Mass spec

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Macromolecular Chemistry
Lecture 5
Step Growth Chain Growth
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Paul Flory Clears Things Up
Polymer Structure is distinct from polymerization
process
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Basic Types of Polymerization Mechanisms
Step-growth
Chain-growth
Ring-opening
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Chain growth system

• The characteristic of a chain polymer is that
  polymer growth takes place by monomer reacting
  only with the reactive centers. Monomer does not
  react with monomer and the different-sized
  species such as dimer, trimer, and n-mer do not
  react with each other. The polymerization ceases
  when the active center is destroyed by
  termination reaction(s).

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Step Growth system

• A condensation takes place between two
  polyfunctional molecules to produce one larger
  polyfunctional molecule with the possible
  elimination of a small molecule such as water.
  The reaction continues until one of the reagents
  is used up.

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The chain growth vs. step growth
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- Step-growth polymerization
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- Chain-growth polymerization
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Chain Growth Polymerization
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1 2 3 4 5 6
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DP No/N 12 / 7 1.7 (for 50, b)
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Step Growth Polymerization
DP No/N 12 / 9 1.3 (for 50, b)
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The chain growth system

• The relationship between Mwt and conversion
• With no termination reactions

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The chain growth system

• The relationship between Mwt and conversion
• With termination reactions

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Step growth system

• The relationship between Mwt and conversion

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Distinguishing features of chain- and
step-polymerization mechanisms
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Lets look at this more closely.
Consider a flask of monomer .If there are No
molecules in the flask at time 0 and N
remaining at time t then the DP at time t is the
average degree of polymerization must just be
N0/N!
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The Carothers Equation
If there are No molecules at time 0 and N
remaining at time t then the amount reacted is
N0-N and we can define p as the conversion or
fraction reacted then as p (No N ) / No or
N No( 1 P) If DP is the average degree of
polymerization N0/N .substituting gives
DP 1 / (1 P) and for P
0.98 (98 conversion), DP only 50!
High Molecular weights are hard to get this way
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It all happens at the end!!!
DP 1 / (1 P)
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A-A, B-B vs A-B ???
A .A
B..................B
A.B
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Chain Growth Polymers

• Chain polymer growth takes place by monomer
  reacting only with the reactive centers. Monomer
  does not react with monomer and species such as
  dimer and trimer do not react with each other.
  The polymerization ceases when the active center
  is destroyed by termination reactions.
• Reactive intermediates in chain-growth
  polymerizations include radicals, carbanions,
  carbocations, and organometallic complexes

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Radical Chain-Growth Polymers

• Look at classical example of polymerizations of
  ethylene and substituted ethylenes

R
n
R
An alkene
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Radical Chain Growth Polymerization

• Among the initiators used for radical
  chain-growth polymerization are diacyl peroxides,
  which decompose as shown on mild heating

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Benzoyl peroxide is a popular pharmaceutical
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Radical Chain Growth Polymerization

• Another common class of initiators are azo
  compounds, which also decompose on mild heating
  or with absorption of UV light

N
N
2

N
N
C
N
C
N
C
N
Alkyl radicals
Azoisobutyronitrile (AIBN)
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Initiation
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Propagation
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Termination
Coupling

Disproportionation