Solvent effect in the polymerization of e‐caprolactone initiated with diethylaluminum ethoxide

Kinetics of e-caprolactone (eCL) polymerization initiated with diethylaluminum ethoxide in benzene (C 6 H 6 ) and acetonitrile (CH 3 CN) as solvents was studied and compared with the previously studied polymerization conducted in tetrahydrofuran (THF) solvent. Kinetic data were analyzed in terms of the kinetic scheme : propagation with aggregation, assuming that actually propagating active species (P n *) aggregate reversibly into the unreactive (dormant) species (P n *) m r ; Kda mP n *, P n * + eCL → kp P* n+1 . The determined equilibrium constants of deaggregation (K da ) decrease with decreasing solvent polarity, namely K da (in mol 2 .L -2 ) = (1.3 ± 0.7) 10 -2 (CH 3 CN), (1.8 ± 0.5). 10 -5 (THF), (4.1 ± 0.7). 10 -6 (C 6 H 6 ), whereas for the rate constants of propagation the opposite is true, kp(in mol -1 . L. s -1 ) = (7.5 ± 0.3). 10 -3 (CH 3 CN), (3.87 ± 0.01). 10 -2 (THF), (8.6 ± 0.9). 10 -2 (C 6 H 6 ) (25°C). The latter effect is explained by a specific solvation (the stronger the higher solvent polarity) of the active species already in the ground state in the elementary reaction of the poly(eCL) chain growth : C 2 H 5 [OC(O)(CH 2 ) 5 ] n O-Al(C 2 H 5 ) 2 + eCL → C 2 H 5 [OC(O)(CH 2 ) 5 ] n+1 O-Al(C 2 H 5 ) 2 .