chapter 31. synthetic polymers
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Chapter 31. Synthetic Polymers
Based on McMurry’s Organic Chemistry, 6th edition
Polymers
• Large molecules built up by repetitive bonding together of monomers
Drawing Polymers
• Indicate repeating unit in parentheses
31.1 Chain-Growth Polymers
• Produced by chain-reaction polymerization• Initiator (radical, acid or anion) adds to a carbon–
carbon double bond of an unsaturated substrate (a vinyl monomer) to yield a reactive intermediate that reacts with a second molecule of monomer and so on
Anionic Polymerization
• Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by anionic catalysts
• Chain-carrying step is nucleophilic addition of an anion to the unsaturated monomer by a Michael reaction
Examples of Anionic Polymerization Products
• Acrylonitrile (H2C=CHCN), methyl methacrylate [H2C=C(CH3)CO2CH3], and styrene (H2C=CHC6H5) react
31.2 Stereochemistry of Polymerization: Ziegler–Natta Catalysts
• Polymerization of a substituted vinyl monomer can lead to numerous chirality centers on the chain
• A polymer having all methyl groups on the same side of the zigzag backbone is called isotactic
• If the methyl groups alternate on opposite sides of the backbone, it is called syndiotactic
• Randomly oriented methyl groups are on atactic polymers
Ziegler–Natta Catalysts
• Allow preparation of isotactic, syndiotactic, and atactic polypropylene
• Prepared by treatment of an alkylaluminum with a titanium compound
• (CH3CH2)3Al + TiCl4 A Ziegler–Natta catalyst
31.3 Copolymers
• Obtained when two or more different monomers polymerize together
• They can be random or alternating
Types of Copolymers
• The exact distribution of monomer units depends on the initial proportions of the two reactant monomers and their relative reactivities
Block copolymers
• Different blocks of identical monomer units alternate with each other
• Prepared by initiating the polymerization of one monomer as if growing a homopolymer chain and then adding an excess of the second monomer to the still-active reaction mix
Graft copolymers
• Homopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unit
• Made by gamma irradiation of a completed homopolymer chain in the presence of the second monomer generating radical sites that can initiate polymerization of the added monomer
31.4 Step-Growth Polymers
• Produced by reactions in which each bond in the polymer is formed independently, typically by reaction between two difunctional reactants
Step-Growth Polymer from a Lactam
• Addition generates new nucleophile• Polyamide from caprolactam is Nylon 6
Polycarbonates
• Carbonyl group is linked to two OR groups, [O=C(OR)2]
Polyurethanes
• Urethane - carbonyl carbon is bonded to both an OR group and an NR2 group
Preparation of Polyurethanes
• Nucleophilic addition of an alcohol to an isocyanate (RN=C=O) gives a urethane
• Reaction between a diol and a diisocyanate gives a polyurethane
31.5 Polymer Structure and Physical Properties
• Polymers experience substantially larger van der Waals forces than do small molecules, producing regions that are crystallites
Heat Transitions
• Heating at the melt transition temperature, Tm, gives an amorphous material
• Heating noncrystalline, amorphous polymers makes the hard amorphous material soft and flexible at the glass transition temperature, Tg
Thermoplastics
• Have a high Tg and are hard at room temperature• Become soft and viscous when heated• Can be molded
Plasticizers
• Small organic molecules that act as lubricants between chains
• Added to thermoplastics to keep them from becoming brittle at room temperature
• Dialkyl phthalates are commonly used for this purpose
Fibers
• Thin threads produced by extruding a molten polymer through small holes in a die, or spinneret
• Fibers are then cooled and drawn out
Elastomers
• Amorphous polymers that have the ability to stretch out and spring back to their original shapes
• When stretched, the randomly coiled chains straighten out and orient along the direction of the pull
Natural Rubber and Gutta-Percha
• The upper structure is rubber, a natural elastomer• The lower structure is the nonelastic gutta-percha
Thermosetting resins
• Polymers that become highly cross-linked and solidify into a hard, insoluble mass when heated
• Bakelite is from reaction of phenol and formaldehyde, widely used for molded parts, adhesives, coatings
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