Polymerization and Properties of Polyurethanes

Urethanes

• Urethanes can be prepared by treating an isocyanate with an alcohol in the presence of a catalyst such as a tertiary amine.
• The most common polyurethanes are prepared by the polymerization of toluene-2,6-diisocyanate and ethylene glycol.

Oriented Polymers

• Polymer chains obtained by conventional polymerization can be stretched and packed together again in a more ordered, parallel arrangement than originally, resulting in polymers that are stronger than steel.
• Dyneema is an oriented polyethylene polymer with a molecular weight 100 times greater than that of high-density polyethylene.
• Dyneema is lighter than Kevlar and at least 40% stronger.
• A rope made of Dyneema can lift almost 119,000 pounds, whereas a steel rope of similar size fails at 13,000 pounds.

Biodegradable Polymers

• Biodegradable polymers can be degraded by microorganisms such as bacteria and fungi.
• Polylactide (PLA) is a biodegradable polymer of lactic acid, widely used.
• Lactic acid is converted to a cyclic dimer, which can form a polymer without loss of water by ring-opening polymerization.
• Polyhydroxyalkanoates (PHAs) are also biodegradable polymers.
• PHAs are condensation polymers of 3-hydroxycarboxylic acids.
• The most common PHA is PHB, a polymer of 3-hydroxybutyric acid; it can be used as polypropylene.
• PHAs are degraded by bacteria to CO2 and H2O.

Anionic Polymerization

• Nucleophilic attack on an alkene does not occur readily because alkenes are electron-rich.
• The 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.
• Living polymers are most common in anionic polymerization because the chains cannot be terminated by proton loss from the polymer.
• Examples of anionic polymerization include the polymerization of methyl α-cyanoacrylate, which is used in Super Glue.

Ring-Opening Polymerizations

• Some compounds can polymerize more readily than ethylene and substituted ethylenes in chain-growth polymerizations.
• Epoxides undergo chain-growth polymerization, which can occur by an anionic mechanism.

Synthetic Polymers

• Synthetic polymers are a multibillion-dollar industry, with over 50 trillion pounds manufactured each year in the United States alone.
• A polymer is a large molecule made by linking together repeating units of small molecules called monomers.
• The process of linking them together is called polymerization.

Polymer Nomenclature

• Polymers can be named according to their structure, but an alternative source-based system is more widely used.
• Source-based names are generated by adding the prefix poly- to the name of the monomer.
• When the name of the monomer consists of two words, both words are enclosed in parentheses following poly-.
• The polymer formulas are written by enclosing the repeating unit within brackets, followed by the letter n to indicate that the number of repeating units is not specified.

Classification of Polymers

• Polymers can be divided into two broad groups: synthetic polymers and biopolymers.
• Synthetic polymers are synthesized by scientists, whereas biopolymers are synthesized by cells.
• Examples of biopolymers include DNA, RNA, proteins, and polysaccharides.