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 (RN=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