Anionic Polymerization: Living Polymers

Anionic Polymerization

• Initiator is a nucleophile that reacts with the monomer to form an anion propagating site
• Nucleophilic attack on an alkene does not occur readily due to the electron-rich nature of alkenes
• Initiator must be a very good nucleophile, such as sodium amide or butyllithium, and the alkene must contain electron-withdrawing substituents to decrease the electron density of the double bond

Mechanism of Anionic Polymerization

• Initiating step: involves the reaction of the nucleophile with the monomer
• Propagation steps: involve the reaction of the anion propagating site with additional monomers

Living Polymers

• Most common in anionic polymerization
• Chains cannot be terminated by proton loss from the polymer (as they can in cationic polymerization) or by chain combination or disproportionation (as they can in radical polymerization)

Examples of Alkenes That Undergo Anionic Polymerization

• Methyl α-cyanoacrylate (Super Glue)

Ring-Opening Polymerizations

• Some compounds can polymerize well rather than ethylene and substituted ethylenes in chain-growth polymerizations
• Epoxides undergo chain-growth polymerization
• If the initiator is a nucleophile, polymerization occurs by an anionic mechanism
• If the initiator is a Lewis acid, epoxides are polymerized by a cationic mechanism

Polymerization of Dienes

• The manufacture of rubber
• Natural rubber is a polymer of 2-methyl-1,3-butadiene (isoprene)
• Synthetic rubbers have been made by radical polymerization of dienes other than isoprene

Vulcanization

• A problem with both natural and most synthetic rubbers is that they are soft and sticky
• They can be hardened by vulcanization (heating of rubber with sulfur)
• Heating rubber with sulfur causes cross-linking of the separate polymer chains through disulfide bonds

Copolymers

• Formed from two or more different monomers
• Both chain-growth polymers and step-growth polymers can be copolymers
• Four types of copolymers:
  • Alternating copolymer: the two monomers alternate
  • Block copolymer: consists of blocks of each kind of monomer
  • Random copolymer: has a random distribution of monomers
  • Graft copolymer: contains branches derived from one monomer grafted onto a backbone from another monomer

Condensation Polymers (Step-Growth Polymers)

• Formed by the intermolecular reaction of molecules with a functional group at each end
• In most cases, a small molecule such as H2O, alcohol, or HCl is lost
• Can be formed by the reaction of a single compound with two different functional groups, A and B, or by the reaction of two different compounds

Classes of Step-Growth Polymers

• Polyamides
• Polyesters
• Epoxy Resins
• Polyurethanes

Polyamides

• Nylon 6: an example of a step-growth polymer formed from a monomer with two different functional groups
• Nylon 6 is formed from the polymerization of 6-aminohexanoic acid, which contains six carbons
• The starting material for the synthesis of nylon 6 is ε-caprolactam

Polyesters

• Polymers containing many ester groups
• Dacron is the most common polyesters polymer
• Dacron is made 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
• They are extensively cross-linked systems
• They can adhere to almost any surface and are resistant to solvents and high temperature