Organic Chemistry

1
Organic Chemistry
William H. Brown Christopher S. Foote
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Organic PolymerChemistry

• Chapter 24

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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

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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

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Notation Nomenclature

• Show the structure by placing parens around the
  repeat unit
• n average degree of polymerization

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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)

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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

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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

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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

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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

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Morphology

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

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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

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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

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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

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Polyamides

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

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Polyamides

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

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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

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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

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Polyesters

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

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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

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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 catchers masks
• it is also used in the manufacture of safety and
  unbreakable windows

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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

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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

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Polyurethanes

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

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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

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Epoxy Resins

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

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Epoxy Resins

• treatment of the diepoxide with a diamine gives
  the resin

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Thermosets

• Baelekite was one of the first thermosets

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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

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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

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Polyethylenes
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Polyethylenes
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Radical Chain-Growth

• Among the initiators used for radical
  chain-growth polymerization are diacyl peroxides,
  which decompose as shown on mild heating

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Radical Chain-Growth

• Another common class of initiators are azo
  compounds, which also decompose on mild heating
  or with absorption of UV light

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Radical Chain-Growth

• Radical polymerization of a substituted ethylene
• chain initiation
• chain propagation

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Radical Chain-Growth

• chain termination

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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

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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

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Radical Chain-Growth
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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

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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

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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

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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!

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Polymer Stereochemistry

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

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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

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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

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Anionic chain-Growth

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

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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

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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

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Anionic Chain-Growth

• the styryl dianion then propagates polymerization
  at both ends simultaneously

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Anionic Chain-Growth

• propagation of the distyryl dianion

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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

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Anionic Chain-Growth

• termination by carboxylation

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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

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Cationic Chain-Growth

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

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Cationic Chain-Growth

• initiation
• propagation

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Cationic Chain-Growth

• chain termination

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Prob 24.5

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

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Prob 24.7

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

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Prob 24.8

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

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Prob 24.9

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

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Prob 24.10

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

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Prob 24.12

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

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Prob 24.13

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

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Prob 24.14

• Draw a structural formula for the repeating unit
  of Nomex.

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Prob 24.15

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

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Prob 24.17

• Propose a mechanism for the formation of this
  polycarbonate.

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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.

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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.

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Prob 24.21

• Propose a mechanism for formation of this
  polymer.

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Prob 24.22

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

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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.

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Prob 24.25

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

74
Prob 24.26

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

75
Prob 24.33

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

76
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.

77
Prob 24.37

• From what two monomer units is this polymer made?

78
Prob 24.38

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

79

• Organic
• Polymer
• Chemistry

End Chapter 24