Synthesis of Ethyl Cellulose Grafted Poly(N-isopropylacrylamide) Copolymer and Its Micellization

Abstract

Thermo-responsive graft copolymers, ethyl cellulose grafting poly(N-isopropylacrylamide) (EC-g-PNIPAm), were synthesized by single-electron transfer living radical polymerization (SET-LRP) in THF/methanol mixed solvent. Ethyl cellulose macro-initiators (EC-Br) for SET-LRP were synthesized by the esterification between the hydroxyl groups on EC backbones and 2-bromoisobutyryl bromides. The degree of substitution of the EC-Br macro-initiator was tailored by varying the feeding mole ratio of 2-bromoisobutyryl bromide to the hydroxyl group on ethyl cellulose. Three EC-Br macro-initiators with different Br substitutions were used as the macro-initiators and it was found that the converting degree of the initiator sites was efficient and the graft density of graft copolymer was controllable. The SET-LRP of NIPAm was efficient that the monomer conversion was above 60% within 25 h. The linear plot of ln((M)0/(M)t) versus the conversion indicated that the reaction was living and controllable. The graft copolymers were characterized by means of gel permeation chromatography (GPC), 1 H NMR and FTIR spectroscopy. GPC peak of the graft copolymer became narrower and shifted to the low elution time with the increase of reactive time. The molecular weight of the side-chain linearly increased with the reaction time. EC-g-PNIPAm copolymers were typical amphiphilic graft copolymers, having the hydrophobic EC backbone and the hydro- philic PNIPAm side chains. They could be self-assembled and form stable micelles in a selective solvent, water. The micelles were in spherical shape with typical diameter around 100 nm. Rod-like and flower shaped particles were also observed, which was probably due to the fusion of the spherical shaped micelles. The structure of the spherical shaped micelles was that the EC backbone collapsed to form the core of the micelles and the hydrophilic PNIPAm chains stayed in the outside of the micelles to stabilize the micelles. Due to the thermoresponsive branch chains of PNIPAm, the micelles could shrink with rising temperature due to the collapses of the PNIPAm side chains. Keywords ethyl cellulose; poly(N-isopropylacrylamide); single-electron transfer living radical polymerization (SET-LRP), graft polymerization; thermoresponsive