Alternating polymer

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Alternating copolymers are polymers which have properties intermediate to those of the corresponding homopolymers A and B.

The alternating copolymer has the formula: -A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)n-. The molar ratios of the monomer in the polymer is close to one, which happens when the reactivity ratios r1 & r2 are close to zero, as given by the Mayo-Lewis equation (also called the copolymerization equation):

\frac {d\left [M_1 \right]}{d\left [M_2\right]}=\frac{\left [M_1\right]\left (r_1\left[M_1\right]+\left [M_2\right]\right)}{\left [M_2\right]\left (\left [M_1\right]+r_2\left [M_2\right]\right)}

where r1 = k11/k12 & r2 = k22/k21

[edit] Example

An example is maleic anhydride and stilbene, with reactivity ratio:

  • Maleic anhydride (r1=0.08) & cis-stilbene (r2=0.07)
  • Maleic anhydride (r1=0.03) & trans-stilbene (r2=0.03)

Both of these compounds do not homopolymerize (i.e. k11 and k22 are close to zero). Instead, they react together to give exclusively alternating copolymer.

[edit] References

  1. Mayo, F. R., and Lewis, F. M., J. Am. Chem. Soc., 66, 1594 (1944).
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