Polymers
Chapter 30
Light weight
Flexible
Easily processable
Transparent (sometimes)
Strong
Elastic
Cheap
1
Polymers
• Macromolecules > 10,000 grams/mole (e.g. proteins, DNA)
poly = many
mer = units or pieces
n
Poly-cis-isoprene
1000 g/mole
Polyisoprene
(natural rubber)
n
2
Polymers in Common Products
They are everywhere
3
Polymers have non-Newtonian Properties
Long macromolecules: 100,000 x longer than diameter
Entanglements are slow to disentangle
Result: Flexible, tough, strong materials
Sticky &
viscous in
solution
or melted
4
Types of Polymers
Elastomers
Thermoplastics
Polyisoprene,
Neoprene,
Spandex
or Lycra
Silicones
Polystyrene
Polycarbonate
Polyethylene
Nylon
Polyester
Rubbery
Elastic
•Tough
•Flexible
•Softens
with heat
Thermosets
Epoxies
Some urethanes
Cured polyesters
Formaldehyde resins
Strong
Inflexible
Insoluble and
does not soften
with heat
5
Polymer Structure and Properties
• The large size of polymer molecules gives them some unique
physical properties compared with small organic molecules.
• Linear and branched polymers do not form crystalline solids
because their long chains prevent efficient packing in a
crystal lattice.
• Most polymers have crystalline regions and amorphous
regions.
6
Crystallites
• Crystallites: These are ordered crystalline regions of the
polymer that lie in close proximity and are held together by
intermolecular interactions, such as van der Waals forces or
hydrogen bonding.
• Crystalline regions impart toughness to a polymer.
• The greater the crystallinity (i.e., the larger the percentage
of ordered regions), the harder the polymer.
7
Amorphous Regions
• Amorphous regions: These are segments of the polymer
structure where the polymer chains are randomly arranged,
resulting in weaker intermolecular interactions.
• Amorphous regions impart flexibility.
• Branched polymers are generally more amorphous, and
since branching prevents chains from packing closely,
they are also softer.
8
Polymer Transition Temperatures
• Two temperatures, Tg and Tm, often characterize a polymer’s
behavior.
• Glass transition temperature (Tg): temperature at which a hard
amorphous polymer becomes soft.
• Melt transition temperature (Tm): temperature at which
crystalline regions of the polymer melt to become
amorphous.
• More ordered polymers have higher Tm values.
9
Processing Thermoplastics
Rule of Thumb
Amorphous: Tg + 80 °C
Crystalline: Tm + 30 °C
10
Chain-Growth and Step-Growth Polymers
• Synthetic polymers may be classified as either chain-growth
(addition) or step-growth (condensation) polymers.
• Chain-growth polymers are prepared by chain reactions.
• Monomers are added to the growing end of a polymer chain.
• The conversion of vinyl chloride to poly(vinyl chloride) is an
example.
11
Step-Growth Polymers
• Step-growth polymers are formed when monomers
containing two functional groups are joined together and
lose a small molecule such as H2O or HCl.
• In this method, any two reactive molecules can combine, so
that monomer is not necessarily added to the end of a
growing chain.
• Step-growth polymerization is used to prepare polyamides
and polyesters.
12
Molecular Formulae of Polymers
• Polymers generally have high molecular weights ranging
from 10,000 to 1,000,000 g/mol.
• Synthetic polymers are really mixtures of individual polymer
chains of varying lengths, so the reported molecular weight
is an average value based on the average size of the polymer
chain.
• By convention, the written structure of a polymer is
simplified by placing brackets around the repeating unit that
forms the chain.
Figure 30.2
Drawing a polymer in a
shorthand representation
13
Chain-Growth (Addition) Polymers
• Chain-growth polymerization is a chain reaction that
converts an organic starting material, usually an alkene, to a
polymer via a reactive intermediate—a radical, cation, or
anion.
