Video lecture for mca
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1 Video Lectures for MBA By: Video.edhole.com
Transcript of Video lecture for mca
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"I just want to say one word to you -- just one word -- 'plastics.'"
Advice to Dustin Hoffman's character in The Graduate
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Polymers: Introduction
• Polymer: High molecular weight molecule made up 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 more
monomers– 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 be
melted (tires, rubber bands)– Thermoplastic: Meltable plastic– Elastomers: Polymers that stretch and then return to
their original form: often thermoset polymers– Thermoplastic elastomers: Elastic polymers that can
be melted (soles of tennis shoes)
• Polymer Families– Polyolefins: made from olefin (alkene) monomers– Polyesters, Amides, Urethanes, etc.: monomers linked
by ester, amide, urethane or other functional groups– Natural Polymers: Polysaccharides, DNA, proteinsVideo.edhole.com
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Common PolyolefinsMonomer Polymer
EthyleneH3C
CH3
nRepeat unitPolyethylene
CH3
CH3n
CH3 CH3 CH3 CH3 CH3 CH3CH3Propylene
Polypropylene
PhCH3
n
Ph Ph Ph Ph Ph PhPhStyrene
Polystyrene
ClCH3
n
Cl Cl Cl Cl Cl ClClVinyl Chloride
Poly(vinyl chloride)
F2C CF2
TetrafluoroethyleneF3C
F2C
CF2
F2C
CF2
F2C
CF2
F2C
CF2
F2C
CF2
F2C
CF2
CF3
nPoly(tetrafluoroethylene): Teflon
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Polyesters, Amides, and UrethanesMonomer Polymer
CO2HHO2CHO
OHO O
HO OH2C
H2C O
nTerephthalic acid
Ethyleneglycol
Poly(ethylene terephthalateH
Ester
HO OH
O O
4H2N NH24
Adipic Acid 1,6-Diaminohexane Nylon 6,6HO N
HNH
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-diisocyantophenylmethane Spandex
H2C
HN
HN
O
HO
O
OH2C
H2C O H
n
Urethane linkageVideo.edhole.com
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Natural PolymersMonomer Polymer
Isoprenen
Polyisoprene:Natural rubber
O
H
HO
H
HO
H
HOHH OH
OH
Poly(ß-D-glycoside):cellulose
O
H
O
H
HO
H
HOHH OH
OH
H
n
ß-D-glucose
H3N
O
O
RPolyamino 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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What Makes Polymers Unique?
• Really big molecules (macromolecules) like polymers have very different properties than small molecules– Chain entanglement: Long
polymer chains get entangled with each other.
• When the polymer is melted, the chains can flow past each other.
• Below the melting point, the chains can move, but only slowly. Thus the plastic is flexible, but cannot be easily stretched.
• Below the glass transition point, the chains become locked and the polymer is rigid
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Physical Properties
Stretch
Linear Polymer
The chains can be stretched, which causes them 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's original form.
Relax
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Polymer Synthesis
• There are two major classes of polymer formation mechanisms– Addition polymerization: The polymer grows by
sequential 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 together to make small oligomers. Small oligomers make bigger 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 PolymerizationPropagation
In*A
InitiationIn A A* A
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Addition PolymerizationPropagation
AIn*A
InitiationIn A A A*
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Addition Polymerization
Propagation
nAIn A A A A
nA*
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 AnA
B A A A Am
Disproportionation
TerminationReactive site is consumed
A
In A A A AnA
A*
Chain TransferNew reactive siteis produced
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MW∝kpropagation
ktermination
MW
% conversion0 100
In*A
InitiationIn A A A A*
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Types of Addition Polymerizations
Ph
Anionic
C3H7 Li C4H9
Ph
Li+ Phn
C4H9
Ph Ph
Li+
n
Ph
Radical
PhCO2•Ph
n
Ph
Cationic
Cl3Al OH2H
PhHOAlCl3
Phn
H
Ph Phn
HOAlCl3
PhCO2
PhPhCO2
Ph Phn
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Step-Growth PolymerizationStage 1
Consumptionof monomer
n n
Stage 2
Combinationof small fragments
Stage 3
Reaction of oligomers to give high molecular weight polymer
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Step-Growth Polymerization
• Because high polymer does not form until the end of the reaction, high molecular weight polymer is not obtained unless high conversion of monomer is achieved.
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Xn = 11−p
Xn = Degree of polymerizationp = mole fraction monomer conversion
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)
Deg
ree o
f P
oly
meri
zati
on
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Nylon-6,6
Cl Cl
O O
4H2N 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 different phases, they can only react at the phase boundary. Once a layer of polymer forms, no more reaction occurs. Removing the polymer allows more reaction to occur.
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Molecular Weight of PolymersUnlike small molecules, polymers are typically a mixture of differently sized 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 # of chains
• Weight average M.W. (Mw): Weighted average. Always larger than Mn
• Viscosity average M.W. (Mv): Average determined by viscosity measurements. Closer to Mw than Mn
# o f m o l e c u l e s
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 2,000,0005 chains: 700,000 Dalton 3,500,00010 chains: 400,000 Dalton 4,000,0004 chains: 100,000 Dalton 400,0002 chains: 50,000 Dalton 100,000
10,000,00010,000,000/23 = 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 polymer fraction by mass.
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2,000,00010,000,000
= 0.20×1,000,000 = 200,000
3,500,00010,000,000
= 0.35× 700,000 = 245,000
4,000,00010,000,000
= 0.40× 400,000 = 160,000
400,00010,000,000
= 0.04 ×100,000 = 4,000
100,00010,000,000
= 0.01× 50,000 = 500
Total = 609,500
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Polymer MicrostructurePolyolefins 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 CH3 region of atactic polypropyleneVideo.edhole.com
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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 more
crystalline, thus harder and less flexible
• Polypropylene (PP) is a good example– Atactic PP is a low melting, gooey material– Isoatactic PP is high melting (176º), crystalline, tough
material that is industrially useful– Syndiotactic PP has similar properties, but is very
clear. It is harder to synthesize
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