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

2. Organic Polymer Chemistry
Chapter 24

3. 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

4. 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

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

6. 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)

7. 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

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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

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

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

17. 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

18. 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

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

20. 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

21. 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

22. 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

23. 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

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

25. 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

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

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

28. Thermosets
Baelekite was one of the first thermosets

29. 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

30. 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

31. Polyethylenes

32. Polyethylenes

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

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

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

36. Radical Chain-Growth
chain termination

37. 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

38. 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

39. Radical Chain-Growth

40. 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

41. 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

42. 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

43. 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!

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

45. 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

46. 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

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

48. 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

49. 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

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

51. Anionic Chain-Growth
propagation of the distyryl dianion

52. 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

53. Anionic Chain-Growth
termination by carboxylation

54. 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

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

56. Cationic Chain-Growth
initiation
propagation

57. Cationic Chain-Growth
chain termination

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

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

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

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

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

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

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

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

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

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

68. 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.

69. 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.

70. Prob 24.21
Propose a mechanism for formation of this polymer.

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

72. 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.

73. 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