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Ch29 Synthetic Polymers

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Organic Chemistry
6th Edition
Chapter 29
Paula Yurkanis Bruice
Synthetic Polymers
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A polymer is a large molecule made by linking together
repeating units of small molecules called monomers:
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Chain-growth polymers, also known as addition
polymers, are made by chain reactions:
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Step-growth polymers, also called condensation
polymers, are made by combining two molecules while
removing a small molecule:
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Chain-Growth Polymers
Chain-growth polymers proceed by one of three
mechanisms:
• Radical polymerization
• Cationic polymerization
• Anionic polymerization
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Radical Polymerization
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The molecular weight of the polymer can be controlled by
a process known as chain transfer:
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Chain-growth polymerization of monosubstituted
ethylenes exhibits a marked preference for head-to-tail
addition:
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Head-to-tail addition of a substituted ethylene results in a
polymer in which every other carbon bears a substituent:
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• Head-to-tail addition is favored for steric reasons.
• Groups that stabilize radicals also favor head-to-tail
addition.
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Branching of the Polymer Chain
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Branched polymers are more flexible
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Cationic Polymerization
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The carbocation intermediates formed during
polymerization can undergo rearrangement to form more
stable carbocations:
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Monomers with electron-donating substituents readily
undergo cationic polymerization:
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Anionic Polymerization
The initiator is a nucleophile, generally a strong base
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Nonterminated chains are called “living polymers”
and remain active until they are “killed.”
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An Example of Anion Polymerization
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What Determines the Mechanism?
• Alkenes with substituents that can stabilize radicals
readily 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
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Stereochemistry of Polymerization
Substituents on the same side:
Substituents regularly alternate on both sides:
Substituents in an atactic polymer are randomly oriented
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Long, unbranched polymers with either isotactic or the
syndiotactic configuration can be prepared using an
aluminum–titanium initiator (Ziegler–Natta catalyst):
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Polymerization of Dienes
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Synthesis of Synthetic Rubber
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The product resulting from polymerization of different
monomers is called a copolymer:
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Four Types of Copolymers
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Progress of a Step-Growth
Polymerization
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Nylon 6 is an example of a step-growth polymer formed
by a monomer with two different functional groups:
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The incorporation of aromatic rings into polymers
improves the physical strength of the polymers:
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Polyesters are step-growth polymers in which the
monomer units are joined together by ester groups:
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Polyesters with two ester groups bonded to the same
carbon are known as polycarbonates:
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Epoxy resins are the strongest adhesive known:
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A urethane (carbamate) is a compound that has an
OR group and an NHR group bonded to the same
carbonyl compound:
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Thermosetting Polymers
Very strong and rigid materials can be obtained by
cross-linking.
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Oriented Polymers
These polymers are stronger than steel and can
conduct electricity
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Biodegradable Polymers
These polymers can be broken into small
molecules by microorganisms
The cyclic dimer of lactic acid undergoes ring-opening
polymerization to afford a high-molecular-weight
polylactide:
Polyhydroxyalkanoates (PHAs) are condensation polymers
of 3-hydroxycarboxylic acids:
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Other Polymers
• Thermoplastic polymers have both ordered
crystalline regions and amorphous, noncrystalline
regions.
• Elastomers are polymers that stretch and then
revert to the original shape.
• Plasticizer is an organic compound that is added to
a polymer to make it more flexible.
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Dental Restorative Resin Components
Photo-curable resin
used by dentists to
restore decayed
teeth
Bisphenol A linker
Bisphenol A may be
hazardous to humans
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Photo-Initiator Components
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Photo-Initiation Process
A blue LED light is used to initiate polymerization
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Radical-Initiated Polymerization
Curing is complete in 12 seconds
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