14
Chain growth or Addition polymerizations:
Monomers & polymers
Me
CN
acrylonitrile
MeO
vinyl chloride
methyl methacrylate
n
poly(acrylonitrile)
Orlon
acrylics
MeO2C
Ph
n
Me
Me
F
Cl
PVC
n
F F
Teflon
CO2Et
O
isoprene
buta-1,3-diene
Me
vinyl acetate
CN
ethyl 2-cyanoacrylate
n
n
n
CH3
polypropylene
tetrafluoroethylene
n
O
propylene
F
PMMA
polystyrene
ethylene
F
F
n
CN
F
Cl
O
styrene
F
n
LDPE
HDPE
polyisoprene
polybutadiene
n
OAc
PVAc
MeO2C
n
N
Superglue
15
16
Radical Polymerization
• Radical polymerization of CH2=CHZ is favored by
Z substituents that stabilize a radical by electron
delocalization.
• Each initiation step occurs to put the intermediate
radical on the carbon bearing the Z substituent.
• With styrene as the starting material, the intermediate
radical is benzylic and highly resonance stabilized.
17
Disproportionation
• Chain termination can occur by radical coupling, or by
disproportionation, a process in which a hydrogen atom is
transferred from one polymer radical to another, forming a
new C–H bond on one polymer chain, and a double bond on
the other.
18
Amorphous
Polystyrene
n
Commercial poly(styrene), PS, is a
substantially linear, atactic polymer. Chain
stiffness induced by the phenyl substituent
creates a high Tg (105°C),
Tensile Strength: 45
MPa, Modulus = 3.2 GPa
Elongation 4%
Styrofoam, molded objects such as tableware
(forks, knives and spoons), trays,
videocassette cases. Styrofoam, molded
objects such as tableware (forks, knives and
spoons), trays, videocassette cases.
19
semicrystalline
Teflon
• PTFE – Polytetrafluoroethylene – aka Teflon
long name, simple structure:
• Exceptional resistance to
solvents, great lubricant, nothing sticks to it!
• The fluorine-carbon bonds are very strong, fluorines protect
carbon backbone.
• High melting point 330 C
• High electrical breakdown – artificial muscle.
• Technically a thermoplastic, but hard to process.
• Opaque due to crystallinity
Tensile Strength: 30 MPa
Modulus: 410 MPa
350% elongation
20
amorphous
Polyvinyl Chloride
Cl
n
PVC
No Plasticizer: Rigid Polymer (pipe)
Tensile Strength: 65 MPa, Modulus = 3.5 GPa
Elongation 10%
Saran Wrap, floor tiles, bottles
40 wt% Plasticizer: soft pliable (Tygon tubing)
Tensile Strength: 15 MPa
Elongation 400%
Synthetic leather, shower curtains
21
Chain Branching
• There are two common types of polyethylene—high-density
polyethylene (HDPE) and low-density polyethylene (LDPE).
• HDPE consists of long chains of CH2 groups joined together
in a linear fashion.
• It is strong and hard because the linear chains pack well,
resulting in stronger van der Waals interactions.
• It is used in milk containers and water jugs.
• LDPE consists of long chains with many branches along the
chain.
• The branching prohibits the chains from packing well, so
LDPE has weaker intermolecular interactions, making it a
much softer and pliable material.
• It is used in plastic bags and insulation.
22
Chain Branching
High density polyethylene
Low density polyethylene
23
Branching in LDPE
high pressure
n H2C CH2
peroxides
heat
0.97n
0.01n
Linear mechanism without branching-note primary radical is propagating the polymerization.
n H C CH
2
2
RO
H2C CH2
RO
RO
n
1
2
H
• primary radicals less stable than secondary
• favorable kinetics for six membered ring
transition state for hydrogen abstraction to
generate a more stable, secondary radical
• This gives rise to butyl groups on the
polyethylene chain
H3C
4
2
3
3
2
3
1
CH3
4
1
CH3
4
24
Chain Branching Mechanism
• Branching occurs when a radical on one growing
polyethylene chain abstracts a hydrogen atom from a CH2
group in another polymer chain.
Incorrect mechanism
25
Cationic Polymerization of C=C monomers
• Cationic polymerization is an example of
electrophilic addition to an alkene involving
carbocations.
• Cationic polymerization occurs with alkene
monomers that have substituents capable of
stabilizing intermediate carbocations, such as
alkyl or other electron-donor groups.
