Well-defined 3, 4 and 6-arm star-shaped poly(ethylene oxide)-b-poly(D or L-lactide) and their stereocomplexes

Abstract

Star-shaped poly(ethylene oxide)-b-poly(D or L-lactide) block copolymers, [PEO-b-PD/L-LA]_x, and their corresponding stereocomplexes (SC) are amphiphilic materials with many potential applications. However, the one-pot synthesis of well-defined [PEO-b-PD/L-LA]_x remains challenging due to the incompatibility between the polymerization of EO and D/L-LA. In addition, the effect of PLA molecular weights and the number of arms on their thermal and micellar properties has yet to be investigated. Herein, we report the synthesis of [PEO-b-PD/L-LA]_x (x = 3, 4, 6) in one pot by using ring-opening polymerization (ROP) and a catalyst switch strategy to obtain star [PEO-b-PD/L-LA]_x block copolymers with multifunctional initiators with narrow molecular weight distributions. ROP of EO was initiated by using multifunctional initiators (containing 3, 4, and 6 primary OH groups) and phosphazene base t-BuP₄ as the catalyst. The base was neutralized by the addition of diphenyl phosphate, and Sn(Oct)₂ was added to promote the ROP of LA from [PEO]_x macroinitiators to afford [PEO-b-PLA]_x in one pot. Polylactide stereocomplexes were prepared by mixing equal amounts of [PEO-b-PDLA]_x and [PEO-b-PLLA]_x having similar molecular weights and numbers of arms in dichloromethane (DCM), followed by slow evaporation of DCM at room temperature. The molecular structure, thermal and micelle properties of the [PEO-b-PD/L-LA]_x and their SCs were characterized using ¹H NMR, size exclusion chromatography (SEC), circular dichroism (CD), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), X-ray diffraction (XRD) spectroscopy, dynamic light scattering (DLS), Transmission Electron Microscopy (TEM), Small Angle X-ray scattering (SAXS), and Polarized Light Optical Microscopy (PLOM). The molecular weights of PLA and the number of arms play significant roles in the thermal and micellar properties of the SC.