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"I just want to say one word toyou -- just one word -- 'plastics.'"
Advice to Dustin Hoffman'scharacter in The Graduate
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Polymers: Introduction
Polymer: High molecular weight molecule madeup of a small repeat unit (monomer).
A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A
Monomer: Low molecular weight compound that
can be connected together to give a poymer Oligomer: Short polymer chain
Copolymer: polymer made up of 2 or moremonomers
Random copolymer: A-B-B-A-A-B-A-B-A-B-B-B-A-A-B
Alternating copolymer: A-B-A-B-A-B-A-B-A-B-A-B-A-B
Block copolymer: A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B
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Types of Polymers
Polymer Classifications
Thermoset: cross-linked polymer that cannot bemelted (tires, rubber bands)
Thermoplastic: Meltable plastic
Elastomers: Polymers that stretch and then return totheir original form: often thermoset polymers
Thermoplastic elastomers: Elastic polymers that canbe melted (soles of tennis shoes)
Polymer Families Polyolefins: made from olefin (alkene) monomers
Polyesters, Amides, Urethanes, etc.: monomers linkedby ester, amide, urethane or other functional groups
Natural Polymers: Polysaccharides, DNA, proteins
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Common PolyolefinsMonomer Polymer
Ethylene
H3C CH3
nRepeat unit
Polyethylene
CH3CH3
n
CH3 CH3 CH3 CH3 CH3 CH3CH3
Propylene
Polypropylene
PhCH3
n
Ph Ph Ph Ph Ph PhPhStyrene
Polystyrene
Cl
CH3
nCl Cl Cl Cl Cl ClCl
Vinyl ChloridePoly(vinyl chloride)
F2C CF2
Tetrafluoroethylene
F3C
2
CCF2
2
CCF2
2
CCF2
2
CCF2
2
CCF2
2
CCF2
CF3n
Poly(tetrafluoroethylene): Teflon
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Polyesters, Amides, and UrethanesMonomer Polymer
CO2HHO2CHO
OHO O
HO OH2C
H2C O
nTerephthalic
acidEthylene
glycol
Poly(ethylene terephthalate
H
Ester
HO OH
O O
4 H2N NH24Adipic Acid 1,6-Diaminohexane Nylon 6,6
HO NH
NH
H
O O
4 4n
CO2HHO2C
Terephthalic
acid
NH2H2N
1,4-Diamino
benzene
Kevlar
O
HO
OHN
HN H
n
Amide
HOOH
Ethyleneglycol
H2COCN NCO
4,4-diisocyantophenylmethaneSpandex
H2
C
H
N
H
N
O
HO
O
O
H2
C
H2
C O Hn
Urethane linkage
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Natural PolymersMonomer Polymer
Isoprenen
Polyisoprene:Natural rubber
O
H
HO
H
HO
H
HOHH
OH
OH
Poly(-D-glycoside):cellulose
O
H
O
H
HO
H
HO
HHOH
H
n
-D-glucose
H3N
O
O
R
Polyamino acid:protein
H3N
OHN
R1
OHN
Rn+1
O
OH
Rn+2n
Amino Acid
BaseO
OH
OP
O
O
O
oligonucleic acidDNA
NucleotideBase = C, G, T, A
BaseO
O
OP
O
O
O
DNA
DNA
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Physical Properties
Stretch
Linear Polymer
The chains can be stretched, which causesthem to flow past each other. When released,the polymer will not return to its original form.
Stretch
Cross-Linked Polymer
The cross-links hold the chains together.When released, the polymer will return to it'soriginal form.
Relax
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Polymer Synthesis
There are two major classes of polymer formationmechanisms
Addition polymerization: The polymer grows bysequential addition of monomers to a reactive site
Chain growth is linear
Maximum molecular weight is obtained early in the reaction
Step-Growth polymerization: Monomers react togetherto make small oligomers. Small oligomers makebigger ones, and big oligomers react to give polymers.
