Methylation of poly(acrylic acid), prepared using RAFT polymerization, with trimethylsilyldiazomethane: A metamorphosis of the thiocarbonyl group to a thiol-end group

Abstract

The Reversible Addition Fragmentation chain Transfer (RAFT) polymerizations of acrylic acid were performed in 1,4-Dioxane solution using three different chain transfer agent (CTA), and 4,4′-azobis(4-cyanopentanoic acid) (ACPA) as initiator. The molar ratios of monomer, CTA and initiator were kept similar for each experiment and all polymerization exhibited high conversion (>90%) at 8 h. Well-defined polyacrylic acids (PAAs) were obtained. Their molecular weights increased linearly with monomer conversion and the dispersity of the molecular weight distribution extended between 1.1 and 1.3. Nuclear magnetic resonance (NMR) spectroscopic studies showed that the PAA prepared are actually random copolymers of AA and the dimer of AA (H2C=CHCO2CH2CH2CO2H), which forms spontaneously from AA, even at room temperature. PAA samples were methylated by addition of trimethylsilyldiazomethane (TMSCHN2 a methylating agent) using a mixture of water/tetrahydrofuran (THF) or methanol/THF. Samples were submitted to different analyses before and after the methylation process in the order to examine the effect of the methylating agent. PAA were characterized by size exclusion chromatography (SEC) in aqueous phase and also in organic phase following methylation. Methylation is accompanied by variable amounts of chain cleavage in the case of PAA prepared using a symmetrical, trithiocarbonate RAFT chain transfer agent. Less extensive cleavage is seen on treatment of poly(methyl acrylate) (PMA), prepared from the trithiocarbonate RAFT reagent with TMSCHN2. Fourier transform infrared (FT-IR), UV-Vis, 1D and 2D NMR spectroscopic studies confirmed that TMSCHN2 reacts with the thiocarbonyl group, though all of the products of this reaction could not be identified. End group analysis using Ellman's reagent demonstrates that some of the polymer chains have a thiol end group, as does a thiol-ene reaction using 9-vinylcarbazole.