Double metal cyanide (DMC) catalysis: detailed in situ NMR kinetics studies on the copolymerization of propylene oxide with substituted epoxides
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Abstract
Recently, the unusual copolymerization behavior of propylene oxide (PO) with ethylene oxide (EO) and glycidyl methyl ether (GME) using double metal cyanide (DMC) catalysis was demonstrated. Inspired by these results, the DMC-catalyzed copolymerization of PO with a large variety of alkyl and aryl epoxides, as well as other glycidyl ethers and -esters, was investigated by in situ1H NMR spectroscopy. This revealed a strong influence of the substituent of the epoxide monomer on the reactivity. With increasing steric demand of the alkyl or aryl group at the oxirane ring, the reactivity difference to PO increased drastically. Reactivity ratios for the copolymerization with PO for butylene oxide (BO) (rPO = 4.7, rBO = 0.21), 1,2-epoxyhexane (HexO) (rPO = 16, rHexO = 0.063), 3-methyl-1,2-epoxybutane (MEB) (rPO = 29, rMEB = 0.034) and styrene oxide (SO) (rPO = 21, rSO = 0.047) were determined. Due to the high steric demand of 3,3-dimethyl-1,2-epoxybutane (DMEB) in the copolymerization experiment with PO, only homopolymerization of PO was observed. Additionally, the copolymerization behavior of PO with various glycidyl ethers (GE) was investigated. This revealed similar reactivity ratios for all glycidyl ethers in a range of rPO = 9.2–21 and rGE = 0.11–0.048. Further investigation of the two glycidyl esters, glycidyl methacrylate (GMA) (rPO = 20, rGMA = 0.050) and glycidyl benzoate (GBz) (rPO = 33, rGBz = 0.031), was conducted as well. The extensive studies give insight into the compatibility of different comonomers in DMC catalysis and confirm its high substrate selectivity, specifically for the PO monomer. The influence of steric demand and electronic structure of individual monomers was assessed using %VBur, Hirshfeld partial charges, and ΔN (electron transfer to Zn2+) and discussed in the context of comonomer reactivity relative to PO under DMC catalysis.