chapter 31. synthetic polymers

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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. - PowerPoint PPT Presentation

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  • Chapter 31. Synthetic PolymersBased on McMurrys Organic Chemistry, 6th edition

  • PolymersLarge molecules built up by repetitive bonding together of monomers

  • Drawing PolymersIndicate repeating unit in parentheses

  • 31.1 Chain-Growth PolymersProduced by chain-reaction polymerization Initiator (radical, acid or anion) adds to a carboncarbon 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 PolymerizationVinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by anionic catalystsChain-carrying step is nucleophilic addition of an anion to the unsaturated monomer by a Michael reaction

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

  • 31.2 Stereochemistry of Polymerization: ZieglerNatta CatalystsPolymerization of a substituted vinyl monomer can lead to numerous chirality centers on the chainA polymer having all methyl groups on the same side of the zigzag backbone is called isotacticIf the methyl groups alternate on opposite sides of the backbone, it is called syndiotacticRandomly oriented methyl groups are on atactic polymers

  • ZieglerNatta CatalystsAllow preparation of isotactic, syndiotactic, and atactic polypropylenePrepared by treatment of an alkylaluminum with a titanium compound(CH3CH2)3Al + TiCl4 A ZieglerNatta catalyst

  • 31.3 CopolymersObtained when two or more different monomers polymerize togetherThey can be random or alternating

  • Types of CopolymersThe exact distribution of monomer units depends on the initial proportions of the two reactant monomers and their relative reactivities

  • Block copolymersDifferent blocks of identical monomer units alternate with each otherPrepared 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 copolymersHomopolymer branches of one monomer unit are grafted onto a homopolymer chain of another monomer unitMade 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 PolymersProduced by reactions in which each bond in the polymer is formed independently, typically by reaction between two difunctional reactants

  • Step-Growth Polymer from a LactamAddition generates new nucleophilePolyamide from caprolactam is Nylon 6

  • PolycarbonatesCarbonyl group is linked to two OR groups, [O=C(OR)2]

  • PolyurethanesUrethane - 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 urethaneReaction between a diol and a diisocyanate gives a polyurethane

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

  • Heat TransitionsHeating at the melt transition temperature, Tm, gives an amorphous materialHeating noncrystalline, amorphous polymers makes the hard amorphous material soft and flexible at the glass transition temperature, Tg

  • ThermoplasticsHave a high Tg and are hard at room temperatureBecome soft and viscous when heatedCan be molded

  • PlasticizersSmall organic molecules that act as lubricants between chainsAdded to thermoplastics to keep them from becoming brittle at room temperatureDialkyl phthalates are commonly used for this purpose

  • FibersThin threads produced by extruding a molten polymer through small holes in a die, or spinneretFibers are then cooled and drawn out

  • Elastomers Amorphous polymers that have the ability to stretch out and spring back to their original shapesWhen stretched, the randomly coiled chains straighten out and orient along the direction of the pull

  • Natural Rubber and Gutta-PerchaThe upper structure is rubber, a natural elastomerThe lower structure is the nonelastic gutta-percha

  • Thermosetting resinsPolymers that become highly cross-linked and solidify into a hard, insoluble mass when heatedBakelite is from reaction of phenol and formaldehyde, widely used for molded parts, adhesives, coatings

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