In chemistry, copolymerization is the process of using more than one monomer to produce a copolymer, which will possess properties different to the homopolymer of either monomer. The resulting product may be a random copolymer, an alternating copolymer, or a block copolymer. In the copolymerization between two monomers, M₁ and M₂, the following reactions may occur:

~~M₁^* + M₁ --> ~~M₁M₁^* (rate constant = k₁₁)

~~M₁^* + M₂ --> ~~M₁M₂^* (rate constant = k₁₂)

~~M₂^* + M₁ --> ~~M₂M₁^* (rate constant = k₂₁)

~~M₂^* + M₂ --> ~~M₂M₂^* (rate constant = k₂₂)

Copolymerization Equation

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The Mayo–Lewis equationMayo, F. R., and Lewis, F. M., J. Am. Chem. Soc., 66, 1594 (1944). (also called the copolymerization equation) gives the molar ratios of the two monomers in the resulting copolymer as:

$\backslash frac\; \{d\backslash left\backslash right\}\{d\backslash left\}=\backslash frac\{\backslash left\backslash left\; (r\_1\backslash left+\backslash left\backslash right)\}\{\backslash left\backslash left\; (\backslash left+r\_2\backslash left\backslash right)\}$

where r₁ and r₂ are the reactivity ratios of the individual monomers and r₁ = k₁₁/k₁₂ & r₂ = k₂₂/k₂₁, i.e. r = the rate of reaction with another monomer of itself / the rate of reaction with a different monomer. r is also called the copolymerization parameter.

k₁₁ and k₂₂ are the rate constants for the homopolymerization of the two monomers. k₁₂ and k₂₁ are the rate constants for the reaction between the two different monomers.

In another form:

$F\_1=1-F\_2=\backslash frac\{r\_1\; f\_1^2+f\_1\; f\_2\}\{r\_1\; f\_1^2+2f\_1\; f\_2+r\_2f\_2^2\}$

where F₁, F₂ are the mole fractions of monomers M₁, M₂ in copolymer:

F₁=1-F₂=d^*/(d^*+d^*)

and f₁, f₂ are the mole fractions of monomers M₁, M₂ in the feed:

f₁=1-f₂=M₁/(M₁+M₂)

When the copolymer composition has the same composition as the feed, this composition is called the azeotrope.

Effect of Reactivity Ratios

• Homopolymer: favored when r = ∞

-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A- (r₁ = ∞)

-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B-B- (r₂ = ∞)

• Random copolymer (or statistical copolymer): favored when r₁, r₂ close to 1.

-A-A-B-B-A-A-A-B-A-A-B-B-A-B-A-A-A-B-A-A-A-B-B-

• Alternating copolymer: favored when r₁, r₂ close to zero.

-A-B-A-B-A-B-A-B-A-B-A-B-A-B-, or -(-A-B-)_n-

• Block copolymer: favored when r₁ >1 r₂ >1

-A-A-A-A-A-A-A-B-B-B-B-B-B-B-A-A-A-A-A-A-A-B-B-...

Examples

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acrylonitrile butadiene styrene (ABS) plastic, styrene-butadiene-styrene (SBS), styrene-isoprene-styrene (SIS), P(VDF-TrFE).

External links

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• http://www.chem.rochester.edu/~chem421/copoly.htm

Further reading

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• Odian, G. Principles of Polymerization, 4th Ed., Wiley-Interscience, Hoboken, NJ 2004, Ch. 6. ISBN 0-47-127400-3

References

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copolymers | polymer chemistry

共重合