• The initiator is an electrophile such as a proton
source or Lewis acid.
• Since cationic polymerization involves
carbocations, addition follows Markovnikov’s
rule to form the more stable carbocation.
• Chain termination occurs by a variety of
pathways, such as loss of a proton to form an
alkene.
26
27
Polymers from Cationic Polymerization
Figure 30.4a
28
Anionic Polymerization
• Alkenes readily react with electron-deficient radicals and
electrophiles, but not (generally) with anions and other
nucleophiles.
• Anionic polymerization takes place only with alkene
monomers that contain electron-withdrawing groups such as
COR, COOR, or CN, which can stabilize an intermediate
negative charge.
• The initiator in anionic polymerization is a strong
nucleophile, such as an organolithium reagent, RLi.
29
30
Anionic Polymerization
• There are no efficient methods of terminating anionic
polymerizations.
• The reaction continues until all the initiator and monomer
have been consumed so that the end of the polymer chain
contains a carbanion.
• Anionic polymerization is called living polymerization
because polymerization will begin again if more monomer is
added at this stage.
• To terminate anionic polymerization an electrophile such as
H2O or CO2 must be added.
• Diene polymerizations, polystyrene
31
Polymers from Anionic Polymerization
Figure 30.4b
NO!!!!!
Water
is the
initiator
32
Copolymers
• Copolymers are polymers prepared by joining two or
more monomers (X and Y) together.
33
Structure of Copolymers
• The structure of a copolymer depends on the relative reactivity
of X and Y, as well as the conditions used for polymerization.
• Several copolymers are commercially important:
• Saran food wrap is made from vinyl chloride and vinylidene
chloride.
• Automobile tires are made from 1,3-butadiene and styrene.
34
ABS:
–High strength, dimensional stability, impact resistance
–Poor UV resistance
C N
–Telephones, PC housing & keyboards, ...
Grafted with polybutadiene
35
Anionic Polymerization of Epoxides
• Anionic polymerization of epoxides can be used to form
polyethers.
• For example, the ring opening of ethylene oxide with OH as
initiator affords an alkoxide nucleophile which propagates the
chain by reacting with more ethylene oxide.
• Polymerization of ethylene oxide forms poly(ethylene glycol),
PEG, a polymer used in lotions and creams.
36
Anionic Polymerization of Epoxides
• Under anionic conditions, the ring opening follows an SN2
mechanism.
• Thus, the ring opening of an unsymmetrical epoxide occurs at
the more accessible, less substituted carbon.
37
Polymer Stereochemistry
• Polymers prepared from monosubstituted alkene monomers
(CH2=CHZ) can exist in three different configurations:
isotactic, syndiotactic, and atactic.
38
Ziegler-Natta Catalysts (Coordination)
• The more regular arrangement of Z substituents makes isotactic
and syndiotactic polymers pack together better, making the
polymer stronger and more rigid.
• Chains of atactic polymer tend to pack less closely together,
resulting in a lower melting point and a softer polymer.
• Radical polymerizations often afford atactic polymers.
• Reaction conditions can greatly affect the stereochemistry of
the polymer formed.
• The use of Ziegler-Natta catalysts permits easy control of
polymer stereochemistry, with the formation of isotactic,
syndiotactic, or atactic polymers dependent on the catalyst
used.
• Most Ziegler-Natta catalysts consist of an organoaluminum
compounds such as (CH3CH2)2AlCl or TiCl4.
39
Polypropylene
semicrystalline
n
H3C
Tensile Strength: 31-41 MPa, Modulus = 1.2-1.7 Gpa
Elongation 100-600%
Living Hinge
40
• Mechanistic details are not known with certainty.
41
Natural Rubbers
• Natural rubber is a terpene composed of repeating isoprene
units, in which all the double bonds have the Z configuration.
• Since natural rubber is a hydrocarbon, it is water insoluble,
making it useful for water proofing.
• The Z double bonds cause bends and kinks in the polymer
chain, making it a soft material.
42
Gutta-Percha Rubber
• The polymerization of isoprene under radical conditions
forms a stereoisomer of natural rubber called gutta-percha, in
which all the double bonds have the E configuration.
