The reactivity‐selectivity principle in polymerization. The case of polymerization of ϵ‐caprolactone

Anionic and pseudoanionic polymerizations of ϵ‐caprolactone (CL) have been compared in terms of the principle relating reactivity with selectivity (reactivity‐selectivity principle, RSP). RSP can be formulated as follows: there are two reagents X and Y; reagent X reacts faster with substrates A and B than reagent Y does (i.e. k xa >k ya and k xb >k yb , where k xa , k ya , k xb and k yb are the respective rate constants); moreover, reactions with A are faster than reactions with B (i.e. k xa >k xb and k ya >k yb ); finally, if RSP is operative the inequality k ya /k yb >k xa /k xb holds. Comparison of the anionic and pseudoanionic polymerizations of CL reveals that this system conforms to the RSP. Propagation on the …‐O − Na + ion pairs proceeds much faster than on the … − OAIR 2 covalent species (k p ∓ = 1,7 l/mol · s vs k p c = 3,0 · 10 −2 l/mol · s, THF solvent, 20°C). Propagation is accompanied by transesterification and cyclization. Formation of the cyclic dimer (with the rate constant k b (2)) was taken as a measure of an extent of these “side” reactions. Following selectivity coefficients ß = k p /k b (2) have been measured (in l/mol): 1, 55 · 10 3 (… − O − Na + ), 4, 6 · 10 4 (…‐OAl(C 2 H 5 ) 2 ), and 7,7 · 10 4 (… − OAl(i‐C 4 H 9 ) 2 ). Thus, k p c /k b c (2)>k p ∓ /k b ∓(2). Therefore formation of the linear, high molar mass polymer is more pronounced in the system with the less reactive active centers.