Organic Chemistry William H. Brown & Christopher S. Foote

Organic Polymer Chemistry Chapter 24

Organic Polymer Chem. Polymer: from the Greek, poly + meros, many parts any long-chain molecule synthesized by bonding together single parts called monomers Monomer: from the Greek, mono + meros, single part the simplest nonredundant unit from which a polymer is synthesized Plastic: a polymer that can be molded when hot and retains its shape when cooled

Organic Polymer Chem Thermoplastic: a polymer that can be melted and molded into a shape that is retained when it is cooled Thermoset plastic: a polymer that can be molded when it is first prepared but, once it is cooled, hardens irreversibly and cannot be remelted

Notation & Nomenclature Show the structure by placing parens around the repeat unit n = average degree of polymerization

Notation & Nomenclature To name a polymer, prefix poly to the name of the monomer from which the it is derived if the name of the monomer is one word, no parens are necessary for more complex monomers or where the name of the monomer is two words, enclose the name of the monomer is parens, as for example poly(vinyl chloride) or poly(ethylene terephthalate)

Molecular Weight All polymers are mixtures of individual polymer molecules of variable MWs number average MW: count the number of chains of a particular MW, multiply each number by the MW, sum these values, and divide by the total number of polymer chains weight average MW: record the weight of each chain of a particular length, sum these weights, and divide by the total weight of the sample

Morphology Polymers tend to crystallize as they precipitate or are cooled from a melt Acting to inhibit crystallization are their very large molecules, often with complicated and irregular shapes, which prevent efficient packing into ordered structures As a result, polymers in the solid state tend to be composed of ordered crystalline domains and disordered amorphous domains

Morphology High degrees of crystallinity are found in polymers with regular, compact structures and strong intermolecular forces, such as hydrogen bonds and dipolar interactions as the degree of crystallinity increases, the polymer becomes more opaque due to scattering of light by the crystalline regions Melt transition temperature, Tm: the temperature at which crystalline regions melt as the degree of crystallinity increases, Tm increases

Morphology Highly amorphous polymers are sometimes referred to as glassy polymers because they lack crystalline domains that scatter light, amorphous polymers are transparent in addition they are weaker polymers, both in terms of their greater flexibility and smaller mechanical strength on heating, amorphous polymers are transformed from a hard glass to a soft, flexible, rubbery state Glass transition temperature, Tg: the temperature at which a polymer undergoes a transition from a hard glass to a rubbery solid

Morphology Example: poly(ethylene terephthalate), abbreviated PET or PETE, can be made with % crystalline domains ranging from 0% to 55%

Morphology Completely amorphous PET is formed by cooling the melt quickly PET with a low degree of crystallinity is used for plastic beverage bottles By prolonging cooling time, more molecular diffusion occurs and crystalline domains form as the chains become more ordered PET with a high degree of crystallinity can be drawn into textile fibers and tire cords

Step-Growth Polymers Step-growth polymerization: a polymerization in which chain growth occurs in a stepwise manner between difunctional monomers we discuss five types of step-growth polymers polyamides polyesters polycarbonates polyurethanes epoxy resins

Polyamides Nylon 66 (from two six-carbon monomers) during fabrication, nylon fibers are cold-drawn to about 4 times their original length, which increases crystallinity, tensile strength, and stiffness

Polyamides the raw material base for the production of nylon 66 is benzene, which is derived from cracking and reforming of petroleum

Polyamides adipic acid is in turn the starting material for the synthesis of hexamethylenediamine

Polyamides Nylons are a family of polymers, the two most widely used of which are nylon 66 and nylon 6 nylon 6 is synthesized from a six-carbon monomer nylon 6 is fabricated into fibers, brush bristles, high-impact moldings, and tire cords

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

Polyesters ethylene glycol is obtained by air oxidation of ethylene followed by hydrolysis to the glycol terephthalic acid is obtained by catalyzed air oxidation of petroleum-derived p-xylene

