Reaktiivisuusluvut

Reaktiivisuusluvut, known in English as reactivity ratios, are fundamental parameters in the study of copolymerization. They describe the tendency of a monomer to add to a growing polymer chain relative to the other monomer present. Specifically, for a copolymerization reaction involving two monomers, M1 and M2, there are two reactivity ratios, r1 and r2. The reactivity ratio r1 is defined as the ratio of the rate constant for the addition of M1 to a chain ending in M1 (k11) to the rate constant for the addition of M2 to a chain ending in M1 (k12). Thus, r1 = k11 / k12. Similarly, r2 is defined as the ratio of the rate constant for the addition of M2 to a chain ending in M2 (k22) to the rate constant for the addition of M1 to a chain ending in M2 (k21), so r2 = k22 / k21.

The values of these reactivity ratios provide crucial information about the composition and microstructure of the resulting copolymer. If r1 > 1 and r2 < 1, monomer M1 is more reactive towards its own kind than M2, and M2 is more reactive towards M1. This typically leads to a tendency for alternating copolymerization. If both r1 < 1 and r2 < 1, both monomers prefer to add to the chain ending in the other monomer, also favoring alternating structures. Conversely, if r1 > 1 and r2 > 1, both monomers preferentially add to their own kind, leading to block copolymer formation. If r1 ≈ 1 and r2 ≈ 1, the monomers add randomly, resulting in a statistically random copolymer. When r1 = 0 and r2 = 0, it indicates a perfectly alternating copolymer. The determination of reactivity ratios is usually achieved through experimental copolymerization studies and subsequent analysis using kinetic models. These values are essential for predicting and controlling copolymer composition and for designing polymers with specific properties.