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Università degli Studi di Salerno
Polimeri
Prof. Pasquale Longo
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Polymers
Introduction
•  Polymers are large organic molecules comprised of
repeating units called monomers that are covalently
bonded together.
•  P o l y m e r s c a n b e n a t u r a l l y o c c u r r i n g ( e . g .
polysaccharides and proteins) or synthesized in a
laboratory (synthetic).
•  Polymerization is the joining together of monomers to
make polymers.
•  Polymers prepared by the polymerization of a single
monomer are called homopolymers.
•  Numerous consumer products are made from
synthetic polymers.
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Polymers
Introduction
Figure 30.1 Polymers in some common consumer products
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Polymers
Introduction
•  Synthetic polymers may be classified as either chaingrowth (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.
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Polymers
Introduction
•  Step-growth polymers are formed when monomers
containing two functional groups come 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.
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Polymers
Introduction
•  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
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Polymers
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:
radical, cation or anion.
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Polymers
Chain-Growth Polymers—Radical Polymerization
•  The initiator is often a peroxy radical (RO•).
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Polymers
Chain-Growth Polymers—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.
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Polymers
Chain-Growth Polymers—Radical Polymerization
•  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.
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Polymers
Chain-Growth Polymers—Radical Polymerization
•  Several monomers can be used in radical
polymerizations
Monomers used in radical
polymerization reactions
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Polymers
Chain-Growth Polymers—Radical Polymerization
Chain Branching
•  There are two common types of polyethylene—highdensity 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
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material. It is used in plastic bags and insulation.
Polymers
Chain-Growth Polymers—Radical Polymerization
Chain Branching
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Polymers
Chain-Growth Polymers—Radical Polymerization
Chain Branching
•  Branching occurs when a radical on one growing
polyethylene chain abstracts a hydrogen atom from a
CH2 group in another polymer chain.
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Polymers
Chain-Growth Polymers—Ionic Polymerization
•  Chain-growth polymerization can also occur by way of
cationic or anionic intermediates.
•  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 groups 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
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of pathways, such as loss of a proton to form an alkene.
Polymers
Chain-Growth Polymers—Ionic Polymerization
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Polymers
Chain-Growth Polymers—Ionic 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.
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Polymers
Chain-Growth Polymers—Ionic Polymerization
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Polymers
Chain-Growth Polymers—Ionic 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.
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Polymers
Chain-Growth Polymers—Ionic Polymerization
Figure 30.4 Common polymers formed by ionic chain-growth polymerization
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Polymers
Chain-Growth Polymers—Ionic Polymerization
Figure 30.4 continued
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Polymers
Chain-Growth Polymers—Ionic Polymerization
Copolymers
Copolymers are polymers prepared by joining two or
more monomers (X and Y) together.
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Polymers
Chain-Growth Polymers—Ionic Polymerization
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.
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Polymers
Chain-Growth Polymers—Ionic Polymerization
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.
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Polymers
Chain-Growth Polymers—Ionic Polymerization
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.
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Polymers
Ziegler-Natta Catalysts and Polymer Stereochemistry
•  Polymers prepared from monosubstituted alkene monomers
(CH2=CHZ) can exist in three different configurations:
isotactic, syndiotactic, and atactic.
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Polymers
Ziegler-Natta Catalysts and Polymer Stereochemistry
•  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.
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Polymers
Ziegler-Natta Catalysts and Polymer Stereochemistry
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Polymers
Natural and Synthetic 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.
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Polymers
Natural and Synthetic Rubbers
•  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.
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Polymers
Natural and Synthetic Rubbers
•  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.
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Polymers
Natural and Synthetic Rubbers
Figure 30.5
Vulcanized rubber
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Polymers
Natural and Synthetic Rubbers
•  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), and polymerization of 2chloro-1,3-butadiene yields neoprene, a polymer used in
wet suits and tires.
