Monodisperse amphiphilic double-crystalline block oligomers composed of linear alkyl chains and poly(vinyl alcohol) segments prepared by aldol-group transfer polymerization and recycling size-exclusion chromatography separation

Abstract

As a model of block copolymers of ethylene and vinyl alcohol, monodisperse amphiphilic double crystalline diblock oligomers composed of linear alkyl chains and poly(vinyl alcohol) segments were prepared using aldol-group transfer polymerization (aldol-GTP) and recycling size-exclusion chromatography (SEC) separation. The aldol-GTP of tert-butyldimethylsilyl vinyl ether (TBSVE) was conducted using octadecyl aldehyde as an initiator and ZnBr₂ as a catalyst for the synthesis of oligo(TBSVE) with controlled molecular weights and a C16 alkyl chain and an aldehyde group at the α- and ω-chain ends, respectively. The obtained oligomers were separated into monodisperse unimers, dimers, trimers, tetramers, and pentamers via preparative recycling SEC. Reduction of the terminal aldehyde group followed by deprotection of the silyl groups resulted in monodisperse amphiphilic oligomers composed of C16 alkyl chains and poly(vinyl alcohol) (PVA) units. The effects of the number of PVA units on the thermal properties were characterized using differential scanning calorimetry. Furthermore, the morphology of the discrete block oligomer was examined by optical microscopy, atomic force microscopy, and small- and wide-angle X-ray scattering. Owing to the high incompatibility between the hydrophobic and hydrophilic blocks, self-assembly was observed despite the very low molar weights of the polymers. Based on our results, we propose that a double-crystalline lamellar structure is formed and consists of alternating hydrophobic (alkyl chains) and hydrophilic (PVA segments) domains (with a period of 6.8 nm), showing a distinct crystallization/melting process.