Polycarbonates 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

Polycarbonates to make Lexan, an aqueous solution of the sodium salt of bisphenol A is brought into contact with a solution of phosgene in CH2Cl2 in the presence of a phase-transfer catalyst

Polyurethanes A urethane, or carbamate, is an ester of carbamic acid, H2NCH2COOH they are most commonly prepared by treatment of an isocyanate with an alcohol Polyurethanes consist of flexible polyester or polyether units (blocks) alternating with rigid urethane units (blocks) the rigid urethane blocks are derived from a diisocyanate

Polyurethanes the more flexible blocks are derived from low MW polyesters or polyethers with -OH groups at the ends of each polymer chain

Epoxy resins Epoxy resins are materials prepared by a polymerization in which one monomer contains at least two epoxy groups within this range, there are a large number of polymeric materials, and epoxy resins are produced in forms ranging from low-viscosity liquids to high-melting solids

Epoxy Resins the most widely used epoxide monomer is the diepoxide prepared by treating 1 mole of bisphenol A with 2 moles of epichlorohydrin

Epoxy Resins treatment of the diepoxide with a diamine gives the resin

Thermosets Baelekite was one of the first thermosets

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 on the following two screens are several important polymers derived from ethylene and substituted ethylenes, along with their most important uses

Polyethylenes

Polyethylenes

Radical Chain-Growth Among the initiators used for radical chain-growth polymerization are diacyl peroxides, which decompose as shown 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

Radical Chain-Growth Radical reactions with double bonds almost always gives the more stable (the more substituted) radical because additions are biased in this fashion, polymerizations of vinyl monomers tend to yield polymers with head-to-tail linkages

Radical Chain-Growth Chain-transfer reaction: the reactivity of an end group is transferred from one chain to another, or from one position on a chain to another position on the same chain polyethylene formed by radical polymerization exhibits a number of butyl branches on the polymer main chain these butyl branches are generated by a “back-biting” chain-transfer reaction in which a 1° radical end group abstracts a hydrogen from the fourth carbon back polymerization then continues from the 2° radical

Radical Chain-Growth

Radical Chain-Growth The first commercial polyethylenes produced by radical polymerization were soft, tough polymers known as low-density polyethylene (LDPE) LDPE chains are highly branched due to chain-transfer reactions because this branching prevents polyethylene chains from packing efficiently, LDPE is largely amorphous and transparent approx. 65% is fabricated into films for consumer items such as baked goods, vegetables and other produce, and trash bags

Ziegler-Natta Polymers Ziegler-Natta chain-growth polymerization is an alternative method that does not involve radicals Ziegler-Natta catalysts are heterogeneous materials composed of a MgCl2 support, a Group 4B transition metal halide such as TiCl4, and an alkylaluminum compound

Ziegler-Natta Polymers Mechanism of Ziegler-Natta polymerization Step 1: formation of a titanium-ethyl bond Step 2: insertion of ethylene into the Ti-C bond

Ziegler-Natta Polymers Polyethylene from Ziegler-Natta systems is termed high-density polyethylene (HDPE) it has a considerably lower degree of chain branching than LDPE and a result has a higher degree of crystallinity, a higher density, a higher melting point, and is several times stronger than LDPE appox. 45% of all HDPE is blow-molded into containers with special fabrication techniques, HDPE chains can be made to adopt an extended zig-zag conformation. HDPE processed in this manner is stiffer than steel and has 4x the tensile strength!

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

Polymer Stereochemistry In general, the more stereoregular the stereocenters are (the more highly isotactic or syndiotactic the polymer is), the more crystalline it is the chains of atactic polyethylene, for example, do not pack well and the polymer is an amorphous glass isotactic polyethylene, on the other hand, is a crystalline, fiber-forming polymer with a high melt transition

Ionic Chain-Growth May be either anionic or cationic polymerizations cationic polymerizations are most common with monomers with electron-donating groups anionic polymerizations: most common with monomers with electron-withdrawing groups

Anionic chain-Growth Anionic polymerization can be initiated by addition of a nucleophile, such as methyl lithium, to an activated alkene

