Synthesis of pH-cleavable poly(trimethylene carbonate)-based block copolymers via ROP and RAFT polymerization

Abstract

In this work, poly(dimethylacrylamide)-b-poly(trimethylene carbonate) (PDMAAm-b-PTMC) block copolymers containing a pH-sensitive imine linkage were successfully synthesized by combination of ring-opening polymerization (ROP) and reversible addition–fragmentation chain transfer (RAFT) polymerization. A variety of vanillin terminated PTMC block copolymers were prepared by ROP of trimethylene carbonate (TMC) initiated by modified vanillin in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an organo-catalyst. The analysis of the resultant homopolymers by ¹H NMR spectroscopy, size exclusion chromatography (SEC) and ESI-ToF masspectrometry (ESI-ToF-MS) revealed the excellent control of molar masses (M_n up to 14 000 g mol⁻¹), dispersities (Đ_M < 1.15) and end groups. The PTMC homopolymer was subsequently end-capped with an amino functionalized RAFT-agent leading to a macroinitiator containing imine group. Using this macroinitiator, PDMAAm-b-PTMC block copolymers with various molar masses were successfully prepared. In aqueous solution at pH 7.4, micelles could be formed by self-assembly of the amphiphilic block copolymer using a dialysis method. The hydrodynamic diameter (D_h) of the micelles was 76.3 ± 2.4 nm with a polydispersity index (PDI) of 0.179 ± 0.002. Under weak acidic conditions (pH 6.0), the hydrophilic PDMAAm block could be cleaved from micelles by hydrolysis of the imine linkage, while PTMC nanoparticles formed in situ with an average size of 149.8 ± 4.5 nm (PDI = 0.287 ± 0.007). Based on our strategy, these pH-cleavable PTMC-based amphiphilic block copolymers will expand the range of biodegradable synthetic polymers available for potential biomedical applications, such as controlled drug and gene delivery.