Maltopentaose-Conjugated CTA for RAFT Polymerization Generating Nanostructured Bioresource-Block Copolymer

Abstract

We now describe the synthesis of a new family of oligosaccharide-conjugated functional molecules, which act as chain transfer agents (CTAs) for the reversible addition–fragmentation chain transfer (RAFT) polymerization. The synthesis was started from the catalyst-free direct <i>N</i>-glycosyl reaction of 5-azidopentylamine onto maltopentaose (Mal₅) in dry methanol at room temperature and subsequent <i>N</i>-protected reaction with acetic anhydride, producing a stable oligosaccharide-building block, such as Mal₅ with an azidopentyl group (Mal₅-N₃). The azido group was hydrogenated using platinum dioxide (PtO₂) as a catalyst to give Mal₅ with aminopentyl group (Mal₅-NH₂), which was then reacted with CTA molecules bearing activated ester moieties. These reactions produced Mal₅-modified macro-CTAs (Mal₅-CTAs, <b>1</b>), which were used for the RAFT polymerizations of styrene (St) and methyl methacrylate (MMA) in DMF. The polymerizations were performed using the [M]₀/[<b>1</b>]₀ values ranging from 50 to 600, affording the Mal₅-hybrid amphiphilic block copolymers (BCPs), such as Mal₅-polystyrene (<b>2</b>) and Mal₅-poly­(methyl methacrylate) (<b>3</b>), with a quantitative end-functionality and the controlled molecular weights between 4310 and 20 300 g mol^–1. The small-angle X-ray scattering (SAXS) measurements were accomplished for <b>2</b> and <b>3</b> to ensure their abilities to form phase separated structures in their bulk states with the increasing temperatures from 30 to 190 °C. The featured results were observed for <b>2</b> (ϕ_Mal5 = 0.14) and <b>3</b> (ϕ_Mal5 = 0.16) at temperatures above 100 °C, where ϕ_Mal5 denotes the volume fraction of the Mal₅ unit in the BCP sample. For both BCP samples, the primary scattering peaks <i>q</i>* were clearly observed together with the higher-ordered scattering peaks √2<i>q</i>* and √3<i>q</i>*. Thus, these Mal₅-hybrid amphiphilic BCP samples have a body centered cubic (BCC) phase morphology. The domain spacing (<i>d</i>) values of the BCC morphology for <b>2</b> (ϕ_Mal5 = 0.14) and <b>3</b> (ϕ_Mal5 = 0.16) were 10.4 and 9.55 nm, respectively, which were determined using Bragg’s relation (<i>d</i> = 2π/<i>q</i>*). The present RAFT agents were shown to eventually provide the phase separated structural polymeric materials in which 5.4 nm bioresource-spherical domains were periodically arrayed at the interval of about 10 nm