Conventional Oxyanionic versus Monomer-Activated Anionic Copolymerization of Ethylene Oxide with Glycidyl Ethers: Striking Differences in Reactivity Ratios

Detailed understanding of the monomer distribution in copolymers is essential to tailor their properties. For the first time, we have been able to utilize in situ H-1 NMR spectroscopy to monitor the monomer-activated anionic ring opening copolymerization (AROP) of ethylene oxide (EO) with a glycidyl ether comonomer, namely, ethoxy ethyl glycidyl ether (EEGE). We determine reactivity ratios and draw a direct comparison to conventional oxyanionic ROP. Surprisingly, the respective monomer reactivities differ strongly between the different types of AROP. Under conventional oxyanionic conditions similar monomer reactivities of EO and EEGE are observed, leading to random structures (r(EO) = 1.05 +/- 0.02, r(EEGE) = 0.94 +/- 0.02). Addition of a cation complexing agent (18-crown-6) showed no influence on the relative reactivity of EO and EEGE (r(EO) = r(EEGE) = 1.00 +/- 0.02). In striking contrast, monomer-activated AROP produces very different monomer reactivities, affording strongly tapered copolymer structures (r(EO) = 8.00 +/- 0.16, r(EEGE) = 0.125 +/- 0.003). These results highlight the importance of understanding reactivity ratios of comonomer pairs under certain polymerization conditions, at the same time demonstrating the ability to generate both random and strongly tapered P(EO-co-EEGE) polyethers by simple one-pot statistical anionic copolymerization. These observations may be generally valid for the copolymerization of EO and glycidyl ethers.