1 polymers chapter 30 light weight flexible easily processable transparent (sometimes) strong...
TRANSCRIPT
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Polymers
Chapter 30
Light weightFlexibleEasily processableTransparent (sometimes)StrongElasticCheap
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Polymers
• Macromolecules > 10,000 grams/mole (e.g. proteins, DNA)
poly = many
mer = units or pieces
Poly-cis-isoprene
n
1000 g/mole
n
Polyisoprene(natural rubber)
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Polymers in Common Products
They are everywhere
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Polymers have non-Newtonian Properties
Long macromolecules: 100,000 x longer than diameterEntanglements are slow to disentangleResult: Flexible, tough, strong materials
Sticky & viscous in solution or melted
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Types of Polymers
Elastomers Thermoplastics Thermosets
RubberyElastic
Polyisoprene,Neoprene,Spandex or LycraSilicones
PolystyrenePolycarbonatePolyethyleneNylonPolyester
EpoxiesSome urethanesCured polyestersFormaldehyde resins
•Tough•Flexible•Softens with heat
StrongInflexibleInsoluble and does not soften with heat
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• 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.
Polymer Structure and Properties
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• 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.
Crystallites
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• 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.
Amorphous Regions
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• 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.
Polymer Transition Temperatures
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Processing Thermoplastics
Rule of ThumbAmorphous: Tg + 80 °CCrystalline: Tm + 30 °C
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• 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.
Chain-Growth and Step-Growth Polymers
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• 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.
Step-Growth Polymers
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• 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 30.2Drawing a polymer in a
shorthand representation
Molecular Formulae of Polymers
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• 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.
Chain-Growth (Addition) Polymers
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Chain growth or Addition polymerizations: Monomers & polymers
styrene
acrylonitrile
CNO
MeO
Me
methyl methacrylate
Cl
vinyl chloride
FF
F F
tetrafluoroethylene
MeO
OMe
CO2Et
CN
ethyl 2-cyanoacrylatevinyl acetatebuta-1,3-dieneisoprenepropylene ethylene
CN
PhMeMeO2C
Cl F F
F Fn
n nn
n
poly(acrylonitrile)Orlonacrylics polystyrene PMMA
PVC Teflon
CH3
n
OAcn
NMeO2C
n
SupergluePVAc
nn
n
polypropylene LDPEHDPE
polyisoprenepolybutadiene
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• 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.
Radical Polymerization
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• 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.
Disproportionation
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Polystyrene
Tensile Strength: 45 MPa, Modulus = 3.2 GPaElongation 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.
n
Commercial poly(styrene), PS, is a substantially linear, atactic polymer. Chainstiffness induced by the phenyl substituentcreates a high Tg (105°C),
Amorphous
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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
semicrystalline
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Polyvinyl ChlorideCl
n
No Plasticizer: Rigid Polymer (pipe) Tensile Strength: 65 MPa, Modulus = 3.5 GPaElongation 10%Saran Wrap, floor tiles, bottles
40 wt% Plasticizer: soft pliable (Tygon tubing)Tensile Strength: 15 MPaElongation 400%Synthetic leather, shower curtains
amorphous
PVC
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• 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.
Chain Branching
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Chain Branching
Low density polyethyleneHigh density polyethylene
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H2C CH2
high pressure
peroxidesheat
0.97n 0.01n
HH3C
12
3
4
CH3
12
3
4
CH3
12
3
4
n
ROROH2C CH2
H2C CH2
nRO
n
Linear mechanism without branching-note primary radical is propagating the polymerization.
• 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
Branching in LDPE
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• Branching occurs when a radical on one growing polyethylene chain abstracts a hydrogen atom from a CH2 group in another polymer chain.
Chain Branching Mechanism
Incorrect mechanism
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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.
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Figure 30.4a
Polymers from Cationic Polymerization
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• 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.
Anionic Polymerization
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• 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
Anionic Polymerization
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Figure 30.4b
Polymers from Anionic Polymerization
NO!!!!!
Water is the initiator
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• Copolymers are polymers prepared by joining two or more monomers (X and Y) together.
Copolymers
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• 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.
Structure of Copolymers
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ABS:–High strength, dimensional stability, impact resistance–Poor UV resistance–Telephones, PC housing & keyboards, ... C N
Grafted with polybutadiene
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• 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.
Anionic Polymerization of Epoxides
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• 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.
Anionic Polymerization of Epoxides
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• Polymers prepared from monosubstituted alkene monomers (CH2=CHZ) can exist in three different configurations: isotactic, syndiotactic, and atactic.
Polymer Stereochemistry
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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.
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Polypropylene
Tensile Strength: 31-41 MPa, Modulus = 1.2-1.7 GpaElongation 100-600%
Living Hinge
semicrystalline
H3Cn
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• Mechanistic details are not known with certainty.
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• 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.
Natural Rubbers
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• 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.
Gutta-Percha Rubber
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• 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.
Polymer Stereochemistry
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Elastomers
Polychloroprene
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cln
Neoprene
n
Poly-cis-isoprene Poly-1,3-butadiene
nm
OO
O
NH
NH
NH
OHN
O
HN
NH
O
O
Spandex or Lycra
n
n = 40
mn
Block copolymer elastomers
SiO
Me Me
n
polydimethylsiloxane
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Figure 30.5
Vulcanized Rubber
No, common mistake!!!!!
