Chem 421: Introduction to Polymer Chemistry

Statistical Copolymerization


Different Kinds of Copolymers

Block copolymer by sequential addition during living polymerization:

Block copolymer

Graft copolymer from an active site on original polymer:

Graft copolymer

Statistical copolymer from mixture of monomers during polymerization:

Statistical copolymerization


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?

Copolymerization

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

Case 2: r1 > 1 and r2 > 1

Case 3: r1 * r2=1

Extreme example:

Alternating copolymerization


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.

Copolymerization equation

Example for Acrylonitrile (AN) with Styrene (S)

Copolymerization graph


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