating site removing a hydrogen atom from the same polymer chain?
Chain branching
Why are linear polyethylene chains able to pack together more closely than branched polyethylene chains?
Because linear chains have a more regular structure
What is the type of reaction that governs cationic polymerization?
Electrophilic addition reaction
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 addition polymers, their mechanisms, and uses. It explains radical, cationic, and anionic polymerization, including initiation, propagation, and termination steps.
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