Dual role of methyl-β-cyclodextrin in the emulsion polymerization of highly hydrophobic plant oil-based monomers with various unsaturations

Abstract

Although series of latexes from plant oil-based acrylic monomers (POBMs) were recently synthesized in emulsion, two factors—allylic termination (the chain transfer reaction to the fatty acid double bonds) and the poor aqueous solubility of POBMs—lead to the lower total monomer conversion and molecular weight of the resulting latex polymers.In this study, amphiphilic oligosaccharide, methyl-β-cyclodextrin (M-β-CD), was used to improve the polymerizability of monomers from high oleic soybean (HO-SBM) and linseed (LSM) oil in copolymerization with styrene. Using X-ray diffractometry and differential scanning calorimetry, interactions between the monomers and M-β-CD were confirmed, while the formation of 1:1 complex from oligosaccharide and each monomer molecules was demonstrated by mass spectrometry.In the presence of “host-guest” complexes, polymer yield increases as the coagulum amount drops during the emulsion polymerization of both plant oil-based monomers with styrene indicating their enhanced aqueous solubility. Remarkably, latex polymers with a consistently higher molecular weight were obtained in the presence of M-β-CD. The complex formation and incorporation of monomer molecules into the oligosaccharide cavities protect the fatty acid moieties and diminishes chain transfer. The latter assumption was quantitatively confirmed in 1H NMR spectroscopy by determining the number of protons of the alkyl carbon−carbon double bonds (–CH = CH–) and the bisallylic hydrogen atoms (–CH = CH–CH2–CH = CH–) in the unsaturated fatty acid moieties of the latex copolymers. The observed effect is more pronounced for more unsaturated monomer from linseed oil.Based on the obtained results, M-β-CD plays a dual role in both enhancing POBM polymerizability as well as protecting against allylic termination chain transfer.