PEGMAs with short and long side chains: what is the effect in the formation of stars and brushes by RAFT polymerization?

Abstract

Reversible addition fragmentation chain transfer (RAFT) polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA_n, where n is the PEG side chain length) for the formation of star-like molecules and bottlebrushes was systematically studied using ethanol as solvent at 70 °C. The effect of PEG units in the side chain on the kinetics of the (macro)monomers 2-ethoxy ethyl methacrylate (EEMA₁), PEGMA₉, and PEGMA₂₃ was investigated in the presence of 4-cyano-4-(dodecylsulfanylthiocarbonyl) pentanoic acid (CDP) as RAFT agent and 4,4′-azobis(4-cyanopentanoic acid) (ACPA) as initiator. Two periods could be distinguished in the kinetics: (i) the initialization stage related to the pre-equilibrium and competition between the propagation and the first addition of monomer to primary radicals, and (ii) a main equilibrium process; both were quantified via the apparent rate constants k^app₁ and k^app₂, respectively. While EEMA₁ and PEGMA₉ RAFT polymerization presented an initiator concentration-dependent acceleration in the first stage, PEGMA₂₃ exhibited inhibition, with practically no polymerization, when the CDP amount was decreased. A retardation effect was observed during the main stage, which decreased at longer PEG side chains of the (macro)monomer. Finally, 1,4-dioxane was used as a solvent for PEGMA₂₃ polymerization and the resulting data were compared with those for the kinetics using ethanol; the former presented not only a strong acceleration in the first stage of the polymerization, reaching 60% conversion at a short time, attributed to the enhanced solubility of PEGMA₂₃, but also stronger discrepancies for RAFT polymerization from experimental and theoretical data.