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Chapter 31. Synthetic Polymers

Chapter 31. Synthetic Polymers. Based on McMurry’s Organic Chemistry , 6 th edition. Polymers. Large molecules built up by repetitive bonding together of monomers. Drawing Polymers. Indicate repeating unit in parentheses . 31.1 Chain-Growth Polymers.

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Chapter 31. Synthetic Polymers

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  1. Chapter 31. Synthetic Polymers Based on McMurry’s Organic Chemistry, 6th edition

  2. Polymers • Large molecules built up by repetitive bonding together of monomers

  3. Drawing Polymers • Indicate repeating unit in parentheses

  4. 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

  5. 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

  6. Examples of Anionic Polymerization Products • Acrylonitrile (H2C=CHCN), methyl methacrylate [H2C=C(CH3)CO2CH3], and styrene (H2C=CHC6H5) react

  7. 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

  8. 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

  9. 31.3 Copolymers • Obtained when two or more different monomers polymerize together • They can be random or alternating

  10. Types of Copolymers • The exact distribution of monomer units depends on the initial proportions of the two reactant monomers and their relative reactivities

  11. 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

  12. 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

  13. 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

  14. Step-Growth Polymer from a Lactam • Addition generates new nucleophile • Polyamide from caprolactam is Nylon 6

  15. Polycarbonates • Carbonyl group is linked to two OR groups, [O=C(OR)2]

  16. Polyurethanes • Urethane - carbonyl carbon is bonded to both an OR group and an NR2 group

  17. 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

  18. 31.5 Polymer Structure and Physical Properties • Polymers experience substantially larger van der Waals forces than do small molecules, producing regions that are crystallites

  19. 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

  20. Thermoplastics • Have a high Tg and are hard at room temperature • Become soft and viscous when heated • Can be molded

  21. 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

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

  23. 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

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

  25. 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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