2.3. Determination of the Monomer Reactivity Ratio

The monomer reactivity ratio (r), namely, the ratio of chain radical homopolymerization rate constant to the copolymerization rate constant reflects the relative polymerization activity [14]. It was defined based on the following equations:

where k₁₁ and k₁₂ are the rate constants of homopolymerization and copolymerization, respectively. When r is more than 1, the monomer tends to homopolymerize with the same monomers; otherwise, the monomer tends to copolymerize with other monomers. Besides, the larger the value is, the higher the relative activity will be. The monomer reactivity ratio is a crucial parameter to control polymer composition and structure in binary copolymerization [15].

In this experiment, two series of polymer products, each containing eight PAB or PAD samples, were prepared under the same conditions described in Section 2.2 except that the monomer conversions of all samples were controlled to less than 10%. Prior to synthesis, the reaction molar ratios of AM and cationic monomers were controlled at 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8 and 1:9. Afterward, the cationic degree of polymer was measured by colloid titration. The monomer reactivity ratios were calculated with Fineman–Ross, Kelen–Tüdö and Yezrielev−Brokhina−Roskin (Y−B−R) methods of which the detailed procedures are shown in Supplementary Text S1–S3 and Figures S1–S4 [16].