24 Questions
What type of polymerization is common for alkene monomers?
Anionic polymerization
Which mechanism involves a carbocation intermediate?
Cationic polymerization
Why does methyl α-cyanoacrylate undergo anionic polymerization?
Because it has two electron-withdrawing groups
What is the role of surface-absorbed water in anionic polymerization?
It initiates the polymerization reaction
Which type of polymerization is characterized by a living polymer?
Anionic polymerization
What is the initiating step in anionic polymerization?
Initiating step
What type of compounds undergo ring-opening polymerization?
Epoxides
What is the function of a nucleophile in anionic polymerization?
It initiates the polymerization reaction
What is the primary role of substituents in monomers during cationic polymerization?
To stabilize the positive charge at the propagating site by hyperconjugation or resonance
What is the nature of the carbocation intermediate formed during cationic polymerization?
Secondary carbocation
What is the primary reason for the requirement of a strong nucleophile as an initiator in anionic polymerization?
To facilitate nucleophilic attack on the alkene
What is the result of a 1,2-hydride shift in the carbocation intermediate during cationic polymerization?
Formation of a more stable tertiary carbocation
What is the mechanism of stabilization of the positive charge at the propagating site in cationic polymerization?
By hyperconjugation and donating electrons by resonance
What is the primary characteristic of the initiator in anionic polymerization?
It is a strong nucleophile
What is the role of electron-withdrawing substituents in the alkene during anionic polymerization?
To decrease the electron density of the double bond
What is the type of polymerization that involves the formation of a carbocation intermediate?
Cationic polymerization
What is the effect of branching on the physical properties of a polymer?
It increases the flexibility of the polymer
What is the mechanism of cationic polymerization?
The initiator is an electrophile that adds to the monomer
What is the advantage of using a Lewis acid as an initiator in cationic polymerization?
It does not have an accompanying nucleophile that could act as a chain terminator
What is the difference between high-density polyethylene and low-density polyethylene?
High-density polyethylene is used for artificial hip joints while low-density polyethylene is used for trash bags
What is the role of the initiator in polymerization?
It adds to the monomer to form a carbocation
What is the rule that governs electrophilic addition reactions?
The electrophile adds to the sp2 carbon bonded to the most hydrogens
What is the effect of removing a hydrogen atom from a carbon near the end of a chain?
It leads to the formation of short branches
What is the difference between the propagation site and the initiator?
The propagating site is the site where the polymer chain grows
Study Notes
Cationic Polymerization
- The mechanism involves a chain-initiating step and chain-propagating steps
- Carbocation intermediates formed during cationic polymerization can undergo rearrangement by 1,2-hydride shift or 1,2-methyl shift to form a more stable carbocation
- The unrearranged propagating site is a secondary carbocation, while the rearranged propagating site is a more stable tertiary carbocation
- The best monomers have substituents that stabilize the positive charge at the propagating site through hyperconjugation or resonance
- Examples of alkenes that undergo cationic polymerization are given
Anionic Polymerization (Living Polymers)
- The initiator is a nucleophile that reacts with the monomer to form an anion propagating site
- Nucleophilic attack on an alkene requires a very good nucleophile, such as sodium amide or butyllithium, and an alkene with electron-withdrawing substituents
- Examples of radical initiators are given
- Living polymers are most common in anionic polymerization because the chains cannot be terminated by proton loss or chain combination/disproportionation
Branching of the Polymer Chain
- Branching occurs when the propagating site removes a hydrogen atom from a chain
- The propagating site can remove a hydrogen atom from a different polymer chain or from the same polymer chain
- Removing a hydrogen atom from a carbon near the end of a chain leads to short branches, while 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
- Branching affects the physical properties of the polymer, with unbranched chains packing more closely together than branched chains
Properties of Polymers
- Linear polyethylene (high-density polyethylene) is a hard plastic, used for production of artificial hip joints
- Branched polyethylene (low-density polyethylene) is more flexible and used for trash bags and dry-cleaning bags
Ring-Opening Polymerizations
- Some compounds can polymerize well, such as epoxides, which undergo chain-growth polymerization
- If the initiator is a nucleophile, polymerization occurs by an anionic mechanism
- The nucleophile attacks the less sterically hindered carbon of the epoxide
This quiz covers the mechanism of cationic polymerization, including the chain-initiating step, chain-propagating steps, and termination. It also discusses the carbocation intermediates formed during cationic polymerization.
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