Ethylene carbonate/cyclic ester random copolymers synthesized by ring-opening polymerization

Abstract

The diaminophenolate and β-diketiminate zinc complexes [(NNO)ZnEt] ((NNO)⁻ = 2,4-di-tert-butyl-6-{[(2′-dimethylaminoethyl)-methylamino]methyl}phenolate)) and [(BDI^iPr)Zn{N(SiMe₃)₂}] (BDI^iPr = CH(CMeNC₆H₃-2,6-iPr₂)₂), respectively, the Lewis acidic triflate salt Al(OTf)₃, and the guanidine TBD (= 1,5,7-triazabicyclo[4.4.0]dec-5-ene), combined to a protic source as initiator, typically benzyl alcohol (BnOH), enabled the successful copolymerization of ethylene carbonate (EC) with various cyclic esters such as β-butyrolactone (BL), δ-valerolactone (VL), ε-caprolactone (CL) or L-lactide (LLA). The random copolymerizations proceeded smoothly under mild operating conditions, preferentially from [(NNO)ZnEt]/BnOH at 60 °C in toluene within a few hours, affording the corresponding copolymers void of ether units, with M_n,SEC values in the range ca. 6000–93 350 g mol⁻¹ and with unimodal, moderately broad dispersity values (Đ_M = 1.3–2.1). Under the same experimental conditions, the homopolymerization of EC did not proceed. The first EC/BL random copolymers were thus synthesized with up to 26 mol% of EC inserted within the polyester, while the second example of P(EC-co-VL) was isolated. P(EC-co-VL), P(EC-co-CL), and P(EC-co-LLA) copolymers were prepared with higher than previously reported EC content, namely 23, 37, and 17 mol% vs. 10, 31, and 4 mol%, respectively. In contrast to other catalyst systems, the Al(OTf)₃/BnOH system promoted CO₂ elimination from the copolymers, thereby leading to ether defects. Microstructural analysis of the copolymers by ¹³C{¹H} NMR spectroscopy revealed the presence of signals previously never described and possibly arising from consecutive EC units within the random copolymers. Thermal transition temperatures measured by DSC further supported the random nature of these copolymers.