Thermally-labile segmented hyperbranched copolymers: using reversible-covalent chemistry to investigate the mechanism of self-condensing vinyl copolymerization

Abstract

A thermally-reversible inimer was used to confirm the controlled growth of individual branches during self-condensing vinyl atom transfer radical polymerization (ATRP). Segmented hyperbranched polymers were synthesized by ATRP of methyl methacrylate (MMA) and a novel inimer that contained a thermally labile Diels–Alder linkage between its initiating and polymerizable moieties. Three distinct feed ratios of MMA to inimer (15 : 1, 30 : 1, and 60 : 1) yielded hyperbranched polymers with variable degrees of branching and molecular weights in the range of 120 000 to 515 000 g mol⁻¹. The resulting hyperbranched polymers contained thermally-reversible branch points that were cleaved quantitatively on heating to yield linear polymers with molecular weights that were similar to the theoretical values that would be expected based on controlled chain growth of individual branches during self-condensing vinyl polymerization (SCVP). The cleaved linear polymers contained pendant furan and terminal maleimide functionalities that allowed reassembly at 50 °C to form “healed” hyperbranched polymers. The healing efficiency was determined by ¹H NMR spectroscopy, and the molecular weights of the repaired hyperbranched polymers were characterized by gel permeation chromatography. A segmented hyperbranched polymer was employed as a multifunctional macroinitiator to prepare an amphiphilic “hyper-star” via chain extension with poly(ethylene glycol) methyl ether methacrylate. Assembly of these “hyper-stars” into well-defined micelles (∼23 nm) in neutral water was confirmed by transmission electron microscopy and dynamic light scattering.