A Study Toward Using ATRP As End-capping Method For Preparation Of Cyclodextrin-based Polyrotaxanes

In the 1990 s, α-cyclodextrin(α-CD) was first found to enable to accommodate polymer chain forming a kind of unique "necklace-like" supramolecular polymers, namely polypseudorotaxanes(PPRs). Accordingly, the polyrotaxanes(PRs) are obtained by attaching bulky groups to both ends of PPRs to block α-CD falls from the polymer chain. The CD-based PRs have attracted tremendous attention for their potential to be used as smart soft materials, for example, supramolecular hydrogel, molecular sensor, drug delivery and gene transfer vector, shape memory and self-healing material, insulated molecular wire, etc. Currently, the coupling reaction between polymer axle chain molecules and bulky groups is still the most commonly used method for the synthesis of PRs. Alternatively, atom transfer radical polymerization(ATRP) has many advantages over conventional radical polymerization, such as for controlling polymer molecular weight, lowering narrow molecular weight distribution, offering various topological homo- and copolymers. ATRP is also suitable for aqueous polymerization techniques. In recent years, ATRP was success- fully employed to prepare CD-based PRs. Using polymer segments as capping groups can not only end-cap the PPRs, but also to endow them the block copolymer characters.To this end, a kind of PR-based block copolymers was synthesized via in situ ATRP of hydroxylethylene acrylate(HEA) initiated by PPRs built from 2-bromoisobutyryl endcapped polypropylene glycol-polyethylene glycol-polypropylene glycol(BriB-PEP-iBBr) with α-CDs in the presence of Cu(I)Cl and N,N,N’,N’,N’’-pentamethyldiethylenetriamine(PMDETA) at 20 °C in aqueous solution. The structure was characterized in detail by means of 1H NMR, GPC, WXRD and DSC measurements. The entrapped number of α-CDs in the PPRs can reach about 85 % of the feed one. However, after end-capped with PHEA segments, the entrapped number became very small: only about 5 %. Even so, it was also observed that the DP of PHEA segments was tunable by varying its feed molar ratio.Furthermore, another kind of PR-based block copolymers was prepared via the same ATRP of HEA, but initiated by PPRs made from BriB-PEP-iBBr with β-CDs. The structure of the resutlign copolymers was also characterized in detail. The entrapped number after end-capped with PHEA segments is about 30 % in this case. The PHEA segments were attached to two ends of β-CD-BriB-PEP-iBBr PPRs to give rise to the PR-based block copolymers showing a channel-type crystal structure. The entrapped β-CD number and the polymerization degree(DP) of PHEA were controllable by changing their molar feed ratio. β-CDs were observed to favorably aggregate around the PPO segment after incubation in water and freeze-drying. However, there were about one-third of the threaded β-CDs to slide to the PEO segments after treated with DMF and anhydrous ether. It suggested that these PR-based block copolymers exhibit the solvent-responsibility.