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