Polymerization of ε‐Caprolactone and its Copolymerization with γ‐Butyrolactone using Metal Complexes

Solution polymerization of ε‐caprolactone (ε‐CL) was performed using four different initiators namely: tin(II) octanoate (Sn(Oct) 2 )/ethanolamine, aluminium Schiff's base complex‐HAPENAlO i Pr, lithium diisopropyl amide (LDA) and aluminium isopropoxide. The reaction conditions varied with the initiator used. LDA gave rise to the most rapid polymerization with the highest amount of cyclic species as detected by 13 C NMR. However, no cyclic species were detected when HAPENAlO i Pr was used as initiator. The tin(II) octanoate/ethanolamine system lead to an α,ω‐dihydroxy‐polycaprolactone (PCL). The copolymerization of ε‐CL was then performed with the hard to oligomerize γ‐butyrolactone using the four initiators. GPC (Gel Permeation Chromatography) analyses showed the formation of copolymers. The highest incorporation of polybutyrolactone (PBL) in the copolymer was obtained using HAPENAlO i Pr as evidenced by 1 H NMR. 13 C NMR indicated the presence of pseudoperiodic random copolymers with short blocks of PCL whose block length varied with initiator used. The longest and shortest block length were obtained using Sn(Oct) 2 and HAPENAlO i Pr respectively as calculated from 13 C NMR. The reactivity ratios were determined using the Finemann‐Ross method at low conversion with HAPENAlO i Pr as initiator. The values obtained, r CL  = 19.4 and r BL  = 0.11, confirmed the presence of long blocks of CL units in the copolymer.