Hetero-Grafted Block Brushes with PCL and PBA Side Chains

Abstract

Giant amphiphilic macromolecules, hetero-grafted block brushes with a crystalline poly(ε-caprolactone) (PCL) brushlike block and an amorphous poly(n-butyl acrylate) (PBA) brush as the second block, were synthesized entirely by the “grafting from” approach using a combination of atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP). The following steps were employed for the preparation of the brush: (1) A well-defined poly(2-hydroxyethyl methacrylate) (PHEMA) was synthesized by ATRP. (2) PHEMA was used as a macroinitiator for chain extension with 2-(trimethylsilyloxy)ethyl methacrylate (HEMATMS) via ATRP to provide PHEMA-b-PHEMATMS. (3) The hydroxyl groups of PHEMA were transformed to bromoisobutyryl ATRP initiating groups to give poly[{2-(2-bromoisobutyryloxy)ethyl methacrylate}-block-(HEMATMS)], PBiBEM-b-PHEMATMS. (4) PBiBEM-b-PHEMATMS was transformed to PBiBEM-b-PHEMA by treatment with tetrabutylammonium fluoride (TBAF) in THF. (5) PCL was grafted from the PBiBEM-b-PHEMA macroinitiator by ROP in the presence of tin(II) 2-ethylhexanoate (Sn(EH)2) catalyst. (6) The PBA chains were grafted from the brush macroinitiators by ATRP. Molecular weight, molecular weight distribution, and the degree of polymerization (DP) of the backbone and the PBA and PCL side chains were determined by gel permeation chromatography (GPC) and 1H NMR spectroscopy. Thermal transition of the hetero-grafted block brush was investigated by differential scanning calorimetry (DSC). Molecular imaging by atomic force microscopy was used to verify the success of the synthetic strategy for this complex molecule. Because of their amphiphilic structure, the hetero-grafted diblock brushes demonstrated association into flower-like or dumbbell-like structures.