Download Polymers: Introduction

"I just want to say one word to
you -- just one word -- 'plastics.'"
Advice to Dustin Hoffman's
character in The Graduate
1
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, proteins
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Common Polyolefins
Monomer
Ethylene
Polymer
Polyethylene
CH3
H3C
n
Repeat unit
CH3
CH3
n
Polypropylene
Propylene
CH3 CH3 CH3 CH3 CH3 CH3 CH3
CH3
Ph
n
Polystyrene
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Styrene
CH3
Cl
n
Poly(vinyl chloride)
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Vinyl Chloride
F2C CF2
Tetrafluoroethylene
F3C
Poly(tetrafluoroethylene): Teflon
F2
C
C
F2
F2
C
C
F2
F2
C
C
F2
F2
C
C
nF
2
F2
C
C
F2
F2
C
C
F2
CF3
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Polyesters, Amides, and Urethanes
Monomer
Polymer
O
HO2C
CO2H
Terephthalic
acid
O
OH
HO
Ethylene
glycol
Poly(ethylene terephthalate
n
Ester
O
NH2
OH H2N
4
1,6-Diaminohexane
HO
Nylon 6,6
O
CO2H H2N
HO2C
NH2
1,4-Diamino
benzene
Terephthalic
acid
H2
C
OCN
NCO
4,4-diisocyantophenylmethane
O
HO
H
N
H2
C
H2 H2
O C C O H
HO
O
HO
4
Adipic Acid
O
HO
O
4
N
H
N
4
H
Amide
O
H
N
H
n
H
N H
Kevlar
n
OH
HO
Spandex
Ethylene
glycol
O
H2 H2
H
N
O C C O H
n
Urethane linkage
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Natural Polymers
Polymer
Monomer
Isoprene
Polyisoprene:
Natural rubber
n
H OH
H OH
HO
HO
HO
H
H
OH
H
H
ß-D-glucose
OH
Poly(ß-D-glycoside):
cellulose
O
O
Polyamino acid:
protein
R
Amino Acid
O
O
H3N
O
OH
Nucleotide
Base = C, G, T, A
O
Rn+1
O
n
OH
Rn+2
O
O
O
oligonucleic acid
DNA
n
H
N
O P O
Base
OH
OH
H
H
N
R1
DNA
O P O
H
H
O
H3N
HO
O
HO
O
Base
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
Linear Polymer
Stretch
The chains can be stretched, which causes
them to flow past each other. When released,
the polymer will not return to its original form.
Cross-Linked Polymer
Stretch
Relax
The cross-links hold the chains together.
When released, the polymer will return to it's
original form.
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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 Polymerization
In*
A
Initiation
In
A*
A
10
Addition Polymerization
Propagation
In*
A
In
A A*
A
Initiation
11
Addition Polymerization
Propagation
A
In*
Initiation
In
A A A*
A
12
Addition Polymerization
nA
A
In*
In
A A A A*
Initiation
In
Propagation
A A A A A*
n
*A
A
*A
A A A A
m
A A A A
m
In
In
A A A A A
A A A A A
n
n
A*
In
A A A A A
A A A A A
n
m
Combination
B A A A A
m
Chain Transfer
New reactive site
is produced
Disproportionation
Termination
Reactive site is consumed
MW
MW 
0
100
% conversion
k propagation
k ter mination
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Types of Addition Polymerizations
Anionic
Ph
C3H7
Li
n
Li+
C4H9
Ph
Li+
C4H9
n
Ph
Ph
Ph
Radical
PhCO2•
Ph
n
Ph
PhCO2
n
Ph
Cationic
Ph
Cl3Al OH2
PhCO2
Ph
Ph
n
Ph
H
Ph
HOAlCl3
HOAlCl3
H
n
Ph
Ph
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Step-Growth Polymerization
Stage 1
n
n
Consumption
of monomer
Stage 2
Combination
of 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.
Degree of Polymerization
1000
Xn = Degree of polymerization
p = mole fraction monomer
conversion
Xn 
1
1 p
100

10
1
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
O
O
O
NaOH
Cl
4
Cl
Adipoyl chloride
H2N
4
NH2
Cl
O
N
H
4
1,6-Diaminohexane
O
Adipoyl chloride
in hexane
HO
N
H
4
O
N
H
4
4
Nylon 6,6
Diamine, NaOH, in H2O
H
6 carbon
diacid
N
H
H
n
6 carbon
diamine
Nylon-6,6
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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.
Adipoyl chloride
in hexane
Nylon 6,6
Diamine, NaOH, in H2O
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Molecular Weight of Polymers
Unlike small molecules, polymers are typically a mixture of differently
sized molecules. Only an average molecular weight can be defined.
Mv Mn
Mw
#
o
f
m
o
le
cu
le
s
• 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
increasing molecular weight
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What the Weights Mean
Mn: 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
100,000
10,000,000
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 polymer
fraction by mass.
2,000,000
 0.20  1,000,000  200,000
10,000,000
3,500,000
 0.35  700,000  245,000
10,000,000
4,000,000
 0.40  400,000  160,000
10,000,000
400,000
 0.04  100,000  4,000
10,000,000
100,000
 0.01 50,000  500
10,000,000
Total  609,500
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Polymer Microstructure
Polyolefins with side chains have stereocenters on every other carbon
CH3
n
CH3 CH3 CH3 CH3 CH3 CH3 CH3
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.
2
1
1
2
13C
NMR shift is sensitive to the two
stereocenters on either side on sptectrometers
> 300 MHz. This is called pentad resolution.
r
m
m
r
m
r
mmrm pentad
m = meso (same orientation)
r = racemic (opposite orientation)
13C
NMR spectrum of CH3 region
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 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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