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Synthetic Polymers

Paula Yurkanis BruiceUniversity of California,

Santa Barbara

Chapter 27

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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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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.