Aqueous Cu(0) Mediated Reversible Deactivation Radical Polymerization of 2-Hydroxyethyl Acrylate
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Reversible deactivation radical polymerization (RDRP) of 2-hydroxyethyl acrylate in D2O was studied using Cu(0) wire and Me6TREN, with different target chain lengths (TCL), amount of initial Cu(II)Br2, and the addition sequence of ligand. Gel formation occurred under most conditions, with the amount of gel formation reduced with short target chain lengths, increased concentration of initial Cu(II)Br2, and when ligand was added with the Cu(II)Br2. This result was explained by the reduced concentration of adsorbed active polymer species on the Cu(0) wire surface through increased desorption rate and deactivation rate on the Cu(0) wire surface, respectively. Gel formation was eliminated using a two-step Cu(0) in situ mediation process, with experiments focusing on the effects of chain length, initial Cu(I)Br, excess NaBr, temperature and residual O2 (brought by syringe for monomer and initiator transferring) on the kinetics and molecular weight (MW) control of the system. There is no visible gel formation with the TCL range from 20 to 800 using this procedure, a result mainly attributed to the lowered number of adsorbed active species per Cu(0) particle, which are highly dispersed, greatly reducing the probability of crosslinking. The concentration of adsorbed active species on Cu(0) particle surface is the key factor controlling polymerization control and the formation of a high molecular weight (MW) shoulder seen under many conditions. Thus, physical processes such as adsorption and desorption combine with kinetic processes such as activation, propagation and deactivation on the Cu(0) surface and in the solution to influence the ability to control chain growth. Shorter target chain lengths, low activator concentration, high deactivator concentration and higher temperature (22 ℃ rather than 0 ℃) all reduce the concentration of adsorbed active species on the Cu(0) surface during the polymerization, and thus improve control. The study indicates that lower concentration of adsorbed active species is the requirement for synthesizing well defined P(HEA) in the aqueous solution, with low Đ and without high MW shoulder and insoluble gel. Based on this insight, well defined P(HEA) with low Đ (~1.2) and no high MW shoulder was synthesized when DPn was less than 160 in pure aqueous solvent (D2O and H2O). Higher MW P(HEA) (TCL=400, 87% conversion) with Đ value of 1.16 was synthesized within 75 min using ca. 250 ppm copper in D2O at 22 ℃, however with a high MW shoulder observed, the first time that high MW P(HEA) with low Đ has been synthesized using such low copper levels in the purely aqueous environment.