Transcript Polymer Properties and Structure

Polymer Properties and
Structure
The age of the plastic fantastic
Learning objectives
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Describe basic features of a polymer
Name three steps in the addition
polymerization process
Distinguish between addition and condensation
polymerization
Describe essential features of condensation
polymerization
Identify polymerization process used on basis
of polymer composition
Polymers are large molecules made by
concatenation of many small units
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Building blocks are monomers
A polymer may contain thousands of
monomers and have a molar mass of hundreds
of thousands
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Homopolymer – the monomers are identical
Copolymer – the monomers are different (2 or
more)
Monomer
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A monomer must be capable of forming two
bonds to generate a chain
A double bond meets that requirement
Two main synthetic
approaches
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Addition polymerization
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Simply adding monomers together –
synthetic plastics
Condensation polymerization
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Combination by exclusion of a small
molecule (usually water) –
extensively used by nature
Reactive radical to initiate the
process
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Radicals are reactive – contain unpaired electrons
H
H
H
H
H
R●
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H
R
●
H
H
H
H
H
H
R
Odd + even = odd: unpaired electron survives
Addition polymerization has three steps
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C2H4 is stable – does not spontaneously change
into polyethylene – requires severe conditions
1. Initiation – create reactive species by
formation of free radical (unpaired electron)
2. Propagation – As chain grows by addition of
C2H4 units, the radical is preserved
3. Termination – radicals eliminated when they
meet
Polymer properties tuned by
modifying added groups
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Density
Polarity
Chain branching
Cross linking
Chain length
Structure is function
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HDPE: the strands pack closely together
LDPE: branches prevent close packing
Polymers have revolutionized containers
and other household items
Common addition polymers cont’d
Condensation polymers: another
route to making chains
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Two types of monomer required – each
capable of making two bonds
Bonding occurs between them by elimination
of components of water
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OH + H = H2O
Lower temperature process than addition
polymerization
Condensation polymer products
Building Blocks of Proteins
Nature’s building blocks contain –CO-NHC- groups – amide links. The monomers are
amino acids. Glycine is the simplest. There
are twenty altogether
Alanine
Leucine
-CO2H
-NH2
Glycine
Polypeptide Chains
Condensation of amino acids creates
polypeptide chains. Hemoglobin
contains four chains, which imprison an
iron atom. The iron atom binds with
oxygen (or CO or cyanide ion)
The amide link
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Why –CO-NH-C- (amide) rather than –CO-OC- (ester)?
No facility for H-bonding in –CO-O-CH atom in –CO-NH-C- provides H-bonding for
secondary structure control
Hair and Wool
Secondary and tertiary structures of peptide chains
(α-keratin) in the structure of hair. Hydrogen
bonding plays a crucial role in this process.
Glucose and Cellulose
Cellulose is a condensation polymer of the simplest
carbohydrate, glucose. Only C,H and O are involved.
Cellulose is the most abundant organic chemical on earth.
Polymer design is unlimited
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Applications
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Nanoelectronics
Sensors
Catalysis
Dendrimers