Copolymer Composition Drift

Copolymer Composition Drift jls164

For most copolymerizations, fA ≠ FA and so, one monomer is preferentially consumed during polymerization. This means that as the reaction proceeds, the overall composition of the comonomer mixtures changes, i.e., fA and fB change over the course of the polymerization. And if fA and fB change, then FA and FB must also change as a function of monomer conversion! This process, which the monomer composition and polymer composition change over the course of the reaction, is called copolymer composition drift. Copolymer composition drift can lead to synthesis of polymers having very different composition over their lengths, and can become especially significant at higher monomer conversions.

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Figure 8.1: Schematic representation of how two different monomers can be incorporated at different rates into the copolymer and the effect that could have on the polymer composition
Source: Lauren Zarzar

PROBLEM


If rA>1 and rB<1 then fA…

  1. Increases with higher extents of reaction
  2. Decreases with higher extents of reaction
  3. Does not change as a function of extent of reaction

ANSWER


B. Decreases with higher extents of reaction

Because A is preferentially consumed (which we know, because of the reactivity ratios, and in turn fA<FA), A will be used up more quickly than B, and so will become depleted in the commoner mixture at higher extents of reaction.

Given below are some example values of reactivity ratios for pairs of monomers for free radical polymerization. Notice that you need to consider the monomers together, and not individually; the reactivity ratio is not inherent to the monomer, but rather is a function of what you are trying to copolymerize and the conditions of the reaction.

Table 8.2: Some Typical Values of Reactivity Ratios for Free-Radical Copolymerization at 60 °C
Monomer AMonomer BrArBrArB
StyreneButadine0.781.391.08
StyreneMethyl methacrylate0.520.460.24
StyreneMethyl acrylate0.750.180.14
StyreneAcrylonitrile0.400.040.02
StyreneMaleic anhydride0.0200
StyreneVinyl chloride170.020.34
Vinyl acetateVinyl chloride0.231.680.39
Vinyl acetateAcrylonitrite0.064.050.24
Vinyl acetateStyrene0.01550.55
Methyl methacrylateMethyl acrylate1.690.340.57
Methyl methacrylaten-Butyl acrylate1.80.370.67
Methyl methacrylateAcrylonitrile1.200.150.18
Methyl methacrylateVinyl acetate200.00150.30
trans-StilbeneMaleic anhydride0.030.030.001
Source: Table 9.1 from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

The exact reactivity ratios are often hard to predict, and therefore are mostly determined experimentally. We can get a sense of how well various monomers stabilize the radical active center in comparison to one another based on these reactivity ratios; do the trends match what we predict? Resonance enhances radical stability; if we polymerize a monomer that stabilizes a radical well via resonance with one that doesn’t (such as, styrene with vinyl chloride) we find that the more stabilizing monomer (styrene) is preferentially incorporated (rstyrene>>1, r­vinyl chloride <<1). More alkyl substituents also help to stabilize radicals (i.e., a radical on a tertiary carbon is more stable than a secondary carbon), so let’s compare methyl methacrylate versus methyl acrylate. The methyl methacylate should stabilize the radical better on a tertiary carbon, and thus we find that the methyl methacrylate is more readily incorporated into the polymer, as predicted.

PROBLEM 2


Consider the cationic polymerization of nitroethane (monomer A) and chloroprene (monomer B) with rA=33 and rB=0.15. How could you describe the structure of the polymer formed based on these reactivity ratios?

  1. Random structure is promoted
  2. Alternating structure is promoted
  3. “blocky” structure is promoted

ANSWER 2


C. “blocky” structure is promoted

Monomer A is much more reactive than the monomer B, rArB>1. This favors homopolymerization and would lead to formation of blocks.