1. Chapter 17: Organic Polymer Chemistry

2. Polymers
Polymer: any long-chain molecule synthesized by linking together single parts called monomers
Monomer: 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
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 first prepared, but once it is cooled, hardens irreversibly and cannot be remelted

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

4. Nomenclature
To name a polymer, prefix poly to the name of the monomer from which the polymer 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 in parens, as for example poly(vinyl chloride)

5. 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

6. Morphology
High degrees of crystallinity are found in polymers with regular, compact structures and strong intermolecular forces such as hydrogen bonds
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

7. 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 high flexibility and low 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

8. Morphology
example: poly(ethylene terephthalate) (PET) can be made with % crystalline domains ranging from 0% to 55%

9. 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

10. 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

11. Polyamides
Nylon 66

12. Nylon 66
during fabrication, nylon fibers are cold-drawn to about 4 times their original length, which increases crystallinity, tensile strength, and stiffness

13. Nylon 66
the raw material base for the production of nylon 66 is benzene, which is derived from cracking and reforming of petroleum
catalytic reduction of benzene followed by air oxidation gives a mixture of cyclohexanol and cyclohexanone
oxidation of the mixture by nitric acid gives adipic acid

14. Nylon 66
adipic acid is in turn used for the synthesis of hexamethylenediamine

15. Nylon 6
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

16. 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

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

18. Polyesters ethylene glycol is synthesized from ethylene
terephthalic acid is synthesized from p-xylene, which is obtained from petroleum refining

19. Polycarbonates
Polycarbonates, the most familiar of which is Lexan, are important engineering plastics
Lexan is made from the disodium salt of bisphenol A and phosgene

20. 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

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

22. Polyurethanes
polyurethane fibers are fairly soft and elastic and are used in “stretch” fabrics such as spandex and Lycra

23. Epoxy Resins
Epoxy resins are 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
epoxy resins are produced in forms ranging from low viscosity liquids to high melting solids
the most widely used epoxide monomer is the diepoxide prepared by treating 1 mol of bisphenol A with 2 mol of epichlorohydrin

24. Epoxy Resins
formation of a diepoxide

25. Epoxy Resins
the diepoxide monomer is then treated with a diamine

26. Chain-Growth Polymers
Chain-growth polymerization: a polymerization in which monomer units are joined together without loss of atoms
for example:
from the perspective of the chemical industry, chain-growth polymerization is the single most important reaction of alkenes

27. Chain-Growth Polymers
Table 17.1

28. Chain-Growth Polymers
Table 17.1 (cont’d)

29. Radical Chain Growth
Radical: a molecule or ion containing one or more unpaired electrons
To account for the polymerization of alkenes in the presence of peroxides, chemists propose a three-step radical chain mechanism
(1) chain initiation
(2) chain propagation
(3) chain termination

30. Radical Chain Growth
Among the initiators used for radical chain-growth polymerization are organic peroxides, which decompose as shown on mild heating
fishhook arrow: a curved and barbed (fishhook) arrow used to show the repositioning of a single electron

31. Radical Chain-Growth Step 1: chain initiation
a step in a radical chain reaction characterized by the formation of radicals from nonradical compounds
Step 2: chain propagation
Chain propagation: a step in a radical chain reaction characterized by the reaction of a radical and a molecule to give a new radical
Chain length, n: the number of times the cycle of chain propagation steps repeats in a chain reaction

32. Radical Chain Growth
chain propagation (cont’d)

33. Radical Chain Growth Step 3: chain termination
a step in a radical chain mechanism that involves destruction of radicals
one type of chain termination is radical coupling

34. Radical Chain Growth
radical reactions with double bonds almost always give 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

35. 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

36. 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
polymerization then continues at the 2° radical

37. Ziegler-Natta Polymerization
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 such as diethylaluminum chloride, Al(CH2CH3)2Cl

38. Ziegler-Natta Polymerization
Step 1: formation of a titanium-ethyl bond
Step 2: insertion of ethylene into the titanium-carbon bond

39. Ziegler-Natta Polymerization
Polyethylene from Ziegler-Natta systems is termed high-density polyethylene (HDPE)
it has a considerably lower degree of chain branching than LDPE and, as 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!

40. Recycling Codes

41. Recycling Codes

42. Organic Polymer Chemistry
End Chapter17