Condensation Polymers

Ring-Opening Polymerizations

• Epoxides undergo chain-growth polymerization, which can occur through an anionic mechanism if the initiator is a nucleophile, or through a cationic mechanism if the initiator is a Lewis acid.
• In an anionic mechanism, the nucleophile attacks the less sterically hindered carbon of the epoxide.

Polymerization of Dienes

• Natural rubber is a polymer of 2-methyl-1,3-butadiene (isoprene).
• Synthetic rubbers can be made by radical polymerization of dienes other than isoprene.
• A problem with natural and synthetic rubbers is that they are soft and sticky, but they can be hardened by vulcanization (heating with sulfur), which causes cross-linking of the separate polymer chains through disulfide bonds.

Copolymers

• Copolymers are formed from two or more different monomers.
• Both chain-growth polymers and step-growth polymers can be copolymers.
• There are four types of copolymers: alternating, block, random, and graft copolymers.

Condensation Polymers (Step-Growth Polymers)

• Condensation polymers are formed by the intermolecular reaction of molecules with a functional group at each end.
• A small molecule, such as H2O, alcohol, or HCl, is often lost during the reaction.
• Condensation polymers can be formed from a single compound with two different functional groups or from two different compounds.

Classes of Step-Growth Polymers

• Polyamides
• Polyesters
• Epoxy Resins
• Polyurethanes

Polyamides

• Nylon 6 is a step-growth polymer formed from a monomer with two different functional groups.
• The carboxylic acid group of one monomer reacts with the amino group of another monomer to form an amide.
• Nylon 6 is formed from the polymerization of 6-aminohexanoic acid, which contains six carbons.
• Nylon 66 is formed from the reaction of adipoyl chloride and 1,6-hexanediamine.

Polyesters

• Polymers containing many ester groups.
• Dacron is a common polyester formed by the transesterification of dimethyl terephthalate with ethylene glycol.
• Kodel polyester is formed by the transesterification of dimethyl terephthalate with 1,4-di(hydroxymethyl)cyclohexane.

Epoxy Resins

• They are the strongest adhesives known, with extensive cross-linking.
• They adhere to almost any surface and are resistant to solvents and high temperatures.

Anionic Polymerization (Living Polymers)

• The initiator is a nucleophile that reacts with the monomer to form an anion propagating site.
• Nucleophilic attack on an alkene requires a very good nucleophile and an alkene with an electron-withdrawing substituent.
• Living polymers are common in anionic polymerization because the chains cannot be terminated by proton loss or chain combination or disproportionation.
• Examples of alkenes that undergo anionic polymerization include methyl α-cyanoacrylate (Super Glue).