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Chapter 27. Synthetic Polymers. Paula Yurkanis Bruice University of California, Santa Barbara. Polymerization. A polymer is a large molecule made by linking together repeating units of small molecules called monomers. A Chain-Growth Polymer (polystyrene).
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Chapter 27 Synthetic Polymers Paula Yurkanis Bruice University of California, Santa Barbara
Polymerization A polymer is a large molecule made by linking together repeating units of small molecules called monomers.
A Chain-Growth Polymer (polystyrene) Chain-growth polymers (also known as addition polymers), are made by chain reactions.
Step-Growth Polymer (Dacron) Step-growth polymers (also called condensation polymers), are made by combining two molecules while removing a small molecule.
Chain-Growth Polymers Chain-growth polymers are formed by one of three mechanisms. • Radical polymerization • Cationic polymerization • Anionic polymerization
Radical Polymerization The initiator is a radical.
Chain Transfer The molecular weight of the polymer can be controlled by a process known as chain transfer.
Head-to-Tail Linkage Chain-growth polymerization of monosubstituted ethylenes exhibits a marked preference for head-to-tail addition.
Poly(vinyl chloride) Head-to-tail addition forms a polymer with a substituent on every other carbon.
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.
Cationic Polymerization The initiator is an electrophile.
Alkenes Used in Cationic Polymerization Monomers with electron-donating substituents undergo cationic polymerization.
Anionic Polymerization The initiator is a nucleophile.
Propagating Steps Nonterminated chains are called “living polymers”and remain active until they are “killed.”
Alkenes Used in Anionic Polymerization Monomers with electron-withdrawing substituents undergo anionic polymerization.
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.
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).
Polymerization of Dienes These rubbers have cis double bonds.
Neoprene(a synthetic rubber) Neoprene has trans double bonds.
Disulfide Bonds in Rubber The more sulfur used in vulcanization, the more rigid the polymer.
Step-Growth Polymers The monomer can have two different functional groups. The monomer can be two different bifunctional compounds.
Step-Growth Polymerization Requires High Yields for Long Chains
Nylon is a Polyamide Nylon 6 is an example of a step-growth polymer formed by a monomer with two different functional groups.
Nylon is a Polyamide Nylon 66 is an example of a step-growth polymer formed by two different bifunctional monomers.
Aromatic Polyamides are Called Aramides The incorporation of aromatic rings into polymers improves the physical strength of the polymers.
Dacron is a Polyester Polyesters are step-growth polymers in which the monomer units are joined together by ester groups.
Lexan is a Polycarbonate Polycarbonates have two ester groups bonded to the same carbon.
An Epoxy Resin is a Cross-Linked Polymer(it is the strongest adhesive known)
A Urethane A urethane has an OR and an NHR bonded to the same carbonyl carbon.