WRONG!!!!!
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Poly-1,3-butadiene
SS
S
S
Vulcanization of dienes with sulfur
Allylic sites react with sulfur by alder-ene chemistry
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Elasticity of polymers
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
High entropy
Low entropy
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• The degree of cross-linking affects the rubber’s properties.
• Harder rubber used for automobile tires has more cross-linking 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.
Synthetic Rubber
NO!!Free radical
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• 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
Step-Growth Polymers
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• 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 BrØnsted-Lowry acid–base reaction forms a diammonium salt which loses H2O at high temperature.
Polyamides
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• 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.
Nylon 6
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Nylon 6,6:–Excellent wear resistance & slick surface–Poor dimensional stability & high cost–Gear, engine fan
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%
Limitations:
Strong acidic environments
Areas where moisture absorption is of concern
-20% strength with humid environment
Areas experiencing high operating temperatures
Strengths:Good Toughness & StrengthGood Chemical resistance
Interchangable for most applications
semicrystalline
NH
HN
O
O n
Nylon 6,6
mp 265 °Ctg 50 °C
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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.
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• 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.
Polyesters
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Polyester films:
O
O
O
O n
Mylar = PETE Film
Dacron = PETE fiber
n
O
O
O O
Teonex = PEN Film
Tg = 80 °C Tm = 260 °C
Tg = 120 °C Tm = 262 °C
Tensile Strength: 48-72 MPa, Modulus = 2.7-4.1 Gpa50-300% elongation
semicrystalline
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• 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.
Biodegradible Plastic
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• 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.
Urethanes
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• 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.
Polyurethanes
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• 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.
Polycarbonates
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Polycarbonates
Hot water = gradual embrittlementCrazed surface with exposure to organic solvents
Excellent clarity
Excellent toughness
Good heat resistance
Excellent electrical properties
Intrinsic flame-retardancy
Excellent strength
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• 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.
Epoxy Resins
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• 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.
Formation of the Fluid Prepolymer
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Formation of an Epoxy Resin
OHHO
MeMe
bisphenol A
Cl O(n + 2)(n + 1)
epichlorohydrin
(n + 2) base
OO
MeMe
MeO OO
MeMe
O
n
Epoxy pre-polymer
OO
MeMe
MeO OO
MeMe
O
n
H2N
Me
O
R
NH2x
R = Mex = 1,2 Jeffamine D230x = 4,5 Jeffamine D400x = 32 Jeffamine D2000
OO
MeMe
MeOO
MeMe
OH
n
HO HN
Me
O
R
HN
x
m
Linear Cured Epoxy
catalyst
Epoxy
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Me
Me
OO O
OH
Me
Me
O
OH
OH
N
O
O
NN O
O N
Me
Me
O
O
O
HO
Me
Me
O
HO
OH
Me
Me
OO O
OH
Me
Me
OOH
OH
Me
MeO
OO
OH
MeMe
O
OH
HO
N
NN O
ON
O
O
Me
Me
O
O
O
HO
OH
Me
Me
O
“Infinite” network
One macromolecule
Epoxy coats inside of steel cans to prevent heavy metals from contaminating food
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Figure 30.7
Synthesis of Bakelite
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• 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.
Plasticizers
O
O
O
O
phthalate plasticizer
O
O
O
O
bis(2-methylhexyl) phthalatebis(2-methylhexyl) adipate
Not new car smell
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New car smellO
SiO
Si
O
SiO
Si
H3CCH3
H3C
H3C
H3CCH3 CH3
CH3
octamethylcyclotetrasiloxane
Si
OSi
O
SiO
H3C CH3
CH3
CH3
H3C
H3C
hexamethylcyclotrisiloxane
H3C
CH3
CH3
p-cymene
NCH3
O
N-methylpyrrolidin-2-one
styrene
(E)-tetradec-5-ene
dodecane
CH3
OH CH3CH3
CH3
H3CH3C
H3C
2,6-di-tert-butyl-4-methylphenol
BHT
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• 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.
Plasticizers—Dibutyl Phthalate
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• 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?
Environmental Impact of Polymers
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• 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 30.8Synthesis of adipic acid and
1,6-diaminohexane fornylon 6,6 synthesis
Where do Polymers Come From?
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• 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.
Problems with Polymer Synthesis
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• 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 by-products.
• Carrying out reactions in the absence of solvent or in aqueous solution (instead of an organic solvent).
Green Polymer Synthesis
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• Chemists at Michigan State University have devised a two-step 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.
Examples of Green Polymer Synthesis
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• 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 ofPoly(trimethylene terephthalate) from 1,3-propanediol derived from corn
Green Polyester Synthesis
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• 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.
Avoiding Solvent Use
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• 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.
Problems with Polymer Disposal
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• 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.
Polymer Recycling
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• 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.
Chemical Polymer Recycling
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Examples of Chemical Polymer Recycling
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• 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].
Biodegradable Polymers
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• 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.
Biodegradable Polymers—PHAs
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• 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.
PHAs
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• 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.
Biodegradable Polymers
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n
CH3
n
Cl
n
n
n n/20
O
O
O
On