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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 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 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 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?
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?
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?
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?
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?
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?
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?
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?
What is the characteristic of a good monomer in cationic polymerization?
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What is the role of hyperconjugation in cationic polymerization?
What is the role of hyperconjugation in cationic polymerization?
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What type of carbocation is formed initially in cationic polymerization?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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.
