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© 2014 Pearson Education, Inc.
Synthetic Polymers
Paula Yurkanis BruiceUniversity of California,
Santa Barbara
Chapter 27
© 2014 Pearson Education, Inc.
Polymerization
A polymer is a large molecule made by linking together repeating units of small molecules called monomers.
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A Chain-Growth Polymer(polystyrene)
Chain-growth polymers (also known as addition polymers), are made by chain reactions.
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Step-Growth Polymer(Dacron)
Step-growth polymers (also called condensation polymers), are made by combining two molecules while removing a small molecule.
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© 2014 Pearson Education, Inc.
Chain-Growth Polymers
Chain-growth polymers are formed by one of three mechanisms.
• Radical polymerization
• Cationic polymerization
• Anionic polymerization
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Radical Polymerization
The initiator is a radical.
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Propagation Steps
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Termination Steps
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Chain Transfer
The molecular weight of the polymer can be controlled by a process known as chain transfer.
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Head-to-Tail Linkage
Chain-growth polymerization of monosubstituted ethylenes exhibits a marked preference for head-to-tail addition.
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Poly(vinyl chloride)
Head-to-tail addition forms a polymer with a substituent on every other carbon.
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Stabilization of the Propagating Site
Head-to-tail addition is favored for steric reasons.
Head-to-tail addition is favored by groups that stabilize radicals.
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Alkenes Used in Radical Polymerization
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Radical Initiators
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Branching of the Polymer Chain
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Branched Polymers are More Flexible
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Recycling Symbols
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Cationic Polymerization
The initiator is an electrophile.
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Propagating Steps
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Three Ways to Terminate the Chain
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A Carbocation Monomer Can Undergo Carbocation Rearrangement
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Alkenes Used in Cationic Polymerization
Monomers with electron-donating substituents undergo cationic polymerization.
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Anionic Polymerization
The initiator is a nucleophile.
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Propagating Steps
Nonterminated chains are called “living polymers” and remain active until they are “killed.”
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Alkenes Used in Anionic Polymerization
Monomers with electron-withdrawing substituents undergo anionic polymerization.
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Super Glue(an example of anionic polymerization)
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What Determines the Mechanism?
Alkenes with substituents that can stabilize radicals undergo radical polymerization.
Alkenes with electron-donating substituents that can stabilize cations undergo cationic polymerization.
Alkenes with electron-withdrawing substituents that can stabilize anions undergo anionic polymerization.
Some alkenes undergo polymerization by more than one mechanism.
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Ring-Opening Polymerization (anionic)
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Ring-Opening Polymerization(cationic)
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Stereochemistry of Polymerization
Long, unbranched polymers with either isotactic or the syndiotactic configuration can be prepared using an aluminum–titanium initiator (a Ziegler–Natta catalyst).
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Mechanism for Ziegler–Natta-Catalyzed Polymerization
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Polymerization of Dienes
These rubbers have cis double bonds.
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Neoprene(a synthetic rubber)
Neoprene has trans double bonds.
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Disulfide Bonds in Rubber
The more sulfur used in vulcanization, the more rigid the polymer.
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Four Types of Copolymers
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Step-Growth Polymers
The monomer can have two different functional groups.
The monomer can be two different bifunctional compounds.
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Step-Growth Polymerization Requires High Yields for Long Chains
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Nylon is a Polyamide
Nylon 6 is an example of a step-growth polymer formed by a monomer with two different functional groups.
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Commercial Synthesis of Nylon 6
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Nylon is a Polyamide
Nylon 66 is an example of a step-growth polymer formed by two different bifunctional monomers.
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Aromatic Polyamides are Called Aramides
The incorporation of aromatic rings into polymers improves the physical strength of the polymers.
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Kevlar
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Dacron is a Polyester
Polyesters are step-growth polymers in which the monomer units are joined together by ester
groups.
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Kodel is a Polyester
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Lexan is a Polycarbonate
Polycarbonates have two ester groups bonded to the same carbon.
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An Epoxy Resin is a Cross-Linked Polymer(it is the strongest adhesive known)
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A Urethane
A urethane has an OR and an NHR bonded to the same carbonyl carbon.
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A Polyurethane
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Crystallites are Highly Ordered Regions
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The More Crystalline the Polymer is, the Denser, Harder, and More Resistant it is to Heat
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A Thermosetting Polymer
Thermosetting polymers are cross-linked.
The greater the degree of cross-linking, the more rigid the polymer.
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An Oriented Polymer
These polymers are stronger than steel and can conduct electricity.
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A Plasticizer
A plasticizer is an organic compound that is added to a polymer tomake it more flexible.
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Recycling a Polymer
This is the reverse of the transesterification reaction that formed the polymer.
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Biodegradable PolymersPolylactic Acid (PLA)
Biodegradable polymers can be broken into small molecules by microorganisms.
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PLA Cannot Be Used for Hot Drinks
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Biodegradable Polymers(polyhydroxyalkanoates)
Polyhydroxyalkanoates (PHAs) are condensation polymers of 3-hydroxycarboxylic acids.
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