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What is the mechanism by which a growing chain reacts with a molecule XY, resulting in termination of the chain and initiation of a new chain?
What is the result of head-to-tail addition in radical polymerization?
What is the role of the substituent attached to the other sp2 carbon in radical polymerization?
What is the result of head-to-head addition and some tail-to-tail addition in radical polymerization?
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What is required for a compound to act as an initiator in radical polymerization?
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What is the mechanism by which two radicals combine to form a single, non-radical species?
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What is the result of disproportionation in radical polymerization?
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What is the condition required for a molecule XY to undergo chain transfer in radical polymerization?
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What is the role of the initiator in anionic polymerization?
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What is the result of a 1,2-hydride shift in cationic polymerization?
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What is the characteristic of a good monomer in cationic polymerization?
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What is the role of hyperconjugation in cationic polymerization?
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What type of carbocation is formed initially in cationic polymerization?
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What is the requirement for the alkene to undergo anionic polymerization?
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What is the characteristic of the initiator in anionic polymerization?
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What is the result of the rearrangement of the propagating site in cationic polymerization?
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What occurs when the propagating site removes a hydrogen atom from a chain?
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What is the effect of branching on the physical properties of the polymer?
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What type of initiator is often used in cationic polymerization?
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What is the rule that governs electrophilic addition reactions in cationic polymerization?
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What is the difference between linear polyethylene and branched polyethylene?
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What is the result of removing a hydrogen atom from a carbon near the end of a chain?
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What is the advantage of using Lewis acid initiators in cationic polymerization?
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What occurs when the propagating site removes a hydrogen atom from a different polymer chain?
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Study Notes
Branching of the Polymer Chain
- Branching occurs when the propagating site removes a hydrogen atom from a chain, allowing a new branch to grow.
- 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 it 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 Polymers
- Branching affects the physical properties of the polymer.
- Unbranched chains can pack together more closely than branched chains, making them harder and less flexible.
- Linear polyethylene (high-density polyethylene) is a hard plastic used for artificial hip joints, while branched polyethylene (low-density polyethylene) is more flexible and 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.
- Lewis acids, such as BF3 or AlCl3, are commonly used as initiators.
- The initiator cannot be an acid such as HCl.
- The reaction follows the rule that governs electrophilic addition reactions.
Radical Polymerization
- Termination steps may occur by chain transfer, where the growing chain reacts with a molecule XY to terminate the chain.
- Molecule XY can be a solvent, a radical initiator, or any molecule with a bond that can readily be cleaved homolytically.
- Chain-growth polymerization of substituted ethylenes can occur via three different ways.
- Head-to-tail addition is favored due to less steric hindrance at the unsubstituted sp2 carbon of the alkene and stabilization of radicals formed by addition to the unsubstituted sp2 carbon.
- When Z is a small substituent, head-to-head addition and some tail-to-tail addition occurs.
Initiators for Radical Polymerization
- The initiator for radical polymerization can be any compound with a weak bond that undergoes homolytic cleavage by heat or ultraviolet light.
Cationic Polymerization Mechanism
- Chain-initiating step: the initiator reacts with the monomer to form a carbocation.
- Chain-propagating steps: the carbocation reacts with more monomers to form a polymer chain.
- Termination: the carbocation is neutralized, stopping the polymerization reaction.
Rearrangement of Carbocation Intermediates
- Carbocation intermediates formed during cationic polymerization can undergo rearrangement by either a 1,2-hydride shift or a 1,2-methyl shift if rearrangement leads to a more stable carbocation.
- The unrearranged propagating site is a secondary carbocation, whereas the rearranged propagating site, obtained by a 1,2-hydride shift, is a more stable tertiary carbocation.
Anionic Polymerization
- The initiator is a nucleophile that reacts with the monomer to form an anion propagating site.
- Nucleophilic attack on an alkene does not occur readily because alkenes are electron-rich.
- Therefore, the initiator must be a very good nucleophile, such as sodium amide or butyllithium, and the alkene must contain electron-withdrawing substituents to decrease the electron density of the double bond.
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Description
This quiz covers the concept of branching of the polymer chain in addition polymers, including the removal of hydrogen atoms from chains and the growth of new branches.
