John E. McMurry Paul D. Adams University of Arkansas Chapter 31 Synthetic Polymers

 Large molecules built up by repetitive bonding together of monomers  Indicate repeating unit in parentheses Polymers

 To take a more comprehensive view of polymers  Look into more detail how polymers are made  Look at how polymer structure correlates with their physical properties Why This Chapter?

 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 31.1 Chain-Growth Polymers

 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 Anionic Polymerization

 Acrylonitrile (H 2 C=CHCN), methyl methacrylate [H 2 C=C(CH 3 )CO 2 CH 3 ], and styrene (H 2 C=CHC 6 H 5 ) react Examples of Anionic Polymerization Products

 Polymerization of a substituted vinyl monomer can lead to numerous chirality centers on the chain  A polymer having all groups on the same side of the backbone is called isotactic  If the groups alternate on opposite sides of the backbone, it is called syndiotactic  Randomly oriented groups are on atactic polymers 31.2 Stereochemistry of Polymerization: Ziegler–Natta Catalysts

 Allow preparation of isotactic, syndiotactic, and atactic polypropylene  Prepared by treatment of a titanium compound with an alkylaluminum  (CH 3 CH 2 ) 3 Al + TiCl 4  A Ziegler–Natta catalyst Ziegler–Natta Catalysts

 Obtained when two or more different monomers polymerize together  They can be random or alternating 31.3 Copolymers

 The exact distribution of monomer units depends on the initial proportions of the two reactant monomers and their relative reactivities Types of 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  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 Block and Graft Copolymers

 Produced by reactions in which each bond in the polymer is formed independently, typically by reaction between two different functional reactants 31.4 Step-Growth Polymers

 Addition generates new nucleophile  Polyamide from caprolactam is Nylon 66 Chain-Growth Polymer from a Lactam

 Carbonyl group is linked to two –OR groups, [O=C(OR) 2 ] Polycarbonates

 Urethane - carbonyl carbon is bonded to both an –OR group and an –NR 2 group Polyurethanes

 They are often prepared by nucleophilic addition of an alcohol to an isocyanate (R–N=C=O) to give a urethane  Reaction between a diol and a diisocyanate gives a polyurethane Preparation of Polyurethanes

 Polymers experience substantially larger van der Waals forces than do small molecules, producing regions that are crystallites 31.5 Polymer Structure and Physical Properties

 Heating at the melt transition temperature, T m, gives an amorphous material  Heating noncrystalline, amorphous polymers makes the hard amorphous material soft and flexible at the glass transition temperature, T g Heat Transitions

 Have a low T g and are hard at room temperature  Become soft and viscous when heated  Can be molded Thermoplastics

 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 Plasticizers

 Thin threads produced by extruding a molten polymer through small holes in a die, or spinneret  Fibers are then cooled and drawn out Fibers

 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 Elastomers

 The upper structure is rubber, a natural elastomer  The lower structure is the nonelastic gutta-percha Natural Rubber and Gutta- Percha

 Polymers that become highly cross-linked and solidify into a hard, insoluble mass  Bakelite is from reaction of phenol and formaldehyde, widely used for molded parts, adhesives, coatings Thermosetting Resins

Let’s Work a Problem Identify the monomer unit from the polymer given below. In addition, state whether the polymer is formed from a chain-growth or a step-growth polymer.

Answer We should notice from the structure that the monomeric unit is a cyclohexane-diol (OHs in para position relative to each other). The polymer is also formed via step-growth polymerization.