Anionic Chain-Growth An alternative method for initiation involves a one-electron reduction of the monomer by Li or Na to form a radical anion which is either reduced or dimerized to a dianion

Anionic Chain-Growth To improve the efficiency of anionic polymerizations, soluble reducing agents such as sodium naphthalide are used the naphthalide radical anion is a powerful reducing agent and, for example, reduces styrene to a radical anion which couples to give a dianion

Anionic Chain-Growth the styryl dianion then propagates polymerization at both ends simultaneously

Anionic Chain-Growth propagation of the distyryl dianion

Anionic Chain-Growth Living polymer: a polymer chain that continues to grow without chain-termination steps until either all of the monomer is consumed or some external agent is added to terminate the chains after consumption of the monomer under living anionic conditions, electrophilic agents such as CO2 or ethylene oxide are added to functionalize the chain ends

Anionic Chain-Growth termination by carboxylation

Cationic Chain-Growth The two most common methods for initiating cationic polymerization are reaction of a strong protic acid with the monomer abstraction of a halide from the organic initiator by a Lewis acid Initiation by a protic acid requires a strong acid with a nonnucleophilic anion in order to avoid addition to the double bond suitable acids include HF/AsF5 and HF/BF3

Cationic Chain-Growth initiation by a protic acid Lewis acids used for initiation include BF3, SnCl4, AlCl3, Al(CH3) 2Cl, and ZnCl2

Cationic Chain-Growth initiation propagation

Cationic Chain-Growth chain termination

Prob 24.5 Name each polymer, and draw the structure of the monomer(s) that might be used to make it.

Prob 24.7 Draw a structural formula for the polymer formed in each reaction.

Prob 24.8 Propose reagents and experimental conditions for the conversion of furan to hexamethylenediamine.

Prob 24.9 Propose reagents for the conversion of 1,3-butadiene to hexamethylenediamine.

Prob 24.10 Propose reagents and experimental conditions for the conversion of butadiene to adipic acid.

Prob 24.12 Propose a mechanism for the step-growth reaction in this polymerization.

Prob 24.13 Identify the monomer(s) required for the synthesis of each step-growth polymer.

Prob 24.14 Draw a structural formula for the repeating unit of Nomex.

Prob 24.15 Propose a mechanism for this Beckmann rearrangement which converts cyclohexanone oxime to caprolactam, the monomer from which nylon 6 is synthesized.

Prob 24.17 Propose a mechanism for the formation of this polycarbonate.

Prob 24.18 Propose a mechanism for the formation of this polyurea. To simplify, consider the reaction of one -NCO group with one -NH2 group.

Prob 24.19 When equal molar amounts of these two monomers are heated, they form an amorphous polyester. Under these conditions, polymerization is regioselective for the 1°-OH groups. Draw a structural formula for the repeat unit of this polyester.

Prob 24.21 Propose a mechanism for formation of this polymer.

Prob 24.22 Draw a structural formula for the polymer resulting from base-catalyzed polymerization of each monomer. Will the polymer by optically active?

Prob 24.23 The polymer on the left is an insoluble, opaque material that is difficult to process into shapes. The polymer on the right is an transparent material that is soluble in a number of organic solvents. Explain the difference in physical properties between the two in terms of their structural formulas.

Prob 24.25 Draw a structural formula for the repeat unit of the polymer formed in each reaction.

Prob 24.26 Select the member of each pair that is more reactive toward cationic polymerization.

Prob 24.33 Natural rubber is the all-cis polymer of isoprene. Draw a structural formula for the repeat unit of natural rubber.

Prob 24.34 Radical polymerization of styrene gives a linear polymer. Show by drawing structural formulas how incorporation of a few percent 1,4-divinylbenzene in the polymerization mixture gives a cross-linked polymer.

Prob 24.37 From what two monomer units is this polymer made?

Prob 24.38 Draw a structural formula for the repeat unit in the polymer formed by ring-opening metathesis polymerization of each monomer.

Organic Polymer Chemistry End Chapter 24