Chapter 29 Organic Chemistry Synthetic Polymers 6th Edition Paula Yurkanis Bruice

A polymer is a large molecule made by linking together repeating units of small molecules called monomers:

Chain-growth polymers, also known as addition polymers, are made by chain reactions:

Step-growth polymers, also called condensation polymers, are made by combining two molecules while removing a small molecule:

Chain-Growth Polymers Chain-growth polymers proceed by one of three mechanisms: • Radical polymerization • Cationic polymerization • Anionic polymerization

Radical Polymerization

The molecular weight of the polymer can be controlled by a process known as chain transfer:

Chain-growth polymerization of monosubstituted ethylenes exhibits a marked preference for head-to-tail addition:

Head-to-tail addition of a substituted ethylene results in a polymer in which every other carbon bears a substituent:

Head-to-tail addition is favored for steric reasons. Groups that stabilize radicals also favor head-to-tail addition.

Branching of the Polymer Chain

Branched polymers are more flexible

Cationic Polymerization

The carbocation intermediates formed during polymerization can undergo rearrangement to form more stable carbocations:

Monomers with electron-donating substituents readily undergo cationic polymerization:

Anionic Polymerization The initiator is a nucleophile, generally a strong base

Nonterminated chains are called “living polymers” and remain active until they are “killed.”

An Example of Anion Polymerization

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.

Ring-Opening Polymerization

Stereochemistry of Polymerization Substituents on the same side: Substituents regularly alternate on both sides: Substituents in an atactic polymer are randomly oriented

Long, unbranched polymers with either isotactic or the syndiotactic configuration can be prepared using an aluminum–titanium initiator (Ziegler–Natta catalyst):

Polymerization of Dienes

Synthesis of Synthetic Rubber

The product resulting from polymerization of different monomers is called a copolymer:

Four Types of Copolymers

Progress of a Step-Growth Polymerization

Nylon 6 is an example of a step-growth polymer formed by a monomer with two different functional groups:

The incorporation of aromatic rings into polymers improves the physical strength of the polymers:

Polyesters are step-growth polymers in which the monomer units are joined together by ester groups:

Polyesters with two ester groups bonded to the same carbon are known as polycarbonates:

Epoxy resins are the strongest adhesive known:

A urethane (carbamate) is a compound that has an OR group and an NHR group bonded to the same carbonyl compound:

Thermosetting Polymers Very strong and rigid materials can be obtained by cross-linking.

Oriented Polymers These polymers are stronger than steel and can conduct electricity

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:

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.

Dental Restorative Resin Components Photo-curable resin used by dentists to restore decayed teeth Bisphenol A linker Bisphenol A may be hazardous to humans

Photo-Initiator Components

Photo-Initiation Process A blue LED light is used to initiate polymerization

Radical-Initiated Polymerization Curing is complete in 12 seconds