Organic Polymer Chemistry BY Dr Ghulam Abbas Some

Organic Polymer Chemistry BY Dr. Ghulam Abbas

Some Definitions • Polymer: From the Greek, poly + meros, meros many parts. Any long-chain molecule synthesized by bonding together single parts called monomers. • Monomer: meros Monomer From the Greek, mono + meros, single part. The simplest necessary unit from which a polymer is synthesized. • Plastic: A polymer that can be molded when hot and retains its shape when cooled.

Conti… Thermoplastic: A polymer that can be melted and molded into a shape that is retained when it is cooled (recycle) e. g. Nylon, polycarbonate (eyeglass lens), polyethylene (shampoo bottles). Thermoset polymer: A polymer that can be molded when it is first prepared but, once it is cooled, hardens irreversibly and cannot be remelted. Examples are synthetic fibers, print roller, duroplast.

Polymerization Reactions • Polymerization = Reaction to convert monomers to a polymer. There are two types of polymerization reaction; 1 - Step-Growth Polymerization (Condensation Polymers): It applies to monomers with functional groups such as: -COOH, -COOR, -COOOC-, -COCl, -OH, -NH 2, -CHO, -NCO, epoxy. Chain Polymerization (Addition Polymers): It applies to monomers having double bonds or ring structure. • Both classes of reaction can lead to the formation of either linear polymers or polymer networks. It only depends on the number of reactive entities per monomer.

Step-Growth Polymerization Basic Reactions • Condensation: coupling reaction with loss of small molecule Example: CH 3 -NH 2 + HOOC-CH 2 -CH 3 Amine + Acid CH 3 -NH-CO-CH 2 -CH 3 + H 2 O Amide + Water • Addition: coupling reaction without loss of small molecule Example: CH 3 -N=C=O + HO-CH 2 -CH 3 Isocyanate + Alcohol CH 3 -NH-CO-O-CH 2 -CH 3 Urethane

Step-Growth Polymers • Step-growth polymerization: A polymerization in which chain growth occurs in a stepwise manner between difunctional monomers. • Five types of step-growth polymers: 1 - Polyamides 2 - Polyesters 3 - Polycarbonates 4 - Polyurethanes 5 - epoxy resins

Polyamides Nylon 6 -6 • Nylon 6 -6 (from two six-carbon monomers). During fabrication, nylon fibers are cold-drawn to about 4 times their original length, which increases alignment, crystallinity, tensile strength, and stiffness.

NYLON PRODUCTS

Polyamides, Nylon 6 • Nylons are a family of polymers, the two most widely used of which are nylon 6 -6 and nylon 6. Nylon 6 is synthesized from a six-carbon monomer. Nylon 6 is fabricated into fibers, brush bristles, and tire cords.

Polyamides, Kevlar • Kevlar is a polyaromatic amide (an aramid). Cables of Kevlar are as strong as cables of steel, but only about 20% the weight. Kevlar fabric is used for bulletproof vests, jackets, and raincoats.

Polyesters, PET • Poly (ethylene terephthalate), abbreviated PET or PETE, is fabricated into Dacron fibers, Mylar films, and plastic beverage containers. Mylar films Dacron fibers

Polycarbonates, Lexan To make Lexan, an aqueous solution of the sodium salt of bisphenol A (BPA) is brought into contact with a solution of phosgene in CH 2 Cl 2.

Polycarbonates, Lexan • Lexan is a tough transparent polymer with high impact and tensile strengths and retains its shape over a wide temperature range. It is used in sporting equipment, such as bicycle, football, and snowmobile helmets as well as hockey and baseball catcher’s masks. It is also used in the manufacture of safety and unbreakable windows.

Polyurethanes A urethane, urethane or carbamate, is an ester of carbamic acid, H 2 NCH 2 COOH. They are most commonly prepared by treatment of an isocyanate with an alcohol. Polyurethanes consist of flexible polyester or polyether units alternating with rigid urethane units. The rigid urethane units are derived from a diisocyanate.

Epoxy Resins • Epoxy resins are materials prepared by a polymerization in which one monomer contains at least two epoxy groups. Epoxy resins are produced in forms ranging from low-viscosity liquids to high-melting solids.

Epoxy Resins – prepared by treating one mole of bisphenol A with two moles of epichlorohydrin.

Chain-Growth Polymers • Chain-growth polymerization: A polymerization that involves sequential addition reactions, either to unsaturated monomers or to monomers possessing other reactive functional groups. • Reactive intermediates in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes.

Chain-Growth Polymers We concentrate on chain-growth polymerizations of ethylene and substituted ethylenes. Several important polymers derived from ethylene and substituted ethylenes, along with their most important Uses are shown as;

Polyethylenes

Radical Chain-Growth Among the initiators used for radical chain-growth polymerization are diacyl peroxides, which decompose On mild heating.

Radical Chain-Growth Another common class of initiators are azo compounds, Which also decompose on mild heating or with absorption of UV light.

Radical Chain-Growth Radical polymerization of a substituted ethylene. Chain initiation: Chain propagation:

Radical Chain-Growth Chain termination

Elastomeric Materials Ø Common characteristics; – Large elastic elongation (i. e. 200%) – Can be stretched and then immediately return to their original length when the load was released. Ø Elastomers are sometimes called rubber or rubbery materials. Ø The term elastomer is often used interchangeably with the term rubber. Ø Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic.

Natural Rubber • Rubber tree (Hevea Braziliensis) • Natural rubber is obtained by drying a latex rubber (milk in which the butter fat component is suspended in water salution) • High temperature stability – cooking the crude natural rubber with sulphur (vulcanization) • Vulcanization creates crosslinking between rubber molecules • Natural rubber is highly elastomeric (elongation 1000% for vulcanized natural rubber). • Compared to other elastomeric materials, natural rubber shows higher tensile strength, high tear strength, high resilience, resistance to wear, etc

Polyisoprene structure Cis-poliisoprena Trans-poliisoprena (Hevea rubber) (Gutta percha)

Butadiene Rubber (BR) • • Synthetic rubber Repeating units of both have a backbone of four carbon atoms including C-C double bond Polybutadiene has just two hydrogen attached to the C-C double bond. Absence of methyl group in polybutadiene results in strength & tear strength than would polyisoprena.

Styrene Butandiene Rubber (SBR)

Polymer Stereochemistry • There are three alternatives for the relative configurations of stereocenters along the chain of a substituted ethylene polymer.

Polymers Applications Polymers (or plastics as they are also called) are known to have good insulating properties. • Polymers are one of the most used materials in the modern world. Their uses and application range from containers to clothing. They are used to coat metal wires to prevent electric shocks.

Conclusion • For conductance free electrons are needed. • Conjugated polymers are semiconductor materials while doped polymers are conductors. • The conductivity of conductive polymers decreases with falling temperature in contrast to the conductivities of typical metals, e. g. silver, which increase with falling temperature. • Today conductive plastics are being developed for many uses.

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