Chem 421: Introduction to Polymer Chemistry
Statistical Copolymerization
Different Kinds of Copolymers
Block copolymer by sequential addition during living polymerization:
Graft copolymer from an active site on original polymer:
Statistical copolymer from mixture of monomers during polymerization:
Essential Point
The composition of a copolymer does not necessarily match the composition of
the comonomers used in the reaction.
Chain End Model
How does a monomer repeat unit at the growing chain end tend to react?
- With more of itself?
- With the other monomer(s)?
Copolymerization Reactivity Ratios
These values are tabulated as r1 and r2 for pairs of
monomers.
|
r1 |
|
r2 |
| Styrene |
0.80 |
Isoprene |
1.68 |
| " |
0.52 |
Methyl methacrylate |
0.46 |
| " |
55 |
Vinyl Acetate |
0.01 |
| " |
0.04 |
Acrylonitrile |
0.4 |
| " |
0.04 |
Maleic anhydride |
0.015 |
Note: Data are for free radical copolymerization under standard conditions.
Values will be very different for ionic polymerization.
Sample cases:
Case 1: r1=0 and r2=0
- Each comonomer prefers to react with the other.
- Perfectly alternating copolymer.
Case 2: r1 > 1 and r2 > 1
- Each comonomer prefers to react with others of its own kind.
- Tendency to form block copolymers.
Case 3: r1 * r2=1
- There is no preference due to the chain ends.
- Random incorporation of comonomers.
- "Ideal" copolymerization.
Extreme example:
- Neither monomer homopolymerizes very well.
- Forms almost perfectly alternating copolymer of high MW.
- Alternating copolymerization is common in free radical reactions when one
monomer has electron withdrawing groups and the other has electron donating
groups.
Instantaneous Composition Equation:
Tells composition of polymer being formed at a given comonomer feed. Note
that in most cases, the composition of the monomer mixture changes as the
reaction proceeds because one monomer if consumed fatser than the other.
Example for Acrylonitrile (AN) with Styrene (S)
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