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

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Abstract
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. kxa>kya and kxb>kyb, where kxa, kya, kxb and kyb are the respective rate constants); moreover, reactions with A are faster than reactions with B (i.e. kxa>kxb and kya>kyb); finally, if RSP is operative the inequality kya/kyb>kxa/kxb 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 …−OAIR2 covalent species (kp∓ = 1,7 l/mol · s vs kpc = 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 kb (2)) was taken as a measure of an extent of these “side” reactions. Following selectivity coefficients ß = kp/kb(2) have been measured (in l/mol): 1, 55 · 103 (…−O−Na+), 4, 6 · 104 (…‐OAl(C2H5)2), and 7,7 · 104 (…−OAl(i‐C4H9)2). Thus, kpc/kbc(2)>kp∓/kb∓(2). Therefore formation of the linear, high molar mass polymer is more pronounced in the system with the less reactive active centers.