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Polymers
Step-Growth Polymers—Condensation 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
Polyesters
Polyurethanes
Polycarbonates
Epoxy resins
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Polymers
Step-Growth Polymers—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
1,6-diaminohexane. A Br Ø nsted-Lowry acid-base
reaction forms a diammonium salt which loses H2O at
high temperature.
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Polymers
Step-Growth Polymers—Polyamides
•  Nylon 6 is another polyamide which is made by
heating an aqueous solution of ε-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.
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Polymers
Step-Growth Polymers—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.
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Polymers
Step-Growth Polymers—Polyesters
•  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 show degradation is
useful.
•  Copolymerization of glycolic acid and lactic acid
forms a copolymer used by surgeons in dissolving
sutures.
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Polymers
Step-Growth Polymers—Polyurethanes
•  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.
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Polymers
Step-Growth Polymers—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 .
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Polymers
Step-Growth Polymers—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.
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Polymers
Step-Growth Polymers—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.
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Polymers
Step-Growth Polymers—Epoxy Resins
•  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.
•  W h e n b i s p h e n o l A i s t r e a t e d w i t h e x c e s s
epichlorohydrin, this step-wise process continues until
all the phenolic OH groups have been used in ringopening reactions, leaving epoxy groups on both ends
of the polymer chains. This constitutes the fluid
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copolymer.
Polymers
Step-Growth Polymers—Epoxy Resins
•  When the prepolymer is mixed with a di- or triamine (the hardener),
the reactive epoxides can be ring opened by the nucleophilic amino
groups to cross-link polymer chains together, causing the polymer
to harden.
Figure 30.6
Formation of an epoxy resin
from a prepolymer and a
hardening agent
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Polymers
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.
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Polymers
Polymer Structure and Properties
•  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.
•  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 are64also
softer.
Polymers
Polymer Structure and Properties
•  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.
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Polymers
Polymer Structure and Properties
•  Thermoplastics are polymers that can be melted and
then molded into shapes that are retained when the
polymer is cooled.
•  They have high Tg values and are hard at room temperature,
but heating causes individual polymer chains to slip past each
other, causing the material to soften.
•  Thermosetting polymers are complex networks of crosslinked polymers.
•  They are formed by chemical reactions that occur when
monomers are heated together to form a network of covalent
bonds.
•  They cannot be re-melted to form a liquid phase because
covalent bonds hold the network together. An example is
Bakelite, formed from phenol and formaldehyde.
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Polymers
Polymer Structure and Properties
Figure 30.7
The synthesis of Bakelite from
phenol and formaldehyde
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Polymers
Polymer Structure and Properties—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, making it more amorphous and
softer.
•  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 making the polymer
brittle and easily cracked.
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Polymers
Polymer Structure and Properties—Plasticizers
•  Plasticizers like dibutyl phthalate that contain
hydrolyzable functional groups are also slowly
degraded by chemical reactions.
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Polymers
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.
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Polymers
The Problems with Polymer Synthesis
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
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Polymers
The Problems with Polymer Synthesis
Where do Polymers Come From?
•  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.
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Polymers
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).
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Polymers
Green Polymer Synthesis—Some Examples
•  Chemists at Michigan State University have devised a
two-step synthesis of adipic acid 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.
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Polymers
Green Polymer Synthesis—Some Examples
•  S o r o n a , D u P o n t s t r a d e n a m e f o r p o l y p r o p y l e n e
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
polypropylene terephthalate from 1,3-propanediol derived from corn
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Polymers
Green Polymer Synthesis—Some Examples
•  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.
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Polymers
The 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.
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Polymers
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
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are melted and molded for reuse.
Polymers
Polymer Recycling
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Polymers
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 εcaprolactam, which can be purified and re-converted to
nylon 6.
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Polymers
Examples of Chemical Polymer Recycling
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Polymers
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].
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Polymers
Biodegradable Polymers
•  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.
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Polymers
Biodegradable Polymers
•  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.
•  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.
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