24 Questions
What type of initiator is often used in cationic polymerization?
A Lewis acid such as BF3 or AlCl3
What is the role of the electrophile in cationic polymerization?
It adds to the sp2 carbon bonded to the most hydrogens
What is the consequence of branching in polymer chains?
It increases the flexibility of the polymer
What is the difference between linear and branched polyethylene?
Linear polyethylene is harder, while branched polyethylene is more flexible
What is the mechanism of branching in polymer chains?
The propagating site removes a hydrogen atom from a chain
What is the advantage of using Lewis acids as initiators in cationic polymerization?
They do not have an accompanying nucleophile that could act as a chain terminator
What is the intermediate formed in cationic polymerization?
A carbocation
What is the difference between high-density and low-density polyethylene?
High-density polyethylene is harder, while low-density polyethylene is more flexible
What is the primary function of the substituents in the best monomers for cationic polymerization?
to stabilize the positive charge at the propagating site
What is the result of a 1,2-hydride shift in the carbocation intermediate during cationic polymerization?
a more stable tertiary carbocation
What is the characteristic of the initiator in anionic polymerization?
a very good nucleophile
What is the reason for the requirement of a very good nucleophile as an initiator in anionic polymerization?
because alkenes are electron-rich
What is the type of polymerization that involves the formation of an anion propagating site?
anionic polymerization
What is the primary role of the 1,2-methyl shift in the carbocation intermediate during cationic polymerization?
to lead to a more stable carbocation
What is the characteristic of the alkene monomer in anionic polymerization?
containing electron-withdrawing substituents
What is the primary function of the chain-initiating step in cationic polymerization?
to form a carbocation intermediate
What distinguishes anionic polymerization from cationic polymerization?
Chain termination by proton loss from the polymer
What is the role of surface-absorbed water in the anionic polymerization of Super Glue?
It acts as a nucleophile to initiate the polymerization reaction
Why are living polymers most common in anionic polymerization?
Because the chains cannot be terminated by proton loss from the polymer or by chain combination or disproportionation
What is the characteristic of the monomer in Super Glue that allows it to undergo anionic polymerization?
It has two electron-withdrawing groups
What is the mechanism of polymerization of epoxides?
Anionic mechanism
What is the role of the nucleophile in the ring-opening polymerization of epoxides?
It attacks the less sterically hindered carbon of the epoxide
Why does anionic polymerization require a moderately good nucleophile?
Because the monomer has two electron-withdrawing groups
What is the result of the anionic polymerization reaction of Super Glue on the surface of the skin?
Two fingers can become firmly glued together
Study Notes
Branching of the Polymer Chain
- Branching occurs when a propagating site removes a hydrogen atom from a chain, allowing a branch to grow at that point.
- The propagating site can remove 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 Branched Polymers
- Branching affects the physical properties of the polymer, as unbranched chains can pack together more closely than branched chains can.
- 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 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; instead, a Lewis acid like BF3 or AlCl3 is often used.
- The advantage of these initiators is that they do not have an accompanying nucleophile that could act as a chain terminator.
Mechanism of Cationic Polymerization
- Chain-initiating step: the electrophile adds to the sp2 carbon bonded to the most hydrogens.
- Chain-propagating steps: the carbocation intermediate reacts with the monomer to form a new carbocation.
Termination in Cationic Polymerization
- The reaction terminates when the carbocation intermediate is neutralized.
Rearrangement of Carbocation Intermediates
- The carbocation intermediates can undergo rearrangement by either a 1,2-hydride shift or a 1,2-methyl shift to form a more stable carbocation.
Monomers in Cationic Polymerization
- The best monomers in cationic polymerization have substituents that stabilize the positive charge at the propagating site, either by hyperconjugation or by donating electrons by resonance.
Anionic Polymerization
- The initiator is a nucleophile that reacts with the monomer to form an anion propagating site.
- The initiator must be a very good nucleophile, and the alkene must contain electron-withdrawing substituents to decrease the electron density of the double bond.
Mechanism of Anionic Polymerization
- Initiating step: the nucleophile reacts with the monomer to form an anion propagating site.
- Propagation steps: the anion propagating site reacts with the monomer to form a new anion propagating site.
Living Polymers
- Living polymers are common in anionic polymerization because the chains cannot be terminated by proton loss, chain combination, or disproportionation.
- Examples of alkenes that undergo anionic polymerization are methyl α-cyanoacrylate.
Ring-Opening Polymerizations
- Some compounds, such as epoxides, can polymerize through a ring-opening mechanism.
- The initiator is a nucleophile that attacks the less sterically hindered carbon of the epoxide.
This quiz covers the basics of addition polymers, including the branching of the polymer chain and the role of propagating sites in removing hydrogen atoms.
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