Synthesis of double hydrophilic block copolymers and induced assembly with oligochitosan for the preparation of polyion complex micelles

J Reboul,T Nugay,V Ponsinet,C Gérardin,P Lacroix-Desmazes,N Anik,H Cottet,M In

doi:10.1039/c1sm05230f

Abstract

This paper reports on the polyion complex micelles (PIC micelles) formed between neutral-ionizable double hydrophilic block copolymers (DHBC), poly(ethylene oxide)-block-poly(acrylic acid) (PEO-b-PAA), and oligochitosan, a natural polyamine. The controlled synthesis of PEO-b-PAA polymers was achieved by atom transfer radical polymerization (ATRP) of tert-butyl acrylate with ω-bromide-functionalized PEO macroinitiators (Mw = 2000 and 5000 g mol−1) and the subsequent deprotection reaction under acidic conditions. A series of copolymers with a narrow molecular weight distribution (Mw/Mn ≤ 1.2) and varied PAA block lengths was synthesized. Capillary electrophoresis (CE) was shown to unambiguously prove the blocky structure of the copolymers. It also showed that about 60% of the sodium counter ions were condensed onto the polyacrylate block in the pure diblock copolymer solution, which is consistent with the formation of polyion complex micelles triggered by counter-ion release in the presence of oligochitosan. The formation of oligochitosan/PAA–PEOcore–corona micelles has been investigated by dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). A minimum length of the PAA block is necessary to ensure micelle formation. The range of pH, where PIC micelles form, critically depends on the PAA block length, which also determines the size of the micelles. Micelles can be dissociated at ionic strength above 0.4 mol L−1. Since these PIC micelles have been used as recyclable structuring agents for the formation of ordered mesoporous materials, the reversibilty of the assembling process was studied upon pH and ionic strength cyclic variations. A hysteresis of stability was observed at low pH, probably due to hydrogen bonding.

Highlights

The absence of any isolated peak corresponding to poly(ethylene oxide) (PEO) and poly(methyl acrylate) homopolymer chains on the Size exclusion chromatography (SEC) spectra confirmed that no significant degradation of the blocky structure of the copolymer occurred during hydrolysis with TFA
Another key result is that about 60% of the sodium counter-ions of the polyelectrolyte block are condensed on the polyion, which is consistent with the polyion complex (PIC) micelle formation triggered by the counter-ions release
The electrostatic complex micelles are studied in detail as a function of different parameters: the pH, the ionic strength and the polymer block lengths

Summary

Introduction

As the assembling process of DHBC can be triggered by variations of physico-chemical conditions of the aqueous medium, DHBC micelles are good candidates for controlled drug release systems or reversible structure directing agents (SDA) for the synthesis of ordered mesoporous materials (OMMs). The PEO macroinitiator was synthesized by modifying the hydroxyl extremity of a-methoxy-u-hydroxypoly(ethylene oxide) according to a procedure adapted from Ranger et al.[33] Esterification of this hydroxyl function with 2bromoisobutyryl bromide leads to the macroinitiator formation This macroinitiator contains an activated alkyl bromide moiety, which is suitable as an effective initiator for the ATRP of tertbutyl acrylate monomer as the second block. The absence of any isolated peak corresponding to PEO and poly(methyl acrylate) homopolymer chains on the SEC spectra confirmed that no significant degradation of the blocky structure of the copolymer occurred during hydrolysis with TFA All these results are in accordance with those found by Hou et al.[35] and Guillemet[36] who demonstrated that the ester linkage of similar copolymers resisted to higher TFA concentrations than ours. The comparison between the experimental feff values and the f values can bring valuable information on the macromolecular architecture (i.e. on the repartition of the charged monomers in the chain)

Used coded

Copolymers f aSa aAa

Findings

Conclusions