Importance of Monomer Reactivity Ratios
and Their Products
Let A and B be two chemically different vinyl monomers. Then the tendency of monomer A and B to form an ordered or random sequences depends on their reactivities; if, for example, radical A· reacts preferably with monomer B and vice versa, an alternating sequence is observed. This is the case if the product of reactivities r1r2 approaches zero. If, on the other hand, the reactivity product r1r2 approaches one1, each radical has no preference and the copolymer has a completely random sequence of monomers. However, one monomer is usually more reactive than the other. Consequently, the copolymer will consist of a greater portion of the more reactive specimen in the random sequence of repeat units.
| Butadiene | |||||||
| 1.0 | Styrene | ||||||
| 0.71 | 0.55 | Vinyl Acetate | |||||
| 0.30 | 0.34 | 0.39 | Vinyl Chloride | ||||
| 0.19 | 0.24 | 0.30 | 1.0 | Methyl Methacrylate | |||
| 0.10 | 0.16 | 0.12 | 0.96 | 0.61 | Vinylidene Chloride | ||
| 0.04 | 0.14 | 0.90 | 0.75 | 0.96 | 0.80 | Methyl Acrylate | |
| 0.02 | 0.02 | 0.25 | 0.13 | 0.24 | 0.34 | 0.84 | Acrylonitrile |
The table above shows the product of reactivity ratios r1r2 for a number of common monomers. This table was first constructed by Mayo and Walling.2 The monomers are arrange in order of their r1r2 values such that the farther apart any pair of monomers, the greater their tendency for alternating copolymerization. However, there exist some exceptions, like vinyl chloride and styrene that has a stronger tendenct for alternation than vinyl chloride and vinyl acetate, even though the later pair of monomers is farther apart. These exceptions are the result of steric hindrance.
According to Majo and Walling, the table above represents a sequence of electrical polarities; monomers with very different polarity have a greater tendency to react with one another than monomers with similar polarity. For example, butadiene and styrene have similar low polarity and the product of the monomer reactivity ratios is close to one, whereas butadiene and acrylonitrile have very different polarities and the product of reactivity ratios is close to zero. Thus, the later pair of monomers forms a more or less alternating sequence whereas butadiene and styrene form a random sequence of monomers.