Chain growth is exponential
Maximum molecular weight is obtained late in the reaction
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Addition Polymerization
In* AInitiation
In A* A
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Addition PolymerizationPropagation
In*A
InitiationIn A A* A
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Addition PolymerizationPropagation
AIn*Initiation
In A A A*
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Addition Polymerization
Propagation
n
In A A A AnA*
A A A A Am
In A A A AnA
*A A A A Am
Combination
*A A A A Am
In A A A A n A
B A A A Am
Disproportionation
Termination
Reactive site is consumed
A
In A A A AnA
A*
Chain TransferNew reactive siteis produced
MWkpropagation
ktermination
MW
% conversion0 100
In*Initiation
In A A A A*
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Types of Addition Polymerizations
Ph
Anionic
C3H7 LiC4H9
Ph
Li+ Phn
C4H9
Ph Ph
Li+
n
Ph
Radical
PhCO2Ph
n
Ph
Cationic
Cl3Al OH2H
PhHOAlCl3
PhnH
Ph Phn
HOAlCl3
PhCO2
Ph
PhCO2
Ph Phn
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Step-Growth Polymerization
Stage 1
Consumptionof monomer
n n
Stage 2
Combinationof small fragments
Stage 3
Reaction ofoligomers to givehigh molecularweight polymer
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Step-Growth Polymerization
Because high polymer does not form until the end
of the reaction, high molecular weight polymer isnot obtained unless high conversion of monomeris achieved.
Xn1
1p
Xn = Degree of polymerizationp= mole fraction monomerconversion
1
10
100
1000
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Mole Fraction Conversion (p)
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Nylon-6,6
Cl Cl4
H2N NH24
Adipoyl chloride 1,6-Diaminohexane
Cl NH
NH
H
O O
4 4
NaOH
HO NH
NH
H
O O
4 4n
6 carbondiacid
6 carbondiamine
Nylon-6,6Diamine, NaOH, in H2O
Adipoyl chloridein hexane
Nylon 6,6
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Nylon-6,6
Diamine, NaOH, in H2O
Adipoyl chloridein hexane
Nylon 6,6
Since the reactants are in differentphases, they can only react at the
phase boundary. Once a layer ofpolymer forms, no more reactionoccurs. Removing the polymer allowsmore reaction to occur.
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Molecular Weight of PolymersUnlike small molecules, polymers are typically a mixture of differentlysized molecules. Only an average molecular weight can be defined.
Measuring molecular weight
Size exclusion chromatography
Viscosity
Measurements of average molecular
weight (M.W.) Number average M.W. (Mn): Total
weight of all chains divided by # ofchains
Weight average M.W. (Mw):
Weighted average. Always largerthan Mn
Viscosity average M.W. (Mv):Average determined by viscositymeasurements. Closer to Mwthan
Mn
# of molecules
Mn
Mw
increasing molecular weight
Mv
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What the Weights MeanMn: This gives you the true average weight
Let's say you had the following polymer sample:2 chains: 1,000,000 Dalton 2,000,000
5 chains: 700,000 Dalton 3,500,000
10 chains: 400,000 Dalton 4,000,000
4 chains: 100,000 Dalton 400,000
2 chains: 50,000 Dalton 100,00010,000,000
10,000,000/23 = 435,000 Dalton
1 Dalton = 1 g/mole
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Weight Average Molecular Weight
Mw: Since most of the polymer mass is in the heavier fractions, this
gives the average molecular weight of the most abundant polymerfraction by mass.
2,000,000
10,000,000 0.201,000,000 200,000
3,500,00010,000,000
0.35 700,000 245,000
4,000,000
10,000,000 0.40 400,000 160,000
400,000
10,000,000 0.04100,000 4,000
100,000
10,000,000 0.01 50,000 500
Total 609,500
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Polymer Microstructure
Polyolefins with side chains have stereocenters on every other carbon
CH3n
CH3 CH3 CH3 CH3 CH3 CH3CH3
With so many stereocenters, the stereochemistry can be complex.
There are three main stereochemical classifications for polymers.Atactic: random orientation
Isotactic: All stereocenters have same orientation
Syndiotactic: Alternating stereochemistry
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How to Determine Microstructure?
13C NMR is a very powerful way to determine the microstructure of
a polymer.
13C NMR shift is sensitive to the two
stereocenters on either side on sptectrometers
> 300 MHz. This is called pentad resolution.
r mm rmr
mmrm pentad
m = meso (same orientation)r = racemic (opposite orientation)
12 1 2
13C NMR spectrum of CH3region
of atactic polypropylene
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Why is this important?
Tacticity affects the physical properties
Atactic polymers will generally be amorphous, soft,flexible materials
Isotactic and syndiotactic polymers will be morecrystalline, thus harder and less flexible
Polypropylene (PP) is a good example
Atactic PP is a low melting, gooey material
Isoatactic PP is high melting (176), crystalline, tough
material that is industrially useful Syndiotactic PP has similar properties, but is very
clear. It is harder to synthesize