• Gutta-percha is also naturally occurring, but is less common
than its Z stereoisomer.
• Polymerization of isoprene with a Ziegler-Natta catalyst forms
natural rubber with all the double bonds having the desired Z
configuration.
43
Polymer Stereochemistry
• Natural rubber is too soft to be used in most applications.
• When natural rubber is stretched, the chains become
elongated and slide past each other until the material pulls
apart.
• In 1939, Charles Goodyear discovered that mixing hot rubber
with sulfur produced a stronger more elastic material.
• This process is called vulcanization.
• Vulcanization results in cross-linking of the hydrocarbon
chains by disulfide bonds.
• When the polymer is stretched, the chains no longer can slide
past each other, and tearing does not occur.
• Vulcanized rubber is an elastomer, a polymer that stretches
when stressed but then returns to its original shape when the
stress is alleviated.
44
Elastomers
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
n
Cl
Cl
Polychloroprene
Neoprene
Cl
O
O
n
Poly-cis-isoprene
O
n
O
N
H
N
H
N
H
n
n = 40
H
N
H
N
O
O
N
H
O
m
Spandex or Lycra
Poly-1,3-butadiene
Me
Me
Si
n
m
O
n
polydimethylsiloxane
Block copolymer elastomers
45
Vulcanized Rubber
Figure 30.5
No, common mistake!!!!!
WRONG!!!!!
46
Vulcanization of dienes with sulfur
Poly-1,3-butadiene
S
S
S
S
Allylic sites react with sulfur by alder-ene chemistry
47
High entropy
Elasticity of polymers
Low entropy
At temperatures above a polymers glass transition
temperature it is a rubber
Under stress, the polymer chains elongate, but are
held in check by entanglements or crosslinks that
prevent the bulk polymer from breaking.
Entropy spring
48
Synthetic Rubber
• The degree of cross-linking affects the rubber’s properties.
• Harder rubber used for automobile tires has more crosslinking than the softer rubber used for rubber bands.
• Other synthetic rubbers can be prepared by the
polymerization of different 1,3-dienes using Ziegler-Natta
catalysts.
• For example, polymerization of 1,3-butadiene affords
(Z)-poly(1,3-butadiene).
• Polymerization of 2-chloro-1,3-butadiene yields neoprene,
a polymer used in wet suits and tires.
NO!!
Free radical
49
Step-Growth Polymers
• Step-growth polymers are formed when monomers containing
two functional groups come together with loss of a small
molecule such as H2O or HCl.
• Commercially important step-growth polymers include:
• Polyamides (can also be chain growth)
• Polyesters
• Polyurethanes
• Polycarbonates
• Epoxy resins
50
Polyamides
• Nylons are polyamides formed from step-growth
polymerization.
• Nylon 6,6 can be prepared by the reaction of a diacid chloride
with a diamine, or by heating adipic acid and 1,6diaminohexane.
• A BrØnsted-Lowry acid–base reaction forms a diammonium
salt which loses H2O at high temperature.
51
Nylon 6
• Nylon 6 is another polyamide which is made by heating an
aqueous solution of -caprolactam.
• The seven-membered ring of the lactam is ring opened to
form 6-aminohexanoic acid, the monomer that reacts with
more lactam to form the polyamide chain.
52
semicrystalline
Nylon 6,6:
–Excellent wear resistance & slick surface
–Poor dimensional stability & high cost
–Gear, engine fan
Strengths:
Good Toughness & Strength
Good Chemical resistance
O
N
H
H
N
n
O
Nylon 6,6
mp 265 °C
Limitations:
tg 50 °C
Strong acidic environments
Areas where moisture absorption is of concern
-20% strength with humid environment
Areas experiencing high operating temperatures
Nylon 6: Tensile yield 76 MPa; Tensile modulus 1.4 GPa, elongation 250%
Nylon 6,6: Tensile yield 80 MPa; Tensile modulus 2 GPa, elongation 200%
Interchangable for most applications
53
Kevlar
• Kevlar is a polyamide formed from
terephthalic acid and 1,4-diaminobenzene.
• The aromatic rings of the polymer
backbone make the chains less flexible,
resulting in a very strong material.
