29_Lecture.ppt

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Chapter 29 Synthetic Polymers
Organic Chemistry 6th Edition Paula Yurkanis
Bruice
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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 aluminumtitanium initiator
(ZieglerNatta 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
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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