Azeotropic Copolymerization

Another special case of copolymerization is that of azeotropic copolymerization, where r_A and r_B are both less than 1, or, r_A and r_B are both greater than one (this case is rarer). Two examples are highlighted below. Notice that these curves cross the line f_A=F_A at a point called the azeotropic composition (f_A)_azeo. These points have been noted with green dots in figure 8.5. The name “azeotropic” comes from the analogy with phase diagrams of liquid-liquid mixtures, where an azeotropic mixture is characterized by having a vapor phase upon boiling with the same ratio of components as the liquid mixture. Similarly, for copolymerization, at the azeotropic composition, (f_A)_azeo, we find f_A=F_A indicating that monomer A is being incorporated into the polymer in the same proportion in which A exists in the monomer solution. Substituting the fact that f_A=F_A into our copolymer equation (equation 8.8) we can simplify to get an expression for the azeotropic composition:

$${\left( f_A\right)}_{azeo}=\frac{1-r_B}{2-r_A-r_B}$$

Figure 8.5: Highlighted curves are example conditions for azeotropic copolymerizations,
and the green dots indicate the azeotropic composition.

Source: Modified from Figure 9.1 from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

PROBLEM

A special case of azeotropic copolymerization is where both r_A and r_B=0. What would be the structure of this polymer?

Source: Modified from Figure 9.1 from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

1. Poly(A)-block-poly(B)
2. Poly(A)-rand-poly(B)
3. Poly(A)-graft-poly(B)
4. Poly(A)-alt-poly(B)

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ANSWER

D. Poly(A)-alt-poly(B)

Because both reactivity ratios are 0

$$\frac{k_{AA}}{k_{AB}}=\frac{k_{BB}}{k_{BA}}=0$$

This means that both k_AA and k_BB must be zero. Since homopropagations can’t occur, the sequence MUST alternate!