• Kevlar is light in weight compared to other
materials of similar strength.
• It is used for bulletproof vests, army
helmets and protective clothing used by
firefighters.
54
Polyesters
• Polyesters are formed using nucleophilic acyl substitution
reactions.
• For example, the reaction of terephthalic acid and ethylene
glycol forms polyethylene terephthalate (PET), a polymer
commonly used in plastic soda bottles.
• It is also sold as Dacron, a lightweight and durable material
used in textile manufacturing.
55
semicrystalline
Polyester films:
O
O
O
O
Mylar = PETE Film
n
Tg = 80 °C
Dacron = PETE
fiber
Tm = 260 °C
Tensile Strength: 48-72 MPa, Modulus = 2.7-4.1 Gpa
50-300% elongation
O
O
O
Teonex = PEN Film
n
O
Tg = 120 °C
Tm = 262 °C
56
Biodegradible Plastic
• Although PET is a very stable material, some polyesters are
more readily hydrolyzed to carboxylic acids and alcohols in
aqueous medium, making them useful in applications where
slow degradation is useful.
• Copolymerization of glycolic acid and lactic acid forms a
copolymer used by surgeons in dissolving sutures.
57
Urethanes
• A urethane (also called a carbamate) is a compound that
contains a carbonyl group bonded to both an OR group and
an NHR or NR2 group.
• Urethanes are prepared by the nucleophilic addition of an
alcohol to the carboxyl group of an isocyanate, RN=C=O.
58
Polyurethanes
• Polyurethanes are formed by the reaction of a diisocyanate
and a diol.
• A well-known polyurethane that illustrates how the
macroscopic properties of a polymer depend on its structure
at the molecular level is Spandex.
• At the molecular level, it has rigid regions that are joined
together by soft flexible segments.
• Spandex is routinely used in both men’s and women’s active
wear.
59
Polycarbonates
• A polycarbonate is a compound that contains a carbonyl
group bonded to two OR groups.
• Carbonates can be prepared by the reaction of phosgene
(Cl2C=O) with two equivalents of an alcohol (ROH).
• Polycarbonates are formed from phosgene and a diol.
• The most widely used polycarbonate is Lexan, used in bike
helmets, goggles, and bulletproof glass.
60
Polycarbonates
Excellent clarity
Excellent toughness
Good heat resistance
Excellent electrical properties
Intrinsic flame-retardancy
Excellent strength
Hot water = gradual embrittlement
Crazed surface with exposure to organic solvents
61
Epoxy Resins
• Epoxy resins are the material of which “epoxy glue” is
comprised.
• Epoxy resins consist of two components: A fluid prepolymer
composed of short polymer chains with reactive epoxides on
each end, and a hardener, usually a diamine or triamine that
ring opens the epoxides and cross-links the chains together.
• The prepolymer is formed by reacting two different functional
monomers, bisphenol A and epichlorohydrin.
62
Formation of the Fluid Prepolymer
• Nucleophilic attack by the phenolic OH groups on the
strained epoxide ring affords an alkoxide that displaces Cl by
an intramolecular SN2 reaction, forming a new epoxide.
• Ring opening with a second nucleophile gives a 2° alcohol.
• When bisphenol A is treated with excess epichlorohydrin, this
step-wise process continues until all the phenolic OH groups
have been used in ring-opening reactions, leaving epoxy
groups on both ends of the polymer chains.
• This constitutes the fluid prepolymer.
63
Formation of an Epoxy Resin
Me
OH
HO
O
(n + 2) Cl
(n + 1)
O
O
O
O
O
O
(n + 2) base
Me
Me
Me
n
Me
Me
Me
epichlorohydrin
bisphenol A
Epoxy pre-polymer
Me
O
O
Me
O
O
O
n
Me
Me
Me
O
O
O
Epoxy
OH
Me
HO
O
O
H
N
O
Me
Me
O
R = Me
x = 1,2
x = 4,5
x = 32
n
Me
R
Me
x NH2
Linear Cured Epoxy
Jeffamine D230
Jeffamine D400
Jeffamine D2000
catalyst
x
m
R
Me
H2N
Me
H
N
64
“Infinite” network
OH
N
Me
Me
O
O
O
Me
O
N
O
O
OH
Me
OH
O
OH
O
O
O
HO
OH
Me
O
Me
N
N
Me
OH
Me
Me
Me
O
HO
O
HO
Me
Me
Me
O
O
O
O
O
Me
Me
Me
O
O
One macromolecule
N
OH
OH
HO
O
OH
O
O
N
O
O
O
Me
N
Me
Me
Me
OH
O
O
N
Epoxy coats inside of steel cans to prevent heavy metals from contaminating food
65
Synthesis of Bakelite
Figure 30.7
66
Plasticizers
• If a polymer is too stiff and brittle to be used in practical
applications, low molecular weight compounds called
plasticizers can be added to soften the polymer and give it
flexibility.
• The plasticizer interacts with the polymer chains, replacing
some of the intermolecular interactions between the polymer
chains.
• This lowers the crystallinity of the polymer, making it more
amorphous and softer.
O
O
O
O
O
O
O
O
bis(2-methylhexyl) adipate
bis(2-methylhexyl) phthalate
phthalate plasticizer
Not new car smell
67
H3C H3C
CH3
H3C Si O Si
New car smell
O
H3C
styrene
CH3
H3C
CH3
p-cymene
O
Si
O
Si
CH3
CH3
CH3
octamethylcyclotetrasiloxane
H 3C
H3C
H3C
OH
CH3
CH3
CH3
CH3
2,6-di-tert-butyl-4-methylphenol
H 3C
H3C
Si
O
O
CH3
Si
O
CH3
Si
H3C CH
3
hexamethylcyclotrisiloxane
BHT
N
CH3
O
N-methylpyrrolidin-2-one
(E)-tetradec-5-ene
dodecane
68
Plasticizers—Dibutyl Phthalate
• Dibutyl phthalate is a plasticizer added to poly(vinyl
chloride) used in vinyl upholstery and garden hoses.
• Since plasticizers are more volatile than the high molecular
weight polymers, they slowly evaporate eventually making
the polymer brittle and easily cracked.
• Plasticizers like dibutyl phthalate that contain hydrolyzable
functional groups are also slowly degraded by chemical
reactions.
69
Environmental Impact of Polymers
• Polymer synthesis and disposal have a tremendous impact
on the environment, and have created two central issues:
• Where do polymers come from?
• What raw materials are used for polymer synthesis
and what environmental consequences result from
their manufacture?
• What happens to polymers once they are used?
• How does polymer disposal affect the environment,
and what can be done to minimize its negative
impact?
70
Where do Polymers Come From?
• Until recently, the feedstock for all polymer synthesis has
been petroleum.
• The monomers of virtually all polymer syntheses are
made from crude oil, a nonrenewable raw material.
• For example, nylon 6,6 is prepared industrially from
adipic acid and 1,6-diaminohexane, both of which
originate from benzene, a product of petroleum refining.
Figure 30.8
Synthesis of adipic acid and
1,6-diaminohexane for
nylon 6,6 synthesis
71
Problems with Polymer Synthesis
• The adipic acid synthesis of nylon 6,6 has other problems.
• The use of benzene (a carcinogen and liver toxin) is
undesirable, particularly in the large quantities demanded
by large scale industrial reactions.
• The required oxidation with HNO3 in step 3 produces N2O
as a by-product.
• N2O depletes ozone in the stratosphere.
• It also absorbs thermal energy from the earth surface
like CO2, and may thus contribute to global warming.
72
Green Polymer Synthesis
• The negative environmental impact of polymer synthesis has
prompted the development of Green Polymer Syntheses—the
use of more environmentally benign methods to synthesize
polymers.
• To date, green polymer synthesis has been approached in a
variety of ways:
• Using starting materials that are derived from renewable
sources, rather than petroleum.
• Using safer less toxic reagents that form fewer byproducts.
• Carrying out reactions in the absence of solvent or in
aqueous solution (instead of an organic solvent).
73
Examples of Green Polymer Synthesis
• Chemists at Michigan State University have devised a twostep synthesis of adipic acid (used to make nylon) from
glucose.
• The synthesis uses a genetically altered E. coli strain (called
a biocatalyst) to convert D-glucose to (2Z,4Z)-2,4-hexadienoic
acid, which is then hydrogenated to adipic acid.
74
Green Polyester Synthesis
• Sorona, DuPont’s trade name for polypropylene terephthalate,
can now be made at least in part from glucose derived from a
plant source such as corn.
• A biocatalyst converts D-glucose to 1,3-propanediol, which
forms polypropylene terephthalate on reaction with
terephthalic acid.
Figure 30.9 A swimsuit made (in part) from corn—The synthesis of
Poly(trimethylene terephthalate) from 1,3-propanediol derived from corn
75
Avoiding Solvent Use
• Other approaches have concentrated on using less
hazardous reagents and avoiding solvents.
• Lexan can now be prepared by using bisphenol A with
diphenyl carbonate in the absence of solvent.
• This avoids the use of phosgene, an acutely toxic reagent.
76
Problems with Polymer Disposal
•
•
•
The same desirable characteristics that make polymers
popular materials for consumer products—durability,
strength, and lack of reactivity—also contribute to
environmental problems.
Because polymers do not degrade readily, billions of
pounds of them end up in landfills every year.
Two solutions to address the waste problem are:
1. Recycling existing polymer types to make new materials
2. Using biodegradable polymers that will decompose in a
finite and limited time span.
77
Polymer Recycling
• Currently, ~23% of all plastics are recycled in the United
States.
• Although thousands of different synthetic polymers have
now been prepared, six compounds called the “Big Six,”
account for 76% of the synthetic polymers produced in the
U.S. each year.
• Each polymer is assigned a recycling code (1–6) that
indicates its ease of recycling; the lower the number, the
easier it is to recycle.
• Recycling begins with sorting plastics by type, shredding the
plastics into small chips, and washing the chips to remove
adhesives and labels.
• After the chips are dried and any metal caps or rings are
removed, the polymer chips are melted and molded for
reuse.
78
79
Chemical Polymer Recycling
• An alternative recycling process is to re-convert polymers
back to the monomers from which they were made, a
process that has been successful with acyl compounds that
contain C–O or C–N bonds in the polymer backbone.
• For example, heating PET with CH3OH cleaves the esters of
the polymer chain to give ethylene glycol and dimethyl
terephthalate.
• These monomers can serve as starting materials for more
PET.
• Similar treatment of discarded nylon 6 polymer with NH3
cleaves the polyamide backbone, forming -caprolactam,
which can be purified and re-converted to nylon 6.
80
Examples of Chemical Polymer Recycling
81
Biodegradable Polymers
• Another solution to the accumulation of waste polymers in
landfills is to design biodegradable polymers.
• A biodegradable polymer is a polymer that can be degraded
by microorganisms—bacteria, fungi, or algae—naturally
present in the environment.
• Several biodegradable polyesters have now been developed
[e.g., polyhydroxyalkanoates (PHAs), which are polymers of
3-hydroxybutyric acid or 3-hydroxyvaleric acid].
82
Biodegradable Polymers—PHAs
• The two most common PHAs are polyhydroxybutyrate (PHB)
and a copolymer of polyhydroxybutyrate and
polyhydroxyvalerate (PHBV).
• PHAs can be used as films, fibers, and coatings for hot
beverage cups made of paper.
• Bacteria in the soil readily degrade PHAs, and in the
presence of oxygen, the final degradation products are CO2
and H2O.
83
PHAs
• An additional advantage of the PHAs is the polymers can
be produced by fermentation.
• Certain bacteria produce PHAs for energy storage when
they are grown in glucose solution in the absence of
certain nutrients.
• The polymer forms as discrete granules within the
bacterial cell.
• These are removed by extraction to give a white powder
that can be melted and modified into a variety of different
products.
84
Biodegradable Polymers
• Biodegradable polyamides have also been prepared from
amino acids (e.g., aspartic acid can be converted to
polyaspartate, abbreviated TPA).
• It is a commonly used alternative to poly(acrylic acid),
which is used to line pumps and boilers of wastewater
treatment facilities.
85
O
O
n
O
O
n
n
Cl
n
n/20
n
CH3
n
86